Evaluation of Carbon Isotopic Chemostratigraphy of the Cedar Mountain Formation of Utah

The Lower Cretaceous rocks of Utah preserve the origin of a multitude of dinosaur taxa, spread of angiosperms on the North American continent, and evolution of marsupial and eutherian mammals. However, the timing of deposition of these rocks is not well understood and must be determined to understand climatic controls on these biological events. Estimates of the age of the Cedar Mountain Formation and its constituent members range from Late Jurassic to early Late Cretaceous. Understanding the timing of deposition of the Cedar Mountain Formation is critical to dinosaurian and associated taxa studies as well as paleoclimate reconstructions from within the Cedar Mountain Formation. It is also critical to understanding the present climate as the earliest Cretaceous is an example of a “coolhouse” world transitioning to a “warmhouse” world similar to our current world transition and future climate. Stable carbon isotope chemostratigraphy of two outcrops of the Yellow Cat Member and an exposure of the Mussentuchit Member at Moore Cutoff Road and east of Green River, Utah are reported on here. This study identifies two major positive carbon isotope excursions with one occurring in the Yellow Cat Member with a magnitude of ~+4‰ and one in the Mussentuchit Member with a magnitude of ~+3‰. We interpret that the Yellow Cat Member records a terrestrial response to the oceanic anoxic event known as the Weissert Event, based on comparison with similar C-isotope chemostratigraphic curves from other oceanic and terrestrial records. This oceanic C-isotope record is associated with the coeval Paraná-Etendeka large igneous province emplacement. The Mussentuchit Member, like the Yellow Cat Member, includes a terrestrial response to an oceanic anoxic event, in this case, the Mid-Cenomanian Event. These results imply the potential existence of terrestrial evidence of other anoxic events OAE 1a, OAE 1b (i.e., “Leenhardt” and “Paquier” events), Jassines (OAE 1c), and OAE 1d within the Ruby Ranch Member, the age of the Yellow Cat Member spans from the Berriasian to Aptian, and the Mussentuchit Member chemostratigraphy aligns with ages from detrital zircon analyses pointing to deposition occurring within the Cenomanian