ICHNOLOGY OF SILICEOUS FACIES IN THE EOCENE TALLAHATTA FORMATION (EASTERN UNITED STATES GULF COASTAL PLAIN): IMPLICATIONS FOR DEPOSITIONAL CONDITIONS, STORM PROCESSES, AND DIAGENESIS

Abstract

The Eocene Tallahatta Formation exposed in the eastern United States Gulf coastal plain includes enigmatic siliceous facies characterized by variably indurated claystone, porcelanite, and subordinate thin sandstone and siltstone beds. Ichnosedimentologic studies at localities in eastern Mississippi and Alabama indicate that these strata accumulated in middle-shelf settings (offshore transition). Sandstone and siltstone beds record storm events of variable magnitude. Moderately diverse Cruziana ichnofacies assemblages that reflect overall hospitable fair-weather conditions characterize most mudrock intervals; however, periodically oxygen-deficient or otherwise unfavorable conditions are indicated locally by unbioturbated mudrock. Storm beds associated with bioturbated mud typically contain mixed Cruziana-Skolithos ichnofacies assemblages reflecting the work of both opportunistic(?) and fair-weather tracemakers. In contrast, ichnofabrics of storm beds that were not disrupted by fair-weather bioturbation likely record short-term colonization of substrates by organisms that were transported, sorted, and redeposited by storm currents. Ichnofabrics and diagenesis of the siliceous facies are linked. Ichnofossils locally influenced silica cementation in some storm beds, whereas preferential conversion to porcelanite of siliceous clay intervals immediately below and above many storm beds dramatically enhanced the visibility of fair-weather ichnofabrics.