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Abstract

In the eastern Gulf Coastal Plain, four, third-order unconformity-bounded depositional sequences are recognized for upper Paleocene and lower Eocene marginal marine to marine shelf strata of the Wilcox Group. In this area, the Wilcox Group includes the Nanafalia Formation (TAGC-2.1 depositional sequence), the Tuscahoma Sand (in part, the TAGC-2.1 and the TAGC-2.2 and TAGC-2.3 sequences), and the Hatchetigbee Formation (TAGC-2.4 depositional sequence). These cycles are interpreted to result from changes in sea level and coastal onlap during the late Paleocene and early Eocene epochs. The Nanafalia Formation and lower part of the Tuscahoma Sand are within the upper Paleocene (Selandian Stage) Planorotalites pseudomenardii Range Zone, whereas the middle and upper parts of the Tuscahoma Sand have been assigned to the upper Paleocene (Selandian Stage) Morozovella velascoensis Interval Zone. The Hatchetigbee Formation contains planktonic foraminifera diagnostic of the lower Eocene (Ypresian Stage) Morozovella subbotinae Interval Zone.

The Paleocene-Eocene Epoch boundary (ca. 54–55 Ma) occurs in the Wilcox Group and coincides with the lithostratigraphic contact of the upper Paleocene Tuscahoma Sand with the lower Eocene Hatchetigbee Formation. Palynological studies of the marginal marine strata of the uppermost clay and silt beds of the Tuscahoma Sand and the lowermost sand beds of the Hatchetigbee Formation indicate late Paleocene and early Eocene ages, respectively, for these formations.

Planktonic foraminiferal zone boundaries generally occur at depositional sequence boundaries dividing the upper Paleocene and lower Eocene marginal marine to marine shelf strata in the eastern Gulf Coastal Plain. Factors that affect the stratigraphic position of biozone boundaries in these strata include the presence of unconformities and associated biostratigraphic discontinuities resulting from nondeposition or erosion, the presence or absence of lowstand systems tract strata, the differential amounts and rates of sedimentation associated with paleobathymetry and/or distance from the shoreline at various depositional sites, differential subsidence within the depositional basin, and paleoenvironmental conditions. The development of a major fluvial-dominated delta complex in this area during Paleocene and early Eocene time had a significant influence on depositional conditions.

An integrated geochronologic, biostratigraphic, and sequence stratigraphic approach provides a useful mechanism for correlation of marginal marine and marine shelf strata of the Wilcox Group in the eastern Gulf Coastal Plain. The use of physical surfaces associated with depositional sequences, such as sequence bounding unconformities, transgressive surfaces, and surfaces of maximum transgression/sediment starvation, in conjunction with the first and last occurrences of age diagnostic microfossils and radiometric data, provides a framework for local, regional, and worldwide correlation.

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