Abstract

Recent work through geologic mapping, measured sections, and facies analysis in the Uinta Mountain Group (UMG) allows further subdivision of the eastern UMG, and a better understanding of how these strata correlate with the western UMG and regionally with units in the Wasatch Range, Death Valley, and Grand Canyon. New environmental interpretations of the eastern strata suggest deposition in fluvial, shoreline, and offshore (marine?) environments. Recent paleoenvironmental interpretation of the upper unit in the western Uinta Mountians (Red Pine Shale), suggests marine deltaic conditions. The merging paleoenvironmental interpretations suggest nearly the entire Uinta Mountain Group represents a fluvio-delatic marine system, with rivers feeding the basin from the north and east. The new ~770 Ma date from the eastern UMG and previous correlation with the well dated Chuar Group (~770–742 Ma) indicate that the majority, if not all, of the UMG is 770–740 Ma in age. Long time correlations with undated units, Big Cottonwood Formation in the Wasatch Range and the middle Pahrump Group in Death Valley, suggest that they are also middle Neoproterozoic. All but the middle-Pahrump Group record intracratonic rifting, and the Chuar and Uinta Mountain groups record fluctuating climate regimes with no evidence of glaciation. This indicates a pre-Sturtian rifting episode, suggesting at least three phases of rifting prior to the development of the Cordilleran miogeocline, and that there was no low-latitude glaciation affecting any of these basins at ~740 Ma.