UPPER CRETACEOUS AMMONITE BIOSTRATIGRAPHY AND PALEOCEANOGRAPHY OF THE MANCOS SHALE IN THE SAN RAFAEL SWELL, EAST-CENTRAL UTAH

During the Late Cretaceous, most of eastern Utah was covered by the Western Interior Seaway, an epeiric sea that stretched across much of central North America, from the Arctic Ocean to the Gulf of Mexico. A multitude of ammonites, bivalves, and other creatures inhabited this seaway. The fine-resolution stratigraphy of biotic and abiotic depositional and environmental changes seen within the basal 40 m of the Blue Gate Shale Member (BGSM) of the Mancos Shale in the San Rafael Swell has not been previously documented for the area around Grassy, Utah. While these rocks were being deposited (Upper Turonian, ~90 Ma) global records indicate large fluctuations in oxygen content of seawater that may have contributed to extinctions, extirpations, and shifts in ecosystem dynamics of the region. Here, we attempt to document the effects of anoxic events by using biostratigraphic observations, specifically with regard to the ammonites. Based on previous studies, beginning ~1 to 2 m below the base of the BGSM, Prionocyclus wyomingensis is observed as the first dominant large ammonite species in the BGSM. We tentatively mark this as the first appearance datum (FAD) of this species in this section. The dominant species changes to Prionocyclus novimexicanus with a FAD at ~15 m, continuing until ~40 m. Within the Prionocyclus novimexicanus taxon range zone, a FAD for Prionocyclus quadratus is observed. Other fossils that were observed were scaphitid ammonites (in concretions at ~7 m) and multiple species of inoceramid bivalves (throughout the observed section). Local facies changes include shifts from shale to silt back to shale seen multiple times throughout the analyzed 40 m. In addition to the biostratigraphic features, the section here also contains two possible bentonite beds which lie at ~27 m and ~31 m. Isotopic analyses of δ13C and δ18O are currently being pursued in an effort to tie the documented biostratigraphy with larger environmental changes and possible regional marine anoxic events.