AN OVERVIEW OF THE FUNDAMENTALS OF SEQUENCE STRATIGRAPHY AND KEY DEFINITIONS
J. C. VAN WAGONER, 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 documentationand interaction with co-workers, our ideas have evolved be-yond those presented earlier, making another presentationdesirable. The following nine papers reflect current think-ing about the concepts of sequence stratigraphy and 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 and subsidence. A fourth paper (Sarg) describes theapplication of sequence stratigraphy to the interpretation ofcarbonate rocks, documenting with outcrop, well-log, andseismic examples most aspects of the conceptual models.Greenlee and Moore relate regional sequence distribution,derived from seismic data, to a coastal-onlap curve. Thelast four papers (Haq and others; Loutit and others; Baumand Vail; and Donovan and others) describe application ofsequence-stratigraphic concepts to chronostratigraphy andbiostratigraphy.
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 tracts,which are defined by their position within the sequence andby the stacking patterns of parasequence sets and parase-quences bounded by marine-flooding surfaces. Boundariesof sequences, parasequence sets, and parasequences pro-vide a chronostratigraphic framework for correlating andmapping sedimentary rocks. Sequences, parasequence sets,and parasequences are defined and identified by the phys-ical relationships of strata, including the lateral continuityand geometry of the surfaces bounding the units, verticaland 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; 2Department of Geology, Rice University,Houston, Texas 77251.
predict stratal relationships and to infer ages in areas wheregeological data are limited.
The following paragraphs define and briefly explain theterms important for the communication of sequence stratig-raphy concepts. Each term will be discussed more fully inthe nine papers previously mentioned.
Parasequences and parasequence sets are the fundamentalbuilding 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 andalso shoal upward. A marine-flooding surface is a surfacethat separates younger from older strata, across which thereis evidence of an abrupt increase in water depth. This deep-ening is commonly accompanied by minor submarine ero-sion (but no subaerial erosion or basinward shift in facies)and nondeposition, 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 andcommonly exhibit only very minor topographic relief rang-ing 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 many cases, by major marine-flooding sur-faces and their correlative surfaces (Van Wagoner, 1985).Parasequence set boundaries (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-ing patterns are predictable within a sequence.
A sequence is a relatively conformable succession of ge-netically related strata bounded by unconformities and theircorrelative conformities (Mitchum, 1977). An unconform-ity is a surface separating younger from older strata, along
OVERVIEW OF THE FUNDAMENTALS 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 and represents asignificant hiatus" (p. 211). This earlier, broader definitionencompasses both subaerial and submarine surfaces and doesnot sufficiently differentiate between sequence and paras-equence boundaries. Local, contemporaneous erosion anddeposition associated with geological processes, such aspoint-bar development, distributary-channel erosion, or dunemigration, are excluded from the 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 and 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, and 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 di-rectly overlie deeper water marine rocks, such as lowershoreface sandstones or shelf mudstones 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, and seawardof which the depositional surface is below base level (Po-
FIG. 2.—Stratal patterns in a type 1 sequence deposited in a basin with a shelf break.
42 J. C. VAN WAGONER 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 publications (Vail and 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 dip 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 and 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 se-quence (Fig. 4).
Systems tracts are defined objectively on the basis of typesof bounding surfaces, their position within a sequence, and
parasequence and parasequence set stacking patterns. Sys-tems tracts are also characterized by geometry and faciesassociations. When referring to systems tracts, the termslowstand and highstand are not meant to imply a uniqueperiod 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, and 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).
The 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. Fan 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 fan deposition, canyon formation, and incised-valleyerosion are interpreted to occur during 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.
The lowstand wedge is characterized on the shelf by in-
44 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 and 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.
The 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 fall insea level during which the shoreline steps rapidly basinwarduntil the relative fall 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 downdip. The top of the lowstand wedgeis the transgressive surface; the base of the lowstand wedgeis the lower sequence boundary.
The 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. The downlap sur-face is a marine-flooding surface onto which the toes ofprograding clinoforms in the overlying highstand systemstract 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). Condensed sections are most extensive duringthe 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 tract—during a timeof rapid eustatic fall;
• slope fan of lowstand systems tract—during the late eus-tatic fall or early eustatic rise;
• lowstand wedge of lowstand systems tract—during thelate eustatic fall or early rise;
• transgressive-systems tract—during a rapid eustatic rise;• highstand systems tract—during 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 environments.
REFERENCES
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FISHER, W. 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.
HEEZEN, B. C., THARP, M., AND EWING, M., 1959, The floors of theocean, I. The North Atlantic: Geological Society of America SpecialPaper 65, 122 p.
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VAIL, P. R., MITCHUM, R. M., AND THOMPSON, S., 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-tigraphy—Applications to Hydrocarbon Exploration: American Asso-ciation of Petroleum Geologists Memoir 26, p. 83-97.•, AND TODD, G. R., 1981, North Sea Jurassic unconformities,
OVERVIEW OF THE FUNDAMENTALS OF SEQUENCE STRATIGRAPHY 45
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