Our investigation focuses on the depositional systems that formed in different sectors of the northern Gulf of Mexico shelf during the last glacial-eustatic sea level cycle, from approximately 125,000 years to present.

We have adopted conventional sequence stratigraphic terminology in our research for sake of uniformity (e. g. Van Wagoner, et al., 1988). This may well lead to some confusion because our paleogeographic maps are well-constrained in terms of the age of depositional systems and changes in sea level. The systems tract designation used by the Exxon researchers was developed without the advantages of "real" sea level information. The terms highstand, lowstand and transgressive systems tracts do not always conform to what sea level was actually doing in a particular area at a specific time interval. The sequence stratigraphic approach is still used because it divides the stratigraphy into units which are useful as analogs to older strata. The detailed sea level information is also presented.

Correlation of strata between the different areas is accomplished using three prominent stratigraphic surfaces. The oldest surface is the Stage 5e maximum flooding surface which is associated with the last highstand (approximately 125,000 BP). It is a downlap surface in most of the study areas (see examples in Regional and Reservoir Studies) and is recognized in cores as a sharp increase in planktonic formanifera (1996 ISG Report). The highstand systems tract occurs between this flooding surface and the Stage 2 sequence boundary (see examples in Regional Differences in Depositional Systems and in Regional and Reservoir Studies). The Stage 2 sequence boundary is a prominent erosional surface and is manifested in cores and platform borings as an abrupt change in lithology and general absence of fossils. The upper surface is the transgressive ravinement surface. This surface often occurs near or at the present sea floor and is manifested in cores as a sharp lithologic change and increase in marine fossils.

Chronostratigraphic ground truth for our seismic stratigraphic correlations is obtained using oxygen isotope curves, micropaleontological analyses and radiocarbon ages from long sediment cores. These chronostratigraphic tools have proven to be reliable dating methods when placed within a seismic stratigraphic framework so that autocyclic (delta lobe shifting) and condensed intervals and unconformities are identified. Biostratigraphic analyses include establishment of modified Ericson-Wollin zones, based on the relative proportions of Globorotalia menardii and Globorotalia inflata (Ericson and Wollin, 1968; Kennett and Huddlestun, 1972; Thunnell, 1984) and extinction of Globorotalia menardii flexuosa, which indicates an age of approximately 85,000 yrs. BP (Poag and Valentine, 1976; Kohl, 1986). The isotopic curves from DSDP Leg 96, Site 619 in Pigmy Basin on the Louisiana continental slope provides a bench mark for interpreting our oxygen isotope curves (Williams and Kohl, 1986). To date, we have generated oxygen isotope curves for the eastern and south Texas shelf using Globogerinoides ruber and for the central Texas shelf using G. menardii. Our oxygen isotope curves show a good match to the Pigmy Basin curve when the Stage 4 and Stage 2 exposure surfaces are identified using seismic records. Two meltwater anomalies that have been age dated at 42,800 to 44,300 and 60,700 yrs. BP (Williams and Kohl, 1986) are also identified. In addition to our own data, we rely on existing chronostratigraphic data from studies on the Louisiana shelf by Coleman and Roberts (1990) and Sydow and Roberts (1994).

Seismic Line R 90-5B is used to illustrate the three major seismic surfaces that are used to correlate across the northern Gulf of Mexico shelf and upper slope. The Stage 5e maximum flooding surface is a prominent downlap surface in areas where deltas exist and was mapped across the entire margin. It is the basal surface for the units mapped in this project. The Stage 2 sequence boundary is the sequence boundary formed during the last eustatic lowstand and is marked by considerable relief over most of the inner shelf and by a lowstand wedge at the outer shelf and upper slope. The transgressive surface is more difficult to recognize, except on the outer shelf where it is marked by onlap.

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