Savrda, Charles E.1, Richard G. Urash1, John Counts1
(1) Auburn University, Auburn, AL
ABSTRACT: Ichnology of a Passive-Margin Condensed Section, Eocene Tallahatta Formation, Alabama
Ichnology has been employed extensively in sequence stratigraphic studies. The role of substrate-controlled ichnofacies in delineating key stratal surfaces and the use of vertical ichnofaunal changes as indicators of sea-level-mediated facies changes are particularly well established. However, comparatively little work has focused on ichnologic signatures of condensed sections. This prompted ichnologic studies of an interval of the Eocene Tallahatta Formation exposed along the Conecuh River, southern Alabama, that records transgression-related stratigraphic condensation in a relatively shallow, passive-margin shelf setting. The condensed interval is dominated by weakly consolidated, thoroughly bioturbated, variably fossiliferous, glauconitic, locally concretionary, muddy sands; thin mud beds and storm-related shell beds are subordinate. Differential erosion of these hybrid sands and concretion surfaces reveal a softground Cruziana ichnofacies assemblage that 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. More significantly, composite structures and other ichnofabric evidence for repeated burrow filling and re-excavation are common. These features, which we also recognize in glauconitic condensed sections within other coastal plain shelf sequences, apparently testify to 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.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.