ROLE OF ALLOCHTHONOUS TRACEMAKERS IN STORM-BED ICHNOFABRICS

Models for storm-bed ichnofabrics generally recognize two main temporally disjunct ichnofossil suites, one produced by opportunistic organisms that temporarily exploit tempestites immediately following deposition and the other produced in storm beds after re-establishment of fair-weather benthic infauna. Traces produced by storm-transported burrowers have been recognized but have not held a prominent place in ichnofossil successional models. However, ongoing studies of tempestite-bearing Cretaceous and Tertiary sequences in the eastern Gulf coastal plain indicate that allochthonous tracemakers may be the dominant element of some storm-bed ichnofabrics and may be responsible for some structures (e.g., Ophiomorpha) that typically have been attributed to opportunistic organisms. Estuarine central-bay storm beds in the Cretaceous Eutaw Formation commonly consist of two parts that appear to represent a depositional continuum; a wavy laminated or hummocky cross-stratified sand deposited during peak storm phases, and an upper massive clay deposited from suspension during waning stages. Clay beds appear to be bioturbated only near their tops. Although thick waning-stage clay blankets precluded bioturbation of storm sands by fair-weather tracemakers, sparse thickly clay-lined and locally clay-filled Ophiomorpha occur throughout the sands. This suggests that the crustacean tracemakers were introduced with the sand by storm currents. Storm sands in shelf deposits of the Eocene Tallahatta Formation lack fair-weather ichnofossil suites owing to oxygen-deficient or otherwise inhospitable background conditions. Nonetheless, sands and immediately subjacent siliceous claystones are weakly disrupted by Ophiomorpha, compound Ophiomorpha/Teichichnus, and/or other structures. Relationships between inferred storm magnitude, ichnofossil assemblages, and burrow size may reflect hydraulic sorting by storm-transported tracemakers. The role of allochthonous tracemakers in the generation of storm-bed ichnofabrics is apparently better manifest in storm-influenced sequences that are not extensively overprinted by fair-weather bioturbators.