NEW U-PB AGE CONTROL FOR THE LOWER CEDAR MOUNTAIN FORMATION AND AN EVALUATION OF THE MORRISON FORMATION/CEDAR MOUNTAIN FORMATION BOUNDARY, UTAH

The Lower Cretaceous Cedar Mountain Fm. (CMF) preserves several vertebrate faunas and has the potential of providing critical timing and deformational information pertaining to the shift from Nevadan to Sevier events. Historically, the contact between the CMF and Morrison Fm. (MF) and the duration of the unconformity between them have been difficult or impossible to determine because 1) the formations were deposited in similar environments, 2) the basal CMF is composed of reworked MF and 3) there are no radiometric ages for the lower CMF. A stratigraphic study through central Utah reveals a diagnostic suite of pedogenic and sedimentologic characters across the previously enigmatic boundary. These findings are supported by new U-Pb zircon ages in the lower CMF. The uppermost MF is characterized by hydroximorphic and redoximorphic paleosol features, including Fe concretions and stains, Mn-coated grains, and intense red-purple-green mottling. Upsection increases in chert-pebble lags and channelized conglomerates within the paleosol section indicate a period of reduced accommodation space in the Tithonian. The paleosol package is usually capped by a groundwater or pedogenic carbonate. This unit is consistently expressed from Green River, Utah to the Utah-Colorado border. The lower CMF above this package is a poorly sorted mixture of fine-grained material and sand-gravel sized chert grains. This material buried the MF in response to renewed tectonic activity to the SW. U-Pb geochronology of zircon crystals from an ash from the top of the MF near Moab, Utah gives a youngest age peak of 151 ± 3 Ma (1σ). This age is consistent with published ⁴⁰Ar/³⁹Ar ages for the uppermost MF throughout the Colorado Plateau. A detrital zircon sample from the lower CMF yields a maximum depositional age of 126 ± 2.5 Ma. This is the first radiometric age for the lower CMF and indicates that the duration of the unconformity is ~25 Ma. This age, along with published ages from the top of the CMF of ~98 Ma, shows that the CMF was deposited over a period of ~28 Ma.