RECOGNITION OF BACK-BARRIER ISLAND HURRICANE-GENERATED PALEOTEMPESTITES: AN ACTUALISTIC TAPHONOMIC STUDY OF NEARSHORE, OVERWASH, AND LAGOON DEATH ASSEMBLAGES

Thin shell beds within back-barrier island lagoon cores are often interpreted as paleotempestites, generated by hurricane overwash. This inference was validated by developing a comparative taphonomic model. The spatial fidelity of paleotempestites should be poor relative to lagoon assemblages and mix nearshore and bay faunas; and the taphonomic grade of overwash faunas should be similar to nearshore assemblages and of poorer grade and greater variability than lagoon faunas. These hypotheses were tested on Lovers Key, Southwest Florida, by comparing overwash assemblages produced by Hurricane Charley (2004) and Tropical Storm Debby (2012) with laterally adjacent, modern foreshore and lagoon assemblages. Bivalve shells were collected from 3 transects set on Charley and Debby overwash terraces. A 4-point grading scheme (4, most degraded; 1, most pristine) was developed for biological (bioerosion / encrustation), chemical (luster / color), and physical (fragmentation) factors. Each shell was graded independently by 3 persons, and each shell’s taxonomic identity was determined. Modern foreshore and lagoon assemblages were bulk sampled, and shells were characterized similarly.

Shells from both Charley fans exhibit similar degrees of degradation for all factors. Debby shells show less physical degradation than Charley assemblages, though they are similar in chemical and biological taphonomic scores. Scores for the foreshore assemblage are very similar to Debby’s, though less similar to Charley’s. The foreshore and fan assemblages are significantly different from the lagoon assemblage, which scored better for all taphonomic factors. Foreshore and fan assemblages have bimodal distributions for the physical factor, while biological grades are generally pristine. The chemical grades are poorest for the fan assemblages, but these shells have been exposed to significant post-depositional subaerial influence. The fan and foreshore assemblages have similar species compositions and rank order abundances. Little faunal overlap exists with the lagoonal assemblage. These taphonomic and faunal distinctions are obvious and vastly employable when identifying suspect paleotempestites. Preliminary research on back-barrier cores in Southwest Florida shows these distinctions are readily detectable.