Abstract In many ways, sequence stratigraphy’s effect on stratigraphic interpretation is comparable to that of plate tectonics on structural geology. These are markers in the history of geology upon which talented minds can build the next advances. However, concepts that seem self-evident to today’s students faced painful periods of ridicule and resistance when first systematized by Peter Vail in the 1960s. The history of this slow acceptance is marked by a gradual evolution of concepts that bring basin tectonics, sea-level fluctuations, and sediment supply into an integrated stratigraphic solution. One basic idea is the time-significance of stratal surfaces and surfaces of discontinuity, and of the seismic reflections generated by them. Another concept is that cyclic sedimentary sequences form in response to varying rates of eustatic changes, tectonic subsidence, and sedimentary supply. The sequence model is highly variable but astonishingly robust in predicting facies and environments in a wide variety of basin and tectonic settings. Applications in industry and academia are widespread, especially in predicting deep-marine sands, seals, and source rocks in offshore settings. Other specific applications include sequence stratigraphy of carbonates, estuarine sands, incised valleys, forced regressions, and well-log and outcrop analysis. Development of eustatic cycle charts needs high-quality biostratigraphy for dating and environmental analysis. Advent of 3-D seismic data opens a myriad of uses involving attributes of the seismic signal. No matter how specialized, however, the good interpreter always starts with a rigorously defined chronostratigraphic framework of sequences and systems tracts for proper interpretation.

Abstract Observations of geologically synchronous basinward shifts in coastal onlap patterns on seismic reflection profiles, from varied tectonic settings and in widespread areas, led Vail and others (1977) to suggest the use of these shifts as a means of global correlation. Sea-level curves based on the interpretation of strati- graphic data are being rigorously tested on the eastern U.S. Continental Margin. The availability of outcrop, well, and seismic data in this region, as well as the general lack of structural overprint over much of the margin, has resulted in its being a focus of research on the effects of sea-level fluctuations on the sedimentary development of basins. Here we present the results of a study in which we apply seismic stratigraphic and geohistory analysis techniques to data from the Baltimore Canyon trough to interpret sea-level changes during the Tertiary. We first develop a stratigraphic framework for the study area through the interpretation of a regional grid of seismic reflection data tied to available well control. We then use the ages of the major depositional sequence boundaries at the COST B-2 well site to predict the thermotectonic subsidence of the basin since the Early Jurassic and to estimate long-term sea- level changes. Finally, we analyze the Tertiary sedimentary patterns expressed in seismic and well data to derive a detailed curve of eustatic changes of sea level.

Abstract: The objectives of this overview are to establish fundamental concepts of sequence stratigraphy and to define terminology critical for the communication of these concepts. Many of these concepts have already been presented in earlier articles on seismic stratigraphy (Vail and others, 1977). In the years following, driven by additional documentation and interaction with co-workers, our ideas have evolved beyond those presented earlier, making another presentation desirable. The following nine papers reflect current thinking about the concepts of sequence stratigraphy and their applications to outcrops, well logs, and seismic sections. Three papers (Jervey, Posamentier and Vail, and Posamentier and others) present conceptual models describing the relationships between stratal patterns and rates of eustatic change and subsidence. A fourth paper (Sarg) describes the application of sequence stratigraphy to the interpretation of carbonate rocks, documenting with outcrop, well-log, and seismic examples most aspects of the conceptual models. Greenlee and Moore relate regional sequence distribution, derived from seismic data, to a coastal-onlap curve. The ast four papers (Haq and others; Loutit and others; Baum and Vail; and Donovan and others) describe application of sequence-stratigraphic concepts to chronostratigraphy and biostratigraphy.

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: (1) a type 1 sequence, which is bounded at the base by a type 1 unconformity and 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 is bounded at the base by a type 2 unconformity and at the top by either a type 1 or type 2 unconformity and has no lowstand deposits. Each sequence is composed of three systems tracts; the type 1 sequence is composed of lowstand, transgressive-, and highstand systems tracts, and the type 2 sequence is composed of shelf-margin, transgressive-, and highstand systems tracts. The type 1 sequence is associated 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 are the onlapping and offlapping depositional stratal patterns observed on basin margins. Locally, conditions of subsidence and/or uplift and 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 of eustatic fall-and-rise inflection points is considered with regard to the occurrence of unconformities and condensed sections, respectively. Type 1 unconformities are related to rapid eustatic falls, and type 2 unconformities are related to slow eustatic falls.

Abstract Depositional sequences are composed of genetically related sediments bounded by unconformities or their correlative conformities and are related to cycles of eustatic change. The bounding unconformities are inferred to be related to eustatic-fall inflection points. They are either type 1 or type 2 unconformities, depending on whether sea-level fall was rapid (i.e., rate of eustatic fall exceeded subsidence rate at the depositional shoreline break) or slow (i.e., rate of eustatic fall was less than subsidence rate at the depositional shoreline break). Each sequence is composed of a succession of systems tracts. Each systems tract is composed of a linkage of contemporaneous depositional systems. Four systems tracts are recognized: lowstand, transgressive, highstand, and shelf margin. The lowstand systems tract is divided into two parts: lowstand fan followed by lowstand wedge, where the basin margin is characterized by a discrete physiographic shelf edge, or lower followed by upper wedge, where the basin margin is characterized by a ramp physiography.Two sequence types are recognized: a type 1 sequence composed of lowstand, transgressive-, and highstand systems tracts, and a type 2 sequence composed of shelf margin, transgressive-, and highstand systems tracts. Type 1 and type 2 unconformities are each characterized by a basinward shift of coastal onlap concomitant with a cessation of fluvial deposition. The style of subaerial erosion characterizing each unconformity is different. Type 1 unconformities are characterized by stream rejuvenation and incision, whereas type 2 unconformities typically are characterized by widespread erosion accompanying gradual denudation or degradation of the landscape. Stream rejuvenation and incision are not associated with this type of unconformity. On the slope and in the basin, type 1 unconformities typically are overlain by lowstand fan or lowstand wedge deposits, whereas type 2 unconformities are overlain by shelf margin systems tract deposits. Within incised valleys on the shelf, type 1 unconformities are overlain by either fluvial (lowstand wedge) or estuarine (transgressive) deposits. Type 2 unconformities typically are characterized by a change in parasequence stacking pattern from progradational to aggradational. Timing of fluvial deposition is also a function of eustatic change insofar as global sea level is the ultimate base level to which streams will adjust. The elevations of stream equilibrium profiles are affected by eustatic change, and, assuming constant sediment supply, streams will aggrade or degrade in response to eustatically induced shifts in these profiles. Fluvial deposition occurs at different times in type 1 and type 2 sequences and is characterized by different geometries within each type of sequence. In type 1 sequences, fluvial deposits occur as linear, incised-valley fill during the time of lowstand wedge and transgressive deposition. Fluvial deposits also may occur during highstand deposition as more widespread floodplain deposits within the late highstand systems tract. Fluvial deposits in type 2 sequences are usually limited to widespread floodplain deposits occurring within the late highstand systems tract.

Abstract: In central Alabama, near the town of Braggs, a complete section across the Cretaceous-Tertiary (K-T) boundary is present within the lower portion of the Clayton Formation. The K-T microfauna and microfloral transition occurs within a 2.5-m (8 ft) section of interbedded sandstones and limestones that directly overlies a sequence boundary, marked by regional truncation of the underlying Prairie Bluff Formation. This sequence boundary is related to a major eustatic fall in the late Maastrichtian (67 Ma). The interbedded sandstones and limestones in the basal Clayton Formation are interpreted as two backstepping marine parasequences deposited on the inner shelf during the subsequent relative rise in sea level. These two backstepping parasequences are overlain, in turn, by 1.5 m (5 ft) of glauconite-rich strata representing a condensed section produced during a period of slow terrigenous deposition, continued par-asequence backstepping, and shoreline retreat. Three small iridium anomalies have been identified at the Braggs locality. These anomalies occur at marine-flooding surfaces, interpreted to be parasequence boundaries, in the uppermost Prairie Bluff and basal Clayton formations. The uppermost of these anomalies also coincides with the base of the well-developed condensed section in the basal Clayton Formation. The concurrence of iridium concentrations with marine-flooding surfaces at Braggs suggests that iridium was present in the open ocean during the latest Maastrichtian through earliest Danian but concentrated only during periods of terrigenous-sediment starvation. Thus, variations in sediment supply and possibly basin location are critical factors controlling iridium enrichment across the K-T boundary.

Abstract We propose a thermal-mechanical model of the rifting process that differs from previous models of lithospheric thinning (McKenzie, 1978; Sclater and Christie, 1980) by including the effect of the mechanical heterogeneity of the lithosphere. We divide the prerift continental lithosphere into ten horizontal mechanical layers that obey power-law creep. The ten layers are deformed in finite steps by extensile forces that remain constant at great distances from the rift. Power-law creep during extension causes necking of the lithosphere at mechanical instabilities beneath the rift axis. Thus, aesthenospheric upwelling is concentrated at an ever greater rate at the rift axis. We compute surface topography by assuming local isostatic compensation. At the rift axis, thinning of the crustal layers causes subsidence initially, but in some cases acceleration of aesthenospheric upwelling eventually causes thermal uplift late in the rifting process. This deformation sequence can explain the origin of one class of outer highs observed on passive continental margins (Schuepbach and Vail, 1980). Our model is consistent with data on mechanical properties of the lithosphere and with geological knowledge of rift valleys and passive margins. Furthermore, it satisfies our measurements of the scale and timing of formation of outer highs.

Abstract Cycles of relative change of sea level on a global scale are evident throughout Phanerozoic time (Vail et al, 1977). The evidence is based on the facts that many regional cycles determined on different continents are simultaneous, and that the relative magnitudes of the changes generally are similar. Because global cycles are records of geotectonic, glacial, and other large-scale processes, they reflect major events of Phanerozoic history. A global cycle of relative change of sea level is an interval of geologic time during which a relative rise and fall of mean sea level takes place on a global scale. A global cycle may be determined from a modal average of correlative regional cycles derived from seismic stratigraphic studies. On a global cycle curve for Phanerozoic time, three major orders of cycles are superimposed on the sea-level curve. Cycles of first, second, and third order have durations of 200 to 300 million, 10 to 80 million, and 1 to 10 million years, respectively (Figs. 1,2, and 3). Two cycles of the first order, over 14 of the second order and approximately 80 of the third order, are present in the Phanerozoic (not counting late Paleozoic cyclothems). Third-order cycles for the pre-Jurassic and Cretaceous are not shown. Sea-level changes from Cambrian through Early Triassic are not as well documented globally as are those from Late Triassic through Holocene. Relative changes of sea level from Late Triassic to the present are reasonably well documented with respect to the ages, durations, and relative magnitudes of the second- and third-order cycles. Magnitudes of eustatic changes of sea level are only approximations. Our best estimate is that sea level reached a high point near the end of the Campanian (Late Cretaceous) about 350 m above present sea level, and had low points during the Early Jurassic, middle Oligocene, and late Miocene about 150, 250, and 200 m, respectively, below present sea level. Facies and general patterns of distribution of many depositional sequences are related to cycles of global highstands and lowstands of sea level. Interregional unconformities occur at times of low-stand. Geotectonic and glacial phenomena are the most likely causes of the sea-level cycles, although there may be other as yet unrecognized factors. Major applications of the global cycle chart include (1) improved stratigraphic and structural analysis within a basin, (2) estimation of the geological age of strata prior to drilling, and (3) development of a global system of geochronology.

Abstract The depositional facies on slopes and rises depend in large part on the tectonic processes and sea level changes, as well as rate and type of sediment supply. Seismic examples of slope and rise depositional facies from divergent, convergent, and strike-slip continental margins are presented and discussed in terms of sea level changes and sediment supply. Seismic sequence and seismic facies analyses are effective methods for recognition of depositional facies on slopes and rises and studying the interrelationships between tectonic processes, sea level changes, and sediment supply. Seismic sequence analysis is based on the identification of stratigraphic units composed of a relatively conformable succession of genetically related strata termed depositional sequences, Fig. 1. The upper and lower boundaries of depositional sequences are unconformities or their correlative conformities. The time interval represented by strata of a given sequence may differ from place to place, but the range is confined to synchronous limits marked by ages of the sequence boundaries where they become conformities. Depositional sequence boundaries are recognized on seismic data by identifying reflections caused by lateral terminations of strata termed onlap, downlap, toplap, and truncation, Fig. 1. The depositional sequences, because they consist of genetically related strata having Chronographic significance, provide an ideal stratigraphic interval for seismic facies analysis. Seismic facies analysis is the deliniation and interpretation of reflection geometry, continuity, amplitude, frequence, and interval velocity, as well as the external form and associations of seismic facies units. Once the seismic facies parameters are described and mapped, an interpretation of

Abstract Seismic stratigraphy is basically a geologic approach to the Stratigraphic interpretation of seismic data. The unique properties of seismic reflections allow the direct application of geologic concepts based on physical stratigraphy. Primary seismic reflections are generated by physical surfaces in the rocks, consisting mainly of stratal (bedding) surfaces and unconformities with velocity- density contrasts. Therefore, primary seismic reflections parallel stratal surfaces and unconformities. Whereas all the rocks above a stratal or uniformity surface are younger than those below it, the resulting seismic section is a record of the chronostratigraphic (time-stratigraphic) depositional and structural patterns and not a record of the time-transgressive lithostratigraphy (rock-stratigraphy). Because seismic reflections follow chronostratigraphic correlations, it is not only possible to interpret postdepositional structural deformation, but also it is possible to make the following types of Stratigraphic interpretations from the geometry of seismic reflection correlation patterns: (1) geologic time correlations, (2) definition of genetic depositional units, (3) thickness and depositional environment of genetic units, (4) paleobathymetry, (5) burial history, (6) relief and topography on unconformities, and (7) paleogeography and geologic history when combined with geologic data. However, one limiting factor is that lithofacies and rock type can not be determined directly from the geometry of reflection correlation patterns. To accomplish the geologic objectives just listed, we recommend the following three-step interpretational procedure: (1) seismic sequence analysis; (2) seismic facies analysis; and (3) analysis of relative changes of sea level. Seismic sequence analysis is based on the identification of stratigraphic units composed of a relatively conformable succession

Abstract A depositional sequence is a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities. This concept of a “sequence” is modified from Sloss. A depositional sequence is determined by a sin gle objective criterion, the physical relations of the strata themselves. The combination of objective determination of sequence boundaries and the systematic patterns of deposition of the genetically related strata within sequences makes the sequence concept a fundamental and extremely practical basis for the interpretation of stratigraphy and depositional facies. Because distribution and facies of many sequences are controlled by global changes of sea level, sequences also provide an ideal basis for establishing comprehensive stratigraphic frameworks on regional or global scales. A depositional sequence is chronostratigraphically significant because it was deposited during a given interval of geologic time limited by ages of the sequence boundaries where the boundaries are conformities; however the age range of the strata within the sequence may differ from place to place where the boundaries are unconformities. The hiatus along the unconformable part of a sequence boundary generally is variable in duration. The hiatus along the conformable part is not measureable, and the surface is practically synchronous. Stratal surfaces within a sequence are essentially synchronous in terms of geologic time. Depositional sequences may range in thickness from hundreds of meters to a few centimeters. Sequences of different magnitudes may be recognized on seismic sections, well-log sections, and surface outcrops. To define and correlate a depositional sequence accurately, the sequence boundaries must be defined and traced precisely. Usually the boundaries are defined at unconformities and traced to their correlative conformities. Discordance of strata is the main criterion used in the determination of sequence boundaries, and the type of discordant relation is the best indicator of whether an unconformity results from erosion or nondeposition. Onlap, downlap, and toplap indicate nondepositional hiatuses; truncation indicates an erosional hiatus unless the truncation is a result of structural disruption. Examples of depositional sequences are presented on well-log and seismic sections. Both examples depend primarily on correlation of physical stratigraphic surfaces for identification of the unconformities bounding the sequences, and on biostratigraphic zonation for determ

Abstract Relative changes of sea level can be determined from the onlap of coastal deposits in maritime sequences. The durations and magnitudes of these changes can be used to construct charts showing cy-cles of the relative rises and falls of sea level. Such charts summarize the history of the fluctuations of base level that control the distribution of the sequences and the strata within them. A relative rise of sea level is indicated by coastal onlap, which is the landward onlap of littoral and/or nonmarine coastal deposits. The vertical component, coastal aggradation, can be used to measure a relative rise, but it should be adjusted for any thickening due to differential basinward subsidence. During a relative rise of sea level, a transgression or regression of the shoreline, and a deepening or shallowing of the sea bottom may take place. A common misconception is that transgression and deepening are synonymous with a relative rise, and that regression and shallowing are synonymous with a relative fall. A relative stillstand is indicated by coastal toplap; intermittent stillstands between rapid rises are characteristic of a cumulative rise. A relative fall of sea level is indicated by a downward shift in coastal onlap from the highest position in a sequence to the lowest position in the overlying sequence. After a major relative fall of sea level, the shelf tends to be bypassed, and the coastal onlap may be restricted to the apex of a fan at the basin margin. Seismic sections provide the best means of determining the onlap and toplap patterns within the de-positional sequences, and well control can provide the determinations of coastal and marine facies. Each cycle is plotted on a chart in chronologic order, dating and measuring the relative rise by increments of coastal aggradation, dating any relative stillstands by the duration of coastal toplap, and dating and measuring the relative fall by the downward shift of coastal onlap. Seismic examples illustrate the procedures and some of the problems encountered.

Abstract Cycles of relative change of sea level on a global scale are evident throughout Phanerozoic time. The evidence is based on the facts that many regional cycles determined on different continental margins are simultaneous, and that the relative magnitudes of the changes generally are similar. Because global cycles are records of geotectonic, glacial, and other largescale processes, they reflect major events of Phanerozoic history. A global cycle of relative change of sea level is an interval of geologic time during which a relative rise and fall of mean sea level takes place on a global scale. A global cycle may be determined from a modal average of correlative regional cycles derived from seismic stratigraphic studies. On a global cycle curve for Phanerozoic time, three major orders of cycles are superimposed on the sealevel curve. Cycles of first, second, and third order have durations of 200 to 300 million, 10 to 80 million, and 1 to 10 million years, respectively. Two cycles of the first order, over 14 of the second order, and approximately 80 of the third order are present in the Phanerozoic, not counting late Paleozoic cyclothems. Third-order cycles for the pre-Jurassic and Cretaceous are not shown. Sea-level changes from Cambrian through Early Triassic are not as well documented globally as are those from Late Triassic through Holocene. Relative changes of sea level from Late Triassic to the present are reasonably well documented with respect to the ages, durations, and relative amplitudes of the second- and third-order cycles, but the amplitudes of the eustatic changes of sea level are only approximations. Our best estimate is that sea level reached a high point near the end of the Campanian (Late Cretaceous) about 350 m above present sea level, and had low points during the Early Jurassic, middle Oligocene, and late Miocene about 150, 250, and 200 m, respectively, below present sea level. Interregional unconformities are related to cycles of global highstands and lowstands of sea level, as are the facies and general patterns of distribution of many depositional sequences. Geotectonic and glacial phenomena are the most likely causes of the sea-level cycles. Major applications of the global cycle chart include (1) improved stratigraphic and structural analyses within a basin, (2) estimation of the geologic age of strata prior to drilling, and (3) development of a global system of geochronology.

Abstract Primary seismic reflections follow chronostratigraphic (time-stratigraphic) correlation patterns rather than time-transgressive lithostratigraphic (rockstratigraphic) units. Physical surfaces that cause seismic reflections are primarily stratal surfaces and unconformities with velocity-density contrasts. Stratal surfaces are major bedding surfaces and thus represent ancient depositional surfaces. Unconformities are surfaces of erosion or nondeposition that represent significant chronostratigraphic gaps. Both stratal surfaces and unconformities have time significance because of the Law of Superposition. In terms of geologic time, reflections from stratal surfaces approximate time-synchronous events, where reflections from unconformities are commonly time-variable. However, unconformity reflections are time-significant because all the strata below the unconformity are older than all the strata above the unconformity. No physical surface that could generate a reflection parallel with the top of a time-transgressive formation (lithostratigraphic unit) exists in nature. The continuity of the seismic reflection follows the stratal surfaces across the time-transgressive formation boundaries, although reflection character (amplitude, cycle breadth, and waveform) will change as the reflection coefficients and spacing of the stratal surfaces change laterally. Commonly, a given seismic reflection character transgresses reflection continuity as a rock formation transgresses geologic time. Geologic time correlations made from paleontologic data tie with seismic-reflection correlations even where the latter cross major facies boundaries. In addition, unconformities or their correlative conformities that bound sequences, also commonly bound paleontologic zones especially in the Paleozoic and Mesozoic. Understanding the chronostratigraphic significance of seismic-reflection correlations and relating them to available well control is essential for Stratigraphic trap exploration. There also are other types of continuous physical surfaces that are locally present in sedimentary rocks. Most significant of these are gas-water, gas-oil, and oil-water fluid contacts and gas hydrate zones. Reflections from these types of physical surfaces cut across the reflections originating from the stratal surfaces, if they are at an angle to each other. Chronostratigraphic correlations of seismic data with well data are accurate only to ± ½ cycle breadth owing to possible changes of reflection character caused by changes in bed spacing and reflection coefficients.

Abstract Seismic stratigraphy is the study of stratigraphy and depositional facies as interpreted from seismic data. Seismic reflection terminations and configurations are interpreted as stratification patterns, and are used for recognition and correlation of depositiona' sequences, interpretation of depositional environment, and estimation of lithofacies. Seismic sequence analysis subdivides the seismic section into packages of concordant reflections, which are separated by surfaces of discontinuity defined by systematic reflection terminations. These packages of concordant reflections (seismic sequences) are interpreted as depositional sequences consisting of genetically related strata and bounded at their top and base by unconformities or their correlative conformities. Reflection terminations interpreted as stratal terminations include erosional truncation, toplap, onlap, and downlap. Seismic facies analysis interprets environmental setting and lithofacies from seismic data. Seismic facies units are groups of seismic reflections whose parameters (configuration, amplitude, continuity, frequency, and interval velocity) differ from adjacent groups. After seismic facies units are recognized, their limits defined, and areal associations mapped, they are interpreted to express certain stratification, lithologic, and depositional features of the deposits that generated the reflections within the units. Major groups of reflection configurations include parallel, subparallel, divergent, prograding, chaotic, and reflection-free patterns. Prograding configurations may be subdivided into sigmoid, oblique, complex sigmoid-oblique, shingled, and hummocky clinoform configurations. External forms of seismic facies units include sheet, sheet drape, wedge, bank, lens, mound, and fill forms. Seismic facies units are interpreted in terms of the depositional environments, the energy of the depositing medium, and the potential lithologic content of the strata generating the seismic facies reflection pattern.

Abstract A generalized procedure for making regional Stratigraphic studies using seismic data involves analysis of seismic sequences and seismic facies, which are interpreted from terminations and configurations of seismic reflections generated by sedimentary strata. Generalized steps in the procedure include (1) recognition, correlation, and age determination of seismic sequences; (2) recognition, mapping, and interpretation of seismic facies; and (3) regional analysis of relative changes of sea level.