STRATAL GEOMETRY AND FACIES RELATIONSHIPS OF THE MIDDLE EOCENE UPPER CLAIBORNE GROUP IN THE SUBSURFACE OF SOUTHEASTERN MISSISSIPPI AND SOUTHWESTERN ALABAMA

New subsurface geologic cross sections oriented along approximate depositional strike and dip of the middle Eocene upper Claiborne Group in southeastern Mississippi and southwestern Alabama reveal thickness trends and facies relationships consistent with depositional models of earlier workers while providing additional stratigraphic constraints within the study area. In general, units thicken down-dip into the Perry Basin and thin distally on the north limb of the Wiggins Uplift. The shelf-bank carbonate facies of the equivalent Cook Mountain and Upper Lisbon Formations are overlain by the prodelta muds of the encroaching Cockfield delta complex. These muds downlap onto a maximum flooding surface, thinning considerably in the vicinity of southern Washington County, AL and Greene County, MS. Prodelta deposits are overlain by the shelly glauconitic sands and intercalated muds of the Gosport Sand, differentiated from the Upper Lisbon (here for the first time) in the subsurface of southwestern Alabama by correlation with laterally equivalent facies in Wayne County, MS. While prior outcrop-based stratigraphic and paleoenvironmental interpretations of the Gosport have cited its relatively restricted thickness and geographic extent, current data suggest that approximately equivalent facies thicken to two or three times those exposed at the surface and persist at least 50 kilometers into the subsurface. These Gosport fossiliferous glauconitic sands and muds represent relatively shallow, marine interdeltaic facies that grade up-dip into the nonfossiliferous delta front sands of the Cockfield Formation in Mississippi. Glauconitic, shelly sands are in turn overlain by deltaic muds which thicken down-dip in the vicinity of central Washington County and thin to the south, where they downlap onto a maximum flooding surface. These observations are consistent with the overall seaward shift of relative sea level during this interval.