THE SIGNIFICANCE OF A FOSSILIFEROUS SPHERULE BED AT THE K/PG BOUNDARY IN NORTHERN MISSISSIPPI

The K/Pg boundary is well exposed in a temporary outcrop due to road construction of Rt 15 near New Albany, Union County, Mississippi. It consists of the upper Maastrichtian Owl Creek Formation and the lower Danian Clayton Formation. At the base of the Clayton Fm is preserved a 15-30 cm thick muddy, poorly sorted quartz sand that contains abundant impact spherules and fossils. The preservation of an abundant macrofaunal fossil assemblage with impact spherules gives a unique picture of the organisms alive during or shortly before the impact, as well as sedimentological processes occurring post impact. The spherules range in size from 0.5 mm to 1 mm in diameter and are hollow with multiple smaller spherical globules typical of impact spherules throughout the Gulf Coast region. The spherules are scattered throughout the bed with an average density of 4.2 per cm². A negative shift in C isotope ratios typical of K/Pg boundary sites is noted within the spherule bed. The macrofossils within the shell bed represent a rich fauna yielding 18 species of bivalves, 13 species of gastropods, 4 species of ammonites (Eubaculites carinatus, Eubaculites latecarinatus, Baculites sp. and Discoscaphites iris) and 1 species of crab. Fossil preservation ranges from whole to fragmentary, and commonly consists of molds. The infill of the shells is lithologically identical to the surrounding matrix of the spherule bed and differs from the underlying Owl Creek. This suggests they existed on the surface of the ocean bed during or shortly before (100 yrs?) the Chicxulub asteroid impact. The fossils show imbrication throughout the bed with gastropods oriented in a similar direction possibly related to strong currents. The taphonomical evidence is consistent with post impact tsunamis as a likely depositional process. Although impact spherules are not uncommon at the K/Pg boundary worldwide, it is rare to find them mixed with abundant macrofossils.