COMPARING FAUNAS AND FLORAS FROM THE LATEST CRETACEOUS AND EARLY PALEOCENE IN SOUTHWESTERN NORTH DAKOTA: A MICROSCOPIC ANALYSIS OF ENVIRONMENTAL AND TAXONOMIC COMPOSITION

Surface collection of microsites provides a basis for determining the presence or absence of taxa near the upper limit of the Cretaceous and the basal Paleocene. Screen washing sites previously sampled only by surface collecting allowed us to recover new taxa. Together with sedimentological observations, these new occurrence data contribute to a more comprehensive, environmentally controlled view of changes in taxonomic composition across the Cretaceous-Tertiary (K/T) boundary. A 100 kg control sample of sediments from sites stratigraphically placed at 8.85, 8.40, 3.40, 3.17 and 2.37 meters below the Hell Creek/Fort Union formational contact and at 0-4.00 and 16.00 meters above this contact, was washed through a 380 micron screen, sorted under microscopes in the lab and compared. The total specimen collection has indicated the presence of 54 taxa within the uppermost 9 meters of Hell Creek, (36 surface and 33 screen wash taxa). There are 25 taxa present in the basal Fort Union (19 taxa at the base and 15 taxa 16 meters up section). The change from 54 taxa in the Hell Creek to 25 taxa in the Fort Union is a result of the conspicuous absence of sharks and rays, non-avian dinosaurs and Lancian mammals and the addition of small aquatic taxa and Puercan mammals. At site V02017, located 0-4.00 meters above the formational contact, the entire 4 meter section representing in excess of 6500 kg was screen washed. The data from this site was used to indicate the presence of faunal and floral taxa relative to each other and the formational contact. Stratigraphic positions and depositional environments were identified during the matrix collection, allowing comparisons to be made between the specimens and the facies from which they were found. Sediments analyzed at site V02017 indicate no significant change in environments crossing the formational contact. The sequence of floodplains overlain by small-scale channels and over-bank deposits occur below, and up to, 2.45 meters above the formational contact. Plants and mammals characteristic of the Paleocene are also present within this interval. Sediments indicative of ponded water begin at 2.45 meters, which is above the first occurrence of Paleocene-aspect faunas and floras, suggesting this environmental change did not promote the presence or absence of particular taxa.