REVISITING THE ICHNOLOGY OF K-PG BOUNDARY EVENT BEDS AT MOSCOW LANDING, WESTERN ALABAMA: DID TRACEMAKERS SURVIVE TRANSPORT VIA MEGAWAVE BACKWASH?

Ichnology has been employed in studies of K-Pg boundary intervals comprising various lithofacies at numerous localities around the globe. Ichnofossils have been used to evaluate the role of bioturbation on biostratigraphic mixing, to gauge how infauna were impacted by and recovered from boundary events, to interpret environmental changes across the boundary, and, as exemplified here, to decipher mechanisms responsible for emplacement of coarse-grained clastic deposits manifest at the boundary at localities around the Gulf of Mexico. The latter include discontinuous clastic sand bodies, known as Clayton sands, that occur at the contact between the Maastrichtian Prairie Bluff Chalk and the Danian Clayton Formation at several sites in the eastern U.S. Gulf Coastal Plain and are particularly well exposed at Moscow Landing in western Alabama. Thin, channel-form or wedge-shaped and fault-bounded bodies at Moscow Landing have been interpreted alternately as incised valley fills linked to sea-level dynamics, impact-induced megawave deposits, or a combination of both. Pervasive Thalassinoides burrow systems mark sand-body tops and, in the absence of Clayton sand, are contiguous with firmground burrow systems emplaced into Prairie Bluff Chalk from the base of the Clayton Formation. These burrows systems mark a widespread transgressive erosion surface formed after Clayton sand deposition. Sedimentologic characteristics of most Clayton sand bodies are consistent with catastrophic deposition associated with megawaves oscillating through the Gulf of Mexico. However, some sand bodies do contain internal horizons of Thalassinoides systems. These horizons could be construed to reflect infaunal occupation of reworked megawave deposits during early transgression, in line with the incised valley-fill interpretation. However, Thalassinoides horizons in some sand bodies may reflect the work of crustaceans that survived energetic transport by megawave backwash and burrowed into sands during short periods of relative quiescence prior to subsequent shoreward passage of rebounding megawaves.