Sedimentation as a three-component system: organic carbon, carbonate, noncarbonate

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Author: Werner Ricken

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Lecture Notes in Earth Sciences Editors: S. Bhattacharji, Brooklyn G. M. Friedman, Brooklyn and Troy H. J, Neugebauer, Bonn A. Seilacher, Tuebingen and Yale

51

Werner Ricken

Sedimentation as a Three-Component System Organic Carbon, Carbonate, Noncarbonate

Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo Hong Kong Barcelona

Budapest

Author Prof. Dr. Werner Ricken Geological Institute. University of Cologne Ztilpicher Str. 49 a, D-50674 KtSIn, Germany

"For all Lecture Notes in Earth Sciences published till now please see final pages of the book"

ISBN 3-540-57386-0 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-57386-0 Springer-Verlag New York Berlin Heidelberg

This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. 9 Springer-Verlag Berlin Heidelberg 1993 Printed in Germany Typesetting: Camera ready by author 32/3140-543210 - Printed on acid-free paper

PREFACE

Sedimentation as a Three-Component System describes the most common styles of deposition in marine environments as they relate to sediment composition. Three components, organic matter, carbonate, and siliciclastic sediment, may settle concurrently, but at different rates, intermixing on the sea floor to form a particular sediment composition. A change in the flux of one component is capable of relatively diluting or concentrating the other two components, which can be expressed in the characteristic ratio of organic carbon to carbonate in the resulting sediment. The basic concept of this book is to address organic carbon-carbonate associations in terms of depositional inputs and time spans. In addition, the three-component system describes organic carbon changes related to major facies transitions. Examples include models of the genesis of carbonaceous sediments, with their various laminated to bioturbated lithotypes, and numerical organic carbon prediction. I hope that this book will encourage stimulating discussions and promote a new approach to quantitative stratigraphy. I would like to thank G. Einsele for his encouragement, exchange of ideas, and unflagging support throughout the development of this work. Many friends and colleagues helped me with their criticisms, which kept me thinking straight and on my toes. These are T. Aigner, R.G.C. Bathurst, W.H. Berger, D.J. Bottjer, P.L. de Boer, F.W. Eder, W.P. Elder, K.C. Emeis, A.G. Fischer, K.B. F611mi, G.M. Harwood, Ch. Hemleben, J.D. Hudson, E.G. Kauffman, W. Kuhnt, D. Leythaeuser, B. Ligouis, H.-P. Luterbacher, J.D. Milliman, G. M/iller, F. Niessen, L.M. Pratt, R. Riding, J. Rullk/Stter, B.B. Sageman, P.A. Sandberg, C.E. Savrda, A. Seilacher, M. Sibuet, R.H. Stein, E. Suess, R. Tada, J. Thurow, H. Weissert, G. Wefer, and A. Wetzel. Their help is gratefully acknowledged. Last but not least, I am indebted to the several kind persons who gave so generously of their time to read this text critically. The written comments of G. Einsele, L. Hobert, B.B. Sageman, and P.A. Sandberg improved this work enormously. From the beginning of this project, I enjoyed the cooperation with W. Engel and the Springer-Verlag. To all of them I give my grateful thanks.

Cologne, October 1993 Werner Ricken

CONTENTS

List of the most commonly used parameters . . . . . . . . . . .

XI

P A R T I T H E T H R E E - C O M P O N E N T SYSTEM . . . . . . . . .

1

C h a p t e r 1 Sediment recipes . . . . . . . . . . . . . . . . . .

3

Chapter 2 Depositional dilution processes with three c o m p o n e n t s . . 10 2.1 2.1. I 2.1.2 2.2

Definitions and stipulations . . . . . . . . . . . . . . The three sediment fractions . . . . . . . . . . . . . . The "main sediment" and the "background s e d i m e n t " . . . . Basic types of deposition and related Cor~-CaCO3 patterns . . . . . . . . . . . . . . . . . . . . 2.3 Identification of carbonate and siliciclastic Cor.-CaCO3 curves . . . . . . . . . . . . . . . . 2.4 Relative sedlmentataon rates using organic carbon dilution equations . . . . . . . . . . . . . . . . . 2.4.1 Combining sedimentation rates from different Corg-CaCO3 curves . . . . . . . . . . . . . . . . 2.5 Conclusions . . . . . . . . . . . . . . . . . . . .

Chapter 3 Factors influencing the three-component system . . . . 3.1 3.2 3.2.1 3.2.2

The role of organic carbon preservation . . . . . . . . . The influence of early and late diagenetic processes . . . . Diagenetic loss of organic matter . . . . . . . . . . . . Differential carbonate diagenesis . . . . . . . . . . . .

10 10 12 12 16 19 26 28

30 30 33 33 36

VIII

3.3 Decoupling o f sediment fluxes . . . . . . . . . . . 3.3.1 The interrelationship between marine organic matter and calcium carbonate fluxes . . . . . . . 3.3.2 Partial flux decoupling for carbonate and siliciclastic deposition . . . . . . . . . . . . . 3.3.3 Organic matter in the main sediment and in the background sediment . . . . . . . . . . . . . . 3.3.4 Alternative interpretations . . . . . . . . . . . . . 3.4 The validity and limitations o f the threecomponent system . . . . . . . . . . . . . . .

.

36

. .

36

.

38

.

.

42 44

.

45

Chapter 4 Relative time span assessment . . . . . . . . . . . . 4.1 4.2 4.3 4.4

Conventional time span determination . . . . Time span determination using sedimentation rates . . . . . . . . . . . . . . . . . Time span assessment using relative sedimentation rates . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . .

47

. . . . .

48

. . . .

48

. . . . . . . .

52 55

P A R T II C A R B O N A T E - O R G A N I C C A R B O N D I S T R I B U T I O N IN B E D S AND S E Q U E N C E S . . . . . . . . . . . .

57

C h a p t e r 5 Input variation in rhythmically bedded sediment

59

5.1 5.2 5.2.1 5.2.2 5.3 5.3.1 5.3.2 5.4 5.4.1 5.4.2 5.5

.

Idealized bedding rhythms for carbonate, siliciclastic and organic matter deposition . . . . . . . . Sediment input pattern as expressed in Corg-CaCO 3 data for various marine environments . . . . . . . . Environments with variation of a single component . . . . Environments with simultaneous variation of several components . . . . . . . . . . . . . . . Relative sedimentation rates and time spans inherent in beds . . . . . . . . . . . . . . . . . Rhythms due to depositional variation of one dominant parameter . . . . . . . . . . . . . . . Rhythms due to simultaneous variation in several depositional parameters . . . . . . . . . . . . . . Statistical evaluation o f carbonate contents in rhythmically bedded sediment . . . . . . . . . . . Carbonate differences between alternating beds . . . . . . Variation in the sedimentation rates o f deep sea bedding rhythms . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . .

60 63 63 66 66 66 67 70 70 73 76

C h a p t e r 6 Combined input pattern of transgressive-regressive

cycles, Upper Cretaceous, U.S. Western Interior . . . .

80

6.1 6.2

Geologic setting and source of data . . . . . . . . . . 80 Contrasting relationships between carbonate and organic carbon content . . . . . . . . . . . . . . 82 6.3 A model for combined changes in depositional input: fractional sedimentation rates for entire sequences . . . 84 6.4 Diagenetic influences . . . . . . . . . . . . . . . . 88 6.5 Fractional sediment inputs in the Greenhorn and N i o b r a r a T R cycles . . . . . . . . . . . . . . . 89 6.5.1 G r e e n h o r n T R cycle . . . . . . . . . . . . . . . . . 89 6.5.2 N i o b r a r a T R cycle . . . . . . . . . . . . . . . . . 93 6.6 Testing the combined three-component model: cross-checking with time scales . . . . . . . . . . . 94 6.7 Conclusions . . . . . . . . . . . . . . . . . . . . 94

PART III CARBONATE - ORGANIC CARBON CHANGES IN FACIES TRANSITIONS . . . . . . . . . . . .

97

C h a p t e r 7 C a r b o n a t e - clastic s y s t e m s . . . . . . . . . . . . .

99

7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.2 7.2.1 7.3 7.3.l 7.3.2 7.3.3 7.3.4 7.4 7.4.1 7.5

Interrelationship between sedimentation rate and carbonate content . . . . . . . . . . . . . . . . Carbonate content - sedimentation rate relation . . . . . Basic facies transitions and standard equations . . . . . . Simultaneous variation of carbonate and siliciclastic deposition . . . . . . . . . . . . . . . . Recognition of beds with high and low carbonate contents: the chalk - clay issue . . . . . . . . . . Facies transitions and associated changes in organic carbon content . . . . . . . . . . . . . Overlapping Cor~-CaCO3 trends . . . . . . . . . . . . Vertical facies changes in sedimentary sequences . . . . Carbonate content distributions . . . . . . . . . . . . A s y m m e t r y relation . . . . . . . . . . . . . . . . C a r b o n a t e - p o o r zones at the base of calcareous sections . . . . . . . . . . . . . . . . . . . . Modelling carbonate input in vertical sequences (Gubbio section) . . . . . . . . . . . . . . . . Lateral facies changes . . . . . . . . . . . . . . . Calcareous and siliciclastic sediment wedges . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . .

100 104 104 1 11 112 I16 119 120 120 122 124 126 131 131 136

Chapter 8 Systems rich and poor in organic carbon . . . . . . Forecasting the organic carbon content . . . . . . . . 8.1 8.1.1 Organic matter in the background sediment: a critical factor for organic carbon prediction . . . . . . . . 8.1.2 Assessment of sedimentary organic carbon contents for calcareous and siliciclastic facies systems . . . . 8.1.3 Average organic carbon contents in carbonate-rich and carbonate-poor sediments . . . . . . . . . . . 8.2 The formation of bioturbated and laminated lithotypes . 8.2.1 Expression of varying organic matter supply . . . . . . 8.2.2 The slopes of C~g-CaCO3 trend lines: indications of bottom water oxygenation? . . . . . . . . . . . 8.2.3 Laminated to bioturbated lithotypes with low to high carbonate and organic carbon contents . . . . . . . Carbonaceous lithotypes controlled by background 8.3 deposition . . . . . . . . . . . . . . . . . . . 8.3.1 The black shale and the plattenkalk lithotype associations . . . . . . . . . . . . . . . . . . 8.3.2 Carbonaceous facies related to sedimentary condensation . . . . . . . . . . . . . . . . . . 8.4. Conclusions . . . . . . . . . . . . . . . . . . .

139 140 140 146 152 155 155 157 164 169 170 174 177

Epilogue . . . . . . . . . . . . . . . . . . . . . . . . .

181

References . . . . . . . . . . . . . . . . . . . . . . . . .

183

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . .

203

Subject Index . . . . . . . . . . . . . . . . . . . . . . .

207

LIST OF THE MOST COMMONLY USED PARAMETERS

C

Carbonate content in wt. %; subscripts (1, 2 to n) indicate sample numbers

C[vol.%]

Carbonate volume of the solid sediment or rock fraction

C I to Clv

Major facies associations with changing carbonate deposition, defined by different amounts of noncalcareous background sedimentation, increasing from Ct to Cry

Co,~

Organic carbon content in wt.%; subscripts (1, 2) indicate organic carbon contents of different samples being compared

Co,#c

Organic carbon content of the noncalcareous background sediment (composed of siliciclastic and organic matter), in wt.%

CorgNs

Organic carbon content of the nonclastic background sediment (composed of carbonate and organic matter), in wt. %

F

Dimensionless factor to describe by what magnitude a given sediment fraction is changed

h

Height or thickness of a stratigraphic unit (in m)

hc, hs

Height of newly-deposited sediment (in m), by carbonate or siliciclastic deposition, respectively

h~um

Cumulative thickness of time-equivalent depositional intervals

hr~c, hss

Height of the noncalcareous or nonelastic background sediment, respectively

J

Flux ~or rain) of inorganic or organic matter through the water column, in gm-2a-.

OM

Organic matter content of the sediment solids in wt. %; for most of the semilithified to lithified samples investigated here, OM is 1.3 * Cotz

OMNc

Organic matter content (wt.%) of the noncalcareous background sediment (composed of siliciclastic and organic matter)

OMr~s

Organic matter content (wt. %) of the nonclastic background sediment (composed of carbonate and organic matter)

OM[vol. %] Organic matter volume of the total sediment solids 0M[vol. %,bg] Organic matter volume of the background sediment @

Porosity of a bulk sediment or rock sample in vol. %

PP

Primary productivity of organic carbon in the surface ocean, in gCor~ m% ~

xII Pc, Rs

Rate of increasing (or decreasing) carbonate (C) or siliciclastic (S) deposition with constantly deposited background sediment; expressed as a percentage of the height of an initial sediment interval (h,); used for modelling sections

Pc,~, PoM

Grain density of carbonate, siliciclastic, and organic matter, in g/era 3

S

Siliciclastic proportion of sediment in wt. %; includes clays, quartz, and, occasionally, a minor amount of biogenous, siliceous sediment

S~ to Sw

Major facies associations with changing siliciclastie deposition, defined by different amounts of nonelastic background sedimentation, increasing from S x to Siv

s

Linear sedimentation rate for a porous or nonporous sediment or rock, in m/Ma; for all standard equations, sedimentation rate is expressed for solid, nonporous sediment.

Sc, Ss

Sedimentation rate of the carbonate and siliciclastic fraction (solid), respectively, in m/Ma

SNc

Sedimentation rate of the solid noncarbonate background fraction, including clastics and organic matter, in m/Ma

Sr~s

Sedimentation rate of the solid nonclastic background fraction, including calcareous and organic matter, in m/Ma

soM

Sedimentation or input rate of solid organic matter, in m/Ma, found in diagenetically altered, semilithified to lithified sediments; subindices [C] and IS] indicate carbonate and siliciclastic deposition, respectively

sr

Relative sedimentation rate between two samples (dimensionless, bulk sediment)

sR

Standardized, relative sedimentation rate of bulk sediment, determined by multiplying various, successive, relative sedimentation rates; dimensionless

S,c, Srs, S~oM Relative sedimentation rates of carbonate, siliciclastic or organic matter fractions (i.e., fractional sedimentation rates, dimensionless) sac, SR~, S~OMStandardized, relative sedimentation rate for the carbonate, siliciclastic, and organic fractions; dimensionless sTrqc, srss

Relative sedimentation rate of the noncarbonate and nonclastic fraction, respectively; dimensionless

T

Time span inherent in a stratigraphic sediment or rock interval

TR

Standardized, relative time span inherent in a stratigraphic interval, derived from standardized, relative sedimentation rates; dimensionless

Chapter 1

SEDIMENT

RECIPES

Sediment is deposited in the space provided between the subsiding sea floor and sea level. Climatic and eustatic variations leave their imprints and control the occurrence of calcareous, noncalcareous, or carbonaceous sediments. Sequences or cycles are thereby formed. Such facies successions are created by

THREE-COMPONENT SYSTEM A

A

sea surface

sediment surface

diagenetic 9

zone

"

I:':':':':':':'~ I ,::::::::.: :::i

(m-km) ;::i:i~i::iiki]

:':':':' I::::1 |::::::::/~:-.

FLUX Si;:;:;:i!i:: Si:i:i:i::

SEDIM ENT .......

S

~

:~i~ii:!~/ i ,i!?,- ,i ;~!i,i!~ ~,~i:'~~,i ~!, ,,,:: :,i;~.- ~:~i ! i : i~i~iiOrgarlloilCarDON ! !:!:!:~

' . ./

i.~ .;~ :;

.

~.i: ;;i~.i ~.i?.!!~.~..~ !.~:~ ;~ :.i. :i :~ ~ " :ii: :iiI

, ,i:andCa i

9

. i .II.

r bonatei!ii ,, ,:! i: i

Fig. 1-I Three-component system and related dilution and concentration processes. The combination of three predominant fluxes (carbonate, %; silieielastic sediment, s~; and organic matter, SoM) controls the concentration of carbonate and organic carbon m the resulting sediment. Fluxes originate somewhere in the water column and pass through a diagenetie zone, before they reach the partly lithified sediments investigated here. Note that a chan~e in one of these fluxes is capable of influencing the concentrations of the two remaining tractions.

changes in the flux of incoming sediment components. Without deciphering this flux pattern, depositional processes and the resulting changes in composition cannot be fully understood. Luckily, there are tools available for determining these fluxes in geologically old, lithified sediments, where time control is difficult. Consider the three-component system: a flux pattern may be identified by its characteristic dilution and concentration processes, as expressed in the organic carbon and carbonate contents in the resulting sediment. Dilution and concentration mechanisms are the prime processes during deposition. Sediment grains settle, but they may contain various components, each with a different flux, which intermix on the sea floor to form a specific sediment (Fig. 1-1). Instead of considering the great number of different potential constituents, we can deal here only with three major components: the calcareous, siliciclastic, and organic fractions. The first two components represent major sediment sources, while the organic fraction, which makes it to the sea floor is relatively small. Its concentration in the sediment is partly determined by the flux of carbonate and clastics. Using a three-component flux model, we can describe the major sediment groups occurring between the calcareous and noncalcareous end members, as well as their associated changes in organic carbon content and sedimentation rate. Why are depositional dilution and concentration processes related to changes in organic carbon content and sedimentation rate? Let us start with an example from the kitchen. Eggs and salt are the basic solid ingredients needed to make an omelette (let's pretend for the moment that fluids don't come into the picture). Take three eggs and one quarter spoon of salt and mix them together in a bowl. Now we have just enough for one omelette (Fig. 1-2). Eggs and salt are our basic ingredients. What happens when we add a third component, flour? According to my cookbook, we have to add 250 grams of flour to get the right mixture for making pancakes. This introduction of flour, however, means that you can make two or three pancakes (instead of just one omelette). If more and more flour is added to the bowl, we will finally end up with a mixture which is something like bread dough. Then, about ten loaves of bread can be produced. The omelette-pancake-bread system follows a simple dilution process, expressed in the addition (or reduction) of flour. This system has two end products, omelettes and bread, andtransitional products, such as various types of pancakes with different flour contents. Each of these products represents a different food facies, and each food facies is supplied in a different quantity. The supply rate of food (equivalent to the total sedimentation rate in the marine environment) increases with the addition of flour. Specifically, we have high supply rates of ingredients when the flour content of the food is large, or when our basic ingredients, eggs and salt, have low concentrations (Fig. 1-2). Try to imagine one of the ten breads after it is baked. The bread is not salty and has no egg taste at all. The bread is kosher. The introduction of so much flour diluted the concentration of eggs and salt. We have learned from our short cooking course that the food type and supply rate of ingredients are interrelated factors characterizing a three-component system. Let's go back to the depositional environment and consider a sediment

"'

..........

~

r--z

~ ~ ~

~.-'~=

(/)O

~

.-~. I~1

o:~ cc I " ~ "

_~

-/~

~= Fig. i-2 Dilution and concentration processes in the kitchen as an analogy to those in the geologic environment. Increasing flour (or carbonate) input to a background mixture gcnerams a succession of relatedfood (or sediment) types. These products denor~ a decreasing salt (or organic carbon) concentration with increasing flour (or carbonate) content.

composed of carbonate, siliciclastics (e.g., silt and clay), and organic matter. Silieiclastics and organic matter represent the background input, equivalent to eggs and salt. As with the addition of flour, when more and more carbonate is supplied to the sediment, different lithologies or facies types are formed (Fig. 1-2). Low rates of carbonate input may produce a shaly, relatively organic carbon-rich facies, representing background deposition of clasties and organic matter. When the input of carbonate is equivalent or somewhat greater than that of the siliciclastic fraction, marly sediment layers are generated, which are approximately three times thicker than the shale. For a good limestone with 90% carbonate, the carbonate supply must be nine times greater than that of the background input. Analogous to the supply rate of food, deposition (or sedimentation rate) increases with higher carbonate contents in the generated facies types. Increasing rates of carbonate deposition dilute the background sediment, and reduce the weight percentages of siliciclastics and organic matter, as illustrated in the following example.

EFFECT OF DILUTION BY CARBONATE DEPOSITION? C0rg -CaCO 3 CONTENTS, NIOBFIARA MARLS TO CHALKS

l-Z 4

I

uJ I-z 03 O z O m2 E: < O 01 z

0i 0

f [3o

q o

! 20

40 60 80 100 CARBONATE CONTENT [wt. %]

Fig. 1-3 Scatter diagram showin~ an inverse correlation between carbonate and organic carbon values. Such data may indicate an environment where carbonate supply shows significant variation, while clay and organic matter are more constantly delivered. Santonian

marls and chalks, Niobrara Formation, Western Interior Seaway (Lyons Quarry, Colorado, U.S.). Data: Rodriguez (1985) and author's own determinations.

During a research fellowship in Boulder, Colorado, I investigated Cretaceous shales and chalks deposited in the former Western Interior Seaway. There were a lot of suggestions as to why organic carbon-rich facies occurred in some of the lithologic units and not in others, but no satisfying explanation was offered. It was conspicuous, however, that most of the Cretaceous rocks showed an inverse relationship between carbonate and organic carbon contents. This was obvious, when carbonate and organic carbon data from short stratigraphic intervals were plotted in xy scatter diagrams; I was surprised by the basically straight-line relationships between the two concentrations (Fig. 1-3). High organic carbon concentrations occurred with low carbonate contents; but with increasing carbonate content, progressively lower organic carbon contents were obtained. Why was the organic carbon content so obviously related to the amount of carbonate? Why should a complicated depositional environment behave so systematically? Was this solely an effect of depositional dilution? Consider the shale-marl-limestone system (Fig. 1-2). The shale lithofacies represents basically background sediment. Therefore, this shaly lithofacies is relatively rich in organic carbon but very poor in carbonate content. The marl facies is the product of increased carbonate deposition, which, in turn, reduces the weight percentage of organic carbon (when the supply rate of organic matter remains unchanged). This reduction in organic carbon concentration is even more drastic in the limestone facies. Plotting carbonate and organic carbon in a scatter diagram, an inverse, straight-line correlation between the two parameters is obtained. Such a relationship indicates a sedimentary system which is controlled by changing carbonate deposition. Working things from the other end, we can deduce the type of deposition from the type of correlation between carbonate and organic carbon contents. So far, we have only considered the shale-marl-limestone system, in which changing calcareous deposition generates an inverse relationship between carbonate and organic carbon contents. But there are also other depositional dilution-concentration processes which are documented by other Co~-CaCO3 correlations, such as variations in siliciclastic deposition and in orgamc matter deposition, or combinations thereof. It is beyond this introduction to describe all of them here, but the main idea is that the distinct pattern of Corg-CaCO3 relationships can be used to identify the type of deposition. Relative changes in sedimentation rates can then be derived from the Cor data using dilution equations. Sedimentation rate changes, in turn, are used to assess the time spans inherent in depositional units such as individual beds and sequences of larger thickness. This integrated theory of depositional dilution-concentration processes can consequently explain deposition and facies types, organic carbon concentrations, sedimentation rates, and high resolution time spans (Fig. I-4). The transititon from omelettes to bread is accompanied by dilution of the smallest ingredient, salt. In contrast, salt is enriched when less flour is used (Fig. 1-2). In the equivalent shale-marl-limestone system, the transition from shale to limestone is accompanied by diluting the smallest fraction, organic carbon, while a limestone to shale transition is attended by concentrating the organic carbon. These fundamental dilution and concentration processes influence the occurrence of carbonaceous strata, for instance, in the sediments of

the Western Interior Seaway, where carbonate production is controlled by third order sea level stands. Additionally, other depositional styles (e.g., siliciclastic or organic matter deposition) create other facies transitions with other organic carbon distributions. What are the combined effects of these various depositional styles? Can they be quantified and predicted in facies investigations and source rock evaluations?

T H R E E - C O M P O N E N T SYSTEM AND RELATED C O N C E P T S ONCENTRATION AND DILUTION OCESSES OF NIC CARBON CONTENTS E-COMPONENT SYSTEM)

PREDICTION OF ORGANIC ~ETERMINATIONZ.7 '~' CARBON

~F.ASiC

/

co~rrE~s

pEPOSITIONAL ! ~ I ~ o I

[TIME SPAN

I

I MAJOR FACIES ASSOCIATIONS / (CARBONATE t"1 SILICICLASTIC TRANSITIONS)

"~1

] ISEOiMENTATION

I

Fig. 1-4 Summaryof concepts addressed in this book. The admixture of one ingredient results in a family of food products, similar to a family of related facies types in sedimentary geology, called a *facies association" (e.g., the shale-marl-limestone association). Both the carbonate and elastic depositional systems have several facies associations, which are related to the different inputs of background components. A statistical evaluation of

several thousand organic carbon values shows that each of these facies associations is connected with a given range of organic matter concentration. This is the basis for further numerical analyses in order to forecast sedimentary organic carbon contents. But geologists are not only interested in forecasting organic carbon concentrations (e.g., for basin analysis and source rock evaluation). They want to understand the specific conditions of deposition (e.g., levels of water mass oxygenation) of the various laminated to bioturbated lithotypes rich to poor in organic carbon and carbonate content. Again using salt as an analogy to organic carbon, a quick taste test of the different food products will soon confLrm that the omelette is saltiest, because there is little or no flour to dilute it. The omelette becomes even overly salty if only one egg is used instead of three, or when more salt is added. Indeed, many sediments rich in organic carbon are formed during conditions of relatively low background sedimentation rates. But the highest concentrations of organic carbon occur when additional dilution by the main sediment fraction is low, and when a high amount of organic matter is supplied, e.g., in situations of high productivity or low levels of bottom water oxygenation.

Chapter 2 D E P O S I T I O N A L D I L U T I O N P R O C E S S E S W I T H THRF~E COMPONENTS

It is well known that depositional dilution-concentration processes influence organic carbon contents. For example, dilution by siliciclastics occurs in areas of high terrigenous input, as in the northern Gulf of Mexico (Dow and Pearson, 1975), the northwest African margin (lbach, 1982), the Niger Delta (Tissot and Welte, 1984), the southwest margin of South America (Pederson and Calvert, 1990), and in anoxic sediments in the Mediterranean and Black Seas (Stein, 1986, 1991). Dilution by evaporites is observed in Tertiary sediments of the Rhine Graben (Hofrnann et al., 1994). Dilution processes have only been described qualitatively in the literature, with emphasis on siliciclastic dilution. As apparently no systematic quantification had been attempted, a three-component flux model was developed as presented here, in which the various dilution and concentration processes are quantitatively described for idealized conditions. 2.1 Def'mitions and stipulations 2.1.1 The three sediment fractions

Marine sediments, be they porous and soft or lithified, are composed of three major components, which constitute their solid fraction. These are carbonate, (silici)clastic sediment, and organic matter. 1. The carbonate fraction in fine-grained sediment is composed primarily of pelagic (calcite) ooze, while shallow water carbonates contain aragonite, calcite and Mg calcite. Additionally, the calcareous fraction may contain other types of marine carbonate, such as bioelastie carbonate; in exceptional eases detrital carbonate from terrigenous sources may also be present. In the oceanic and shelf environments, variations in carbonate deposition can be related to changes in primary productivity (Berger and Diester-Haass, 1988; Berger et al., 1989); but in the deep sea below the lysoeline, CaCO3 dissolution affects the carbonate supply reaching the sea floor (Berger, 1970, 1976; Berger et al., 1978, 1982; Peterson and Prell, 1985; Farell and Prell, 1987; Gr6tsch et al., 1991). 2. The silieiclastie or elastic fraction is composed of mostly fine-gained silicates, such as clay minerals and quartz, but also of coarser-grained, terrigenous sediment (Wedepohl, 1970; Bausch, 1982). Most of the sediment is provided by riverine, and to a lesser degree, eolian input. Variation in elastic deposition reflects variations in climate, sea level, and tectonic activity. Only small admixtures of biogenic siliceous ooze (e.g., Bohrmarm et al. 1990) is included in the noncarbonate fraction and is thus understood here to belong to

11

the "siliciclastic" fraction. 3. The much smaller organic fraction is composed of any possible composition and from any possible source, including matter primarily produced in the marine environment and imported terrestrial organic matter, as well as their diagenetic modifications, up to the onset of the oil window (e.g., Tissot et al., 1979, 1980; Degens et al., 1986; Stein et al., 1986, 1989). Variations in the organic matter supply reflect, on the one hand, changing primary productivity of the surface waters (e.g., Berger et al., 1989; Wefer, 1989), and, on the other hand, different degrees of decomposition, as related to water mass oxygenation, water depth, and sedimentation rate (MfiUer and Suess, 1979; Demaison and Moore, 1980; Suess, 1980; Bralower and Thierstein, 1984; Demaison, 1990; Stein, 1991; see Chap. 3.2).

THREE-COMPONENTSYSTEM '~Z---Oz~_l ml

IORGANIC 'l

< m I.-I

IMATmR I pEpOSmONI ' CA~IBONA'rE CONTENT

I .,UC,CL S CI fCAR.O] CARBONATE CONTENT ~

CONTENT

Fig. 2-1a The basic flux pattern in the three-component system. Each apex of the triangle represents one of the basic types of deposition (deposition dominated by varying carbonate, siliciclastic, and orgamc matter), resulting in distinctive Co,,-CaCO~ associations (schematic). These basic types have an idealized flux pattern as illffstrated m Fig. 2-Ib.

12

2.1.2 The "main sediment" and the "background sediment" In a sediment system with three components, the various depositional co-fluxes can be fairly complicated. Therefore, idealized concentration-dilutionprocesses must first be considered, expressed in terms of carbonate, siliciclastic and organic matter deposition. In such idealized types of deposition, the input of only one fraction varies, while the input of the two remaining fractions remains approximately constant, for instance in the scenario illustrated in Fig. 12. Hence, the idealized sediment is viewed as composed of a main and a background fraction: The "main sediment" is supplied at a varying rate, whereas the "background sediment" is ideally supplied at a constant rate. Depending on the mode of deposition, different components constitute these two divisions. Variations in calcareous input (hereafter called "carbonate deposition"), implies that the main sediment is composed of carbonate, whereas the background sediment is composed of clastics and organic matter (Fig. 2-1). Variations in siliciclastie input (hereafter called "silicielastie deposition") is characterized by the terrigenous fraction forming the main sediment, and carbonate and organic matter as the background sediment. When only the input of organic matter fluctuates (hereafter called "organic matter deposition"), the main sediment is composed of organics, while the background sediment is comprised of clastic and calcareous sediment (Fig. 2-1). Note that the main sediment is not necessarily identical with the largest fraction; it is only supplied in a more varying input compared to the more constantly delivered background fraction. 2.2 Basic types of deposition and related Cor~-CaCO3 patterns The basic types of deposition are recognized by distinctive Cocg-CaCO3 relationships, as outlined in the following overview and schematically illustrated in Fig. 2-1: Variation in carbonate deposition: The principles of carbonate deposition were already described in Chap. 1 (see Fig. 1-2). Organic carbon concentrations are either increased or decreased by decreasing or increasing carbonate supply, respectively, so that the carbonate content shows an inverse relation with the organic matter content in the resulting sediment (Fig. 2-1). Variation in silieiclastie deposition: Superimposed on constantly supplied background deposition of carbonate and organic matter, elastic deposition dilutes or concentrates both background fractions proportionally. One can observe a covariation in the weight percentages of carbonate and organic carbon in the sediment. Variation in organic matter deposition: Changes in the organic matter supply, which are usually small compared to the more pronounced changes in the supply of the two other, larger fractions, only slightly influence the concentration of CaCO3. The organic carbon content largely fluctuates independently of the carbonate content. These three basic relationships between organic carbon and carbonate content can be explained by assuming idealized depositional inputs. The manner

13

CARBONATE DEPOSITION ORGANIC~o..~

9

CONTENT MATTER I ~ [voL%] ~

o~

z=

RELATIONSHIP INVERSE CARBONATEORGANIC CARBON CONTENT

o

ao

~ T E

CON1T=NT[~%]

5% 2,5% ORGANICMATTER CONTENTIVY.%}

MATTER 7.5%

SILICICLASTIC DEPOSITION ORGANICtc cot,r r ~

,

~-~/':~,

I

NORMAL

CONTENT CAR~TE

i

SEGMENT

l~

C O N ~ N T [~,%]

I

~

~

OROAMC

2.s% 5% 7.5% P,u,-n~ ORGANICMATTER CONTENT[~t.%]

ORGANIC MATrER DEPOSITION ORGANIC ta [voL%] 1

INDEPENDENT OFCARBONATE CONTENT

0

CARBONAI~CONTENT[~.%1

I

r-"

'

SEDIMENT MA1"r~

5"%

~).5%

14%

MAIN $EDIMB~T

ORQAI~CMAi~P.H CONTENT[v=~.%]

Fig. 2-1b The three basic types of deposition and the resulting principal trends between carbonate content and organic carbon content (vol. %). The main sediment is characterized by a varied input of one fraction, while the background sediment is ideally defined by a constant input of two fractions.

14

in which they are quantified is shown with the example illustrated in Fig. 2-2, which describes the effect on composition and sedimentation rate of systematic changes in carbonate, siliciclastic, and organic matter deposition. Consider a hypothetical sediment sample (or sedimentary rock) composed of 40 vol.% carbonate, 40 vol.% siliciclasties, 10 vol. % organic matter, and 10 vol. % pore space (Fig. 2-2a, arrow; the porosity chosen was so low to describe the conditions of most sedimentary rocks investigated). Due to their different grain densities, the weight percent of carbonate and organic carbon are 47.8 wt. % and 3.4 wt. %, respectively. The actual sedimentation rate is unknown for this sample, but its relative sedimentation rate (st) is defined as one. Carbonate deposition: How will the composition of the initial sediment change, if the carbonate input during the same interval were increased by a factor of two and the other inputs (i.e., the background sediment) were kept constant? The sediment then contains two volumes of carbonate and unchanged volumes of elastic and organic matter. When these volumes are expressed as weight percentages, the new sample has a higher carbonate content (64.7 wt.%) and a lower organic carbon content (2.3 wt.%), while the relative sedimentation rate, st, is higher than for the first sample by a factor of 1.4. Note that increasing or decreasing carbonate input yields a characteristic family of points (on a C.,z-CaCO3 diagram), along which the weight percent of organic carbon decreases as the carbonate content and relative sedimentation rate increase. The opposite applies if the carbonate input and the relative sedimentation rate decrease. Silicielastic deposition: Variation in siliciclastic input results in similar dilution or enrichment of organic carbon wt. % in the sediment. However, wt. % organic carbon and carbonate vary directly (Fig. 2-2b), rather than inversely, as they do with carbonate dilution. For example, if the siliciclastic input increases by a factor of two, the weight percent of organic carbon decreases from 3.4 to 2.3 wt. %, and, simultaneously, the weight percent of carbonate decreases (as a result of the higher amount of elastics) from 47.8 to 32.3 wt. %. Thus, variations in the siliciclastic input (with constant inputs of the other components) results in linear trends with positive slopes in a C~s-CaCO 3 diagram. Increasing sedimentation rates are attended by decreasing C~-CaCO3 contents. Organic matter deposition: As mentioned above, fluctuations in the input of organic matter predominantly influence the organic carbon content because the organic fraction is usually too small to significantly affect the percentage of the two other components. Consequently, the organic carbon dilution curve follows a straight line almost parallel to the y axis. Varying the input of organic matter will change the sedimentation rate only slightly. On the C,~( CaCO3 diagram in Fig. 2-2e, increasing the organic matter supply by a factor of two results in small increases in relative sedimentation rates by a factor of 0.02 to 0.10 (these numbers depend on the initial amount of organic matter).

15 MAJOR INPUT TYPES, Corg-CaCO 3 AND SEDIMENTATION RATE CHANGES "2 t'~ CARBONATE s I~..~.~,, ~-61 I DEPOSITION 'I "

y

~

'42~t~

~ t

,,~,._.

C volumes

st--/.4 ;'2-~-3"

t

Xf ~3

Sr=2_Y

;'17Z/.- ]

~ " ~:' - ~' ~:." .

1:1 CaCO3 186

-.

31.4

47`8

64.7

7 8 5 weight percent

CorgS4 Eorg~5 Cora3.4 forqZ3 Corgi4.

w'loCnrn

~" ~67 SIUCICLASTIC , /./~. :.~ DEPOSITION

0 ~ 100

zl ~'"=~ w%EaIs3 Sr =.67

b CaCO3

74.4

~

Sr=1

~IS

Sr=23

C

m.

volumes

,,=

Sr=.~ . . . . . .r'.lN--'l :,7:,-:

r16(

4Z8

628'

1

i;.-,i;i S =Io,, OH

OMJ 32.3

19.6 ~ weight percent

Corgi4 Corg4.5 Corg34 CorgZ3 Corgl.4j ORGANIC MATrER DEPOSITION st=.92

. . . .

st=.94 s. =1.

Sr--tl

~11.1 ~;~ volumes

c

.% 1~176c~C~ C=CO]

oM/

45.7~ weight 489 47.8 CorgO.9 [orgZ8 Corg34 Corg6,6Jpercent

Fig. 2-2 A detailed description of the lhrea types of deposition, involving the volumes of the three solid sediment fractions (C, S, and OM) and the porosity (~) typical of the lithified sediment. Sedimentation rates are expressed relative to the initial composition (arrow), which is assigned a relative sedimentation rate of s, = 1. Note that increasing or decreasing relative sedimentation rates (compared to the initial composition) determine the direction of shift along the Co,z-CaCO3 lines.

16

2.3 Identification of carbonate and siliciclastic Cm-CaCOs curves So far we have used the characteristic C,~-CaCO3 relationships to identify the basic types of deposition. Here, we want to quantify steeply sloping and flatly sloping dilution lines for a given type of deposition. Consider Fig. 2-3, where different background inputs generate a family of associated dilution curves or lines. These lines show the characteristic inverse and normal C.~.-CaCO~ relationships for carbonate and elastic deposition, respectively, but ~,ith different

ORGANIC MATTER CONTENT IN BACKGROUND SEDIMENT SlLICICLASTIC DEPOSITION 100%50%

10%

5%

r

I I

CALCAREOUS DEPOSITION

10%

t

/

50%100%

b'%

SEDIMENTJ OMNC

Fig. 2-3 How the percentage of organic matter in the background sediment controls the slopes of the Cor,-CaCO3 lines. With higher organic matter percentages in the background sediment (wt. %~ and OMNc), slopes become steeper. The background sediment in siliciclastic dep?sition is composed of carbonate and organic matter, whereas that in calcareous deposition contains silieiclastics and organic matter. Note the theoretical upper limits, when the background sediment is composed entirely of organic matter. angles of slope for background sediments with different organic matter contents. A high amount of organic matter in the background sediment (i.e., high OMNc or OMss values) represents steepl)" sloping Co~-CaCO3 lines. Such lines reflect either a large supply of orgamc matter, or a small supply of the nonorganic portion in the background fraction (see Chap. 8.1.1). As the organic matter content in the background sediment increases, the dilution line slopes steepen; this effect becoming less pronounced as organic matter percentages increase (OMr~c, OM~, Fig. 2-3). In calcareous deposition,

17

there is a theoretical upper limit for this slope, when the entire background deposition is composed of organic matter (i.e., 85 ~ in Fig. 1-3). But in most of the investigated sediments, background organic matter content is below

Corg-CaCO 3 PATTERN OF CARBONATE AND CLASTIC DEPOSITION Corg [wt.%], OM[wt.%]

5 /.

3 2

0

u

20

40

60

80 100 % COCO3

Fig. 2-4 Two suites of intersecting C~-CaCO 3 dilution lines, indicating carbonate (solid lines) and siliciclastie deposition (dash*ed lines), expressed in a Co,s-CaCO3 coordinate system. Small numbers on the dilution lines denote the percentage of organic matter in the background sediment (OMr~c and OMm, respectively). 50% and rarely reaches the theoretical upper limit (see Chap. 8.1). In silieiclastic deposition, the theoretical limit (with OMr~s = 100%) is identical to the y axis (i.e., organic carbon axis, Fig. 2-3).

18

The various differently sloping Co~-CaCO3 lines are defined by the various different quantities of organic matter contained in the background sediment. This means that the OMNc or OMNs value is identical for all possible C_CaCO3 pairs defining one given dilution line. For carbonate deposition, th-"e weight percent of organic matter (OMNc) in the noncalcareous background sediment is 1.3 Con OMNc [wt. %] -

,

(2-1)

1-0.01C where C.~. is the organic carbon and C the carbonate content; the number 1.3 denotes th'e conversion factor between the organic matter and organic carbon contents for semilithified to lithified sediments (see below). The OM~c value can be taken from the scatter diagrams by extrapolating a C,~-CaCO 3 trend (i.e., a dilution-concentration line) towards zero carbonate content. The resulting background organic matter content is read on the C,~ axis, multiplied by 1.3. In siliciclastic deposition, the background sediment is composed of carbonate and organic matter (see Fig. 2-1). The weight percent of organic matter (OMm) in this background sediment follows the equation 130 C~l OMNs [wt. %] =

,

(2-2)

1.3 Co,z + C where Co,z is the organic carbon content and C the carbonate content (both in wt. %). When a data trend is extrapolated towards a high carbonate content (approaching 100%CaCO3), the OM~s value is read at its intersection with the dilution line which represents the theoretical upper limit for carbonate deposition (Fig. 2-3). In a C.,.-CaCO3 coordinate system, the graphic representation of Eqs. 2-1 and 2-2 rdults in two intersecting sets of lines with various OM~c and OMNs values (Fig. 2-4). One set, carbonate deposition, intersects the point at 0 % Con and 100% CaCO3; the other set of dilution lines, siliciclastie deposition, intersects the origin of the C~-CaCO3 graph. Theoretically, almost every given C,,~-CaCO3 pair for a single sample could either lie on a carbonate or on a elastic Free (and on an organic matter line, which is not shown in Fig. 2-4). But the assignment to one of these lines cannot be accomplished using the Co~CaCO3 content of a single sample. The assignment to depositional types is only achieved if several successive samples (vertically or laterally) are used, in order to get C,,,-CaCO3 scatter diagrams, as shown in Fig. 1-3. Identifying the type of C~-CaCO3 association then allows definition of the type of deposition and related-OM~c and OM~ values. As demonstrated in the following section, determination of the depositional type is critical for assessing relative changes in sedimentation rates.

19

2.4 Relative sedimentation rates using organic carbon dilution equations Systematically changing sediment inputs control not only the compositions but also the thicknesses of dine-equivalent sediment units. These differently thick sediment columns (e.g., see Fig. 2-2) can be translated into their corresponding sedimentation rates for carbonate, siliciclastic, and organic matter deposition. As already mentioned, sedimentation rates are here not expressed in absolute numbers. Instead, two vertically or laterally succeeding sediment compositions are compared, and their relative changes in sedimentation rate are evaluated. In Fig. 2-5, one sample (A) is set as the standard, which is as-

RELATIVE SEDIMENTATION RATE CARBONATE

DEPOSITION

$ r 1/4

:-:: :::":::~::" CARBONATE VOLUME

~2

t~

, . . . . ,.... 8e.e

/

'. . . . . . .

..

'

CLASTIC AND ORGANIC MATTER VOLUME

A

B

[~I~]V

D

rellUvl sedlmentatlOnl i o~OM 1 01~]l

Ir a t e

I

C

=

SILIClCLASTIC DEPOSITIO N~,,,,"J~r 4

Z

iii

CLASTIC VOLUME

W rJ9

Srl

s~ "-

113

,

,~ CARBONATE A N D MATTER

t::::s:~.::::.:-:-I ORGANIC Ijiiiiiiiiii iiii!::iiii::il. q•R liiiii::i!:iii::il 22.5 ~, ~ ............... VOLUME

.. A

/ relative N d l m e n t M I o n

B

C

D

C 1 [vol%] I

Fig. 2-5 Determination of relative sedimentation rates (%) using the volume percents of the sediment fractions. Carbonate and siliciclastic deposition. When A is defined as the standard sediment, D has a relative sedimentation rate of s, = 4. When D is the standard, A has a relative sedimentation rate of s, = 1/4.

20

signed a relative, dimensionless sedimentation rate of s r = 1. For another sampie (e.g., D), we calculate by which quantity the rate of deposition is different compared with the f'u'st sample. Factors larger than one tell us that the second sample has a larger sedimentation rate compared to the first sample, whereas factors smaller than one indicate lower sedimentation rates. In Fig. 2-5, the relative sedimentation rate is sr = 4, when the composition changes from sample A to D due to the addition of either calcareous or siliciclastic sediment; if we consider D as the standard (with sr = 1), we obtain for sample A a relative sedimentation rate of s, = 1/4. Consider the sedimentation rate changes illustrated for the siliciclastic system in Fig. 2-5. Here, terrigenous sediment is added to calcareous background sediment. The addition of siJiciclastics leads to an increase in thickness (equivalent to increasing sedimentation rate), combined with a simultaneous decrease in the carbonate concentration. The relative sedimentation rate between two successive samples is expressed by their ratio of carbonate, expressed in volume percent (Fig. 2-5):

GRAIN DENSITY Corg-RICH SEDIMENTS 2.8

~o 2.7 2.6 mz 2 . 5 a

2.4 mulation, grain densities

2.3

r = -0.89

minem~ frac~n 2.70 g/cm 3 organic fra~on 1.01 g/cn13

2.2

I

0

1

I

I

!

2 3 4 5 ORGANIC CARBON CONTENT [wt.%]

Fig. 2-6 Bulk grain densities (g/cm3) of sediments (shales to chalks) with various organic carbon contents. The correlation line (solid curve, r=-0.89) was simulated using assumed densities of 2.70 and 1.01 g/cm~ for the mineralic and organic fractions, respectively (lower dashed curve). Samples from Upper Cretaceous Greenhorn and Niobrara TR cycles, Western Interior Basin, Colorado, USA. Finely dashed line indicates grain aensity relationship given by Schmoker et al. (1983).

21

WEIGHT TO VOLUME TRANSFORMATION SOLIDS EXPRESSED AS WEIGHT PERCENTAGES

ALL PARTS EXPRESSED AS VOLUME PERCENTAGES ! 'I

a

POROSITY

t

C

U'J "O ~

20 POROSITY

C

U'J

'4-- tJ3 O.,m O

O

Em =E

S

S

>

I

OM Corg

O O

e

,'g-E "5~3

O U}

tO,

T

! I,

oM

t

7.7

0

0

_k

Fig. 2-7 Compacted sediment with reduced porosity, where the solids are expressed either in weight or volume percent. Note differently expressed quantities of organic matter (OM). C and S denote the carbonate and siliciclastic fractions.

Ct[vol. %] Sr[S,OM ] ~

(2-3)

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

C2[VO1. % ]

where s,,s oM, is the relative sedimentation rate between the f'n'st and second samples '~or 'siliciclastic or organic matter deposition, with Ct[vo[.%] and C2[vol. %] representing the volume percent of carbonate. For practical purposes, Eq. 2-3 must be rewritten, because carbonate contents are conventionally expressed as weight percent; thus, volumes are transformed into their equivalent weight percents (see Fig. 2-7). The volume percent of carbonate (C[vol. %]) of a porous sediment is (volume of solids * absolute carbonate volume) / (sum of absolute volumes of all solid fractions), (100-~) * Vc C[vol. %] =

; V c + Vs + VoM

(2-4)

22

where 9 is the porosity (as a percent of the bulk volume), and V c, V s, and VoM are the absolute volumes of carbonate, silieiclastic, and organic matter, respectively. The absolute volume of the solid sediment fraction is equivalent to the weight percent of that sediment fraction divided by its grain density. The volume carbonate (Vc), for instance, is written Vc =

C --

,

(2-5)

where C is the carbonate content (wt. %), and P, is the grain densitiy of calcite. Here, average grain densities were taken to be 2.7 glcm 3 for carbonate and siliciclastie grains, and 1.01 g/era3 for the organic fraction (Fig. 2-6). These values were obtained by carrying out 32 grain density determinations (Lewis, 1984) on samples with varying Co,, contents, taken from an epeiric sea setting (Cretaceous Western Interior Basih, USA). Bulk grain densities decline with increasing organic carbon content; this trend was numerically simulated, with an assumed specific density of 1.01 g/cm 3 for organic matter (see curve in Fig. 2-6). According to Tissot and Welte (1984), a factor of 1.3 was used to convert the organic carbon content (wt. % C ~ to the total amount of organic matter (wt. % OM). When the transformation represented in Eq. 2-5 is performed for all the absolute volumes given in Eq. 2-4, the following expression is obtained C/2.7 * (I00-~) C[vol. %] =

(2-6) C/2.7 § S/2.7 + OMI1.01

If we write 1.3 Co,. for OM, and write for the mineralic solids (C + S) the expression (100 - 1.~3 Cork), we obtain C * (100-~) C[vol. %] =

,

(2-7)

100 + 2.175 C.~ where 9 is porosity (vol.%), C carbonate content (wt.%), and Co,g organic carbon content (wt. %). To determine relative sedimentation rates, the volume percent of carbonate (C[vol. %]) in Eq. 2-7 can be substituted for their two equivalent parameters in Eq. 2-3. By dividing the two carbonate volumes, and eliminating the factor of 100 in the numerator in Eq. 2-7, the Co,z content in Eq. 2-8 obtains a factor of 0.0218. Hence, the resultant relative sedimentation rate (st) between two samples is written as the product of three ratios. The first ratio is formed between the weight percent of carbonate (Cl and C0, while the others are corrections for the differing grain densities of organic matter and the mineralic components, and for different sediment volumes (or porosities),

23

Cl SrfS.OMl

=

---

C2

1 + 0.0218 Co,z2 *

100- @t *

1 + 0.0218 Co,,,

(2-8)

I00 - 4'2

Equation 2-8 is designed to determine relative sedimentation rates for silicielastic (Srfsl) and organic matter deposition (%tOMl)- Co,,1 and Co,,; are the organic carbon contents (wt. %), and ~ and r are the porosities (vol. %) of the two samples being compared. The relative sedimentation rates for carbonate and siliciclastic deposition (%tc,s0 are expressed as in Eq. 2-3. Here, the organic matter volumes of two samples are compared (OM t, OM 2 [vol. %]). OMt[vol. %] %tc.sl=

(2-9)

OMz[vol. %]

As in the procedures performed in Eqs. 2-4 to 2-6, the volume percent of organic matter (OM[vol. %]) is written (100-~) * 1.3Co~JI.O1 OM[vol. %] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (100-1.3C0~)/2.7 + 1.3Co,a/1.01

(2-1o)

The substitution for OM[vol. %] in Eq. 2-9 results in a formular structured as in Eq. 2-8. The relative sedimentation rate (%) is the ratio between the organic carbon contents of two samples (Co,~1 and Co~, wt.%), with two correction factors, one for the grain density and the other for porosity difference S,tc,s~=

Co~,~ 1 + 0.0218 Cowa 100 - ~ l ..... * ................... * ........... Co~ja 1 + 0.0218 C~, i 100 - ff~2

(2-1 I)

Equations 2-8 and 2-11 are the principal formulae for the determination of relative sedimentation rates used throughout this book, which I will refer to as "organic carbon dilution equations". These equations allow us to calculate the difference in sedimentation rates between two vertically or laterally successive samples. The actual sedimentation rate of these two samples is usually unknown, but their relative sedimentation rate (i.e., the difference between their sedimentation rates) can be precisely determined when the sediment input follows one of the above-described principal types of deposition, or a combination thereof. Equations 2-8 and 2-11 can be simplified when porosity differences are small, because the third ratio in the equations [(100-~1)/(100-~2) ] approaches one and thus can be neglected. This simplification is valid for most lithified rocks, where porosities are low and variations in porosity are usually small. Consider the Co,,-CaCOj data from DSDP Site 535 in the Straits of Florida, an Upper Cretaceous sequence of marls and chalks (Fig. 2-8). The inversely

24

ESTIMATION OF RELATIVE SEDIMENTATION RATES

SITE 535, UNIT III o~10

Sr=2.4

C O .D

I I I I

t_

U C

porosity

o c7~ o

5O

Car

carbonate sediment

sr=l

st= 0.4

ii!i!iiiiiiiiiiiii!!!

I11111111111111111111

IIIIlllllllilllllllll

Corg3.1 wt.%

e-.org1~ wt.%

C

.4

clastic Corg0.3 wt.%" matter~ B

Fig. 2-8 Organic carbon and carbonate contents from Cenomanian sediments, DSDP Site 535, Straits of Florida. Columns show inferred changes in carbonate deposition, reflecting changes in sediment thickness (compacted, with 20% porosity), as related to different relative sedimentation rates (s~).

correlated cluster of points in the scatter diagram represents a dilutionconcentration line with an OM~c value of 8.2 %, suggesting c a r b o n a t e deposition and thus the application of Eq. 2-11. Let us assume that our standard sediment, with a relative sedimentation rate of sr = 1, represents a point on the dilution line, with a CaCO 3 content of 80% and a C,~ content of 1.2% (Sample A). Relative to this standard, a sample with 95 % CaCO~ and 0.3 % C~s indicates an increase in the sedimentation rate by a factor of S,ic] = 2.4 (Sample

25 B, Fig. 2-8). For Sample C, with 50% carbonate and 3.1% Co,g, the relative sedimentation rate, calculated using Eq. 2-11, is S~c] = 0.4, significantly lower than our standard. When the entire range of 50 to 95 % CaCO3 is considered (from C to B), changes in the relative sedimentation rate constitute a factor of S,~cl = 5.8. Besides indicating carbonate deposition, a considerable change in the-sedimentation rate is documented in the Co_-CaCO 3 pattern of Site 535. Next we will investigate, how changes in'sedimentation rates are documented by changes along Co,:CaCO~ lines. Equal increases in carbonate or elastic deposition are expressed by unequal movements along a given dilution line and are characterized by increasingly smaller changes in the Cog and CaCO3 contents. An example is given in Fig. 2-9, illustrating this aspect of increasing carbonate deposition, where the initial composition is assumed to represent a carbonate-free background sediment with an organic carbon content

SEDIMENTATION RATE CHANGE: SHIFT ALONG Corg- CaCO 3 LINE 10_

8-

.:i!i

--

ii!~i

Q:>

r~ n,,,

-[~ I 0

........ t ! 10 20

, t 30

I 40

! 50

I 60

I-

,S O M I I Ill I 170 { 80 ] I 90 100 CARBONATE CONTENT

== r~ 0.5-

g 0

L i

10

20

i

30

i

40

D' 'i

50

60

I 9O 100 80 70 CARBONATE CONTENT

Fig. 2-9 N0n-linear relationshipbetween changingdeposition and associated shifts along an organic carbon - carbonate correlation line. For constantly increasing deposition, expressed by increasing relative sedimentation rates (s,), shifts along a Co,:CaCO3 line become smaller. Diagram shows conditions for carbonate deposition.

26

of 1%. Each doubling of the sedimentation rate leads to a relative increase in carbonate content by 50% and a simultaneous 50% reduction in the organic carbon content. After the first doubling (st = 2), the carbonate content increases from 0 to 50%, while the organic carbon content decreases from 1 to 0.5 %. After the second doubling, when the relative sedimentation rate is 4, the carbonate content increases to 75 %, while the organic carbon content is halved a second time to 0.25 %. When the sedimentation rate is doubled a third time (s~ = 8), concentrations are 87.5% and 0.125% for the carbonate and organic carbon contents, respectively. Consequently, two intervals with identical lengths on a given dilution line, but one representing low, and the other high, carbonate contents, document different sedimentation rates. In carbonate deposition, relative sedimentation rates are small when carbonate contents are low but are large when carbonate contents are high. In siliciclastic deposition, these conditions are inverted (see Fig. 2-2). One consequence of this difference between sedimentation rate changes and associated C~-CaCO 3 changes is the likelihood of generating bedded sediments with aifferent carbonate contents (see Chap. 5.3). 2.4.1 Combinig sedimentation rates from different Ca~-CaCO 3 curves

So far we have considered the difference in sedimentation rate (i.e., relative sedimentation rate) between two samples situated on a single Co~-CaCO3 dilution line (see Fig. 2-8 for example). Relative sedimentation rates can be calculated not only for two samples with different composition on one dilution line, but also for two (or several) samples located on different lines, which overlap each other. As relative sedimentation rates are factors, they can be multiplied to give a combined value of the relative change in the rate of deposition. This concept of combining or standardizing relative sedimentation rates is fully addressed in Chap. 6. For the moment, only a brief outline is presented. Four points (P~ to P4) are presented in a C,~-CaCO3 coordinate system illustrated in Fig. 2-10. Each two of the four points lie on carbonate (solid lines) and siliciclastic Cor~-CaCO3 curves (dashed lines). Sedimentation rates are calculated between P~ and P2, and Pz and P3; they are finally compared with the resultant sedimentation rate between Pl and P3- First, between Pt and P2, the sediment composition moves along a line of decreasing silicielastie deposition, which reduces the relative sedimentation rate by a factor of s~t -- 0.67. Then, between Pz and P3, the sedimentation rate again increases (sr~ = 1.49), because of calcareous deposition (Fig. 2-10). When the relative sedimentation rate at P3 is expressed in terms of the standard at PI (with s~ = 1), the two relative sedimentation rates must be multiplied (srt * sa). This multiplication results here in a value of one. Decreasing elastic deposition is balanced by a succeeding increase in the carbonate supply. Numerically, the relative sedimentation rate at P3 is standardized to that at P~, therefore, this combined sedimentation rate is also referred to as the "standardized relative sedimentation rate", sR (see Chap. 6.3). An identical standardized sedimentation rate is obtained when two other

27

intersecting dilution lines are used, for example between points P~, P4, and P3The resultant sedimentation rate (between Pt and P3) is identical to that determined above, as long as the considered points lie on the two sets of crossed CorECaCO3 lines, as indicated in Fig. 2-4.

COMBINING Corg-CaCO 3 REGRESSION LINES

%OM .~O

3

! p~O"r

-

t+-

2t 1"I 0

2b

E0

6b OXoco{03

Fig. 2-10 How relative sedimentation rates between several samples, which lie on differnt Cot.-CaCO3 dilution lines, can be combined. Silicielastic deposition is represented by the linds through compositions PI and P2, as well as P3 and P4; calcareous deposition is indicated by the dilution lines through P, and P3, as well as through PI and P4- Standardization of the relative sedimentation rate~ (which are dimensionless factors) allows expression of the rate of deposition in sample P3 relative to that in the initial composition, P~.

There are two ramifications of this combination of various relative sedimentation rates: 1. The assessment of depositional inputs and sedimentation rates can be performed not only for individual lithologies representing small stratigraphic intervals, but also for longer sections (see Chap. 6). 2. A C.~-CaCO3 trend, which does not exactly follow one of the major dilution-concentration lines for the three basic types of deposition, is understood to be the result of two simultaneously changing fluxes of individual components. These aspects of deposition with complex flux pattern are addressed in Chap. 5.3. However, before the three component-system is applied, some of the basic premises have to be discussed, which is presented in the next chapter.

28

2.5 Conclusions

The basic premise of the three-component system is that, in marine environments, major depositional types are documented by different relationships between carbonam and organic carbon contents in the deposited sediments. These basic styles of deposition, as well as associated flux patterns and sedimentation rate changes, are quantifiable. The three-component system serves as an integrated instrument connecting carbonate - organic carbon contents with depositional input patterns and relative sedimentation rates. 1. Defining the three-component system: The three-component system simplifies the complex flux patterns occurring in marine sediments. Fluxes are grouped into three major families - carbonate, siliciclastic and organic matter fluxes. As a first approach, only the simplest flux patterns are analyzed, in which one flax varies greatly, but the remaining two fluxes are relatively constant. Carbonate deposition is characterized by varying carbonate input but a relatively constant supply of siliciclastic and organic sediment; siliciclastic deposition is characterized by a changing supply of silt and clay with more constantly delivered carbonate and organic carbon components; and organic matter deposition is characterized by a varying organic matter supply, related to changing productivity or oxygen deficiency, with more constantly delivered carbonate and siliciclastic sediments. 2. Basic types of C~,-CaCO3 associations: As documented in various parts of this book, the three iSasic flux styles of the three-component system can be recognized by distinctive types of organic carbon-carbonate regression lines when expressed in CaCO3-Co~ coordinate systems. Carbonate deposition is characterized by negatively sloping Co~_-CaCO3 lines, whereas siliciclastic deposition is recognized by positively slol~ing regression lines. Organic matter deposition is indicated by Cor,-CaCO~ relationships occurring largely parallel to the organic carbon axis. These three basic types of Co~-CaCO3 relationships reflect different sedimentation patterns and, thus, different dilution or concentration processes which lead to reduction or augmentation of organic carbon contents, respectively. 3. Determining relative sedimentation rates: Relative changes in sedimentation rate can be estimated when a particular sediment composition shifts along a given Co~-CaCO3 regression line. This shifting is accompanied by organic carbon dilution-concentration processes, which is a direct function of the increasing or decreasing main sediment suppl),, respectively. Sedimentation rates are usually calculated for small stratigraphlc intervals in which the basic flux pattern remains largely constant. Additionally, sedimentation rates are determined by relative means, comparing one composition (which is used as a standard) to a second composition (which is taken as a result of changing deposition of the main fraction). This procedure allows both estimation of relative sediment inputs or sedimentation rates despite various diagenetic changes, and achievement of a resolution which is much higher than those of conventional approaches. 4. Combining C~42. aCO 3 regression lines: Relative sedimentation rates can be analyzed in vartous ways. If a sediment composition shifts from one C~-CaCO3 regression line to another, the changes in individual sedimentation

29

rates calculated for each of these regression lines can be combined and standardized. In this way, the more complicated input patterns of larger sequences can be evaluated. Another application is that a composite C~-CaCOj pattern reflecting two simultaneously varying fluxes can be analyzed by graphically separating the complex Cor~-CaCO3 pattern into the changes observed in the individual depositional components.

Chapter 3 FACTORS INFLUENCING THE THREE-COMPONENT SYSTEM

3.1 The role of organic carbon preservation Most authors assume that an increasing bulk sedimentation rate enhances organic carbon preservation because a higher rate of burial may prevent degradation of the organic matter at the sediment-water interface (e.g., Toth and Lerman, 1977; Heath et al., 1977; Mfiller and Suess, 1979; Ibach, 1982; Betzer et al., 1984; Stein, 1986; Berger, 1989; Einsele, 1992; Kuehl et al., 1993). For the three-component system, a significant sealing effect has the consequence that the organic matter flux, which is assumed to be at a roughly constant rate (over a small stratigraphic interval), increases with increasing deposition. In their classical paper on organic carbon preservation, MfiUer and Suess (1979) found a strong correlation between increasing sedimentation rate and increasing organic carbon content. This relationship was established using C ~ contents and sedimentation rates from surface sediments in different marine environments ranging from the continental shelf to the deep sea, including marginal seas (i.e., Baltic Sea, Black Sea), continental margins with upwelling (i.e., Peru, Oregon, NW Africa), and the deep ocean (e.g., Argentine Basin, Central Pacific). Despite modified interpretation by other authors (e.g., Betzer et al., 1984; Stein, 1986), the general interpretation of these data was that they largely represented the sealing effect. However, such an interpretation is difficult, because the Miiller-Suess relationship is influenced by several other factors. The trend of increasing organic carbon contents with increasing sedimentation rates is accompanied by a ten-fold increase in primary productivity between the central oceanic gyres and marginal seas (Romankevich, 1984; Berger et al., 1989). ParaUelling this trend from deep to marginal seas is an increase in the supply of terrigenous organic matter, a decrease in water depth, and, commonly, a decrease in bottom water oxygenation. The latter three parameters diminish the degradation of organic matter; thus they further enhance the Cm content in the sediment (Betzer, et al. 1984; Sarnthein, 1987; Berger et al., 1989; Stein 1991). The combined result of all these factors (more productivity and less degradation in shallow waters) is that the organic carbon content increases from the deep oceans to the highly sedimented shelves and marginal seas (Calvert, 1987); this also implies a connection between the organic carbon content and the sedimentation rate (Stein, 1991). The magnitude of true preservational effects, created solely by changing the amount of nonorganic deposition, while maintaining constant productivity, water depth, and bottom water oxygenation, is difficult to assess from the data published by Mfiller and Suess (1 979). Most probably, such an effect is much smaller than

31

previously suggested, as demonstrated by several authors (see below). Emerson (1985) reappraised the Mfiller-Suess relationship through applicadon of a numerical flux model for marine organic carbon contents. This model quantifies organic matter degradation in the surface sediment, as it relates to oxygenation and bioturbation processes (Emerson et al., 1985; Emerson and Hedges, 1988). The major result of the Emerson model is that the effect of increasing nonorganic sedimentation is basically dilution of the organic carbon

I

i

1

'

Z UJ pZ

0 0

1.4

1.2

Z

0

rn "I.0

<

0 Z < nO 0.4 L

10

20

30

~0

SEDIMENTATION RATE [m/Ma] Fig. 3-1 Relationship between the organic carbon content (wt. %) and sedimentation rate (m/Ma) according to the Emerson (1985) model. Upper curve: Increasing organic carbon content with increasing sedimentation rate is a consequence of the concomitant increase in or~.anic carbon flux to the sediment surface; this curve is parallel to that expressing the Miilter-Suess (1979) relationship. Lower curve: Organic carbon dilution with increasing sedimentation rate for a constant supply of organic matter. After Emerson (1985).

concentration in the sediment; thus, unlike the Mfiller-Suess relationship, orgarlic carbon contents would decrease with increasing sedimentation rates (Fig. 3-1). Emerson's calculations indicate a positive association between C m content and sedimentation rate (as does the MfiUer-Suess relationship), when the organic matter sedimentation rate increases along with the sedimentation of nonorganic sediment (Fig. 3-1). These findings suggest that the validity of the

32

Mfiller-Suess relationship depends on many factors and the nature of the depositional environment. Emerson's (1985) study therefore indicated that the true sealing effect is apparently much smaller than that of concurrent dilution by the nonorganic fraction. In order to identify the magnitude of a possible sealing effect, Arthur et al. (1984) have investigated the influences of changing sedimentation rates on organic carbon contents in Middle Cretaceous black shale sequences from the North Atlantic (Fig. 3-2). When the sedimentation rate is plotted against orgO 81TE 3818 O

~.m

o SITE I05

A SITE IOtA SITE 39 t

9 SITE 387 9 SITES 417D.418A&S

Z

3; Z

O,

0

rr"

~ O

<3

0 0 Z

t

0

iS

o

11{2

o t,

0 o

<

13

v

o . . o

C

O

reJllllontl~p illuming ~ SO'Ii, co~nq~Ctl~n o ~

0

M"S

0 !

0

10

20

i

|

30

i

!

40

SEDIMENTATION RATE [m/Ma] Fig. 3-2 Average org~anic carbon content per core versus sed.imentation rate for given stratigraphie intervals m DSDP sites from the Western North Atlantic. The dashed line re~resents the MLtller-Suess (1979) relationship for Recent marine sediments adjusted for 0% compaction. Note large scatter of points, and a tendency for the organic carbon content to decrease with increasing sedimentation rate. After Arthur et al. (1984). Compare with Fig. 8-3. anic carbon content, the points show considerable scatter, with a tendency for organic carbon content to decrease with increasing sedimentation ram. A positive relationship between the Co,zcontent and sedimentation rate, as proposed by MfiLler and Suess, was not observed (Fig. 3-2, dashed line). Such results are supported by my own statistical analyses presented in Chap. 8.1. I. In the

33

analyzed data, a decreasing organic carbon content accomplished by an increasing sedimentation rate is interpreted as the consequence of mineralic dilution, because the mean organic matter flux is statistically nearly constant and thus independent of the rate of mineralie deposition (see Fig. 8-3 and 8-4). In a more recent extension of the data set used by Mfiller and Suess (1979), Stein (1986, 1990, 1991) showed that oxic and anoxie open marine sediments can be distinguished by the general relationship between organic carbon content and sedimentation rate ("OCSR diagram"). As a whole (across the ocean), oxic open marine sediments follow the Mfrller-Suess relationship and show a positive correlation between the organic carbon content and sedimentation rate. By contrast, in anaerobic to dysaerobic environments, the organic carbon content, within a factor of 10, is generally little influenced by the sedimentation rate, or it may decrease slightly with increasing sedimentation rate. Ibach (1982) and Einsele (1992) have taken these arguments further that, above a certain boundary sedimentation rate of 10 to 100 m/Ma, deposits with both oxic and anoxic bottom waters are dominated by dilution processes. In the investigations performed in this book, a significant increase in organic carbon flux with increasing sedimentation rate was not detected (i.e., through evaluation of C~s-CaCO3 scatter diagrams). Preservational effects by solely increasing minerahc deposition are therefore thought to be generally smaller than dilution effects (Emerson, 1985). Moreover, the changes in sedimentation rates, which are described in this study, are relatively small (having factors of 2 to 10); many sediments analyzed here indicate anaerobic and dysaerobic bottom waters, which are, on a whole, less affected by changing sedimentation rates (Stein, 1986; Canfield, 1989). For all these reasons, no correction for preservational effects, as a function of sedimentation rates, was carried out in this book.

3.2 The influence of early and late diagenetic processes Diagenetic changes of the organic carbon and carbonate fractions influences both the slope and length of the Coa-CaCO 3 curves. A comparison of the two sediment components suggests that diagenesis of the organic matter has a more systematic effect on the three-component system than that of the carbonate fraction.

3.2.1 Diagenetic loss of organic matter Aerobic oxidation in surface sediment is followed by sulfate reduction and fermentation in anaerobic bottom or pore waters (e.g., Curtis, 1980; Demaison and Moore, 1980). In aerobic bottom waters, most of the easily degradable organic matter is already oxidized in the surface sediments. In the equatorial Pacific Ocean, Emerson et al. (1985) found that C,r, decreased by more than 50% in the uppermost 1 cm of sediment. Stein (1991) calculated that the degradation of organic matter in surface sediment is between 60 and 70% for coastal upwelling environments, and between 70 and 80% for non-upwelling

34

coasts. In the open ocean, 90 to 98 % of the organic carbon reaching the sediment surface is decomposed. Thus, compared to the primary production of organic matter in surface waters, only a minute percentage is commonly embedded in the sediment (from 0.001 to 10%; for most aerobic environments below 1%; Bralower and Thierstein, 1984, 1987).

D I A G E N E T I C O R G A N I C MATTER LOSS INFLUENCE ON RELATIVE SEDIMENTATION RATE CARBONATE DEPOSITION PRE.

A

Sr (A-C) ~

O

POST.

oiAaeNe'nc mAGeN C ',

I

Sr (a-c)

B

q C

& carbonate clasflc

organic matter

dlage .netlc

~) o ~ nic maltar

Fig. 3-3 How degradation and diagenetic organic matter loss influence the determination of relative sedimentation rates (s,), here illustrated for carbonate deposition. Original variation in sedimentation rate is expressed by a C~.,-CaCO3 dilution lira: through compositions A, B, and C. Equal loss of organic m ~ c r generates a tater dilution line (a, b, and c). The relativechange in carbonate deposition along each of the individualC ~ F C a C O 3 linesremains largelyconstant;the relativesedimentationratebetween A and C is approximately identical to that between a and c.

In principle, organic matter degradation is thought to be less effective in anaerobic and dysaerobic bottom waters than in aerobic bottom waters. Hazl~ann et el. 0973) estimate that degradation differs by more than a factor of two between these categories. However, other authors assume that bacterial sulfate reduction affects the organic matter content to a significantamount or even the same degree as does oxic degradation (e.g., Jorgensen, 1982; Henrichs and Rccburgh, 1987; Pedersen and Calvert, 1990; Littke et al., 1991; Martens et

35

al., 1992; Calvert and Pedersen, 1992). Below the sulfate reduction and fermentation zones (generally below 10 to several 100m of overburden), degradation of organic matter continues. Using the S/C relationship in various earbonaceous sediments, RaisweU and Berner (1987) have estimated that organic matter loss is approximately 30% between the end of the sulfate reduction zone and the onset of the oil window. All these diagenetic losses of organic carbon, due to various early and late degradation processes, are thought to have affected the different rock types in any given section similarly, thus systematically flattening the slopes of all Co~CaCO3 curves, and lowering the apparent organic carbon input (Fig. 3-3). This lowering, however, has little influence on calculating sedimentation rates using the dilution equations (Eqs. 2-8, 2-11), because these sedimentation rates are determined by relative means. When the absolute amount of organic carbon, contained in a time-equivalent sediment interval, is lowered equally for different lithologies both rich and poor in carbonate content, then the calculation of relative sedimentation rates is little affected for both carbonate and elastic deposition.

DIFFERENTIAL

CARBONATE

DIAGENESIS

ELONGATION OF CARBONATE DILUTION UNES DISSOLVED CARBONATE

ORIGINAL PQROSITY

OMSCI

I .

, !

MARL

|

0

,-=_1--..-~. CARBONATE ; ~1 ",'X [)ISSOLUTION

~

;--~1~

.

:

C

::~:::1. . . . . . i

ORIGINAL CARBONATE SOUDS CEMENT

j

I

__

~

OM S

I I

. . . .

t

,~N.C~BONATE

, I CARBONATE CONTENT

Fig. 3-4 Influence of differential dissolution and cementation (arrow) on the length of Co~-CaCO3 lines (carbonate deposition). Original dilution lines elongate because organic ma(Ier is diquted as a result of carbonate cementation (e.g., forming a limestone bed), and concentrated as a result of carbonate dissolution (e.g., forming a marl bed). The slope of a pre-dia~enetic C~-CaCO~ line, which is determined by the ratio of organic matter to silicielastle sedimen[, remains essentially unaffected. Both organic matter and siliciclastie sediment are not involved in diagenetic carbonate redistributmn processes.

36

3.2.2 Differential carbonate diagenesis Carbonate diagenesis affects the lengths of the carbonate dilution curves, but not their slopes (Fig. 3-4). In bedded marls and especially in fine-grained limestones, differential cementation and dissolution processes between carbonate-rich and carbonate-poor beds are frequently observed (e.g. Einsele, 1982; Hallam, 1986; Ricken 1986, 1987; Bathurst 1987, 1991; Leythaeuser, 1993). Studies based on rock compaction (Ricken, 1986) and observations from chalks encountered in DSDP drillholes (Garrison, 1981) indicate that this process begins in pelagic carbonates after an overburden of several I00 m has accumulated (creating mechanical compaction). Beds originally richer in earbonate become cemented, as additional carbonate fills the pore spaces of what are later to become limestone beds. As a result, the weight percent of carbonate increases while the weight percent of organic carbon decreases. In marl beds, the carbonate donors, the opposite occurs. Thus, diagenetic carbonate redistribution causes the original carbonate dilution curves to become longer by increasing the differences in composition of alternating beds (Fig. 3-4). A correction for these diagenetically elongated dilution curves is carried out by applying carbonate mass balance calculations (Ricken, 1986, 1987). The effects of differential compaction and diagenetic enrichment of organic matter in highly compacted calcareous beds is discussed in Ricken (1992, 1993). Corrections for the effects of differential carbonate diagenesis, which are based on evaluating rock compaction, were carried out only for the highly lithified and cemented marl-limestone sequences of the Western Interior Basin (see Figs. 62 and 6-3). For most of the investigated soft sediments from DSDP and ODP Sites, diagenetic effects were assumed to be low enough to be neglected.

3.3 Decoupling of sediment fluxes In the idealized three-component model described in Chap. 2.2, the three different sediment fluxes (carbonate, siliciclastics, and organic matter) were assumed to be genetically unrelated. The supply of the main sediment varied without any connection to the fluxes of the background components, which were delivered at a constant rate. Is such a decoupling of the various sediment fluxes realistic (e.g., the carbonate supply de.coupled from that of organic matter and elastic sediment)? Are these fluxes, which reach the sea floor and the sediment, so closely interrelated that the application of the three-component system is significantly restricted?

3.3.1 The interrelationship between marine organic matter and calcium carbonate fluxes In the oceans, the fluxes of marine organic matter and coecolithic to foraminiferal carbonate are principally related, because they both originate in the photic zone by phytoplankton productivity. Even so, these fluxes have different histories during their paths through the water column and into the sediment.

37

While the carbonate flux remains largely unaffected in environments above the lysocline, the organic matter flux is substantially affected and diminished; consequently these fluxes essentially become decoupled. Planktonic algae (phytoplankton) are the primar~ producers of organic matter in the modern oceans (i.e., they constitute the primary productivity ). As a "biological pump', these planktonic algae remove carbon by photosynthesis from the ocean surface layer, the photic zone, transporting it to deeper zones in the ocean (Berger et al., 1988). The planktonic algae are composed of a wide range of species; the most important are diatoms, silicoflagellates, dinoflagellates, and coccolithophorids. Secondary producers, the zooplankton, recycle most of this organic matter, so that, below the surface zone, only 6% to approximately 30% of the primary production (i.e., the "new production') is transferred to the deeper ocean, depending on the magnitude of the primary productivity (Eppley and Peterson, 1979; Sarnthein et al., 1987; Berger et al., 1989, Eppley, 1989). When sinking through the water column, this "newly produced" organic matter is further diminished, to form the water depth dependent "export production" (Burland et al., 1989). Measurements of sediment fluxes at different water depths have shown that export production comprises approximately 10% of the primary production at a water depth of 400m, and only 1% at 4000m (Suess, 1980). Betzer et al. (1984) refined this estimate and showed that the export production is non-linearly related to primary productivity (PP) Jco~g = 0 . 4 0 9 ppZ.4t / z0.62s ,

(3-1)

where Jco,,is the (export) flux of organic carbon, PP is the primary productivity, (bofla rates in g Co~m2a_~), and z the water depth (in m; Betzer et al., 1984). This equation is valid mostly for oxic water masses. These findings document that in oxic to suboxic environments only a small fraction of the primarily produced organic carbon is transferred from the oceanic surface layer to the sediment-water interface. As already discussed in Chaps. 3.1 and 3.2. I, the incoming organic matter is further diminished within the uppermost sediment layer, so that only 0.001 to 10% of the primarily produced organic matter is embedded in the sediment (Mfiller and Suess, 1979; Bralower and Thierstein, 1984, 1987; Aller and Mackin, 1984; Berger et al., 1988; Jumars et al., 1989; Einsele, 1992). In the oceanic environment, the production of calcium carbonate is associated with two trophic levels. The largest quantity of CaCO3 is generated by one phytoplankton group, the coccolithophorids (primary producers), whereas planktonic foraminifera (zooplankton, or secondary producers) generate a minor, but significant, portion of the carbonate. These conditions, characterized by the dominance of coccolithophorids relative to foraminifera, are even more prevalent for many Jurassic and Cretaceous limestones and chalks (Roth and Bowdler, 1981; Roth, 1986). Carbon, which is on the average removed from the ocean surface, shows a ratio of organic carbon to carbonate carbon of approximately 4:1, equivalent a OM-CaCO3 ratio of I: 1.2 (Broecker and Peng, 1982). Since substantial amounts of carbonate dissolution can be

38

excluded in environments above the lysoeline, most of this carbonate carbon is embedded as CaCO3 in the sediment, whereas the organic matter carbon is greatly diminished. In the following, several scenarios with partly decoupled fluxes (organic matter and CaCO3) are described.

3.3.2 Partial flux decoupling for carbonate and siliciclastic deposition The ratio in which coccolithophorids produce carbonate and organic matter varies from species to species. One of the most common modem coccolithophorids, Emiliania huxleyi, produces an estimated 25 wt.% organic matter with CaCO3 constituting 75 wt. %. This estimation is based on the observation of recent plankton blooms; an average Emiliania huxleyi is 4Ix in diameter, each with 20 coccoliths with a content of 10 pg Ca (Holligan et al., 1983). This value is approximately similar to Broecker's average OM-CaCO3 ratio of the surface ocean mentioned above. Other authors estimate a much higher organic matter - CaCO3 ratio for coccolithophorids. J.D. Milliman and M. Sibuet (both personal communication) even assume that this ratio is of the order of 1:10; implying that carbonate production could be changed without significantly changing the production of organic matter. How the abundance of coccolithophorids depends on overall surface water productivity is not well understood (Berger et al., 1989). In situations of coastal upwelling and high productivity (200 to 250 gCor~m2aX), diatoms are the prevailing phytoplankton group; increased productivity would tend to decrease the abundance of coccolithophorids and thus the production of carbonate (Dymond and Collier, 1988; Williams et al., 1989). In the surface waters of subtropical gyres and intraoceanic upwelling zones with a low to medium productivity of 25 to 100 gCo_m'2a"1, coccolithophorids and planktonic foraminifera consta'tute a greater l~rtion. Here, a relative increase in productivity tends to increase the abundance of coccolithophorids and thus the export of carbonate (G. Wefer, personal communication). Large seasonal variations in productivity, such as winter or spring maximum, more episodic events, and plankton blooms all play an important role in contributing to the sediment (e.g., Honjo, 1982; Holligan et al., 1983; Deuser, 1987; Fischer et al., 1988; Wefer et al., 1988; Wefer, 1989). In the course of a year, one can observe a succession of phytoplankton blooms of different species, where the increase in one species is accompanied by a simultaneous decrease in the others (e.g., Izdar et al., 1987; Kempe, et al. 1988).

Carbonate deposition Carbonate deposition is illustrated in Fig. 3-5 for a deep water environment above the lysocline, with an assumed moderate 19rimary productivity of 100 gCovgm'2al . Coccolithophorids comprise 10% ot~ the total phytoplankton, a value which has been observed in the central Black Sea (lzdar et al., 1987). As a conservative estimate, coccolithophorids produce a flux of organic matter and carbonate in a ratio of 25:75, which equals a C,~s-Cc~co3 flux ratio of 6:4 (see

39

above). In oxic to suboxic conditions and at a water depth of 2000m, only 2 % of the primary productivity reaches the sediment surface (Eq. 3-1, Betzer et al., 1984; Fig. 3-5 A). An additional C _ flux originates from terrestrially derived organic matter, which is coupled~'~vith the elastic input and which is assumed to be 50% of the total organic matter supply. Degradation in the surface sediment is estimated to be 80% of the total incoming organic matter flux (Stein, 1991). The resulting sediment in Fig. 3-5A contains 33 vol.% of CaCO3 and 1.6 vol. % of organic matter, with a background organic matter content of OMr~c = 2.4% (Eq. 2-1). Principally, there are two ways of changing carbonate production. An increase in coccolithophorids (here a five-fold increase) is achieved when the productivity of the other phytoplankton species remains constant (Fig. 3-5B), or when they proportionally decrease (Fig. 3-5 C). The former occurs with higher primary organic carbon productivity, so that the organic carbon flux is slightly augmented, whereas the latter occurs with lower organic carbon productivity, so that the carbon flux is slightly diminished. In any case, the overall effect is dilution of the organic carbon concentration in the sediment. The organic matter content in the noncalcareous background sediment (OMr~c, Eq. 2-1) is slightly higher in the scenario depicted in Fig. 3-5 B, and slightly lower in Fig. 3-5 C. The background sediment remains essentially unchanged when natural conditions fall somewhere between the two alternatives. A decoupling of the carbonate and organic matter fluxes is even more significant, when two additional conditions are fulfilled. 1) The organic fraction is composed predominantly of terrigenous organic matter, which is supplied simultaneously with the clastic background sediment. Variations in the marine organic matter flux, as related to changing carbonate production, are then "buffered ~ or subdued by a large quantity of terrigenous organic matter. These conditions are verified for some Aptian to Cenomanian organic carbon-rich strata in the North and South Atlantic that contain mostly terrigenous organic matter (Tissot et al., 1979, 1980; Summerhayes, 1981; de Graciansky et al., 1982; Arthur et al., 1984; Stein et al., 1986, 1989; Emeis, 1987). However, many calcareous-carbonaceous sequences studied here, with pronounced negative Cor,-CaCO 3 correlations, are characterized by marine organic matter, such as tile Posidonia Shale (Prauss et al., 1991), the Upper Cretaceous sediments of the Western Interior Basin (Pratt, 1984), the Middle and Upper Cretaceous in central Italy (Pratt and King, 1986), and various deep sea sequences (Arthur et al., 1988). 2) Varying carbonate deposition is not a consequence of productivity changes, but of greatly fluctuating carbonate dissolution. Under these conditions, carbonate input shows considerable dissolution-related variations, while the organic matter supply is more or less constant (when primary productivity in surface waters is equal). In Quaternary climatic cycles, fluctuating carbonate dissolution is well known to occur between the lysocline and the CCD, although the primary productivity was not entirely constant (e.g., Berger, 1979; Berger et al., 1982; Peterson and Prell, 1985; Gr6tsch et al., 1991). In addition, dissolution processes have been reported for special conditions (CO2 rich bottom waters) in bedded sediments above the lysocline (Emerson and Bender, 1981; Diester-Haass, 1991). All these factors may contribute to the

40

CARBONATE DEPOSITION A

INITIAL CONDITIONS other phyt0piankton

c o c r

o ~:i::::~:'~7::!~:!:~::~:!:!~i:i:i:~::::~:?Ji90!::i:i::~:i:i~!:i:i:i~i~::i:i:~::i:i:~;~i~ pdmary p r o d ~

I00

o,]r 65.1

N

2.2

33.3

2.2 80% dlageneffc loss of org, matter

organic matter 1.6%

65.1% B

1"11 Jc.,

surface

cad0onm sediment volumes

IOMNc = 2.4 !

33.3%

INCREASING COCCOUTHOPHORIDS WITHOUT C H A N G I N G

THE PRODUCTIVITY OF OTHER PHYTOPLANK'rON SPECIES

o~er phyto#ankton

65.1

2.2

dastics 27.9%

C

cr

3.0 80% dlagenetJc toss of org. matter

organic matter 0.8%

165.6

surface

carbonate sediment volumes

.. IOMNc =2.7 I

71.3%

INCREASING COCCOUTHOPHORIDS WITH DECREASING THE PRODUCTIVITY OF OTHER PHYTOPLANKTON SPECIES water

o ~ e r ~ytol~nkton o -: ..:.:..:.:.:::.:: ::.:.::.:::.:::'; ~ i: ,i: ~i!51;I , :.~ i "

Js J c o ~ t ~ I

65,1

2.2

clast~= 27.9%

ocganic matter 0.6%

Fig. 3-5

pr~locoducli~

100

ccaco3 VOLUME,~

80% dJagenetlc Io~i of 0~1. r n a t ~ ~ vo~

166.6

~

9

AD

"'lo " ~

"l"

"~ "

CARB(~TE VOLUME

ca-bona, 71.5%

i.-,OM

NC

'

1 2.~

41

~ ~,~

A

o. . . . .

~ " ~6o d~

SILICICLASTIC DEPOSITION

,

.

9

INITIAL C O N D m O N S

~r,~ ~ other phytop~nklon

o I::. :.:::::::::':::.::!..::':':".:::::..:..:::: :: ::':>.".,:::. : :-: :::::"> ..:::..::!i!:.::..:.:.~

:i::. :i::::(::::: :::::::::::::::::::::::::::::::::::::::::::::::::: !i:i':~.'. '"""":": ">:'::'>::>":"::;:":::>:": :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: c~ Ccac%

ol]iiii:ii:!iii!:::ii:i::i:!::i:i:i::i:ii

JS JCorgterr

z~ Jco~ 13 ctlmlRm

J

cadoonate IOMNS ~ 16.3] 11.5%

~

2.2%

~

CLASTIC FLUX INCREASE

~ olher i:#~ytc~a~klon

~ JC~

.... { ~ ~ L ~ 5

~

zoo

g(Cocg+Ccac03) m'2 a-1

o !!~ii!i!i~ii!:ii~i~!i::i~:i%ii~i:i~:~:::i:iS:!~:i~:~!::iiii::!ii~i!~iiii:iiii!iii~i~i~ii:i!!~ii!::i!ii:i::i:h: ii!~.!i~iiiii o,

JS

s~dim~t

Ioa oi Org. maitre'

~

86.3%

B

2o0

g(Corg+CC,aC03] m'2 a"1 ....~. : : ~

..... ::::: .......................................... I

~

19.5 crg~c

!,i!Li.... 12 ~

s

c ~ ~il Cc~co~ =:.,J =~..=t--

~ o. ~. 2, , _ =~ . .JCorg ....

250O

: i : ..........................

70% dlagenel~ I0~ ol org. m,~t~ sediment

66,6 c~bc~e ~6~

saxtace OM

- ~S'

I

Fig. 3-6 Siliciclastic deposition in a shelf environment. Combination of high primary pro- 9 (200 gCm-2a-I ) and low water depth (250m) results m elevated marine organic ductavxty matter supply. A five-fold increase in terrigenous input (B) leads to increasing siliciclastic and non-marine organic matter flux. Note slight changes in the background organic matter content (OMNs) between scenarios A and B but see the general normal correlation between carbonate and organic matter volumes (inset).

9 Fig. 3-5 Calcareous deposition in the oceanic environment above the iysocline. A) Initial conditions with a water depth of 2000m and a primary productivity of 100 gC mZa'L Coccolithophorids are assumed to comprise 10% of the total phytoplankton. Most organic matter is oxidized while settling through the water column and recycled in the upper sediment layer, so that the marine organic carbon flux (]c,~) and the carbonate flux (Jcc,co3) are essentially deeoupled. The terrigenous input is composed of silieielastic (Js) and-terrigenous organic fluxes (ICorg~). B) Five-fold increase in the carbonate flux by enhanein~ only the production of coccolithophorids, while keeping the vroduetivity of the remainmg phytoplankton constant (total primary productivity 1~10gCmaaa). C) Five-fold increase in the carbonate flux (coceolithophorids) by simultaneously reducing other phytoplankton species. Compare the slightly different (background) organic matter contents (OMNc) for scenarios A, B, and C but see the inverse trend between carbonate and organic matter volumes (inset).

42

fact that the inversely correlated C~s-CaCO a associations, interpreted as representing carbonate deposition, are more frequently found than the normally correlated associations that are thought to indicate siliciclastic deposition. SiUciclastie deposition

The typical environment for siliciclastic deposition is characterized by relatively shallow water depths (within several 100m), as well as high primary productivity (approximately 200 .gCorzm'~a"~, Fig. 3-6). Due to these conditions, a relatively large amount of marine organic matter survives decomposition and is embedded in the surface sediment (Eq. 3-1). Additionally, ter-rigenous organic matter is supplied, coupled as it is with the elastic influx (lttekkot, 1988). As much as 70% of the incoming flux of both organic matter types undergoes early degradation and depletion (Stein, 1991). For the conditions depicted in Fig. 3-6A, the resulting sediment contains 11.5 vol. % of carbonate and 2.2 vol. % of organic matter. Silieiclastic deposition is described by assuming that the supply of terrigenous sediment significantly increases (here, a five-fold increase, Fig. 3-6B). This higher input of terrigenous sediment has little or no effect on pelagic carbonate production, so that a higher terrigenous flux will basically dilute both the supply of carbonate and marine organic carbon. But a higher elastic supply will also enhance the terrigenous organic matter in the same proportion. In consequence, the organic carbon content increases slightly, and the organic matter contained in the nonelastic background fraction (OMss, Eq. 2-2) is somewhat higher than the initial value (Fig. 3-6 B). But these differences in the OMNs values have very little effect on the slope of the resulting dilution line. For high background organic matter contents, characteristic of silieiclastic deposition, the slopes have relatively low resolutions (see Fig. 2-3). Both scenarios for carbonate and siliciclastic deposition suggest that the various genetically related sediment fluxes are largely decoupled, which supports the assumptions here made regarding the general flux patterns for the above-described, idealized types of deposition. 3.3.3 Organic matter in the main sediment and in the background sediment

In this section we want to investigate how a partial deeoupling of genetically connected fluxes is related to the concept of the main and background sediment described earlier. In Chap. 2.1.2, the 'main sediment" was def'med as the sediment fraction with the largest variation in deposition, whereas the "background sediment", composed of two fractions, was defined as that which shows only minor variations in deposition. In carbonate deposition, genetically related fluxes are grouped in a different way into the main and the background fractions. As an effect of flux decoupling, variations in carbonate supply are left unchanged while variations m organic matter supply are surpressed. Thus, carbonate is assigned to the main

43

sediment fraction, whereas siliciclastic and organic matter constitute the background sediment. The allocation of a depositional component to either a main or a background sediment does not give an answer as to how fluxes are genetically connected. This is illustrated in Fig. 3-7 for carbonate deposition, where the different sediment combinations suggest that organic matter is apparently part of either the siliciclastic or calcareous fractions, or both. Instead, the only requirement is that the input of organic matter and siliciclastic sediment is approximately constant.

CALCAREOUS DEPOSITION

SILICICLASTIC DEPOSITION

gmN

Fig. 3-7 Possible combinations of sediment flux resulting in differently composed sediment fractions. Carbonate deposition. Organic matter can be theoretically contained in the siliciclastic background fraction (A), in the carbonate fraction (B), and in both fractions (C). Note the identical organie carbon content in the carbonatefree sediment (Co,~C).

Fig. 3-8 Possible combinations of sediment flux resulting in differently composed sediment fractions. Siliciclast~c deposition. Organic matter can be deposited simultaneously with the carbonate fraction (A), the siliciclastie fraction ('B), and with both fractions (C). Note identical organic carbon contents in the nonelastic sediment (C,,~s).

In the siliciclastic system, the supposed association between fluxes is more consistent with their assignment to either the main or background sediments. When the input of terrigenous organic matter is relatively small, the silicielastic fraction represents the main sediment, while carbonate and marine organic matter constitute the background sediment. Thus, natural fluxes are approximately identical to the conditions depicted in Fig. 3-8A. In diagenetically altered sediments, the organic matter is contained in the fraction representing the smallest grain size. This was demonstrated by Hunt

44

(1961) for the Viking Shale (Alberta), where most of the organic carbon was contained in the clay fraction. Similar results were found for Recent carbonate sediments and ancient rocks with different noncarbonate contents (Gehman, 1962; Hunt, 1979; Jones, 1984; Anton et al., 1993). This phenomenon, obviously related to the small size of the organic particles, may be an additional argument for the allocation of elastic and organic matter to the background fraction (for calcareous deposition).

3.3.4 Alternative interpretations One of the most critical aspects of the three-component system is the use of .Co~g-CaCO3 associations to recognize the type of deposition, and, even more ~mportant, to estimate changes in sedimentation rate. In addition to the mode of deposition assumed here, more controversial interpretations may be considered. As illustrated in Fig. 3-9, a trend denoting increasing carbonate content with decreasing organic matter content could theoretically represent a decrease in the sedimentation rate (Fig. 3-9A), a constant sedimentation rate (Fig. 3-9B), or an increase in the sedimentation rate, as suggested here (i.e., carbonate deposition, Fig. 3-9C). One only has to choose the right combinations among the three fractions, so that their carbonate and organic matter contents fall on the organic matter - CaCO 3 trend line depicted in Fig. 3-9. Are these alternative interpretations likely to be realistic? The first two models addressing decreasing and constant sedimentation rates suggest that all three fluxes are precisely coupled. The marine organic matter flux is either related to the elastic fraction (Fig. 3-gA,B), because the silicielastic and organic matter fluxes change proportionally, or it is inversely correlated with the carbonate flux (Fig. 3-9B). Several assumptions seem to be unrealistic. It is hard to see why the flux of marine organic matter should be associated with the flux of siliciclastic sediment, and why the organic matter input should be inversely related to the carbonate input. This suggests, for most of the marine environments studied here that higher carbonate productivity is associated with an unusually large decrease in the organic matter supply (Fig. 3-9B). In the scenarios illustrated in Fig. 3-5, it was demonstrated that increasing carbonate flux is either related to higher or slightly lower organic matter input. The only explanation for a coupling of the organic and elastic fluxes is that all of the organic matter is of terrigenous origin. This assumption is difficult to achieve for marine marls and chalks, where coccolithie CaCO3 is a major sediment constituent. Moreover, it is not clear, why the carbonate and elastic inputs (plus organic matter) show such a perfectly inverse correlation (Fig. 3-9B). In a productivity model, an increase in the supply of nannopankton carbonate in order to reduce the input of silieiclastic sediment is hard to imagine (e.g., Eicher and Diner, 1991). Hence, the alternative interpretations suggested above (Fig. 3-9A,B) seem valid only for very restricted conditions.

45

ALTERNATIVE

EXPLANATIONS

ORGANIC20 .~ MATTER [voh%] 15 10 CARBONATE I~:~.~::..iFLUX 5 ~ SIUQCLASTIC FLUX I 10 l ORO , FLUX DECREASING SEDIMENTATION RATE, CONSTANTCARBONATE FLUX, SIMULTANEOUSCHANGE OF CLASTICAND ORGANIC I MATTERFLUX

o

CONSTANT SEDIMENTATION RATE, ~ SIMULTANEOUS CHANGES OF THREE FLUXES

I

I

I

I

I

I

I

~ I

I

I

20 30 40 50 60 70j80 90 100 ~ 2 5 ~60CARBONATE~CONTENT [voh%]

I

;'s20s A

I

5(1 40 10

20

D

75 20

5

B

21!!i!iiiiii!ii!il !::';i.i~J::i:.~' :'!!':?!:!?

i;!~i;~il)!i:i75 [ SEDIMENTATIONRATE, [ CONSTANT CLASTIC AND ORGANIC MATTERFLUX

~

N

20

~

15

C

5

Fig. 3-9 Three alternatives for a t~pical inverse relationship between carbonate and organic matter volumes. (A) Decreasing sedimentation rate with increasing carbonate content; constant carbonate flux, but decreasing silieiclastic and organic matter fluxes. (B) Constant sedimentation rate; increasing carbonate supply with simultaneously decreasing silieielastie and organic matter supplies. (C) Increasing sedimentation rate with increasing carbonate content; constant supphes of elastics and organic matter. Scenario (A) seems unlikely, scenario (B) is probably valid only in special situations, while scenario (C) seems more realistic. Small numbers indicate sediment fractions in vol. %.

3.4 The validity and limitations of the three-component system The theory proposed here describes concentration and dilution processes for a three component flux system. This theory is based on various assumptions and simplifications of the more complex flux patterns that occur under natural, marine conditions, as briefly summarized below:

46

I. In the marine environment, individual fluxes of the various sediment constituents can be arranged into three major components, representing carbonate, siliciclastics, and organic matter. 2. How these major components reach the sediment, can be further simplified. It is thought that idealized flux patterns often develop, in which one of these fluxes shows more change (i.e., the main sediment) compared to the two remaining fluxes, which are more constantly delivered (i.e., the background sediment). The main sediment undergoes essentially independent input variation compared to the background sediment; thus, the fluxes between these sediment groups seem either genetically, unrelated (i.e., siliciclastic deposition) or largely decoupled (carbonate deposition). In the oceanic environment, carbonate and organic matter are both generated by nannoplankton productivity in the photic zone. The carbonate flux remains essentially unaffected (above the while the organic flux is greatly reduced and decoupled by various gradation processes. 3. The idealized fluxes of carbonate, siliciclasties, and organic matter undergo dilution and concentration processes, reflecting characteristic C~CaCO3 relationships, which in turn can be used to recognize the basic types6f deposition in ancient sediments and rocks. 4. Additionally, Co~-CaCO3 associations can be employed to derive relative fluxes, sedimentation rates and time spans. Errors include deviations from the general flux pattern, preservational effects, and diagenetic alterations of organic matter (Fig. 3-10). However, this organic matter alteration has only a moderate influence on determining sedimentation rates, as sedimentation rates and associated flux changes are derived by relative means. 5. Carbonate, siliciclastic, and organic deposition (with the variation of one fraction dominating) is only valid for small stratigraphic intervals, where the supply of background sediment is relatively continuous. The smaller a selected stratigraphic interval, the higher is the chance to approach such ideal conditions. Individual beds and stratigraphic units a few meters thick are thought to give appropriate results (see Chap. 5). 6. Combinations of carbonate, siliciclastic, and organic matter deposition denote varying supplies of both the "main" and the "background" sediments. Concurrent variation of two fluxes is thought to generate a resultant C,nCaCO3 pattern. This pattern can be resolved into the individual types of deposition (see the example in Fig. 2-10). In larger sedimentary sequences, changhag relative sedimentation rates, derived from a stacked pattern of different C~-CaCO~ associations, can be numerically connected and standardized. Detail&l flux modelling of larger sequences is possible. This is discussed in greater detail in Chap. 6. Although the three-component system is based on the above simplifications, it can nonetheless serve as an instrument in understanding not only the basic styles of deposition and their related organic carbon and carbonate contents, but how they represent various sediments and facies associations. The accuracy of this method must not be overstressed; nevertheless, insights and quantitative analyses of a whole complex of interrelated processes can be achieved.

Chapter 4 R E L A T I V E T I M E SPAN A S S E S S M E N T

Time spans inherent in small lithostratigraphic units can be assessed by using the flux rates derived here (i.e., the "timing"). Unlike conventional time span determinations, these timing methods are performed by relative means, because the absolute amount of time is difficult to evaluate but the relative change is usually more easily discernible.

THICKNESS

TIME

rgap or diastem !short-term sedimentation

long-term sedimentotion

Fig. 4-1 Basic relationshipsbetween stratigraphie thicknessand time span. Although conventional timing provides the basis for the investigation of depositional, cyclical, and biological processes, timing has remained an elusive problem (e,g., Behrensmeyer, I983; Mc Kinney, 1985). Too many inaccuracies are involved in resolving stratigraphic durations, including large ranges of error in radiometric age determinations, poor biostratigraphic as well as magnetostratigraphic resolution.s, and incomplete sedimentary sections (Sadler, 1981; Schindel, 1982; Schwarzacher, 1986; Anders et al., 1987; Algeo and Wilkinson, 1988; Rieken, 1991b). As a result, time estimates are commonly inprecise, and the range of error is often larger than the actual time span considered, especially when the time spans of small intervals, such as beds, have to be evaluated.

48

It seems therefore appropriate to find other means for estimating time spans than those usually applied. The relative timing methods performed here are based on dividing a given stratigraphic thickness by its relative sedimentation rate. Multiple methods are available through the use of different types of relative sedimentation rates. These include fractional sedimentation rates, which are individually derived for the main and background sediments; relative sedimentation rates, calculated from Cm-CaCO3 associations, according to the methods outlined in Chap. 2; and relative sedimentation rates, derived by comparing the thicknesses of laterally correlated, isochronous sections. 4.1 Conventional time span determination

The determination of the time span inherent in any given stratigraphic interval is commonly performed by comparing two radiometric data at the bottom and top of a given interval, or by using biostratigraphic, magnetostratigraphie and isotopic data (e.g., STSr/~Sr ratio) that axe tied to conventional radiometric time scales (e.g., Odin, 1981; Harland et al., 1982; Palmer, 1983; Berggren et al., 1985; Salvador, 1985; Kent and Gradstein, 1985; Haq et al., 1986; Bayer, 1987). There are two sources of error involved in such a conventional time span determination. First, the radiometrie ages and time scales have a considerable range of error themselves; and second, the time span error between the two radiometric dates or biostratigraphie zone boundaries is often much larger than the interval of the section which is to be dated. To interpolate down to smaller intervals, it is usually assumed that time is linearly distributed (Fig. 4-1); in consequence, the precise timing of smaller stratigraphic intervals is a difficult procedure. 4.2 Time span determination using sedimentation rates

Unlike the timing methods explained above, the time span represented in stratigraphic intervals can be estimated by employing standard, long-term sedimentation rates. This is achieved by dividing the thickness of a given stratigraphic interval by the typical sedimentation rate for the sediment or rock type under consideration (Fig. 4-I). The time span (T) of a sedimentary interval equals h T =-

h or

s

s=-

,

(4-1)

T

where h is the rock or sediment thickness, and s the average long-term sedimentation rate for a given environment. In this simple approach to time span estimation, sedimentation rates are used that are derived from compacted rocks in various environments, averaged over long time intervals. In Fig. 4-2, long-term sedimentation rates are compiled according to data

4g

presented by Reineck (1960), Seibold (1974), Schwab (1976), Sadler (1981), Seibold and Berger (1982), Scholle et al. (1983), Stow et al. (1985), and Anders et al. (1987). These rates, representing a variety of depositional environments, span four orders of magnitude, from 1 to 10,000 m/Ma. In contrast, most sedimentation rates for indiviual environments vary by only 1 to 2 orders of magnitude, except deep sea clay and siliceous ooze, which have smaller ranges, spanning 0.3 to 0.8 orders of magnitude. Thus, the error from using average sedimentation rates for time estimations may'be large, because long-term sedimentation rates are highly variable within most individual environments.

mime

SEDIMENTATION RATES time span contained In a 30 cm-thick bed

10000-

0.1 k a

II.-I

1000"-

q

W

100-

-lo oE 10-

:ii-~!i~ <':':

~:

-

~ .::~.:..:::::::.::: ::;.: :

........ ~"11~ ?.i~::ii ~ ' iiii!i~iiiii~=.

~E 1

!.i;!:ii!i~

ii~i~i~i~.~:ii!i;i!~!iiil;i:.ii~iiiiiii!iiiiiiii~ii :ii!ili~i'~

i

10ko ko I~ iii!H;iiiiiiii!i!iil ig-l-iiiiii!iiiill iiii! 100ka

ii!ilil ii! iiiii!iiiil

Fig. 4-2 Major long-term sedimentation rates (in m/Ma) compiled after several authors as referenced in the text. Horizontal lines indicate time spans (in ka) inherent in 30 era-thick beds for environments with various sedimentation rates. "MFB" (shaded) is the Milankoviteh frequency band.

Time spans can be more accurately assessed by using fractional sedimentation rates, instead of the average, whole rock sedimentation rates as utilized above. Such fractional sedimentation rates are defined as representing the individual sedimentation rates of two sediment fractions. Fractional sedimentation rates are especially useful for rocks composed of a carbonate and a non-

50

carbonate fraction. Such fractional sedimentation rates can be transformed into conventional, whole rock sedimentation rates (s) by emplo},ing the mean carbonate content (C) of the stratigraphic interval under consideration. When one knows the individual sedimentation rate of either the noncarbonate fraction (Sr~c) or the carbonate fraction (Sc), the sedimentation rate can be expressed as 100 SNc s =

;

(4-2)

100-C or

100 Sc s -

(4-3) C

The time span inherent in a given stratigraphic interval can be derived when Eqs. 4-2 and 4-3 are substituted for the s value used in Eq. 4-1. Fractional sedimentation rates can be used when the sedimentation rate of one fraction can be more precisely estimated than that of the other fraction; or when the sedimentation rate of one fraction is relatively constant, while that of the other fraction varies greatly, as defined for calcareous or siliciclastic deposition (see Chap. 2). Eqs. 4-2 and 4-3 suggest that sedimentation rates are highly dependent on the carbonate content of the stratigraphic unit (i.e., "carbonate content - sedimentation rate relation"). In Chap. 7.1, this concept is applied further to determine standard sedimentation rates, facies transitions, and perform sequence modelling. An instructive example for the application of fractional sedimentation rates can be taken from the Albian to Eocene Gubbio section in central Italy (Fig. 4-3). Mean carbonate contents in this entirely pelagic sequence range from 70 to 95 % (Arthur, 1979). These values have been averaged over all stratigraphic stages, in order to smooth diagenetically enhanced and lowered carbonate contents found in individual marl-limestone couplets (Arthur, 1979; Ricken, 1986). For the calculation of modelled sedimentation rates and time spans (Fig. 4-3, right graph, Eqs. 4-1 and 4-2), it was assumed that the noncarbonate content of the sequence is equivalent to typical deep sea clay with a long-term sedimentation rate of Sr~c = 1m/Ma (see Fig. 4-2). The results are roughly equivalent to sedimentation rates calculated using the time scales developed by Berggren et al. (1985) and Kent and Gradstein (1985); see Fig. 4-3 (left graph). This indicates that the carbonate input for the Gubbio sequence varied, while the clay input remained relatively constant, especially for the lower and upper parts of the sequence (i.e., carbonate deposition; Chap. 2. Further evidence for this interpretation is presented in Chap. 7.3. In siliciclastic deposition, these relationships would be reversed. Theoretically, both types of sequence can develop, with deposition dominated by either carbonate or elastic (i.e., terrigenous) variations, and with transitions between these basic types (see Chap. 7). Such end members form sequences for which time spans and carbonate contents are easily determined. This aspect is discussed further in the following section, with respect to relative sedimentation rates.

51

o

a

.-

z

o_

-- ~

t~

~

,',-

~

~r

~

05

o o

,,..1:

,~i~::~zliiiil ii~ i!i!!ii!!

,,.-:

t'<

E

!i

"r

~~ ~t

r,~

z

~i

-~I

.,~ I--:

~l.ul

~

iiii!iiii:

_

~

,., __.~ ~

o_

F-

--,,,

-~

.~

t~

~

:~_~

;;i~:.:.!::!ii ~:.:: o

I /-" f'q

m

ix.

5

r

tool-

r

.r

Fig. 4-3 Time scale sedimentation rates and sedimentation rates derived from model for the carbonate (shaded) and noncarbonate fractions (black) of the Gubbio section, Italy, based on carbonate data from Arthur (1979). Numbers on the left sides of the two diagrams indicate time spans for stratigraphic stages (T, in Ma), while numbers within shaded parts of histogram indicate mean carbonate content (vet. %). Model sedimentation rates and time spans are determined by assuming a constant sedimentation rate of lm/Ma for the noncarbonate fraction. Inset presents the Albian with smaller-scale carbonate variations and variations in sedimentation rates and time spans, based on an assumedly constant noncarbonate input.

52

4.3 Time span assessment using relative sedimentation rates The time span estimated for a given sedimentary unit in Eq. 4-1 becomes a relative number when the relative sedimentation rate (Sr) is used instead of the absolute sedimentation rate (s). h Tr

~

(4-4)

m

Sr

where T, is the relative time span, and h the thickness of a considered stratigraphic unit. Because the relative sedimentation rate is a dimensionless factor, T, denotes the relative amount of time in a given lithologie interval. Three types of relative sedimentation rates can be determined:

LATERAL CORRELATION S "=2

h2H\'r WhlN_ section 3

2

1

_

h2

Sr2- hl

/\

SNC = constant

sC = constant

sr2=I00-[I

[I

I00_C2 sr2=~

sr2

sr2

,.i i increasing

carbonate content

increasing

carbonate content

Fig. 4-4 Determination of sedimentation rates relative to a time-equivalent reference section. Left side of diagram shows lateral correlation of three sections with different thicknesses (h) and relative sedimentation rates (s,, expressed relative to Section 1). Right side of diagram indicates the relationship between relative sedimentation rate and carbonate content (C) for two idealized end members, with either laterally constant nonearbonate (Ssc, black) or carbonate sedimentation rate (Sc, shaded).

53

1. Relative sedimentation rates, as defined in Chap. 2, which use characteristic Cor.-CaCO~ associations to estimate the relative difference in deposition (se'e Eqs. 2-8 and 2-11). 2. Standardized sedimentation rates (s~ that are determined from a succession of relative sedimentation rates; standardisation is performed by multiplication of the various relative sedimentation rates (which are factors). A detailed record of standardized inputs can be obtained for larger sequences (Chap. 6). 3. Relative sedimentation rates that are derived from laterally correlated sections, as explained below. Lateral correlation procedures allow one to determine precisely the relative values of stratigraphic time spans and sediment inputs. The methods are based on accurate, lateral correlation of stratigraphic intervals, either by performing graphic correlations (Shaw, 1964) or commonly-used lithostratigraphic and biostratigraphic high-resolution correlations. Refined correlation methods presented by Freyberg (1966), Hattin (1971, 1985), and Kauffman et al. (1991) demonstrate that lateral correlations can be performed very accurately, spanning tens to thousands of kilometers. As follows from such precise lateral correlations, an equivalent amount of time must be represented by the various correlated units each differently thick (Fig. 4-4). Hence, when differences in porosity and compaction between sections are small enough to be negligible, variations in thickness are directly related to variations in sedimentation rate. Such variations can be expressed relative to a time-equivalent reference section. As described in Fig. 4-4, the relative sedimentation rate of a given laterally correlated section can be defined by comparing its thickness 0az) with that of such a reference section (hi); the latter is assigned a relative sedimentation rate of sr = 1. Hence, the relative sedimentation rate of a correlated section (st) is expressed by h2

sr -

(4-5) hi

Next, individual sedimentation rates for the noncarbonate (S~c). and carbonate fractions (Sc) can be calculated in order to document changes m deposltional input. Such calculations are performed by substituting the relative sedimentation rate (st) obtained in Eq. 4-5 for the sedimentation rates expressed in Eqs. 4-2 and 4-3, and by solving these equations for SrNc and S~c (Fig. 4-4). An example of lateral correlation and the determination of related relative inputs is provided by an E-W transect through the Western Interior Basin, U.S.A. In Fig. 4-5, a 1500 kin-long, time-equivalent cross section through the western part of this basin is presented, spanning the Cenomanian-Turonian boundary. Individual sections were correlated precisely, controlled by bentonite beds and biostratigraphic data (Hattin, 1971; Elder, 1987a,b). In this transect, mid-basin pelagic chalks grade into marls and silty clays towards the

54

WESTERN INTERIOR BASIN TRANSECT RELATIVE CARBONATE AND NONCARBONATE INPUTS A1 A,~ o so ~c~co3 " ' L

,~ 9 "::!:!::i:i: ::::::::::::::::::::::::::::: :: :i: :i ~i~iiZ!!i!i!~!~!~ii?i!i!!!i~i!i~i?i!ii3i~!iii~!ii!i?!

=.~~iiii!!iiiiii!!

~,~: ~

~iiiiiiiiiiiil

~-."" -: o.:, =================================== .!:!:i:i?i:i -~'~iii~!iii:iii!

klkA.

~

"

J ....

~" "')

~'L

so','.o,co3

o

--.,,z.-- f

*

~, ','.c~co3

ELOER. I987

Sr Z/,4 m'----.._ srt97 r N C ~

S

_

_

5m[ Src

_

_

~

l~,:,i'~a~!~4 -lii i~a~ili;I 10.8 26.8

8.7 3/.6

FLUX PATTERN C,~qBO~TE V V V V v ' v v v v SNPUT

5,75 /.1.3

,,,

-
4,42 66m thickness 62.7 Z,8.5% mean CaC03 standard section B =bentonites A /

INPUT

Fig. 4-5 Time-equivalent E-W transect through the Western Interior Basin, U.S., spanning the Cenornanlan-Turonian boundary. Sections AI to K I show variations in carbonate content (wt.%). Lower part of diagram schematically depicts section thicknesses with relative sedimentation rates (s,, expressed relative to the standard section with s,= 1), as well as relative sedimentation rates for the nonearbonate (S,No black) and carbonate fractions (S,o shaded). 'B indicates bentonite marker bed. Inset on the lower left side exhibits schematic interpretation of sediment flux pattern. Carbonate data and section correlation according to Elder (1987a). western shore zone. In the chalk, correlated time-equivalent sections are relatively thin, while sections become thicker towards the western shore zone. Such increasing thickness is reflected by increasing relative sedimentation rates in the individual sections, when a section in mid-basin is chosen for the reference section (i.e., the Colorado 2 section). The depositional input pattern causing this nearshore increase in the sedimentation rate is also expressed in the relative fractional sedimentation rates (s~c, sw_.),determined using the mean carboaate content of the individual sections as explained above. The noncarbonate input (i.e., siliciclastic sediment) steadily increases as the western shore

55

zone is approached, but the carbonate input (i.e., coccolithic calcareous ooze) is basically the same in all sections (Fig. 4-5). Hence, sections become thicker towards the west and more depleted in carbonate due solely to increased terrigenous input; the constant carbonate input reflects laterally constant surface water productivity. 4.4 Conclusions Time span estimations serve as basic instruments in sedimentary geology, providing the scale on which various depositional, cyclical, and evolutionary processes occur. As demonstrated here, time span estimations are closely associated with indirect procedures, such as deriving relative sedimentation rates and fluxes in either vertically or laterally correlated sections. Despite their indirect and comparative character, some of these timing methods have higher resolution than conventional time span determinations. 1. Timing by fractional sedimentation rates: For sequences composed of both carbonate and noncarbonate fractions, a relatively precise time span estimation can be achieved when the sedimentation rate of the carbonate or noncarbonate fraction can be derived individually. Such fractional sedimentation rates can be obtained for vertically successive stratigraphic intervals and laterally correlated isochronous sections. In lateral correlation, sedimentation rates and the carbonate and noncarbonate fluxes are accurately expressed relative to a reference section by comparing the thicknesses of time-equivalent stratigraphic units. Thus, fractional sedimentation rates provide ultimate clues as to the style and processes of deposition, expressed by vertical and lateral variations in the relative input pattern. 2. Timing by relative sedimentation rates: An additional timing method is based on relative sedimentation rates derived by applying equations (Eqs. 2-8 and 2-11), as described in Chap. 2. Evaluation of C~.-CaCO3 associations allows the calculation of relative sedimentation rates f6~r small stratigraphic units (e.g., beds and bundles) and larger units, when the individual relative sedimentation rates are numerically connected and standardized. Examples of the relative timing of individual beds are given in Chap. 5, while those for combined changes in depositional inputs in larger sections are presented in Chap. 6.

Chapter 5 INPUT VARIATION IN RHYTHMICALLY BEDDED SEDIMENT

In this chapter, the effects of depositional dilution and concentration processes on bedded, "pre-diagenetic" sediment are investigated. This demonstrates different styles of deposition governing bedding rhythms. Different depositional styles are related not only to different Corz-CaC03 relationships, but also to different bedding types. The evaluation of depositional processes allows deter-

ASMMETRY RELATION OF RHYTHMIC SEDIMENTATION SINUSOIDAL SEDIMENT FLUX

i!~

i:.:i:i:!:i:i:::::.:"9

TRANSLATION INTO SEDIMENT

FLUX,~ INTERVAL WITH LOW i" [i.::;:i:i!i!::.:i:i:!:::il ;:::i::i| .-----'-"-'-"~;:;iiii;ii:::i::i;i:i;i:ii:~:i SEDIMENTATION RATE

m FLUX PATi'ERN _z o l::::.::::.::.,::::::.:: :,l

Fig. 5-1 How a sinusoidally fluctuating sediment input is transformed into a rhythmically bedded sediment sequence with alternating thick and thin beds (asymmetry relation). A. Supply of sediment with alternating periods of low and high sediment supply with identical length of time. B. Resulting rhythmically-bedded sequence in which periods of low and high sediment supply are transformed into thin and thick beds, respectively.

60

ruination of relative sedimentation rate changes between alternating beds according to the organic carbon dilution equations introduced in Chap. 2. As it turns out, changes in relative sedimentation rates are fairly large, but these large changes are translated only to small carbonate-organic carbon differences in the deposited sediment, regardless of whether the carbonate content is either low or high. Interrelationships between the several involved parameters, such as depositional input styles, type of bedding rhythms, carbonate differences between beds, and related variations in sedimentation rates, are addressed here. These topics are treated through the investigation of three major subjects: 1. Introduction of procedures which show idealized bedding rhythms for the three basic types of deposition, according to the three-component system. 2. Analysis of sediment inputs and determination of relative sedimentation rates based on carbonate - organic carbon data of simple and composite depositional styles. 3. Modelling of sedimentation rate changes and their influence on carbonate differences between beds for non-lithified, deep sea sediment.

5.1 Idealized bedding rhythms for carbonate, siliciclastic and organic matter deposition Rhythmically bedded sediment with oscillating carbonate content can be explained either by fluctuating deposition from individual sediment inputs, or by a combination of several, simultaneously varying inputs. Rhythms produced by the three basic types of deposition, defined in Chap. 2.2, contain distinct, alternating, thick and thin beds, which represent successive periods of high and low sedimentation rates, respectively (de Boer, 1980, 1983, 1991; Arthur et al., 1984a; Arthur and Dean, 1991; Cotillon, 1985, 1991). These different expressions of bedding rhythms are characterized using the "asymmetry relation", as it is decribed in Chap. 7.3.2. This relation stipulates that changes in the sediment supply are expressed in the resulting sediment sequence, which becomes distorted or asymmetrical compared to the original signal. A rhythmically fluctuating sediment supply, following a sine function with identically long alternating periods of low and high sediment supply, will be translated into a rhythmically bedded alternation, but with thicker beds and thinner interbeds (Fig. 5-1). In an interval with low sediment supply the beds become relatively thin, whereas the opposite is true in a period with high sediment supply.

Fig. 5-2 Distinctive types of bedding rhythms formed by varying deposition in either I~ .c~3na.te (a.), silici.clastic Co), or organic ..matter(c). Bedding rhythms show carbonate tt-at:u~ aria organlc caruon (C~,) curves (in wt.%) for alternationswith low, medium,

and high.car.bo.natecontents(A..B,.and C). Changing.sedimentationrates are expressed by me,v-~-y.mgto~Crnessesor. sectmn intervals representingequal time spans (horizontalbars on fen siae o curves). R~ght-hand graphs represent the carbonate - organic carbon relau.'onshipfor each bedding rhythm (m wt. %). Diagrams are based on depositional inputs t~t vary sinusoidally by a factor of 4 (see Fig. 7-17"o).

61 a q

10 2.0 30 30 /.0 50 60 70 70 80 90 100%C~CO3 i

i

i

i

9

:.~ =..!;..~!i ~;:~i.!.:;:~..... ~ ................. ~ ~ ;~ : ~i~ii!iii!~ii~!;i;i:~i;i;..... i~

A

B

-- ;~i:!:!:;:!:i:::~.:

t'~g

....

:::.~ii:!i:~!iiiiiiiiiiiiii!iiiii;iiii!i~i~!;~:~ ....

- ~aco3

_Z.!

-~ )

-::~:iiiiii;~:~...........

A

50

0

100

C~--,O3 [W%]

6o 7o 8o 9o ~,oo%c_~co3 .,,.s

--

V A R I A T I O N IN CARBONATE oco3 D E P O S I T I O N

~TE~;'WLSwrrH ~u~L r ~ r SPAN

~0 20 20 3o z,o 59 ~

bg

=

~ iii;::i~!;~?::~co3

V A R I A T I O N IN CLASTIC DEPOSITION Coro[w%] B

B

0

C

5(3

C !

I00 CaCO3[w%]

V A R I A T I O N IN O R G A N I C D E P O S I T I O N 3,o ~o 5o 60 10 20 ~o 8,o 9o lqo~ coco3

C 0

i

~.~ ~

_..... _

i

'- 6 e 1. IZro,.,

O ~o rZGrg

CaCO3

:

- ' . ~ ~ a m. 1?_co,.g

.

2co0o3 ,,.

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-i!i!i::i ,; J

:;:: ; ~ Corg

org

--~

.

.

.

.

.

.

.

-~ -".::;:::.:.: ..:.:.:.:

..

-~.::::::.: ~...: :.:.:. : :.:.:.:.:

.

i

.

I

I

-~:::.::.:.:

A

B

F;~..~-2

C

0

50

t 100

82

Such effects of thickness distortion are combined here with Co~-CaCO3 curves, which are used as criteriafor distinguishingbetween the differenttypes of rhythmic bedding (Fig. 3-2). A n additional factor comes into play as the appearance of the rhythms also depends on the overall carbonate content of the environment, leading to differentbedding rhythms with dominantly low, medium, or high carbonate contents. In a carbonate-poor environment, where the constantly supplied clay flux is much more dominant than the rhythmically varying carbonate flux, thickness distortionand carbonate difference between beds are relatively small (Fig. 5-2a). In an highly calcareous environment, however, thickness distortionis significantbecause the large carbonam fraction is subjected to rhythmic fluctuations.A more comprehensive explanation of these aspects is provided in Chap. 5.4.2. Bedded sediment with variable carbonate deposition (Fig. 5-2a): Ideally, the carbonate curves of alternatingbeds have broad, convex maxima and smaller, sharper minima, because the sedimentation rate and carbonate input are higher in the CaCO3-rich bed. This effectis developed best in alternationswith overall high carbonate contents. Since the organic carbon and carbonate contents are inversely correlated, the lowest organic carbon values occur in the middle of the carbonate-rich layers.

iiiiii:"::ii::iiiOS, T ~2030~0 .3p~.0.506p~ ~. ~P9~%CoCO3

Fig. 5-3 Bundle ~ for carbonat~ and siliciclastlc deposition. The effect of bundlin~ is shown for alternations oscillating around low, malium, and high carbonaW contents. Depositional inp.ut varies sinusoidaily for bundles by a factor of 5 and for smaller, superimposed osc111ations by a factor of 2.

63

Bedded sediment with variable siliciclastic deposition (Fig. 5-2b): The bedding rhythms here have essentially opposite curve forms compared to alternations with varying carbonate deposition. Carbonate maxima are narrow and distinct, whereas minima are broad and concave. This pattern is most obvious where carbonate contents are low and terrigenous dilution is high, which increases the thickness of the carbonate-poor beds (Fig. 5-2b). As already explained in Chap. 2.2, the organic carbon content is positively correlated with carbonate content: Carbonate-rich layers have high organic carbon contents, while the opposite is true for carbonate-poor beds. The influence on the weight percent of organic carbon is great for alternations with low carbonate contents. Bedded sediment with variable organic matter deposition (Fig. 5-2e): Sedimentation rates and carbonate contents are influenced little by variations in the small amount of organic matter deposition. Carbonate contents are significantly affected by variations of organic matter only when the carbonate contents are very high (Fig. 5-2c, curve C). Similar depositional processes also control the grouping of beds into bundles (Fig. 5-3). Bundles are understood here as being formed by regular variation of only one type of input (either carbonate or clastic), but with varying frequency. Bundles related to carbonate variation are composed of several carbonate-rich layers, with thin, carbonate-poor beds forming the upper and lower boundaries of the bundle. Such a bedding rhythm occurs especially when overall carbonate content is high. The opposite pattern is observed for siliciclastic variations. Here, bundles show relatively thin carbonate-rich beds in the middie which are surrounded by thick carbonate-poor zones. Hence, carbonate and organic carbon relationships, bedding rhythms, and bundling of beds are all integrated instruments which allow the distinction of the three basic types of depositional input. What they all have in common is that regular variations in deposition are expressed in unequal, distinctive types of bedding rhythm. The marine environments of such bedding rhythms are discussed below with their characteristic organic carbon - carbonate relationships. 5.2 Sediment input pattern as expressed in Co~-CaCO3 data for various marine environments Marine environments can show practically any style and combination of depositional inputs that lead to rhythmic bedding. These rhythms are presented in Fig. 5-4 through a schematic marine facies transect depicting idealized Co,CaCO3 trends. Two major groups of rhythms can generally be discriminate. These are (1) variation of a single component (i.e., the basic types of deposition), and (2) simultaneous variation of several components. The latter group can be distinguished from the first by the development of two or more Corf CaCO3 regression curves, indicating a superposition of several input types.

5.2.1 Environments with variation of a single component Evaluation of a large data set of Corg-CaCO3 contents from sediments encoun-

64

tered in the deep sea and epicontinental basins suggests that variation in carbonate input is the dominant factor above the lysocline in producing rhythmically bedded sediment (Fig. 5-4). Obviously, varying planktonic carbonate production in pelagic environments and epeiric seas since the Jurassic seems to be the most important (npre-diageneticD bed-forming process (i.e., productivity cycles, Herbert et al., 1986; Eicher and Diner, 1991). Such a view may be supported by the substantial productivity fluctuations observed in the modern surface ocean (Deuser, 1987; Wefer et al., 1988; Wefer, 1989). Seaward of carbonate platforms and calcareous shelves, variations in carbonate productivity are superimposed on fluctuations in lateral transport of shallow water, aragonite-rich sediment, forming aragonite-chalk rhythms (e.g., Kier and Pilkey, 1971; Droxler et al., 1985; Haak and Schlager, 1989). Between the lysocline and the CCD, varying carbonate dissolution is expressed by large CaCO3 differences in succeeding beds (i.e., dissolution cycles, Berger, 1973; Emerson and Bender, 1981; Peterson and Prell, 1985; Farell and Prell, 1987; Grttsch et al., 1991). For all of these environments, the correlation curves between Co~gand CaCO3 are negative, with various slopes representing various ratios between organic matter supply and terrigenous sediment (see Chap. 2.3). For instance, environments with high organic matter inputs, as in the oxygen minimum zone or in other oxygen-deficient bottom waters, show usually steep correlation curves (Fig. 5-4). Environments with variations in terrigenous input show a positive correlation between organic carbon and carbonate content (i.e., dilution cycles, Fig. 5-4). This group includes siliciclastic dominated environments with minor admixtures in carbonate, for example in some epicontinental seas, deep clastic shelves and upper continental slopes. Variation in the terrigenous fraction also occurs below the CCD and in largely carbonate-depleted sediment below the lysocline. A positive Cor.-CaCO3 correlation pattern is also assumed for siliceous ooze productivit~ cycles containig a constant supply of carbonate (Decker, 1991). Varying organic matter input, with the two other fractions deposited at a constant rate, is rarely observed under oceanic and epeiric sea conditions. Usually, the much larger carbonate and siliciclastic depositional fluctuations influence the smaller variation of the organic fraction, causing simple or composite bedding rhythms. However, in deeper waters far away from terrigenous sources, wthin oscillating oxygen minimum zones, alternations may only show varying organic matter input (Fig. 5-4). It seems that such types of redox cycle were more common in the middle Cretaceous than in the modem seas, when possibly sluggish oceanic circulation, and a higher input of terrestrial organic matter formed extended, but varying oxygen minimum zones (Wyrtld, 1962; Bralower and Thierstein, 1984; Dean and Arthur, 1986; de Boer, 1986; Stein, 1986, 1989; Emeis, 1987; Sarmiento et al., 1988). Techniques for distinguishing between redox and productivity cycles combine sedimentation rates and Corg-CaCO3 trends with bioturbation patterns (see Chap. 8.2.3).

65

Z

0 Fn

0 iii

(J

:I

"r I I I

"1-

~I ~

I!

0

9~-.u

(/)

"1-

i,,,,,, i::

O--l~o o

(/)

o,... N

Z

~9

0

0=.~

~

I--

E~.-'~-~ ~_

9~- ~ "

~'~1 o~ ~

~i~,~

0

~

>,E

~-~..~.~

c~

0 0

~,'o

.

.)e-

66

5.2.2 Environments with simultaneous variation of several components Composite rhythms with simultaneous variations in two or more depositional components are typical of many marine systems. A common situation is carbonate production in the upper water zone which is underlain by oxygendeficient bottom waters. Such situations occur either locally (e.g., stagnant basins, see Fig. 8-22) or over large areas, when the oxygen mimimum zone impinges on the sea floor (Fig. 5-4). In this case, rhythms indicate concurrent carbonate and organic matter deposition. In upweiling areas, the production of carbonate and siliceous ooze and the terrestrial and marine inputs of organic matter may vary simultaneously with the transport of terrigenous sediment from nearby land sources, leading to a complex organic carbon - carbonate correlation pattern (Fig. 5-4). 5.3 Relative sedimentation rates and time spans inherent in beds Figures 5-5 and 5-6 present a few examples of simple and composite bedding types, illustrating the primarily theoretical considerations made so far. According to Eqs. 2-8 and 2-11, variations in relative sedimentation rates are determined between alternating carbonate-poor and carbonate-rich beds. This is performed by comparing the compositions defined by the maximum and minimum Co~-CaCO3 values, as indicated in Figs. 5-5 and 5-6. In addition, the time span inherent in the beds can be estimated. The bed thickness must be divided by the relative sedimentation rate, assuming that differential compaction and diagenetic carbonate redistribution have not yet occurred (see Chap. 4). 5.3.1 Rhythms due to depositional variation of one dominant parameter Fort Hays Limestone, Upper Cretaceous, U.S. Western Interior Seaway (Fig. 5-5): Here a negative correlation exists between C _ and CaCOj, and the cyclicity is caused by depositional variation in caT'~nate input, probably related to changes in productivity. Sedimentation rates differ by a factor of 7.9 across the entire Co,~-CaCO~ curve (F-xl. 2-11); however, when correcting for carbonate diagenesis-by performing mass balance calculations, carbonate-rich layers have a sedimentation rate only 2 times higher than the marl beds. The organic carbon - carbonate data investigated here do not indicate bottom water redox eyclicity, as proposed by Savrda and Bottjer (1986"), as would be evidenced by changing bioturbation styles (Ricken, 1993). A detailed explanation on this aspect is presented in Chap. 8.2.3. DSDP Site 535, Valanginian to Cenomanian, Straits of Florida (Buffler et al., 1984; Cotillon and Rio, 1984; Fig. 5-5): These Lower to Upper Cretaceous alternations are described as representing redox rhythms, with alternating beds of chalk and organic carbon-rich shale. The negative C---CaCO3 correlations indicate that varying carbonate input repeatedly d-il'uted or concentrated the organic carbon content of the sediment, leading to periodical-

67

ly changing redox conditions in the uppermost sediment layer. As a result, a higher degree of bioturbation is observed in carbonate-rich beds compared to beds with lower carbonate contents (see Fig. 8-19). The correlation curves show various clustering of the C,rFCaCO3 data, with a sedimentation rate 1.8 to 4.8 times higher in the carbonate-rich beds. The Cenomanian alternations have dominantly lower Co,~ inputs than the Valanginian to Albian alternations, as indicated by an additional, flatly sloping Co,FCaCOa regression line. Fairport Shale, Cenomanian, U.S. Western Interior Seaway (Fig. 5-5): This sequence consists of little to intensively bioturbated transgressive shales with minor bedding variations (Glenister and Kauffman, 1985). From the positive Co,z-CaCO3 slope, terrigenous dilution can be inferred, with sedimenta-tion rates up to 6.7 times greater in carbonate-poor beds. DSDP Site 603, Cenomanian, North Atlantic Ocean off New Jersey (van Hinte et al., 1987; Fig. 5-5): Alternating Upper Cretaceous black shale and claystone show background variation in terrigenous input su.perimposed with oscillating organic matter input. Although organic matter varies by 10%, the effect on the sedimentation rate is insignificant. 5.3.2 Rhythms due to simultaneous variation in several depositional parameters

Rhythms formed by the simultaneous variation in several inputs can be recognized in Co_-CaCO 3 diagrams by the superimposition of different types of organic cari~on - carbonate relationships (Fig. 5-6). This is the case when several regression curves are obviously present, or when one composite regression curve is formed which does not intersect the CaCOa axis at 0 or 100 percent, in other words, at pure siliciclastic or carbonate depositions, respectively. The composite regression lines can be treated numerically as the result of two (or more) Cor~-CaCO3 relationships, from which relative sedimentation rates can be derived. Multiplication of these individual rates then gives the relative variation in sedimentation rate for the resultant composite regression curves (see Chap. 2.4.1). DSDP Site 532, Miocene to Pleistocene, South Atlantic Ocean, Walvis Ridge (Gardner et al., 1984; Dean and Gardner, 1986; Fig. 5-6): As suggested by the negative Cor--CaCO3 correlation and the broad, convex carbonate maximum, carbonate v~ariation is the dominant type of depositional input. A detailed investigation by Gardner et al. (1984) shows that under conditions of coastal upwelling, a complicated interaction between carbonate productivity and terrigenous dilution occurred. Separation of the composite C,~z-CaCO3 relationship into individual carbonate and siliciclastic depositions shows that periods with high carbonate productivity are contemporaneous with increased terrigenous sediment input. Obviously, periods representing higher current activity, intensified coastal upwelling, higher carbonate production, and larger terrigenous input alternate with contrasting periods of low current activity. As a result, the carbonate bed depicted in Fig. 5-6 has a maximum sedimentation rate 3.4 times higher than that of the carbonate-poor bed; this proportion is reflected in the greater thickness of the carbonate-rich bed. Therefore, the thin-

68

SIMPLE DEPOSITION CARBONATE DEPOSITION coco3 coCK :0

~

M

J01~ O

,

:ii'

Forf Hays Limesfones Corg z~

r t ~

:

l,r!l ".....,2_~

s r = relative sedimentation rate

O.Sm

L

o

~ Cat03 ~

Corg

OSDP Sire 535 Valonge - A Ib Cenomon

10

'N

0

o

,.

0.3-1.2m J.

@

o

,s,~,.~Sr=Z5

T 0.2-O.Sm

I

0 0

CLASTIC

s ~~

s'o COCO3 ~o

DEPOSITION

s6 Cat03 loo

ORGANIC MATTER DEPOSITION

1 OI

o

s'o Cat03 ~

o

~

"

i

2s Cat03

so

Hg. 5-5 Formation of bedding rhythms by the variation of one dominant parameter and associated organic carbon - carbonate relationships (wt. %). Arrows denote variations in carbonate, organic carbon contents, and relative sedimentation rates (s,). Numbers show factors by which the relative sedimentation rates change between succeeding beds; stars indicate sedimentation rates after correction for differential carbonate diagenesis; small arrows denote direction of sedimentation rate increase.

69

COMPOSITE DEPOSITION CARBONATE AND CLASTIC DEPOSITION

CaC03 Cora Corg 20 t,,060 2.5 "5

Corg

i "

_oNN I

r

soCat03

"G~

COCO3

Corg

zo~o 60 Bo |

o

.Iol ~

0

D

0

I

o

i

2

soCaC03

ODP I Siie722 Corg

~

o

2.

=

.

m

0 o

g

I) O (~

o

so tag03 .1~176

CARBONATE AND ORGANIC CARBON DEPOSITION

CaC03 Corg

rg BridgeCreekLimestone u

*"

9

I

Q

.[*

*,. Sr:1.1 T

sedimentaUon

OSm

I

0

sr~Z7~

t00 rate

Fig. 5-6 Formation of bedding rhythms by the variation of several parameters and elated carbonate - organic carbon relationships (wt. %). For symbols see Fig. 5-5.

aSSO"

70

ner carbonate-poor and the thicker carbonate-rich beds probably represent similar or identical lengths of time. This scenario was already described in the ideal alternations discussed in Chap. 5.1 (Fig. 5-2). ODP Site 722, Miocene, Western Arabian Sea, Owen Ridge (Prell et al., 1989; Fig. 5-6): Rhythmic bedding is related to changes in monsoonal upwelling intensities. Cor,-CaCO3 data express a carbonate dilution pattern combined with variation in orgamc carbon or clastic/silicate matter. Differences in relative sedimentation rates are largely through variation in carbonate productivity, with the sedimentation rates in the middles of the carbonate-rich beds being about 2.4 times higher relative to the carbonate-poor beds. Bridge Creek Limestone, epicontinental chalk, Cenomanian-Turonian boundary, U.S. Western Interior (Pratt, 1984; Elder, 1987; Arthur et al., 1985, 1991; Either and Diner, 1989, 1991): This alternation consists of bioturbated limestones and laminated marls. The inversely, correlated Co~-CaCO3 data and the broader carbonate-rich peak point to varying carbonate supply as the main depositionai process (Fig. 5-6). After correcting for diagenetic carbonate redistribution, the limestone has a higher sedimentation rate by a factor of 1.7 compared to that of the marl. Superimposed on this variation is fluctuating organic matter input, with virtually no effect on sedimentation rate. These data suggest that the Bridge Creek Limestone combines productivity variations (e.g., Eicher and Diner, 1991) with those of water mass oxygenation (Ricken, 1993). The previous examples correspond closely to the theoretical considerations made thus far. The data presented in Figs. 5-5 and 5-6 evidently support the following arguments: (I) In principle, the organic carbon and carbonate relationships found in bedding rhythms from various environments show a pattern equivalent to that observed in the idealized dilution processes for carbonate, siliciclastic, and organic matter deposition (see Chap. 2.2, Fig. 2-1b); (2) in additon, the observed bedding rhythms, with their characteristic carbonate and organic carbon curves, are similar to the idealized rhythms presented in Fig. 5-2; and (3) the application of organic carbon dilution equations shows that variations in sedimentation rates associated with rhythmic bedding are relatively large, ranging from a factor of 1.7 to 6.7 for carbonate and siliciclastic deposition. Such large differences in sedimentation rate are also supported by theoretical considerations of bedded sediments performed by de Boer (1980, 1991), Dean et al. (1981), Einsele (1982), Arthur et al. (1984a), Arthur and Dean (1991), Eicher and Diner (1991) and Cotillon (1985, 1991). 5.4 Statistical evaluation of carbonate contents in rhythmically bedded sediment 5.4.1 Carbonate differences between alternating beds stcmatic variations in sedimentation rate not only control the thicknesses of mating beds but also influence the carbonate differences between them. This CaCO3 difference depends primarily on the amplitude of the oscillating flux, but is severely modified by the overall carbonate content of the environment. A regularly oscillating flux results in various CaCO3 differences depend-

71

CARBONATE DIFFERENCES CARBONATE S P A N BETWEEN SUCCEEDING BEDS (~._.x~CO3)

8oI

801

ACaCO3

QUATERNARY

TERTIARY

6O

'~

I

__~_____ ....

20

40

ACaCO3 80

60

----2='----__ --;-------- ..... ~_

20

O

0

r

60-

-

-

9

.

'

~CaCO3 L0 3O As 20

9

.

40

20

80 100 %COCO3

CRETACEOUS

~-_-~_~_~:__

.-_. . . .

.

.

60

.

"

KEY

3 40 |

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,

!

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'~~

~

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100 80 %COCO3

9

40' 20' 9

,

20

,

,

,

4o

!

'"%

go

go

:oo

10 20 30

%cQco3

50

70 80 95 %CaCO3

MAJOR D~POSfriONAL TYPES OF 8ECON6 . . . . . E4RBONATE DEPOSITION .... ......

Ot$..~..ODON CYCLES

9ELASTIE OEPOSITION CQI'It~[251TEDEPOSITION

Fig. 5-7 Carbonate differences between succeeding beds in various deep sea alternations of Cretaceous to Quaternary age. X-axis: carbonate contents in the middle of succeeding carbonate-rich and carbonate-poor beds, expressed by the ends of a horizontal bar. Y-axis: absolute carbonate span between succeeding beds (CaCO3). Note that the carbonate difference is largest when cycles have medium carbonate contents. Source of data: Quaternary, DSDP Sites 418, 474, 503, 530, 532, and 593, Berger (1973) and Hays et al. (1969); Tertiary, DSDP Sites 366, 329, 474, 530, 532, 540, and 593; Cretaceous, DSDP Sites 398, 461,463,511,540, and 535.

72

ing on whether an interbedded sediment forms in a shaly, marly, or a chalky environment. In the following, deep sea bedding rhythms are statistically evaluated to further quantify these relationships. Such an evaluation is based on carbonate and organic carbon data from soft sediments described by various authors in the Initial Reports of the DSDP (Fig. 5-7), where the carbonate content is thought to be less influenced by differential diagenesis than in lithified alternations. A statistical technique used to demonstrate the CaCO3 span employs the absolute and relative CaCO3 differences between beds, as depicted in an xy graph (Fig. 5-7). While the absolute CaCO 3 difference (or span) is plotted on the y axis, its relative expression is denoted by a horizontal bar parallel to the x axis. The absolute CaCO3 difference represents the carbonate span between succeeding beds and interbeds, comprising for example a span of 15%. The relative span, on the other hand, indicates that the CaCO~ content varies according to the average carbonate content of the environment, between I0 and 25 %, or 80 and 95 % CaCOa, etc. Consequently, alternations with small carbonate differences between beds are characterized by small horizontal bars close to the x axis, whereas those with large carbonate differences have long horizontal bars located in the upper part of the diagram (see key to Fig. 5-7). By evaluating deep sea bedding rhythms in such a manner, it can be shown at which overall carbonate content the largest carbonate differences between beds occur (i.e., rhythms with shale, marl or chalk sediment). The carbonate difference observed between beds in deep sea sediment is assumed to be dominantly due to variations in carbonate or siliciclastic deposition (see symbols in Fig. 5-7), based on Co~g-CaCO3 relationships, bedding rhythms, and other data reported in the literature. Three major results were obtained: 1. The carbonate spans between beds in Cretaceous to Quaternary deep sea alternations are found to be much larger in sequences representing environmerits with generally medium carbonate contents (i.e., marl and marly chalk rhythms) than in those with low or high carbonate contents (i.e., rhythms in shales and chalks, respectively; Fig. 5-7). 2. Rhythms related to dominant variation in siliciclastic deposition occur more frequently when carbonate contents are low (from 0 to 30%), while carbonate deposition is dominantly observed in alternations with medium to high carbonate contents (30 to 100%). 3. Alternations described in the literature as dissolution cycles have larger carbonate variations between succeeding beds than alternations formed above the lysocline. To explain carbonate differences observed between succeeding beds, an equivalent CaCO3 pattern can be generated and plotted on the same type of diagram as that presented in Fig. 5-7. Such theoretical carbonate spans are determined by assuming that either the elastic or carbonate fraction varies (between beds) by a constant factor (F) for rhythms with various carbonate contents. An example is given in Fig. 5-8a. The resulting pattern of theoretical CaCO3 spans between succeeding beds is represented in Fig. 5-8"o. The carbonate spans generated with regularly varying siliciclastic or carbonate deposition are obviously similar to the observed

73

deep sea CaCO3 spans shown in Fig. 5-7. They are small with both low and high carbonate contents and large with medium carbonate contents. Comparison of the observed and calculated CaCO~ differences demonstrates that carbonate or clastic deposition in interbedded sediment above the lysocline varies by average factors of 3 to 6, and by a factor of approximately 10 in rhythms deposited between the lysocline and the CCD. In the next section, a more comprehensive explanation of the observed CaCO3 variation pattern is discussed which involves changes in sedimentation rate.

5.4.2 Variation in the sedimentation rates of deep sea bedding rhythms Considering the fact that the magnitude of carbonate spans between succeeding beds is largest with medium carbonate content, a pattern in sedimentation rate change can be identified. Let us first consider how variations in carbonate input affect sedimentation rates, assuming, for example that the amount of carbonate deposition between alternating beds varies by a factor of Fc = 5 for various alternations with low to high carbonate contents (Fig. 5-9). With low carbonate content, the carbonate fraction is so small that after multiplication by a factor of 5, to obtain the carbonate-rich bed, both the CaCO 3 content and the newly formed sample volume do not increase significantly. As a result, the carbonate difference and change in relative sedimentation rate (st, Eq. 2-I 1) between beds are small. With medium carbonate content, an already large carbonate fraction increases, resulting in a considerable shift in the CaCO 3 content. Augmentation of the sedimentation rate is moderate, namely by a factor of approximately 2. When the carbonate content is very high and comprises most of the sediment volume, multiplication of the carbonate fraction by a factor of 5 results in a large sediment volume, and therefore in a significant increase in the sedimentation rate (Fig. 5-9). However, this large increase in carbonate volume can only cause a small increase in the carbonate percentage. This example demonstrates a contradiction between the carbonate differences (spans) of alternating beds and their related variations in sedimentation rates. Carbonate differences are large when the carbonate content is medium as a whole, whereas associated variations in sedimentation rate are large with overall high carbonate contents. The same pattern is also observed for silicielastic deposition. However, variations in sedimentation rate show the opposite patterns, with small variations with high, and large variations with low carbonate contents (see Figs. 2-1b and 2-2). As a result, carbonate differences between succeeding beds are associated with sedimentation rates in two very different fashions, depending on whether carbonate or elastic deposition is varying. As mentioned earlier, the prediagenetie CaCO3 differences observed in deep sea sediment show that, statistically, most alternations with a carbonate content below ca. 30% are dominated by depositional variation in siliciclastic input, while above 30%, alternations are dominated by variation in carbonate input. This means that most alternations with small CaCO3 differences oscillating around either low or high carbonate contents can be related to clastic or carbonate variations, respective-

74

CARBONATE DIFFERENCES (a)

,, CaCO3 CARBONATESPAN BETWEEN SUEEEEDIN5 BEDS

,o!

.

i

ot/t

. . . . . . .

010 20 30140

50

.

.

.

.

.

.

.

!'-:

6O 7O 8O9O

INCREASING CLASTIC I DEPOSITION B

.

'--.. "'LC',,

Io41 ~

IV'!

p.

100 % CaC03

INCREASING CARBONATE DEPOSITION

i

t

B

:.:.:.:.:.:.:.: [

:---S

A

~

/

;:::;:::::::::: ..-...-....... :::::::::::::::

C iiii!iiiiiiiiii

i!iii~iiiiiiiii iJiiiiHiS

10

35

65

9o %caco3

Fig. 5-8 Model for carbonate differences between succeeding beds. (a): Carbonate differences are generated by varying the siliciclastic or carbonate deposition by a constant factor (F,, Fo =5) for alternations with varioas carbonate contents. Differences in CaCO3 are expressed by dcpositional changes from A to B. C = carbonate, S = siliciclastic sediment. (b) Resulting pattern of theoretical carbonate differences between succeeding bq:ls; variations in silicielastic or carbonate deposition by factors (F, or F,.) of 1.25 to lu. Carbonate differences are large for alternations with medium carbonate contents, but low for alternations with low or high carbonate contents. Note close correspondence to CaCO3 patterns as depicted in Fig. 5-7.

75

ly. Both types of alternations are associated with large variations in sedimentation rate with maximum factors of 3 to 6 for alternations above the lysocline. On the other hand, alternations with medium carbonate contents are related to smaller variations in sedimentation rates, with factors of 1.5 to 2.5

CARBONATE

(b)

DIFFERENCES

CARBONATE SPAN BETWEEN SUCCEEDING BEDS (zxCoCO3)

.

30

t

20-

/

' "

-~. - -

.

.

.

/

-

/

_-._..l ' = I O

.._.___ "-,-

/

~,

"-,,

10-

\,,,-<~%,

40-

I--

3o- / /

-._

-.., F=5 .~..j_--_-_-... / "~- ...... \

/

20-/

"~. . . . . ---,,

1o-/~ 20] 1 0

Fig. 5-8b

0

"~" . . . . ~

------

~

-.z~...F= 2

~

10 20 30 40 50 50 70

"-~-~--

80 90 %CctC03

100

(Fig. 5-10). Thus, environmental changes must have the largest effect on bedding variations where medium carbonate contents occur. Here, Milankovitchtype, orbital-climatic variations have the potential o f being prominently expressed in bedding rhythms, likewise disturbing variations related to the noncyclic deposition. On the other hand, depositional variations in environments with low or high carbonate contents will have much smaller expressions in bedding variation, although they are associated with the highest variation in sedimentation rate.

76

5.5 Conclusions Here we attempted to quantify how depositional dilution and concentration processes control the formation of interbedded, "prediagenetic" sediment. Variations in the major types of deposition are related to characteristic changes in

CONTRADICTION BETWEEN CARBONATE AND SEDIMENTATION RATE DIFFERENCE

CARBONATE SPAN BETWEENSUEEEE9

BEDS (~CnC03)

i

30.

A.

\

2 0 " ~ 10"

"""',,,.,,,

F st=4.6 e

iiii!ii;i!ii ::i iii!iiii!iil;

%CuCO3

ki:ii~;~ki!iiS~!

~-22

Illllflllll_ltllltltlll illllllllll, lllllllllli ........... 10 '

35 25 -'-~

30 '

68

90

38-'-~

98 %Cat03

8~

A CoCO3

Pig. 5-9 Contradiction between the carbonate span of succeeding beds and related changes

m sca~mentation rates. Example shows three marl bed sediments with low, medium and high carbonate contents (A, C, and E); each of the carbonate fractions is augmented by a factor ofF~ = 5 to produce the carbonate-rich beds (B, D, and F). Maximum increase in relative sedimentation rates (s,) occurs in bedding rhythms with generally high carbonate contents, whereas maximum carbonate differences between beds (CaCO3) exist in rhythms with medium carbonate contents. Upper graph is equivalent to graphs depicted m Figs. 5-7 and 5-8. sedimentation rates, stratification rhythms, and C_-CaCO~ associations Even more, carbonate differences between beds are systematically associated w~th both variation in input and the overall carbonate content of the environment. ~ 6

.

.

"

9

77

VARIATION IN SILIClCLASTIC DEPOSITION CARBONATE SPAN BETWEENSUCCEEDN IG BEDS (z~CoC03)

C CaEO3..'

4o-

A

.

%>I

30"

E=Sr=~" n

.

10' 9

10

9

9

8

9

I

7

6

'"

|

5

"

9

9

|

4

3

2

RELATIVE SEDIMENTATION RATE {st)

VARIATION IN CARBONATE DEPOSITION CARBONATE SPAN BETWEENSUECEEDN IG BEDS(z~EaCO3 )

C~ 3

so,40. ~*,,

"

A~ii',i;!iO":,ili'

20"

10" I

~ 2

3

Z,

5

6

7

8

9

10

RELATIVE SEDIMENTATION

RATE(st) Fig. 5-10 Magnitude of the carbonate span between succeeding beds (CaCOa) and associated variations in relative sedimentation rate (s,), depicted for variations in siliciclastic and carbonate deposition by factors (F,, F,) of 2 to 10. Note that maximum carbonate differences between beds occur with moderate changes in sedimentation rates. Inset of each diagram schematically shows changes in relative sedimentation rate between a layer rich in carbonate and carbonate-poor beds (arrows).

78

1. Carbonate - organic carbon relationships and the determination of re-. iative sedimentation rates: Uncemented, bedded sediments that are formed by carbonate, siliciclastic, and organic carbon deposition can be related to distinctive Co~.-CaCO3 correlation patterns reflecting various types of deposi-tional dilution an'd concentration processes. They can be quantified by ehang-ing sedimentation rates as outlined theoretically in Chap. 2.2. 2. Asymmetry relation and distinction of various bedding rhythm types: In rhythmically bedded sediment, thin and thick beds commonly represent different sedimentation rates but equal time spans, indicating a general contradiction between bed thickness and the associated amount of time. Regular variations in sediment supply do not result in equally regular variations in sedimentary rhythms. Instead, variations in sediment supply influence the resulting bedding rhythms related to carbonate, silieiclastic or organic deposition in a contrasting and distinctive manner (see Fig. 5-2): (i) Variations in carbonate input are expressed by bedding rhythms with broad carbonate maxima, small minima, and inversely correlated Cor.-CaCO 3 trends. Bundles formed by variations in carbonate deposition have'a broad zone of carbonate-rich layers which are separated from other bundles by small, carbonate-poor zones. (ii) Variations in siliciclastie deposition are expressed by rhythms with sharp, small carbonate maxima, wide minima, and parallel organic carbon and carbonate curves. Bundles are characterized by sharp, small CaCO3 peaks separated by relatively thick CaCO3-poor zones. (iii) Variations in deposition of organic matter are encountered in alternations with rhythmic Corg variations essentially unrelated to the slightly changing carbonate content. 3. Simple and composite rhythms: Alternations with one varying parameter (i.e., the basic types of deposition as defined in Chap. 2) can be distinguished from those with simultaneous variations in several parameters by using single and composite Co,,-CaCO~ correlation patterns, respectively. In marine environments, nearly all types of depositional variations and combinations can be found (see Figs. 5-4 to 5-6). The most common type of alternations encountered in oceanic and epicontinental Cretaceous to Quaternary sediments shows either pure variations in carbonate input, or composite variations, in which carbonate input is combined with contemporaneous organic or silicictastic fluctuations, Rhythms with generally low carbonate contents may express variations in siliciclastic input or combinations with other simultaneously varying fractions. They are found on deep elastic shelves and in environments deep below the lysocline. Rhythms related to pure redox cycles are seldom encountered, but combinations with other types of input are very common. 4. Deep sea carbonate variations and associated variations in sedimentation rate: Statistically, alternating deep sea sediments of Cretaceous to Quaternary age show the largest carbonate differences between succeeding beds when overall carbonate contents are medium, whereas the smallest earbonate differences are encountered in alternations with either low or high carbonate contents. This conspicuous pattern is explained by the assumption that the amount of carbonate or elastic deposition fluctuates in various oceanic

?9

bedding rhythms by a roughly constant factor. For alternations above the lysocline, carbonate or clastic deposition varies by a factor of 3 to 6, while higher factors, around 10, are obtained for alternations situated between the lysocline and the CCD. Such a systematic pattern in carbonate differences between succeeding beds is predictively related to variations in sedimentation rate. In bedding rhythms with low and high carbonate contents, carbonate differences between beds are relatively small, but rhythms are thought to be associated with considerable variations in sedimentation rate. In bedding rhythms with medium carbonate contents, carbonate differences between beds are relatively large, and variations in sedimentation rate range between factors of only 1.5 and 2.5 for both types of input. 5. Obliteration versus expression of bedding rhythms: Bedding rhythms with low or high carbonate contents are interpreted as being associated with the largest variations in sedimentation rate. In spite of that, rhythms are usually not detectable in outcrops or drilling cores with these carbonate contents, as they give rise only to minor shifts in CaCO3 content unless redox and color cycles are developed. For example, in some European chalks with approximately 98% CaCO~, bedding cyclicity is not clearly visible although sedimentation rates must have varied considerably in order to generate the small variations in the carbonate content. On the other hand, rhythms in environments with overall medium carbonate contents express striking carbonate differences between beds, although associated variations in sedimentation rate are much smaller. Consequently, environments with generally medium carbonate contents are most sensitive to translating smaller depositional variations into bedding-related CaCO 3 oscillations (see Chap. 7.1.4). We have seen in this chapter that the basic premises of the three-component system are applicable to interbedded sediment. The following chapter shows that not only individual beds, but also larger sequences, which are characterized by complex, concurrently changing input patterns, can be addressed by using a combined version of the three-component system.

Oapter6

COMBINED INPUT PATTERN OF TRANSGRESSIVER E G R E S S I V E C Y C L E S , U P P E R C R E T A C E O U S , U.S. WESTERN INTERIOR

In this chapter, an advanced approach is outlined for the determination of fluxes in entire, larger sequences which lack high-resolution time control, or even any time control. It is argued here that in such older sequences, depositional fluxes can nevertheless be derived by means of a combined input model for three components. The combined three-component model is designed to evaluate observed systematic changes in carbonate and organic carbon data in order to reconstruct concurrent changes in deposifional inputs, as they relate to the sea level history o f the Western Interior Seaway in North America. This model is based on the interpretation of Upper Cretaceous transgression-regression cycles (hereafter called TR cycles) that arc taken to be the product of three simultaneously changing fluxes: carbonate, siliciclastics, and organic matter. In the combined three-component model, a continuous succession of these basic depositional variations is numerically combined or standardized. Such a standardization allows us to derive a detailed pattern of rclative, but combined, fluxes throughout a sequence. 6.1 Geologic setting and source of data The combined three-component model is applied to mid-basin sequences of the Greenhorn and Niobrara TR cycles (Cenomanian-Campanian, Fig. 6-I). These two TR cycles, in the investigated localities 170 and more than 80m thick, respectively, contain a variety of facies types (nearshore sandstones at the bases and tops of the cycles, and organic-rich and organic-poor silts and shales, marls, and pelagic limestones in the middles of the cycles.) These facies represent a wide range of sedimentation rates. The two TR cycles show relatively -,vide spans of organic carbon (0.1-5%) and carbonate content (0-95%). Members which represent peak transgression possess conspicuous, well-developed rhythmic bedding, which allows examination of high frequency changes in sediment input (see Chap. 5). The rocks are well-studied in terms of petrology, stratigraphy, and organic carbon content (e.g., Weimer, 1960, 198//; Hattin, F!g. 6-1 Stratigraphy and study localitiesof the Upper Cretaceous Greenhorn (G) and ~. Nlobrara (N) TR cycles. Paleogeographyof the Western Interior Seaway (WIS) and the North Atlantic Ocean (NA) at peak transgressionof the GreenhornTR cycle. After Scott and Cobban (1964), Kauffman (1984), and Pratt et al. (1985).

81

Ill

I

Illl

I

t

I

3~•

...,.,I

!

IIH

'''' ' wV~OiN

II

N~O.,N 33

',.,,"~ iv-, ~, ,~ I~I

~%~

t~1~tt.~ ~ ~ I ~ I-

~._~ ~

~

I~.1~11~ l_-..v. ~ x

"moo

~1}1

I I

uoluas ~.uoo uoanl

,,,ueo "V Fig. 6-1

82

1964, 1981; Scott and Cobban, 1964; Either and Worstell, 1970; Kauffman, 1984; Pratt, 1984; Pratt and Threlkeld, 1984; Sageman, 1989). In addition, special studies on bedding cycles were carried out by Hattin (1971), Pratt (1984), Eicher and Diner (1985, 1989, 1991), Arthur et al. (1985, 1986, 1991), Barron et al. (1985), Savrda and Bottjer (1986), Bottjer et al. (1986), Elder (1987), Laferriere et al. (1987), Pratt et al. (1991), and Ricken (1993). Values for Cor~-CaCO3 obtained from 576 samples from the Greenhorn and Niobrara TR cycles (Pueblo and Lyons, Colorado) were plotted in xy-diagrams for various lithologies (Figs. 6-2 and 6-3). The sources of the data are 258 of my own determinations and 318 values taken from the literature (Arthur et al., 1985; Barlow and Kauffman, 1985; Glenister and Kauffman, 1985; Kauffman, 1985; Rodriguez, 1985; Sageman, 1985; and Sageman and Johnson, 1985) as indicated in Figs. 6-2 and 6-3. My own Co~s-CaCO3 analyses were performed at the Oil and Gas Branch of the U.S.G.S.in Denver using a LECO induction furnace and a volumetric CO2 analyzer (Pratt, 1984). 6.2 Contrasting relationships between carbonate and organic carbon content The various lithologies of the Greenhorn and Niobrara TR cycles display conspicuously linear relationships between organic carbon and carbonate content (Figs. 6-2 and 6-3, see Fig. 1-3), although some scattering is apparent. Two major data trends correspond to carbonate-rich and carbonate-poor rocks. Most of the data from limestones and marls follow regression lines in which the weight percent of organic carbon approaches zero as carbonate content approaches 100% (i.e., carbonate deposition, see Chap. 2.2). In contrast, in shaly, silty, and partly marly sequences, the organic carbon content increases with increasing carbonate content, and the regression lines intersect the origins on Co,,-CaCO3 graphs (i.e., siliciclastic deposition). Occasionally, there is a third ~ of regression trend which runs parallel to the Co~gaxis, indicating that in these intervals the organic carbon content varies independently of the carbonate content (i.e., organic matter deposition). The dominant patterns observed in the Co~s-CaCO3 diagrams for the studied Cretaceous lithologies (Figs. 6-2 and 6-3) are consistent with the Cor,-CaCO3 associations, theoretically derived in the methodology chapter (Chap. 2.2, Fig. 2-1b). It is therefore assumed that the depositional processes responsible for the observed variations in the organic carbon contents of the Greenhorn and Niobrara TR cycles are significantly influenced by depositional dilution and concentration processes similar to those indicated in the idealized models intro-

Fig. 6-2 Weight percent of organiccarbon versus carbonate content and resultingdilution curves for lithologiesof the Greenhorn TR cycle. Bold numbers represent OMNcand OM~t values describing organic matter content in the backgroundsedimentfor carbonate and sdiciclastic deposition. Small numbers indicate sample points correlating with the stratigraphy displayedin Fig. 6-5. Arrows indicatechangesin composition.Hatched areas show values affected by diagencticcarbonate redistribution.

83

GREENHORN TR CYCLE %Corq 4-

%Cot

~ 3~11

9

~RAN~ROSs.~,E

Cor920 40 6O

Corcj20 40 60 80 100 L-

3:

80 100

~2~9.5 9

8.5~,,

LOWER 2 HARTLAND MEMBER

i

3 UPPER 13~' ~ ,,88 r HARTLAND .I " ~ * . ~

2:

'v,\

~,

DATA: SAGEM,NN1985

"

LINCOLN

9 M.

2 0 9x

_1,

"~('<,~

9

Co~2.0 ~.0'60 80i00 co~~

~.'0' 60

80 i00

z

m

-1-

4

9

LOWER

BRIDGECREEK .,\, 3 MARL-LIMESTONE ~23 "1"..j ALTERNATION F\ ,,, 2

>

3:

2:

5.5.E~,~,

Corg9 2"0"/~0'

14.(

L.

.... \'~5

i'~

MIDDLE "" BRIDGECREEK ~ - ALTERNATION

60" "' " 100 Cor~

20 /.,0 60

7 za 100 z

14.3 9

4: UPPER BRIDGE CREEK ~.,~.~

3: ALTERNATION

BL(

RILL.

404

"20 " t.'O' 60 8029100 Fig. 6-2

% CaC03

II /["

IRPORTMEMBER

11.7/9,3z

.. . . . . . . . .

~

20 z.O 60 80 100 %

CaC03

t'~

84

duced here. Different lithoiogies in the investigated cycles are the products of different styles of deposition. Transgressive and regressive muddy sandstones and silty shales, located at the bases and tops of the TR cycles, indicate by their C,rf-CaCO3 data the predominance of siliciclastie deposition On the contrary, sea level hlghstands lead to carbonate deposltton, with some concurrent changes in the supply of organic matter. Understanding the relationships observed in the data can be facilitated by simulating the combined types of sediment input, as discussed below. .

.

+

.

*

6.3 A model for combined changes in depositional input: fractional sedimentation rates for entire sequences The combined three-component model is an attempt to quantitatively combine the various Cor~-CaCO3 relationships in the investigated TR sequences. Relative depositional inputs (or sedimentation rates) are first derived for single lithologies and short rock intervals, using the procedure described in Chaps. 2.2 to 2.4, Eqs. 2-8 and 2-11). These relative sedimentation rates are then standardized to derive a continuous plot of inputs for the three sediment fractions throughout the two TR cycles, which results in two plots, contrasting the weight percent of Cor,-CaCOa with the obtained standardized sedimentation rates (Figs. 6-5 and 6-6). In the following, the standardization of fractional sedimentation rates for these two TR cycles is given without considering the effects of diagenesis, which are discussed later. The whole procedure is explained in four main steps as illustrated in Fig. 6-4. 1. For various smaller stratigraphic intervals representing various individual lithologies, values of organic carbon and carbonate are plotted in xy diagrams (Figs. 6-2 and 6-3). This is performed in order to determine the different types of deposition, as documented by their characteristic types of Cors-CaCO3 relationship (Chap. 2.2). 2. Relative sedimentation rates are calculated using the various Cor--CaCO3 relationships by applying Eqs. 2-8 and 2-11. These procedures are rel~ated for the entire sequences of the Greenhorn and Niobrara cycles, using 40 and 37 representative sample points for the two cycles, respectively, and 33 additional, non-numbered (aiding) sample points. The representative samples are numbered on the Co,8-CaCO3 plots (Figs. 6-2 and 6-3) and are indicated on the stratigraphic sequences in the resulting diagrams (Figs. 6-5 and 6-6). During this procedure, changes in the composition along the established dilution curves, as well as from one dilution curve to another, are encountered (see Fig. 2-10). These shifts are represented graphically by arrows in the C~-CaCO3

Fig. 6-3 Weight percent of orl~aniecarbon versus carbonatecontent and resulting dilutionI~, curves for lithologiesof the Nlobrara TR cycle, Bold numbersrepresent OMsc and OMss values describing organic matter content m the background sediment for carbonate arid silicictastie deposition. Small numbers indicate sample points correlating with the stratigraphy displayedin Fig. 6-6. Arrows indicatech.~ges in composition.Hatched areas show values affected by diageneticcarbonate redistribution.

85

NIOBRARA TR CYCLE %Corc~. 1

%,Co~

~ .

-,l

FORTHAYS I LIMESTONE"|

~.'S~

o >,,,

,,.I

-I

3t

~w~RSHALE~ . LIMESTONEMEMBER

I '

Corq

2o" ~b' 6o "8o ~1oo Cor920 " 9 MIDOLEI SHALE&LIMESTONEM.J (M-LALT.) I

I R-I LIJ - /

40 60 80 100 .~t.~,~Jo (MARL-LALT.}I

t t 6 +N q g~N"-.

2

|THIS .

PA PE R ~/-//'j Ir

.

.

.

"~l~'~ " " -...~_~ I

.

.

.

I . ~ z~

OWER

SHALEMEMBEI; BAgLDW 1 9 6 5

OAT/e

"" 801oo LO,~R

,.t

:t9 /, 1 ,,,~,~,; ""

I

I

.

~Cor~ 20 40 60 80 100 Co~92o~o 5 179~ sl ;~

L

I

o

21 /I ++I' .1 LOwER

~,,+.25 ',,

11 LIMESTONEM. =e',~.24 ,

:

Cors

40 60 80 100 Corg20 40 60 80 100

3

. 9

12.8

t TRANSITION 9.6~ l 3

M,mE ~ . ~ . &

2 MIDDLE .I CHALKt-6 ' .MBER. . . . 3~ l 1{ IMARL-LIMESTOm! ALt) F~,,, I

t~

UPPERCHALKY ~ , , ~ SHALE "~,

g

""q

Com20 40 60 80 100 cor~~ 2'o' ~ ' 6'o ' 8o' lOO "0 0.2 21 UPPER

" ~'~$

i

'~'X

I CHALKYSHALE

J

"1 1~2 TRANSITION \ ' ~ . 1 /ZONENIOBRARA! ~

H'mERRE

l o~,' r~~o~oJ~z ~! , , , ,

,

,

,

,

"l

20 4O 60 80 100 Fig. 6-3 % CoCO3

"~l

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>-I I

UPPER', I

CHALK M.', I

?s . . . . .

"1

20 40 600 80 100 YoCoCO3

t,~ LU

0 uJ er

86

lOts (Figs. 6-2 and 6-3). Normally, shifts from one dilution curve to another How a carbonate, silicilastic or organic matter dilution line; consequently, relative sedimentation rates between a point on one curve and a point on another can be easily calculated by applying Eqs. 2-8 and 2-11. On the other hand, a few transitions between dilution curves occur which do not follow the simple C~-CaCO3 relationships for the three basic types of deposition. They can be interpreted as indicating a combination of two simultaneous dilution processes, as discussed in Chap. 5.3.2.

STANDARDIZING INPUTS OF SEDIMENT COMPONENTS

s
A

' V--C

ft S"

B

D

~,~7--II

s~.mp~e-

I

li

i

"

F

"! ~,o~

/!

'

/

Corg CaCO3 concentrations

C

L, retalive input s~dim~'il lraciior~i

Fig. 6-4 Schematic representation of procedures used to perform the combined input model estimating simultaneous flux pattern with three components. A: Vertical distribution of organic carbon and carbonate contents (wt. %) in a sequence. B: Predominant flux pattern of sequence intervals is characterized by different C=.,-CaCO3 correlation styles, related to organic matter deposition, and siliclclastic and c~rbonate deposition. C: Cor.reeting inyerse, elongated C~-CaCO3 trends for the effects of diagenetic carbonate reoistritmtion. -l-t~se trends originally indicate carbonate deposition. D: Determination of relative sedimentation rates (g) between each two samples, which are allocated to either organic matter, siliciclastic, or carbonate deposition. E: Combining relaUy,.esedimentation rates (s,~, s,: .... ), which become standardized (sal, s~ ..... ) to m.r specmc .s~lmen.~.tmn ra.te at the sequence base (sai). F: Transforming standardized sedLrnentation rates into fractional sedimentation rates gives flux pattern withcon.current supply of organic matter (Saou), calcareous (Sac), and sfliciclast~c seoiment (s~. 3. In order to obtain a continuous record of deposition, individual sedimentation rates (s~) are successively multiplied (i.e., standardized). Such multiplication is possible, because relative sedimentation rates represent dimensionless factors (see Chap. 2.4.1). The first relative sedimentation rate

87

is located at the base of the sequence at sample point 1, which has, by deftnition, the relative rate of s,~ = 1. Consequently, the various individual sedimentation rates are expressed relative to the rate of deposition at the sequence base. The standardized, relative sedimentation rate (s~) at stratigraphic position n is therefore sh

=

s,i * s , 2 . . . * s,~

(6-1)

For instance, the standardized sedimentation rate of a sample at stratigraphic level 4 (s~4) in the Graneros Shale at the base of the Greenhorn TR cycle is SR4 =

I * 0.25 * 2.25 * 0.36 = 0.20

(compare the sg curve in Fig. 6-5c). This standardization for all 110 sample points in the two investigated TR cycles is essential for the combined input model, allowing direct comparison of sedimentation rates that were previously calculated only for successive pairs of samples from small intervals representing individual lithoiogies. As outlined in Chap. 4, relative time spans can be determined from a detailed plot of the total standardized sedimentation rates, by dividing interval thicknesses by these sedimentation rates. 4. Standardized sedimentation rates for the entire cycles can be subdivided into their fractional sedimentation rates; thus, depositional inputs can be obtained (Fig. 6-4). Separation into sedimentation rates for the carbonate, siliciclastic, and organic fractions (sRc,n; sR,,n; and Saou,n) for each sample point (n) follows the equations: s~ * C(I-0.01~) SRc,n

~

(6-2)

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

100 + 2.18 Co,g s~, * (100-C-1.3Co,~)(1-0.010) sRs,n = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 + 2.18 C o r '

, and

(6-3)

s~ * 3.475Cor~(1-0.01 r SRoM,n =

(6-4)

100 + 2.18 Co,g In Eqs. 6-2 to 6-4, sm is the standardized relative sedimentation rate at stratigraphic level n. This bulk sedimentation rate for all fractions is subdivided into the relative sedimentation rates (or inputs) for the individual fractions Sac,n; s~s,n; and sRou,n. C a represents the weight percent of or-ganic carbon, C the carbonate content, and q~ the sediment or rock porosity in volume percent. In soft sediments this final transformation allows the determination of fractional inputs, but in cemented carbonates a correction for

88

diagenetic influences on the three-component model must be performed (see the next section). The whole procedure is then applied to the Greenhorn and Niobrara TR cycles.

6.4 Diagenetic influences The role of diagenetic influences was already outlined in Chap. 3.2. It is thought that diagentic loss of organic matter changes the calculated relative sedimentation rates for carbonate and elastic deposition only slightly, when the various lithologies (shales, marls, and limestones) have been influenced by apTable 6-1 Parameters describing diagenetic enhancement of carbonate and organic carbon contents in marl-limestone alternations for small stratigraphic members in the Greenhorn and Niobrara cycles. Average values for composition, cement content, and porosity in percent. Brige Creek Fort Hays Limestone Limestone Member

Shale and Limestone Member

Middle Chalk

primary composition marl beds

CaC03 Cor~

71.4 1.4

78.2 0.3

50.2 1.0

63.3 2.6

primary composition limestones

CaCOj C.~

78.2 1.1

87.9 0.2

59.6 0.8

77.9 2.1

present composition marl beds

CaC03 C~g

63.0 1.8

42.6 0.9

45.9 1.1

57.9 3.0

present composition limestones

CaCO3 Co~s

81.9 0.9

93.0 0.1

62.5 0.8

81.4 1.8

factor of diagenetic en- F-CaCO3 hancement F-Co~s

2.8 2.9

5.2 4.8

1.8 1.8

1.6 2.7

cement content of limestone (% of carbonate fraction)

21.1

45.2

11.4

19.5

61.8

63.5

76.0

72.6

mean

decompaction porosity (%)

89

proximately identical quantities of diagenetic organic matter loss. Differential carbonate diagenesis affects the lengths of the carbonate dilution lines but not their slopes (see Fig. 3-4, Chap. 3.2.2). Changes in carbonate deposition would be overestimated, if limestone sequences were not corrected for diagenetic carbonate redistribution. In order to quantify these diagenetic effects, carbonate mass balance calculations were performed according to the procedure described in Ricken (1986). Four short sections with characteristic, diversified lithologies were selected; calculations were based on 258 determinations of carbonate content, organic carbon content, and porosity. Compaction was evaluated for 179 samples by measuring the degree of deformation, in vertical cross sections, of originally circular, horizontal burrows of Thalassinoides, Teichichnus, Chondrites, and Planolites (Ricken, 1987). These calculations (Table 6-1) indicate that, for carbonate-rich layers in a section of predominantly marly shales, only I 1 to 21% of the total carbonate is cement. On the other hand, substantial carbonate redistribution was encountered in the Fort Hays Limestone, where cement in the limestone layers constitutes, on the average, 45% of the total carbonate. Here, carbonate dissolution in the marly layers is documented by numerous wispy dissolution seams and by microstylolites observed at the marl-limestone transitions (Precht and Pollastro, 1985). As mass balance calculations show, primary variations of around 10 to 20% in carbonate content among beds of the Greenhorn and Niobrara sediments were diagenetically augmented by average factors of 1.6 to 2.8. Only in the Fort Hays Limestone does the diagenetic enhancement of primary carbonate fluctuations reach a significant level (a factor of 5.2; Table 6-1). All these enhancement factors are used to correct for samples that have been altered by carbonate loss or gain during diagenesis. In the C__-CaCO3 diagrams of the studied rocks (see Figs. 6-2 and 6-3), these augmente-'d values are indicated by hatched fields; they are not used for the combined threecomponent model. Instead, only the non-hatched parts of the Co,t-CaCO3 curves are utilized for determinations of relative sedimentation rates and inputs (see also Fig. 6-4).

6.5 Fractional sediment inputs in the Greenhorn and Niobrara TR cycles Application of the combined input model to the Cretaceous Greenhorn and Niobrara TR cycles produces a detailed record of the relationship between the weight percent of carbonate and organic carbon and the standardized, relative sedimentation rates of the three components (Figs. 6-5, 6-6). 6.5.1 Greenhorn TR cycle The Greenhorn cycle, late Albian to middle Turonian in age, shows an almost symmetric carbonate content distribution, with little to no CaCOa in early transgressive and late regressive shale members and more than 60% CaCO 3 in the pelagic to hemipelagic Bridge Creek Limestone in the middle of the cycle

90

__.E8 =r-

GREENHORN TR CYCLE _L_a

ff Z 0

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~ ,,[N-,Ch.,........,~ o

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l~

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~

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Zx

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91

(Fig. 6-5a). Unlike carbonate content, the weight percent of organic carbon exhibits several maxima and minima; it fluctuates between a low of 0.1 and a high of 5 % and is low at the base, the middle, and the top of the cycle. According to the three-component model, the following depositional inputs can be obtained thus: 1. The relative carbonate input (Fig. 6-5b) was low during deposition of the early and late transgressive and regressive units. Except in layers representing calcarenitic depositional events, as in the Lincoln and Hartland members, the highest rate of carbonate deposition occurs during the middle of the cycle. This maximum of CaCO3 input indicates considerable planktonic carbonate production during peak transgression (e.g., Either, 1969; Kauffman, 1984; Eicher and Diner, 1985). 2. The relative siliciclastic input pattern (Fig. 6-5c) is essentially the inverse of that for carbonate deposition. SUiciclastic deposition was lowest during the middle of the cycle and highest at the beginning and end. The late regressive part of the cycle, however, has a much higher elastic flux compared with the early transgressive part. These patterns are assumed to reflect changing shoreline distances (e.g., Hattin, 1964; Weimer, 1984; Kauffman, 1984) as well as high elastic sediment input during regression due to basinward progradation of siliciclastic wedges (e.g., Posamentier and Vail, I988; Einsele and Bayer, 1991; see also Chap. 7.4.1). 3. The relative input of organic matter (Fig. 6-5b) is high in the transgressive and regressive portions but is at a minimum and varies rhythmically in the middle of the cycle (i.e., during sea level highstand). This pattern is interpreted as reflecting primarily the position of the aerobic/dysaerobic interface relative to the sediment surface. During sea level highstand, stagnant bottom waters with the aerobic/dysaerobic interface at or above the sediment surface were repeatedly replaced by oxidized bottom waters due to the mixing of water masses (Pratt, 1984; Eicher and Diner, 1985, 1989; Elder, 1987; Wright, 1987; Watldns, 1989; Arthur and Dean, 1991; Ricken, 1993). The weight percent of organic carbon is considerably different from the calculated input of organic matter. Consequently, the organic carbon content is affected largely by changes in the siliciclastic and carbonate inputs: 1. Dilution by the siliciclastic fraction occurred in nearshore environments during the beginning and end of the Greenhorn TR cycle. When transgression began, the terrigenous flux decreased, whereas during the subsequent regression it increased. The result is that the organic carbon concentration increases upward at the base, and decreases upward near the top of the TR cycle. This pattern of elastic dilution causes changes of up to 4.5 wt. % of organic carbon in the deposited sediment. 4Fig. 6-5 Results of the three-componentmodel for the Greenhorn TR cycle, depicting carbonate and organic carbon concentrations versus standardized sedimentationrates, a: Weight percentages of carbonate (CaCO3) and of organic carbon content (C~); numbers refer to Co~-CaCO3curves shown in Fig. 6-2. b: Standardized sedimentationrates for the organic ( ~ ) and carbonate fractions (S~c). c: St~r~ardized sedimentationrates for the siliciclastic fraction (s~s) and for the total rock volume (sO including 10% porosity. Numbers on the right display averaged standardized sedimentationrates (s~) and re|ative time spans (TR).

92

8

~

~

I.~-,1~1 ~ I ~

a,.

I~:~le:~l

~1

,

~

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I

/

l

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~/

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..

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~i

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Fig. 6 - 6

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NOLNVS N

~ "3"1

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93

2. In most of the studied stratigraphic intervals, we see both large-scale variations in carbonate input and small-scale, bedding-related carbonate variations. The former is expressed in the mid cycle carbonate production peak, while the latter is especially important in some beds of the Bridge Creek Limestone and calcarenitic layers of the Lincoln and Hartland Shales (Sageman and Johnson, 1985; Sageman, 1985), whose relatively low organic carbon contents (0.5 to 1.0 wt. %) are explained by dilution processes during events of rapid carbonate deposition (see the carbonate dilution lines in Fig. 6-2, numbers 14, 15 and 16, 17). 3. Generally, the carbonate content of the Greenhorn TR cycle shows a pattern similar to that of the carbonate input. It is less influenced by changes in the clastic input because the two inputs are derived from different reservoirs (pelagic and continental-fluvial, respectively), whose sediment supplies are largely inverse (see Chap. 3.3). However, in the uppermost Fairport member, a sudden drop in the siliciclastic input leads to a marked, parallel increase in the weight percents of carbonate and organic carbon which increase from 20 to 60% and from 2 to 5%, respectively (see siliciclastic dilution curve in Fig. 6-2, sample 36, and Fig. 6-5). 6 . 5 . 2 Niobrara TR cycle

The Niobrara TR cycle (Fig. 6-6), Upper Turonian to Lower Campanian in age, is highly asymmetric in the study area (Kauffman, 1984; Weimer, 1984). Unlike the Greenhorn cycle, it lacks a transgressive part. Thus, the hemipelagic limestones of the Fort Hays member lie directly above a transgressive unconformity over the nearshore Codell Sandstone (see Fig. 6-1). Because of a lack of outcrops in the middle of the Niobrara cycle, independent sedimentation rate standardizations (Eq. 6-1) had to be carried out for two subsequence bases, the base of the Fort Hays Limestone and that of the Middle Chalk member (Fig. 6-6). 1. The relative carbonate input (Fig. 6-6b) shows two large peaks, one in the Fort Hays Limestone and the other in the Upper Chalk member. Towards the regressive part of the cycle, the Transition Zone and the lower Pierre Shale, the rate of carbonate deposition drops markedly and the rate of silicielastic deposition rises simultaneously (Fig. 6-6c). As in the Greenhorn cycle, this pattern is thought to reflect the inversely correlated sediment input of the pelagic calcareous (Hattin, 1981) and the contrasting terrigenous reservoirs, which are both governed by water depth and shoreline distance (Weimer, 198t,; Kauffman, 1984; Posamentier and Vail, 1988). 9 Fig. 6-6 Results of the three-componentmodel for the Niobrara TR cycle, depicting carbonate and organic carbon concentrations versus standardized sedimentation rates, a: Weight percentages of carbonate (CaCO3) and of organic carbon content (C.~); numbers refer to C~-CaCO3 curves shown in Fig. 6-3. b: Standardized sedimentation rates for the organic (sR~) and carbonate fractions (sRc). c: Standardized sedimentation rates for the sihciclastic fraction (s~) and for the total rock volume (sa) including 10% porosity. Numbers on the right display averaged standardized sedimentation rates (sR) and relative time spans (Ts).

94

2. The relative input of organic matter (Fig. 6-6b) reaches a maximum in the Lower Shale member. This maximum corresponds to high organic carbon contents of up to 5%. In addition to a higher organic matter supply, however, the carbonate and elastic inputs decrease in this unit, so that the organic carbon contents become further concentrated. A different pattern is observed in the upper part of the Niobrara cycle, where the effect of decreasing carbonate deposition (i.e., Co~ concentration) is balanced by that of increasing silicielastic deposition 0"-e., C~s dilution), so that the resultant organic carbon content remains essentially constant. In the Fort Hays Limestone, the welldeveloped variation in organic carbon content is not reflected by fluctuating organic matter input (compare Fig. 6-6a with 6-6b). Instead, the variation in Co,, content is controlled by substantial fluctuations in the rate of carbonate det~osition. This surprising result is also documented in Fig. 6-3, where the Co,,-CaCO3 data from succeeding marl and limestone layers in both the lower and upper parts of the Fort Hays Limestone lie on one carbonate dilution line. Further explanations, namely sedimentation rate changes and bioturbation patterns in the Fort Hays Limestone, are given in Chaps. 5.3.1, 8.2.2, and 8.2.3 (see also Ricken 1993). 6.6 Testing the combined three-component model: cross-checking with time scales

In order to evaluate the validity and accuracy of the combined three-component model, standardized sedimentation rates obtained by the model were compared with averaged sedimentation rates derived using the time scale by Kent and Gradstein (1985). This crosscheck was carried out only for the Greenhorn cycle because of the exposure gap and uncertainties in stratigraphy associated with the Niobrara cycle. Calculated fluxes had to be averaged for time intervals of 0.8 to 2.2 Ma, the maximum achievable time resolution (Fig. 6-7). A fairly good agreement in the overall flux pattern can be obtained when the time scale derived flux pattern is compared with that derived from the combined three-component system (see Figs. 6-5 and 6-7). Both diagrams denote the highest inputs of siliciclastic and organic matter in the transgressive and regressive parts of the Greenhorn cycle, whereas carbonate deposition reaches its maximum during sea level highstand in the middle of this cycle. The general agreement in the flux pattern between the two independently derived methods is evident and supports the validity of the three-component Compared to the combined three-component model, time scale derived xes not only have a much weaker resolution, but are highly dependent on the quality of the stratigraphic data and the time scale used. 6.7 Conclusions

1. Co,,-CaCO3 contents and basic depositional styles: The various lithologies of the-upper Cretaceous Greenhorn and Niobrara TR cycles are characterized by three distinct straight-line relationships in their Co,s-CaCO3 contents, which

95

GREENHORN TR CYCLE TIME

[Ma]

TOTAL ORGANIC CLASTIC SEDIMENT CARBONATE MATTER SEDIMENT 10 10^ lO 20 30 40 50 Bo 70 DEPOSITED 6-

MIDDLE TURON

4-

4

E.-M.

CENOMAN

[m]

,.

EARLY TURON

LATE CENOMAN

~ S

3

3-

2

2-

1

0

,,

0

Fig. 6-7 Average sediment input for the Greenhorn TR cycle, expressed as the solid sediment (in m) supplied for different time intervals (in Ma). Individual compo,nents denote the organic matter (OM), carbonate, siliciclastic (S), and total fractions (sohd curve). Note general correspondence with standardized fluxes that were independently determined according to the combined three-component model (see Fig. 6-5).

support the three-component model as outlined in Chap. 2. In limestones and marls, the organic carbon content increases as carbonate content decreases, reflecting carbonate deposition. In contrast, organic carbon concentration increases with increasing carbonate content in siltstones, shales, and laminated and nonlaminated marly shales, indicating siliciclastie deposition. Some lithologies infrequently show variations in organic carbon concentration with approximately constant carbonate content, which is interpreted as reflecting organic matter deposition. 2. Standardized high-resolution flux pattern: Relative sedimentation rates were determined and numerically standardized for the various different C.~CaCO3 associations mentioned under 1. above. The resulting high resolution plot of relative fluxes agrees fairly well with averaged fluxes derived through stratigraphic time scales; this supports the three-component model and the standardization technique of relative sedimentation rates. For entire sequences, the

96

weight percent of organic carbon differs considerably from the calculated input of organic matter; instead, large variations in organic carbon content (ranging form 0.1 to 4.5 wt. %) are influenced by concurrently changing carbonate and siliciclastic deposition. Both inputs largely govern the distribution of organic carbon-rich and organic carbon-poor lithologies. 3. Relation between sediment inputs: Each of the TR cycles demonstrates two general flux patterns. The overall ~ n e r n is characterized by a correspondence between the various fluxes, as expressed by the roughly inverse covariation of the carbonate and siliciclastic inputs, which are governed by the changing distance to the shoreline and varying water depth created by the sea level history of the Western Interior Basin. In contrast, small-scale flux variations for individual lithologies over short stratigraphic intervals are characterized by an independent supply of carbonate, clastics, and organic matter (see Chap. 3.3). In principle, each of the three fractions is capable of influencing the weight percentages of the remaining two fractions. However, because both the siliciclastic and carbonate inputs are greater and undergo much larger variations than the organic fraction, the weight percent of organic carbon depends largely on these two inputs. 4. Dominance of simple types of deposition: One of the most striking features of the investigated TR cycles is the predominant occurrence of the basic types of deposition. According to C,~-CaCO3 correlations, variations in either carbonate or siliciclastic inputs are frequently observed, but a simultaneous change in such inputs is found only in a few cases. Such simultaneous changes can be recognized by their composite Co~-CaCO3 curves, as mentioned previously (Chap. 2.4.1). In the investigated epeiric basin setting deposition occurred primarily in two major environments, shallow water and relatively deep water, separated by a small transitional zone. The shallow water environment, which is represented by the lower and upper thirds of the Greenhorn cycle (i.e., Graneros Shale, Blue Hill Shale) and the Transition Zone of the Niobrara cycle, is characterized by siliciclastic deposition, while the deeper water environment is delineated by carbonate deposition. Thus, the two investigated TR cycles represent relative simple variations in input, with rapid transgressional-regressional changes between nearshore, siliciclastic and pelagic, calcareous environments. We have seen in this chapter that individual Co~.-CaCO3 lines can be combined m order to determine the sediment inputs of larger sections. It is assumed that the methodology of this combined three-component model is valid for most marine sediments, when narrowly spaced carbonate-organic carbon data are available and when stratigraphic gaps are not significant. In addition to determinig the inputs of individual beds or small cycles and sequences, as outlined in Chap. 5, the standadized input model allows us to develop a continuous, detailed record of relative supply rates for the individual sediment components.

Chapter 7 CARBONATE-CLASTIC SYSTEMS

It is well known that mixed carbonate-elastic depositional systems develop in shelf seas and in other marine basins to various degrees (Mount, 1984; Doyle and Roberts, 1988). Calcareous shelf areas or isolated carbonate platforms are contrasted by nearshore zones with dominantly fluvial sediment input (e.g., Holmes, 1988; Sussko and Davis, 1992). The resulting sediments consist, in various proportions, of carbonates and siliciclastics. Not only nearshore sediments, but also most of the fine-grained sediments deposited in deeper basins represent carbonate-clastic mixtures. This is obvious for marls and many marly shales which are usually composed of carbonate and clay. But seemingly pure

CARBONATE DEPOSITION CARBORA~

~:i i:~i:iiii:;!: :::?~~i:I!~: ?.::::!iii:.!.:;ii ! :..:;: ~i:ii::i :?..

~ i~. :i:. :::! i::iil]:::::i::~::i:, FRACTION ....

FRACTION C a C O 3 0%

CaCO 3 50%

C a C O 3 66.6%

CaCO 3 75%

SILICICLASTIC DEPOSITION NONCARBONATE

CARBONATE I!i!ii~!i~!iiii!i!iiii::]i:iiii~Iili:.iiii :i] iiiii]iiii]iiiiiiii'iiiiiii/ [iiiiiii:ii:ili!iiiiiiiiiii!iii:ilili~!!iil FRACTION C a C O 3 100% CaCO3 50% c,,c% ~ . 3 % CaC% 26%

Fig. 7-1 Principleof carbonate and siliciclastic deposition with associated changes in sediment height and carbonate content. Organic fraction is omitted.

tOO

elastics or pure carbonates also often contain a small portion of calcareous or siliciclastic sediment. "Pure" limestones and chalks contain 3 to 10 % clay (Ffichtbauer, 1950; Bausch, 1968; Bathurst, 1976), while "pure" elastics, such as sandstones, siltstones, and clays, include usually a few percent carbonate (Yaalon, 1962; Shaw and Weaver, 1965). Hence, even the so-called monocompositional sediments may be viewed as representing end members of mixed carbonate-elastic deposition. In this chapter, an integrated concept of carbonate-elastic systems is introduced, which is based on the major processes of deposition addressed in Chap. 2.2. This concept is somewhat simplified compared to natural conditions, insofar as it deals with a carbonate-dominated or a siliciclastic-dominated depositional type. Carbonate deposition is defined as a situation in which variable rates of calcareous deposition are superimposed on a dominantly constant rate of siliciclastic background sediment. On the contrary, clastic deposition is accomplished when varying rates of siliciclastic deposition are superimposed on constant calcareous background deposition. The combination of these two depositional types allows us to describe a variety of carbonateelastic facies types that are grouped into major facies associations, with distinctive relations between carbonate-organic carbon contents and sedimentation rates. These relations are expressed in vertical and lateral successions, quantifying the formation of vertical sequences and prograding sediment wedges. 7.1 Interrelationship between sedimentation rate and carbonate content During the past 30 years, several attempts were made to systematically study sedimentation rates. Sedimentation rates were statistically evaluated in order to find a correlation with the specific environments or basin types in which the sediments were deposited (e.g., Schwab, 1976). All these attempts met with little success, as sedimentation rates were found to vary by one to two orders of magnitude for each environment or sediment type (e.g., Sadler, 1981). Only Seibold (1952) gave attention to the fact that sedimentation rates and carbonate contents are interrelated (see Chap. 4.2). Analyzing the effects of this interrelation is the key point in the following section. Carbonate deposition: Idealized, stepwise-increasing carbonate deposition is superimposed on a uniform rate of noncarbonate material (Fig. 7-I). For each isochronous sedimentation step, the thickness of the deposited sediment (or the sedimentation rate) increases steadily, while the carbonate content increases nonlinearly. This nonlinear increase in sedimentation rate (Sic~) and carbonate content (C) is expressed through the equation already used in Chap. 4 (Eq. 4-2) stq = (100 sr~7) / (100-C) ,

(7-1)

where Ssc is the sedimentation rate of the noncalcareous background deposition, including silieiclastic and organic sediment.

101 Siliciclastic deposition: This type of deposition is ideally represented by the superimposition of a stepwise-increasing siliciclastic sedimention on a steady supply of nonclastic background sediment, composed of carbonate and organic matter (Fig. 7-1). The sedimentation rate increases as it does for calcareous deposition, but the carbonate content decreases nonlinearly with increasing sedimentation rate. Sedimentation rate (Sisl) and carbonate content (C) follow the relationship stsl = (100 SNs) / C ,

(7-2)

where Sss is the sedimentation rate of the nonelastic background fraction, composed of carbonate and organic matter (see Chap. 3.3.3).

CARBONATE

DEPOSITION

SILICICLASTIC

DEPOSITION RATIO S:C

RATIO C:S

,~i10-1

I~1: r-9

~i 41

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Fig. 7-2 Relationships between sedimentation rate (or sediment thickness) and carbonate content for carbonate and siliciclastic deposition (based on Fig. 7-1). The initial composition is identical to the background sediment (SNc, SNs = 1). Dashed curves depict relationships between sedimentation rate and carbonate content for halved background deposition. The geological consequences of variable carbonate or variable siliciclastic deposition are addressed in the succeeding sections. Here only one aspect is mentioned, the characteristic sharply bended curve of sedimentation rate versus carbonate content (Fig. 7-2). With increasing (or decreasing) carbonate content

102

INTERRELATION OF CARBONATE CONTENT ON SEDIMENTATION RATE

~" 100

CARBONATE]

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i01

EPICON~NENTAL SETTINGS

=60. o

I

. . . .

40

50

///~ i../z-/ ;Li~ ' CI 7

~"G'-..~" ._~'.7 e"

..,,,~,.4~""

.--'~" -

s I9

.

"

60

~-%1o$~

2~ ..... '

70

CAFtBONATE

_

o,~a-,

--zx

80

+

90

CONTENT

100 [w%]

Fig. 7-3 Trend curves for the relationship between sedimentation rate (m/Ma) and mean carbonate content (wt.%) averaged for stratigraphie intervals. Sedimentation rates are expressed on the basis of nonporous sediment, using sequences from deep-sea, slope and shelf environments. Sequences characterized by carbonate deposition are represented by facies associations C, and C,. the resulting sedimentation rates first increase slightly so that one branch of the curve is more parallel to the x-axis; but for high (or low) carbonate contents sedimentation rates increase considerably, so that another branch is roughly parallel to the carbonate axis (y-axis). For the former, the sedimentation rate is determined largely by the rate of background deposition, whereas for the latter, the sedimentation rate depends on the carbonate content. The lower the background deposition, the clearer is the bending effect.

103

I N T E R R E L A T I O N OF C A R B O N A T E C O N T E N T O N S E D I M E N T A T I O N RATE ,,-

"~ "'

180. 160

"I"A

Isluc~c~sTm!

~ I I

~DEPOSITION

I

Z

I

o p--

Jl

D~EP,-SF_~,SEI-nNGS

I

140

Z

I

m ,?~IE ~ 1 o SITES~2

I

-t- 8ri~ 511

LU

~

--~ Q

=l

,,, 120

I

+o

O wr=.~'~P.HNINTERK)R BASIN CRETACEOUS, GERMANY o,MIDDLE EARLY CRETACEOUS, GERMANY

I

~

~oo

SII"E 4 7 4

EPtCON'nNENTALsEr~NC~

il;.

'

~ ,,sin ;'S ',?"....k: \ , ~ - . . ~o~--.-x._ . . ....

20 \ , l

9

,,~ "'---~..~,.,.:

+,~,'~4rL )~-..., 9

0 ?

+._ i

20

sNS

--o----_o_

....

i

40

tr

60

~"

80

i

100

CARBONATE CONTENT [w%] Fig. 7-4 Trend curves for the relationship between sedimentation rate (m/Ma) and mean carbonate content (wt.%) averaged for strafigraphic intervals. Sedimentation rates are expressed on the basis of nonporous sediment, using sequences from deep-sea, slope and shelf environments. Sequences characterized by siliciclastic deposition are represented by facies associations S~, Sn, and Sin.

104

7.1.1 Carbonate content - sedimentation rate relation

In order to test whether this simplified view of calcareous or siliciclastic deposition is applicable to geologic systems, average sedimentation rates for lithologic intervals were plotted versus the average carbonate contents of these intervals (Fig. 7-3 and 7-4). For this purpose, sequences were selected with well-defined biostratigraphy, sufficient carbonate, and, when possible, with organic carbon data, as reported by various authors in the literature. These sequences reflect different carbonate-clastic environments ranging from the shelf to the deep sea, with ages from the Lower Jurassic to the Quaternary. To compare lithified sections on land with their nonlithified counterparts below the sea floor, sedimentation rates from sequences described in the Initial Reports of the DSDP/ODP were determined on the basis of pore-free sediment, using the porosity data given in the site reports. Sedimentation rates were calculated by applying two related and compatible time scales, for the Jurassic to Cretaceous (Kent and Gradstein, 1985), and for the Tertiary to Recent (Berggren et al., 1985). Sedimentation rates reported in the literature based on other time scales were recalculated accordingly, to keep errors in the data set as few as possible. The carbonate-sedimentation rate data for each of the investigated sequences were grouped into two summary diagrams, representing carbonate and siliciclastic deposition (Figs. 7-3 and 7-4). Despite scattering of individual values, the various sections show characteristic curves of carbonate content versus sedimentation rate. Chalks, limestones, marls, and clay sequences follow principally the curves of idealized carbonate deposition as documented in Fig. 7-2. Sandy sequences, silty clays and their transitions tomarls basically obey the curves of idealized siliciclastic deposition. Hence, many sedimentary systems can be grouped into a dominantly calcareous or siliciclastic sedimentation type, which indicates concurrent, varying deposition of main sediment and largely constant background deposition. Thus, the idealized flux patterns for carbonate and clastic deposition outlined in Chap. 2.2 are well documented in the rock record; accordingly, Eqs. 7-1 and 7-2 are hereafter called the "carbonate content - sedimentation rate relation". This relation implies that sedimentation rates either directly obey these equations, or they obey a system with two varying sediment components, where the variations of the individual components can nevertheless be determined. 7.1.2 Basic facies transitions and standard equations

Sediments with calcareous-clastic compositions undergo major facies changes when their compositions shift along the sedimentation rate-carbonate content curves (Figs. 7-3 and 7-4). These facies changes result in a suite of characteristic sediment types which represent commonly found transitions between carbonate-rich and carbonate-poor environments. For each of the two basic depositional styles (i.e, carbonate or siliciclasfic deposition), different facies successions can be distinguished through various levels of background sediment.

105

Calcareous deposition Evaluation of the data given in Fig. 7-3 shows that calcareous deposition can be grouped into two facies trends (Ct and Ca), representing the transition from highly calcareous sediments with high sedimentation rates to marls and clays with low sedimentation rates. C~ and Cn are def'med by low and medium low background deposition, respectively. But data on sedimentation rates reported in the literature suggest the additional existence of associations C m and Cw, with medium high and high rates of noncalcareous background deposition, respectively (SNc, Fig. 7-5). 9 Calcareous facies association C~: Facies association with low noncarbonate background deposition (highly calcareous chalk - pelagic marl - clay and deep sea clay). Highly calcareous nannoplankton chalks and limestones (e.g., the "Schreibkreide" of northern Europe; or the "Scalia" and "Maiolica" of southem Europe) with nonporous sedimentation rates of several 10 m/Ma change to chalks with higher clay contents (about 10 m/Ma), pelagic marls (less than 5 m/Ma), and, below the CCD, to deep sea clay. An evaluation of sedimentation rate - carbonate trends plotted in Fig. 7-3 shows that noncalcareous background deposition ranges from Sr~c = 0.5 to 4 m/Ma, which is the same order of magnitude as that reported for deep sea clay (e.g., Scholle et al., 1983; Stow et al., 1985; see Fig. 4-2). According to the carbonate content sedimentation rate relation (Eq. 7-1), the nonporous sedimentation rate (s) of this facies trend follows the equation s [m/Ma] = [100"(0.5 to 4SNc)] / (100-C),

(7-3)

where Ssc is the noncarbonate sedimentation rate (in m/Ma), and C is the average carbonate content (in wt. %) of a section interval. Calcareous facies associations have upper limits for both sedimentation rate and carbonate content, depending on the different potentials for carbonate production. In association C~ the limiting factor is determined by the flux of pelagic carbonate leading to an average sedimentation rate of 10 to 50m/Ma for pelagic chalks (Davies and Worsley, 1981). These values are in accordance with data presented here, which show that sedimentation rates for facies association C l are mostly below 50m/Ma (except one zone in the Schreibkreide). Thus, the corresponding maximum carbonate content in association C~ is between 92 and 99 % (Eq. 7-3). Calcareous facies association Cu: Facies association with fairly low noncarbonate background deposition (hemipelagic chalk - marl - clay). Impure, pelagic to hemipelagic chalks or limestones with a nonporous sedimentation rate of 40 to 80m/Ma change to hemipelagic chalks (20 to 30m/Ma), and hemipelagic marls and clays (sedimentation rate below 10m/Ma). The rate of background deposition ranges between SNc = 4 and 9 m/Ma. Nonporous sedimentation rates for this facies trend are expressed by the equation s [m/Ma] = [100"(4 to 9SNc)] / (100-12).

(7-4)

Association Ca represents the facies types in typical epicontinental basins

106

with transitions to the deeper sea. There is an upper limit for sedimentation rates, due to the higher productivity of shelf and slope environments. In Fig 73, the maximum sedimentation rate observed in this facies association is 80 m/Ma, which restricts the maximum carbonate content to values between 89 and 95%.

CALCAREOUS FACIES ASSOCIATIONS Z ,--,100

C/)

tr

..ou.

e. I ~111/

cart)on, ate mud to chalk I

[

9

40-

30 -

wackestone / to mudstone i

. ~ "

heml-

0

10

I

I

I

I

l

20

30

40

50

60

/ purl)

~mlk

i i'1"0"" ...'...hemipelagic clay._......-,,, ~ a r l .nelagi. c marP ~ ' ' ~..C.]!.....deep sea clay I

to_._.

.... I

I

I

70

80

90

CARBONATE

100

CONTENT

Fig. 7-5 Schematic depiction of calcareous facies associatiom (Cj, C.. and Ci~) with their principle facies successions for pore-free, long-term sedimentation rates. Facies associations are defined by different rates of noncalcareous background sedimentation (S~c), while individual facies types are defined by typical carbonate contents and sedi-menta.on rates.

Calcareous facies association Cm: Facies association with fairly high noncarbonate background deposition (reefal carbonate - calcareous shelf - silty marl and clay). Reefal and shallow water carbonates and calcareous shelf sediment with nonporous depositional rates between 50 and 150 m/Ma pass into calcareous wackestones and mudstones (30 to 50m/Ma) and into silty marls and silty clays with rates between 10 and 30m/Ma, representing slope and deeper basin environments. The rate of noncalcareous background deposition is estimated to be in the range of 9 to 17 m/Ma; thus nonporous sedimentation rates are described by the following relationship

107

s [m/Ma] = [100"(9 to 17s~)] / (100-C).

(7-5)

Association Cm is thought to be typical of more highly sedimented, mixed calcareous-elastic shelf and slope environments. The upper sedimentation rate limit is in the range of several 100 m/Ma, due to the high production rates of shallow water carbonates, with a maximum carbonate content of around 90 to 95%. Calcareous facies association Cry: Assumed facies association with high noncarbonate background deposition (some reefal carbonates - highly sedimented shelf and slope sequences). Abundant carbonate deposition is superimposed on high terrigenous background sedimentation related to reef carbonate production (several 100 m/Ma), with a transition to carbonate slopes and to deeper water environments (e.g., some periplafform oozes and marls, around 50 m/Ma). The background sedimentation rate is estimated to be larger than 17m/Ma, resulting in s [m/Ma] = [100"(> 17sNc)] / (IO0-C) .

(7-6)

Unlike isolated, intraoceanic carbonate platforms, which belong to the associations CI and Cn, reefs and their related sediments seem to belong mainly to the associations Cm to Crr because they can still incorporate a high amount of fine-grained, terrigenous sediment, provided through the back-reef lagoons, as observed in the northern Great Barrier Reef, Australia (Flood and Orme, 1988) or in Barbados (Aeker and Steam, 1990). Average Holocene reef growth rates are on the order of 0.5 to 1.5 mm/a leading to theoretical rates of 500 to 1500 m/Ma, regardless of the types of organisms involved (Smith, 1973; Enos, 1977; Hallock, 1981; Bosence, 1985; Tucker and Wright, 1990). In back-reef lagoons this value is lower, and higher on the reef front (Longman, 1981). Assuming an average carbonate content of 98%, as observed for a completely cemented Devonian reef (Franke, 1973), the resulting background sedimentation rate of 10 to 30 m/Ma is relatively large (see also SNc for Solnhofen limestone, Chap. 8.3.1). The extremely high sedimentation rates at the reef front (progradation or accomodation-related aggradation) determine the upper sedimentation rate limit for this facies association. Siliciclastic

deposition

Silieiclastic deposition is characterized by the general transition from carbonate-poor sands, silts, and silty clays with high sedimentation rates to condensed marls and chalks with low sedimentation rates. According to the data compiled in Fig. 7-4, three major silieiclastic facies associations (S~ to Sial) are distinguished, which correspond to low, medium, and fairly high nonelastic background deposition (SNs, Fig 7-6). Additionally, the existence of an association (S~v) with a high background deposition is assumed, based on data reported in the literature. Siliciclastic facies association S~: Facies association with low nonelastic (i.e., carbonate dominated) background deposition (silty clay - clayey marl -

108

chalky marl). Silty clays deposited in isolated, deeper basins and hemipelagic clays in boreal, epicontinental basins, with nonporous sedimentation rates between 20 and 60 m/Ma, pass into clayey and then chalky marl with sedimentation rates below 10m/Ma. This facies trend is related to nonelastic background deposition between sm = 0.5 and 6 m/Ma. According to Eq. 7-2, the nonporous sedimentation rate (s) is related to carbonate content (C) by s [m/Ma] = [100"(0.5 to 6sm)] / (2,

(7-7)

where s~ and C denote the background deposition (in m/Ma) and the carbonate content, respectively. Silieiclastic facies association Sn: Facies association with fairly low nonelastic background deposition (highly sedimented clayey silt - silty marl chalky marl). Thick clayey silt sequences characteristic of the deeper zone in boreal shelf and slope basins, with nonporous sedimentation rates around 100 m/Ma, pass into marly silts (30 to 60m/Ma) and chalky marls (about 20m/Ma) which represent low sedimented areas far from terrigenous sources. The nonelastic background sedimentation rate ranges between SNs = 6 to 15 m/Ma, so that the nonporous sedimentation rate is s [m/Ma] = [100"(6 to 15SNs)] / C .

(7-8)

Siliciclastic facies association Sat: Facies association with fairly high nonelastic background deposition (sandy sequence- sandy marl - condensed marly chalk). Highly sedimented, delta front and nearshore sands and deposits in deeper basins, e.g., turbidite sequences with sedimentation rates in the range of 100 to more than 200m/Ma pass into silty to marly deeper shelf and slope sediments (50 to 100m/Ma) and condensed marl and chalk sequences (20 to 40m/Ma). The nonelastic background rate lies between S~s = 15 and 30 m/Ma, and the nonporous sedimentation rate follows the equation s [m/Ma] = [100"(15 to 30SNs)] / C .

(7-9)

Siliciclastic facies association S~v: Assumed facies association with very high nonelastic deposition, representing transitions from highly sedimented, carbonate-elastic shelves (nonporous sedimentation rate around 150 to 500 m/Ma) to calcareous-dominated basins (less than 75 m/Ma). Delta front sands, which pass into marly prodelta sediments, and turbidite sequences derived from a carbonate-elastic sediment source may belong to this association, where the nonelastic background sediment is assumed to be greater than 30 m/Ma; again, the nonporous sedimentation rate can be expressed as s [m/Ma] = [100*(>30%s)] / C.

(7-I0)

All these different facies types, controlled either by carbonate or siliciclastic deposition, are generated by the combination of two factors: 1. The rate of background deposition designates the particular type of facies association (CI to Cw or $1 to Sw).

109

SILICICLASTIC FACIES ASSOCIATIONS

~ ~

, 200

E18o~uJ

sillciclastic 1. turbidltes , ~ deltaic to I nearshore

~.,~ 160" ~rr" m

/ sands

"

140" 120 100-

6~ 40

l

rharly

~

silt

mad~

silty clay r

20

~

OI

ilty marl~

I 10

I 20

san~-

~

~nt marl ~

"

~'~clayey

0

r

"~,,c

chalky-siL~

...... c u,..,,.,_ nuenseo ~.aJK u.u carbonates

m a r l . _ . _ c~a~ky m a r l

I 30

I 40

1 50

L 60

i 70

CARBONATE

--

( 80

I 90

.....i sNS

100

CONTENT

Fig. 7-6 Schematic depiction of siliciclastic facies associations (St, S., and S.~) with their principle facies successions for pore-free, long-term sedimentation rates. Facies associations are defined by different rates of nonclasttc background sediment (sN~)while individual facies types are defined by typical carbonate contents and sedimentauon rates. 2. Individual facies types included in one of these associations are defined by their typical carbonate - sedimentation rate values. Consequently, the application of standard equations (Eqs. 7-3 to 7-10) allows a relatively precise assessment of sedimentation rates and associated time spans (Eq. 4-1) using mean carbonate contents and typical values for the rate of background deposition (Fig. 7-7), despite weakly established stratigraphy or a lack of time control. This is especially true, when sequences with a succession of genetically connected facies types are investigated and related to one of the standard facies associations. However, it may be difficult to determine to which system, calcareous or siliciclastic, a single facies may belong, since the facies trends of both groups overlap (see Fig. 7-25). Many individual facies types with medium carbonate contents can theoretically belong to both systems.

110

CARBONATE CONTENTSEDIMENTATION RATE RELATION ILl

CARBONATE DEPOSITION

S~

tti

S

CLASTIC DEPOSITION

I~ ~C

S~

IOO-C

I~SHs .......

C

= SED~F-NTATION RATE

SNC = ~

SEDIMENTATION RATE

(NON<~SONATEFR/CnX~

LU

SNS = BACKGROUND SED~ENTATK)N RATE $NC

C

0~ONcuwnc FR~mO~ = C.ARBONATECONTENT

CARBONATE CONTENT

Fig. 7-7 Carbonate content - sedimentation rate relation. Determination of sedimentation rates (s) is achieved by using the mean carbonate content ((2) of an interval and a standard rate of background deposition, either ,%c, or sin. Diagram shows conditions for carbonate deposition.

SIMULTANEOUS DEPOSITION CARBONATE-SILIClCLASTIC SEDIMENTATION RATE (m/Ma)

2O 10 sNC 5

5 sNS 0

25

50

75

CARBONATE

100 CONTENT

Fig. 7-8 Simultaneous change in carbonate and. silicicl.~tic...dej~, it!on: Dec.teasing carbonate deposition (so!id lines) is aeeomp~,.".ed by mereasm~g smetctastle, aetx~ltapn.~.aas .n~_~ line). Resultant trena is exp_ressm" oy mlmng from .a calcare~s ra.c.les ~soc~auonwim low background deposition (I) to umt of a higher o ~ g r o u n a oe .posmon.t,J~.. t nts start ts expressed by moving the sediment along a curve ot increasing sihciclasuc deposit,on (2).

111

As explained below (Chap. 7.3), the carbonate distribution of beds and sequences provides hints on the general flux pattern; carbonate-organic carbon associations characterize and quantify the basic types of deposition (Table 7-1). Table 7-1 How to distinguish between carbonate and siliciclastic deposition II

Carbonate deposition: Carbonate-rich lithologies occur with higher sedimentation rates and smaller organic carbon contents than shaly lithologies. Thick, carbonate-rich sequences are replaced by thin, carbonate-poor ones. Laterally correlated sections that are time equivalent show transitions from thick, carbonate-rich to thin, carbonate-poor sections. Unlike chalks, current-related stratification processes, including any style of cross-bedding and storm dominated events, may be more intense in carbonate-rich arenites, while either bioturbation or organic carbon enrichment occurs in the fine-grained, slowly sedimented marls and clays. The thickness of rhythmically bedded strata (i.e., beds or parasequences) increases with increasing carbonate content; carbonate-rich beds tend to be thicker than the enclosing carbonate-poor beds (see Figs. 5-2 and 7-17). As a general tendency, organic carbon - carbonate concentrations are inversely correlated. Siliciclastie deposition: Carbonate-poor, silty or sandy sequences show higher sedimentation rates than organic carbon-rich marls. Thick, sandy sequences change to more condensed, marly ones. Laterally correlated, time-equivalent sequences show transition to thinner sequences, which are more calcareous and organic carbon-rich. Cross-stratification, storm beds, and turbidities are more significant in the carbonate-poor sands and silts, while either bioturbation or organic carbon enrichment occur in the fine-grained marls and chalks. Rhythmically stratified beds or parasequences increase in thickness in the carbonatepoor intervals. In rhythmically bedded hemipelagic marls, siliciclastic deposition is recognized by thin, carbonate-rich beds with thick, carbonatepoor interbeds (see Figs. 5-2 and 7-17). As a general tendency, organic carbon - carbonate concentrations are positively correlated. im

i

7.1.3 Simultaneous variation of carbonate and siliciclastic deposition In addition to shifts along the general facies trend curves depicted in Figs. 7-5 and 7-6, shifts from one facies curve to another are related to combined changes in carbonate and siliciclastic deposition, and thus, to transitional deposition types. Despite such combined changes, carbonate contents and sedimentation

112

rates can still be determined, because the resultant facies change can be described in terms of the individual amounts of carbonate and siliciclastic deposition (Fig. 7-8). 7.1.4 Recognition of beds with high and low carbonate contents: the chalk - clay issue The carbonate content - sedimentation rate relation demonstrates that sedimentation rates substantially increase with either high or low carbonate contents,

LAYER THICKNESS

CA."O.ATEI 7

// 86, 4-

J VARVES 0

20

4.0

60

80

100

CARBONATE CONTENT

Fig. 7-9 Increasing thicknesses of time-ecquivalentlayers, when carbonate deposition occurs with a constant background input. D~agn~. shows typical relationsnips rot parasequences, bundles, beds, and varves, with imdal (noncalcareous) thicknesses of 5m, lm, 0.1m and 0.001m, respectively. representing calcareous or siliciclasdc deposition, respectively (see Fig. 7-2). Such elevated sedimentation rates have ramifications on the recognition of typical stratigraphic units, such as beds and parasequences. In chalks and clays, beds may become too thick, and their difference in carbonate content too low, in order to be recognized in the field (see Chap. 5.4.1). Figure %9

113

illustrates these conditions for carbonate sedimentation. A noncalcareous, isochronous bed with an initial thickness of 0.1m (i.e., the background sediment) will only gradually increase in thickness when the carbonate content is between 0 and 80%, but above 95% it will greatly expand, becoming several meters thick (Fig. 7-9). A bundle of carbonate-free sediment 1 m thick grows to 20 m when the carbonate content is 95%; in chalk~ with 97% CaCO3, this sequence will be more than 30 m thick. Thus, above a certain threshold, the original thickness increases dramatically such that the typical characteristics of beds and bundles disappear. Thinner units, such as varves, tend to maintain their typical thicknesses throughout a wider carbonate range and may be identified even in deposits with very high carbonate contents.

CARBONATE

RELATED

LAMINA THICKNESS

i

i

0.5-]CARBONATEJ Z

0.4-

(j

9

0.2"

9

9

0.1-

_

NONCARBONATE SEDIMENTATION

lmmJ

/

Sn'E 535

0.3-

~

o~

_B

]DEPOSITION]

.t"

~

A

"

oo:o :

o

2o

,o'go'go'loo CARBONATE CONTENT (w%)

Fig. 7-10 Thickness increase of varve-like laminations in Valanginian marls and chalks, indicating predominantly carbonate deposition. See Fig. 5-5 for the inverse Co,,-CaCO3 association. DSDP Site 535, Straits of Florida. Carbonate - thickness data after*Cotillon 0985). The threshold, beyond which a given sedimentary unit becomes difficult to recognize occurs at a carbonate content of 95 % for varves, 90% for beds, and 70% for bundles. These values denote carbonate deposition. Not only does the expansion of familiar thicknesses contribute to these obliteration processes, but also the diminution of carbonate differences between beds with overall low or high carbonate contents. A detailed discussion of the latter aspect was given in Chap. 5.4.1. The theoretical relationship between carbonate content and thicknesses of

114

CARBONATE DEPOSITION

/ ::::~'.::::~:

III

INTERRELATIONSHIP BEI3NEEN I

sezauE~anON ~ S ,

c a m . O N A ~ CONmS'rS, ~ O

I

I

ORC-~I,NIC C A R B O N CONTENTS

::::~:::::

rw

:~:!:i:i:i:

Z

carbonate sediment

~ clasticsediment organic matter 9

o

9

9

9

2o

~o.~-I o

I, 0

9

4o

I

_

6~

!

,

9

I

!

18~ I 7oo

III

'%~.&">,.

; I I

.,,,,I,.'%! 2O

60

60

8O

100

CARBONATE CONTENT wt.% Fig. 7-11 Principal intcrr~latioaships between scdimealation rate, carbonate coatent, aad organic carbon content, illustyated for calcareous facies association (i.e., carbonate deposition). Small arrows connect oepositional units with related C=fCaCO 3 contents on a given carbonate dilution line, with an assumed Ceackground) organic carbon content of 1%.

isochronous units (Fig. 7-9) is documented in varve-like laminations in Valanginian marls and chalks (DSDP Site 535, Cotillon, 1985; Fig. 7-10). Cotillon's data indicate two types of lamination (A and B), with dominantly carbonate deposition but with somewhat different rates of noncalcare,ous back-

115

SILIClCLASTIC DEPOSITION

I IINTERRELATIONSHIPBETWEEN ILl

ISEDIMENTATIONRATES, ICARBONATECONTENTS,AND

~

IORGANICCARBONCONTENTS

~'q

s,uocu~s~c FAC,ES ~ L ~ - -A~,~IATION$

, . x ~.\'~

siiicici~tic J

!

o

201

J

.

!

40

I

I

!

60

I

80

9

carbonate sediment organic matter

100

iI CARBONATE CONTENT wt.% Z

q

1.o.

I

i

~

rr-

<06.

=

I

I

~_ 0.z,.

, Ir

,"~ I

0

I

20

I

I

z,O

I

60

I

I

80

I

100

CARBONATE CONTENT wt.% Fig. 7-12 Principal interrelationships between sedimentation rate, carbonate content, and organic carbon content, illustrated for silicielastie facies association (i.e., siliciclastic deposition). Small arrows connect depositional units with related C.,s-CaCO3 contents on given silieiclastic dilution line, with an assumed (background)organic carbon content of %_ ground sediment supply. Two lines of evidence show that fluctuating carbonate supply is the dominant depositional process forming these laminae: (1) Carbonate deposition is indicated by an exponential thickening of laminae with rising carbonate content, and carbonate dissolution processes are not observed

116

(Cotillon and Rio, 1984; Cotillon, 1985); (2) in addition, carbonate deposition is suggested by a negative Co,.-CaCO~ trend, which is observed in various units throughout the recovered ~ection at Site 535, and which was previously interpreted as representing carbonate deposition on a larger scale (e.g., beds, see the discussion in Chap. 5.3.1, Fig. 5-5). Hence, CotiUon's example strongly supports the existence of a flux pattern dominated by carbonate, as documented by the carbonate content - sedimentation rate relationship and the distinctive (i.e., inverse) C~-CaCO3 association, which was theoretically proposed in Chaps. 2 and 3. 7.2 Facies transitions and associated changes in organic carbon content Changes along a succession of related facies types (i.e., facies association) are accompanied by changing organic carbon contents, here expressed by shifting along a Cor~-CaCO3 trend line. Controlled b), dilution and concentration pro-cesses, these trends commonly span up to hve or more percent by weight of organic carbon. Transitions between carbonate and siliciclastic facies types are expressed by the transition from organic carbon-poor sediments to organic carbon-rich counterparts and vice versa. These processes influence the vertical and lateral distribution of lithologies rich or poor in organic carbon. Carbonate deposition: A facies transition from shaly to marly and calcareous sediment (i.e., increasing carbonate deposition) is attended by dilution of the organic carbon content in the initial (background) sediment (Chap. 2.2). Increasing carbonate deposition diminishes the organic carbon concentration; simultaneously, the sedimentation rate increases with increasing carbonate and decreasing organic carbon content (Fig. 7-11). Both, the weight percentages of organic carbon and carbonate are non-linearly related to the sedimentation rate (see Eq. 7-I), whereas the carbonate and organic carbon contents show a linear, inverse relation. Silieiclastic deposition: A facies transition from marly to clastic sediment (i.e., increasing siliciclastic deposition) is accompanied by organic carbon dilution, resulting in a "normal" C~-CaCOj correlation (see Chap. 2.2). The weight percentages of carbonate and organic carbon decrease simultaneously with increasing siliciclastic input, whereas the sedimentation rate increases non-linearly with decreasing carbonate and organic carbon contents (Fig. 7-12, Eq. 7-2). The interrelationships proposed here between carbonate or siliciclastic facies changes, sedimentation rates, and C.,~-CaCO~ associations are documented in the rock record. Sexluenees that are primarily related either to dominantly carbonate or elastic deposition, based on their distinct carbonate-sedimentation rate trends summarized in Figs. 7-3 and 7-4, approximate the theoretically established C.,s-CaCO~ associations (Chap. 2.2). Sequences assigned to carbonate deposition show an inverse C.,,-CaCO 3 trend (Fig. 7-13), whereas those assigned to silieielastie deposition show a positive C~-CaCO3 trend (Fig. 714). Scattering of the data, though, is large, and some-diagrams contain several regression lines. This is because the graphs summarize the C,,,~-CaC03 data for entire sequences over a vertical range of several 100m, over which the

117

basic style of sedimentation remained the same, but additional fluctuations in organic carbon deposition (and in the supply of nonorganic background sediment) created differently sloping C=g-CaCO3 regression lines (see Chap. 2.3). When plotted for smaller sections, e.g., a few to 10 meters thick, the variation in slope and the scattering of organic carbon - carbonate contents is substantially reduced (see Chap. 6.2). INVERTED C o r g - C a C O 3 RELATIONS: C A R B O N A T E DEPOSITION c ~g

KEY

C

*~ "

= Cmbbio fhalks

9

.sito s93 I ~ R ~ q A T E CONTENT [w*~l

a 9

9

76 0 C~xg

20

~,0

60

80 p I

lO0 ~CO

s~e~o~

~\. o,& 0

_g

2O r \,

~0

60

80

'"~,..

, "u

'1-..2:__. ' \ - \ ~fn'*"

I '

:'-3-~... *-.222....

0 20 Co~ ' .

02

S~te

t~

60

540

80

I00

x u =

CaCO:l

40

60

80

C

iQ''.'4.'

=

'

'

700 CaCO

sit~65a

.,.y,, %b.'" ",'~"

100

9

0

20

C,,g

o\ " " \

40

60

Sire 535

80

~,e

,

20

100

* I,,

I~$

.

OaCU

5ires 662, 66r

9

~--.a~.~. ,

ZO

. r?

~O

*'q

.-',~:-~--,

60

40

80

~,v~, -*

100

CaCO~

0

9

ZO

60

80

\

~00 CaCG

.

\ o

9 9

"

"~"

. . . . . . . .

*0

6#

%

x

O

20

\ qo

...

"~""-"-~--~

o

00 SWGermany \ ~ Toorcian

,

sit~ 638

Zl

[

~ ]

"-%

0 c otg

9~ 96 I00 Site 530

Pliensbnchtan I

"'~

a6

80 84 86

C

% 31 ~.

80

.

0.~

I00

CaGO s

0

ZO

60

60

80

100

CaC03

Fig. 7-13 Inverse trends in organic carbon and carbonate contents for sequences which were previously interpreted as representing carbonate deposition, based on their relationship between sedimentation rate and carbonate content, see Fig. 7-3. The data presented here indicate that facies transitions from highly-sedimented calcareous or siliciclastic types to their corresponding shaly or chalky background sediments axe generally associated with a tendency organic carbon concentrations to increase. In calcareous systems, bioturbated, carbonate-rich and organic carbon-poor sediments with high sedimentation rates pass into

118

carbonaceous marly and shaly background sediments. In siliciclastic systems, biotuxbated, organic carbon-poor sands and silts transform into organic carbonrich marly and chalky background sediments. A method for quantifying these general trends is outlined in Chap. 8.1.

N O R M A L C o r g - C a C O 3 RELATIONS: SIUCICLASTIC DEPOSmON C

Site

/ /

Corg

X

Site 651

/

4." ? 3/ 2-

650

a

/

l

/

/

/

.

o o

1"

p =

a

=-----~ ,,•...=_ nDO T

i

D

zo

o

I

D~ O o

~o

io'

i

80

" 100 CaCO 3

C.g Site r

o

zo

511

!

4.o

Lo ' ~o '1oo CaCO

" [ ; i ~ ' ~ , + .~+ +

org

~, 9

9

BasinWeSten Interior Fairport, Blue Hill Shales

I

5/

%

I 9.

,

,

20

0

,

..,-,-,-

40

~

,

100

60

CaCO3

Cofg 3t

~

/ z~ A

Site 674.

A

o

io

4'o "6"o '

o CaCO3

1o

20

~o

4o

CaCO~

Fig. 7-14 Normal trends in organic carbon and carbonate contents for sequences which were previously assigned to siliciclastic deposition, based on the relationship between sedimentation rate and carbonate content, see Fig. 7-4.

119

7.2.1 Overlapping C ~ - C a C O 3 trends

Under certain conditions, major trends in organic carbon contents, associated with the facies changes as discussed above, may be differently or unequally expressed on a Co,~-CaCO3 diagram. This may be related to simultaneous changes in deposition or to the fact that the flux styles of different depositional processes are integrated in the same data set. Simultaneous depositional changes in two or more components result in composite Co~g-CaCO3 trends, whereas superimposing the different flux patterns of beds, parasequences, and sequences is characterized by overlapping C.,.-CaCO3 patterns. A simultaneous flux change can be analyzed by separa~Jng the individually varying components, which allows determination of relative sedimentation rates and fluxes in a straightforward manner (see Chap. 2.4.1).

TREND OVERLAPPING IN Corg- C a C 0 3 DATA parallel

!

!!i:ii:iiiiii::i:i:.i ii;::

..',

[ i .!~iii:ii:~i .ili~ii~i~::.~ii~i ~i~::i;::~:.

,;,/

9.

~Z

uence-

elated

Z

bedding~ rslated

o

""~;~i!i i !i !~:!ili:!ii:i i i!!!:~ reladt~dmg

' CARBONATECONTENT jCorg.CaC03 TREN"D

ii~iiiiiz.ii!iiiiii~ iii!:;i!i;~

IOF BEDS IS

l.

inverted

oC oc

~!i! ii i:!ii!ill!i i!ii:!i i:! ~!i! ii i:!ii!ill

/

z IOZ nn~ n'P"

~n, uence-

I PARAI i FL TO THAT I OF SEQUENCE

CARBONATECONTENT oCOrg-CaCO3 TREND I

OFBEDSIS

I

INVERTED TO THAT I F SEQUENCE l

Fig. 7-15 Principal conditions of overlapping C,n-CaCO3 values, expressed by superimposing bedding-related and sequence-related flux patterns (see Fig. 7-24).

Overlapping C~g-CaCO3 patterns originate when a bedding-related change in deposition is superimposed on a sequence-related one (Fig. 7-15). If both changes reflect the same type of deposition, recognized by parallel Co~-CaCO3 patterns, it may be difficult to distinguish between these processes, unless additional, more detailed investigations are carried out. If the overlapping trends belong to different types of deposition, as recognized by an inverted C~-CaCO3 pattern, fluxes can be better recognized and identified. In lithified deep sea sequences, the diagenetically enhanced, bedding-related Corg-CaCO3 trend is often more significant than that of the whole sequence. One example of this is the Lower Turonian Bridge Creek Limestone, which shows an inverted Cor,-CaCOj trend indicating overlapping depositions in the

120

whole sequence and in individual beds. Lateral correlation of different sections shows basinward-decreasing siliciclastic dilution, represented by a positively sloping Co_-CaC03 line, which is overlapped by a bedding-related, negatively sloping lin~ of carbonate deposition (see Fig. 7-24). 7.3 Vertical facies changes in sedimentary sequences

Vertical facies changes in many sedimentary sequences correspond to the major carbonate-clastic types of deposition, or to combinations of these two types. Facies changes are accompanied by distinct changes in carbonate content and interval thickness. When the style of deposition is relatively simple, i.e., dominated by either calcareous or siliciclastic deposition, a predictable pattern of facies successions, carbonate contents, sedimentation rates, and time spans arises. 7.3.1 Carbonate content distributions

The simpliest way to investigate the vertical distribution of CaCO3 is to assume carbonate or siliciclastic deposition. This was expressed earlier in the basic depositional models depicted in Fig. 7-1. The laterally arranged sediment units in these figures must be vertically stacked, however, in order to simulate the C a C O 3 distribution of the resulting sediment pile. An example is depicted in Fig. 7-16, where a stepwise-increasing carbonate supply is superimposed on an initial sediment with 0% CaCO~. The resulting sequence shows a sharp, prominent increase in carbonate content near the base, contrasted by a moderate to insignificant CaCO~ increase near the sequence top. Numerically, the carbonate content at a certain level in a given sequence is expressed by the cumulative addition of the individual depositional elements (Fig. 7-16, elements 1 to 5). As shown in this figure, these elements increase their thicknesses linearly, but not their carbonate contents. The relationship between these factors is formulated as in Eqs. 7-1 and 7-2, where the various sedimentation rates (s) are equivalent to the thicknesses of the isochronous depositional elements (h). The cumulative sequence thickness, or height (h~ is expressed thus for carbonate deposition n

hc,o=

=

[(100hNc) / (I00-C,)1

,

(7-11)

n=l whereas for siliciclastic deposition we have n

hs.c=

=

~. [(100hss) / CA n=l

(7-12)

The thickness of the background sediment is denoted by h ~ and h~, while the

121

mean carbonate content in the various depositional intervals from 1 to n is given by C,. Theoretically, the initial depositional element (hi) can be composed of either the calcareous or the elastic background sediment, or both. Consider an example of carbonate deposition (Eq. 7-11), in which the carbonate content increases in each sedimentation interval by an amount equivalent to the initial sediment thickness (h~). If this initial sediment is carbonate-free, its

INCREASING CARBONATE DEPOSITION time equivalent Intervals

dominantly increasing section thickness

3

2

I

I

0

10

i

I

i

20

w dominantly Increasing carbonate content 1

3O

t'

40

'

50

60

70

I

I

I

80

90

100

CARBONATE CONTENT CALCAREOUS SEDIMENT

SIUCICLASTIC SEDIMENT

Fig. 7-16 The vertical effect of carbonate deposition. Stacked depositional units (1 to 5) increase their carbonate content by an equal amount for each isochronous depositional element, but form a convex-shaped carbonate distribution curve. thickness equals that of the background sediment (h~ = hsc). For the second sedimentation interval, the carbonate content is 50%, and its thickness amounts to 0az = 2*hsc), etc., see Fig. 7-16. When the initial sediment contains carbonate, e.g. 75%, it is composed of one part noncarbonate and three parts carbonate with an intitial thickness of h2 = 4*hsc. Then, the hsc value, used to calculate the cumulative sediment thickness in Eq. 7-I 1, has to be divided by four in order to obtain the initial thickness of the carbonate-free sediment.

122

7.3.2 Asynunetry relation The "asymmetry relation" stipulates that a linearly changing sediment supply is expressed non-linearly in the carbonate contents of the deposited strata. Steadily rising carbonate deposition (above clastic-dominated background sedimentation) results in a steadily increasing sediment column, but with ever smaller increases in carbonate content. A convex-shaped CaCO~ curve is thereby formed through the vertical sequence (Fig. 7-16). This principle of asymmetry between deposition and its expression in the resulting strata is valid for both types of deposition. Steadily rising carbonate or siliciclastic deposition

VERTICAL CARBONATE DISTRIBUTION

INCREASING CARBONATE INCREASING SIUCICLASTIC DEPOSITION DEPOSITION Ul

th,cknesst/ increasJ

~!~!~!

0

increase Tthickness

I,ow

initial

carbonate content UI G') U) Ill Z

I-, Z

o

........~j'. increase in carbonate u~ CARBONATE CONTENT

CARBONATE CONTENT

=1I ttthickness I/increase ~l | ' ~1 t

I\

~) I ~decrease

high initial

in

carbonate

content

I,0 Ul

CARBONATE CONTENT CARBONATE CONTENT Fig. 7-17a Steadily increasing carbonate or siliciclastic dept. ition generates different types of vertical carbonate distribution curves; expressed for initial sediments with either low or high carbonate contents.

is documented by more slowly increasing or decreasing carbonate content, respectively. Therefore, sedimentary sections seldom show a linear carbonate content-thickness relationship. Some basic ideas of the asymmetry relation were already discussed for rhythmic bedding in Chap. 5.1. Sequences with carbonate deposition: When the carbonate content in the initial sediment is small, steadily increasing carbonate deposition is characterized by prominent augmentation of the CaCO3 content, followed by more

123

slowly increasing carbonate content higher up in the sequence (Fig. 7-17a). However, when the initial carbonate content is high, steadily increasing carbonate deposition mainly produces a thickening upward sequence with a slightly increasing CaCO3 content. Steadily increasing and then decreasing carbonate deposition is represented by a sequence with a convex-shaped carbonate maximum (Fig. 7-17b). In contrast, upwardly decreasing and then increasing carbonate deposition is associated with a sharp CaCO 3 minimum. All these patterns of vertical carbonate distribution indicate, generally, an inverse covariance between carbonate and organic carbon contents.

CARBONATE-ORGANIC CARBON DISTRIBUTION carbonate clastic supply CARBONATE DEPOSITION supplYcLASTIC DEPOSITION ~/h~rp minimum " " wide minimum

i

/

//

/

I[

ilJ~

~decreasmg

noncarbonate

\ carbonate

ORGANIC ~ ' deposition CARBON I I I convex I maximum

I l

I \

~reasing

\

\

~

tl

11 CARBONATE CONTENT / l ...,.= ~/I increasing =colgrd

~n;Ite carbonate eposition ,,n

Inverse carbonate - organic carbon relationship

l

I

I

maximum j /

El

I

~[

I

II/

/ ,

~CARBONATE .~.^r^ao;n.. / CONTENT

,,JT~.. ~

o, ~

>

noncarbonate

0#os.on

. . . . . . . . . . .

parallel carbonate - organic carbon relationship

I J

Fig. 7-17b Schematic diagram showing how the vertical carbonate distribution (solid curve) and the organic carbon distribution (dashed curve) differ for carbonate and silieiclastic deposition. Carbonate deposition: increasingand then decreasing supply of earbonate. Silieiclasticdeposition: decreasing and then increasingsupply of elastic sediment. The distinctive effects on bedding rhythms are illustrated in Figs. 5-I and 5-2. Sequence with siliciclastic deposition: The CaCO3 distribution found for siliciclastic deposition is the reverse of that for calcareous deposition. For an initial, carbonate-poor sediment, increasing siliciclastic deposition leads essentially to an increase in section thickness (Fig. 7-17a). However, when the

124

initial sediment is carbonate-rich, a thickening-upward sequence is generated, with an exponential decline in CaCO3 content. A decrease in elastic deposition followed by an increase is expressed by a sequence with a sharp CaCO3 maximum (Fig. 7-17b). In contrast, increasing and then decreasing siliciclastic deposition forms a broad, concave CaCO3 minimum. One can observe a tendency for a general correlation between carbonate and organic carbon contents. 7.3.3 Carbonate-poor zones at the base of cakareous sections

The effect of increasing carbonate deposition on the vertical CaCO 3 distribution in a given section can be illustrated by the diagrams presented in Fig. 7-18. These diagrams show various CaCO3 curve forms that depend on different rates of carbonate deposition (PC) and different initial carbonate contents as described in Eq. 7-11. Dashed time lines connect isochronous section intervals, representing the various vertically stacked depositional elements. Analysis of the CaCO 3 pattern allows us to determine both the type and quantity of sediment input, as well as the related distribution of time-equivalent intervals in a sequence. Whether a vertical carbonate distribution curve shows a slight or a substantial CaCO3 rise depends on the carbonate increase rate per time-equivalent depositional element (Pc), 100

Pc[%] =

.......

(7-13)

hi where hi is the height, or thickness, of the initial sediment and hc is the height of newly added calcareous sediment for each time interval. Thus, a rate of Rc = 100% indicates an increase in thickness (as an effect of carbonate deposition) in every new depositional element equivalent to the initial sediment height; a rate of Pc=500% means that every element increases 5 times the initial thickness, etc. Only a very high rate of carbonate deposition is capable of significantly augmenting the carbonate content at the base of an initially CaCO3-poor sedimentary sequence; increasingly higher carbonate input rates are needed to shift the carbonate curve to higher values (Fig. 7-18). On a larger scale, if smaller, bedding related CaCO3 variations are neglected, calcareous sequences seldom have an abrupt increase in their carbonate contents. The bases of such sections commonly represent zones where carbonate contents are restricted to lower values. These basal zones are thick when the rate of carbonate increase is small. Fig. 7-18 Representation of the effects of steadily increasing carbonate deposition, documented by various carbonate distribution curves (solid) for cumulative section heights or thicknesses (dimensionless numbers). Time lines connect time-equivalent intervals (dashed). Carbonate distribution curves depend on various carbonate increase rates CRY, and different carbonate contents in the initial sediment (0, 25, 50, and 75% CaCO3). Ndte the general difficulty in achieving high carbonate contents at the section base, which leads to the formation of a carbonate-poor basal zone.

125

CUMULATIVE CARBONATE DEPOSITION RATE OF CARBONATE INCREASE RATEOF CARBONATE INCREASE 1 Z5 5 /0 20 50110:.~0 / Z 5 5 f020:;O..~

~176

,o , , ~" 18"

6

, ,,,.,

,,

"

i

~8.

~6;

14~'~"~/:~"/~"~/iftl''-,''~'~.''i

1L,:

14.

~2: ,~Lr..~.;.i;.f,,~_ .~. . . ~o.

' ,:: ,!/i, ,

.".,'7,'!

~"':~ffll Il

6 '20 '/.,b'

,,'t;,tI ,;II ',t,I1111 ,~,l,ll

~:

~:

,,

, ///' // ,,'l

,"

I

I11

7~It"i",iI'li'" ' " '

!; 2s, A/i/W'

,i

~.,

6'0'" 8'0 '100 0 20 40 60 80 100 1O5050O #2_.55 201OO

_ 18

~,'~ I I'II

~I

':~4,fld/l/,l'r

~.

;,[,I,'Vh~TIl

I01

~':J/Vlil II

~

?:i%tllill

8~

Fig. 7-18

3O5OO 310170

0

i i

"I.V'F/,'I'II

20 40 60

80 100 0

20 40

60

80 100

CARBONATE CONTENT [wt.%] CARBONATE CONTENT [wt.%]

126

Example: The Lower Jurassic shales (50 m thick) at the base of the entire Jurassic carbonate sequence in southern Germany (ca. 750 m thick) can be viewed as representing such a zone restricted to low CaCO3 content. Carbonate deposition increases regularly while the elastic background supply rate remains approximately constant, so that higher parts in the sequence become richer in carbonate content. This increase in CaCO3 is not only related to a major transgression (e.g., Jenkyns, 1985; Haq et al., 1987; Hallam, 1988), but also to the concomitant evolution of calcareous nannoplankton (Roth and Bowdler, 1981; Roth, 1986). Very typically, this carbonate-poor basal zone is, on the whole, organic carbon-rich. If these interpretations are correct, the Jurassic sequence should follow the carbonate content - sedimentation rate relation, with the least deposition in the Liassic and and the most deposition in the carbonate-rich members of the Kimmeridgian and Tithonian, as documented in Fig. 7-3. Only one member of the sequence, the Aalenian Opalinus clay, does not meet the carbonate content - sedimentation rate relation for CaCO3 deposition, but rather indicates siliciclastic supply. Other carbonate-limited zones have much smaller thicknesses, due to the higher rates of carbonate deposition relative to the initial sediment. One example, which is outlined in the following section, is the transition from the BonareUi black shale horizon into the Upper Cretaceous chalks of the Gubbio section (Italy).

7.3.4 Modelling carbonate input in vertical sequences (Gubbio section) The Middle Cretaceous chalks of the Gubbio sequence (Umbria, Italy) provide instructive examples for modelling carbonate deposition and estimating the associated time spans inherent in various intervals of the section. The occurrence of a Lower Turonian black shale horizon (Bonarelli horizon) in this sequence documents organic carbon enrichment in a carbonate-poor zone with a low sedimentation rate. The overall sedimentation pattern in the Gubbio sequence is dominated by carbonate deposition, which is superimposed on slow, relatively constant background deposition of clay with sedimentation rates on the order of deep sea clay (see Fig. 4-3, Chap. 4.2). Carbonate deposition is also documented by the facies succession developed during the transition from the Bonarelli black shale into the Turonian-Santonian chalk sequence, which is similar to the low elastic background deposition depicted in facies association C~ (see Fig. 7-3). Above the lm thick Lower Turonian black shale layer (i.e., the BonareUi horizon), carbonate content increases from 0 to 95 % within an interval of 70m in a characteristically convex form as described in Figs. 7-16 and 7-18. This carbonate curve (Arthur, 1979) was simulated using the depositional model described in Eq. 7-11, using an initial interval thickness of 0. lm and a rate of increase in carbonate of P.,c=50% and, later, of Rc=25% (Fig. 7-19). In adclifton, the lower part of the Gubbio sequence (110m from the Albian to the Bonarelli horizon) was tentatively modelled, using various rates of increasing or deca'easing carbonate deposition (Fig. 7-20). Both numerical simulations (Figs. 7-19, 7-20) document that sedimentary sections have CaCO3 distributions similar to those theoretically expressed in

127

GUBBIO SEQUENCE Intervals with equivalent time span

CARBONATE CONTENT

0

20

40

60

80

100 cn uJ

reefer

z

-200

0 carbonate curve (measured data)

-- 1~0

"1" I-"

Z
Z

0

Z O I-.

- 180

0

Z

.,

ILl

r

(/)

- 170

m

- 160

Z

1

0 < Z O O

- 140

Z

9- 130

-150

m

,.

- iC25

Z

0n.

[

. - 120

.,

F-

~

curl/e

- 110 '1

0'

"

20

9

60

I

ii

i I

6O

i

80

i

100

CARBONATE CONTENT

carbonate content data (Arthur, 1979) Fig. %19 Carbonate curve (solid circles) for an 80 m thick section of the Gubhio sequence, compared with the modelled carbonate curve (open circles) which assumes two rates of increasing carbonate deposition (Pc = 50% and, since the Coaiacian, Rc = 25%). Intervals between vertical lines are interpreted to be time-equivalent, each representing five depositional elements, with an initial thickness of 0. lm. Carbonate data from Arthur (1979).

128

Figs. 7-16 and 7-18. This is especially obvious for the succession of shales, marls, and chalks deposited subsequent to the Bonarelli black shale event. The convex-shaped CaCO 3 curve in Fig. 7-19, with a carbonate-poor Zone at the sequence base, indicates a steady increase in carbonate deposition at a rate of Rc = 50%. In the Lower Coniacian, this rate declines to half of its original value, and diminishes by the end of the Santonian. Thus, the oceanographicecologic "relaxation" after the BonareHi event is expressed by increasing calcareous nannoplankton productivity for a time span of about 5 Ma (Fig. 7t9). In the model simulations (Figs. 7-19, 7-20), each depositional element represents an equivalent time span, even when the supply rate of carbonate (Pc) changes. For increasing carbonate deposition, as expressed in the sequence above the Bonarelli horizon, such time-equivalent elements represent increasingly thicker section intervals, as schematically shown in Fig. 7-16. The distribution of these intervals in the sequence allows us to determine relative time spans as indicated in Table 7-2. Such simulations can permit time spans to be modelled for section intervals below the level of normal biostratigraphic or radiometric resolution. Table 7-2 Time spans in the Gubbio sequence, according to time scales and simulation models number of timestages time equivalent span [Ma] "> simulation intervals --> Simulation A (see Fig. 7-19)

Santonian Coniacian Turonian

3.5 1 2.5

3.3 2.5 >6

Simulation B (see Fig. 7-20)

Cenomanian Albian

6.5 15.5

8.5 38

") according to Kent and Gradstein (1985) **)relative intervals, time-equivalent for each simulation Absolute time spans from radiometric time scales and the resulting number of time-equivalent simulation intervals shoud be proportional, if the time scale used is correct, and when there are no additional changes in deposition of the clastic background fraction (Table 7-2). As the investigated sections are relatively thick (90 and 110 m), small changes in the supply of siliciclastic background sediment probably did occur. In addition, it is difficult to model a sequence with slightly increasing carbonate content (e.g., the Santonian). The best agreement between the simulation model and radiometric time scale is for the curved CaCO3 increase in the Turonian and Coniacian (simulation A, Fig. 7-19). According to simulation A (Fig. 7-19), the time span inherent in the lm thick, nonealcareous Bonarelli horizon is equivalent to: a thickness of 1.3m immediately above the Bonarelli which contains 0 to 71% carbonate; a thick-

129

GUBBIO S E Q U E N C E CARBONATE CONTENT

Intervals with equivalent time span

0

20

I

rr

1

= I-._.

<: ~ Z0

!11

0

!

I

J

40 I

60 I

I

-11o

I I J

Z

I

-

I"'

80 *

I

100 I

.-~ Rc - loo

100 J

Rco -

90

(I) U)

I

I ,, I

!

-80m Z ~ --------de , ~ -

~6 40

I

-70~ .,

(measured data)

Z

..

-60o_

I

l--

(J

Rr'8

ill

Z ,r m _1

-5001

m

<

-40

-30 -20.

I

.

.

~ ~ ~ _ . ~ x c~rbonate c u r v e

, .

Rc7O

.

-lo~;:~~Rc~3 mefer 0 'i0

~0

do

do

100

CARBONATE CONTENT Fig. 7-20 First attempt to simulate the carbonate curve in a 110 m thick interval below the Bonarelli horizon, using various rates of carbonate supply (Rc). Vertical lines show time-equivalent intervals. Measured carbonate curve shown by solid dots (from Arthur, 1979), while simulated carbonate distribution curves are depicted by crossed symbols.

130

ness of 3.7m for a carbonate interval between 83 and 88%; and a thickness of 7.5m for a carbonate interval between 92 and 94% at the top of the Turonian. A combination of the models in Figs. 7-19 and 7-20 gives us 10 time-equivalent intervals for the Turonian, with a total time span of 2.5 Ma (Kent and Gradstein, 1985). One of these intervals represents the Bonarelli horizon, which would then have a duration of 0.25 Ma. In this case, the Bonarelli horizon can be assumed to represent the noncalcareous background deposition of the Turonian chalks at Gubbio, composed of clay and radiolarian ooze (Arthur, 1979). A crosscheck is performed by solving Eq. 7-1 for SNc; the investigated Turonian chalks have a mean sedimentation rate of 13 m/Ma and a mean carbonate content of 84% (see Chap. 4.2, Fig. 4-3). The resulting average Ssc value is 2.1 m/Ma, and according to this value, the 1 m thick Bonarelli horizon is assumed to have lasted for 0.48 Ma. For the "Black Band" in England, supposedly the same stratigraphic position as the Bonarelli horizon spanning the W. archeocretacea zone, Schlanger et al. (1987) have even estimated a duration of 1.5 Ma. Therefore, the two methods applied here both indicate that the Bonarelli black shale represents a short time interval, where the oxygen minimum zone became so intensified that diluting carbonate productivity was greatly reduced; thus essentially background sediment was deposited, composed of carbonaceous clay and some radiolarian ooze. After this drop in CaCO3, the rate of carbonate deposition steadily increased by approximately 5cm per 25 000 to 48 000 years, which is on the order of 2.8 to 5.4 gCaCO~m2a' , as documented by the convex-shaped CaCO3 increase above the Bonarelli event (initial depositional element is h~ = 0.1 m which represents one tenth of the Bonareiti hori-

SYSTEMS TRACTS HIGH STAND

\-.-..__~

SYSTEMSTRACT Toplap

Truncatlon

~

~

Senuence B o u n d a ~ /__ -- uowmap

;

~

~

!

]

.....

Apparent

Downlao Surface Truncation afterVAIL et al., 1991

I

i~:ondensedl I section

I

Fig. 7-21 Schematicrepresentation of a highstand systems tract with progradafion and lateral transition along downlap surfaces into a condensed section (after Vail et al., 1991).

131

zon time span with a rate Rc = 50%). As also observed in other condensed sections, the Bonarelli horizon with its low sedimentation rate shows an enrichment of organic carbon (Loutit et al., 1988). The carbonate-organic carbon data given by Arthur (1979) indicate that about one third of the organic carbon increase in the Bonarelli horizon is due to decreasing carbonate deposition, while two thirds are related to a simultaneous, higher rate of organic matter supply, probably related to reduced organic matter degradation in an extensifled oxygen minimum zone (see Chap. 8.3.2).

7.4 Lateral facies changes The major calcareous-clastic facies types can be arranged not only in vertical but also in lateral (isochronous) successions, with respect to three-dimensional basin fill. In lateral successions, facies transitions, expressed by simultaneous changes in sedimentation rate and carbonate content, indicate general trends from shallower to deeper water environments and vice versa. Many of these transitions represent depositional wedges with either a calcareous-dominated or a siliciclastic-dominated style of deposition.

7.4.1 Calcareous and siliciclastic sediment wedges Sediment prisms occur lateral of high sediment input zones related to shorelines, deltas, shelf edges, and aprons on reefs and carbonate platforms. In most cases, however, sediment prisms represent progradational systems tracts, related to sea level highstands and the early parts of lowstands, when subsidence is approximately equal to sea level fall (Posamentier et al., 1988; Einsele and Bayer, 1991). Progradational systems are characterized by the lateral transition from areas of high sedimentation to those of low sedimentation, and finally, in deeper water, to zones of condensation that are vertically expressed by "condensed sections" (Fig. 7-21; Posamentier and Vail, 1988; Loutit et al., 1988; Kidwell, 1991a,b). Not all progradational systems, however, can be simply grouped into calcareous or siliciclastic styles of deposition, because some of these systems show changes in lateral sediment supply from both of these styles. Here, only sediment prisms are considered which are thought to represent basic types of deposition (see Chap. 2.1), where either the calcareous or siliciclastic sediment supply changes laterally more rapidly than the corresponding background fraction. Calcareous sediment wedge: In a prograding calcareous wedge, facies types in the carbonate system succeed from highly calcareous sediments with high sedimentation rates to carbonate-poor, deeper water deposits with low sedimentation rates. Along the progradational prism, shallow water carbonates or shelf chalks pass into more slowly deposited marly or even shaly sediments (Fig. 7-22). This decrease in sedimentation rate is accompanied by a tendency for increasing organic carbon content, which ideally shows its highest concentration in the condensed section. Typical calcareous wedges can be observed particularly in Paleozoic tran-

132

sitions from shallow to deeper water. Paleozoic carbonates, largely restricted to shelf, platform, and schwellen environments, progress into shaly, basinal deposits (Wen& and Aigner, 1985; Tucker and Wright, 1990; Ricken and Eder, 1991). With the onset of substantial planktonic carbonate production in the lower Jurassic (Roth, 1986), deeper water sediments became more calcareous, so that shallow to deeper water facies transitions became restricted to a narrower CaCO3 span. Even so, with the appearence of pelagic carbonates in large quantities, a new oeeanie facies succession came into existence, the lysocline - CCD transition, expressed by the carbonate-poor to noncalcareous facies types of association C~.

CALCAREOUS SEDIMENT WEDGE ~ ~ ' ~ - - , ~ z : . : . : ~:.:.:..:.:...~.:...~.:.:..-.,.,.. Carbonates

~ d " " " " "':'::::::::::':'::'::::!::i:::!!i"::::i:i~i:':':':"'" Shale

SIMPUFIED FLUX PATTERN :

Condensed Section

!::!~iii::iiiiii!i}::!ii::i!!::i::iii::?ii!i!~ii{i~ Carbonates Silicidastics

CARBONATE CONTENT ORGANIC CARBON

100"

50.

CONTENT(noscate)

Carbonate Content

Organic Carbon Content I highest ceme.rCtationpotenthai ,,in organic caroorvpoor sediments [

Fig. 7-22 Facies successionsalonga calcareous sedimentwedge,with the principal trends in carbonate and organic carbon contents (schematic). Siliciclastic sediment wedge: A siliciclastic sediment wedge may ideally comprise the entire facies suite from highly-sedimented sandy sequences to carbonate mud or marly and chalky basin sediments (Sussko and Davis, 1992). This is usually accompanied by increasing organic carbon content (Loutit et al., 1988; Fig. 7-23). Typical siliciclastic sediment wedges are observed for the post-Jurassic, when elastic shoreline deposits and deltaic sediments pass laterally into marls and chalks relatively rich in organic carbon, Clastic

133

shorelines of Cretaceous epeiric seas and foreland basins in Europe and North America follow this type of deposition. In deeper seas, turbidite sequences, with transitions into pelagic marls and chalks, can be viewed as representing a special case of siliciclastic wedge sedimentation. In the Paleozoic, siliciclastic sediment was transported to basins relatively poor in carbonate content (Tucker and Wright, 1990), so that only minor facies changes occurred, restricted to a smaller range of low CaCO3 values. Condensed sections, lateral of calcareous and siliciclastic sediment wedges, are different in their hydrocarbon source rock potential (Taguchi and Mori, 1992; Leythaeuser, 1993). In a calcareous wedge, the organic matter is domin-

SlLIClCLASTIC SEDIMENT WEDGE

~

hale

SIMPUFIEDFLUXPATTERN .

.

.

.

Marl Chalk Condensed Section

Carbonates

CARBONATECONTENT

100

ORGANICCARBON CONTENT

Carbonate/ Conte~ /

(no scale)

50

~ _

J

J/Organic Carbon ~..,.t ~:Jntent

highestcementation I potentialin organic carbon-richsediments Fig. %23 Facies sucessions along a silieiclastic sediment wedge, with the principal trends in carbonate and organic carbon contents (schematic).

anfly of marine origin (i.e., kerogen types I and II); the associated condensed section is relatively carbonate-poor, thus it is not affected by immobilization of organic carbon by early carbonate cementation. The opposite conditions are found for siliciclastic wedges. Here, the condensed section contains a higher proportion of terrestrially derived organic matter (i.e., kerogen type III), reducing its potential for hydrocarbon generation. This tendency is further amplified, because carbonate contents in these condensed sections are relatively

134

high; hence, early cementation prior to hydrocarbon generation may fix a part of the organic matter and prevent compaction, thus further reducing its source rock potential. However, another factor which may partly offset these effects is related to the fact that siliciclastic deposition has, on the average, an organic matter supply which is five times higher than that of carbonate deposition (see Chap. 8).

Siliciclastic wedge deposition: Transition from elastic shoreline sediments to mid basin chalks, Cretaceous Seaway, North America The two margins of the Upper Cretaceous Western Interior Seaway (North America) provide instructive examples of both calcareous and siliciclasticdominated types of sedimentation (Chap. 4.3). The eastern basin margin is epicontinental in character, with prevailing calcareous deposition which passes laterally into marly to chalky basin sediments. In contrast, the western margin has the attributes of a foreland basin with siliciclastic wedge deposition, documented by clastic shorelines and their lateral transition into hemipelagic, carbonaceous marls and chalks (Weimer, 1960, 1983; Kauffman, 1984). For a time slice spanning the Cenomanian/Turonian boundary, Hattin (1964, 1981) and Elder (1987a) established a 1500km wide, isochronous, bentonite-controlled transect from the western shore zone to the mid-basin area. During this interval (i.e., the Bridge Creek Limestones) maximum transgression occurred in the Western Interior Seaway (Kauffman, 1984; Either and Diner, 1985; Vail et al., 1991; see Chap. 6.5.1). The pattern of relative calcareous and clastic sediment flux in Elder's transect was already determined in Chap. 4.3, by introducing relative sedimentat.ion rates for laterally-correlated, isochronous intervals (see Fig. 4-5). Due to decreasing siliciclastic deposition, relative sedimentation rates decline by a factor of 2.5 from the western margin to the middle of the basin (from section A~ to CO, while the concurrent planktonic carbonate deposition remains nearly constant along the 1500 km long transect (from A t to K t, see Fig. 4-5). The average organic carbon content follows the predicted Cot~ trend for siliciclastic wedge deposition; it increases with decreasing sedimentation rate and increasing carbonate content towards the middle of the basin (Fig. 7-24). With the exception of section NM~, which has lower Cm values, there is good agreement between the observed data and the theoretically predicted straight line Co,z-CaCO3 relationship. Thus, increasing organic carbon contents in this transect (from the sections A~ to C2) strongly reflect the decrease in clastic input, and to a lesser degree, the transition into oxygen-poorer bottom waters. Only at the section Kansas 1 (see Fig. 4-5 for Kt) have highly dysaerobic bottom waters probably existed. Similar results can be obtained with the C~-CaCO3 data derived from the laterally correlated individual sections (Fig. 7-24, lower graphs). The scatter diagrams show changes in siliciclastie deposition, expressed by positive Co~gCaCO 3 associations, on which the pattern of larger, bedding-related carbonate variations is superimposed (i.e., the well-bedded Bridge Creek Limestone). With increasing siliciclastic influence from the center of the basin (CO to the

135

SILIClCLASTIC WEDGE DEPOSITION WESTERN INTERIOR BASIN RELATIVE SEDIMENTATION

RATE(st) 3

-

CUNING CLASTIC DEPOSITION

, lb

eXl'CO

20 ]3b ~b gb 6ol 7o CARBONATE

tS"

sr~' 3 ~

1

org

,'~ ~~

Ar~z~;.,~ ~ookg

AVERAGE Corg CONTENT

c

?',:C~oroao

~

i

0.5

f2

ORGANICCARBON CONCENTRATION

..,-." s s

~

4o 5b 6bl 70 I

8ED~NG-REI~1ED ~ 8 0 1 ~ T E DEI~OSmOIq

('

5

.~5

'

/

..5

ILL. 9 20 40

60 80 100 ~

T

E

CARBONATE

1

.9

H

20 60 60

80 100

CARBONATE

.... 20

60 60 80 100 ~ ' I ~

~ T E

Fig. 7-24 Siliciclastic wedge deposition. We.stern haterior Basin, isochronous cross section from the shore zone (section A I) to the middle of the basin (section C,). Declining elastic deposition is accompanied by increasing organic carbon concentrations. Note the general agreement between the different relative sedimentation rams (s,), calculated from laterally correlated sections (upper graph) and from carbonate-organic carbon data (middle and lower graphs). Cenomanmn/Turonian boundary. Se~ Fig. 4-5 for individual sections, calculations of relative sedimentation rates and basic flux patt=ra, and see Fig. 7-15 for trend overlapping. Co,gCaCO3 data from Elder (1987a). shore zone (A~), the C ~ - C a C O 3 trends shift along a siliciclastic line towards the origin of the C=,-CaCO3 graph. In consequence, the slopes of the superimposed, bedding-related, carbonate trend lines become smaller. The relative sedimentation rates calculated by using the average and individual Co=-CaCO3 data (Eq. 2-8, Fig. 7-24 middle and lower graphs) are in accordance'with the

138 carbonate content - sedimentation rate relation (F_xt. 7-1, Fig. 7-24 upper graph), and with the relative sedimentation rates derived by the lateral correlation of isochronous sections (Eq. 4-5, Chap. 4.3). 7.5 Conclusions Most marine sediments can be characterized by facies types that are interpreted as obeying carbonate dominated and siliciclastic dominated sedimentation. Under such conditions, sedimentation rates, time spans, carbonate and organic

MAJOR DEPOSITIONAL TRENDS I S~UCtCLAS:riC DEPOSmON I CARBONATE EPOSmON

:, s|lidclasLic shelf, "turb|dite$

shallow water carbonates,

i' Z ILl U)

S "-.."~ "".............

/

hemlpelagic marls 2 II ~176

Corq-dch chalk

~ me. - ~ 1 7 6 1 7 6

clay, I,!i

3

I

CARBONATE CONTENT v

black shale

Corg-dch chalk

....... 0 Z < nO

~

.

O~ bioturbated . . sands and sifts 1

11V~al~t~

bloturbated chalk I , ~ _ CARBONATE CONTENT

Fig. 7-2.5 Combined sndlmentological effects of carbonate and siliciclastic deposition (schematic). a) Most important trends in facies transitions, sedimentation rates and carbonate contents, b) The associated tendencies of organic carbon concentrations. Direction of facies change from high to low sedimentation rates (arrows). Siliciclastic depos,itioa: dashnd curves with facies types 1, 2, and 3. Carbonate deposition: solid curve w~th facies types I, II, and RI. carbon contents, and facies successions form integrated, numerically deterruinable systems. 1. C a r b o n a t e content - sedimentation rate relation: A fundamental principle for carbonate or siliciclastic deposition is the relationship between the

137

average carbonate content of a stratigraphic unit and its sedimentation rate. Data compilation shows that the sedimentation rate in sequences with carbonate deposition increases with increasing carbonate content, while for siliciclastic deposition this increase is related to decreasing carbonate content. Sedimentation rates are numerically expressed as a function of the carbonate content and the corresponding rate of background deposition. 2. Development of facies associations: The interrelationships among sedimentation rates and carbonate contents are expressed in facies associations, defined by different rates of background deposition. For each of the basic styles of deposition, four standard facies associations (S! to Srv , d C~ to Cry) are established. This allows precise estimation of sedimentation rates on a pore-free basis by using the average carbonate content of a sediment or rock unit.

M A J O R LATERAL FACIES T R E N D S

SI JClCLASTIC

CARBONATE DEPOSITION

I

DEPOSITION

l ~ U) I siliciclastic uJ ~ I shelf ZI'~r'--~I hemipelagic

shallow water carbonates, ] _ u.~

~

hemipelagic ! , . marls ~-~s ~:: , ' , ' ,

Corg-rich ---"~"-_,_ ' , ' , ~ ' , : clay~ . . . . - ~ ' ~ - - -" -,i , , :

CARBONATE CONTENT

CARBONATE CONTENT

organic carbon-rich laminated chalks 6Qee weakly ..-" bioturbated

ee

marls"

oo. e*

bioturbated O

. ~

cnalk

ZIO z muJ O o O o

t

.silts, sands

black shales

~weakly

bioturbated marls bloturbated chalks, limestones

I

CARBONATE CONTENT

CARBONATE CONTENT ~ 4 P

Fig. %26 Principal facies transitions in siliciclastic and calcareous sediment wedges and associated trends in organic carbon contents. Dashed line, siliciclastic deposition; solid line, carbonate deposition. Siliciclastic facies associations are characterized by sandy and silty sediments with high rates of deposition and relatively low organic carbon contents that pass into condensed, hemipelagic marly and then chalky sediments. This trend is accompanied by a general increase in organic carbon content (Fig. 725). In contrast, calcareous facies associations are characterized by transitions from organic carbon-poor, shallow water carbonates and thick epeiric chalk se-

quences to weakly bioturbated marls and organic carbon-rich shales and clays with low rates of deposition. Such facies changes may also include transitions from chalks to shales and clays below the lysocline. All these facies associations occur vertically and laterally with respect to the three-dimensional basin fill. 3. Carbonate distribution in sedimentary sequences: The carbonate content - sedimentation rate relation controls the vertical carbonate content distribution. Constantly changing input of one of the two sediment fractions leads to a non-linear CaCO3 distribution in the deposited strata (i.e., the "asymmetry relation"). Changing carbonate deposition is recognized by wide, convex CaCO3 maxima and sharp minima which tend to be organic carbonrich, whereas siliciclastic deposition is denoted by the opposite. Carbonate deposition is documented by a Jurassic shaly to calcareous sequence in southem Germany and by the facies succession above the Bonarelli black shale horizon (Turonian-Santonian sequence in Italy). Simulations of vertical carbonate content curves allow estimation of carbonate and siliciclastic fluxes and permits relative time spans to be modelled below the normal level of biostratigraphic or radiometric resolution. 4. Silicilastic and calcareous sediment wedges: Laterally, major facies successions are expressed in the form of siliciclastic and calcareous sediment wedges (Fig. 7-26). Prograding siliciclastic wedges pass basinwards into increasingly calcareous sediments with higher organic carbon values and reduced sedimentation rates (e.g., condensed sections). Calcareous sediment wedges, including shallow water carbonates and epeiric chalks, change basinwards into shaly, more organic carbon-rich sequences with low rates of deposition. In both types of wedges, the whole suite of facies successions over the entire carbonate span may not develop, depending on local conditions and on changes in the style of carbonate deposition over Earth's history. Siliciclastic wedges occur mostly after the early Jurassic, where clastic nearshore sediments change into pelagic chalks; before the Jurassic, calcareous wedges were prominent, with calcareous shelf and platform sediments interfingered with carbonate-poor basin sediments. Condensed sections lateral of calcareous wedges have a higher hydrocarbon potential than do equivalent condensed sections in silicielastic wedges.

Chapter 8 SYSTEMS

RICH AND POOR

IN ORGANIC

CARBON

The prediction of organic carbon content is pertinent to various sedimentological investigations as well as practical applications. Bottom water oxygenation and organic carbon-carbonate contents are the main parameters controlling the occurrence of bioturbated to laminated, shaly to calcareous lithotypes and their associations. Therefore, in this chapter we will address three general topics related to organic carbon concentration: (1) the forecasting of sedimentary organic carbon contents, (2) determination of bottom water oxygenation, and (3) definition of various lithotypes both rich and poor in carbonate and organic carbon contents (Fig. 8-1).

FORECASTING ORGANIC CARBON CONTENT I TYPE AND RATE OF I BACKGROUND - I DEPOSITION, _ _ I TYPE OF FACIES ASSOCIATION Corg CONTENT

IO~aAN,C CAnBON CONTE~, IDILUTION BY

AND I Corg-CaC03

OF BACKGROUND, , SEDIMENNT]" 7 MAIN SEDIMENT 1.4 RELATION IN SEDIMENTS A

/

I'OR~,ANIC MATTER F~U~I I PRIMARYPROOUCTI~TYII I---I rl BOTTOM WATER /I OXYGENATION I

/

BIOTURBAT1ON

'

l

FACIES TRANSITIONS I F.~ IBY B' CARBONATE OR

CLASTIC DEPOSmON I CI

I

LITHOTYPE CONCEPT

II

'

'

,.IVARIOUS Ln~o~'PES PJCHTOI c J POOR IN ORGANIC CARBON "lC~Bo"*~c~ ~ .

BU~CKSHALEL r r ~ o ~

I

J I

I

ASSOCIATION I PLATTENKALK LITHOTYPEI ASSOCIATION ,I

Fig. 8-I The most important parameters controlling sedimentary organic carbon content and determining different lithotypes rich to poor in organic carbon content.

140

8.1 Forecasting the organic carbon content Quantifying and forecasting the sedimentary organic carbon content is a longstanding goal in sedimentary geology and basin analysis. In the literature, various efforts have been made to determine organic carbon content based on primary productivity, decomposition potential, and flux rate, as previously discussed here in earlier chapters (Mfiller and Suess, 1979; Suess, 1980; Betzer et al., 1984; Emerson, 1985; Stein, 1986, 1991; Sarnthein, 1987; Berger et al., 1988; see also Chaps. 3.1 and 3.3). In this chapter, three factors are used to quantify organic carbon content, such as the supply of organic matter (i.e., flux), the rate of background sedimentation, and dilution processes through the main sediment fractions. The first two parameters determine the organic carbon content in the background sediment, while the third factor controls to what degree this background sediment is diluted to lower organic carbon values by deposition of the main sediment (Fig. 8-1). 8.1.1 Organic matter in the background sediment: a critical factor for organic carbon prediction It is demonstrated in this book that depositional variations in the main sediment can either dilute or concentrate the organic carbon which is contained in the background sediment (e.g., see Chap. 2.2.2). Thus, quantifying this background organic carbon content is essential in forecasting the organic carbon content. In the first step we want to investigate how the organic carbon content in the background sediment changes when the nonorganic portion in it is supplied at either a low or high rate, leading to additional concentrationdilution processes, respectively. The simplest way to analyze such effects is to assume that the input of main sediment would be so low that essentially background sediment is deposited. Under such conditions, increasing deposition in the nonorganic portion of the background sediment results in additional organic carbon dilution, as schematically explained in Fig. 8-2. Such an increase in non-organic background deposition is associated with a characteristically curved decrease in (background) organic carbon content (Fig. 8-2). The significance of such dilution processes, which occur in addition to dilution processes in the main sediment, is statistically documented by evaluating the organic carbon content in the background sediment for different rates of background deposition. This is performed by expressing the organic carbon content of the background sediment (Cocoa:, Co,zss) from various sequence intervals against the corresponding sedimentation Fates of the background fractions (Ssc, s~s). The Co~c and C - ~ s values can be determined either graphically by extrapolatin~ the swarm o points to low or high carbonate contents (see resets m Figs. 8-3 and 8-4), or by employing Eqs. 2-1 and 2-2 (see Chap. 2.3). This procedure was performed for sequences or individual stratigraphic intervals with established carbonate and organic carbon data, representing environments from the deep sea to epicontinental seas, as documented in Figs. 8-3 and 8-4. The sedimentation rate of the background fraction (sNc, sss) was calculated according

141 to Eqs. 7-1 and 7-2 for nonporous sediments, based on the time scales of Kent and Gradstein (1985) and Berggren et al. (1985) and the mean carbonate contents of the considered sediment intervals.

INFLUENCE OF CHANGING BACKGROUND DEPOSI?ION !

i

,

!

!

I PERCENTAGEOF ORGANICMATTER IN BACKGROUND SEDIMENT

,

I ~

,

MA,N

9 SEDIMENT ! i

!

N

IJ]ACKGROUND SEDIMENT

~

,

I

. . . . . . . . '~,/ORGANIC MA]-rER | ]LIowOMsupply LhighOMsupply 1

! ! i t i ! t

I

. . . . . .

Q. . . . . . . . . . . . . . . . . . . . . . . .

SEDIMENTATIONRATE BACKGROUNDSEDIMENT

Fig. 8-2 How changing non-organic (clastic or carbonate) background deposition influences background organic carbon concentration (C~sc, CmNs)- Upper curve (solid) shows conditions for constantly, high organic matter supply; lower curve (dashed) shows the conditions for constantly, low orgame matter supply.

The two resulting diagrams (Figs. 8-3 and 8-4) for carbonate and siliciclastic deposition contain considerable scattering; in addition, organic carbon contents vary distinctively from relatively low to relatively high values. Even so, both diagrams demonstrate the general tendency for decreasing organic carbon content with increasing background deposition, as theoretically established in Fig. 8-2. Determination of organic matter flux rates (E,qs. 8-1 to 8-4) shows that this decrease occurs within the limits of low and high organic matter deposition (SoM, in m/Ma, see the overlying curves on the data sets). How these curves were quantified is explained below. For carbonate deposition, the organic matter sedimentation rate (SoM~c~ is expressed as a function in the organic matter volume of the background sedi-

142

OM NC C o r g N c

[voL%]

[wt.%]

I,-

ORGANIC CARBON CONTENT IN BACKGROUND SEDIMENT CARBONATE DEPOSITION

w ffl Q z Q ~

n-

z

/.0--15 .

30-10.i

w

.{),1.C.e~real1

z O

0.05

z O

5-

P..

10-z nO

,=____.~

0---0

~

5

10

E

0.08p_.,,~,~2;*

~

15

F '

20

S E D I M E N T A T I O N RATE B A C K G R O U N D S E D I M E N T

2~ sNC [m/Ma]

D S D P / O D P SITES

D S D P / O D P SITES

^ 329 Miocene I] 370 Neocom, Paleocene n 398 Apt - Cenornan == 461 Eocene- Miocene Thithon - VaJange . 463Apt . " ocee 511 L Jutassm - E. Olag n 9 530 Turon - Pleistocene | 532 Pliocene - Quartemary O 535 Bernas - Cenoman + 5~3 Miocefle x 540 AIb ~Pateogene 547 Jurassic, Cretaceous, Tertiary 9 603 Neocome

! 638 Valange - Ptiocene ~ 6 5 8 Pliocene - Pleistocene ~'662 Pleistocene- Pliocene <~663 Pleistocene 9 664 Pliocene - Pleistocene r~ 721-722, 730-731 P l i o c e n e Pleistocene

Fig. 8-3

~-

[ ] Blue Lias, Britain 0 Plie~sbach - Toarc, SW Germany v Apt - AIb, SE France AIb - Turon, Ce.u,=l =lt~dy =t Cenoman - Turon, U.S. Western Intedor X Coniac - Santon, U.S. Western Interior

14,3

ment (OM~L~.bs) and the background sedimentation rate (Ssc) thus Ssc * OMI~,~.,.~~ So~c~ =

(8-1) 100

The organic matter volume (OM,~ ~ b~) is expressed in analogy to Eq. 2-10 using specific densities of 1.01 g/cmT'(or organic matter and 2.7 g/cm 3 for the siliciclastic fraction (S), 100 * (OMl,~.~.bs): 1.01) OMt,ol.~.bs I =

(8-2) ($1,,.~.~:2.7) + (OMj,,,.t,b~:l.01)

Several transformations are performed in Eq. 8-2, such as expressing SI,~ 9 bg) as 100-OMI~, bea, eliminating the two terms OMI,,~, b,)/1.01, and writi/~g OMt,,~ ~ b J as I" ~Co~ Nc This results then in the equatmn i3"M,,oI ~ b- = 100/[(10 l- 1.31Co~,Nc)/(3.51 ~o,,Nc) + 1]. When this expres-slon is s u b s t t t u t ~ f o r OM,o~.,.bg in Eq. 8-1, the sed.ihaentation, rate. for organic matter deposition in the carb6nate system (SoM~c), In m/Ma) Is wntten as 9

g

"

"

.

9

101-1.31 CorgNc SOMIC) = S~c" [ . . . . . . . . . . . . . . . . . . . + 1 ] , 3.51 Co~ir~c

.

I.

.

,

(8-3)

and, accordingly, in the siliciclastic system SoMIs)=Sss:

101-1.31 Cor~S [ . . . . . . . . . . . . . . . . . . . + 1] 3.51 Cor~NS

,

(8-4)

where C ~ c and Cor,NS are the organic matter content of the noncalcareous and noncla-stic backg]'ound sediments, respectively. Note that the organic matter flux rates, obtained here using semilithified to lithified sediments, are diagenetically affected. Compared to the original sediment these flux rates are reduced (see Chap. 3.2.1). Eqs. 8-3 and 8-4 are used

.~ Fig. 8-3 Organic carbon content in the noncalcareous background sediment (Co,~c in wt.%; OM,c in voi.%) for various sequences with different background sedimenhtion rates (Ssc, m m/Ma); carbonate deposition. Curves show the theoretical rates for organic matter supply expressed as the. (nonporous) sedimentation rate (SoM, in m/Ma) and the organic carbon flux (gC~,, m"a") for semilithified to lithified sediments. Note that organic carbon contents in the b~ckground sediment scatter approximately between curves o f constant low and high organic matter supply, suggesting depositional conditions largely equivalent to those shown in Fig. 8-2. Inset describes determination of the background Coq content by extrapolating a given data set to zero carbonate content.

OMNS

ORGANIC CARBON C O N T E N T CorgNS IN BACKGROUND SEDIMENT

[voL%] [wL%]

~Z

<~ m

!

S0

SILICICLASTIC

DEPOSITION

/

KEY

60_30"

c=~.s

CARBCNATECONTENT

5o--2o. i

C3 0.06C~.;z;~

,.o- IL

O

13

6

10

20

30

/.13

SEDIMENTATION RATE BACKGROUND SEDIMENT

50 sNS

[m/Ma]

D S D P / O D P SITES D 9 A [] x 0

463 Apt 474 Pliocene - Pleistocene 532 Pliocene - Pleistocene 603 Cenoman - Turon 650 Pliocene - Reistocene 651 Reistocene 720 Pleistocene

,,- Cefloman - Turon, U.S. Western Interior I Campan, U.S. Western Interior

Fig. 8-4 The organic matter content in nonelastic background_ sediment (C=.,Ns in wt. %; OMm in vol. %), for sequences with silieiclastie deposition. Curves show th~ theoretical rates for orgamc matter supply expressed a s ~ e nonporous sedimentation rate (SoM, in m/Ma) and the organic carbon flux (gC~ m"a") for semilithified to lithified sediments. Data suggest a similar relationship between background organic carbon content and sedimentation rate as found for calcareous deposition, but with a generally higher rate of organic matter supply. Inset shows the determination of the backgroundC.,= content by extrapolating a given data set to high carbonate content.

145

to determine "iso-curves" denoting an equal degree of organic matter deposition represented on the scatter diagrams (Figs. 8-3 and 8-4). These curves approximate the swarm of points representing the background sedimentation rate versus organic carbon content, and show that the supply of organic matter varied considerably by factors of 40 and 100 for carbonate and siliciclastic deposition, respectively. The siliciclastic system has an organic matter flux 2 ORGANIC CARBON - CARBONATE FIELDS OF CALCAREOUS FACIES ASSOCIATIONS f/CARBONATE [i ~ FRACTION ; ' (MAIN SEDIMENT)

' ._| ' , iIi ~ ~,

I

'

~ ~

eLASTIC FRACTION

I I

ii~

~

~

,ORGANIC

"MATTER

SEDIMENTATIONRATElm/Ma] .

.

.

.

.

I5 -

I

C~o~'rE

FACIES ASSOCIATIONS CI

. . . . . . _

. 10

.,

I SEDIMENTATION RATEOFTHE[SNc]

I

[~1

CARBONATEl ~ t

CARBONATE[ ~ 1

Cll

C ll I

CARBONATE[ ~ 1

C IV

Fig. 8-5 How a varying supply of background sediment (sNc) controls the size of fields defining the statistical ranges ofCo~-CaCO3 values. Carbonate deposition. The amount of background deposition controls not only the size of C,,,-CaCOj fields via the background organic carbon content (Conr but also determineS" the particular calcareous facies association (C~ to Cw). Compare with Fig. 8-2.

to 5 times higher than the carbonate system, related to higher terrigenous organic matter contents and smaller water depths (see Chap. 3.3.2). Despite these variations, the organic matter supply is statistically almost constant or increases only slightly with increasing background deposition, which is especially obvious in the carbonate system (Fig. 8-3). This supports the assumption

146

made here that the organic matter content of the background sediment is progressively diluted when the background deposition increases, and thus, its mineralic portion. Consequently, depositional processes are nearly identical to those expressed in Fig. 8-2. These findings are supported not only by Emerson's (1985) theoretical model, but also by the data set provided by Arthur et al. (1984), which shows that the effect of increasing deposition is mainly dilution of the C.rs content (see Chap. 3.1, Fig. 3-2). Hence, two concurrent depositional dilution processes, namely the supply of mineralic background and main fractions (i.e., carbonate and elastics, respectively), are the key mechanisms controlling the concentration of organic carbon. Their combined effect is schematically represented in Fig. 8-5, where the organic carbon content is expressed in relation to the amount of background sediment, using the parameters as in Figs. 8-2 and 8-3 and assuming carbonate deposition. When background deposition is low, the organic carbon content is little diluted and is, on the average, relatively high; the opposite occurs when background deposition is substantial. Because the rates of organic matter supply vary, it is possible to establish lower and upper Cot. limits for the background sediment. Their organic carbon contents are furthe{ diluted by increasing carbonate deposition, so that triangle-shaped fields of possible carbonate and organic carbon contents are each defined through an upper and a lower Corz-CaCO 3 trend line. For low background deposition, these fields are large, but they are small for sediments with high background deposition. Note that the background sedimentation rate, which controls the sizes of these fields, also defines the various calcareous facies associations (C~ to Crr as discussed in Chap. 7.1.2). One can briefly summarize: The various calcareous and siliciclastic facies associations (C~ to Cry and St to S~v) are statistically connected with different graphical fields of carbonate versus organic carbon contents (e.g., Fig. 8-5). Facies associations defined by low background deposition (CI, Cn; S~, Sn) are characterized by relatively high Co,- contents in the background sediment, whereas facies associations with substantial background deposition show the opposite trend (Cm, Civ; Sat, S~v). All these different background organic carbon contents are usually diminished further by increasing carbonate (or siliciclastic) deposition by the main sediment, expressed in various graphical Cor~-CaCO 3 fields.

8.1.2 Assessment of sedimentary organic carbon contents for calcareous and siliciclastic facies systems We have seen that the sedimentary concentration of organic carbon is controlled by three major factors, the background sedimentation rate, the rate of organic matter supply, and the degree of dilution by the main sediment. In the following we see how these factors can be quantified so that we can assess the organic carbon content. 1. As the first step in this procedure, the type of deposition for a given lithology must be determined (i.e., calcareous or clastic deposition). One can use successions of facies types in a given sedimentary sequence and assign

147

them to the standard facies associations introduced in Chap. 7.1.2; or one can apply the distinctive pattern of vertical carbonate distribution for beds (noncemented) or sequences, as adressed in Chaps. 5.1 and 7.3. In addition one can use the characteristic Co~s-CaCO3 correlation styles (see Chap. 2.2). If assignment to one of the major styles of deposition is possible, the sedimentation rate of the background sediment can be estimated by solving Eqs. 7-1 and 7-2 (the carbonate content - sedimentation rate relation) for either SNC or SNS. 2. The next step is to estimate the amount of organic carbon contained in the background sediment (Co,~Nc, Co~Ns). Eqs. 8-3 and 8-4 are solved for Co,sSc and C~r~s, respectively, or these values are read directly from the diagrams depicted in Figs. 8-6 and 8-7. For both methods one must select an appropriate Table 8-1 Average organic carbon contents in the background fraction of similithified to lithified sediments (carbonate deposition) facies association

rate of background deposition [m/Ma] range mean

organic carbon content in background sediment [wt. %] ") range mean

Ct Ca Cm Cw

0.5 to 4 2.25 4 to 9 6.5 9 to 17 13 > 17 ca.22

0.66 to 20 0.22 to 7.8 0.11 to 3.6 0.065 to 2.0

7.75 3.4 1.6 1.0

") Lower limit, upper limit, and mean are determined using solid organic matter sedimentation rates of SOM = 0.05, 1.5, and 0.7 m/Ma, respectively, except the mean and upper limit for Ct, for which smaller SOMvalues are used (see Fig. 8-6). Table 8-2 Average organic carbon contents in the background fraction of semilithified to iithified sediments (siliciclastic deposition) facies association

rate of background deposition [m/Ma] range mean

organic carbon content in background sediment [wt. %] ~ range mean

S~ Sn Sm Sw

0.5 to 6 3.25 6 to 15 10.5 15 to 30 22.5 > 30 ca.40

0.9 to 50 0.28 to 30 0.13 to 13.3 0.07 to 6.3

18 12 4.9 2.3

"> Lower limit, upper limit, and mean are determined using solid organic matter sedimentation rates of sou = 0.1, 3.5, and 7 m/Ma, respectively, except the mean and upper limit for S~ and the upper limit for Sn, for which smaller SOMvalues are used (see Fig. 8-7).

148

value for the rate of organic matter deposition (soM). This is difficult because the supply of organic matter undergoes substantial variations. As shown in Figs. 8-3 and 8-4, and in Figs. 8-6 and 8-7, organic matter deposition varies in the carbonate-dominated system approximately from 0.05 to 1.5 m/Ma (0.04 to I. 15 gCo,=m'Za~), but in the elastic-dominated system from 0.1 to 7 m/Ma (0.08 to 5.4 gCo,,mZat). These sedimentation, or flux, rates represent the solid organic fraction for diagenetically altered, semilithified to lithified, marine sediments.

AVERAGE ORGANIC CARBON CONTENTS IN BACKGROUND SEDIMENT

C~ [wt.%]

9 ml,

z ,,, )...

~20,,, 9

z

i

N ~i!)!i~r,

CALCAREOUS

FACIES ASSOCIATIONS

oOoz~c~ = 15" ~~J

(.9 n"

rn

.

.

O

so.o4c~0 "'!i'=2: ~ k:2!5!~2iSi?Sii!?i

0

S

C ~V

,, 10

15

;tO

75

SEDIMENTATION RATE BACKGROUND SEDIMENT sNC [m/Ma]

Fig. 8-6 Estimation of mean organic carbon content of the background sediment (C,,~c, in wt.%), with lower and upper limits(dots), used to predict the organic carbon contents of calcareous facies associations C~to Cry. Noncalcareousbackground deposition, S~o in m/Ma; organic matter supply, sou, in m/Ma and C,,, flux in g~o,8m2al. The mean organic matter supply is approximately 0.7 m/Ma (0.5 gCo~tmZa1) and 3.5 m/Ma (2.7 gC,~,m"a") for the carbonate and siliciclastic systems, respectively. These values are used to derive average organic carbon contents in the background sediments, determined for the standard facies associations C~ to Cry and S~ to Sw, which in turn are defined by characteristic rates of background deposition (see Chap. 7.1.2; Tables 8-1, 8-2). Positive and negative

149

deviations from the mean are expressed in the lower and upper background Cors limits. Such variations are related to changing productivity and redox conditions, addressed in Chap. 8.2. In addition, the organic matter content of the background sediment has varied through Earth's history, associated with global changes in productivity, anoxia, and the mineralic background supply (e.g., Jenkyns, 1980; Schlanger et al., 1987; Arthur et al., 1990). The compi-

CorgNsAVERAGE ORGANIC

CARBON CONTENTS [wt.O/o] IN BACKGROUND SEDIMENT

I,-. Z LLI s LLI O3 Q Z O n'-

(3 ,v, < t-Z uJ F-Z

o

Z O rn oz <

Z .< nO

SEDIMENTATION RATE BACKGROUND SEDIMENT sNS [m/Ma] Fig. 8-7 Estimation of mean organic carbon content of the background sediment (Corgss , in wt. %), with lower and upper limits (dots), used to predict the organic carbon contents of siliciclastic facies assoctations S~ to Sw. Nonelastic background deposition, SNs, in m/Ma; organic matter supply, SOM,in m/Ma and Co,s flux in gCo,s m2a"r. lation shown in Fig. 8-8, which is based on the same data set as Fig. 8-3, may be used to correct selected average Co,~c values for a given time interval. 3. The last step concerns the question of to what degree the above-determined background Co,- content is diluted by concurrent deposition of the main sediment (i.e., carbonate or clastic deposition). As shown in Chap. 2.2, dilu-

150 tion through either the calcareous or siliciclastic fraction is characterized by a straight-line relationship between organic carbon and the carbonate content, expressed by the general equation (3, = ax + b). For carbonate deposition, where the C~.-CaCO3 relationship is negatively correlated, the organic carbon content (Co~v"wt. %) is expressed as -Co,~c Co.,~c) -

C.,.~c (IO0-C) *C+Co.,,c

,

=

I00

(84)

I00

where C is carbonate content and C., ~ is the organic carbon content of the noncalcareous background sediment. ~ i s relationship is then used to develop standard formulas for assessing of average organic carbon contents, applicable for semilithified to lithified sediments. Standard equations derive the average organic carbon contents for the four calcareous facies associations (C~ to Ctv), including their lower and upper organic carbon limits, using the various C ~ c values presented in Table 8-1. Facies Association Ct: C~g [wt.%] = -0.078 C + 7.8 lower limit: C~, [wt.%] = -0.007 C + 0.7 upper limit: Co~z [wt. %] = -0.2 C + 20

(8-6)

Facies Association Cn: C ~ [wt. %] = -0.034 C + 3.4 lower limit: Co,~ [wt.%l = -0.002 C + 0.2 upper limit: Co~ [wt. %] = -0.078 C + 7.8

(8-7)

Facies Association C.I: C~. [wt.%] = -0.016 C + 1.6 lower limit: Cor~ [wt.~] = 43.001 C + 0.1 upper limit: Cor~ [wt. %1 = -0.036 C + 3.6

(8-8)

Facies Association C,v: Co,, [wt.%] = -0.010 C + 1.0 lower limit: Co,g [wt. %] = -0.0007 C + 0.07 upper limit: Co~ [wt.%] = -0.02 C + 2.0

(8-9)

Sedimentary systems with siliciclastic deposition show a positive correlation between the carbonate and organic carbon content (see Chap. 2.2), which can be expressed in the general equation

CorgN$ C~,[sl -

* C

;

(8-10)

100

Fig. 8-8 The change in organic carbon concentrations in the background sediment over P, the last 200 Ma. Carbonate deposition. Trends towards higher C ~ concentrations for the Early Jurassic, Lower to Middle Cretaceous, and the Pliocene-Piei'stocene; the latter peak may be due to incomplete decomposition. Symbols and data are identical to those used in Fig. 8-3.

151

ORGANIC CARBON CONTENT IN BACKGROUND SEDIMENT FOR THE LAST 220 Ma

CorgN( [wt.%

IX

F,z

CARBONATE DEPOSITION

15 CN~ONAI~

~10 =<

A

A

~

+

g~ ~5

9

0 x

~0

5 |'

. m

~,

,,

"~x^

| "

~-=-^=

~

,:

,,

~o~

v

="

9

,=e=

|

eO~

@

~ 0 '.' MU0'0U' E0 'PA'hA'CA'"IC~,ALB'A~~Xv~fi' '~C~:BJ ~1 's=''RH J 50

0

0 DSDP/ODP

.~ J BAR 100

SITES

^ 329 Miocene [I 370 Neocom, Paleocene [3 398 A p t - Cenoman == 461 E o c e n e - M i o c e n e Thithon - Valange 463 Apt 511 L Jurassic - E. Oligocene 9 530 Turon - Pleistocene | 532 Pliocene - Quartemary 0 535 Benias - Cenoman

+ 593 Miocene x 540 AIb -paleogene 547 Jurassic, Cretaceous, Tertianj 4> 603 Neocome

IKI 150

DSDP/ODP

A PIPET 200 Ma

SITES

1638 Valange - Ptiocene ~,658 Pliocene - Pleistocene V 6 6 2 Pleistocene- Pliocene <~ 663 Pleistocene 9 664 Pliocene - Pleistocene E~721-722, 730-731 Pliocene Pleistocene Blue Lias, Britain 0 Pliensbach - Toarc, SW Germany V Apt - AIb, SE France z~ AIb - Turon, Central Italy Cenoman - Turon, U.S. Western Interior ,k Coniac - Santon, U.S. Western Interior

152

where Co~ is the organic carbon content, C the carbonate content, and CoaNS the orgamc carbon content of the nonelastic background fraction (all in wt. %). According to Eq. 8-10 and the numbers given in Table 8-2, standard equations for the four siliciclastic facies associations (St to Sly) can be derived, which give us the average organic carbon contents in semilithified to lithified rocks. Facies Association S~: Co,, [wt.%] = 0.18 C lower limit: Co"s [wt.%] = 0.01 C upper limit: Co~ [wt. %] = 0.5 C

(8-11)

Facies Association Su: Co',[wt.%] = 0.12 C lower limit: Co,, [wt. %] = 0.003 C upper limit: Co'~ [wt. %] = 0.3 C

(8-12)

Facies Association Sin: Co~=[wt. %] = 0.05 C lower limit: Co's [wt.%] = 0.001 C upper limit: Co'~ [wt.%] = 0.13 C

(8-13)

Facies Association S~v: Co~ [wt.%] = 0.02 C lower limit: Co'g [wt.-%] = 0.0007 C upper limit: Co~ [wt. %] = 0.06 C

(8-14)

We have seen that ranges of average organic carbon content can be forecasted for various siliciclastic and calcareous facies associations. Such ranges are broad, when background deposition is low; hence, the possible error in forecasting organic carbon content is relatively large for facies associations CI, CII and St, Sa, but small for associations with high background deposition (Cm, Cw and Sin, Srv). The siliciclastic system is generally characterized by higher organic carbon contents, but the carbonate system occurs more frequently. The silieiclastic system is essentially restricted to sediments between 0 and 35 % CaCO3, while the calcareous system occurs between 0 and 100% CaCO3, and, most commonly, between 35 and 100% CaCO 3 (see Fig. 5-4). Hence, the carbonate system plays a more important role in the overall distribution of organic carbon than does the clastic system; this is discussed further in the next section.

8.1.3 Average organic carbon contents in carbonate-rich and carbonatepoor sediments When forecasting Co~ contents using the above approach, one should also compare the results w~th average Co'= data reported in the literature. Because calcareous deposition is more common in many marine environments, rather than elastic deposition, only the carbonate system is considered here. According to Fig. 4-2, the average sedimentation rate for calcareous pelagic and shelf environments is estimated to range from 2 to 50 m/Ma (with a mean rate of 26 m/Ma). Assuming that these environments have typical mean carbonate contents of around 60%, an average background sedimentation rate of 10

153

m/Ma can be derived (Eq. 7-1). Using our calculated average organic matter supply of SOM = 0.7 m/Ma (see Chap. 8.1.2), the background organic carbon content is estimated at Co,,Nc = 2.1% (Eq. 8-3), which is in turn diluted by the carbonate fraction (Eq.8-'5). In shaly and calcareous sediments (with assumed carbonate contents of 15% and 85%, respectively), the estimated average organic carbon content amounts to 1.8% and 0.3%, respectively. These numbers are confirmed by average Co~ values reported in the literature.

AVERAGE Corg CONTENTS IN SHALES AND LIMESTONES

Corg carbonatepoor

carbonaterich

3 --__--_ 2- TW---- . . . . . ::~_---_-

H61-- >""~'S. "'""

i,...,.., H61 G V TW t"t72

I ""

~<- G

H72"1~/; 5 H61 V

CaC03

Fig. 8-9 Average organic carbon contents for carbonate-rich and carbonate-poor sediments as given in statistical investigations reL~orted in the literature. Small vertical bars show average Co,,-CaCO3 data from Uspenskii and Chernysheva (1951). Horizontal bars denote average or~ganiccarbon contents of various authors, and the assumed corresponding carbonate contents, as explained in the text. Authors: H61, H72: Hunt (1961, 1972); G: Gehman (1962); V: Vassoevich (1967); TW: Tissot and Welte (1984). All authors who have established large organic carbon data sets agree that limestones are substantially lower in Coa content than their shaley counterparts (e.g., Uspenskii and Chernysheva, 1951; Hunt, 1961, 1972; Gehman, 1962; Vassoevich, 1967; Tissot and Welte, 1984). No explanation is offered as to why this is the case, except by Uspenskii and Chernysheva (1951) and Hunt (1961), who show that the organic matter is concentrated mostly in the clay fraction, obviously related to carbonate dilution processes (see Chap. 3.3.3). Average organic carbon values reported by individual authors range between 0.9 and 2.2% Co,g for shales, and between 0.18 and 0.33% Co,g for carbonates (Table 8-3).

154

Table 8-3 Average organic carbon contents for shales and carbonates [wt. %] Authors

number of samples

shales

Uspenskii and Chernysheva (1951) Hunt (1961)

279 Paleozoic

1.1

0.7

0.3

1.65

-

0.18

1.2 0.9

-

0.24 0.2

0.99

-

0.33

2.16

1.9

0.67

1072 60 basins Gehman (1962) 1066 Vassoevich very large (1967) Hunt (1972) Tissot and 633 Welte (1984) soure rocks

calcareous shales

carbonates

If the Co~. difference between carbonate-rich and carbonate-poor sediments is solely an ~ffect of carbonate dilution, the average C.~ contents of the shale and corresponding carbonate samples examined by the d~fferent authors should each lie on a carbonate trend line. However, this is difficult to evaluate, be-

INTEGRATED CARBONATE-CLASTIC SYSTEMS LOW BACKGROUND DEPOSITION lZ ILl I-Z 0

....

:i:.!il :

HIGH BACKGROUNDDEPOSITION

Z 0 n~ n.,r CO

t O ne

_o Z < 0 nO

Iii

i.-::.-:::::"::: ::',

!ii..::?:ii~j " " IM. :iii i ! : i i i : : ~ : : i i i : i t'~ , , i~!i:i:ii:i:iO. -. ' 5 : : i :

.::. ::".'.

CARBONATE CONTENT

i

W d n-

L : i . : i O i~i i!i!il:

~../~.,~

o.._

O

CARBONATE CONTENT

Fig. 8-10 Interrelationship between carbonate and siliciclastic deposition for low and high background sedimentation rates, expressed by overlapping Con-CaCOs fields. Variation m organic matter deposition can be associated with e~ther the siliciclastie or calcareous system, restricting Co,s variations (about vertical arrows) to the upper and lower limits of the C~-CaC03 fields. Note that constant variations in the orgamc matter supply are represented by increasingly smaller Co~s variations towards the apexes of these fields.

155

cause authors have commonly not provided the corresponding carbonate contents by which the mean Co~ values were calculated. Only Uspenskii and Chernysheva (1951) presented the carbonate contents in their investigation of 279 Paleozoic to Tertiary rocks from the Russian Platform. They found a clear dependency on carbonate content, interpreted as a product of depositional CaCO 3 dilution (Fig. 8-9). Additionally, the C ~ data from the other authors may be interpreted as having been influenced by'carbonate dilution processes. In an attempt to draw a carbonate trend line through each shale-carbonate pair of avarage C,~z values (with the unknown, but corresponding CaCO3 contents estimated here) a symmetrical carbonate distribution was attained for carbonate-rich and carbonate-poor samples (e.g., 20%-80% or 10%-90% CaCO3, etc.; Fig. 8-9). The supposed average carbonate contents with their corresponding Cor~values, which each fall on one trend line, appear reasonable, with 10 to 25% CaCO3 for shales and 75 to 90% for limestones (see arrows in Fig. 8-9). All these findings suggest that changing carbonate deposition is a major process controlling the average organic carbon content. Carbonate deposition is responsible for a major inverse relationship between the carbonate and organic carbon contents found in shaley to calcareous sediments and rocks.

8.2 The formation of bioturbated and laminated lithotypes 8.2.1 Expression of varying organic matter supply It was shown in the previous section that the sedimentary organic carbon content is determined by the interaction of various factors, such as background deposition, organic matter flux, and deposition of the main fraction, all of which control the size of the Co,g-CaCO3 fields (see Fig. 8-5). These fields are inversely correlated with the amount of background deposition, and are defined by lower and upper calcareous or siliciclastic trend lines. When expressed in one diagram, the carbonate and clastic dominated Co~-CaCO3 fields overlap each other (Fig. 8-10). Such overlapping is asymmetric, because the organic matter input in the elastic system is 5 times higher than that in the carbonate system and more restricted to lower CaCO3 values; whereas the commonly occurring carbonate system, on the other hand, occurs over the entire CaCO3 range. In the calcareous system, the Co~.-CaCO3 fields are triangular, with one apex in the lower right corner of the coordinate system 0.e., 100% CaCO~ / 0% Cor~). Changes in the organic matter supply are expressed by near-vertical variations within these fields parallel to the C ~ axis (the y axis, Fig. 8-10). Hence, changes in organic carbon content (verttcal lines) are largest for low CaCO3 contents, become smaller with increasing CaCO 3 content, and are smallest immediately in the right-hand apex of the Co,~-CaCO3 triangle. Thus, an identical variation in the organic matter supply may be expressed either as a large or a small C~_ variation, depending on whether the carbonate content is low or high. For cl~stic deposition, the shape of the triangle is inverted. One apex coincides with the origin of the coordinate system, such that larger

158 organic carbon variations occur with higher carbonate contents. The upper and lower boundaries of the Co~-CaCO3 fields determine the magnitude of organic carbon variations (Fig. 8-1"0. The Corn variations on the near-vertical lines in the diagram are associated with, and controlled by, various calcareous and siliciclastic trend lines. The dominantly carbonate or elastic styles of deposition are superimposed by concurrent variations in the organic matter supply. Hence, the magnitude of the Con variations depends on the background deposition controlling the size of the Con-CaCO3 fields, and the carbonate content controlling the amount of dilution by the main fraction.

ORGANIC CARBON DEPOSITION AS RELATED TO SILICICLASTIC OR CARBONATE Corg-CaCO 3 TRENDS SILICICLASTlC DEPOSITION DSDP SITES

FZ LU

o

--4

Is.

46~

tJi'

CALCAREOUS DEPOSITION

lip

]

TO.ARC, SW GERMANY

- - * - - ~ M ~ ' ) I L ~ A N I . T I IICle'~kl I I

~603 - x - 651

9

Z

o I I I

5"

0

iJl

J,

50 100 CARBONATE CONTENT [wt.%]

)

CARBONATE CONTENT [wt,%]

Fig.: 8-11 Examples of 9rsan!c matter variation (approximately vertical lines) associated w~m catcareous aria SlllClClaStlCt~o,~-t;aCU3 trend hnes. Diagrams summarize data trends from sequences with different background deposition rates. Organic carbon variations become generally smaller with increasing CaCO3 content for carbonate deposition (see Fig. g-to).

157

8.2.2 The slopes of Co~-CaCO3 trend lines: indications of bottom water oxygenation? The above arguments show that variations in organic matter supply are expressed by vertical shifting from one carbonate or siliciclastic Co,~-CaCO3 line to another (see Figs. 8-10 and 8-11). Each of these differently sloping lines represent an equal amount of organic matter supply for constant background sedimentation (i.e., one of facies associations C~ to Cw or S~ to Sw). Steeply sloping Co_-CaCOa lines indicate a high rate of organic matter deposition, while flatly~ sloping lines indicate the opposite (see Fig. 2-3). The following discussion is focused dominantly on carbonate deposition.

TRENDS OF INCREASING BIOTURBATION CARBONATE DEPOSITION 2 FOR EQUALDEGREESOF BOTTOMWATER OXYGENATION,VARIOUS

,

"K

INTENSITIES O F

B,O R+AT,ONARE

ASSOCIATEDWITH DIFFERENTORGANIC CARBON CONTENTS,

INFLUENCINGTHE

% ~ , k upper ' \ ~'6 ~- \timit " \ ~ ~"O.k

_

Z

2

O /~ ~ ~ '~ ~r ~

9 ~&~'\

",,X'O\ ,-00,

COl " ~ ? ~ i ~ -'o6~~

OXYGENCONTENTOF COl THE POREWATERS. ~ 1 ' ' - .

\

"\ 1 k ~ \

~ f ~ ,

\

\

~

1 ANGLEOF SLOPEREPRESENTS LARGELYAMONTOF ORGANIC MAT]'ER DEPOSITION. THIS IS ROUGHLY RELATEDTO BOTTOM WATEROXYGENATION WHICH IN TURN CONTROLS THE DEGREEOF BtOTURBAT~ONAND THE DISTRIBUTION OF BENTHICEPIFAUNA. SNC = COnstant

I~ limit

CARBONATE

Fig. 8-12 Principal bioturbation trends for aerobic to anaerobic sediments with carbonate deposition. For a given environment and background deposition, the primary increase in bioturbation (large arrow, 1) is related to increasing bottom water oxygenation, expressed in decreasing organic matter deposition, which in turn is associated with more flatly sloping Co,g-CaCO3lines. For a given aerobic or dysaerobic bottom water, a smaller increase in bioturbation (small arrow, 2) is related to decreasing organic matter content in the sediment, reflecting increasing oxygen in the pore waters of the surface sediment. This trend occurs parallel to carbonate dilution lines, towards rising CaCO3 and declining Co,s contents. Differently sloping carbonate trend lines, represent different rates of organic matter flux. As outlined in Chap. 3.3.1, such flux changes are a function of both changing primary productivity and different degradation degrees of organic matter, related to various degrees of bottom water oxygenation (Demaison and Moore, 1980; Calvert and Pedersen, 1992). In Jurassic and Cretaceous epicontinental sea settings investigated here, the organic matter flux seems to be controlled more by changing bottom water oxygenation than by changing

158

productivity (Demaison, 1990; Paropkari et al., 1992, 1993). This can be inferred from the conditions in the present shelf seas, where long-term primary productivities vary between 200 and 400 gCor,m'Za"~, thus by a maximum factor of 2 (Romankevich, 1984; Berger et al., 1988). But, according to the data provided by Bralower and Thierstein (1984), the preservation potential from oxic to anoxic conditions in these environments varies by more than a factor of 10. Consequently, for epicontinental sea sediments, steeply sloping carbonate trend lines are thought mainly to indicate more preservation, and thus, less oxygen in the bottom waters; while flatly sloping lines are thought to denote the opposite. This view is strengthened by the lesser degrees of bioturbation observed in sediments with st~ply sloping Cm-CaCO~ lines. In an attempt to determine bottom water oxygenation as a function of the calcareous dilution line slopes, the bioturbation patterns associated with these lines were evaluated for various epeiric sea settings. This investigation involved a variety of strata, including marl to limestone sequences, shales, and black shales, such as the Liassic in southwestern Germany and the Upper Cretaceous Greenhorn and Niobrara formations of the Western Interior Seaway (i.e., foreland basin epeiric in character, see Chap. 6). The intensity of bioturbation was scaled as discussed by various authors in the literature (Droser and Bottjer, 1986; Savrda and Bottjer, 1986); here, seven bioturbation levels were used, as applied by Kauffman and Sageman (1990) for various members of the Greenhorn and Niobrara TR cycles: (1) (2) (3) (4) (5)

No bioturbation, very well laminated sediment; microbioturbation, millimeter-scaled disturbances of strata; rare subcentimeter horizontal burrows; scattered subcentimeter burrows such as Planolites and Chondrites; scattered to common subcentimeter to centimeter-sized burrows including Chondrites, Planolites, and Thalassinoides; (6) strongly burrowed sediment fabric with discrete burrows visible, including Chondrites, Planolites, and Thalassinoides; and (7) completely homogenized sediment fabric. As indicated in Figs. 8-12 and 8-13, two principal bioturbation trends can be observed in the studied rocks. 1. Bioturbation increases with decreasing carbonate dilution line slopes. As outlined above, these slopes represent different amounts of organic matter supply and are thought to predominantly indicate different levels of bottom water

Fig. 8-13 Assessment of bottom water oxygenationfor various epicontinentalsea to fore- I~ land basin settings (Lower Jurassic, Germany;Upper Cretaceous Greenhorn and Niobrara cycles, U.S. Western Interior). Stratigraphie intervals with their carbonate dilution lines are plotted on Co~-CaCO3 diagrams (lower graphs). Bioturbation--derivedoxygenation connects different background organic carbon levels (Co,,Nc)according to the different rates of background deposition (snc, in m/Ma; upper graph). Carbonate deposition. Numbers 1 to 6 represent increasingdegrees of bioturbation as explained in the text. Note that bottom water oxygenationfields become narrower with increasing background sedimentation rate.

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Fig. 8-13

160

oxygenation; flat slopes denote aerobic bottom waters, whereas steep slopes indicate the opposite. 2. The intensity of bioturbation mottling also changes along individual Co~F CaCO3 lines. For aerobic to dysaerobic bottom waters, bioturbation increases when sediments contain smaller amounts of organic carbon (and larger carbonate contents). This second bioturbation trend is interpreted as an effect of

BOTTOM WATER OXYGENATION

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SEDIMENTATION RATE BACKGROUND SEDIMENT sNC [mlMa]

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Fig. 8-14 Tentative trends of aerobic, dysaerobic, and anaerobic bottom waters for the four calcareous facies associations, Cj to Cry, as a function of the organic carbon content (C c) and sedimentation rate of the background sediment (~c, upper graph), and of Co~C a ~ 3 contents (lower graphs). F_,picontinentalsea setting. Boundartes between the variou's subfields represent transitional zones. Assessment of bottom water oxygenations is performed using the background deposition, or facies association, ano the Co,8-CaCO3 contents for a gtven lithified sediment.

161

changing redox conditions in the pore water rather than in the bottom water, because the pore water oxygen content is influenced by the organic carbon content of the sediment. Burrowing activity becomes restricted when the sedimentary organic carbon content is concentrated by declining carbonate deposition. Consider a medium-sloping carbonate line delineating dysaerobic bottom waters. The maximum possible degree of bioturbation for dysaerobic bottom waters, restricted burrowing, occurs when the organic carbon content is low and carbonate content high. When carbonate deposition decreases, the sediment composition shifts along the Corg-CaCO3 line towards increased organic matter concentration, so that the pore waters become so diminished in dissolved oxygen (as an effect of aerobic oxidation, Curtis, 1980) that it is difficult to impossible for the infauna to penetrate and live in the sediment. Thus bioturbation declines, and a sharp interface develops at the sediment surface, separating nearly anoxic pore waters from the overlying dysaerobic water mass (Sageman, 1989; Savrada et al., 1991). The resulting sediments are therefore laminated, carbonaceous shales with virtually no bioturbation; but they may contain shelly epifauna adapted to low oxygen conditions, as observed in many black shale sequences (Kauffman, 1982, 1988; Wignall and Myers, 1988; Sageman, 1989; Sageman et aI., 1991). Some of these shelly faunas, "flat clams", seem to reflect bacterial chemosymbioses of the "exaerobic zone" (Savrda and Bottjer, 1987, 1991; Kauffman, 1988; Savrda et al., 1991). In the following, five examples of epicontinental basin sequenes are investigated in order to tentatively assess the degree of bottom water oxygenation using the different bioturbation levels associated with the various calcareous dilution lines in these sequences (Fig. 8-13). This assessment is difficult because of the existence of the two overlapping, principal bioturbation trends mentioned above in Fig. 8-12. As a consequence of these overlapping bioturbation trends, only that portion of the calcareous dilution lines were used for assessing bottom water oxygenations denoting low organic carbon and high carbonate content, where the degree of bioturbation is thought as largely reflecting bottom water oxygenation (i.e., the basic bioturbation trend 1). Under these conditions, fully bioturbated sediments indicate aerobic bottom waters ( > 1mg/l dissolved O2) , while restricted and nonbioturbated sediments represent dysaerobic (ca. 0.2 to 1 mg/l dissolved O2) and anaerobic bottom waters ( < 0.2 rag/1 dissolved Oz), respectively (Rhoads and Morse, 1971; Arthur et al., 1984; Thompson et al., 1985; Savrda and Bottjer, 1986, 1987, 1991; Savrada et al., 1991; Sageman, 1989; Sageman et al., 1991). The following attribution of bottom water oxygenations to the seven bioturbation levels mentioned above (Kauffman and Sageman, 1990) is based on epeiric sea settings with carbonate deposition which represent the facies associations C~ and C a . Aerobic bottom waters: With low organic carbon content, below 1%, the level of bioturbation lies between 5 and 7 (on Kauffman and Sageman's scale from 1 to 7), while for higher organic carbon content, above 1%, bioturbation is around 4. Dysaerobic bottom waters: When the organic carbon content is below 1%, burrow mottling has a degree of 4, while for higher Co~gcontents of 2 to 4%

162

Corg-CaCO 3 FIELDS FOR SILICICLASTIC FACIES ASSOCIATIONS CorgNS ~z so i~ ~IUCICLASTIC I EPOSlTION I

ffl 7

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Fig. 8-15 Model for siliciclastic facies associations (S~ to S~v), related to organic carbon content (C~,~s), the sedimentation rate of the background sediment (SNs, upper diagram), and C~-CaCO~ fields (lower diagram). Theoretically, Cm-CaCO3.fieldsspan the entire carbonate range, but in many environments, siliciclastic oeposition has a L:aCO3 range or 0 to 40%.

the bioturbation level declines to 2 or 3, or even to 1 when the sediment contains higher C~g values o f more than 4 to 5 %.

t63

Anaerobic bottom waters: Here the degree of bioturbation is 1, independent of whether the organic carbon and carbonate content are high or low. These methods connecting Co~-CaC03 trend lines with bioturbation patterns allow a rough estimation of boftom water oxygenation for the five investigated examples of epicontinental sea environments (i.e., Posidonia Shale and litholo-

LAMINATED TO B I O T U R B A T E D L I T H O T Y P E S C A L C A R E O U S FACIES A S S O C I A T I O N C I Z I-- 14 0 z rnUJ rr~12 < O0

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0 0

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['7-]

20

40

60

80

RESTRICTED

BIOTURBATION

100

CARBONATE CONTENT

8,OTU aAT1ON

Fig. 8-16 Idealized pattern of bioturbation and bottom water oxygenation for calcareous facies association CI, expressed in a Con-CaCO3diagram. Epieontmentalseas. Anaerobic bottom waters are characterized by laminated sediments, and dysaerobic bottom waters by laminated, microbioturbated, and restricted bioturbated sediments with some shelly vpifauna. Aerobic bottom waters show incompleteto complete bioturbation. Boundaries betwren fields indicate transitional zones.

gies from the Greenhorn and Niobrara formations; Fig. 8-13). Using the above assigned bottom water oxygenation levels, oxygenation fields were established in the various Co~s-CaCO3 diagrams, expressed by differently sloping carbonate lines. With increasing background sedimentation, these oxygenation fields become smaller (see the Co~,-CaCO3 diagrams with increasing Ssc values from left to right, Fig. 8-13). Depending on the background deposition rate (s~), the various boundaries between the oxygenation fields are transformed into the corresponding rates of organic matter supply (Fig. 8-13, upper diagram). As

164

a result, the different organic matter supplies can be ap-proximately related to different oxygenation levels. The following rates of organic matter deposition were estimated (Fig. 8-14) from the conditions depicted in 8-13: Aerobic bottom waters have rates of organic matter deposition from 0.05 m/Ma to approximately 0.2 m/Ma (0.04 to ca. 0.15 gCo,,mZa~). Dysaerobic bottom waters have rates of ca. 0.2m/Ma to approximatel~ 0.8 m/Ma (ca. 0.15 to ca. 0.6 gCo,,m'2al), while anaerobic bottom waters have rates between approximately 0.8in/Ma and 1.5m/Ma (ca. 0.6 to 1.15 gCo,~m"aL). These values represent rates of solid organic matter deposition obtained for diagenetically altered, lithified sediments in epeiric seas in the carbonate system. A lithified sediment previously deposited in an environment representing the transition from dyserobic to anaerobic waters (with a diagenetically-reduced C _ flux of 0.6 gmZa~) would show a preservation factor of 0.3%, if a forme-*r primary productivity of 200 gC_m2a ~ is assumed. Compared to the present surface sediments, this valuV~ is approximately one order of magnitude lower than the preservation factor given by Bralower and Thierstein (1984); in order to explain this discrepancy one has to assume an additional organic matter decomposition of approximately 90% below the surface sediment, as observed by Emerson (1985), Jumars et al. (1989), Stein (1991); see Chap. 3.2.1. A similar diagram as in Fig. 8-14 can be established for siliciclastic deposition (Fig. 8-15). Here, subdivision of bottom water oxygenation fields was not performed due to lack of data. A synthesis of these two factors, bottom water oxygenation and bioturbation pattern, leads to a basic diagram combining sedimentary structures, Com-CaCO3 contents, and oxygenation levels (Fig. 8-16). Bottom water oxygenation is expressed by triangle-shaped subfields (i.e., aerobic, dysaerobic, and anaerobic; Fig. 8-14), while the superimposed bioturbation patterns show four main classes, bioturbation, restricted bioturbation, microbioturbadon, and lamination (Fig. 8-16, facies association CO. As the facies association shift from Ct to Ca, etc. and as background deposition increases, the CO,,-CaCO3 fields and the bioturbation patterns become narrower and more restricted to lower Co,- contents. Obviously, many different facies (sub)types are represented ti'y the various Co~-CaCO 3 fields with their different bioturbation levels. This aspect is used to develop the concept of lithotypes outlined in the next section.

8.2.3 Laminated to bioturbated lithotypes with low to high carbonate and organic carbon contents In this book sediments are expressed as genetically-related systems, as described below and depicted in Fig. 8-17: 1. Facies type: A facies type represents a given lithology which is developed in environmental systems with carbonate or siliciclastic deposition. A given facies type is defined by a characteristic rate of background deposition and average carbonate content of a stratigraphic interval (Chap. 7.1.2). 2. Facies association: A facies association represents a succession of related facies types that have the same range of background deposition (Chap. 7.1.2, Figs. 7-5 and 7-6).

165

3. Lithotype: A lithotype is a carbonate or siliciclastic facies type with a defined organic matter content. Each individual facies type in one facies association contains various lithotypes with different Cot~ contents and bioturbation patterns. 4. Lithotype association: A lithotype association represents a genetically connected succession of iithotypes that are formed by varying carbonate and by varying organic matter deposition.

FACIES- LITHOTYPE CONCEPT ZI.U

I-Z UJ

5 'CO "

FACIES ASSOCIATION

I CARBONATE

'

Utho-

I

CARBONATE

types CONTENT Fig. 8-17 Schematic interrelationship between facies association, facies type, lithotype, and lithotype association, expressed in carbonate - sedimentation rate and carbonate organic carbon diagrams. Definitions in text. In the following, the formation of various lithotypes is addressed, as they form bioturbated to laminated sediments, which are both rich to poor in carbonate and organic carbon contents, representing different degrees of bottom water oxygenation (Fig. 8-18). Consider the various lithotypes in the field representing anaerobic bottom waters (calcareous facies association Cx, Fig. 8-18). All the lithotypes in this field lack bioturbation and epibenthos, generating a suite of laminated sediments with decreasing Co,- but increasing CaCO3 content, such as black shale, laminated organic carbon'-rich marl, and laminated limestone. Hence, black shales with high C.~ contents and laminated limestones with low Co~ contents belong to an anoxic lithotype association, representing increasing amounts of carbonate deposition. A more complicated pattern is found for dysaerobic bottom waters because the suite of lithotypes, formed by increasing carbonate

166

deposition, is characterized by different styles of bioturbation. The resulting lithotypes are laminated to slightly mottled, including shales and black shales with a restricted epifauna, organic carbon-rich marls with micromottling, and organic carbon-poor limestones with micromottling to restricted mottling (Fig. 8-18). Aerobic bottom waters contain a. succession of shales and limestones

LAMINATED TO BIOTURBATED LITHOTYPES CALCAREOUS

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ASSOCIATION

CI

14

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Fig. 8-18. Laminated to.bioturb.ated .lithotypes. of calcareous facies association Ct, as ooserv~a in eplcontmentaJ seas. one aegree of bottom water oxygenation is represented by various lithotypes. Note the various types of laminated to weakly bioturbated liraestones, which are the carbonate-diluted counterparts (with identical oxygenation) of different shales rich to poor in organic carbon content. When facies associations shift from Ci to Ca, etc. the various fields have smaller organic carbon ranges (see Fig. 8-14). that are relatively organic-carbon poor, contain epibenthos, and show restricted to normal bioturbation. Very conspicuous are the different laminated to bioturbated lithotypes obtained with high carbonate contents. Here, the various lithotypo fields are spaced so closely that small variations in orl~anic carbon or carbonate content are sufficient to create shifts from one hthotype to another (Fig. 8-18). However, one must keep in mind that these small C a shifts require the same

167

variation in organic matter supply as for.large Co~gshifts at low carbonate contents (see Chap. 8.2.1, Fig. 8-10). Similarly, small CaCO 3 shifts, when carbonate contents are high, require large changes in the carbonate supply as previously discussed in Chap. 5.4.2, Fig. 5-9. The lithotype concept proposed above shows that it is difficult establishing simple relationships between the meaning of bioturbation styles, the expression

REDOX AND PRODUCTIVITY RHYTHMS LAIBSTEIN BEDS POSIDONIASHALE

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MIDDLE CHALK --NIOBRARA

O=,o-O MIDt~E CRETACEOUS

0

.,

~ ~.

BRIDGE CREEK LIMESTONE GREENHORN

CARBONATEtCONTENT1 UPPER SHALE AND UMESTONE MEMBER NIOBRARA

NUMISMALIS MARL PUENSBACHIAN

FORT HAYS UMESTONE NIOBRARA

Fig. 8-19 Distinction between productivity (carbonate productivity) and redox rhythms as indicated by either inverse or near vertical C~t-CaCO3 trends; values, corrected for differential carbonate diagenesis, are expressed on the lithotype diagram for calcareous facies association C~(see Fig. 8-18). Note that both tyl~s of rhythms show different bioturbation styles between beds. Dashed Co,s-CaCO3 trends of the Fort Hays Limestone and the Shale and Limestone Member belong actually to tacies association Cn with smaller Co,=-CaCO~ distribution fields (see Fig. 8-14). Further explanation in text. of bottom water oxygen contents, and the use of organic carbon contents as environmental indicators. This is discussed in items 1. and 2. below: 1. There is no simple relation between bottom water oxygenation and its expression in the corresponding sediment. Equal amounts of bottom water oxygenation are represented by different lithotypes, expressed in both different Co,g-

CaCO3 contents and different bioturbation styles. 2. For a given calcareous facies association (C 1 to CIv), the various lithotypes are defined by both the organic carbon and carbonate contents. That both of these parameters are used here to determine the different lithotypes is the principal difference with the various earlier approaches described in the literature, which attempt to relate oxygenation and associated bioturbation styles only to the organic carbon content (e.g., Kauffman and Sageman, 1990). Two samples with identical organic carbon contents, one with a low, the other with a high carbonate content, represent different bottom water oxygenation and bioturbation patterns. Hence, only using the organic carbon content to estimate the degree of oxygenation is inadequate. Principal transitions from one lithotype to another occur either along carbonate dilution lines or parallel to the organic carbon axis (i.e., near vertical). When along carbonate dilution lines, variations in carbonate deposition are represented, whereas when parallel to the C.~. axis organic matter deposition is indicated. Of course, combinations betweez{ these types occur; however, it is essential that both styles of deposition are related with changes in the level of bioturbation. When the supply of either organic matter or carbonate oscillates, bedding rhythms are generated, which develop alternating bioturbation patterns between beds, as documented by the various examples in Fig. 8-19. Only rhythms with variations in the organic matter supply (i.e., vertical data trends in the Co,,-CaCO3 diagram) are considered true redox rhythms. In these examples, alternating high and low degrees of burrowing usually represent true changes in bottom water oxygenation. In contrast, carbonate productivity rhythms (e.g., Fischer et al., 1990; Einsele and Ricken, 1991) are related to varying carbonate production in the photic zone, but are little affected by changing bottom water oxygenation (see Chap. 5.2; Ricken, 1993). Oscillating lower and higher inputs of CaCO3 lead to higher and lower organic carbon contents in the sediment, which in turn influence the oxygen content in the pore waters of the surface sediment, thus controlling the capacity of the infauna to penetrate the sediment (see Chap. 8.2.2). The conspicuous bedding rhythm of the Fort Hays Limestone, for example, where Savrda and Bottjer (1986) determined changing degrees of oxygenation based on changing bioturbation patterns, is such a diagenetically overprinted productivity rhythm with an inverse, straight-line C,,~-CaCO3 relationship (Figs. 8-19 and 5-5). Here, the changing style of bioturbation seems dominantly to reflect changing oxygenation in the pore water of the surface sediment instead in the bottom water (see Chap. 5.3.1). We have seen from the previous discussions that no simple relationship can be observed between water mass oxygenation, bioturbation pattern, and organic carbon content. Instead, a system is introduced in which the bottom water oxygenation in various epeirie sea settings is represented by different slopes of calcareous Con~-CaCO3 dilution lines. But, as these slopes also depend on the type and amount of background deposition, bottom water oxygenation can only be evaluated when the interrelation between carbonate - organic carbon content and facies association is taken into account.

169

8.3 Carbonaceous iithotypes controlled by background deposition As shown in Fig. 8-20, different rates of background deposition not only define different calcareous facies associations, but also determine the range of sedimentary organic carbon content. This range is different for facies associa-

L I T H O T Y P E S - R E L A T I O N TO F A C I E S A S S O C I A T I O N S ANAEROBIC

BOTTOM WATER OXYGENATION

ORGANIC CARBON CONTENT

DYSAEROBIC

AEROBIC

black shale, no epJbeflthos

\

org. carbon-rich laminated mad, laminated shale no epibenthos, tO black shale, orna=niccza~on-dch restricted ~ =~'" ,~. epibenthos '~Pmad restricted b~on ~ L . and epibenthos, org. carbon-rich_=limestone, laminated ~ t o microbioturbated, shale, restricted low tO no epibenthos bioturbation~mad, bioturbated, " ~ epibentlx~ epibenth~ bi~um~ed. --I~ CARBONATE CONTENT

SPAN OF ORGANIC

CARBON CONTENTS

10 ORGANIC

e_~Bc~ CONTENT CAI~ONATECONTENT

FACIES ASSOCIATIONS

TAT1ON RATE CARBONATECONTENT

Fig. 8-20 Interrelationship between lithotypes and facies associations. Schematic representation of the main lithotypes for the carbonate system, expressed in a Co~z-CaCO3graph (upper diagram). Lower diagrams demonstrate how the Cm-CaCO3 fields with these lithotypes become narrower from facies association C~ to Ctv. Whether a calcareous facies transition is expressed by lithotypes along a flatly sloping or a steeply sloping dilution line, is interpreted here as largely a consequence of changing bottom water oxygenation. tions with low or high background sedimentation, due to an organic carbon threshold effect, as shown in Fig. 8-14: Low background deposition (facies associations CI and C~1) is associated with large Co~- spans, while high background deposition (facies associations Cm and Cry) i~ associated with small C spans, resulting in two major groups of lithotype associations (Fig. 8-21). ,o7 each of these two groups, the combination of the effects of oxic or anoxic bottom water with low or high degrees of carbonate dilution generates four

170

lithotype end members. These end members form the "black shale" and the "plattenkalk lithotype associations', named after the most conspicuous anoxic lithotype in each association.

8.3.1 The black shale and the plattenkalk lithotype associations The black shale iithotype association (C~, CO is characterized by relatively organic carbon-rich, laminated to hioturbated shales to limestones, deposited in environments with extremely low background sedimentation rates (Fig. 821). Hence, changes in organic matter supply have a large effect on organic carbon content. For low carbonate deposition rates the lithotypes may have either low or high organic carbon contents, resulting in shales with restricted bioturbation or laminated black shales, respectively. When carbonate deposition is high, laminated to bioturbated limestones are generated with relatively low organic carbon contents. Note that true, Cor~-rich black shales represent only one lithotype, which combines anaerobic bottom waters with low concurrent carbonate deposition. The black shale lithotype association characteristically shows interlayered lithologies, formed by simultaneous changes in redox conditions and the supply of carbonate. Examples are the black shales to limestones of the FrasnianFamennian Kellwasser Event (Europe and North Africa), the Toarcian Posidonia Shale (central Europe) and the organic-carbon rich intervals of the Cenomanian to Santonian Greenhorn and Niobrara Cycles of the Western Interior Basin (see Figs. 6-1, 6-5, and 6-6; e.g., Hattin, 1981; Pratt, 1984; Arthur et al., 1985; Jenkyns, 1985; Riegraf, 1985; Riegel et al., 1986; Sageman, 1989; Eicher and Diner, 1989, 1991; Brumsack, 1991; Buggisch, 1991; Hudson and Martill, 1991; Littke, et al., 1991; Oschmann, 1991; Prauss et al., 1991). All these sequences contain typical black shales, deposited under anaerobic and dysaerobic conditions, but also laminated to bioturbated, organic carbon-poor limestones, representing an identical amount of oxygenation, but an increased, concurrent supply of carbonate (e.g., the primary sediment of the concretionary Laibstein beds, Posidonia Shale; the Fort Hays Limestone, and the Middle and Upper Chalk member, Niobrara cycle; see Fig. 8-19). In addition, bioturbated marls to limestones form in periods with more oxidized, but fluctuating, bottom waters (e.g., the limestone beds of the Bridge Creek as well as the Upper Shale and Limestone member, Greenhorn and Niobrara cycles, respectively). Not only do sediments of epicontinental seas belong to the black shale association, but also pelagic, carbonaceous strata of the "oceanic anoxic events", characterized by expanded oxygen minimum zones and low rates of deposition (Schlanger and Jenkyns, 1976; Jenkyns, 1980; De Graciansky et al., 1984; Schlanger et al., 1987; Arthur et al., 1987, 1990). Very typically, many of the Aptian-Albian carbonaceous sequences of the North Atlantic Ocean are interlayered, often on the scale of beds, with bioturbated to non-bioturbated intervals poorer to richer in carbonate content (Jenkyns, 1980; Weissert, 1981; Summerhayes, 1981; Southam et al., 1982; Arthur et al., 1984, 1988; Herbin et al., 1986). Changing carbonate dilution may be one reason why some pa-

171

ckages of organic carbon-rich facies can not be precisely correlated between different basin parts (Waples, 1983). The plattenkalk lithotype association (Cm to C w) is characterized by laminated to bioturbated shales and limestones from environments with relatively high background sedimentation (Fig. 8-21). Consequently, organic carbon con-

BLACK SHALE AND PLATTENKALK ASSOCIATIONS BACKGROUND DEPOSITION low (C I' C II )

c~

B A C K G R O U N D DEPOSITION high {S III'S IV )

L--'1 Cad:tonal,.

Con~nt

BOTTOM_WATER OXYGENATION

~ohic

~a~'obic

resll'ic~d bioturbe, ted, laminab~, laminated, bk~Jrbation, Corg-poor, veryCorg- medium medium Co.j, =l~a/ma eO/~rm

r~

Corg

BLACK SHALE UTHOTYPE ASSOCIATION

B O T I ' O M ) N A T E R OXYGENATION aeFobie

shale, limestone, bioturbated, bioturbamd,

epifauna

an~bbic

shale, laminatad, rr.gtktm to

poor Corg

limestone.

(plat~nkalk)

PLA [ie:NKAU( UTHOTYPE ASSOCIATION

Fig. 8-21 The black shale and the plattenkalk lithotype associations, related to either low (Cx, Ca) or high background deposition (Cm, Cry), respectively. For each association, four lithotype end members are generated by the combination of aerobic or anaerobic bot-tom waters with either low or high carbonate deposition. The black shale lithotype association is composed of sediments with a wide range of organic carbon content, including true black shales, shales, and laminated to bioturbated limestones, The plattenkalk lithotype association is composed of sediments with a narrow range of relatively low organic carbon content, including laminated to bioturbated shales, and lithographic to bioturbated limestones. Note that true black shales are only generated under conditions of low background deposition, anaerobic bottom waters, and low carbonate dilution (further explanations see text).

tents are generally low, which leads to the deposition of sediments with more equal, and smaller, CO_ variations, compared to the black shale association. For anaerobic or dysa'~robic bottom waters, the plattenkalk lithotype association is comprised of laminated shales to laminated lithographic limestones with low Cot= contents (i.e., the "plattenkalks"). In oxic bottom waters, Corf poor bioturb/lted shales and limestones are formed. All these different litho-

172 types are found in highly-sedimented, oxic to anoxic environments, such as enclosed basins of all sizes, as well as outer shelf and slope environments (see Chap. 7. 1.2).

BLACK SEA PRIMARY PRODUCTIVITY

PLATTENKALK UTHOTYPE ASSOCIATION

Corg [wt.%]PISTON CORE 1474 20

.I

15111

SAPROPEL EARLY HOLOCENE

lO-! 9

III

, o . 3.0 l ~ o

=. l o r----lo.3s .o s----] o.,s- o=s

:t

=

DailyPrimaryProduction(gC/m;~)

ORGANIC CARBON CONTENT N

~

u

LAMINATED COCCOUTH MARL SNC = 12.8m/Me PRESENT

CARBONATE [wt.%]

13-4g4.sll.s

Fig. 8-22 Organic carbon - carbonate sedimentation in the Black Sea. Left side of figure shows the primary productivity of the Black Sea (after Izdar et al., 1987), and the organic carbon distribution of the surface sediments (Shim'us and Tdmonis, 1974). Right side of figure shows sediment history of the last 10ka. In the early Holocene, a sapropel was deposited with Co~, contents of up to 20% (solid symbols), probably related to facies association C[ or Sv Under present conditions, laminated coccolith marls and oozes with medium organic carbon contents are deposited (light symbols). High noncalcareous backg.rouod deposition of ssc = 13 m/Ma (C o denotes the plattenkalk hthotype association for me present situation. Co,~-CaCO3 data from Degens and Ross (1974).

173

Examples of the plattenkalk iithotype association: Solnhofen Limestone and the Black Sea The small, inter-reef basins of the laminated Soinhofen Lithographic Limestone (Lower Tithonian, southern Germany) may be considered a typical example of the plattenkalk lithotype association, where calcareous deposition is superimposed with a main sediment which has an even higher carbonate content (Hemleben, 1977; Keupp, 1977; Barthel et al., 1990). The whole Lower Tithonian limestone sequence (Zeiss, 1977), which contains, among other lithologies, the Solnhofen Limestone, indicates an extremely high average sedimentation rate of 130m/Ma. However, Barthel (1978) has estimated that the Solnhofen Limestone shows even a higher rate of deposition of about 180 m/Ma. The average carbonate content of the Solnhofen Limestone is 94% (carbonate-rich beds with 95-98 % CaCO~, carbonate-poor beds with 77-87% CaCO~, which represent 15% of the total rock mass, Barthel et al., 1990). The resulting noncalcareous background sedimentation amounts to Ssc = 11 m/Ma (Eq. 7-1), which suggests the existence of calcareous facies association Clu. Accordingly, the organic carbon content is relatively low, at 0.9% in the carbonate-poor beds and 0.2 to 0.4% in the carbonate-rich beds (Hiickel, 1974). These values represent the anoxic bottom water field for facies association Cm (see Fig. 8-14). Authors have reported that some lithographic limestone settings show transitions to organic carbon-rich sediment (Barthel, 1976; Hemleben and Swinburne, 1991). Such transitions are consistent with this theory when carbonate deposition declines with persisting anoxic conditions. Under such circumstances the sediment shifts along a carbonate dilution line towards higher organic carbon and lower carbonate contents (Fig. 8-21). The Black Sea, considered the schoolbook example of an anoxic, stagnant basin with free H2S in the water column, represents in its central part the plattenkalk lithotype association. The Black Sea has a productivity distribution typical of ocean basins, with high primary productivities in the coastal zones and low productivities in the central parts of the basin (Fig. 8-22). Despite this, the sedimentary organic carbon distribution shows an inverted pattern, with the highest Co~. contents in the middle of the basin, due to a siliciclastic wedge transition frdm highly sedimented marginal zones towards Cor.-richer chalky sediment (Ross et al., 1970; Sorokin, 1982; Glenn and Arthur~ 1985; Degens, 1989; Stein, 1991; see Chap. 7.4.1). In a vertical profile (piston core 1474, central basin; Degens and Ross, 1974), the Holocene sediment is a laminated, coccolithic marl with a Co,~ content of only 4 to 5% and an average carbonate content of 50%. Such a surprisingly low organic carbon content can be explained not only by nannoplankton calcite dilution, but also by the relatively high background deposition of 12.8 m/Ma (PC 1474), restricting the possible Co,g range to intermediate values (calcareous facies association Cm, see Fig. 8-1~). 9Lack of highly carbonaceous sediments in the Black Sea shows that Co,.-rich black shales are only formed when three conditions are fulfilled: Low background deposition, anaerobic to dysaerobic bottom waters, and lack of dilution by the main sediment. Even when only one of these parameters cannot be satisfied, black shales cannot form. Instead, a variety of different lithotypes

174

is generated, either by dilution of high organic carbon contents with carbonate (see Fig. 8-21) or by a high background sedimentation rate reducing the organic carbon content, even for anaerobic bottom waters and low concurrent carbonate deposition.

8.3.2 Carbonaceous facies related to sedimentary condensation The role of condensation in the formation of organic carbon-rich sediments has already been outlined in various parts of this book. Condensation, and thus the relative enrichment of organic carbon contents, is related to a substantial decrease in deposition rate. Geologically, this is achieved by different processes, such as sea level rises trapping sediment on the inner shelf, reduction of the elastic sediment supply into a basin, and by various ways of diminishing CaCO3 production. Condensed sections, formed laterally ofprograding systems, are an essential element in the recognition of former sea level highstands (Chap. 7.4; see also Posamentier and Vail, 1988). Organic carbon enrichment by condensation processes is controlled by reducing the influence of two factors, background deposition and dilution by the main sediment. In Fig. 8-23, the first factor is expressed in a diagram which shows the rate of background deposition versus the corresponding organic carbon content (upper graph, see also Figs. 8-6 and 8-7), while the second factor is represented through the typical Co,g-CaCO3 diagrams presented earlier (lower graphs in Fig. 8-23). During condensation, the sediment shifts from higher to lower facies association along the curve relating background organic carbon content to the sedimentation rate, e.g., from facies association Ctu to facies association CI (upper graph in Fig. 8-23). During this shift, the background sedimentation rate must decrease considerably for a significant increase in organic carbon content (i.e., "organic carbon threshold effect"). With high organic matter supply, interpreted here as dominantly reflecting anoxic conditions, this threshold occurs with high sedimentation rates, while the opposite is true for low organic matter supply or oxic conditions.

Condensation during maximum transgression Let us first consider the formation of condensed sections which show significant organic carbon concentrations (Loutit et al., 1988; Posamentier and Vail, 1988). When condensation is well developed, background deposition can move towards lower facies associations (e.g., C0 and thus below the critical threshold below which organic carbon content significantly increases. This C ~ content must remain essentially undiluted by the main sediment in oder to remain high (Fig. 8-23). Because condensed sections are the distal portions of silieiclastie or calcareous sediment wedges (see Chap. 7.4), the seaward facies change along such a wedge may be attended by a decrease in both the rates of background and main sediment deposition, resulting in prominent organic carbon concentrations. Many typical black shales, relatively low in carbonate, may represent ex-

175

amples of condensed sections formed in downlapping positions lateral of calcareous sediment wedges (Fig. 8-23A, right hand side). These black shales with their related lithotypes (i.e., the black shale association) are often inter-

TRANSGRESSION-RELATED O R G A N I C C A R B O N DISTRIBUTION

SlUCICLASTICDEPOSITION

CARBONATE DEPOSITION

c~ N c l i ~~ESSION' '~" I BACXC_.~qOUND SED RATES NC

8ACJ(GROUND SED RATE 8 N8

"CONDENSED SECTION" (MAXIMUM TRANSGRESSION) Sill

''''-''''',V

,

,

,,

"CONDENSED SECTION" (MAXIMUM TRANSGRESSION)

~

i

4 "me~6oF,;E.~TE~ ,.osmo.

CARBONKI'F ~

EARLY TRANSGRESSION

EARLY TRANSGRESSION

m 1 v'" "Ib C,,t~bDNA~

~

~i C

~ T I s

TREND OF D E P O S m O N

OF DEP'O61TION

" a r . - . -]L

c,I I

I\ i

"

~

1

C"ll

c,,,I

"~"-,..J I'm""--.-,-dl

O TRF_NO OF IMfoOSlTION

Fig. 8-23 Organic carbon concentrations related to transgressions. Diagrams combine the relationship between the organic carbon content and sedimentation rate of the background sediment (C,,,•s, Co,~C and S~s, SNc; upperpart of figure) with Co,=-CaCO3 diagrams (lower part of figure)', for silictclastie (left side) and calcareous deposition (right s*de of figure). Arrows indicate the development of changing sediment compositions. A) Condensed section, with decreasing rates of both background deposition (transition from Sm to St or Cm to C~) and main sediment deposition (i.e., either elastic or calcareous sediment). B) Early transgression causing C~,s enrichment in the background sediment, which is partly balanced by dilution by the mare sediment. calated with marly to calcareous shelf sequences. The Toarcian Posidonia Shale, spanning the falcifer and bifrons zones, seems to represent such shaly, Co~,-rich condensation, related to pronounced eustatic highstand CRiegraf, 1985; Riegel et al., 1986; Haq et al., 1987; Hallam, 1988; Prauss et al.,

176

1991). But Co,=-CaCOj data indicate that, in addition to condensation, the change from su-boxic to dysoxic and anoxic bottom waters had a significant influence on organic carbon content. One example of siliciclastic wedge deposition (Fig. 8-23A, left hand side) is the Lower Turonian Bridge Creek Limestone, representing organic carbon enrichment during a sea level highstand (Kauffman, 1984; Arthur et al., 1987; Elder, 1987b; Eicher and Diner, 1991; Ricken, 1993; see Chap. 6.5). Nearshore and deeper water shales were replaced basinwards by marls and chalks rich in organic carbon and with low sedimentation rates (see Chap. 7.4.1, Fig. 7-24).

Early transgression: Co~ enrichment balanced by dilution During transgressions, the sedimentation rate of the background sediment can be so greatly reduced that organic carbon-rich sediments may form as in condensed sections. But this Cot- enrichment is usually balanced by increasing deposition of the major sedim[nt fraction (Fig. 8-23B). In the calcareous system, relatively carbonate-poor nearshore sediments with high rates of deposition are usually replaced by more pelagic, calcareous deposition. Sediments rich in organic carbon form only when the addition of carbonate is out of phase with the decrease in background deposition. An example would be when, during a transgression, siliciclastic background input is significantly reduced, but diluting carbonates are not produced until a critical water depth or time span is reached. Then, for a short interval at the base of the transgressive calcareous sequence, an organic carbon-rich layer may form (see also the carbonate-poor zone at the base of a calcareous section, Chap. 7.3.3). A prominent example of such a transgression-related black shale is the Permian Kupferschiefer (Paul, 1982; Oszczepalski and Rydzewski, 1987).

Oceanic condensation processes During periods of Oceanic Anoxic Events ("OAEs"; Jenkyns, 1980; Schlanger and Jenkyns, 1976), organic carbon condensation is accomplished by reducing siliciclastic input and by diminishing the production of nannoplankton ooze. Cretaceous OAEs are thought to be correlated with major sea level rises, reducing the siliciclastic supply from the continents (e.g. Arthur et al., 1987; Schlanger et al., 1987; Funnell, 1990). Sediments of OAEs generally show low rates of deposition, especially those spanning the CenomanianJTuronian boundary (de Graciansky et al., 1987). Facies association C~ is predominant; anaerobic to dysaerobic sediments are organic carbon-rich, while concurrent dilution by carbonate is low. Although some of the OAE sequences are accompartied by various degrees of carbonate deposition, the highest organic carbon contents are associated with very low CaCO3 contents (e.g., de Graciansky et al., 1987). Low carbonate production may be related to lower productivity, or to yet higher productivity (e.g., Weissert, 1989; Arthur et al., 1990; Stein, 1991), when nannoplankton carbonate production is partly replaced by radiolarian production (Wonders, 1980; Thurow and Kuhnt, 1986). In the deep-sea,

177

a rising C C D may further contribute to low carbonate content causing organic carbon enrichment. In summary, organic carbon enrichment is an inherent process in the formation of carbonaceous sediments. On the other hand, such organic carbon enrichment only has a significant effect when organic matter is supplied at a high rate, reflecting increased productivity or decreased oxygenation (see the different Cot- contents for aerobic and anaerobic bottom waters for facies association C~'in Fig. 8-14). 8.4 Conclusions Quantification of organic carbon is based on statistically evaluating the influence of changing background deposition; this is combined with the various sedimentary dilution-concentration processes described earlier. Organic carbon concentration can be predicted, within statistical limits, for different facies associations. In addition, organic carbon is sensitive to facies changes and bottom water oxygenation, expressed by various lithotypes rich to poor in carbonate and organic carbon contents. 1. Quantification of organic carbon concentration in the background sediment: The basic idea in quantifying organic carbon content is to determine its concentration in the background sediment, which is then subjected to various dilution processes by the main sediment. This procedure is performed by using a large data set of organic carbon contents from environments from the deep sea to epicontinental basins, ranging in age from the Lower Jurassic to the Quaternary. Though scattering of organic carbon values is large as a result of changing productivity, oxygenation, and diagenetic influences, organic carbon contents statistically decrease with increasing background sedimentation rates, indicating that depositional dilution processes are a major control on the weight percent of organic carbon. 2. Forecasting organic carbon contents: Standard equations for organic carbon contents have been established for semilithified to lithified sediments. These equations are based on the degree of background deposition, the average organic carbon supply according to the statistical investigation carried out in item 1, and the various degrees of calcareous or siliciclastic dilution. They are expressed through the carbonate content of the sediment. As background deposition also controls the individual facies associations, organic carbon forecasting is achieved by integrating the statistical interrelations between organic carbon contents and sedimentation rates with the previously established standard equations for carbonate and siliciclastic deposition (see Chap. 7.1.2). 3. Bottom water oxygenation and bioturbation styles: The graphical representation of the standard equations shows that organic carbon contents can be expressed in Co,~-CaCO3 diagrams by distribution fields that are shaped like triangles. These trtangles reflect the statistically lower and upper values for organic carbon in the background sediment and its dilution by main sediment deposition (i.e., siliciclastic or carbonate deposition). For environments in the calcareous system, the distribution fields are determined by a lower and upper Cor:CaCO 3 regression line representing low or high organic matter input, re-

178

spectively. Investigation of various shelf to epicontinental sea environments indicates that flatly sloping, bioturbated and steeply sloping, non-bioturbated C~z-CaCO 3 regression lines are mainly an effect of oxle and anoxic bottom waters, respectively. This allows us, as a first approach, to establish zones of bottom water oxygenation for the Corg-CaCOa diagrams of the various facies associations. Two trends of bioturbation are generally observed in environments in the calcareous system. A major trend of increasing bioturbation is associated with more flatly sloping Cor.-CaCO3 regression lines, indicating dominantly higher bottom water oxygenation. A minor trend in bioturbation is observerd which is parallel to the Co~g-CaCO3regression lines, and which is thought to indicate changing oxygenation of pore waters, owing to higher and lower C,,g contents in the sediment. 4. Lithotypes rich and poor in organic carbon: Lithotypes are defined as varieties of the previously described facies types (Chap. 7.1.2) with different organic carbon contents and bioturbation levels. In the carbonate system, one oxygenation level is represented by sediments with different degrees of bioturbation and Cor~-CaC03 contents, reflecting changing lithotypes along a carbonate dilution hne. Laminated, organic carbon-rich shales deposited in anoxic bottom waters have their calcareous counterparts in laminated, Cof_-poor limestones; dysoxic, organic carbon-rich and laminated shales have their" calcareous counterparts in limestones with low Cor.-contents and restricted bioturbation, while organic carbon-poor shales with re~strictedbioturbation deposited in oxic bottom waters have their calcareous conterpart in fully bioturbated limestones extremely low in organic carbon. To use only the organic carbon content as an indication of bottom water oxygenation, as often found in the literature, is invalid. The combined use of organic carbon and carbonate contents allows more realistic characterization of bottom waters. 5. Black shale and plattenkalk lit_hotype associations: The various lithotypes of the calcareous system can be grouped into two major families, the black shale and plattenkalk lithotype associations. The black shale lithotype association is defined by low rates of noncalcareous background deposition with low to very high Co,~ contents. Lithotypes are characterized by varying organic carbon and carbonate contents, and include true black shales, laminated limestones, and organic carbon-poor bioturbated shales and limestones. In contrast, the plattenkalk association is defined by high rates of background deposition, thereby restricting lithotypes to those with generally low organic carbon contents, such as laminated shales and lithographic limestones (i.e., plattenkalks). True, highly carbonaceous black shales are only formed when the organic matter supply is high, carbonate-free background deposition is extremely low, and when diluting deposition of carbonate is low enough to be neglected. 6. Depositional organic carbon enrichment: Processes of organic carbon enrichment are inherent in anoxic and dysoxie sedimentation and contribute significantly to elevated organic carbon contents. In anoxic environments, often related to sea level highstands, the supply of background sediment ma~. become significantly reduced, leadin~gto high organic carbon contents when dilution by the main sediment (i.e., either siliciclastic or carbonate supply) is simul-

179

taneously reduced. Examples of such processes are condensations lateral of calcareous and siliciclastic sediment wedges, transgressions, and Oceanic Anoxic Events.

EPILOGUE

The three-component system connects organic carbon - carbonate content with changes in sedimentation rates and facies transitions, and integrates these with organic carbon prediction and the explanation of oxygen deficient, shaly to calcareous sediment. As a comprehensive approach, the three-component system offers new insights into organic carbon content related to sediment flux, facies change, and oxygenation levels. An essential conclusion is that organic matter contained in marine sediments cannot be regarded as a unique parameter independent of other depositional components. Instead, the organic carbon content depends substantially on the carbonate and siliciclastic fluxes which can either dilute or concentrate the relative amount of organic carbon. Organic carbon content is generally dependent on carbonate content. This dependency can either be positive Co,gC a C O 3 correlation, as in siliciclastic-dominated deposition, or inverse Co~gCaCO3 correlation, as in carbonate-dominated deposition (Fig. 9-I). t--Ill i--Z

O O Z O

m rr < O O Z m

<

nO

CARBONATE CONTENT Fig. 9-1 General Co,g-CaC03 trends for carbonate and siliciclastic deposition. The three-component system stipulates that Co,g-CaCO3 correlations that vary with sediment type, allow recognition and quantification of the depositional flux pattern by reflecting relative changes in sedimentation rate. This provides the basis for further calculation of high-resolution relative flux changes and time spans in individual, rythmically bedded strata and in entire sequences in

182

which stratigraphic time control is poor or absent. These calculations can be performed laterally or vertically with respect to three-dimensional basin fill. The three-component system facilitates time span assessment because Co~,CaCO 3 content reflects the dilution or concentration resulting from the thr~e fluxes. Thus, the carbonate content distribution in sequences indicates time spans and sedimentation rates. Inverse correlation of Co~-CaCO3, reflecting variations in carbonate deposition, is more frequently observed than the positive C~.-CaCO 3 relationship in siliciclastic deposition. That organic carbon concentr'ation is so strongly influenced by carbonate content is a result of partial flux deeoupling between the organic matter and pelagic carbonate fluxes. Both fluxes originate in the photic zone but become de,coupled because the organic matter is substantially degraded, while the carbonate flux remains largely unaffected above the lysocline. An important result of the three-component system is that sedimentation rate in carbonate- or siliciclastic-dominated sequences is clearly reflected by carbonate content. This can be expressed by the general carbonate content sedimentation rate relationship, which defines suites of related facies types. These facies associations are numerically described through standard equations for various rates of calcareous or siliciclastic deposition. Facies associations play a central role not only in understanding lithologic successions vertically or laterally with respect to basin fill, but also in numerically quantifying the organic carbon content and in applying a lithotype concept in assessing bottom water oxygenation. In the carbonate system, a given level of bottom water oxygenation is not indicated by a single lithotype but by a succession of lithotypes, characterized by a predominant bioturbation or lamination style, including transitions from organic carbon-rich shales to marls and limestones. Depending on whether the rates of deposition are generally low or high, lithotypes of the carbonate system form the black shale and the plattenkalk lithotype associations, respectively. The black shale association contains lithotypes with low to high carbonate and organic carbon contents. The plattenkalk association contains lithotypes with low to high carbonate content but generally low organic carbon content. Changes from the plattenkalk to the black shale lithotype association are accompanied by organic carbon condensation, which is inherent to many dysaerobic and anaerobic environments. The three-component system not only precisely demonstrates particular depositional processes, but also changing sedimentation rates, relative fluxes and time spans. All these factors are associated with the C~-CaCO3 ratio in sediments and sedimentary rocks, which reflects differentia[ dilution of the carbonate, silicielastic, and organic matter components. As a whole, the threecomponent system ]provides an integrated model that permits quantification of the factors controlhng carbonate and organic carbon concentration. These relate directly to both depositional flux and facies change.

REFERENCES

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SUBJECT INDEX

asymmetry relation (see glossary) 5960, Fig. 5-1, 112-124, Fig. 7-7 background sediment (see glossary) 12 allocation of organic matter 42-44, Fig. 3-7, Fig. 3-8 effect on slopes of Co,.-CaCO3 curves 16-18, Fig.~2-3 organic matter content 140-146. Fig. 8-2, Fig. 8-3, Fig. 8-4 bedding reduction in chalks and clays (see glossary) 76-79, Fig. 5-9, 112116, Fig. 7-9, Fig. 7-10 bioturbation expression in Co,,-CaCO3 trends 157164, Fig. 8-12, Fig. 8-13, Fig. 816 intensity scale 158 Black Sea 172-174, Fig. 8-22 black shale lithotype association 169172, Fig. 8-20, 8-21 bundles 62-63, Fig. 5-3, 112, Fig. 7-9 calcareous sediment wedge 131-134, Fig. 7-22 carbonate cementation 36 difference between beds, deep sea rhythms 70-73, Fig. 5-7 difference versus sedimentation rate 76-79, Fig. 5-9 fraction 10 carbonate content - sedimentation rate relation (see glossary) concepts 100-104, Fig. 7-1, Fig. 72, 110, Fig. 7-7 in beds and varves 112-116, Fig. 79, Fig. 7-10 in sedimentary record 102-104, Fig. 7-3, Fig. 7-4 carbonate dependency of organic carbon concept 6-8, Fig. 1-3, 24, Fig. 2-8 facies transitions 114-118, Fig. 7-11, Fig. 7-12, Fig. 7-13, Fig. 7-14, 136-138, Fig. 7-25, Fig. 7-26 in lithotypes 164-168, Fig. 8-18, Fig. 8-19

rhythmic bedding 60-63, Fig. 5-2, Fig. 5-3, Fig. 5-4, 167, Fig. 8-19 sediments of Western Interior Basin 82-84, Fig. 6-2, Fig. 6-3 carbonate deposition associated facies change 114-I 18,

Fig. 7-11, Fig. 7-13

beds and varves 112-116, Fig. 7-9,

Fig. 7-10

black shale and plattenkalk lithotype associations 169-174, Fig. 8-20, Fig. 8-21, Fig. 8-22 calcareous facies associations 105107, Fig. 7-5 calcareous sediment wedge 131-134, Fig. 7-22 carbonate content - sedimentation rate relation 104, Fig. 7-1, Fig. 7-2, Fig. 7-3 concept 12-14, Fig. 2-1b distinguishing carbonate and siliciclastic deposition 111, 119-120, Fig. 7-15 flux decoupling 38-42, Fig. 3-5 influence on lithotypes 164-168, Fig. 8-18, Fig. 8-19 influence on relative sedimentation rates 19, Fig. 2-5, 24, Fig. 2-8, 99-100, Fig. 7-1, Fig. 7-2 rhythmic bedding 60-63, Fig. 5-2, Fig. 5-3 vertical sequences 120-131, Fig. 716, Fig. 7-17, Fig. 7-18, Fig. 719, Fig. 7-20 clastic deposition see siliciclastic deposition combined deposition see simultaneous deposition condensation and organic carbon enrichment calcareous and siliciclastic sediment wedges 131-136, Fig. 7-22, Fig. 7-23, Fig. 7-24 Condensed Section 130, Fig. 7-21 oceanic processes 176-177 transgression related processes 174176, Fig. 8-23

208 C,nr 3 curves (see glossary) bedding rhythms 63-70, Fig. 5-4, Fig. 5-5, Fig. 5-6, 167-168, Fig. 8-19 bottom water oxygenation and bioturbation 157-164, Fig. 8-12, Fig. 8-13, Fig. 8.-14, Fig. 8-15, Fig. 8-16 combinations 26-27, Fig. 2-10 definitions 16-18, Fig. 2-3, Fig. 2-4 flux decoupling 38-42, Fig. 3-5, Fig. 3-6 influence on relative sedimentation rates 19, Fig. 2-5, 24, Fig. 2-8, 25-26, Fig. 2-9 overlapping trends 119-120, Fig. 715, 134-136, Fig. 7-24 sediments of Western Interior Basin, 82-84, Fig. 6-2, Fig. 6-3 Co~-CaCO3 distribution fields black shale and plattenkalk lithotype associations 169-174, Fig. 8-20, Fig. 8-21, Fig. 8-22 bottom water oxygenation and bioturbation t57-164, Fig. 8-12, Fig. 8-13, Fig. 8-14, Fig. 8-15, Fig. 8-16 formation of lithotypes 164-168, Fig. 8-18, Fig. 8-19 influence of facies associations 145152, Fig. 8-5 organic matter deposition 154-156, Fig. 8-10, Fig. 8-11 depositional dilution processes 3-10, Fig. 1-2 flux decoupling 36-42, Fig. 3-5, Fig. 3-6 organic carbon dilution equations 2023 diagenesis carbonate, differential 36, Fig. 3-4, 88-89 eompactional enrichment of organic matter 36, Fig. 3-4, 88-89 organic matter loss 33-35, Fig. 3-3 Emiliania huxleyi 38 facies association (see glossary) bottom water oxygenations 159-164, Fig. 8-13, Fig. 8-14, Fig. 8-15

carbonate content sedimentation rate relation, data 102-104, Fig. 7-3, Fig. 7-4 changes in organic carbon content concepts 114-118, Fig. 7-11, Fig. 7-12, Fig. 7-13, Fig. 7-14, 136-138, Fig. 7-25, Fig. 7-26 quantification 145-152, Fig. 8-5, Fig. 8-6, Fig. 8-7 distinguishing carbonate and silicielastic deposition 111 standard equations 104-109, Fig. 75, Fig. 7-6 subdivision of lithotypes 164-169, . Fig. 8-17, Fig. 8-18, Fig. 8-20 facies change, lateral calcareous and siliciclastic sediment wedges 131-134, Fig. 7-22, Fig. 7-23 major trends, 136-138 Upper Cretaceous, Western Interior Basin 53-55, Fig. 4-5, 134-136, F'~g. 7-24 facies change, vertical asymmetry relation 59-60, Fig. 5-1, 112-124, Fig. 7-17 carbonate content distributions 120126, Fig. 7-16, Fig. 7-17, Fig. 718 carbonate-poor zone at sequence base 124-126, Fig. 7-18 condensation processes 174-177, Fig. 8-23 Gubbio sequence 126-131, Fig. 719, Fig. 7-20 facies type (see glossary) 106, Fig. 7-5, 209, Fig. 7-6, 164-165, Fig. 8-17 flux asessment using laterally correlated sections 53-55, Fig. 4-5 change in lateral sequences 131-136, Fig. 7-22, Fig. 7-23, Fig. 7-24 change in vertical sequences 120131, Fig. 7-16, Fig. %17, Fig. 718, Fig. 7-19, Fig. 7-20 deeoupling (see glossary) 36-42, Fig. 3-5, Fig. 3-6 dependency on water depth, organic matter 31, Fig. 3-5, Fig. 3-6, 40-41 determination in varves 112-116, Fig. 7-9, Fig. %10 major components 12-14, Fig. 2-1a, Fig. 2-1b -

209 marine carbonate and organic matter 37-38 pattern of sediment components 4244, Fig. 3-7, Fig. 3 - 8 standardization of concurrently changing sediment components 8488, Fig. 6-4, 89-94, Fig. 6-5, Fig. 6-6 variation in rhythmic bedding 66-70, Fig. 5-5, Fig. 5-6, 73-77, Fig. 58, Fig. 5-9, Fig. 5-10 forecasting of organic carbon content concepts 139-140, Fig. 8-1 equations for assessing organic carbon contents 150-152 organic carbon content in background sediment 140-146, Fig. 82, Fig. 8-3, Fig. 8-4 organic carbon content reflected by facies associations 146-149, Fig. 8-5, Fig. 8-6, Fig. 8-7

grain density of sediment components 20-22, Fig. 2-6 Gubbio sequence carbonate input 126-131, Fig. 7-19, Fig. 7-20 fractional sedimentation rates 50-51, Fig. 4-3 lateral correlation relative sedimentation rates and timing 52-55, Fig. 4-4 transect, Upper Cretaceous, Western Interior Basin 53-55, Fig. 4-5, 134-136, Fig. 7-24 lithotype (see glossary) association 165, Fig. 8-17, 169-174, Fig. 8-20, Fig. 8-21, Fig. 8-22 definition I64-165, Fig. 8-17 facies association Cl 166-168, Fig. 8-18, Fig. 8-20 rhythmic bedding 167, Fig. 8-19 main sediment (see glossary) 12 allocation of organic matter 42-44, Fig. 3-7, Fig. 3-8 Oceanic Anoxic Events 176-t77

organic carbon average concentration in carbonates and shales 152-155, Fig. 8-9 bioturbation and bottom water oxygenation 157-164, Fig. 8-12, Fig. 8-13, Fig. 8-14, Fig. 8-15, Fig. 8-16 change in facies associations 146152, Fig. 8-5, Fig. 8-6, Fig. 8-7 change in facies transitions 114-118, Fig. 7-11, Fig. 7-12, Fig. 7-13, Fig. 7-14 compactional enrichment 35-36, Fig. 3-4, 88-89 content in background sediment 140146, Fig. 8-2, Fig. 8-3, Fig. 8-4 dilution equations (see glossary) 2023 enrichment see condensation flux related to water depth 37, Fig. 3-5, Fig. 3-6 influence on relative sedimentation rates 33-35, Fig. 3-3 in lithotypes 164-169, Fig. 8-17, Fig. 8-18, Fig. 8-20 prediction see forecasting of organic carbon content preservation in marine sediments 3035 sedimentation rate dependency, sealing effect 30-33, Fig. 3-1, Fig. 3-2, 140-146, Fig. 8-3, Fig. 8-4 trend overlapping 119-120, Fig. 715 organic matter background sediment 16-18, Fig. 23, Fig. 2-4 influence on grain density 20-22, Fig. 2-6 sediment component 11 organic matter deposition (see glossary) concept 12-14, Fig. 2-1b expression in Co~-CaCO~fields 154-156, Fig.*8-10, Fig. 8-11 lithotypes 164-168, Fig. 8-18, Fig. 8-19 rhythmic bedding 60-63, Fig. 5-2 sequences, vertical change 89-95, Fig. 6-5, Fig. 6-6, Fig. 6-7, 174176, Fig. 8-23 oxygenation of bottom waters determination of Co~dCaCO~ fields in facies associations 159-164, Fig. 8-13, Fig. 8-14, Fig. 8-I5

210

slope of Co,,-CaC% curves 16-18, Fig. 2-3, Fig. 2-4 63-65, Fig. 54, 157-158, Fig. 8-12, 163, Fig. 8-16 parasequence 112, Fig. %9 plattenkalk lithotype association Black Sea 173-174, Fig. 8-22 definition 169-171, Fig. 8-20, Fig. 8-21 Soinhofen Limestone 173-174 productivity 3642, Fig. 3-5, Fig. 3-6 organic - inorganic ratio, oceans 3738 organic matter flux 37-38 relative sedimentation rate see sedimentation rate rhythmic bedding asymmetry relation (see glossary) 59-60, Fig. 5-1 basic types of deposition 60-63, Fig. 5-2, Fig. 5-3 bedding reduction in chalks and clays (see glossary) 76-79, Fig. 5-9, Fig. 5-10 bundles 62-63, Fig. 5-3 carbonate difference deep-sea 70-73, Fig. 5-7 distinguishing redox and productivity rhythms 65, Fig. 5-4, 167-168, Fig. 8-19 marine environments 63-65, Fig. 5-4 relative sedimentation rates using carbonate differences, deep-sea 73-77, Fig. 5-8, Fig. 5-9, Fig. 510 relative sedimentation rates using CF~yaCO~ data 66-70, Fig. 5-5, 5-6 variation of one component 63-67, Fig. 5-4, Fig. 5-5 variation of several components 6570, Fig. 5-4, Fig. 5-6 sediment component standardization 84-88, Fig. 6-4 weight to volume transformation 21, Fig. 2-7

sedimentation rates distortion of original flux pattern 5960, Fig. 5-1 fractional 49-50, Fig. 4-3, 84-94, Fig. 64, Fig. 6-6 in facies associations 104-109, Fig. 7-5, Fig. 7-6 influenced by carbonate content 99104, Fig. 7-1, Fig. %2, Fig. 7-3, Fig. 7..4 influence on organic carbon preservation 31-33, Fig. 3-1, Fig. 3- 2 long-term, for different environments 49, Fig. 4-2 sedimentation rates, relative (see glossary) bedding rhythms 66-70, Fig. 5-5, Fig. 5-6 combination 26-27, Fig. 2-10 concepts 15, Fig. 2-2, 19-26, Fig. 25, Fig. 2-8, Fig. 2-9 influence of diagenetic organic matter loss 33-35, Fig. 3-3 organic carbon dilution equations (see glossary) 20-23 standardization of concurrently changing fluxes 84-88, Fig. 6-4, 89-94, Fig. 6-5, Fig. 6-6 using carbonate difference of deep sea bedding rhythms 73-76, Fig. 5-8, Fig. 5-9, Fig. 5-10 using laterally correlated sections 5255, Fig. 4-4, Fig. 4-5 siliciclastic deposition carbonate content - sedimentation rate relation 104, Fig. %1, Fig. 7-2, Fig. 7-3 concept 10-14, Fig. 2-1b distinguishing carbonate and siliciclastie deposition 111, 119-120, Fig. 7-15 flux decoupling 4t-42, Fig. 3-6 relative sedimentation rates, determination 19, Fig. 2-5 rhythmic bedding 60-63, Fig. 5-2, 53 sedimentation rate change 99-101, Fig. 7-1, Fig. 7-2 sediment wedge, laterally 132-136, Fig. 7-23, Fig. 7-24 siliciclastie facies associations 107109, Fig. 7-6, 114-118, Fig. %12, 7-14 vertical sequences 122-124, Fig. % 17a

2tl

siliciclastic sediment wedge 132-136, Fig. 7-23, Fig. 7-24 simultaneous deposition combining Co..CaC03 curves 26-27, Fig. 2-10" combining facies associations 110112, Fig. 7-8 overlapping Co~-CaCO3 trends 119120, Fig. 7-15, 134-136, Fig. 724, 154-156, Fig. 8-10, Fig. 8-11 standardization of concurrently changing sediment components 8494, Fig. 6-4, Fig. 6-5, Fig. 6 Solnhofen Limestone 173-174 thickness change of beds 112-116, Fig. 7-9, Fig. 7-10 three-component system (see glossary) alternative interpretations 44-45, Fig. 3-9 background sediment (see glossary) 12 basic types of deposition 3-6, Fig. 11, Fig. 1-2, 12-14, Fig. 2-1a, Fig. 2-1b components 10-11 determination of relative sedimentation rates 19-27, Fig. 2-5, Fig. 28, Fig. 2-9, Fig. 2-10 enhanced version, for simultaneous flux change 84-88, Fig. 6-4 main sediment (see glossary) 12 related concepts 8, Fig. 1-4 slope of C rg -CaCO3 curves 16-18, . Fig. 2-~, F~g. 2-4 testing using time scales 94-95, Fig. 6-7 validity and limitations 45-46 time span assessment 47-55 bedding rhythms 66-70

conventional approach 48 Gubbio sequence 50-51, $]g. 4-3, 126-131, Fig. 7-19, Fig. 7-20 lateral correlation 52-55,Fig. 4-4, Fig. 5-5, 134-136, Fig. 7-24 using carbonate difference in deep sea beds 73-77, Fig. 5-8, Fig. 5-9, Fig. 5-10 using fractional sedimentation rates 49-51, Fig. 4-3 usincga relative sedimentation rates 52oo

using sedimentation rates 48-49, Fig. 4-2 using standardized, relative sedimentation rates 84-94, Fig. 6-4, Fig. 6-5, Fig. 6-6 vertical sequences 89-95, Fig. 6-5, Fig. 6-6, Fig. 6-7, 120-121, Fig. 7-16, 124-125, Fig. 7-18 Western Interior Basin sediments 5355, Fig. 4-5, 89-95, Fig. 6-5, Fig. 6-6, 134-136, Fig. 7-24 varves 112-116, Fig. 7-9, Fig. 7-10 Western Interior Basin, CenomanianTuronian 81, Fig. 6-1 Co_-CaCO. pattern 82, Fig. 6-2,

"Fig. 6-j

diagenetie overprint 88-89 flux pattern, shoreline to basin transect 53-55, Fig. 4-5, 134136, Fig. 7-24 flux pattern, vertical sequences 8995, Fig. 6-5, Fig. 6-6, Fig. 6-7 rhythmic bedding 66-70, Fig. 5-5, Fig. 5-6

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Vol. 10: T.M. Peryt (Ed.), The Zechstein Facies in Europe. V, 272 pages, t987.

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Vol. 12: S. Turner (Ed.), Applied Geodesy. VIII, 393 pages. 1987. Vol. 13: T. M. Peryt (Ed.), Evaporite Basins. V, 188 pages. 1987.

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Boundaries of Order: Private Property as a Social System

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