TEXTURAL AND PETROGRAPHIC ANALYSES OF SAND AND SANDSTONE SAMPLES FROM THE MERIDIAN SAND, GRENADA COUNTY, MISSISSIPPI: IMPLICATIONS FOR INTERPRETING DEPOSITIONAL ENVIRONMENT AND DIAGENETIC HISTORY

The Meridian Sand Member of the Middle Eocene Tallahatta Formation is considered to be the basal unit of the Claiborne Group in Mississippi and Alabama. In northern Mississippi, the Meridian Sand is a productive aquifer, and yet its lithology and petrographic properties have not been studied in detail. The purpose of this project was to perform a thorough petrographic analysis of the unit in multiple locations across Grenada County, Mississippi. Five outcrop locations were chosen for measuring and describing stratigraphic sections as well as lithofacies analysis. Grain properties and distributions were determined by sieving, petrographic observations and point counting from thin sections.

In general, the Meridian Sand is a very fine to coarse, angular to sub-angular, poorly to moderately well-sorted sand and sandstone interbedded with mud. Nine lithofacies were defined based on lithologic properties (mean grain-size, sorting, grain shape), and field observations. Sedimentary structures include mud drapes, flaser bedding, cross-bedding, herringbone cross-stratification, and burrows. These structures, as well as grain-size trends, suggest that the Meridian Sand reflects deposition in marginal-marine environments, probably tidal flat and shoreface settings. Diagenetic features described from thin-sections include clay and hematite cements, hydrocarbon staining, minor compaction, grain fracturing, tangential grain contact, deformation of muscovite grains, and partial dissolution of quartz grains, indicating eogenesis and early mesogenesis. Furthermore, our interpretation suggests that hematite and clay cement precipitation occurred prior to compaction. The partial dissolution of quartz grains might have resulted from pore fluids capable of dissolving silica, and this dissolution resulted in secondary porosity. The diagenetic history of the Meridian Sand resulted in relatively porous sand and sandstone, and the distribution of cements is an important factor to consider when evaluating the unit’s aquifer characteristics.