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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 δ13C 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.

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