GYROLITHES IN THE EOCENE TALLAHATTA FORMATION: FACIES DISTRIBUTION, MORPHOLOGIES, AND PRESERVATIONAL MODES

The trace fossil Gyrolithes is a common component of ichnofossil assemblages in the Eocene Tallahatta Formation exposed in the Alabama coastal plain. These vertical, coiled burrows were emplaced in a broad range of substrates, including relatively high-energy, cross-stratified, nearshore or marginal marine sands (e.g., Meridian Sand); fully bioturbated muddy glauconitic sands; soft to firm, siliceous marine-shelf clays; and thin silt and sand tempestites. Gyrolithes are variably preserved in full relief as weakly to well-cemented sediment casts that are commonly more resistant than surrounding sediments (all lithologies), as open or unfilled tubes (siliceous claystones), and in cylindrical mummy concretions associated with silica diagenesis of ambient claystone. Burrow morphologies vary from simple to complex, and in sand facies include compound forms (e.g., Gyrolithes-Ophiomorpha). Although the producers of the Tallahatta Gyrolithes likely were crustaceans, composite structures indicate that burrow fills were commonly exploited by worms. The prevalence of Gyrolithes in a wide range of Tallahatta facies evokes questions regarding a possible link between this ichnotaxon and conditions in the Gulf region during the Eocene.