INTEGRATING SUBSURFACE AND OUTCROP DATA TO BUILD A UNIFIED MODEL FOR THE COCKFIELD-GOSPORT SHORE ZONE, MIDDLE EOCENE OF MISSISSIPPI AND ALABAMA

The paleogeography and sequence stratigraphy of the middle Eocene Cockfield Formation of Mississippi and equivalent Gosport Sand of Alabama have been historically contentious topics. This is largely the result of both limited outcrop exposure and striking differences in their respective facies, as observable sediments appear to grade from deltaic sands and lignitic clays to densely fossiliferous, glauconitic sands near the state line. Also, efforts to build process-based models for the formation of shell beds like the Gosport have not received the same degree of attention as some of the classic studies that describe high-constructive deltaic systems like the Cockfield. New isopach maps and stratigraphic cross sections that integrate subsurface as well as outcrop data constrain the paleogeography and stratigraphic architecture of this interval. The shelly Gosport thickens and persists at least 100 km to the west along depositional strike while interfingering up-dip with the typical deltaic Cockfield facies to the north. Taphonomic and paleoecological evidence further suggests a fully marine, inner shelf environment subject to episodic winnowing. Given its disconformable contact with the underlying Upper Lisbon Formation and seemingly limited areal extent, prior sequence stratigraphic interpretations of the Gosport have considered it as an incised valley fill deposited during a sea level lowstand. The geographic persistence of these highly fossiliferous, glauconitic sands and their temporal equivalence to more proximal sediments necessitate an alternative interpretation of this celebrated fossil deposit as a part of a complex shore zone within the Cockfield delta system.