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).