UPPER TRIASSIC STRATA OF THE SOUTHERN UINTA MOUNTAINS, NORTHEASTERN UTAH: A LINK BETWEEN SEDIMENTARY RECORDS OF THE COLORADO PLATEAU AND WYOMING

The western United States preserve one of the most complete non-marine early Mesozoic sedimentary records in North America, yet despite over 100 years of geologic study, significant questions remain as to how individual units correlate across the region. One outstanding puzzle is the stratigraphic relationships between the Popo Agie and Nugget Sandstone formations in Wyoming, and the Chinle Formation and Glen Canyon Group further south on the Colorado Plateau. Many lithostratigraphic and biostratigraphic correlations have been proposed without any clear consensus. Critical to these discussions are Upper Triassic-Lower Jurassic outcrops in the southern Uinta Mountains of northeastern Utah, in and around Dinosaur National Monument. These strata have variously been referred by different authors to both Colorado Plateau and Wyoming lithostratigraphic units, further confusing correlation across the region.

We examined in detail the stratigraphy and paleontology of Upper Triassic-Lower Jurassic strata in the southern Uinta Mountains, with a focus on outcrops within and adjacent to the western portion of Dinosaur National Monument. Our analysis indicates that the slope-forming unit above the Lower Triassic Moenkopi Formation is best assigned to the Chinle Formation, and is entirely Upper Triassic in age. Furthermore, there is no evidence for an unconformity in the middle part of this unit, which removes the main support for referral of the upper part of the formation to the Bell Springs Member of the Nugget Sandstone. It also complicates any direct correlation of these Uinta strata to either the Popo Agie Formation or Bell Springs Member of the Nugget Sandstone in Wyoming, where there is a clear unconformity between the two units. Previous authors have demonstrated that the overlying cliff-forming unit in the Uinta Mountains is equivalent to the entire Glen Canyon Group further south. This unit is best called the Nugget Sandstone for nomenclatural stability, but it is still unclear how these strata relate to those of the same name in Wyoming. Palynomorph and vertebrate paleontological evidence indicate that the lower part of the Nugget Sandstone in the Uinta Mountains is latest Triassic in age, further weakening support for a regional J-0 unconformity.