THE NEW AND IMPROVED NEOPROTEROZOIC JESSE EWING CANYON FORMATION, BASAL UINTA MOUNTAIN GROUP, NORTHEASTERN UTAH AND NORTHWESTERN COLORADO

Detailed mapping of the basal unit of the Uinta Mountain Group, the Jesse Ewing Canyon Formation (JECF), located in northeastern Utah and northwestern Colorado, indicates that previous work (Sanderson and Wiley, 1986) is not entirely representative of the formation. The JECF is ~≤900 m thick as opposed to ≤225 m thick, and the dominant lithology is not conglomerate, but rather finer-grained facies. While only exposed in a relatively small area (56 km2), the JECF reveals multiple alluvial fan point sources feeding a shallow body of water in an active rift basin at ~770 Ma.

Stratigraphic mapping (1:12,000 scale) of the JECF has allowed the designation of two members defined by changes in lithology. The coarse-grained member of the JECF (~0-200 m thick) is defined by the abundance of conglomerate interbedded with lesser variegated shale and sandstone. Sedimentary structures include planar tabular crossbeds, weak clast imbrication, channel forms, and asymmetric ripples. The fine-grained member (~150-900 m thick) is defined by quartz and lesser arkosic sandstone interbedded with variegated shale with sedimentary structures including hummocky-cross stratification, asymmetric and symmetric ripples, mud cracks, trough and planar crossbedding, and soft-sediment deformation. The coarse-grained member represents alluvial fan deposition in agreement with Sanderson and Wiley's (1986) interpretation. The fine-grained member represents predominantly subaqueous deposition in a storm-affected shallow body of water occupying the main basin.

Distribution of the two members of the JECF suggests that sedimentation along the basin-bounding east-trending fault to the north was dominated by alluvial fans that graded laterally into finer sediments basinward. Three point sources have been identified that represent three independent transverse drainages that fed into the main basin to the south. Changes in thickness and (or) lithology across structures are attributed to syntectonic deposition related to an early phase of rifting related to Rodinia breakup. The JECF may require modification under the North American Stratigraphic Code, as the type section (Sanderson and Wiley, 1986) is not representative of the majority of the unit.