INSIGHTS INTO CLIMATIC AND FAUNAL CHANGES AT THE END OF THE MAASTRICTIAN AT THE TYPE LOCALITY OF THE OWL CREEK FORMATION, MISSISSIPPI

The examination of the biostratigraphic distribution of ammonites and nannofossils in the upper 10 m interval of the Owl Creek Formation at its type locality, Tippah County, Mississippi sheds new light on climatic and faunal changes at the end of the Maastrichtian. The investigated unit is correlative with the Discoscaphites minardi and D. iris Zones spanning approximately 500 ky immediately prior to K/Pg boundary. The uppermost Maastrichtian nannofossil markers Micula prinsii and M. murus are present corroborating the ammonite zonation. A sample from near the bottom of the section contains the dinoflagellate Palynodinium grallator, also indicative of the uppermost Maastrichtian. Discoscaphites minardi, D. iris, and Baculites sp. C of Cobban and Kennedy (1995) are dominant in the lower part of the section whereas D. iris, Eubaculites carinatus, and E. latecarinatus are dominant in the upper part of the section. A striking drop in the nannofossil abundance at 6 m coincides with the change in the ammonite fauna possibly due to environmental factors. The shells of ammonites, gastropods, bivalves, and <63µ size fractions were analyzed for oxygen and carbon isotopes. The oxygen isotopic values of ammonites range from – 1.7 to 1.5 ‰ and the carbon isotopic values range from -3.9 to 1.7 ‰_. Converting the oxygen isotopic values to temperature, using d_w of Cretaceous sea water = -1.0 ‰ as an approximation, yields a temperature range of 15.1°C - 25.3°C. This range is nearly constant throughout the stratigraphic section suggesting a relatively stable climate through the last 500 ky of the Maastrichtian. Two specimens of E. latecarinatus and a single specimen of Baculites sp. C were analyzed for seasonality, yielding d¹⁸O values from 0.0 to 0.1‰ (at 0.5 m), from -0.5 to -0.2‰ (at 7.9 m), and -1.1 to -0.8‰ (at 9 m). Although preliminary, a comparison between the oxygen isotopic values of nannofossils (<63µ size fractions) and benthos reveals the similarity in the lower part of the section and difference in the upper part of the section. Such a pattern suggests a change in the water depth.