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

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


.".,:::. : :-: :::::"> ..:::..::!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~

3:

2:

5.5.E~,~,

Corg9 2"0"/~0'

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L.

.... \'~5

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MIDDLE "" BRIDGECREEK ~ - ALTERNATION

60" "' " 100 Cor~

20 /.,0 60

7 za 100 z

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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

9 t":."~z

>-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-.

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

,

~

I~ N I ~

I

/

l

I ~

~/

a---~-.~

--~\

"-'-e~-

~ o~ !"- ~ ~ -

..

Z

~i

, or"

Izi~

- - l ,

I-,,, NVdNV3 ]

Fig. 6 - 6

"

lllll

"i !Lr-

~,. ,,,

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

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 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-._.

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
603 Neocome

IKI 150

DSDP/ODP

A PIPET 200 Ma

SITES

1638 Valange - Ptiocene ~,658 Pliocene - Pleistocene V 6 6 2 Pleistocene- Pliocene