AMMONITE BIOSTRATIGRAPHY AND PALEOCEANOGRAPHY OF THE MANCOS SHALE (UPPER CRETACEOUS) 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 bio- and lithostratigraphic changes seen within the basal 40 m of the Blue Gate Shale Member (BGSM) of the Mancos Shale in the San Rafael Swell have not been previously documented for the area around Grassy in east-central Utah. While these rocks were being deposited (Late 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 understand the effects of ocean anoxic events by linking geochemical changes with fossil observations, specifically with regard to the ammonites. Based on our 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 teil 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 siltstone back to shale seen repeatedly through the analyzed 40 m. In addition, the section here also contains two possible bentonite beds which lie at ~27 m and ~31 m. Preliminary isotopic analyses of δ¹³C and δ¹⁸O indicate our fossil material, all of which was collected at the surface, has been diagenetically altered. We are currently pursuing further geochemical techniques in an effort to tie the biostratigraphy with environmental changes and possible regional marine anoxic events.