EUSTATIC CONTROLS ON CLASTIC DEPOSITION I-CONCEPTUAL FRAMEWORK

H. W. POSAMENTIER'Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77252-2189;

M. T. JERVEY,Canadian Hunter Exploration Ltd., 700, 435 4th Avenue SW, Calgary, Alberta 12P 3A8

AND

P. R. VAleExxon Production Research Company, P.O. Box 2189, Houston, Texas 77252-2189

ABSTRACT: A conceptual framework for understanding the effects of eustatic control on depositional stratal patterns is presented.Eustatic changes result in a succession of systems tracts that combine to form sequences deposited between eustatic-fall inflection

points. Two types of sequences have been recognized: (I) a type I sequence, which is bounded at the base by a type 1 unconformityand at the top by either a type 1 or type 2 unconformity and has lowstand deposits at its base, and (2) a type 2 sequence, which isbounded at the base by a type 2 unconformity and at the top by either a type I or type 2 unconformity and has no lowstand deposits.Each sequence is composed of three systems tracts; the type I sequence is composed of lowstand, transgressive-, and highstand systemstracts, and the type 2 sequence is composed of shelf-margin, transgressive-, and highstand systems tracts. The type I sequence isassociated with stream rejuvenation and incision at its base, whereas the type 2 sequence is not.

Eustacy and subsidence combine to make the space available for sediment to fill. The results of this changing accommodation arethe onlapping and offlapping depositional stratal patterns observed on basin margins. Locally, conditions of subsidence and/or upliftand sediment supply may overprint but usually will not mask the effects of global sea level. Any eustatic variation, however, (e.g.,irregular eustatic rise or fall, asymmetric fall, slow or rapid rise or fall, and so on) will be globally effective. The significance ofeustatic fall-and-rise inflection points is considered with regard to the occurrence of unconformities and condensed sections, respectively.Type I unconformities are related to rapid eustatic falls, and type 2 unconformities are related to slow eustatic falls.

INTRODUCTION

The objective of this paper is to provide a conceptualframework for understanding the relationship between rel­ative sea-level changes and clastic depositional stratal pat­terns on basin margins. An awareness of this relationshipwill provide the geologist with a tool to understand betterthe relationship between depositional sequences and thedistribution of lithofacies within these sequences. Key tennsused in this paper are defined in Table 1. See Van Wagonerand others (this volume) for additional definitions.

This study builds on previous studies by Vail and others(1977); Vail and Todd (1981); Vail and others (1984); andJervey (this volume), which established that a relationshipexists between relative sea-level change and depositionalstratal patterns. This paper analyzes the mechanics of thisstratigraphic relationship in the context of sequence, sys­tems tract, and types of unconfonnity discussed. The paperby Posamentier and Vail (this volume) addresses the se­quence and systems-tract models as well as variations onthe model.

Conceptual models presented in this report represent ananalysis of the effects of accommodation change on clasticsediment deposition and suggest how sedimentary basinswill fill. It must be emphasized that the models are gen­erally applicable. The effects of local factors such as cli­mate, sediment supply, and tectonics must be incorporatedinto the models before these models can be applied to aparticular basin. Once these considerations have been takeninto account, the refined models can then be used predic­tively to simulate local conditions in order to predict lith­ologic succession better.

'Present addresses: Esso Resources Canada Ltd., 237 4th Avenue SW,Calgary, Alberta T2P OH6 (HWP).

2Department of Geology, Rice University, Houston, Texas 77251 (PRY).

ASSUMPTIONS

Geologic and seismic observations of accommodation anddeposition show that a predictable succession of as manyas four systems tracts can be generated by varying globalsea level. These are highstand, lowstand (including low­stand fan and lowstand wedge), transgressive-, and shelf­margin systems tracts, and are shown in block diagrams inFigures 1-6. Each systems tract is composed of one or moredepositional systems (Brown and Fisher, 1977) and each ischaracterized by a set of lithofacies. The timing of uncon­fonnites or surfaces of nondeposition bounding these sys­tems tracts can also be predicted from the sea-level curve.

Locally, however, the models should first be refined bythe incorporation of subsidence and sediment supply infor­mation before predictions are made. Each systems tract willbe considered in greater detail by Posamentier and Vail (thisvolume).

In order to develop generally applicable depositionalmodels, it was assumed that the following conditions wouldbe present:

-The rate of seafloor subsidence an any single location ona profile was held constant. Seafloor subsidence is pri­marily a function of lithospheric cooling and sedimentloading (together they compose total subsidence). Geo­history analyses from a variety of sedimentary basinssuggest that eustatic variations occur with greater fre­quency than subsidence variations. Thus, over a limitedinterval, the assumption of constant subsidence rate seemsacceptable. Nonetheless, when the general model ismodified to account for local conditions, nonunifonnsubsidence can be accommodated.

- Totai subsidence increases in a basinward direction. Thisseems to characterize most divergent basin margins.

-Deposition was occurring along a divergent continentalmargin characterized by a shelf, slope, and basin, where

Sea-Level Changes-An Integrated Approach, SEPM Special Publication No. 42Copyright © 1988, The Society of Economic Paleontologists and Mineralogists, ISBN 0-918985-74-9

EUSTATIC CONTROLS ON CLASTIC DEPOSITION I-CONCEPTUAL FRAMEWORK

H. W. POSAMENTIER'Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77252-2189;

M. T. JERVEY,Canadian Hunter Exploration Ltd., 700, 435 4th Avenue SW, Calgary, Alberta 12P 3A8

AND

P. R. VAleExxon Production Research Company, P.O. Box 2189, Houston, Texas 77252-2189

ABSTRACT: A conceptual framework for understanding the effects of eustatic control on depositional stratal patterns is presented.Eustatic changes result in a succession of systems tracts that combine to form sequences deposited between eustatic-fall inflection

points. Two types of sequences have been recognized: (I) a type I sequence, which is bounded at the base by a type 1 unconformityand at the top by either a type 1 or type 2 unconformity and has lowstand deposits at its base, and (2) a type 2 sequence, which isbounded at the base by a type 2 unconformity and at the top by either a type I or type 2 unconformity and has no lowstand deposits.Each sequence is composed of three systems tracts; the type I sequence is composed of lowstand, transgressive-, and highstand systemstracts, and the type 2 sequence is composed of shelf-margin, transgressive-, and highstand systems tracts. The type I sequence isassociated with stream rejuvenation and incision at its base, whereas the type 2 sequence is not.

Eustacy and subsidence combine to make the space available for sediment to fill. The results of this changing accommodation arethe onlapping and offlapping depositional stratal patterns observed on basin margins. Locally, conditions of subsidence and/or upliftand sediment supply may overprint but usually will not mask the effects of global sea level. Any eustatic variation, however, (e.g.,irregular eustatic rise or fall, asymmetric fall, slow or rapid rise or fall, and so on) will be globally effective. The significance ofeustatic fall-and-rise inflection points is considered with regard to the occurrence of unconformities and condensed sections, respectively.Type I unconformities are related to rapid eustatic falls, and type 2 unconformities are related to slow eustatic falls.

INTRODUCTION

The objective of this paper is to provide a conceptualframework for understanding the relationship between rel­ative sea-level changes and clastic depositional stratal pat­terns on basin margins. An awareness of this relationshipwill provide the geologist with a tool to understand betterthe relationship between depositional sequences and thedistribution of lithofacies within these sequences. Key tennsused in this paper are defined in Table 1. See Van Wagonerand others (this volume) for additional definitions.

This study builds on previous studies by Vail and others(1977); Vail and Todd (1981); Vail and others (1984); andJervey (this volume), which established that a relationshipexists between relative sea-level change and depositionalstratal patterns. This paper analyzes the mechanics of thisstratigraphic relationship in the context of sequence, sys­tems tract, and types of unconfonnity discussed. The paperby Posamentier and Vail (this volume) addresses the se­quence and systems-tract models as well as variations onthe model.

Conceptual models presented in this report represent ananalysis of the effects of accommodation change on clasticsediment deposition and suggest how sedimentary basinswill fill. It must be emphasized that the models are gen­erally applicable. The effects of local factors such as cli­mate, sediment supply, and tectonics must be incorporatedinto the models before these models can be applied to aparticular basin. Once these considerations have been takeninto account, the refined models can then be used predic­tively to simulate local conditions in order to predict lith­ologic succession better.

'Present addresses: Esso Resources Canada Ltd., 237 4th Avenue SW,Calgary, Alberta T2P OH6 (HWP).

2Department of Geology, Rice University, Houston, Texas 77251 (PRY).

ASSUMPTIONS

Geologic and seismic observations of accommodation anddeposition show that a predictable succession of as manyas four systems tracts can be generated by varying globalsea level. These are highstand, lowstand (including low­stand fan and lowstand wedge), transgressive-, and shelf­margin systems tracts, and are shown in block diagrams inFigures 1-6. Each systems tract is composed of one or moredepositional systems (Brown and Fisher, 1977) and each ischaracterized by a set of lithofacies. The timing of uncon­fonnites or surfaces of nondeposition bounding these sys­tems tracts can also be predicted from the sea-level curve.

Locally, however, the models should first be refined bythe incorporation of subsidence and sediment supply infor­mation before predictions are made. Each systems tract willbe considered in greater detail by Posamentier and Vail (thisvolume).

In order to develop generally applicable depositionalmodels, it was assumed that the following conditions wouldbe present:

-The rate of seafloor subsidence an any single location ona profile was held constant. Seafloor subsidence is pri­marily a function of lithospheric cooling and sedimentloading (together they compose total subsidence). Geo­history analyses from a variety of sedimentary basinssuggest that eustatic variations occur with greater fre­quency than subsidence variations. Thus, over a limitedinterval, the assumption of constant subsidence rate seemsacceptable. Nonetheless, when the general model ismodified to account for local conditions, nonunifonnsubsidence can be accommodated.

- Totai subsidence increases in a basinward direction. Thisseems to characterize most divergent basin margins.

-Deposition was occurring along a divergent continentalmargin characterized by a shelf, slope, and basin, where

Sea-Level Changes-An Integrated Approach, SEPM Special Publication No. 42Copyright © 1988, The Society of Economic Paleontologists and Mineralogists, ISBN 0-918985-74-9

110 H. W. POSAMENTIER, M. T. JERVEY AND P. R. VAIL

-, TABLE 1.~DEFINITION OF KEY TERMS

Sequence Stratigraphy:The study of rock relationships within a chronostratigraphic framework whereinthe succession of rocks is cyclic and is composed of genetically related stratalunits (sequences and systems tracts).

Depositional System:A three-dimensional assemblage of lithofacies, genetically linked by active(modem) or inferred (ancient) processes and environments (delta, river, barrierisland, and so on) (Brown and Fisher, 1977).

Systems Tract:A linkage of contemporaneous depositional systems (Brown and Fisher, 1977).Each is defined objectively by stratal geometries at bounding surfaces, positionwithin the sequence, and internal parasequence stacking patterns. Each isinterpreted to be associated with a specific segment of the eustatic curve (i.e.,eustatic lowstand-lowstand wedge; eustatic rise~transgressive; rapid eustaticfall-lowstand fan, and so on), although not defined on the basis of thisassociation.

Sequence:A relatively conformable succession of genetically related strata bounded at itstop and base by unconformities and their correlative conformities (Vail andothers, 1977). It is composed of a succession of systems tracts and is interpretedto be deposited between eustatic-fall inflection points.

Parasequence:A relatively comformable succession of genetically related beds or bedsetsbounded by marine-flooding surfaces and their correlative surfaces (VanWagoner, 1985).

Unconformity:A surface separating younger from older strata, along which there is evidence ofsubaerial erosional truncation (and, in some areas, correlative submarine erosion)or subaerial exposure, with a significant hiatus indicated.

Condensed Section:A thin marine stratigraphic interval characterized by very slow depositional rates<1-10 mm/1000 yr) (Vail and others, 1984). It consists of hemipelagic andpelagic sediments, starved of terrigenous materals, deposited on the middle toouter shelf, slope, and basin floor during a period of maximum relative sea-levelrise and maximum transgression of the shoreline (Loutit, and others, thisvolume).

Accommodation:The space made available for potential sediment accumulation (Jervey, thisvolume).

Equilibrium Point:The point along a depositional profile where the rate of eustatic change equalsthe rate of subsidence/uplift. It separates zones of rising and falling relative sealevel.

Equilibrium Profile:The longitudinal profile of a graded stream or of one whose smooth gradient atevery point is just sufficient to enable the stream to transport the load ofsediment made available to it. It is generallx...-gai'Oed as a smooth, paraboliccurve, gently concave to the sky, practically flat at the mouth and steepeningtoward the source (Gary and others.. 1974).

sediment supply remains constant. In the real world, dif­fering rates of sediment supply affect primarily the sea­ward extent of deposition. In the landward direction, stratalpatterns will usually show onlap and aggradation as thespace between sea floor and base level is filled, whereasthe basinward limit of progradation is a function of sed­iment supply and b,asin margin geometry. Thus, in thelandward direction; where base level (sea level or graded­stream profile) is the controlling factor, the stratal pat­terns on the landward side of identical basins will be thesame regardless of the sediment supply. When local sed­iment supply parameters are incorporated into the model,the effect will be observed primarily at the seaward limitof deposition.

-The trend of eustatic change is curvilinear, approaching

sinusoidal. Although the actual trend of eustatic changeis clearly not sinusoidal, the eustatic curve may none­theless be resolved by a series of sine curves. It will beshown that depositional stratal patterns are directly re­lated to inflection points on this curve.

Again, it must be emphasized that, although these modelsare generally applicable, the overprint of local factors mustbe considered in order to utilize them in a predictive modefor a particular basin. By incorporating local factors, onecan readily refine the models to simulate local conditions.

RELATIVE SEA-LEVEL CHANGE AS A FUNCTION OF EUSTASYAND SUBSIDENCE AND ITS EFFECT ON DEPOSITIONAL

STRATAL PATTERNS

Stratal patterns and facies distribution depend in part upon(1) the amount of space available for the sediment and (2)the rate of change of new space added. Sediment is de­posited in the space between the sea floor and base level(sea level in a marine environment or graded-stream profilein a nonmarine fluvial environment). The space availableis referred to as accommodation, defined by Jervey (thisvolume) as "the space made available for potential sedi­ment accumulation, [which] is a function of both sea-levelfluctuation and subsidence. " In this paper the terms accom­modation and new space added are used. Accommodationrefers to all the space available for sediment to fill, includ­ing old space (leftover space not filled during an earliertime) plus new space added, whereas new space added re­fers only to space contemporaneously being made avail­able. Accommodation may vary as both the upper and lowerboundaries of this space move up or down. The result isan accommodation envelope, which defines the space avail­able for sediment to fill (Fig. 7).

Relative sea-level change, rather than eustatic change only,controls accommodation change. Eustacy (global sea level)refers only to the position of the sea surface with referenceto a fixed datum, such as the center of the earth, and istherefore independent of local factors. Relative sea levelincorporates local subsidence and/or uplift by referring tothe position of the sea surface with respect to the positionof a datum (e.g., basement) at or near the seafloor (Fig.8). Thus, relative sea-level change observed along a profilevaries with local subsidence or uplift. A relative rise or fallof sea level determines whether or not new space is beingmade available for sediment to fill. A relative rise adds space,whereas a relative fall takes space away. Consequently, evenduring a eustatic stillstand or slow eustatic fall, relative sealevel may continue to rise and add new space as a result oflocal subsidence. This parameter is independent of sedi­ment accumulating above the datum plane and should notbe confused with water depth.

Water depth involves the integration of a third parame­ter-sediment supply-with eustacy and tectonics. Waterdepth may be described as relative sea level less accumu­lated sediment, as illustrated in Figure 8. Thus, relative sealevel may continue to rise and add new space to accom­modate sediment, whereas water depth may nonetheless si­multaneously decrease if sediment is accumulating fasterthan relative sea level is rising.

EUSTATIC CONTROLS ON CLASTIC DEPOSITION I

LATE HIGHSTAND

(PREDOMINANTLYf PROGRADATIONAL)

} EARLY HIGHSTAND

(PREDOMINANTLYAGGRADATIONAL)

EUSTACY

111

LITHOLOGY

SUBMARINEFAN

FACIES

COASTALPLAINAND

NEAR SHOREFACIES

..FLUVIAL

FACIES

..OFFSHORE

MARINEFACIES

I~vlLEVEED

CHANNELFACIES

LOW

FiG. I.-Highstand systems tract, 1.

Significance of the inflection point.-

Eustatic change is a curvilinear function punctuated byinflection points. These are points on the curve where ab­solute slope or rate of change is greatest. Figure 9 illustratesa hypothetical sea-level curve with two inflection points.The one on the falling limb will be referred to as the Einflection point, and the one on the rising limb will be re­ferred to as the R inflection point.

Basin-margin depositional stratal patterns depend in largepart on eustacy and seafloor subsidence. Sedimentation onthe shelf involves filling the wedge-shaped space betweenthe sea surface and the seaward-dipping sea floor. The stra­tal pattern which results will depend on the rate at whichspace has been added and how sediment responds to this

addition of space. If the sediment supply is sufficient toallow continued aggradation to base level, then, as the rateof addition of new shelf space slows, the rate of aggrada­tion will gradually decrease. As a result of the decreasedrate of aggradation, progressively less sediment will be re­quired to keep up with slower-rising base level and, con­sequently, progressively more sediment will be availablefor progradation.

Figure 10 illustrates how the rate of accommodation change(i.e., dA/dt or rate of new space added) varies with eus­tacy. At E inflection points, the rate at which new shelfspace is added is least; at R inflection points, the rate isgreatest (Fig. 10). Little or no new shelf space is beingadded at E inflection points, so relatively little new sedi­ment can be accommodated there (assuming that sediment

EUSTATIC CONTROLS ON CLASTIC DEPOSITION I 115

ACCUMULATEDSEDIMENT

CENTER OF THE EARTH

FiG. 8.-Eustacy, relative sea level, and water depth as a function of sea surface, water bottom, and datum position.

builds to base level). Hence, at E inflection points, the rateof aggradation will be least and that of progradation willbe greatest. At R inflection points, the opposite occurs. Thethinnest topset beds (per unit time) occur at the E inflectionpoint (T6, Fig. 11) and, conversely, the thickest topset beds(per unit time) occur at R inflection points. Thus, with con­stant sediment supply, rates of aggradation and prograda­tion are inversely related. As a result, within successiveparasequences, shoreline regression tends to be progres­sively more rapid approaching the E inflection point andgradually less rapid thereafter (Fig. 11). Maximal rate ofaddition of new space at R inflection points commonly re-

F INFLECTION POINT

FiG. 9.-Elements of eustatic change.

One-dimensional model.-

sults in transgression and the development of starved orcondensed sections.

The maximum landward encroachment of the condensedaction or maximum flooding usually occurs sometime afterthe R inflection point during the eustatic rise (Fig. 12). Notethat topset (i.e., shelf) beds approach maximum thickness(per unit time) at the eustatic-rise inflection point, and atthe same time, approach their minimum areal extent. Asthese layers onlap progressively farther landward, the po­sition of the basinward pinchout of each layer also migrateslandward until T9 is reached. This marks the time of max­imum flooding (TMF). After time T9, the seaward limit ofeach time slice migrates progressively basinward as regres­sion resumes.

A basinward shift of coastal onlap characterizes E in­flection points. Coastal onlap may be defined as the land­ward limit on the shelf or upper slope of sediment distri­bution-marine or nonmarine. It has been observed thatinitiation of fluvial erosion resulting in globally synchro­nous subaerial unconformities is associated with these ba­sinward shifts and apparently is controlled by sea-levelchange (Vail and others, 1977). This will be considered inPosamentier and Vail (this volume).

At any point on a continental margin, the rate at whichnew space is made available for sediment to fill is deter­mined by the rate of relative sea-level change and is equalto the rate of eustatic change minus the rate of subsidence(Fig. 10). For example, if global sea level is falling at acertain rate and the sea floor is subsiding at the same rate,relative sea level remains unchanged and no new space isbeing made available. If global sea level is falling, but moreslowly than the sea floor is subsiding, the net effect will

•RISING. LIMB• I-

TIME---

FALLING LIMB..

HIGH

LOW

116 H. W. POSAMENTIER, M. T. JERVEY AND P. R. VAIL

HIGH +EUSTACY

LOW ~

UPLIFT +

SUBSIDENCE

SUBSI DENCE ,

TIME..

[

RATE OF ]EUSTATICCHANGE

MINUS

[RATE OF ]

SUBSIDENCE

EQUALS

[

RATE OF]RELATIVESEA-LEVEL

CHANGE

RISE t

RISE tO~...I..I...L..&.I.o.I-"""""I.o.I-L...L..L...I..'L....L..,jL....L..,jU-l""""'~"~.&..I....I..I.L.L

FALL.

RATE OF ADDITIONOF NEW SPACE

FIa. lO.-Relative sea level as a function of eustacy and subsidence.

be relative sea level rise, and consequently, new space winbe added.

In the example shown in Figure 10, most of the intervalis characterized by a relative rise of sea level since the rateof subsidence exceeds the rate of eustatic change most ofthe time. Thus, new space is being added throughout mostof the interval. The actual accommodation at any given timeis the sum of the new space added plus the left-over unfilledspace and occurs between base level and the sea floor.

Two-dimensional model.-

On passive continental margins, subsidence.gradually in­creases from shelf to basin, resulting in a basinward in­crease in the rate of addition of new space. The effect ofthis differential subsidence is illustrated in Figure 13, wherethe rate of addition of new space is shown for outer, mid­dle, and inner shelf-edge positions. Note that greatest ac-

commodation occurs on the outer shelf where subsidenceis greatest. Conversely, on the inner shelf, where subsi­dence is least, there are intervals when no new shelf spaceis being added.

A continental shelf profile may be subdivided into twozones separated by the equilibrium point. This point is de­fined as the point along a profile where the rate of eustaticchange equals the rate of subsidence. Seaward of this point,the rate of subsidence is greater than the rate of eustaticfall, resulting in the addition of new space, whereas land­ward the opposite occurs. Alternatively, the equilibrium pointdefines two zones: (1) a zone of relative sea-level rise thatis seaward of the equilibrium point and, (2) a zone of rel­ative sea-level fall that is landward of the equilibrium point.Figure 14 illustrates the seaward migration of the equilib­rium point in response to an increasing rate of eustatic fall.The addition of new space over the whole shelf profile is

EUSTATIC CONTROLS ON CLASTIC DEPOSITION I 117

EUSTACY

~RISE

INFLECTION POINT

DECREASING RATEOF REGRESSION

THICKNESS PER UNIT TIME ON SHELF

A>B>C>D>E=F<G<H<I

10

FALL~

TIMEt

T10T9T80T7~T6--..T5-

T4T3

T2

T1

FiG. 11.-Response of topset bed thickness to eustatic fall.

least when the rate of eustatic fall is greatest (at the r: in­flection point). At this time (T4), the equilibrium point hasreached its maximum seaward position. Conversely, theequilibrium point reaches its maximum landward positionat the Be inflection point.

Figure 15 illustrates the effect of different subsidence rateson the zones where new space is being added. Longitudinalprofiles are shown for two basins differing only in theirsubsidence rates. Given the same eustatic change, basin A,

with a high subsidence rate, is characterized by a greateraddition of new shelf space than basin B, where subsidencerate is low. All else being equal, relatively greater aggra­dation will occur in basin A than in basin B, whereas thelatter basin will be characterized by a higher rate of pro­gradation.

The response of sedimentation to an interval of slow eus­tatic fall is shown in Figure 16. A type 2 unconformity isillustrated in this figure. From Tl to T4, the rate of eustatic

INFLECTIONPOINT

T12 J T12

T10 T~: ~ T~~

tT8 T7 ;;2 T6TIME T6· T5 u

:I:T4 T3 f- T4

T2 T2T1 TO- ---HIGH LOW

EUSTACY

EXTENT

----TIME OF MAXIMUM FLOODING8

2

~

BASIN

RELATIVE AREAL EXTENT OF SUCCESSIVETIME·EQUIVALENT LAYERS

ONLAP ONTO AN INCLINE SLOPE

FiG. I2.-Response of topset bed thickness and timing of maximum flooding to eustatic rise.

118 H. W. POSAMENTIER. M. T. JERVEY AND P. R. VAIL

A. INNER SHELF POSITION (POSITION 31 B. MIDDLE SHELF POSITION (POSITION 21

RISE FALL--- ----RISE FALL-- --TECTONIC ­

THERMALSUBSIDENCE

TECTONIC­THERMAL

SUBSIDENCE

+000

FIRST FIRST RATE OFDERIVATIVE DERIVATIVE RELATIVE

OF OF SEA-LEVELEUSTATIC - SUBSIDENCE = CHANGE

CURVE CURVE

[RATE OF ] [ RATEOF ]

EUSTATIC SUBSIDENCECHANGE

EUSTACY

TIME

+o 0 0

FIRST FIRST RATE OFDERIVATIVE DERIVATIVE RELATIVE

OF OF SEA-LEVELEUSTATIC - SUBSIDENCE = CHANGE

CURVE CURVE

[

RATE OF ] [ RATEOF ]EUSTATIC SUBSIDENCECHANGE

EUSTACY

TIME

C. SHELF EDGE POSITION (POSITION 11 D.

TIME

1

~SHELF EDGE

2

~

3 2

+===~~~~CEl==TIME

wLL.()

0 2wWo

f-­<tooa:'":::l

oIl

3

~

RATE OFSUBSIDENCE

o 0FIRST RATE OF

DERIVATIVE RELATIVEOF SEA-LEVE L

- SU BSI DENCE = CHANGECURVE

[RATE OF ]

SUBSIDENCE

oFIRST

DERIVATIVEOF

EUSTATICCURVE

[RATE OF ]EUSTATICCHANGE

TECTONIC ­THERMAL

SUBSIDENCE

EUSTACY

RISE FALL--- ----

TIME

FIG. l3.-Response of relative sea level to differential tectonic thermal subsidence.

fall gradually increases and the equilibrium point migratesbasinward. Prior to T4, the bayline, defined as the demar­cation line between fluvial l and paralic/delta plain envi­ronments, migrates landward in response to slowly risingrelative sea level. The bayline is differentiated from theshoreline, which is the demarcation line between paralic/deltaic and marine environments. Under certain circum­stances, the bayline can be in the same position as the

'Fluvial deposition as used in this report refers to sedimentation abovesea level only and does not include fluvial deposits on the delta plain/coastal plain.

shoreline (e.g., if no bay or lagoon is present). The termbayline, rather than shoreline, is used extensively in thispaper because the points to which stream profiles are ad­justed lie on the bayline rather than on the shoreline. Thus,it is the position and migration direction of the bayline thatis significant when fluvial deposition is considered.

Figure 17 illustrates the landward translation of the bay­line because of this rise of relative sea level. The rate ofthis landward migration is a function of the rate of relativesea-level rise and the slope of the land over which the bay­line travels (e.g., a high rate of relative sea-level rise cou­pled with a low slope will result in rapid migration). Assediment fills the wedge-shaped area on the shelf between

120 H. W. POSAMENTIER. M. T. JERVEY AND P. R. VAIL

BASIN A.(HIGH SUBSIDENCE RATE)

BASIN B.(LOW SUBSIDENCE RATE)

•ZONE WHERE NEWSPACE IS BEING ADDED

•ZONE WHERE NO NEWSPACE IS BEING ADDED

+ SEA SURFACE TENDENCY

SEAFLOOR TENDENCY

FIG. 15.-New space added as a function of subsidence rate.

ceases, the point of coastal onlap abruptly shifts basinward,occurring again at the bayline (see Posamentier and Vail,this volume). At this time, deposition in the absence of thefluvial component is once more restricted to the wedge­shaped space between the sea floor and the sea surface (T6to T8, Fig. 16). It should be noted again that a type 2 un­conformity is shown here. With a rapid eustatic fall gen­erating a type 1 unconformity, cessation of widespread flu­vial deposition is marked by stream rejuvenation and incision.Subsequently, when relative sea-level rise resumes, theseincised valleys (which may be quite extensive) fill with flu­vial and/or estuarine deposits. Thus, incised-valley fill willoverlie the type 1 unconformity in places.

ELEMENTS OF THE COASTAL ONLAP CURVE

The general shape of the coastal onlap curve on the global­cycle chart is based on a set of observations from manysedimentary basins (Vail and others, 1977). It representsthe maximum landward limit of terrigenous deposition andcomprises either nonmarine or marine sediment. Its specificshape is inferred from models of accommodation and de­position. Figure 18 illustrates a hypothetical coastal onlapcurve with two type 2 unconformities. From.the r inflec­tion point to the B inflection point, the rate of addition ofnew space steadily increases (Fig. 10). As a result, the rateof landward migration of the bayline and thus, coastal on­lap, increases. At B inflection point time, the rates of rel­ative sea-level rise at the bayline and the rate of landward

bayline migration are greatest. Thereafter, until the equi­librium point reaches the bayline, the rate of relative sea­level rise decreases at the bayline. In addition, the rate oflandward migration of the bayline decreases as well. Be­cause the point of coastal onlap is at the bayline during thistime, the rate of landward shift of coastal onlap thus alsoappears to decrease. When the equilibrium point reachesthe bayline, the rate of relative sea-level rise there is zero.From this time until r inflection point time, the equilibriumpoint and the bayline move basinward together. The mi­gration rate of the equilibrium point, and hence the bayline,decreases as the equilibrium point gradually reaches its mostbasinward position.

Concomitant fluvial deposition results in a continuedlandward shift of coastal onlap until r inflection point time.The rate of this landward shift of coastal onlap graduallydecreases in response to the decelerating landward shift ofstream equilibrium profiles as the r inflection point isreached. At this time, the equilibrium point and the baylinechange migration direction and once again move landward,resulting in the cessation of widespread fluvial deposition.This, in tum, results in an apparent abrupt basinward shiftof coastal onlap back to the bayline. Again, it should benoted that a type 2 unconformity is described here. Type 1unconformities are often characterized by extensive in­cised-valley fluvial deposits overlying the unconformity(Posamentier and Vail, this volume).

The chronostratigraphic distribution of the condensedsection is also shown in Figure 18. The maximum landward

EUSTATIC CONTROLS ON CLASTIC DEPOSITION I 121

EUSTACY

COASTAL ONLAP

TIMEt

INFLECTION TSPOINT T]t TS .A2.-----__T4 _ TS__~ _

T2 T3 (F:O~o"~~~IN)Tl DEPOSITION

~_TS

ALLUVIUM T- T4

- -LANDWARD BASINWARD

- -RISE FALL

TECTONICHINGE POINT

T]

PROFILE DETERMINEDBY STREAM·STATE

TS PROFI;

TECTONICHINGE POINT

ARROWS INDICATE DIRECTIONALTENDENCYOFTHELANDSURFACEISEAFLOOR)RELATIVE TO SEA LEVEL

EO. PT.

~.

DIRECTION OFEO. PT. BAYLINE

Tl l MIGRATION•TECTONICHINGE POINT

BAYUNE

T2EO. PT.

l •TECTONICHINGE POINT

FIG. 16.-Response of sedimentation to an interval of slow eustatic fall.

extent of the condensed section is shown as a dashed line,because the seaward limit of terrigenous deposition may varysignificantly as sediment supply varies from basin to basin.The maximum seaward limit of significant terrigenous sed­iment distribution occurs at the r inflection point. At thistime, the rate of new shelf space added is at a minimum.The time of maximum regression, however, occurs some­what later when, with continued increase in rate of newshelf space added, progradation finally gives way to retro­gradation. The time represented by the condensed sectiondecreases in a landward direction, with the condensed sec­tion reaching its maximum landward position at the time ofmaximum flooding sometime after the R inflection point(see Fig. 12). The surface corresponding to the time ofmaximum flooding is called the downlap surface (DLS) (Vailand others, 1984) or maximumjlooding surface (MFS). Ingeneral, downlap surface is used where seismic data are

involved and the downlapping toes of clinoforms can beobserved. Maximum flooding surface is more commonlyused when only outcrop or well-log data are involved.

STRATAL PATIERNS (PARASEQUENCE SCALE) ASSOCIATED WITHVARYING RATES OF EUSTATIC RISE OR FALL

This discussion addresses perturbations, or "bumps," onthe eustatic curve, rather than true higher frequency cyclessuperimposed upon a lower frequency eustatic curve. Themodel predicts that all eustatic cycles, regardless of fre­quency, will result in the deposition of sequences composedof a predictable succession of systems tracts. (Sequence type,i.e., type. 1 or 2, will be a function of local subsidencerate.) Unconformities associated with such high-frequencyeustatic cycles superimposed on general eustatic rises andfalls may correspond to fourth or fifth order (in the sense

122 H. W. POSAMENTIER, M. T. JERVEY AND P. R. VAIL

ZONE OF +-FALLING RELATIVE ZONE OF RISING RELATIVE SEA LEVEL .-

SEA LEVEL

LANDWARD TRANSLATION OF BAYLINE DUETO RELATIVE SEA-LEVEL RISE

FIG. 17 .-Effect of relative sea-level rise on coastline position.

of Vail and others, 1977). Higher frequency eustatic cyclesmay be associated with parasequence sets or parsequences(Van Wagoner, 1985). Simple perturbations on the eustaticcurve, however, where there is no change in eustatic ten­dency, (i.e., fall to rise or vice versa) generate a differentstratal response.

When perturbations in overall eustatic rises or falls oc­cur, additional inflection points are generated. If the rateof eustatic fall changes from decreasing to again increasingfollowing an E inflection point on the falling limb of a eus­tatic curve, another inflection point is generated (Fig. 19).This will be referred to as an E' inflection point. Similarly,on the rising limb of a eustatic curve, another inflectionpoint is generated if the rate of eustatic rise changes fromdecreasing to again increasing after an R inflection point.

This will be referred to as an R' inflection point. R' in­flection points are similar to E inflection points since bothare associated with times of farthest basinward position ofthe equilibrium point. Similarly, E' inflection points are as­sociated with times of farthest landward position of theequilibrium point and are therefore similar to R inflectionpoints (Fig. 19).

The effects of perturbations on a eustatic rise may beobserved primarily at the seaward limit of terrigenous de­position. Subaerial unconformities, with concomitant ba­sinward shifts of coastal onlap, typical of type 1 or type 2unconformities, do not occur in association with either Ror R' inflection points. Rather, an uneven eustatic rise ischaracterized by recurrent condensed sections correspond­ing to successive intervals of maximum flooding associated

EUSTACY

~----

DECREASING RATE OFLANDWARD ENCROACHMENTOF COASTAL ONLAP

tTIME

----LOW

INCREASING RATE OF LANDWARDENCROACHMENT OF COASTALONLAP

--­BASINWARD

EQUILIBRIUM POINTLOCATION

-LANDWARD

• FLUVIAL FACIES • PARALIC/DELTAIC AND MARINE FACIES

FIG. IS.-Elements of the coastal-oniap curve.

EUSTATIC CONTROLS ON CLASTIC DEPOSITION I 123

1 OR 2 UNCONFORMITY

til tilUJ UJ...J ...JU U>- >­u urr: rr:UJ UJo 0rr: rr:o 0o Irr: e­M "

~1

IEQUILIBRIUM

POINTPOSITION

(RELATIVE TOTECTONIC

HINGE POINT)

(RELATIVE TOEQUILIBRIUM

POINT) LANDWARD

"SEAWARD LIMITOF TERRIGENOUS SEDIMENT

---.­SEAWARD

~-----1-- - - - -

RATE OF RELATIVESEA-LEVEL

CHANGE

LHS

EHS

LSFLSWLSFLSWLSF

LHS

EHS

--l:IJ»zUl

SM

LHS

LHS - LATE HIGHSTAND SYSTEMS TRACT

EHS EAR LY HIGHSTAND SYSTEMS TRACT

LSIN- LOWSTAND WEDGE

LSF - LOWSTAND FAN

TRANS - TRANSGRESSIVE SYSTEMS TRACT

SM - SHELF MARGIN SYSTEMS TRACT

FIG. 19.-Effect of varying rates of eustatic rise and fall.

with each Eo inflection point. Figure 19 shows two Eo in­flection points and two condensed sections on each of thegeneral eustatic rises shown. Each condensed section is as­sociated with an Eo inflection point when the rate of additionof new space is greatest. Note that the second condensedsection extends farther landward in each case, even thoughthe rate of relative sea-level rise is the same. (This diagramis drawn so that the slope at each Eo inflection point is the

same.) During the overall eustatic rise, the equilibrium pointremains landward of the tectonic hinge point, never reach­ing the bayline (Fig. 19). This assumes that the bayline islocated seaward of the hinge point. Consequently, the bay­line lies in the zone of relative sea-level rise and migrateslandward throughout this interval. No fluvial aggradationwill occur at this time and, therefore, no subaerial uncon­formity or basinward shift of coastal onlap characterizes

124 H. W. POSAMENTlER, M. T. JERVEY AND P. R. VAIL

g' inflection points. Rather, these points are characterizedonly by an increased basinward encroachment of terrigen­ous deposition.

Perturbations on a sea-level fall usually generate the morefamiliar pattern of a succession of type I (or 2) unconform­ities and condensed sections (Fig. 19). An uneven or steppedsea-level fall is characterized by a succession of E and E'inflection points. Each E inflection point will generate atype 1 (or 2) unconformity, provided the equilibrium pointreaches the bay1ine prior to that time so as to initiate fluvialdeposition. E' inflection points, similar to g inflection points,are associated with condensed sections. At these inflectionpoints, the equilibrium point is at a maximum landwardposition, generating conditions analogous to those whichgenerate condensed sections associated with g inflectionpoints. The first basinward shift of coastal onlap occurs atthe first E inflection point and separates the highstand sys­tems tract into early and late sections.

The early section of the highstand systems tract is char­acterized by alternating transgressions and regressions cor­responding to g and g' inflection points, respectively. It iscapped by fluvial deposits associated with the first E in­flection point. The late section of the highstand systemstract is characterized by recurrent basinward shifts of coastalonlap with associated subaerial exposure surfaces that cor­respond to successive E inflection points (see Fig. 1). Be­cause the rate of new space added generally decreasesthroughout the highstand systems tract, the early section ofthis systems tract is usually characterized more by aggra­dation than progradation, whereas the opposite usually ap­plies to the late section of the systems ~ract. The maximumlandward position of coastal onlap steps basinward with eachsuccessive E inflection point. Each E' inflection point re­sults in a condensed section. The maximum landward en­croachment of the condensed section also steps basinwardwith each successive E' inflection point. The dominant Einflection point during an overall eustatic fall is usuallycharacterized by the most pronounced unconformity.

CONCLUSIONS

The observation that similar stratal patterns develop atthe same time in widely varied sedimentary basins suggestsa globally effective control such as eustatic change. Theinteraction of eustacy with local tectonics and sedimentsupply determines local depositional stratal patterns. Theconcepts discussed here and in Jervey (this volume) serve

as the foundation or framework upon which the sequenceand systems tract depositional models discussed in the pa­per by Posamentier and Vail (this volume) are based. Cer­tain simplifying assumptions have been made for the pur­pose of presenting this model in a straightforward andcoherent fashion, but it should be emphasized that theseassumptions can and should be modified to conform to theconditions observed in specific basins before the modelscan be applied.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support and con­tributions from their many co-workers at Exxon ProductionResearch Company who contributed to the development ofthese concepts. We especially thank J. C. Van Wagoner,J. F. Sarg, R. M. Mitchum, and R. A. Hoover for theirhelpful suggestions and constructive critiques of this manu­script at various stages of its evolution. In addition, we thankG. J. Moir, W. A. Burgis, R. D. Erskine, G. Mirkin, V.Kolla, G. R. Baum, C. G. St. C. Kendall, and T. R. Nar­din for their helpful suggestions and comments. Ultimateresponsibility for the material presented herein, however,rests with the authors.

REFERENCES

BROWN, L. F., Jr., AND FiSHER, W. L., 1977. Seismic stratigraphic inter­pretation of depositional systems: Examples from Brazilian rift and pullapart basins, in Clayton, C. E., ed., Seismic Stratigraphy-Applicationsto Hydrocarbon Exploration: American Association of Petroleum Ge­ologists Memoir 26, p. 213-248.

GARY, M., McAFEE, R., Jr., AND WOLF, C. L., 1974, Glossary of Ge­ology: American Geological Institute, Washington, D.C., 805 p.

VAIL, P. R., HARDENBOL, J., AND TODD, R. G., 1984, Jurassic uncon­formities, chronostratigraphy and sea-level changes from seismic stra­tigraphy and biostratigraphy: American Association of Petroleum Ge­ologists Memoir 36, p. 129-144.

VAIL, P. R., MITCHUM, R. M., Jr., TODD, R. G., WIDMIER, J. M., THOMP­SON, S., III, SANGREE, J. B., BUBB, J. N., AND HATELID, W. G., 1977,Seismic stratigraphy and global changes of sea level, in Clayton, C.E., ed., Seismic Stratigraphy-Applications to Hydrocarbon Explora­tion: American Association of Petroleum Geologists Memoir 26, p.49-212.

VAIL, P. R., AND TODD, R. G., 1981, North Sea Jurassic unconformities,chronostratigraphy and sea-level changes from seismic stratigraphy:Petroleum Geology of the Continental Shelf of Northwest Europe, Pro­ceedings, p. 216-235.

VAN WAGONER, J. C., 1985, Reservoir facies distribution as controlledby sea-level change: Abstracts with Programs, Society of EconomicPaleontologists and Mineralogists Midyear Meeting, Golden, Colo­rado, p. 91-92.