SIGNIFICANCE OF FOSSIL CONCENTRATIONS IN INTERPRETING PALEOENVIRONMENTAL CHANGES PRECEDING THE END CRETACEOUS MASS EXTINCTION EVENT ALONG THE GULF COASTAL PLAIN

Five marine fossil concentrations within the Upper Maastrichtian Owl Creek (OC) and Prairie Bluff (PBC) Formations, and the Lower Danian Clayton Fm. were analyzed to determine processes of formation related to environmental changes preceding the K/Pg extinction event. The siliciclastic OC Fm and the carbonate rich PBC represent marginal to offshore shelf deposits. Three localities were sampled to evaluate biodiversity, taphonomy (encrustation, fragmentation, disarticulation, and orientation), sedimentology and placement within a sequence stratigraphic setting. At Mooseland (Wilcox County, AL), a shell horizon occurs at the base of the PBC. High biodiversity (Simpson Diversity index 1-D = 0.93, richness of 73), abundant taphonomic damage, and stratigraphic placement indicate an environmentally mixed lag resulting from shoreface retreat during the onset of a transgressive systems tract (TST). At Moscow Landing (Marengo County, AL), a fossil concentration lies mid-sequence within the PBC, with a less diverse assemblage (1-D= 0.87, richness of 43). This most likely represents a hiatal concentration that formed during maximum flooding at the latest stages of a TST. The overlying Danian clastic unit of the Clayton Fm. contains a shell bed composed of reworked Cretaceous fauna (1-D=0.95, richness of 30), abundant shell hash, and PBC rip-ups. Sedimentologic and biostratinomic evidence suggests that this represents an event-concentration probably resulting from a high energy episode such as a tsunami. The OC type locality (Tippah County, MS) contains two distinct fossil accumulations. Preliminary data suggests that the upper accumulation is a relatively diverse assemblage (1-D=0.93, richness of 43). Biostratigraphic data and taphonomic signatures indicate a within-habitat assemblage with minimal time averaging, resulting from exhumation/redeposition associated with minor sea level fluctuations. The lower accumulation consists of discontinuous, lens shaped pods dominated by the species Turritella tippana. Preferential orientation measurements suggest currents as the main agent associated with accumulation. The concentrations in our study suggest differing processes of formation. This is critical in reconstructing paleoenvironmental changes leading up to the K/Pg extinction event.