Abstract Multidisciplinary investigations based on the lithology, sedimentology, mineralogy, and biostratigraphy of upper Maastrichtian to lower Danian boundary (KTB) sequences along 3.5 km of the Brazos River in Falls County, Texas, reveal depositional sequences, including an impact-spherule-rich sandstone complex, characteristic of sequence stratigraphic models applied to shallow shelf areas, such as incised valleys, lag conglomerate, storm deposits, and repeated bioturbation. The top of the Corsicana Formation coincides with a channel, which we interpret as an incised valley. The erosion surface marks a major depositional sequence boundary (SB) associated with the latest Maastrichtian sea-level fall. Initial channel deposits consist of coarse shelly glauconitic sand with large lithified clasts containing impact spherules and large bored and encrusted phosphatized concretions, which we interpret to indicate that the Chicxulub impact occurred well prior to the lithification, erosion, and redeposition at the base of the channel. The primary Chicxulub ejecta layer lies about 40–65 cm below the sandstone complex in a 3-cm-thick yellow clay layer that consists of cheto smectite (altered impact glass) interbedded in claystones of the Corsicana Formation. Above the sandstone complex, claystones, and mudstones are burrowed and correspond to a condensed interval interpreted as a maximum flooding surface (MFS). Based on biostratigraphy and the δ 13 C shift, the KT boundary is up to 1 m (50–100 ky) above the sandstone complex and coincides with increased sediment accumulation during the early Danian sea-level rise (HST). These features are inconsistent with a single catastrophic bolide impact on Yucatán and associated megatsunami deposition as commonly interpreted. The biostratigraphy and KT characteristic δ 13 C shift of the Brazos sections indicate that the KTB, the sandstone complex, and the Chicxulub impact occurred as three different stratigraphic events during the late Maastrichtian planktic foraminiferal zone CF1. These are represented by: (1) the Chicxulub impact sequence deposited about 200–300 ky prior to the KTB, (2) the sandstone complex with reworked impact spherules deposited in incised valleys during the latest sea-level fall about 100–150 ky prior to the KTB, and 3) the KTB event during the subsequent HST and following the condensed MFS.

ABSTRACT A Cretaceous (Aptian) to Cenozoic composite oxygen isotope curve is presented and correlated to eustatic records and to global tectonic events. The curve was built using deep water benthonic foraminifera from DSDP/ODP sites. In addition, well-dated outcrop and subsurface whole rock samples were used. This composite record provides insight about the evolution of deep-water temperatures and/or ice volume changes from the Aptian to the present. Two important observations can be made from the isotope record. First, three low-frequency isotope cycles are recognized, encompassing most of the Upper Cretaceous (named Uki), most of the Paleogene (named Pi) and most of the Neogene (named Ni) period. These low-frequency cycles correspond well with the sequence stratigraphic supercycle sets Upper Zuni A, Tejas B and Tejas A, respectively. Second, oxygen isotope values for deep-water benthonic foraminifera during the Aptian to lower Albian and Campanian to Maastrichtian are similar to those observed during middle Eocene. Due to the evidence for middle Eocene Antarctic glaciation, similarity between Cretaceous and Eocene isotope values could indicate the presence of polar ice as early as the Aptian.

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 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.

Abstract The development and refinement of seismic stratigraphic techniques over the past decade have added the dimension of large scale stratal geometries to stratigraphic correlations, and given renewed impetus to regional correlations based upon unconformity-bounded (allothemic) sequences (Vail, 1976; Vail et al., 1977; 1980; 1982; Vail and Todd, 1981, Todd and Mitchum, 1977). Seismic data compensate for the generally incomplete rock record in outcrop; however, because of seismic resolution limits, outcrop–based studies have provided a direct method to document the age and physical character of seismic sequence boundaries. The integration of outcrop and seismic observations has provided a framework to further subdivide unconformity-bounded sequences into component parts, as well as refine the coastal onlap chart (Fig. 1). Within the confidence level of paleontological zonations, the synchronism of allothemic units has been demonstrated for the Eocene carbonates of the Carolinas and equivalent clastic sediments of Alabama (Baum et al., 1979; Baum et al., 1982; Powell and Baum, 1982; 1984), and supports the contention that they are caused by eustatic sea level changes (Vail, 1976; Vail et al., 1977; Vail and Hardenbol, 1979). Although structural histories have left an Imprint on facies development, sequence distribution, and geometry of the Eocene carbonates of the Carolinas (Baum, 1981; Baum et al., 1979; Powell, 1985), the overriding control of sequence boundaries appears to be eustatic sea level change. Likewise, sedimentation rates do not seem to affect the time distribution of sequences; however, carbonates tend to react more Immediately (“give up”) to rapid sea level changes.