EXTRACTING THE ICHNOLOGIC RECORD FROM COASTAL PLAIN STRATA: THE EOCENE LISBON TO MOODYS BRANCH SECTION OF MISSISSIPPI AND ALABAMA (USA)

Where body fossils are abundant, trace fossils tend to be overlooked, particularly in sections where traces are hard to discern due to low contrast – as is usual in the unconsolidated sediments of the Atlantic and Gulf Coastal Plains. Considerably more attention has been paid to the ichnology of consolidated Cretaceous strata than to the generally unconsolidated Cenozoic, where it is more difficult to apply the classic approach of collecting and identifying individual traces to establish trace-fossil assemblages, ichnofacies, and tiering relationships. In such cases, the ichnofabric approach, which relies on recognition of distinctive bioturbate textures rather than the identification of individual burrows, may be more appropriate and can aid in the interpretation of depositional environments and construction of regional correlations. Ongoing field study in the Eocene section of Mississippi and Alabama has revealed several characteristic textures within the Kosciusko, Lisbon, Cook Mountain, Cockfield, Gosport, and Moodys Branch Formations. Transgressive sands and marls typically lack any primary depositional fabric due to pervasive bioturbation. Condensed intervals in offshore facies are marked by geographically widespread horizons of large, in-situ oysters bored by pholadid bivalves as well as Teredolities in allochthonous xylic substrates. More proximal condensation is driven in part by repeated storm winnowing, and consequently lacks characteristic traces other than Entobia in amalgamated shell beds. Highstand deposits exhibit moderate to little bioturbation, with small-scale horizontal burrowing that disturbs bedding. The greatest detail is available at stratigraphic boundaries, where firmgrounds were often developed and contrast is enhanced. Examples include horizons burrowed with Gyrolithes, Rosselia, and large network burrows that represent colonization of exhumed substrates during ravinement. The continued integration of ichnofabric data in a detailed stratigraphic framework will add much-needed nuance to paleoenvironmental reconstructions of this famed interval of the fossil record.