SEDIMENTOLOGY AND PETROGRAPHY OF MARIANNA AND GLENDON LIMESTONES OF THE VICKSBURG GROUP IN SMITH COUNTY, MISSISSIPPI

The Vicksburg Group (Oligocene) is a northwest - southeast trending rock unit that outcrops in central Mississippi. From base to top, it consists of the following formations: The Forest Hill (lignitic sand and clay), Mint Spring Formation (sand and marl), Marianna Limestone, Glendon Limestone, Byram Formation (silt, clay, sand, and carbonate), and Bucatunna Formation (clay, silt, and mud). Heavy vegetation covers the majority of exposures of the Vicksburg Group. However, the two limestone formations (Marianna and Glendon) are regularly mined for agricultural lime and building stones. One such operation in Smith County, Mississippi, has resulted in excellent exposures of these two limestones and was investigated for this project.

The contact between the Marianna Limestone and the underlying Mint Spring Formation is not exposed in the Smith County Quarry. Here, the Marianna Limestone is only 5 meters thick. It consists of poorly lithified to unlithified, highly bioturbated, carbonate mud and fossil fragments. Petrographic observations show abundant fossils of bryozoans, mollusks, and forams. Highly weathered glauconite grains and detrital sand-sized quartz are also present. Marianna Limestone lacks any high-energy sedimentary structures suggesting deposition in deep water below wave base.

The overlying Glendon Limestone is also 5 meters thick at the Smith County quarry. It consists of five highly lithified, ledge-forming limestone layers which alternated with poorly lithified marly limestone beds. Each layer varies in thickness from 20 cm to 80 cm. The transition between the two layer types is gradational. Petrogrpahic studies indicate that both layers are highly fossiliferous. In addition, lithification occurred by precipitation micrite cement in ledge-forming layers. Traction current features are absent in the Glendon Limestone suggesting deposition below a wave-based environment.