PRELIMINARY BIOFACIES MODEL FOR THE GOSPORT SAND, UPPER MIDDLE EOCENE, ALABAMA

The Gosport Sand is a notably species-rich, glauconitic shell bed from the upper middle Eocene of Alabama. It has been interpreted as a transgressive sequence deposited in a shallow, deltaic interlobe region. Previous work suggests that it represents an initially current-swept environment with a higher rate of sedimentation that grades into a more quiescent, sediment-starved setting up-section. New preliminary analysis of the individual macrofaunal assemblages found within the Gosport and their dominant ecologies, including ordination and two-way cluster analysis performed on common taxa, suggest the presence of an environmental gradient through time as well as three biofacies, dominated respectively by the bivalve Glycymeris, turritellid gastropods and venerid bivalves, and the bivalve Parmicorbula. Glycymeris and Parmicorbula biofacies are interpreted to represent a more turbulent environment while turritellid-venerid-dominated assemblages are indicative of more stable conditions with higher observed diversity. The stratigraphic position of biofacies is consistent with the proposed transition, with Glycymeris and Parmicorbula making up the lower portion of the section and turritellid-venerid assemblages comprising most of the top. Taken in conjunction with previous sedimentological data, this suggests an environmental shift from tidal inlet or open sound to shallow shelf, driven by coastal flooding during transgression. The return to Glycymeris facies and the overlying echinoid-dominated “Scutella bed” in the uppermost portion of the Gosport suggests shoaling, which continues across the contact of the Gosport with the Moody’s Branch Formation.