Ichnofossils in the Eocene Tallahatta Formation, Eastern Gulf Coastal Plain: Evidence for Depositional Environments of Siliceous Claystones

The Eocene Tallahatta Formation exposed in the eastern Gulf coastal plain is dominated by siliceous claystone (e.g., Basic City Shale), the origins of which have been subject of some debate. Ongoing ichnologic and sedimentologic studies of the siliceous claystone facies at various localities in Alabama and Mississippi are providing clues regarding depositional setting and conditions. At all localities examined, siliceous claystones contain very thin to thin interbeds of very fine- to medium-grained quartzose sands, the character of which reflect the influence of storms. In the upper part of the Tallahatta exposed in east-central Alabama, storm beds are weakly bioturbated yet intercalated claystones apparently lack ichnofossils produced by resident bioturbators. This suggests that these claystones accumulated in oxygen-deficient or otherwise inhospitable settings and that tracemakers in associated sands were delivered to depositional sites by storm currents (i.e., doomed pioneers). At most other study localities, siliceous claystones are apparently thoroughly bioturbated, but ichnofabrics and ichnofossils are generally poorly manifest. However, fair-weather ichnofossil assemblages are very well expressed within storm sands and immediately adjacent claystones, reflecting both bed-junction preservation and enhancement of ichnofossil visibility caused by preferential silica diagenesis. Well-preserved ichnofabrics associated with storm beds contain relatively diverse Cruziana ichnofacies assemblages that indicate that siliceous clays accumulated in relatively well-oxygenated, fully marine, middle to outer shelf settings.