9_Posamentier_Jervey&Vail_1988_EUSTATIC CONTROLS on clastic deposition CONCEPTUAL FRAMEWORK.pdf

8/12/2019 9_Posamentier_Jervey&Vail_1988_EUSTATIC CONTROLS on clastic deposition CONCEPTUAL FRAMEWORK.pdf

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EUST TIC CONTROLS ON CL STIC DEPOSITION ICONCEPTU L FR ME WORKH. W.POSAMENTIER1

Exxon Production Research Company, P.O. Box 2189, Houston, Texas 77252-2189;M.T.JERVEY,

Canadian Hunter Exploration Ltd., 700, 435 4th Avenue SW, Calga ry, Alberta TIP 3A8A N D

P. R. VAIL2Exxon Production Research Company, P.O. ox 2189, Houston, Texas 77252-2189

A B S T R A C T : A conceptual frameworkf orunderstandingthe 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 inflectionpoints. Tw o types of sequences have been recognized: (1) a type 1 sequence, which is bounded at the base by a type 1unconformityand 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 1 or type 2 unconformity and has nolow stand deposits.Each sequence iscomposed ofthree systems tracts;thetype 1sequence iscomposed oflo wstan d, transgressive-,andhighstandsystemstracts, and the type 2 sequence is composed of shelf-margin, transgressive-, and highstand systems tracts. Th e type 1 sequence isassociated with stream rejuvenationand incision at its base, whereas th e type 2 sequence is not.Eustacy and subsidence combine to make the space available fo r sediment to fill. Th e results of this changing accommodation areth e onlapping and offlapping depositional stratal patterns observed on basin margins. Locally, conditions of subsidenceand/or upliftand sediment supply ma y overprint bu t usually will no t mask th e effects of global sea level. A ny 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. Th e significanceofeustatic fall-and-rise inflection pointsisconsidered w ith regardto theoccurrence of unconformities andcondensed sections, respectively.Type 1unconformities are related to rapid eustatic falls, and type 2 unconformities are related to slow eustaticfalls.

I N T R OD UC T I ONThe objective of this paper is to provide a conceptualframework for understanding the relationship between rel-ative sea-level changes and clastic depositional stratal pat-terns on basin margins. A n awareness of this relationshipwill provide thegeologist witha tool to understand betterthe relationship between depositional sequences and thedistribution of lithofacies within these sequences. Key termsused in this paper are defined inTable 1. See Van Wagonerand others (this volume) fo r additional definitions.This study builds on previous studies by V ail and others

(1977); Vail andTodd (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, sys-tems tract, and types of unconformity discussed. The paperby Posamentier and Vail (this volume) addresses the se-quence an d 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 gen-erally applicable. The effects of local factors such as cli-mate, sediment supply,and tectonics mustbe 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 usedpredic-tively to simulate local conditions inorder to predict lith-ologic succession better.

'Presentaddresses:Esso Resources Canada Ltd., 237 4th A v e n ueSW ,Calgary, A lberta T2P OH6 (HWP).DepartmentofGeology, Rice University, Houston, Texas 77251(PRV).

ASSUMPTIONSGeologic 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 low-stand fan and lowstand wedge), transgressive-, an d shelf-margin systems tracts, and are shown inblock diagrams inFigures 1-6. Each systems tractiscomposed of one or moredepositional systems (B rown andFisher, 1977) and each ischaracterized by a set of lithofacies. The timingof uncon-formites or surfaces ofnondeposition bounding these sys-

tems tracts can also be predicted from the sea-level curve.Locally, however, the models should first be refined bythe incorporation of subsidencean d sediment supply infor-mation 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 tha t the following conditions wouldbe present:Therate of seafloor subsidencean anysingle location ona profile was held constant. Seafloor subsidence is pri-marily a function of lithospheric cooling and sedimentloading (together they compose total subsidence). Geo-history analyses from a variety of sedimentary basinssuggest that eustatic variations occur with greater fre-quency than subsidence variations. Thus, over a limitedinterval, the assumption of constant subsidence rate seemsacceptable. N onetheless, w hen the general model ismodified to account for local conditions, nonuniformsubsidencecan be accommodated.Totalsubsid ence increases in a basin ward direction . Thisseems to characterize most divergent basin margins.Deposition was occurring along a divergent continentalmargincharacterized by a shelf,slope, and basin, where

Sea-Level ChangesAn Integrated Approac h, SEPM Special PublicationNo. 42Copyright 1988, The Society of Economic Paleontologists and M ineralogists, ISBN 0-918985-74-9


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110 H. W. POSAMENTIER, M. T. JERVEY AND P.R. VAILTABLE1.DEFINITION OF KEYTERMS

Sequence Stratigraphy:Th e study of rock relationships within a chronostratigraphic framework whereinth e succession of rocks is cyclic and is composed of genetically related stratalunits (sequences and systems tracts).Depositional System:Athree-dimensional assemblageof lithofacies, genetically linkedby active(modern) or inferred (ancient)processes and environments (delta, river, barrieris land, and so on) (Brown andFisher, 1977).SystemsTract:A linkage of contemporaneous depositionalsystems (Brown andFisher, 1977).Each is defined objectively by stratal geometries at bounding surfaces, positionwithin the sequence, and internal parasequence stacking patterns. Each isinterpreted to beassociated with a specific segment of the eustatic curve (i .e . ,eustatic lowstandlowstand wedge; eustaticrisetransgressive; rapid eustaticfalllowstand fan, and soon), although no t 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 iscomposed of a succession of systems tracts and is interpretedto bedeposited between eustatic-fall inflection points.Parasequence:A relatively comformablesuccessionof genetically related beds orbedsetsbounded 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, insome 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


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EUSTAT1C CONTROLS ON CLASTIC DEPOSITION I 111

FIG. 1.Highstand systemstract, I.

Significance of the inflection point.Eustatic change is a curvilinear function punctuated byinflectionpoints.These are points on the curve where ab-soluteslopeor rate ofchangeis greatest. Figure 9illustratesa hypothetical sea-level curve with tw o inflection points.

The one on the falling limb will be referred to as the Finflection point, and the one on the rising limb will be re-ferred to as the R inflectionpoint.Basin-margindepositionalstratalpatternsdependinlargepart on eustacy and seafloor subsidence. Sedimentation onthe shelf involves filling thewedge-shapedspace betweenthesea surfaceand theseaward-dippingseafloor. The stra-tal pattern which results willdepend 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 baselevel, then, as therateof addition of new shelf space slows, therate of aggrada-tion will graduallydecrease. A s a result of the decreasedrate of aggradation,progressivelylesssediment will be re-quired tokeep up with slower-rising baselevel and, con-sequently, progressively more sediment will be availablefor progradation.Figure 10illustrateshow therateofaccommodation change(i.e.,dA/dt orrate of newspaceadded) varies with eus-tacy. A t F inflection points, the rate at which ne w shelfspace is added is least; at R inflection points, the rate isgreatest (Fig. 10). Little or no new shelf space is beingadded at F inflection points, so relatively little new sedi-ment can beaccommodated there (assuming thatsediment


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

FIG. 2.Lowstand systems tractlowstand fan.

FIG. 3.Lowstand systems tractlowstand wedge.


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EUSTATIC CONTROLS ON CLASTIC DEPOSITION I 113

FIG. 4.Transgressive-systems tract.

FIG. 5.H ighstand systems tract, II.


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114 H. W. POSAMENTIER, M. T. J E R V E Y AND P.R. V I L

FIG. 6.Shelf-margin systemstract.

FIG. 7.A ccommodationenvelope as a function of eustacy and subsidence.


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

builds tobase level). Hence, at F inflection points, therateof aggradation will be least and that of progradation willbegreatest.At Rinflectionpoints, theopposite occurs.Thethinnesttopsetbeds(perunittime)occurat the F inflectionpoint (T6,Fig.11) and, conversely, the thickest topset beds(perunittime)occurat R inflection points. Thus, withcon-stantsediment supply, rates of aggradation and prograda-tion are inversely related. A s a result, within successiveparasequences, shoreline regression tends to be progres-sively more rapid approaching the F inflection point andgradually less rapid thereafter (Fig.11). Maximal rate ofadditionof newspace at Rinflectionpoints commonly re-HIGH TIME

LOW

FALLING LIMB RISING LIMB

FIG. 9.Elements ofeustatic change.

suits in transgression and the development of starved orcondensed sections.The maximum landwardencroachmentof thecondensedaction or maximumflooding usually occurs sometime afterthe R inflection pointduring the eustaticrise (Fig. 12). Notethat topset (i.e., shelf) beds approach maximum thickness(per unit time) at the eustatic-rise inflection point, and atth e same time, approach their minimum arealextent. A sthese layers onlap progressively farther landward, the po-sitionof thebasinward pinchoutofeach layer also migrateslandward until T9 isreached. Thismarks the time of max-imumflooding (TMF). A f tertime T9, theseaward limitofeach timeslicemigrates progressivelybasinward asregres-sion resumes.A basinward shift of coastal onlap characterizes F in-flection points. Coastal onlap may be defined as the land-ward limit on the shelf or upper slope of sedimentdistri-butionmarine or nonmarine. It hasbeen observed thatinitiation of fluvial erosion resulting in globally synchro-nous subaerial unconformities is associated with theseba-sinward shifts and apparently is controlled by sea-levelchange(Vail andothers, 1977). Thiswill beconsideredinPosamentier and Vail (this volume).

One-dimensional model.A t any point on a continental margin, therate at whichnew space is made available for sediment to fill is deter-mined by therate ofrelativesea-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 acertainrateand the sea floor is subsiding at the samerate,relative sea level remains unchanged and no newspace isbeing made available. If global sea level is falling, but moreslowly than the sea floor is subsiding, the net effect will


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

FIG. 10.Relative sea level as a function of eustacy and subsidence.

berelativesea levelrise,andconsequently,new spacewillbeadded.In theexample showninFigure 10,mostof theintervalischaracterizedby a relativeriseof sealevel since therateof subsidenceexceeds therate of eustatic change most ofthe time. Thus, new space isbeing added throughout mosto f the interval. Th e actual accommodation at any given timeis the sum of the newspaceaddedplus theleft-over unfilledspace and occursbetween base level and the sea floor.

Two-dimensional model.Onpassive continental margins,subsidence-graduallyin-creases from shelf to basin, resulting in a basinward in-crease in therate of addition of new space. The effect ofthisdifferential subsidenceisillustratedinFigure 13,wheretherate of addition of newspace is shown for outer, mid-dle, an d inner shelf-edge positions. Note thatgreatest ac-

commodation occurs on the outer shelf where subsidenceis greatest. Conversely, on the inner shelf, where subsi-denceisleast,thereare intervals whenno new shelf spaceis beingadded.A continental shelf profile may be subdivided into twozones separated by theequilibriumpoint.This point is de-fined as the point along a profile where therate of eustaticchangeequalstherateof subsidence. Seawardofthispoint,the rate of subsidence is greater than the rate of eustaticfall, resulting in the addition of new space, whereas land-wardtheopposite occurs.Alternatively,theequilibriumpointdefines tw o zones: (1) a zone of relativesea-level rise thatisseawardof the equilibrium pointand,(2) azoneofrel-ativesea-levelfall thatislandwardof the equilibriumpoint.Figure 14 illustrates the seaward migration of the equilib-rium point inresponse to an increasingrateofeustaticfall.The addition of newspaceover the whole shelf profile is


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FIG. 11.Response of topset bed thickness to eustatic fall.

least when therate of eustatic fall is greatest (at theF in-flection point). A tthis time (T4), the equilibriumpointhasreached its maximum seaward position. Conversely, theequilibrium point reaches its maximum landward positionat the R inflection point.Figure 15illustrates the effect of different subsidence rateson thezones wherenewspaceisbeingadded.Longitudinalprofiles are shown for two basins differing only in theirsubsidencerates. Given th esame eustatic change,basin A ,

with a high subsidence rate, is characterized by agreateradditionof new shelf space than basinB,wheresubsidencerateislow.Allelse being equal, relativelygreater aggra-dation willoccur in basin A than in basin B, whereas thelatter basin will be characterized by a higherrate ofpro-gradation.The responseofsedimentation to anintervalof slow eus-tatic fall is shown inFigure 16. Atype 2 unconformity isillustrated in this figure.FromTI toT4, therateof eustatic

FIG. 12.Response of topset bed thickness and timing of maximum flooding to eustaticrise.


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118 H. W. POSAMENT1ER, M. T. JERVEY AND P.R. VAIL

FIG. 13.Response of relative sea level to differential tectonicthermal subsidence.

fall gradually increases and the equilibrium point migratesbasinward.Prior to T4, the bayline, defined as thedemar-cation line between fluvial1 andparalic/delta plain envi-ronments, migrates landward inresponse to slowly risingrelative sea level. The bayline is differentiated from theshoreline, which is the demarcation line betweenparalic/deltaic and marine environments. Under certain circum-stances, the bayline can be in the same position as the

'Fluvial depositionas usedin thisreport refers to sedimentationabovesea level only anddoes not include fluvial deposits on the deltaplain/coastal plain.

shoreline(e.g., if no bay orlagoonispresent). The termbayline, rather than shoreline, is used extensively in thispaperbecausethepointstowhich stream profiles are ad-justedlie on thebayline ratherthan on theshoreline.Thus ,itis the position andmigrationdirectionof thebayline thatis significant when fluvial deposition isconsidered.Figure 17 illustrates the landward translation of thebay-line because of this rise ofrelative sea level. Therate ofthis landwardmigration is afunction of the rateofrelativesea-levelriseand theslopeof the landoverwhich thebay-line travels (e.g., ahigh rate ofrelativesea-level risecou-pled with a low slopewill result in rapid migration). Assediment fills the wedge-shapedarea on the shelf between


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FIG. 14.Relative sea level as a function of eustacy and differential subsidence.

the sea floor and the sea surface basinward of the ba yline,the resulting stratal pattern will show a gradual landwardshift ofcoastalonlap. Th erateo f landward shift of thebay-line, andhence coastal onlap,alsodecreasesas therate ofrelative sea-levelriseat the bayline graduallydecreasesap-proachingT4. Simultaneously, theshoreline/delta front maymigrateeither landward orseaward dependingon sedimentsupply. (Fig. 16 illustrates basinward migration of theshoreline.)A fter the equilibrium pointreachesthe bayline at T4, thebaylinereversesmigrationdirectionand shifts basinward inconjunction with the equilibriumpoint. The points (at thebayline) to which streams in equilibrium aregraded movebasinward, causing a subsequent basinward shift of steady-state streamequilibrium profiles (T4 to T6,Fig. 16). Flu-vialdeposition occurs as streams attempt to maintain steady-state profiles. A t this time, the bayline is the line of de-marcation between zones characterized by subaerial andmarine accommodation.Subaerial accommodationmay bedefined as thespace available for sediment to fill betweenthe oldstreamequilibrium profile and anew,higher stream

equilibrium profile. Consequently, theaccommodation en-velopein this setting is bounded by the oldequilibriumpro-file below and the new one above. Only when the baylinemigratesbasinwardacrossa surface of lowrelief is subaer-ial newspacebeingadded. Sedimentdeposited thereisflu-vial.

The equilibrium point reaches its basinward-most posi-tion at the F inflectionpointandthen graduallymovesland-ward again. Widespread fluvial depositionceases at this timeas thebaylinereverses its migrationdirection and resumesits landward movement (T6 to T7,Fig. 16).A lthough in-stantaneous cessation of fluvial deposition issuggested here,this will occur if fluvialdeposition hasbeen able tokeepup with the basinward-shifting stream equilibrium profile.In mostcases, however, there ismore likely to be an in-determinate lag between the time the equilibrium profileceasesmoving basinward (at the F inflection point) and thetime thatfluvialdepositioncatchesup with the equilibriumprofile, fillingspace previously madeavailable.Localcon-ditions, including sediment supply and climate, will affectthis response time. N onetheless, once fluvial deposition


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

FIG. 15.New space added as a function of subsidencerate.

ceases, thepointofcoastalonlapabruptlyshifts basinward,occurring again at the bay line(see Posamentier andVail,this volume). Atthis time,depositionin theabsenceof thefluvial component is once more restricted to the wedge-shaped space between the sea floor and the seasurface (T6toT8, Fig. 16). It should be noted again that a type 2 un-conformity is shownhere. With a rapid eustatic fall gen-erating atype 1unconformity,cessationofwidespread flu-vialdepositionismarkedbystreamrejuvenation andincision.Subsequently, when relativesea-level rise resumes, theseincisedvalleys(which may bequite extensive)fill withflu-vialand/orestuarinedeposits.Thus, incised-valleyfillwilloverliethe type 1 unconformity inplaces.

ELEMENTS OF THE CO ST L ONL P CURVE

Thegeneralshapeof thecoastalonlap curve on theglobal-cyclechart isbased on a set of observations from manysedimentary basins (Vail andothers, 1977). It representsthe maximum landwardlimito fterrigenous deposition andcompriseseithernonmarineor marine sediment. Its specificshape is inferred from models ofaccommodation and de-position. Figure 18illustratesa hypotheticalcoastal onlapcurve with tw otype 2unconformities. From.the F inflec-tionpoint to the R inflection point, the rate of addition ofnewspace steadily increases (Fig. 10). A s a result, therateof landward migration of the bayline and thus,coastalon-lap, increases. A t R inflection point time, the rates ofrel-ativesea-level rise at the bayline and therateof landward

bayline migration are greatest. Thereafter, until theequi-libriumpoint reaches the bayline, therate ofrelative sea-levelrisedecreases at thebayline.Inaddition, therate oflandward migration of the baylinedecreases aswell. Be-causeth epointofcoastalonlapis at thebayline duringthistime, therate of landward shift ofcoastal onlap thusalsoappears todecrease. When the equilibrium pointreachesthe bayline, therateofrelativesea-level rise thereiszero.From this timeuntilFinflectionpoint time,theequilibriumpoint and thebayline move basinwardtogether. The mi-grationrateof theequilibrium point,andhencethebayline,decreasesas the equilibrium pointgraduallyreaches itsmostbasinwardposition.Concomitant fluvial deposition results in a continuedlandwardshift ofcoastalonlapuntilF inflectionpoint time.Th e rate of this landward shift of coastal onlap graduallydecreases inresponse to thedecelerating landward shift ofstream equilibrium profiles as the F inflection point isreached.A t thistime,th eequilibriumpointand the bay linechangemigration directionandonceagain m ove landward,resulting in thecessationofwidespreadfluvialdeposition.This, in turn,results in anapparent abruptbasinward shiftof coastal onlap back to the bayline. A gain, it should benoted that atype2unconformity isdescribed here. Type 1unconformities are often characterized by extensive in-cised-valley fluvial deposits overlying the unconformity(Posamentier and Vail, this volume).Th e chronostratigraphic distribution of the condensedsectionisalsoshowninFigure18. Themaximumlandward


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

extent of thecondensed section is shown as a dashedline,because the seaward limit ofterrigenous deposition mayvarysignificantly as sediment supplyvariesfrom basin to basin.The maximumseaward limit of significant terrigenoussed-imentdistributionoccursat theF inflection point. At thistime, therate of new shelf space added is at a minimum.The timeof maximum regression,however, occurssome-what later when, with continued increase in rate of newshelfspace added,progradation finallygives way toretro-gradation. The timerepresented by thecondensed sectiondecreasesin alandwarddirection, withthe condensedsec-tion reaching its maximumlandward positionat the time ofmaximum flooding sometime after the R inflection point(see Fig, 12). The surface corresponding to the time ofmaximum flooding iscalledthedownlap surface (DLS) (Vailand others,1984)or maximumflooding surface (MPS). Ingeneral, downlap surface is used where seismic data are

involved and the downlapping toes of clinoforms can beobserved. M ax imu m flooding surface is more commonlyused when only outcrop or well-log data are involved.STRATAL PATTERNS (P A R A SE Q UE N C ESCALE)ASSOCIATED WITH

V A R Y I N GRATES OF EUSTATIC RISE OR FALLThis discussionaddressesperturbations, or bumps, onth eeustatic curve, rather than truehigher frequencycyclessuperimposedupon a lower frequencyeustatic curve. Themodel predicts that all eustatic cycles, regardless of fre-quency,willresult in thedepositionofsequences composedof apredictable successionof systems tracts.(Sequencetype,i.e., type 1 or 2, will be a function of local subsidence

rate.) Unconformities associated with such high-frequencyeustaticcyclessuperimposed on generaleustatic rises andfalls maycorrespond to fourth or fifth order (in thesense


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122 H. W. POSAMENTIER, M. T. JERVEY ANDP R VAIL

FIG. 17.Effect of relative sea-level rise oncoastline position.

ofVailandothers, 1977). Higher frequencyeustaticcyclesmay beassociated withparasequence sets or parsequences(VanWagoner, 1985 .Simpleperturbations on theeustaticcurve, however, where there is no change in eustatic ten-dency, (i.e., fall to rise orvice versa generate a differentstratalresponse.Whenperturbations inoverall eustaticrises or falls oc -cur, additional inflection points are generated. If the rateofeustatic fall changes fromdecreasing to againincreasingfollowing an F inflectionpointon the falling limbo f aeus-taticcurve, another inflectionpoint isgenerated(Fig. 19).Thiswill bereferredto as anF' inflectionpoint. Similarly,on the rising limb of a eustatic curve, another inflectionpoint isgeneratedif therateofeustaticrisechanges fromdecreasing to again increasing after an R inflection point.

This will be referred to as an R' inflection point. R' in-flection points are similar to F inflection points since bothare associated with times of farthest basinward position oftheequilibrium point. Similarly,F' inflectionpoints are as-sociated with times of farthest landward position of theequilibriumpoint and are therefore similarto R inflectionpoints(Fig.19 .The effects of perturbations on a eustatic rise may beobserved primarily at the seaward limit of terrigenous de-position. Subaerial unconformities, with concomitant ba -sinward shifts ofcoastal onlap, typical of type 1 or type 2unconformities, do not occur in association with eitherRorR' inflection points. Rather, an uneven eustaticrise ischaracterizedbyrecurrent condensed sectionscorrespond-ing to successiveintervalsof max imum flooding associated

FIG. 18.Elements of thecoastal-onlapcurve.


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LHS LATEHIGHSTANDSYSTEMSTRACTEH S EARLY HIGHSTANDSYSTEMSTRACTLSW LOWSTANDWEDGELSF LOWSTAND FA NTRANS TRANSGRESSIVE SYSTEMSTRACTSM SHELFMARGINSYSTEMSTRACT

FIG. 19 Effectofvarying r tesof eust tic rise andfall

with each R inflection point. Figure 19 shows two R in-flection pointsand two condensed sectionsoneachof thegeneraleustaticrisesshown.Each condensed section is as-sociatedwith an R inflectionpointwhen therateofadditionof new space isgreatest. Note that the second condensedsection extends farther landward ineachcase, even thoughtherateofrelativesea-level riseis the same.(Thisdiagramis drawn so that the slope at each R inflection point is the

same.) D uring theoverall eustaticrise, the equilibrium pointremainslandwardof the tectonic hingepoint, neverreach-ing the bayline (Fig. 19). Thisassumesthat the bayline islocated seaward of thehinge point. Consequently, the bay-linelies in the zoneofrelative sea-levelriseandmigrateslandward throughout this interval. No fluvial aggradationwill occur at this time and, therefore, no subaerial uncon-formity or basinward shift of coastal onlap characterizes


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124 H. W. POSAMENTIER, M. T. JERVEY AND P. R. VAILR' inflection points. Rather,these points are characterizedonly by an increased basinward encroachment of terrigen-ous deposition.Perturbations on asea-level fall usuallygenerate themorefamiliar pattern of asuccession of type 1 (or 2)unconform-ities andcondensed sections(Fig. 19). A n uneven or steppedsea-level fall is characterized by a succession of F andF'inflection points. Each F inflection point will generate atype 1 (or 2) unconformity, provided th e equilibrium pointreachesthe baylinepriortothat timeso as to initiate fluvialdeposition.F' inflectionpoints, similar to R inflectionpoints,are associated withcondensed sections.Atthese inflectionpoints, the equilibrium point is at a maximum landwardposition, generating conditions analogous to those whichgenerate condensed sections associated with R inflectionpoints. The first basinward shift of coastal onlap occurs atth e first F inflection point andseparates th e highstandsys-tems tract into early and late sections.The early section of the highstand systems tract is char-acterized by alternating transgressions and regressionscor-responding to R andR' inflection points,respectively. It iscapped by fluvial deposits associated with the first F in-flection point. The late section of the highstand systemstractischaracterizedbyrecurrent basinward shifts ofcoastalonlap withassociated subaerial exposure surfaces thatcor-respond to successive F inflection points(see Fig. 1). Be-cause the rate of new space added generally decreasesthroughout the highstand systems tract, the early section ofthis systems tract is usually characterized more by aggra-dation than progradation, whereas th eopposite usually ap -pliesto the late section of the systems tract. The maximumlandwardpositionofcoastalonlap steps basinward witheachsuccessiveF inflection point. EachF' inflection point re-sultsin a condensed section. The maximum landward en-croachmentof thecondensed section alsosteps basinwardwith each successiveF' inflection point. The dominant Finflection point during an overall eustatic fall is usuallycharacterized by the most pronounced unconformity.

C O N C L U S I O N SThe observation that similar stratal patterns develop atthesametime inwidelyvariedsedimentary basinssuggestsa globally effective control such as eustatic change. Theinteraction of eustacy with local tectonics and sedimentsupply determines local depositional stratal patterns. Theconceptsdiscussed hereand in Jervey (thisvolume) serve

as the foundation or framework upon which the sequenceand systems tract depositional models discussed in the pa-per by Posamentierand Vail (this volume) arebased.Cer-tain simplifying assumptions have been made for thepur-pose 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 modelsca n be applied.

A C K N O W L E D G M E N T SThe authorsgratefully acknowledge the support andcon-tributions from theirmanyco-workersatExxonProductionResearchCompany who contributed to thedevelopment oftheseconcepts. W e especially thank J. C. VanWagoner,

J. F. Sarg, R. M. M i tc hum, and R. A. Hoover for theirhelpful suggestionsandconstructive critiquesofthis manu-script atvarious stages of itsevolution. In addition,we thankG. J. Moir , W . A . Burgis, R . D . Erskine,G. M irkin, V.Kolla, G. R. Baum, C. G. St. C. Kendall, and T. R. Na r -din for their helpful suggestions and comments. Ultimateresponsibility for the material presentedherein, however,rests with the authors.

REFERENCESB R O W N L. F., Jr., A N D F I S H E R W. L., 1977. Seismic stratigraphic inter-

pretationo fdepositional systems:Examples from Brazilian rift and pullapartbasins, inClayton, C.E.,ed.,SeismicStratigraphy-Applicationsto Hydrocarbon Exploration: A merican A ssociation ofPetroleum Ge -ologists Memoir26, p.213-248.

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