CONTRIBUTION OF ALLOCHTHONOUS TRACEMAKERS TO STORM-BED ICHNOFABRICS

Models for storm-bed ichnofabrics generally recognize two main temporally disjunct ichnofossil suites: (1) a suite of structures produced by opportunistic organisms that temporarily exploit tempestites immediately following deposition; and (2) a suite of traces subsequently produced in storm beds after re-establishment of fair-weather benthic infauna. Traces produced by storm-transported burrowers—so-called “doomed pioneers”—have not held a prominent place in ichnofossil successional models. However, ongoing studies of storm deposits in eastern Gulf coastal plain sequences indicate that allochthonous tracemakers may be more significant than previously recognized and may be responsible for some structures (e.g., Ophiomorpha) that typically have been attributed to opportunistic organisms. Proximal estuarine central-bay storm beds in the Cretaceous Eutaw Formation, western Georgia, 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. Although thick waning-stage clay blankets precluded bioturbation of storm sands by fair-weather tracemakers, sparse Ophiomorpha occur throughout the sands. This may indicate that the crustacean progenitors were introduced along with the sand by storm currents. Storm sands in shelf deposits of the Eocene Tallahatta Formation, southern Alabama, similarly 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 and/or other traces. Relationships between inferred storm magnitude and burrow size may reflect hydraulic sorting by storm-transported tracemakers. Summarily, structures produced by allochthonous tracemakers may be the dominant element of some storm-bed ichnofabrics and should be considered more fully in tempestite ichnofossil successional models.