SEDIMENTOLOGY, ICHNOLOGY, AND DIAGENESIS OF STORM BEDS WITHIN SILICEOUS SHELF DEPOSITS, EOCENE TALLAHATTA FORMATION, EASTERN GULF COASTAL PLAIN

The Eocene Tallahatta Formation in western and central Alabama is dominated by siliceous claystones that were deposited under productive waters on a passive-margin shelf during sea-level highstand. Claystones and their diagenetic equivalents (e.g., porcelanites) contain numerous thin sand beds that record periodic storm depositional events. Thickness (<1 mm to several decimeters), texture, composition, internal stratification, and ichnofossil content of storm beds vary considerably, reflecting variations in storm magnitude and specific transport and depositional mechanisms. In many intervals, biogenic disruption of even the thinnest tempestites is commonly weak. Trace fossils, mainly Ophiomorpha, Thalassinoides, and Gyrolithes, typically are restricted to storm sands and immediately subjacent claystones; assemblages of ichnofossils representing resident fair-weather faunas are commonly absent. These observations indicate that siliceous mud substrates were generally inhospitable, perhaps due to oxygen-deficient background conditions, and that bioturbation of storm beds was mainly the work of allochthonous crustacean tracemakers (doomed pioneers). The latter inference underscores the need to more fully integrate traces produced by transported organisms into existing tempestite ichnofossil successional models. Thicker storm beds are commonly wholly or partly cemented by silica. Disposition of various silica cement types (e.g., chalcedony, opaline silica) within individual sand beds generally is related to proximity to siliceous clays (sub- and superjacent beds, but also rip-up clasts and clay incorporated into burrow fills and/or linings) and associated silica concentration gradients that existed in pore fluids.