Abstract: Techniques that can be used to determine the relative magnitude of eustatic excursions include the measurement of: (a) the amount of sedimentary onlap onto the continental margins; (b) the thickness of marine sedimentary cycles and the elevation and distance between indicators of old strandlines; (c) the perturbations on individual thermo-tectonic subsidence curves and stacked crustal subsidence curves; (d) the variations in deep-ocean oxygen isotopes found in sediments; and (e) the size of variables, such as rates of tectonic movement, sediment accumulation, and eustatic changes, used in graphical and numerical simulations of basin fill that “invert” the problem. To date, a combination of some or all of these methods can be used to construct relative (tectono/eustatic) sea-level curves; however, these are not unique solutions to absolute eustatic variations. Each method assumes some behavior for two of the three underlying processes (tectonic movement of the basement, sedimentary accumulation, and eustasy), and then determines the third process relative to the assumed model behavior of the other two. The sense of this result is confirmed by mathematical models which suggest that only the sum of tectonic basement subsidence and sea-level variations can be obtained.

Abstract: Thermo-mechanical modeling demonstrates that tectonically induced vertical motions of the lithosphere may provide an explanation for third-order cycles in apparent sea level deduced from the seismic stratigraphic record of passive margins. The interaction of fluctuations in intraplate stresses and the deflection of the lithosphere caused by sedimentary loading can produce apparent sea-level changes of as much as 100m at the flanks of passive margins. In general, stress variations of a few hundred bars associated with local adjustment of stresses at passive margins suffice to explain a significant part of the stratigraphic record associated with short-term variations in sea level on the order of a few tens of meters. To induce short-term apparent sea-level fluctuations with magnitudes on the order of 50m or more, which occur less frequently in the record, changes in stress level in excess of one kbar are required. These larger fluctuations in apparent sea level could be related to major reorganizations of lithospheric stress fields due to rifting and fragmentation of plates, dynamic changes at convergent plate boundaries, or collision processes. A fluctuating horizontal stress field in the lithosphere can explain contemporaneous changes in apparent sea level in neighboring depositional environments. In principle, it implies the possibility of regional correlations in different basin settings. Specific short-term fluctuations in the curves of Vail and others, (1977; 1984) can be associated with particular plate tectonic reorganizations of lithospheric stress fields. The seismic stratigraphic record may provide a new source of information on paleo-stress fields which can be correlated with results of independent numerical modeling of intraplate stresses.

Abstract: The stable oxygen isotope record for the Cenozoic is characterized by a series of large third-order steps of +1 per mil superimposed on a long-term second-order trend. This second-order trend accounts for a δ 18 O change of nearly +4 per mil from the early Eocene into the Neogene. The second- and third-order changes in the δ 18 O signal are driven primarily by a combination of glacio-eustatic sea-level and ocean paleotemperature changes. These changes are global responses to evolving circulation and climate patterns. Timing of the δ 18 O events is in good agreement with the seismically defined changes in the coastal-onlap curve (Vail and others, 1977). Agreement in the timing of events supports a common mechanism, perhaps that glaciation is apparent throughout much of the record and certainly intensified beginning in the Neogene. Agreement is not good between the magnitudes of apparent changes in sea level using the EXXON onlap record and oceanic δ 18 O events. Consideration of the δ 18 O, ice volume, and sea-level relationships during the Pleistocene suggests that sinusoidal eustatics, i.e., the rise and fall of sea level being equal, is not a good assumption at fourth- and fifth-order sea-level events. Although interpretation of the δ 18 O record is not without its assumptions and limitations, it offers an independent geochemical check on seismically defined changes in stratal patterns.

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: In order to clarify the principles that govern the development of siliciclastic sequences and their bounding surfaces, a mathematical model of progradational basin filling was created for Atlantic-type continental margins. This paper discusses the model and its implications with respect to depositional facies, sandstone geometry, and seismic stratigraphic interpretation. Basin filling is modeled as the interaction of subsidence, change in sea level, and sediment influx. The simulations show that seismic-sequence boundaries are located, in time, near inflection points of eustatic sea-level fluctuation, where rates of fall or rise are maximized. Changes in the rate of accommodation development, both in time and space, are believed to play a dominant role in shaping the internal facies distribution, the geometry, and the nature of the bounding surfaces of depositional sequences. The pattern of coastal onlap and offshore condensed sections displayed by global-cycle charts are shown to develop in the context of smoothly fluctuating eustatic and relative sea level.

Abstract: Sequence-stratigraphic concepts are used to identify genetically related strata and their bounding regional unconformities, or their correlative conformities, in seismic, well-log, and outcrop data. Documentation and age dating of these features in marine outcrops in different parts of the world have led to a new generation of Mesozoic and Cenozoic sea-level cycle charts with greater event resolution than that obtainable from seismic data alone. The cycles of sea-level change, interpreted from the rock record, are tied to an integrated chronostratigraphy that combines state-of-the-art geochronologic, magnetostratigraphic and biostratigraphic data. In this article we discuss the reasoning behind integrated chronostratigraphy and list the sources of data used to establish this framework. Once this framework has been constructed, the depositional sequences from sections around the world, interpreted as having been formed in response to sea-level fluctuations, can be tied into the chronostratigraphy. Four cycle charts summarizing the chronostratigraphy, coastal-onlap patterns, and sea-level curves for the Cenozoic, Cretaceous, Jurassic, and Triassic are presented. A large-scale composite-cycle chart for the Mesozoic and Cenozoic is also included (in pocket). The relative magnitudes of sea-level falls, interpreted from sequence boundaries, are classified as major, medium, and minor, as are the condensed sections associated with the intervals of sediment starvation on the shelf and slope during the phase of maximum shelf flooding during each cycle. Generally, only the sequence boundaries produced by major and some medium-scale sea-level falls can be recognized at the level of seismic stratigraphic resolution; detailed well-log and/or outcrop studies are usually necessary to resolve the minor sequences.

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: The major controls on changes in carbonate productivity, as well as platform or bank growth and the resultant facies distribution, are interpreted here to be short-term eustatic changes superimposed on longer term tectonic changes (i.e., relative changes in sea level). Carbonate platforms associated with sea-level highstands are characterized by relatively thick aggradational-to-progra-dational geometry. They are bounded below by the top of a transgressive unit and above by a sequence boundary. Two types of highstand platform, keep-up and catch-up, are differentiated here. (1) A keep-up carbonate highstand platform is interpreted to represent a relatively rapid rate of accumulation that is able to keep pace with periodic rises in relative sea level. A keep-up carbonate is characterized at the platform margin by grain-rich, mud-poor lithofacies and nonpervasive submarine cementation. keep-up platforms display a mounded/oblique stratal configuration at the platform/bank margin and in places on the platform. (2) A keep-up carbonate highstand platform is interpreted to represent a relatively slow rate of accumulation that is characterized by micrite-rich parasequences and pervasive early submarine cementation at the platform margin. A keep-up carbonate displays a sigmoid depositional profile at the platform/bank margin. At the formation of a type 1 sequence boundary, where the rate of eustatic fall is interpreted to be greater than subsidence at the platform/bank margin, two major processes occur: (1) local-to-regional slope front erosion and (2) subaerial exposure of the shelf and major seaward movement of the regional meteoric lens. At a large-scale type 1 sequence boundary , sea level may fall from 75 to 100 m or more and for an extended period of time. When this occurs, the meteoric lens becomes established over the shelf for a long time, and its influence extends well into the subsurface. If there is sufficient rainfall and a permeable section with mineralogically unstable grains, significant solution will occur over the shelf in the shallow portion of the underlying highstand carbonate platform. Precipitation of phreatic cements will occur deeper or downdip in the section. At a small-scale type 1 sequence boundary , where sea level falls less than about 100 m and for a short period of time, the meteoric lens becomes less well established. It remains in a shallow position on the shelf, causing less extensive solution. Mixing and hypersaline dolomitization may be important processes during the late highstand and continuing through the formation of either a large- or small-scale type 1 sequence boundary. At a type 2 sequence boundary , in which the rate of eustatic fall is interpreted to be less than the rate of subsidence at the platform/bank margin, the inner-platform peritidal and outer-platform shoal areas will be exposed. The dominant meteoric effect will be in the inner-platform areas. During sea-level lowstands, three types of carbonate deposits are recognized: (1) allochthonous material derived from erosion of the slope (i.e., debris sheets and allodapic carbonate sands); (2) autochthonous wedges deposited on the upper slope during type 1 sea-level lowstands; and (3) type 2 platform/bank margin wedges. In addition, given the appropriate climatic and hydrographic conditions (i.e., evaporation exceeds influx, and basin is restricted), evaporite lowstand wedges may occur associated with either type 1 or type 2 sequence boundaries. During evaporitic lowstands, hypersaline dolomitization, evaporite replacement, and solution may occur in associated carbonate highstand platforms. Siliciclastic lowstand deposition will occur in areas where an updip-source terrain is available.

Abstract: Condensed sections play a fundamental role in stratigraphic correlation, both regionally and globally. Condensed sections are thin marine stratigraphic units consisting of pelagic to hemipelagic sediments characterized by very low-sedimentation rates. Areally, they are most extensive at the time of maximum regional transgression of the shoreline. Condensed sections are associated commonly with apparent marine hiatuses and often occur as thin, but continuous, zones of burrowed, slightly lithified beds (omission surfaces) or as marine hardgrounds. In addition, condensed sections may be characterized by abundant and diverse planktonic and benthic mi-crofossil assemblages, authigenic minerals (such as glauconite, phosphorite, and siderite), organic matter, and bentonites and may possess greater concentrations of platinum elements such as iridium. Condensed sections are important because they tie the temporal stratigraphic framework provided by open-ocean microfossil zonations to the physical stratigraphy provided by depositional sequences in shallower, more landward sections. Condensed sections represent a physical stratigraphic link between shallow- and deep-water sections and are recognized by the analysis of seismic, well-log, and outcrop data. Within each depositional sequence, condensed sections are best recognized and utilized within an area from the shelf/slope break landward to the distal edge of inner neritic-sand deposition. Where sedimentation rates are generally low, as in the deep ocean, a number of condensed sections may coalesce to form a composite condensed section. Data from detailed analyses of continental-margin condensed sections are presented to illustrate the nature and importance of condensed sections for dating and correlating continental-margin sequences and reconstructing ancient depositional environments.

Abstract: Carbonate sediments of tectonically quiescent continental interiors are nearly ideal for precisely tracing eustatic sea-level change during major transgressions. Over roughly 10 million years during the later Middle Ordovician (Rocklandian through the middle Denmarkian stages), sea levels measured in the American Midwest rose about 10 m relative to the continent. Because the sediment accumulation rate in the epeiric sea was proportional to water depth, the time trend of sea level can be reconstructed from cumulative sediment thickness and from measurements on water depth throughout a stratigraphic section. Sea level is reconstructed as a function of time for six sections in the midwestern United States, and the reconstructed time trends are compared for common eustatic components based on the section time correlations by geochemically fingerprinted volcanic-ash layers. Relative water depth is measured through gradient analysis of fossil assemblages by reciprocal averaging ordination. Sample ordination scores are calibrated as a measure of absolute depth by use of the offshore depth estimated from stratigraphic expressions of the shoreline edge effect in lithospheric flexure. Sea-level change during the Middle Ordovician transgression had at least two components: (1) a steady rise at a slowly varying rate around 1 m per million years (2) pulses no more than 0.1 to 1 million years long during which sea level fell roughly 1 m and then rose about the same amount. The long-term trend is attributable to steady decrease in the mean age of oceanic lithosphere. The pulse correlations from section to section and the remarkably small sea-level changes involved testify to the tectonic quiescence, spatial homogeneity, and essential tidelessness of the epeiric sea, and to the precision of its stratigraphic record. Pulses were probably related to climatic fluctuations, and their association with particularly frequent volcanic-ash deposition is suggestive that climatic effects of increased explosive volcanism may have been controlling factors.

Abstract: Approximately sixty transgressive-regressive depositional sequences are present in Carboniferous and Permian shallow-marine successions on the world's stable cratonic shelves. These sequences were synchronous depositional events that resulted from eustatic sea-level changes. Based on currently available age correlations of rapidly evolved late Paleozoic tropical, subtropical, and temperate shelf faunas, the sequences on different cratonic shelves were time equivalent. These transgressive-regressive sequences averaged about 2 million years and ranged from 1.2 to 4.0 million years in duration. Local depositional conditions are important in controlling sedimentary patterns on different cratonic shelves. These conditions are affected by changes in sea level, strandline position, and drainage base level and are reflected in the sedimentary record. Because midsize sea-level fluctuations are usually widely identifiable in the stratigraphic record, they are useful aids in correlation. They are particularly helpful between regions that have contrasting depositional conditions, such as between a carbonate shelf starved of clastic sediments and a clastic-dominated shelf on which carbonates are rare. The appearance of new species and genera generally occurs above unconformities that signal new marine transgressive events and new depositional sequences. The durations of the hiatuses between these transgressive-regressive sequences are difficult to estimate. The hiatuses may represent cumulatively as much time, if not more, than the rock record. The numerous worldwide synchronous unconformities marking hiatuses of considerable duration within late Paleozoic shelf strata suggest that the fossil record may be very incomplete and preserves mostly biota that were extant during times of high sea level. Such an incomplete fossil record could easily be misinterpreted as a punctuated evolution having a highly irregular mutation rate.

Abstract: Triassic sea-level changes are not well documented because of a scarcity of Triassic marine strata over many of the continental interiors and on passive continental margins. An excellent laboratory for studying Triassic sea-level changes is the Sverdrup Basin, which was a major depocenter in the Canadian Arctic Archipelago from the Carboniferous to early Tertiary. Marine Triassic strata are widespread across the basin and are as thick as 4,000 m. The established stratigraphic pattern for the Triassic succession consists of thick progradational wedges of deltaic and marine strata, alternating with thin, transgressive, clastic units (T-R cycles). On the basin margins, subaerial unconformities cap the progradational wedges, and over much of the basinal area, submarine unconformities form the cycle boundaries. Nine T-R cycles occur in the basin and are interpreted as having been generated by an interplay of eustatic sea-level change, gradually decaying thermal subsidence, and variable rates of sediment supply and load subsidence. In this model, rapid eustatic sea-level rises coincide with major transgressions that occurred in earliest Griesbachian, earliest Smithian, late Smithian, earliest Anisian, early Ladinian, earliest Carnian, mid-Carnian, earliest Norian, earliest Rhaetian, and earliest Jurassic. Progradation occurred in the intervening time intervals under conditions of slow eustatic sea-level rise, stillstand, and fall. The long duration of each of the sea-level cycles (about 5 million years) and the apparent lack of Triassic glacial deposits indicate the cycles had a tectono-eustatic origin that relates sea-level changes to changes in the volume of the ocean basins. Sea-level rises are related to episodes of increased rates of seafloor spreading and oceanic volcanism that resulted in reduced oceanic-basin volume. The intervals of sea-level fall occurred when seafloor spreading and associated volcanism were subdued and the ocean basins gradually enlarged due to thermal subsidence.

Abstract: A comparison is made between the revised Exxon eustatic curve for the Jurassic, based essentially on seismic stratigraphic analysis of North Sea data, and a new curve derived from more conventional stratigraphic analysis. The two curves are broadly similar in that a secular rise of sea level through most of the period is indicated on which about 17 shorter term cycles are superimposed. Both record notable rises in the Sinemurian, Toarcian, Bajocian, Callovian, Oxfordian, and Kimmeridgian. The Exxon curve, however, misses the important event across the Triassic-Jurassic boundary and underestimates the rate of rise in sea level for a number of cycles. In addition, some supposed eustatic events can be discounted as the consequence of regional tectonics. Tectonic activity involving subsidence and uplift, rather than geoid changes, is thought to be the principal cause of regional distortions of the global picture. There is a need for better quantitative data on the amplitude and rate of changes in sea level.

Abstract Three surface and subsurface sections of Lower Cretaceous strata from the Gulf Coast are correlated with three comparable sections in Oman. Detailed fossil ranges and graphic correlation methods resulted in a biostratigraphic data base that could be related to the geologic time scale. Two events of relative sea-level rise are synchronous in the Gulf Coast basin and the southeastern Arabian platform and may represent eustatic sea-level rises. The intra-Aptian rise began about 115.8 Ma and in many places is represented by a sharp lithologic change, by submarine hardgrounds, or by onlap. Deep-water deposition resumed from 115.2 to 113.9 Ma. The intraCenomanian rise began approximately 94.6 Ma. In Oman, this rise is locally represented by a submarine hardground that formed after drowning of a carbonate shelf. In the updip Gulf Coast, mid-Cenomanian paralic and deltaic sediments were deposited upon an Albian-early Cenomanian shallow carbonate shelf. In the downdip Gulf Coast, this event either is not recognizable in deep-water muds or is represented by drowning of shallow-water carbonates. A third, intra-Albian event at 104.3 Ma may also be a eustatic sea-level rise; however, it needs to be identified in other tectonic settings.

Abstract: Two types of criteria are used to recognize relative changes in sea level in the Cretaceous of the Western Interior (Fig. 1). The first type is a highstand condition identified by: (1) highstand regression of the shoreline, depositing widespread shallow-marine sandstone and shale and shoreline sandstone, sometimese overlain by a widespread coal layer (Fig. 2); (2) deposits that fill incised drainage, reflecting rising sea level and landward movement of the shoreline (transgression) associated with coastal onlap; the incised valley fill may be zoned-more freshwater environments in the lower part and brackish to marine environments in the upper part (Figs. 3, 4); (3) recognition of one or more of the following in a marine condensed section: missing faunal zones; concentrations of phosphate nodules and/or glauconite; organic-rich shale with high total organic content; recrystallized shell debris forming thin lenticular limestone layers or shell hash in shale; residual concentrations of coarse-grained sand with chert pebbles and/or bone and teeth fragments on a transgressive surface of erosion. The second type is a lowstand condition recognized by: (1) lowstand surface of erosion with incised drainage; paleosols and root zones (causing zones of early cementation) may be preserved in marine shale or other deposits under an erosional surface (Fig. 3); this type of erosional surface is a major sequence boundary in analyses related to sequence stratigraphy; (2) more or less uniform depth of erosion by streams over large areas because of lowered base level; (3) missing shoreline and shallow marine sandstone facies; freshwater deposits rest on marine shales; (4) correlation with relative lowering of sea level and unconformities on other continents. Intrabasin fault block movement, creating topographic relief, may influence location of incised drainages (Figs. 3, 4).

Abstract: Paleoslope models of foraminifera in the Upper Cretaceous of the New Jersey coastal plain are utilized to estimate paleobathymetric change during cycles of rising and falling sea level. The paleoslope method estimates change in sea level from the distribution of foraminiferal assemblages and species on a baseline parallel to the regional dip. The paleoslope is the restoration of the original depositional slope of the basin. The paleobathymetry of the foraminifera along the paleoslope is estimated from the gradient of the original depositional slope. Application of the paleoslope model to the Campanian of New Jersey indicates a maximum rise of sea level of 90 m and 80 m, respectively, during two cycles of sea-level change. By extension, a paleodepth curve is derived for the other cycles in the Late Cretaceous. Eight cycles are recognized in the Late Cretaceous section of New Jersey.

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 Type 1 and type 2 sequence boundaries can be used for regional correlation in seismic, wireline log, and outcrop data. Marine condensed sections (zones of markedly reduced sedimentation) divide these sequences and are recognized seismically as downlap surfaces. Sequence boundaries can be dated at their basinward correlative conformities. Depositional sequences are not synthems or allostratigraphic units. Synthems or allostratigraphic units are extended only as far as both of the bounding unconformities or discontinuities are identifiable. Sequences are bounded by unconformities and their correlative conformities and so are identifiable beyond the extent of their bounding discontinuities. Because most of the exposed Paleogene units in the Gulf and Atlantic basins were deposited landward of their respective shelf slope breaks, evidence of deposition of deep-sea fans common to type 1 unconformities is precluded. Regional mapping, however, generally reveals discontinuous incised valleys that are indicative of type 1 unconformities. Typically, the incised valleys are onlap-filled with reservoir-prone fluvial-to-estuarine sediments. In additonal, sequence boundaries are characterized by abrupt downward shifts in facies with relatively shallower water facies resting sharply on relatively deeper water facies. In carbonates, subaerial unconformities are typically characterized by mesokarst, phosphate pebble conglomerates, and sediment fill of early moldic porosity. Condensed sections are characterized by anomalous concentrations of mammillated-to-lobate glauconite, planktonic organisms, phosphate, and exotic minerals, and by glauconitized/phosphatized surfaces commonly associated with hardgrounds or burrowed omission surfaces. Hardgrounds are characterized by intercrystalline sediment fill after subaqueous, acicular, bladed, and/or pelloidal marine cements, and by abrupt shifts to more negative δ 13 C values of calcite above the hardgrounds associated with condensed sections. Application of these concepts to outcrop studies reveals that many stage boundaries are typically not placed at sequence boundaries. Rather, they are defined either by micropaleontologic hiatuses and/or planktonic zonal boundaries associated with condensed sections, or by transgressive (flooding) surfaces overlying incised-valley-fill sediments. Also, the currently recognized European and Gulf Coast stages do not adequately reflect the higher frequency coastal-onlap cycles recognized in outcrop. Because most micropaleontologic zones appear to span sequence boundaries, the current micropaleontologic zonations cannot, at present, precisely define a sequence boundary in time. They can approximate sequence position, however. By intergrating physical stratigraphy, seismic stratigraphy, and paleontology, these higher frequency eustatic events can be resolved and fixed in a relative time framework.