CHEMOSTRATIGRAPHY OF UPPER MAASTRICHTIAN SHALLOW MARINE DEPOSITS IN MISSISSIPPI, USA: TEMPORAL FRAMEWORK FOR THE GULF COASTAL PLAIN REGION LEADING UP TO THE END-CRETACEOUS MASS EXTINCTION EVENT

Upper Maastrichtian shallow marine deposits of the Prairie Bluff Chalk Formation (PBC) are well exposed in the Gulf Coastal Plain, USA. In Starkville (Oktibehha County, Mississippi) and the Prairie Bluff Landfill (Chickasaw County, MS), the PBC consists of alternating, decimeter scale chalk and marl couplets. The observed rhythmic pattern is most likely a result of orbitally forced changes in sedimentation. In the study area, the contact between the PBC and the overlying Danian Clayton Formation marks the K-Pg boundary (KPB). To better understand the timing and mechanisms of extinction, a detailed record of changes in the ocean-climate systems is necessary. However, this is often difficult to obtain due to insufficient time control. Here, we present a high-resolution temporal framework to improve interpretations of environmental conditions leading up to the KPB. In the study area, ammonite and microfossil biostratigraphic ranges have been documented. However, a high-resolution chemostratigraphy is lacking. We conducted a geochemical analysis on the PBC Fm to determine if and which Milankovitch cycles were responsible for rhythmic sedimentation. Both sections were sampled in 0.1m increments up to and including the KPB. Stable isotope (δ¹³C_carb, δ ¹⁸O_carb) values, % CaCO₃, and elemental concentrations (Fe, K, Ti, Sr, Ca, Mn) were determined. Data from both sections indicates that cyclical changes in lithology mirror variations in geochemical proxy values. A statistical analysis confirms these observations, showing that most variables indicate orbitally forced sedimentation. Additionally, the KPB in the study area exhibits a negative δ¹³C excursion similar in magnitude to those observed at KPB sites from around the globe. The negative δ¹³C excursion can be attributed to productivity changes that may have been a consequence of the Chicxulub impact. This study will enable global/regional correlation with other KPB sections, and will allow accurate interpretations of environmental change that may have contributed to the end-Cretaceous mass extinction.