STRATIGRAPHY, PALYNOLOGY, AND MATURITY OF THE PROTEROZOIC UINTA MOUNTAIN GROUP, EASTERN UINTA MOUNTAINS, UTAH—IMPLICATIONS FOR UNIT THICKNESS

Recent geologic mapping in the eastern Uinta Mountains, as part of STATEMAP, provided an opportunity to look at the regional stratigraphy of eastern exposures of the Uinta Mountain Group (UMG). The mapping resulted in the division of the UMG into three formations: the basal Jesse Ewing Canyon Formation, a middle unnamed formation, and the overlying Red Pine Shale. In addition, outcrop samples and cuttings from two water wells yielded palynomorphs from all three formations. A sample from basal shale beds of the Jesse Ewing Canyon Formation, as currently defined, contains abundant, well-preserved carbonaceous filaments (algal?) and sphaeromorphs (Leiosphaeridia spp.). Samples from the middle unnamed formation contain these same filaments and sphaeromorphs, and some additional, less abundant taxa identified as Eosaccharomyces sp., Trachysphaeridium laminaritum, and T. laufeldi. Samples from the Red Pine Shale carry a uniquely different assemblage consisting mostly of colonial organisms that have been assigned to Myxococcoides and Eohyella. These fossil assemblages suggest that the age of the eastern UMG is Mesoproterozoic to Middle Neoproterozoic.

The organic material in all samples is well preserved and not significantly altered, with a thermal alteration index (TAI) between 2.8 in the Jesse Ewing Canyon sample and 2.5 in the Red Pine samples. These TAI values equate to a vitrinite reflectance between 0.5% and 1.2%. This raises an interesting question: How can a Proterozoic unit, estimated to be greater than 7,000 m thick, with an additional estimated 3,000 m of overlying Phanerozoic strata, not be more thermally altered? A preliminary burial history curve of the UMG using published heat-flow values for the region resulted in a modeled vitrinite reflectance of 2.4% at the base and 0.6% at the top of the group. To closely match the modeled vitrinite reflectance with the observed data, the eastern UMG could be no more than about 3,500 m thick. These modeled values suggest that either the eastern UMG is thinner than previously estimated or that all samples were collected within the top 3,500 m the group, suggesting that the palynomorph-bearing shale beds of the Jesse Ewing Canyon Formation are part of the overlying unnamed formation in a down-faulted block.