ICHNOLOGY OF TRANSGRESSIVE GLAUCONITIC SANDS, EOCENE TALLAHATTA FORMATION, ALABAMA

Exposures of the Eocene Tallahatta Formation along the Conecuh River, southern Alabama, are providing the opportunity to assess the ichnologic signature of a shallow-marine, passive-margin condensed section. The condensed interval is dominated by weakly consolidated, thoroughly bioturbated, fossiliferous, glauconitic, locally concretionary, muddy sands with subordinate thin mud beds and storm-related(?) shell beds. Differential erosion of the muddy sands and concretion surfaces reveal a softground assemblage diagnostic of the Cruziana ichnofacies. Work to date indicates that this assemblage includes abundant Thalassinoides, Rosselia, and Diopatrichnus, common Asterosoma, Teichichnus, Planolites, and shell-filled burrows, and rare Ophiomorpha. Some of these ichnotaxa commonly occur as parts of compound structures. Composite structures and fabrics indicative of repeated burrow filling and re-excavation also are common. The latter features likely reflect periodically energetic conditions but overall very low net-sediment accumulation rates. Glauconitic muddy sands of the Tallahatta also are host to incongruous Entobia-bored shellgrounds and Teredolites-bored log-grounds, the abundances and character of which also can be linked, in part, to transgression and associated sediment starvation. Comparative studies of other transgressive greensand successions in the coastal plain will help to define ichnologic criteria for identifying condensed intervals and delineating parasequences therein.