MICROFACIES CORRELATION OF THE UPPER EOCENE SANDERSVILLE LIMESTONE MEMBER OF THE TOBACCO ROAD SAND TO THE OCMULGEE FORMATION ON THE COASTAL PLAIN OF GEORGIA

The Ocmulgee Formation of the Upper Eocene in East-Central Georgia has been recognized as a separate unit from the Sandersville Limestone member of the Tobacco Road Sand of Middle Jacksonian age due to lithology differences and geographical separation of these two units. The microfacies classification of the Ocmulgee Formation has shown a relationship to the Sandersville Limestone member based on biostratigraphic, petrographic, x-ray crystallography, insoluble residues, and grain size analyses.

The lower portion of the Ocmulgee Formation is a calcite-cemented sandstone, which grades into a quartz-rich sparse biosparite. The matrix is composed of a microsparite that has replaced the original micrite matrix. The lower portion has a diverse fauna containing bryozoans, gastropods, echinoderms, ostracods, brachiopods, foraminifera, and fish teeth and bones. Many of the fossils show signs of abrasion. The lower portion of the Ocmulgee is interpreted to be a near shore environment.

The upper portion of the Ocmulgee Formation is a quartz-rich, glauconitic, sorted biosparite. The matrix contains both microsparite and micrite. The lithology grades into a packed poorly washed biosparite composed primarily of ostracods and globorotalid foraminifera. The Ocmulgee Formation becomes more calcareous towards the top of the section. The upper portion of the Ocmulgee Formation is interpreted to be a deep shelf margin environment.

The Sandersville Member of the Tobacco Road Sand is a micritic limestone with portions that have been silicified. It is moderately fossiliferous and contains similar faunal as the Ocmulgee. Less biota is found in the upper portion of this unit. Dissolution and other diagenetic alteration features are observed in the upper portion of the Sandersville Limestone. The Sandersville represents a shallow marine environment, which may have gradually deepened towards the top of the section.

Stratigraphically the Ocmulgee and Sandersville underlie the Twiggs Clay Member of the Dry Branch Formation. Both units contain layers that have large percentages of sand and layers that are glauconitic. Both show a rise in sea level through the section, and have a very diverse fauna. Both units have the echinoid species Periarchus quinquefarius, which is only found in the Sandersville Limestone and the Ocmulgee Formation.