EVALUATION OF THE AGE OF DEPOSITION OF THE CEDAR MOUNTAIN FORMATION OF UTAH THROUGH CARBON ISOTOPIC CHEMOSTRATIGRAPHY

The Cedar Mountain Formation (CMF) of Eastern Utah was deposited during the Early Cretaceous. This time was when crown-group angiosperms were radiating within North America and a major influx of Eurasian and Gondwanan dinosaur taxa had occurred. Preserved in the up to 5 different members of CMF are these events and a large portion of the dinosaur taxa representing the Early Cretaceous of Utah. As such, the temporal and paleoclimatic context of these events, and the abiotic/biotic reactions to them in Utah, is dependent on the age of the individual members. Ongoing studies suggest the timing of deposition of the CMF members are within the range of the Late Jurassic to early Late Cretaceous.

Stable carbon isotope chemostratigraphy of organic carbon for each member of the CMF was constructed from outcrops of the Yellow Cat Member (YCM), Ruby Ranch Member (RRM), and Mussentuchit Member (MM) at Moore Cutoff Road, UT and an outcrop of the YCM from east of Green River, Utah. A compendium of the various ages and modes of dating from past literature was also constructed. This carbon chemostratigraphic framework adds stratigraphic and inferred temporal context to the CMF. For this study, within these stratigraphic sections, major positive C-isotope excursions (CIEs) can be observed. In the YCM, a magnitude of +4‰ CIE in both YCM sections is observed. Within the RRM section at Moore Cutoff Road, a magnitude +4‰ CIE is observed. Finally, within the MM at Moore Cutoff Road a magnitude of +3‰ CIE is observed.

Within the YCM sections, some CIEs imply that the YCM records a terrestrial response to the CIE associated with the Late Valanginian ocean anoxic event (135 to 129 Ma) but other sections show varying ages ranging from Berriasian to Aptian. The overlying RRM records a broad positive CIE interpreted to be the C10 positive CIE from Bralower et al. (1999) that occurs in the late Aptian. This infers that a number of short negative CIE spikes are associated with the OAE 1b set (Killian, Paquier, and Leedhardt). The dates for this broad CIE in the literature vary from ~115 Ma near the rising limb of the CIE to ~112 Ma and implies it spans the Aptian-Albian boundary (113.2 Ma). At the top of the CMF, the MM contains a positive CIE defined as the Mid-Cenomanian Event (96.4 to 95.8 Ma).