Ichnology of an Eocene shallow marine passive margin condensed section, eastern Gulf coastal plain, Alabama, U.S.A.

Abstract

Exposures of the Eocene Tallahatta Formation in southern Alabama (U.S.A.) provided the opportunity to document the ichnologic character of a highly condensed, passive-margin transgressive systems tract and its two component parasequences. The studied succession is dominated by variably glauconitic, fossiliferous muddy sands deposited in shallow marine shelf settings. Thorough bioturbation and abundance of mud indicate quiet background conditions, whereas admixed sand and localized scour features indicate periodic influence by bottom currents. Parasequences, delineated based on vertical trends in glauconite abundance, are bounded at their bases by flooding surfaces marked by Thalassinoides-dominated firmground ichnofabrics but internally host moderately diverse Cruziana ichnofacies assemblages. Recurring softground ichnofossils include Asterosoma, Chondrites, Diopatrichnus, Ophiomorpha, Planolites, Rosselia, Teichichnus, Thalassinoides, and enigmatic composite structures, the distributions and abundances of which vary within and between parasequences. Increasing prominence of Diopatrichnus and Rosselia towards parasequence tops reflects organismal responses to environmental changes associated with shallowing and/or enhanced preservation potential of shallow-tier structures in response to increased sedimentation rates. Maximum sediment starvation during deposition of the uppermost parasequence is reflected by peaks in burrow density, softground ichnofossil diversity, and concentrations of Entobia-bored shells and Teredolites-bored logs. Enigmatic composite burrows and common Thalassinoides with linings and/or meniscate backfills are ubiquitous and may be diagnostic of sediment starvation and prolonged habitation of slowly accreting substrates.