an overview of fundamental of sequence stratigraphy and key definition-van wagoner featuring...

8/12/2019 An Overview of Fundamental of Sequence Stratigraphy and Key Definition-Van Wagoner Featuring Posamentier & Mitchum

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AN OVERVIEW OF THE FUNDAMENTALS OF SEQUENCE STRAT IGRAPHY AND KEY DEFINITIONS

J. C. VAN WA G O N E R , H. W. POSAMENTIER, 1 R. M. MITCHUM,P. R. VAIL, 2 J. F. SARG, T. S. LOUTIT, AND J. HARDENBOL

Exxon Production Research Company P.O. Box 2189 Houston Texas 77252-2189

The objectives of this overview are to establish funda-

mental concepts of sequence stratigraphy and to define ter-minology critical for the communication of these concepts.Many of these concepts have already been presented in ear-lier articles on seismic stratigraphy (Vail and others, 1977).In the years following, driven by additional documentationan d interaction with co-workers, our ideas have evolved be-yond those presented earlier, making another presentationdesirable. The following nine papers reflect current think-in g about the concepts of sequence stratigraphy an d theirapplications to outcrops, well logs, and seismic sections.Three papers (Jervey, Posamentier and Vail, and Posamen-tier and others) present conceptual models describing therelationships between stratal patterns and rates of eustaticchange an d subsidence. A fourth paper (Sarg) describes the

application of sequence stratigraphy to the interpretation ofcarbonate rocks, documenting with outcrop, well-log, andseismic examples most aspects of the conceptual models.Greenlee an d Moore relate regional sequence distribution,derived from seismic data, to a coastal-onlap curve. Thelast four papers (Haq an d others; Loutit an d others; Bauman d Vail; and Donovan and others) describe application ofsequence-stratigraphic concepts to chronostratigraphy an dbiostratigraphy.

Sequence stratigraphy is the study of rock relationshipswithin a chronostratigraphic framework of repetitive, ge -netically related strata bounded by surfaces of erosion ornondeposition, or their correlative conformities. The fun-damental unit of sequence stratigraphy is the sequence whichis bounded by unconformities and their correlative con-formities. A sequence can be subdivided into systems tractswhich are defined by their position within the sequence andby the stacking patterns of parasequence sets and parase-quences bounded by marine-flooding surfaces. Bound ariesof sequences, parasequence sets, and parasequences pro-vide a chronostratigraphic framework fo r correlating an dmapping sedimentary rocks. Sequences, parasequence sets,an d parasequences are defined an d identified by the phys-ical relationships of strata, including the lateral continuityan d geometry of the surfaces bounding the units, verticalan d lateral stacking patterns, and the lateral geometry ofthe strata within these units. Absolute thickness, the amountof time during which they form, and interpretation of re-gional or global origin are not used to define sequence-stratigraphic units.

Sequences and their stratal components are interpreted toform in response to the interaction between the rates of eus-tasy, subsidence, and sediment supply. These interactionscan be modeled and the models verified by observations to

Present addresses: Esso Resources Canada Ltd., 237 4th Avenue SW,Calgary, Alberta T2P OH6; 2 Department of Geology, Rice Univers i ty,Houston, Texas 77251.

predict stratal relationships an d to infer ages in areas where

geological data are limited.The following paragraphs define an d briefly explain th eterms important for the communication of sequence stratig-raphy concepts. Each term will be discussed more fully inthe nin e papers previously mentioned.

Parasequences and parasequence sets are the fun dam entalbuilding blocks of sequences. A parasequence is a rela-tively conformable succession of genetically related bedsor bedsets bounded by marine-flooding surfaces and theircorrelative surfaces (Van Wagoner, 1985). Siliciclastic par-asequences are progradational and therefore shoal upward.Carbonate parasequences are commonly aggradational an dalso shoal upward. A marine-flooding surface is a surfacethat separates youn ger from older strata, across which there

is evidence of an abrupt increase in water depth. This deep-ening is commonly accompanied by minor submarine ero-sion (but n o subaerial erosion or basinward shift in facies)an d nond eposition, and a minor hiatus may be indicated.Onlap of overlying strata onto a marine-flooding surfacedoes not occur unless this surface is coincident with a se-quence boundary. Marine-flooding surfaces are planar andcommon ly exhibit only very minor topographic relief rang-in g from several inches to tens of feet, with several feetbeing most common. The marine-flooding surface com-monly has a correlative surface in the coastal plain and acorrelative surface on the shelf. The correlative surface inthe coastal plain is not marked by significant subaerial ero-sion due to stream rejuvenation, a downward shift in coastalonlap, a basinward shift in facies, nor onlap of overlyingstrata. The correlative surface in the coastal plain may bemarked by local erosion due to fluvial processes and minorsubaerial exposure. Facies analysis of the strata across thecorrelative surfaces usually does not indicate a significantchange in water depth; often, the correlative surfaces in thecoastal plain or on shelf can be identified only by corre-lating updip or downdip from a marine-flooding surface.

A parasequence set is a succession of genetically relatedparasequences which form a distinctive stacking pattern thatis bounded, in man y cases, by major marine-flooding sur-faces an d their correlative surfac es (Va n W agon er, 1985).Parasequence set bound aries (1) separate distinctive parase-quence stacking patterns; (2) may be coincident with se-quence boundaries; and (3) may be downlap surfaces andboundaries of systems tracts. Stacking patterns of parase-quences in parasequence sets (Fig. 1) are progradational,retrogradational, or aggradational, depending upon the ratioof depositional rates to accommodation rates. These stack-in g patterns are predictable within a sequence.

A sequence is a relatively conformable succession of genetically related strata bound ed by un conformities and theircorrelative conformities (Mitchum, 1977). A n unconform-ity is a surface separating younger from older strata, along

Sea-Level ChangesAn Integrated Approach, SEPM Special Publication No. 42Copyright 1988, The Society o f Economic Paleontologists and Mineralogists, ISBN 0-918985-74-9

Copyright 2012, The Society of Economic Paleontologists and Mineralogists (SEPM)


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OVERVIEW OF THE FUNDAM ENTALS OF SEQUENCE STRATIGRAPHY 41

which there is evidence of subaerial erosional truncation(and, in some areas, correlative submarine erosion) or sub-aerial exposure, with a significant hiatus indicated. Thisdefinition restricts the usage of the term unconformity tosignificant subaerial surfaces and modifies the definition ofunconformity used by Mitchum (1977). He defined an un-conformity as a surface of erosion or nondeposition thatseparates younger strata from older rocks an d represents a

significant hiatus (p. 211). This earlier, broader definitionencompasses both subaerial and submarine surfaces and doesnot sufficiently differentiate between sequence an d paras-equence boundaries. Local, contemporaneous erosion anddeposition associated with geological processes, such aspoint-bar development, distributary-channel erosion, or dunemigration, are excluded from th e definition of unconform-ity used in this paper.

A conformity is a bedding surface separating youngerfrom older strata, along which there is no evidence of ero-sion (either subaerial or submarine) or nondeposition, andalong which no significant hiatus is indicated. It includessurfaces onto which there is very slow deposition, with longperiods of geologic time represented by very thin deposits.

Type 1 and type 2 sequences are recognized in the rockrecord. A type 1 sequence (Figs. 2, 3) is bounded below

by a type 1 sequence boundary an d above by a type 1 ora type 2 sequence boundary. A type 2 sequence (Fig. 4) isbounded below by a type 2 sequence boundary and aboveby a type 1 or a type 2 sequence boundary. A type 1 se-quence boundary (Figs. 2, 3) is characterized by subaerialexposure and concurrent subaerial erosion associated withstream rejuvenation, a basinward shift of facies, a down-ward shift in coastal onlap, an d onlap of overlying strata.

As a result of the basinward shift in facies, nonmarine orvery shallow-marine rocks, such as braided-stream or es-tuarine sandstones above a sequence boundary, may d i-rectly overlie deeper water marine rocks, such as lowershoreface sand stones or shelf mud stones below a boundary,with no intervening rocks deposited in intermediate depo-sitional environments. A typical well-log response pro-duced by a basinward shift in facies marking a sequenceboundary is illustrated in Figure 2. A type 1 sequenceboundary is interpreted to form when the rate of eustaticfall exceeds the rate of basin subsidence at the deposi-tional-shoreline break producing a relative fall in sea levelat that position. The depositional-shoreline break is a po-sition on the shelf, landward of which the depositional sur-face is at or near base level, usually sea level, an d seawardof which the depositional surface is below base level (Po-

FIG. 2.Strata l patterns in a type 1 sequence deposited in a basin with a shelf break.


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42 J. C. VAN WA G O N E R ET AL.

FIG. 3. Stratal patterns in a type 1 sequence deposited in a basin with a ramp margin.

samentier and others, this volume). This position coincidesapproximately with the seaward end of the stream-mouthbar in a delta or with the upper shoreface in a beach. Inprevious pu blications (Vail an d Todd, 1981; Vail and oth-ers, 1984), the depositional-shoreline break has been re-ferred to as the shelf edge. In many basins, the deposi-tional-shoreline break may be 160 km (100 mi) or morelandward of the shelf break which is marked by a changein d ip from the gently dipping shelf (commonly less than1:1000) landward of the shelf break to the more steeplydipping slope (commonly greater than 1:40) seaward of theshelf break (Heezen and others, 1959). In other basins, thedepositional-shoreline break may be at the shelf break.

A type 2 sequence boundary (Fig. 4) is marked by sub-aerial exposure and a downward shift in coastal onlap land-ward of the depositional-shoreline break; however, it lacksboth subaerial erosion associated with stream rejuvenationand a basinward shift in facies. Onlap of overlying stratalandward of the depositional-shoreline break also marks atype 2 sequence boundary. A type 2 sequence boundary isinterpreted to form when the rate of eustatic fall is less thanthe rate of basin subsidence at the depositional-shorelinebreak, so that no relative fall in sea level occurs at thisshoreline position.

A depositional system is a three-dimensional assem-blage of lithofacies (Fisher and McGowan, 1967). A sys-tems tract is a linkage of contemporaneous depositionalsystems (Brown an d Fisher, 1977). We use the term sys-tems tract to designate three subdivisions within each se-quence: lowstand, transgressive-, and highstand systemstracts in a type 1 sequence (Figs. 2, 3) and shelf-margin,transgressive-, and highstand systems tracts in a type 2 sequence (Fig. 4).

Systems tracts are defined objectively on the basis of typesof bounding surfaces, their position within a sequence, an d

parasequence an d parasequence se t stacking patterns. Sys-tems tracts are also characterized by geometry and faciesassociations. When referring to systems tracts, the termslowstand an d highstand are not meant to imply a un iqueperiod of time or position on a cycle of eustatic or relativechange of sea level. The actual time of initiation of a sys-tems tract is interpreted to be a function of the interactionbetween eustasy, sediment supply, an d tectonics.

The lowermost systems tract is called the lowstand sys-tems tract (Figs. 2, 3) if it lies directly on a type 1 se-quence boundary; however, it is called the shelf-marginsystems tract if it lies directly on a type 2 boundary (Fig.4).

Th e lowstand systems tract if deposited in a basin witha shelf break (Fig. 2), generally can be subdivided intothree separate units, a basin-floor fan a slope fan and alowstand wedge. The basin-floor fan is characterized bydeposition of submarine fans on the lower slope or basinfloor. Fa n formation is associated with the erosion of can-yons into the slope and the incision of fluvial valleys intothe shelf. Siliciclastic sediment bypasses the shelf and slopethrough the valleys and the canyons to feed the basin-floorfan. The base of the basin-floor fan (coincident with thebase of the lowstand systems tract) is the type 1 sequenceboundary; the top of the fan is a downlap surface. Basin-floor fa n deposition, canyon formation, and incised-valleyerosion are interpreted to occur dur ing a relative fall in sealevel.

The slope fan is characterized by turbidite and debris-flow deposition on the middle or the base of the slope. Slope-fan deposition can be coeval with the basin-floor fan orwith the early portion of the lowstand wedge. The top ofthe slope fan is a downlap surface for the middle and upperportions of the lowstand wedge.

Th e lowstand wedge is characterized on the shelf by in-


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J. C. VAN WAGONER ET AL.

cised-valley fill (Figs. 2, 3), which commonly onlaps ontothe sequence boundary, and on the slope by progradationalfill with wedge geometry overlying an d commonly down-lapping onto the basin-floor fan or the slope fan. Lowstandwedge deposition is not coeval with basin-floor deposition.Lowstand wedges are composed of progradational to ag-gradational parasequence sets. The top of the lowstandwedge, coincident with the top of the lowstand systems tract,

is a marine-flooding surface called the transgressive sur-face (Figs. 2-4). The transgressive surface is the first sig-nificant marine-flooding surface across the shelf within thesequence. Lowstand wedge deposition is interpreted to oc-cur during a slow relative rise in sea level.

Th e lowstand systems tract if deposited in a basin witha ramp margin (Fig. 3), consists of a relatively thin low-stand wedge that may contain two parts. The first part ischaracterized by stream incision and sediment bypass of thecoastal plain interpreted to occur during a relative f a l l insea level during which the shoreline steps rapidly basinwarduntil the relative fa l l stabilizes. The second part of the wedgeis characterized by a slow relative rise in sea level, the in-filling of incised valleys, and continued shoreline progra-

dation, resulting in a lowstand wedge composed of incised-valley-fill deposits updip and one or more progradationalparasequence sets downd ip. The top of the lowstand wedgeis the transgressive surface; the base of the lowstand wedgeis the lower sequence boundary.

Th e shelf-margin systems tract (Fig. 4) is the lower-most systems tract associated with a type 2 sequenceboundary. This systems tract is characterized by one or moreweakly progradational to aggradational parasequence sets;the sets onlap onto the sequence boundary in a landwarddirection and downlap onto the sequence boundary in a ba-.sinward direction. The top of the shelf-margin systems tractis the transgressive surface, which also forms the base ofthe transgressive-systems tract. The base of the shelf-mar-gin systems tract is a type 2 sequence boundary.

The transgressive-systems tract (Figs. 2-4) is the mid-dle systems tract of both type 1 and type 2 sequences. It ischaracterized by one or more retrogradational parasequencesets. The base of the transgressive-systems tract is thetransgressive surface at the top of the lowstand or shelf-margin systems tracts. Parasequences within the transgres-sive-systems tract onlap onto the sequence boundary in alandward direction and downlap onto the transgressive sur-face in a basinward direction. The top of the transgressive-systems tract is the downlap surface. he downlap sur-face is a marine-flooding surface onto which the toes ofprograding clinoforms in the overlying highstand systems

tract downlap. This surface marks the change from a retro-gradational to an aggradational parasequence set and is thesurface of maximum flooding. The condensed section (Figs.2-4) occurs largely within the transgressive and distal high-stand systems tracts. The condensed section is a faciesconsisting of thin marine beds of hemipelagic or pelagicsediments deposited at very slow rates (Loutit and others,this volume). Cond ensed sections are most extensive du ringthe time of regional transgression of the shoreline.

The highstand systems tract (Figs. 2-4) is the uppersystems tract in either a type 1 or a type 2 sequence. This

systems tract is commonly widespread on the shelf and maybe characterized by one or more aggradational parase-quence sets that are succeeded by one or more prograda-tional parasequence sets with prograding clinoform geo-metries. Parasequences within the highstand systems tractonlap onto the sequence boundary in a landward directionand downlap onto the top of the transgressive or lowstandsystems tracts in a basinward direction. The highstand sys-

tems tract is bounded at the top by a type 1 or type 2 se-quence boundary and at the bottom by the downlap surface.Systems tracts are interpreted to be deposited during spe-

cific increments of the eustatic curve (Jervey and Posa-mentier and others, this volume).

lowstand fan of lowstand systems tractduring a timeof rapid eustatic fall;

slope fan of lowstand systems tractd uring the late eus-tatic f a l l or early eustatic rise;

lowstand wedge of lowstand systems tractduring thelate eustatic f a l l or early rise;

transgressive-systems tractdu ring a rapid eustatic rise; highstand systems tractduring the late part of a eus-

tatic rise, a eustatic stillstand, and the early part of aeustatic fall.

The subdivision of sedimentary strata into sequences,parasequences, and systems tracts provides a powerfulmethodology for the analysis of time and rock relationshipsin sedimentary strata. Sequences and sequence boundariessubdivide sedimentary rocks into genetically related unitsbounded by surfaces with chronostratigraphic significance.These surfaces provide a framework for correlating andmapping. Interpretation of systems tracts provides a frame-work to predict facies relationships within the sequence.Parasequence sets, parasequences, and their bounding sur-

faces further subdivide the sequence and component sys-tems tracts into smaller genetic units for detailed mapping,correlating, and interpreting depositional en vironments.

REFERENCES

B R O W NL. F., AND F IS H E RW . L., 1977, Seismic-stratigraphic interpre-tation of depositional systems: examples from Brazil rift an d pull-apartbasins, in Payton, C. E., ed., Seismic StratigraphyApplications toHydrocarbon Exploration: American Association of Petroleum Geol-ogists Memoir 26 p. 213-248.

F I S H E RW. L., AND McGowAN, J. H., 1967, Depositional systems in theWilcox Group of Texas and their relationship to occurrence of oil andgas: Gulf Coast Association of Geological Societies, Transactions, v.17, p. 213-248.

H E E Z E NB. C ., T H A R PM ., AN D E W I N GM ., 1959, The floors of the

ocean, I. The North Atlantic: Geological Society of America SpecialPaper 65, 122 p.M I T C H U MR. M ., 1977, Seismic stratigraphy an d global changes of sea

level, Part 1: Glossary of terms used in seismic stratigraphy, in Payton,C. E., ed., Seismic StratigraphyApplications to Hydrocarbon Ex-ploration: Association of Petroleum Geologists Memoir 26, p. 205-212.

VA I L P. R ., M I T C H U MR. M ., AND T H O M P S O NS., Ill, 1977, Seismicstratigraphy and global changes of sea level, Part 3: Relative changesof sea level from coastal onlap, in Payton, C. W., ed., Seismic Stra-t igraphy Applications to Hydrocarbon Exploration: American Asso-ciation of Petroleum Geologists Memoir 26, p. 83-97.

AN D TODD, G. R., 1981, North ea Jurassic unconformities,


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OVERVIEW OF THE FUNDAMEN TALS OF SEQUENCE STRATIGRAPHY 45

chronostrat igraphy and sea-level changes from seismic stratigraphy: formities and Hydrocarbon Accumula t ion: American Association ofPetroleum Geology of the Continental Shelf, Northwest Europe, Pro- Petroleum Geologists Memoir 36, p. 129-144.ceedings, p. 216-235. VA N W A G O N E RJ. C., 1985, Reservoir facies distribution as controlled

- , H A R D E N B O LJ., AN D TODD, R . G., 1984, Jurassic unconformi- by sea-level change: Abstract an d Poster Session, Society of Economicties, chronostratigraphy and sea-level changes from seismic stratigra- Paleontologists and Mineralologists Mid-Year Meeting, Golden, Col-ph y and biostrat igraphy, in Schlee, J. S., ed., Interregional Uncon- orado, p. 91-92.

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