EVALUATION OF CARBON ISOTOPIC CHEMOSTRATIGRAPHY OF THE CEDAR MOUNTAIN FORMATION OF UTAH

During the Early Cretaceous, the spread of crown-group angiosperms within North America, the split of marsupial and eutherian mammals, the immigration of Eurasian and Gondwanan dinosaur taxa into North America, and the preservation of a large percentage of Utah’s Early Cretaceous dinosaur taxa occurred during the deposition of the Cedar Mountain Formation (CMF) in Utah. Thus, the temporal and paleoclimatic context of these events is dependent on the age of the CMF members. Ongoing studies suggest the timing of deposition of the members has been estimated to range from the Late Jurassic to early Late Cretaceous.

To add stratigraphic context to the CMF, 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. Within these outcrops, major positive C-isotope excursions (CIEs) can be observed: a magnitude of +4‰ CIE in both YCM sections, a magnitude +4‰ CIE in the RRM and a magnitude of +3‰ CIE in the MM.

The CIEs in the YCM imply that the YCM records a terrestrial response to the CIE associated with the Late Valanginian ocean anoxic event (135 to 129 Ma). The overlying RRM records a broad positive CIE interpreted to be the C10 positive CIE that occurs in the late Aptian and includes a number of short negative spikes associated with the OAE 1b set (Killian, Paquier, and Leedhardt). Dates reported for this CIE vary in the literature but range from ~115 Ma near the beginning rising limb of the CIE to ~112 Ma and also spans the Aptian-Albian boundary (113.2 Ma). The MM includes the CIE associated with the positive CIE described as the Mid-Cenomanian Event (96.4 to 95.8 Ma).

The depositional period of the CMF then spans from as old as the Berriasian to the Cenomanian, based on a variety of biostratigraphic, detrital minerals, and now revised, chemostratigraphic evidence. The > 20 million years of deposition represented within the CMF suggests it preserves some of the oldest Cretaceous fauna in North America, likely documents the emergence of angiosperms on the North American continent, as well as the early evolution of eutherian mammals.