DINOFLAGELLATE CYST BIOSTRATIGRAPHY ACROSS THE CRETACEOUS-PALEOGENE (K-PG) BOUNDARY NEAR THE CITY OF MALVERN, HOT SPRINGS COUNTY, ARKANSAS, USA

In Hot Spring County, AR, near Malvern, outcrops of the upper Maastrichtian Arkadelphia Formation are best exposed along the banks of the Ouachita River. Sediments collected above and below the Cretaceous-Paleogene (K-Pg) boundary at the locality has yielded palynological slides that preserve abundant dinoflagellate cysts. Evidence for the placement of the K-Pg boundary event deposit at this site is based on the abundant recovery of impact spherules preserved in a bed of poorly sorted, quartz-rich sand that defines the lowermost Danian. A sharp, undulatory contact separates the spherule bed from the underlying Arkadelphia Formation. Here, previous workers have documented the uppermost Maastrichtian index ammonite Discoscaphites iris, 4.5 m below the base of the K-Pg boundary. Collection of this material provides a unique opportunity to study the biostratigraphic distribution of dinoflagellate cysts across the impactite. The following taxa have been identified thus far: Alisocysta sp., Alisogymnium sp., Andalusiella spp., Alterbidinium acutulum, Alterbidinium spp., Cannosphaeropsis utinensis, Carpatella cornuta, Cerodinium diebelii, Cerodinium spp., Cordosphaeridium fibrospinosum, Coronifera sp., Cribroperidinium spp., Cyclonephelium brevireticulatum, Deflandrea galeata, Deflandrea spp., Dinogymnium spp., Diphyes sp., Disphaerogena carposphaeropsis, Fibradinium annetorpense, Florentinia spp., Fromea fragilis, Glaphyrocysta spp., Hafniasphaera fluens, Hystrichokolpoma spp., Ifecysta spp., Lanternosphaeridinum spp., Lejeunecysta spp., Manumiella spp., Oligosphaeridium complex, Palaeotetradinium sp., Palaeocystodinium spp., Palynodinium grallator, Pierceites pentagonus, Phelodinium spp., Senegalinium dilwynense, Senegalinium simplex, Senoniasphaera inornata, Spiniferites spp., Spongodinium delitiense, Tanyosphaeridium xanthiopyxides, Thalassiphora pelagica, Trithyrodinium evittii, Xenicodinium sp., Yolkinigymnium sp., small peridinioids, and the late Maast. pollen Rugubivesiculites spp. The long-term goal of this study is to firmly document the distribution of dinoflagellate cysts, to establish dinoflagellate-based biozonations that can be correlated to the ammonite zones, and to confirm the ages of the units above and below the K-Pg boundary.