Walker J. Facies models, Canada, 1992

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Facies

RESPONSE

SEA LEVEL-

CHANGE

Edited

Rwer

Walker

and

Noel

James

F,$

/jFb>-.7L2

'2,

5.'

Geological Association of Canada

L'Association geologique du Canada

FACIES

MODELS

RESPONSE TO

SEA LEVEL CHANGE

Edited

Roger

Walker

Department of Geology

McMaster University

Hamilton, Ontario

L8S

4M1 Canada

and

Noel

James

Department of Geological Sciences

Queen's University

Kingston, Ontario K7L

3N6

Canada

Geological Association of Canada

L'Association giologique du Canada

June 1992

Canadian

Cataloguing in Publication Data

..=

Main entry under title:

Facies models

response to sea level change

Includes

bibliograph~cal references.

ISBN 0-91 921 6-49-8

1. Facies (Geology).

Walker, Roger G.

11.

James, Noel

111.

Geological Association of Canada

QE651 .F25 1992 551.7 C92-090412-2

Illustration Concept: Peter Russell, Waterloo

Additional copies may be obtained by writing to:

Geological Association of Canada

Publications

Department of Earth Sciences

Memorial University of Newfoundland

St. John's, Newfoundland

A1 B 3x5

Canada

1992

ISBN 0-91 921 6-49-8

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CONTENTS

PART

PRINCIPLES, TOOLS AND CONCEPTS

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

Facies, facies models and modern stratigraphic concepts 1

Roger G. Walker

2. Control of sea level change

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

A. Guy Plint, Nicholas Eyles, Carolyn

Eyles and Roger G. Walker

3. Subsurface facies analysis

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

Douglas J. Cant

4. Trace fossil facies models: environmental and allostratigraphic significance

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

George Pemberton, James A. MacEachern and Robert W. Frey

PART

TERRIGENOUS CLASTIC FACIES MODELS

5. Glacial depositional systems

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

Nicholas Eyles and Carolyn

Eyles

6. Volcaniclastic rocks

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

101

Jean Lajoie and John Stix

7. Alluvial deposits

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

1 19

Andrew D. Miall

Eolian systems

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

143

Michael E. Brookfield

9. Deltas

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

157

Janok P. Bhattacharya and Roger G. Walker

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

10. Transgressive barrier island and estuarine systems 179

Gerald E. Reinson

1 1

Tidal depositional systems

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

195

Robert W. Dalrymple

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

12. Wave- and storm-dominated shallow marine systems 21 9

Roger G. Walker and A. Guy Plint

13. Turbidites and submarine fans

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

239

Roger G. Walker

PART

I11

CARBONATE AND EVAPORITE FACIES MODELS

14. Introduction to carbonate and evaporite facies models

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

265

Noel P. James and Alan C. Kendall

15. Shallow platform carbonates

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

277

Brian Jones and Andre Desrochers

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

16. Peritidal carbonates 303

Brian R. Pratt, Noel P. James and Clinton A. Cowan

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

17.

Reefs and mounds 323

Noel P. James and Pierre-Andre Bourque

18. Carbonate slopes

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

349

Mario Coniglio and George R. Dix

19. Evaporites

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

375

Alan C. Kendall

Preface

In the summer of 1989, the Geological Association of

Canada asked us whether it was worthwhile to reprint the

second edition of

Facies Models.

Most of the authors felt

that the world of sedimentary geology had changed dramat-

ically since the second edition was published in 1984.

Rewriting rather than reprinting was necessary. There is

now much more information on specific facies. Some funda-

mental concepts have changed and, most importantly,

stratigraphy has re-emerged as a primary analytical

ttiol.

Dynamic controls on sedimentation in the form of sea level

change, tectonics, climate and biotic evolution, must be

incorporated into contemporary facies modelling.

This volume is

not

a third edition, nor is it part of the

Geoscience Canada reprint series. Depositional environ-

ments have been subdivided in different ways, authors have

taken on new responsibilities, new authors are involved, and

the volume goes beyond being a mere revision. When pre-

liminary plans for this volume circulated "through the

grapevine", Brian Rust was one of the first to volunteer to

update his chapter. We discussed plans for this on the day

before his trip to Zambia in 1990, where he contracted a

fatal case of malaria. We have missed his enthusiasm and

knowledge. We will also miss, in the future, the contribu-

tions of Bob Frey, who died in January, 1992. His knowl-

edge has enhanced the chapter on Trace Fossils, both in

the second edition and in this book.

Of the new concepts incorporated in this book, perhaps

the most important concerns the dynamic control of sea

level change. Eustatic cycles form the basis of

sequence

stratigraphy,

and the bounding discontinuities in the geologic

record that formed as a response to sea level change allow

allostratigraphic

subdivision of the stratigraphic column.

These fundamental concepts, reviewed in chapter 1, form

the underlying theme of the book. All authors have tried to

emphasize not only the individual depositional environ-

ments, but where appropriate, their response to fluctuations

in sea level and other dynamic factors.

Modelling, however, remains an important focus. Models

are built on the comparison of modern and ancient exam-

ples. In the second edition it seemed possible, when dis-

tilling all variables into a static model, to achieve a certain

sedimentological

synthesis which served as a norm, a

guide for observations, a predictor, and a basis for interpre-

tation. As more variables are introduced, it is harder to

select a group of examples that are homogeneous and can

be combined into a model. For example, you might be able

to combine the sedimentological features of ten

wave-

dominated deltas into one model. If sea level change is

added as an important descriptive parameter, there may be

three deltas that have responded to a relative rise, four to a

fall and three in which sea level has not fluctuated. There

are now insufficient examples to make powerful integrated

models. If tectonic parameters, and the evolving terrestrial

biota are included, the problem becomes much worse. In

this book we have staked the middle gound. We have tried

to explain how the various depositional systems work, and

how they respond in general terms to sea level fluctuation

and where appropriate, other dynamic factors. At the same

time we have still emphasized as much generality as

possible (modelling). The models we do present are more

complex, and harder to illustrate in block diagrams.

The book is aimed at a general audience, and at students.

It is not intended as an advanced research text. All authors

have therefore tried to keep the number of citations to a

minimum, and we have not necessarily cited the source of

every idea presented. The references at the end of each

chapter begin with basic sources of information, where

more detail on the subject can be found; these references

are annotated. The other references are simply in alphabet-

ical order, and are only occasionally annotated.

As editors, we thank the individual authors for building

their contributigns around the ideas of sedimentation dynam-

ics, in particular the response to sea level change. Although

the contributions are largely reviews, there is a lot of original

research incorporated in them. Over the years, much of this

research has been supported by the Natural Sciences and

Engineering Research Council of Canada, and by various

universities and government agencies where the authors

work. Editors and authors gratefully acknowledge this

support.

Bob

Baragar (Geological Association of Canada) was

extremely helpful in getting this book moving, and Monica

Easton took over toward the end and has helped with the

technical side of communicating our wishes to the printer.

However, there are many other large steps to be taken

between writing a manuscript, editing it for technical content,

and seeing it appear between the covers of a book. We par-

ticularly thank Judith James for editing all nineteen chapters

and ensuring that they all follow the same overall style. She

has checked on all of those little details that make a contri-

bution as consistent and readable as possible, despite the

differing styles of the authors.

Roger G. Walker

Hamilton, Ontario

Noel

James

Kingston, Ontario

WALKER

bined (groups) (Fig.

2).

Formal meth-

ods and rules are given in the North

American Stratigraphic Code (NACSN,

1983).

AJernative stratiam

schemes of increasina importance em-

phssize the

bounding discont&ities

rather than the internal jithological

hGmo_geneity: This type of subdivision

has been formalized as

allostrati-

graphy

(Fig.

NACSN,

98'mm

stratigraphy is purely descriptive;

sequence stratigraphy

(Van Wagoner

et a/.,

1990) also recognizes units

defined by discontinuities and uncon-

----

formities

,-..

but relates them to cycles of

sea level fluctuat&n.

Systems tracts

are linkages of contemporaneous de-

positional systems. However, different

depositional systems occur at different

positions of relative sea level, and

allow the definition of three main

systems tracts;

lowstand, transgres-

sive

and

highstand.

Certain facies successions and

facies geometries in the geological

record appear to be characteristic of

certain depositional environments.

Similar successions and geometries of

various ages can be compared with

the facies, facies successions and

facies geometries observed in modern

sediments (Fig. 1). In this way, the

general characteristics of various

depositional environments can be

defined, and used to interpret other

parts of the geological record. The

rphological subdivision of a particular depositional system characterized by a distinctive assemblage of

Depositional Environment

geographic andlor geomorphic area

Depositional System

'three dimensional assemblage of lithofacies, genetically linked by active or inferred processes and environ-

ments" (Posamentier

et a/.,

1988, p. 110). It embraces depositional environments and the processes acting therein.

Downiap

the situation where "an initially inclined layer terminates downdip against an initially horizontal or inclined surface"

(Mitchum

eta/.,

1977, p. 58).

Eustasy -a world-wide change of sea level relative to a fixed point such as the centre of the earth. Eustatic changes result from vari-

ations in the volume of water in the ocean basins (glacial control), or a change in the volume of the basins themselves (related

to rates of ocean ridge building and rates of

seafloor spreading). The eustatic sea level curve describes cyclic changes in sea

Facies

a body of rock characterized by a particular combination of lithology, physical and biological structures that bestow an aspect

("facies") different from the bodies of rock above, below and laterally adjacent.

Facies Association

"groups of facies genetically related to one another and which have some environmental significance"

(Collinson, 1969, p. 207).

Facies Succession

a vertical succession of facies characterized by a progressive change in one or more parameters, e.g., abun-

dance of sand, grain size, or sedimentary structures

Facies Model

a general summary of a particular depositional system, involving many individual examples from recent sediments

and ancient rocks.

Genetic Stratigraphic Sequence

"the sedimentary product of a depositional episode" (Galloway, 1989, p. 125), where a deposi-

tional episode "is bounded by stratal surfaces that reflect major reorganizations in basin paleogeographic framework" (Galloway,

1989, p. 128). These stratal surfaces are maximum flooding surfaces, not the unconformities used to define stratigraphic

Lithostratigraphy

"a defined body of sedimenta ry... strata which is distinguished and delimited on the basis of lithic characteristics

and stratigraphic position" (NACSN, 1983). It is internally

lithologically homogeneous.

Marine Flooding Surface

"a surface separating younger from older strata across which there is evidence of an abrupt increase in

water depth" (Van Wagoner

eta/.,

1990, p. 8).

Maximum Flooding Surface

a surface separating a transgressive systems tract (below) from a highstand systems tract (above). it

is commonly characterized by a condensed horizon reflecting very slow deposition; markers in the overlying systems tract

downlap onto the

MFS.

and their correlative surfaces" (Posamentier

ab,

1988, p. 11 0).

conformable succession of genetically related strata bounded at its top and base by unconformities and their

ties

...

it is composed of a succession of systems tracts and is interpreted to be deposited between eustatic-

(Posamentier

etal.,

1988, p. 110).

WALKER

bined (groups) (Fig.

2).

Formal meth-

ods and rules are given in the North

American Stratigraphic Code (NACSN,

1983).

A-ltevaphic

s&eem_es of increasina importance-em-

phasize the

bounding discontinuities

rather than the internal jithological

homogeneity, This type of subdivision

has been formalized

allostrati-

graphy

(Fig.

NACSN, 198T~-ilo---

stratigraphy

purely descriptive;

sequence stratigraphy

(Van Wagoner

et a/.,

1990) also recognizes units

defined by discontinuities and

uncon-

formities

but relates

_-__---

them to cycles of

sea level

fluctualkon.

Systems tracts

are linkages of contemporaneous de-

positional systems. However, different

depositional systems occur at different

positions of relative sea level, and

allow the definition of three main

systems tracts;

lowstand, transgres-

sive

and

highstand.

Certain facies successions and

facies geometries

the geological

record appear to be characteristic of

certain depositional environments.

Similar successions and geometries of

various ages can be compared with

the facies, facies successions

and

facies geometries observed

modern

sediments (Fig. 1). In this way, the

general characteristics of various

depositional environments can be

defined, and used to interpret other

parts of the geological record. The

Table 1

Glossary

terms used

this chapter and throughout the

book,

.&Yw-m-

subdivision of the stratigraphic record into mappable rock bodies "defined and identified on the basis of their

bounding discontinuities" (NACSN, 1983, p. 865).

'Architectural Element

a morphological subdivision of

particular depositional system characterized by a distinctive assemblage of

facies,

facles geometries, and depositional processes.

Bounding

Discontinui-v =@aterally traceable discontinui

can be an unconformity, ravinement surface,

onlap or downlap surface,

condensed horizon or

hardgroi7W.-

hi@arisksli"l'

Depositional Environment

geographic andlor geomorphic area

Depositional System

"three dimensional assemblage of lithofacies, genetically linked by active or inferred processes and environ-

ments" (Posamentier

eta/.,

1988, p. 110). It embraces depositional environments and the processes acting therein.

Downlap

the situation where "an initially inclined layer terminates downdip against an initially horizontal or inclined surface"

(Mitchum

etal.,

1977, p. 58).

Eustasy

a world-wide change of sea level relative to a fixed point such as the centre of the earth. Eustatic changes result from vari-

ations in the volume of water in the ocean basins (glacial control), or a change in the volume of the basins themselves (related

to rates of ocean ridge building and rates of

seafloor spreading). The eustatic sea level curve describes cyclic changes in sea

level.

Facies

a body of rock characterized by a particular combination of lithology, physical and biological structures that bestow an aspect

("facies") different from the bodies of rock above, below and laterally adjacent.

Facies Association

"groups of facies genetically related to one another and which have some environmental significance"

(Collinson, 1969, p. 207).

Facles Succession

a vertical succession of facies characterized by a progressive change in one or more parameters, e.g., abun-

dance of sand, grain size, or sedimentary structures

Facles Model

a general summary of a particular depositional system, involving many individual examples from recent sediments

and ancient rocks.

Genetic Stratigraphic Sequence

"the sedimentary product of a depositional episode" (Galloway, 1989, p. 125), where a deposi-

tional episode "is bounded by stratal surfaces that reflect major reorganizations in basin paleogeographic framework" (Galloway,

1989, p. 128). These stratal surfaces are maximum flooding surfaces, not the unconformities used to define stratigraphic

sequences.

Lithostratigraphy

"a defined body of sedimenta ry... strata which is distinguished and delimited on the basis of lithic characteristics

and stratigraphic position" (NACSN, 1983). It is internally lithologically homogeneous.

Marine Flooding Surface

"a surface separating younger from older strata across which there is evidence of an abrupt increase in

water depth" (Van Wagoner

etal.,

1990, p. 8).

Maximum Flooding Surface

a surface separating a transgressive systems tract (below) from a highstand systems tract (above). It

is commonly characterized by a condensed horizon reflecting very slow deposition; markers in the overlying systems tract

downlap onto the MFS.

Onlap -the situation where "an initially horizontal stratum laps out against an initially inclined surface" (Mitchum

et a/.,

1977, p. 57-

58).

Parasequence

"a relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces

and their correlative surfaces" (Posamentier

et a/.,

1988, p. 11 0).

Ravinement Surface

an erosion surface produced during marine transgression of a formerly subaerial environment.

Seismic Stratigraphy

"a geological approach to the stratigraphic interpretation of seismic data" (Vail and Mitchum, 1977, p. 51).

Sequence

"a relatively conformable succession of genetically related strata bounded at its top and base by unconformities and their

correlative conformities

...

it is composed of a succession of systems tracts and is interpreted to be deposited between eustatic-

fall inflection points" (Posamentier

et a/.,

1988, p. 110).

S@b/A"~tr~t~~p~+

"the study of rock relationships within a chronostratigraphic framework wherein the succession of rocks is

cyclic and is composed of genetically related stratal units (sequences and systems tracts)" (Posamentier

eta/.,

1988, p. 110).

S)@@@@TrE&t

"a linkage of contemporaneous depositional systems" (Posamentier

etal.,

1988, p. 110).

Unconforniity

"a surface separating younger from older strata, along which there is evidence of subaerial erosional truncation

...

subaerial exposure, with a significant hiatus indicated" (Posamentier

etal.,

1988, p. 110). This is an extremely restricted defini-

tion; Posamentier (personal communication, 1990) now accepts that the "evidence" may be inferred rather than real.

Facies, Facies Models and Modern

Stratigraphic Concepts

OBJECTIVES OF FACIES

MODELLING

Features of recent sediments and

ancient sedimentary rocks can be

combined and condensed into ideal-

izations or models that characterize

particular sedimentary environments.

This combination of features from

modern and ancient situations has

been emphasized from the earliest

days; in 1893 Johannes Walther

(quoted by Middleton, 1973, p. 981)

"explained that the most satisfying

genetic explanations of ancient phe-

nomena were by analogy with modern

geological processes".

good model

embodies a large amount of informa-

tion from different examples of the

same depositional system, for in-

stance, meandering river channels. It

is therefore an excellent point of refer-

ence for the interpretation of new

examples of the same system, and

allows predictions to be made from

limited amounts of data. The predictive

capabilities of models have largely

been used in the exploration for oil and

gas

(e.g., Chapters 3, 12, 13, 16, 17),

and to a lesser extent in exploration for

minerals hosted by sedimentary rocks.

However, the broad understanding of

depositional systems is becoming in-

creasingly important in modelling the

movement of ground water and pollu-

tants through surficial unconsolidated

sediments, where the movement is

partly a function of the geometry of

permeable and impermeable layers

(Chapter

5).

This geometry largely

depends on the depositional pro-

cesses operating in the original sedi-

mentary environment. Facies models

also embody ideas about how natural

sedimentary systems work, and to

what extent they can be "managed".

For example, a general understanding

of beaches and barriers (Chapter 10)

contributes to the solution of coastal

erosion problems. In the Mississippi

Delta, there is a large annual land loss

Roger G. Walker, Department of Geology,

McMaster University,

Hamilton, Ontario L8S 4M1

due to regional subsidence and delta

inundation; these aspects of delta be-

haviour are part of the general deltaic

facies model (Chapter 9).

In the first two editions of

Facies

Models,

we tried to synthesize and

idealizq the features of some modern

deposihonal environments and sys-

tems (terminology defined in Table

I),

and show how their deposits could be

identified in the geological record. The

models we built tended to be snap-

shots of specific systems at one time

(static block diagrams). They empha-

sized the sedimentary processes oper-

ating within the systems rather than

processes external to the environ-

ments, such as fluctuations of relative

ssl,

and tectonics.

The major conceptual change since

the second edition (1984) has been

the recognition of the

impoftance of

relative sea level fluctuation (Chap-

ter 2). This concept now permeates

stratigraphy and sedimentology, and

brings a dynamic quality to models

of depositional environments.

Spemc

st be modelled

videos rather than snapshots. The

Sntheses in this book will

attempt to convey how the environ-

ments respond to sea level fluctuation.

----

A SEDIMENTOLOGICAL

MODUS OPERAND1

Over the years, many different meth-

ods and concepts have been used in

the study of sedimentary rocks. The

modus operandi,

or way of working on

sedimentary rocks, depends on the

objectives. Studies of ancient deposi-

tional environments commonly begin

with stratigraphic measurements and

correlations, in order to define the rock

types present, their three-dimensional

geometry, and their internal sedimen-

tary structures.

Overview of terminology

glossary of terminology used in this

chapter, and throughout the book, is

given in Table 1. Themeasurementof

<vertical

stratigraphic section implies

that it will be subdivided into a series

of different units. each with different

----I"

____-

thicknesses and charaEt~fistics. The

different aspect of each measurement

unit is summarized by the term

facies

(from the Latin word for aspect,

-1

"appearance of" something).

Primay

descriptive characteristics include

lithology, sedimentary structures and

_._

---..

biolpgicam.

During an individual study, facies

can commonly be grouped into

associations

(Fig. 1). On a broader

sc5n associations occur suffi-

ciently commonly throughout time and

space that they can be regarded as

basic

architectural elements,

of a par-

ticular depositional environment. For

example, lateral-accretion deposits

form an architectural element within

meandering river environments, as

discussed in Chapter 7. Facies also

tend to occur in specific

facies succes-

sions

(Fig. I), in which one or more of

th~ch~~gcteristics changes progreg-

sively upsection. Thus a succession

might be termed

coarsennng-upward

---

there is a

proglessive change of grain

size, or

thic~ening-pard

there is a

change in thickness of individual beds.

---

--.*

__-

Progress~ve changes in rock prosr-

ties are also shown up in downhole

surveys of oil and gas wells (Chap-

ter 3).

P~ogre$siy&fdes successions

a_~e__clmmod~terminated

by &-u&t

changes in Ijhdcgy across

digco~u-

iie8f

some type (Fig. 2); these might

. -..

be erosion surfaces, or surfaces of

nondeposition. Such surfaces

com-

monly have a distinct trace fossil

signature (Chapter

4).

The traditional descriptive strati-

graphic scheme for subdividing an-

cient sedimentary rocks involves the

definition of

lithologically homoge-

neous units (formations) that can be

further subdivided (members) or

corn-

1. FACIES MODELS

comparison of modern and ancient de- many different ways since 1838, with nition of facies was given by Middleton

positional environments, and the arguments centering on

whether the (1978), who noted that

search for the processes that control term implies an abstract set of

charac- "the more common (modern) usage

their facies successions and

geome- teristics, as opposed to the rock body is exemplified by de Raaf

et al.

tries, is now termed

facies modelling.

itself,

whether the term should refir (1965) who subdivided a group of

These terms and concepts are

dis-

only to "areally restricted parts of a three formations into a cvclical

cussed in the following text, beginning

designated stratigraphic unit" (Moore,

--petition of a number of facies dis-

at the small scale (facies) and working 1949) or also to stratigraphically un- tinguished by "lithological, st=:

up to the large scale (systems tracts).

confined rock bodies (as originally

tural and organic aspects detect-

implied by Gressly), and

whether able in the field". The facies may

FACIES

the term should be purely descriptive

be given informal designations

The modern geological usage of the

term facies was introduced by Gressly

in 1838, who used

it to imply tke

total of the lithological and paleon&

logical aspects of a

stratigta$bhi-u@t.

x_-*

-1

Translations of Gressly's extended

definition are given by Middleton

(1973). The term has been used in

("mudstone facies") or also interpretive

("~acies

etc.) or brief descriptive

("fluvial facies"). Succinct discussions

designations ("laminated siltstone

of these problems have been given by

facies") and it is understood that

Middleton

(1978), Anderton (1985) and

they are units that will ultimately be

Reading

(1 986).

given an environmental

interpre-

tation; but the facies definition is

workihg definition ofms

itself quite objective and based on

The most useful modern working

defi-

the total field aspect of the rocks

can be combined into

FACIES SUCCESSIONS

modern

and ancient examples

("distillation")

these models

combined with processes characterize

DEPOSITIONAL SYSTEMS

linkages of contemporaneous depositional systems

LOWSTAND AND TRANSGRESSIVE

themselves

...

The kev to the inter-

pretation of facies is to combine

observations

madee~n their spatial

relations and internal

characteris-

-..-

t~cs (lithology and sedimentary

structures) with comparative infor-

mation from other well-studied

stratigraphic units, and particularly

from studies of modern sedimen-

tary environmentsn.

Figure

Relationship between facies, depositional environments and systems, and

DEFINING FACIES

Facies can be defined on many dif-

ferent scales. In a study specifically

devoted to the interpretation of depo-

sitional environments, there is usually

a deliberate attempt to subdivide a

rock body into constituent facies (units

of similar aspect). This is a classifica-

tion procedure, and the degree of sub-

division is governed by the objectives

of the study. If the objective is routine

description and interpretation on a

large scale, a fairly broad facies sub-

division may suffice. If the objective is

more detailed, perhaps involving the

refinement of an existing model or the

definition of a new one, the facies

subdivision must be more detailed.

The

scale

subdivision

is depen-

dant not only on the objectives, but on

the time available in the field, the

degree of preservation, and the abun-

dance of physical and biological struc-

tures in the rocks. A thick sequence of

massive mudstones or thin-bedded

turbidites (Fig. 3) will be difficult to

subdivide, but a similar thickness of

interbedded sandstones and shales

(with abundant and varied examples

of ripples, cross bedding [Fig.

and

systems tracts, as used throughout this book.

trace fossils) might be subdivisible into

WALKER

Flooding Surface and MaxFS

Maximum

Flooding Surface. Solid arrowheads indi-

cate

onlap, and open arrowheads indicate

downlap. Bounding discontinuities are

shown by the small zig-zag serrated symbol

(unconformities); the MarFS and MaxFS

Figure

Upper diagram shows lithostrati-

graphic units

(conglomerate),

(shales)

and

(sandstones). Allostratigraphic units

A through

and systems tracts HST and

surfaces are also bounding discontinuities.

The large serrated symbol separating the

shoreface from the offshore is not a

bounding discontinuity

it represents a

gradual facies change. Lower diagram

shows an interpretation in terms of sea

level (SL) fluctuations. During

SLI, the

shoreface built out to profile

With a drop

in sea level to SL2, a new shoreface was

cut off the right hand edge of the diagram.

During a standstill in transgression, and

incised shoreface formed at profile 2, and

conglomerate was deposited in the

shoreface. When the transgression

resumed (SL2 to

SL3), the top of the con-

glomerate was eroded, and profile

was

cut landward to profile 3, making a

ravine-

ment surface (profile 3). Note that evidence

of subaerial erosion (vegetation on profile

TST are also indicated.

MarFS

Marine

2) is removed during subsequent transgres-

sion. Sea level stabilized at SL3-4, and the

shoreface sand built out to profile

Subsequent fall and rise of sea level re-

sulted in the ravinement surface

floodplain

Figure

Devonian turbidites near Mel-

bourne, Australia. Stratigraphic top to right.

LITHOSTRATIGRAPHY

-Z

ALLOSTRATIGRAPHY A

-E

SYSTEMS TRACTS

HIGHSTAND. HS

TRANSGRESSIVE. TST