DINOFLAGELLATE BIOSTRATIGRAPHY OF THE UPPER CRETACEOUS PRAIRIE BLUFF CHALK AND THE DANIAN CLAYTON FORMATION IN ALABAMA AND MISSISSIPPI, USA

The Owl Creek Formation and the Prairie Bluff Chalk are the highest Upper Cretaceous units in the Mississippi Embayment (ME); and are unconformably overlain by the Paleocene Clayton Formation. Here we utilize the occurrence of key dinoflagellate species at two new sites in the ME K-Pg sequence, Houston, MS, and Mooseland, AL, to date upper Maastrichtian ammonite zones, correlate them with nannofossil zones, and clarify the timing of the post-impact recovery community. At both sites, the Prairie Bluff contains a progression of upper Maastrichtian ammonite zones (Discoscaphites conradi; D. minardi; D. iris) that is consistent with the ammonite zonation in the Atlantic Coastal Plain. At Mooseland, the lower Prairie Bluff contains the Late Cretaceous dinocysts Dinogymnium sp., Pierceites pentagonus, Isabelidinium cooksoniae? and the first occurrence of Deflandrea galeata. These taxa are indicative of the upper Maastrichtian and co-occur with D. conradi and D. minardi. The D. iris zone, the highest Maastrichtian ammonite zone, contains Dinogymnium sp., P. pentagonus, and the first occurrence of Palynodinium grallator. These dinocysts indicate that the D. iris ammonite zone at Mooseland correlates with the top of the uppermost Maastrichtian nannofossil subzone CC26b. At Mooseland, the K-Pg boundary is defined by a sharp, undulating contact separating the Prairie Bluff and Clayton Fms. At its base, the Clayton contains two clastic horizons which we interpret as tsunamites resulting from shelf collapse due to the Chicxulub impact. The lower horizon occurs directly above the K-Pg contact and contains impact spherules and broken shell fragments. The upper horizon occurs ~0.25 m above the first, and contains Prairie Bluff rip-up clasts at its base. Dinocysts taken from these beds reveal no Danian indicators. The absence of Danian forms strengthens the view that the clastic beds represent tsunami deposits. A third horizon located ~1 m above the K-Pg boundary contains the oyster Ostrea pulaskensis and the first occurrence of the Carpatella cornuta?, a dinocyst which has been dated at ~200,000 years into the Danian. This suggests that the recovery community represented by O. pulaskensis first appears within this time frame, and thus that recovery began within a relatively short interval following the K-Pg impact.