ICHNOFABRIC CHARACTER AND PRESERVATION IN STORM-INFLUENCED SHELF DEPOSITS, EOCENE TALLAHATTA FORMATION, EASTERN MISSISSIPPI

Eocene marine shelf deposits of the Tallahatta Formation exposed in eastern Mississippi (Basic City Shale Member) are characterized by centimeter- to decimeter-scale alternation of relatively weakly and well indurated layers. Weakly indurated layers are dominated by siliceous claystone, while well indurated intervals contain one or more silica-cemented, hummocky cross-stratified, very fine- to fine-grained sandstones separated and mantled by porcelanitic claystone. Claystone-dominated layers presumably represent relatively quiet fair-weather conditions. They typically appear homogeneous or exhibit only diffuse burrow mottling, limiting paleoenvironmental interpretation. In contrast, ichnofabrics in indurated layers are very well expressed. In sandstones, which reflect storm depositional events, primary stratification is cut by well-defined trace fossils, including Asterosoma, Chondrites, Gyrolithes, Phycosiphon, Skolithos, Taenidium, Teichichnus, Thalassinoides, and Zoophycos. Notably, these same ichnotaxa also are manifest in associated thoroughly bioturbated porcelanitic claystones. This ichnofossil assemblage is provisionally interpreted to reflect the fair-weather benthic conditions that prevailed during deposition of all claystones. In this case, expression of ichnofabric and, hence, preservation of paleoenvironmental information were enhanced both directly and indirectly by storm beds. Emplacement of storm-event sands resulted in bed-junction preservation. Subsequently, vertical migration of silica diagenetic fronts outward from sandstones preserved or enhanced the contrast between biogenic structures and host sediments in claystone horizons immediately adjacent to sands.