Log-Grounds and Teredolites in Transgressive Deposits, Eocene Tallahatta Formation (Southern Alabama, USA)

摘要

Bored relict log-grounds are locally abundant in hybridized glauconitic fossiliferous muddy sands and associated carbonate concretions in the Eocene Tallahatta Formation, southern Alabama. Log-ground ichnofabrics are dominated by commonly stenomorphic, calcite-lined, variably filled clavate borings assigned to Teredolites longissimus and attributed to teredinid bivalves. Differences in axial morphology and occurrence of accessory tube features (e.g., retrusive calcite caps, concamerations, etc.) within and between borings (1) reflect ontogeny and variable behavioral responses to substrate overcrowding and/or degradation and (2) help to assess previously proposed models for teredinid bivalve life histories. Wood preservational state and significant textural variability among tube-filling sediments reflect extended residence time of log-grounds at or near the sediment-water interface. Like similar accumulations in other marine shelf sequences, the abundance of Teredolites-bearing log-ground accumulations in the Tallahatta can be attributed to both an influx pulse of xylic material and sediment starvation associated with transgression. The low ichnologic diversity (T. longissimus and very rare Thalassinoides) of these and other marine log-grounds may reflect a limited medley of wood-inhabiting tracemakers, but also likely reflects poor substrate preservation associated with physical reworking, biochemical degradation, and the bioerosion process itself.