ABSTRACT

Throughout the Paleogene of the northern Gulf Coastal Plain large deltaic systems dumped thick sequences of lithoclastic sediments onto the shelf of the Gulf of Mexico. In between major deltaic pulses, thick carbonate banks or reefs developed in the vicinity of the broad Wiggins Uplift of southern Mississippi and Alabama. Since the present-day outcrop of the Oligocene Vicksburg Group in Mississippi and Alabama cuts across sedimentary strike, all of the component lithofacies of a typical Paleogene carbonate bank complex of the northern Gulf Coastal Plain are exposed. By relating outcrop lithology to electric log character, the various lithofacies of the Vicksburg Group can be mapped throughout its subsurface extent. Similar inferences and maps can be made for the other Paleogene carbonate complexes.

Outcrop sediments of the Vicksburg Group comprise five lithofacies, all of which show variations:

1. Molluscan, glauconitic, foraminiferal, quartz silty sand/wackstone.
2. Foraminiferal, algal mudstone.
3. (Quartzose) bryozoan, foraminiferal silty sand.
4. Glauconitic, Lepidocyclin, algal (?), silty sand.
5. Skeletal grainstone/coarse sand.

These lithofacies suggest a set of depositional environments: (corresponding lithofacies number in parenthesis)

1. Destructional delta (1)
2. Algal muddy shelf bottom (2)
3. Regressive carbonate shelf (1)
4. Carbonate bank (back-bank primarily) (3,4)
5. Regressive carbonate shoal/shoreline (5)

All of these environments, with their slightly different faunal constituents, can be found in the other carbonate units of the Gulf Coast Paleogene. The Tatum and the Salt Mountain Limestones

Figure 1. Study area and significant structural elements.

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have coral-algal community shelf margins, whereas the Cook Mountain, Ocala, and Vicksburg have slightly deeper water foraminiferal-algal community at the margin. Time-equivalent lithoclastic deposition to the west prevented westward expansion much beyond central Louisiana. During the Miocene, bank complex communities were displaced farther to the east by continued Rocky Mountain-derived sediments coupled with a major epeirogenic uplift of the Southern Appalachians.