LATE CRETACEOUS CALCAREOUS NANNOFOSSIL ASSEMBLAGES OF THE TROPIC SHALE FORMATION DURING OCEANIC ANOXIA EVENT 2 (~93.9 MA) IN THE U.S. WESTERN INTERIOR SEAWAY

Oceanic Anoxic Event 2 (OAE2) occurred at the Cenomanian/Turonian Boundary (CTB; 93.9 MA). This event coincided with global deposition of organic rich sediments that led to a distinctive positive shift in carbon isotope values and significant species turnover, including changes in calcareous nannofossil assemblages. Organic rich sediment deposition is thought to have been triggered by volcanism that led to increased productivity and/or enhanced organic matter preservation. The temporal succession of volcanism, organic matter deposition, and changes in biota such as nannofossils, is an important goal of OAE research because it helps to clarify the dynamics of these major environmental perturbations.

Here we study nannoplankton in an expanded section of the Tropic Shale near the western margin of the Western Interior Seaway to assess the relationships between anoxia, organic matter deposition, and planktic biotas during OAE2. Samples were collected from a 100-m core from southwestern Utah that contains well preserved nannoplankton. Relative abundance data are complemented with Total Organic Carbon values to determine whether organic rich sediments were a response to high surface ocean fertility or water column stratification.

Certain nannofossil species are indicative of high surface ocean fertility. We use statistical techniques to investigate the relationship of these species with organic matter deposition. Relative abundances of Watznaueria barnesae, a dissolution resistant species, indicate well-preserved nannofossils (avg= 10%, min= 3%, max= 30%). Therefore, paleoecological interpretations can faithfully be made from relative abundance data. We find high surface water fertility species Biscutum constans are dominant at this site (avg=20%, min= 5%, max= 43%) indicating high productivity in the surface ocean. We explore the potential sources of high productivity including increased run-off from the Sevier and/or Mogollon Highlands or increased trace metals from volcanism.