UTILITY OF DISCOSCAPHITES IN INTERPRETING THE STRATIGRAPHIC ARCHITECTURE AND FAUNAL CHANGES PRECEEDING THE K/PG MASS EXTINCTION EVENT ACROSS THE GULF COASTAL PLAIN

Recent debate on the completeness of the upper Maastrichtian deposits across the Gulf Coastal Plain (GCP) propose anywhere from a zero to two million year hiatus prior to the K/Pg boundary. These studies primarily utilize microfossil-based biostratigraphy in the GCP. Macrofossils in the GCP also have an excellent record, but have received little attention in this regard. Our study reveals the potential in applying Discoscaphites iris, D. minardi and D. conradi occurrences to extract fine scale temporal changes in the stratigraphic architecture across the basin. This will aid in the correlation of upper Maastrichtian units throughout the GCP facilitating the reconstruction of faunal, depositional, lithological and paleoenvironmental changes leading up to the K/Pg mass extinction event.

Three upper Maastrichtian formations, the Owl Creek Formation, the Arkadelphia Formation, and the Prairie Bluff Chalk, were investigated across the GCP. The Owl Creek Formation was examined in Tippah and Union Counties, MS, and Stoddard County, MO; the Arkadelphia Formation in Jefferson County, AR; and the Prairie Bluff Chalk in Chickasaw and Oktibbeha Counties, MS, and Sumter ("Moscow Landing" site) and Wilcox Counties, AL. Initial investigations of ammonite distribution supported by co-occurring dinoflagellates and nannofosills assemblages suggest that the occurrence of D. iris, D. minardi and D. conradi are isochronous across the Atlantic and Gulf Coastal Plain and are facies-independant varying from siliciclastic to carbonate sediments. The D. iris Zone is correlative with the upper part of calcareous nannofossil Zone CC26b, spanning the upper part of the upper Maastrichtian. For the first time the D. iris Zone has been documented to extend throughout the entire Mississippi Embayment supporting the idea that most sections in the GCP are quite complete with the exception of the Moscow Landing site in AL. A disconformity at the K/Pg boundary was observed at many sites. However, this hiatus appears to represent minimal missing time based on faunal assemblages and sediment thickness. The ammonite record provides an important contribution to understanding the stratigraphic architecture and paleoenvironments in the GCP leading up to the K/Pg extinction event.