DINOFLAGELLATES OF THE K/PG TRANSITION SEQUENCE IN SOUTHEASTERN, MISSOURI

Sedimentary deposits in Stoddard County, southeastern Missouri, near the town of Bloomfield reveal a biostratigraphic record across the K/Pg boundary. The K/Pg transition sequence is represented by the uppermost Maastrichtian Owl Creek Formation and Paleocene Clayton and Porters Creek Formations. The Clayton Formation is characterized by a basal fossiliferous coquinite that contains the late Maastrichtian index ammonites Discoscaphites iris and Eubaculites carinatus. We use dinoflagellate occurrences recovered from these units to determine the timing of the coquinite layer and specifically whether or not it is an end-K tsunamite deposit related to the Chicxulub impact event. Palynological results indicate a mixed assemblage of late Cretaceous and early Danian dinocysts within the basal coquinite of the Clayton Formation. Dinoflagellate taxa restricted to the Maastrichtian include Riculacysta amplexa, Pierceites pentagonum, Phelodinium tricuspe and Dinogymnium sp. Dinoflagellates considered to be global indicators of the basal Danian include: Senoniasphaera inornata, Carpatella cornuta, Damassadinium californicum, and Lanternosphaeridium reinhardtii. According to Williams et al., (2004), C. cornuta and D. californicum appear approximately 200,000 years after S. inornata, which first appears at the base of the Danian, and thus after the K/Pg boundary, and yet all three species are found together in the basal coquinite. We would expect only Maastrichtian macrofossils and microfossils to be preserved in an end-K event tsunami-induced deposit. A gray mud occurring above the coquinite in the middle of the Clayton Formation contains the mid-Danian dinoflagellate Senegalinium interlaanse. Taken together, these data suggest that the coquinite is significantly younger than the K/Pg impact event, but older than middle Danian. The dinoflagellate data supports a Danian age for the coquinite, which also preserves reworked macrofossils of the underlying late Maastrichtian Owl Creek formation. The mixed assemblage of late Cretaceous and early Paleocene dinocysts preserved in the coquinite weakens the argument for an end-K related tsunamite deposit. The coquinite may instead result from a long-term transgressive lag deposit that occurred well after the K/Pg event but before the middle Danian.