STRATIGRAPHIC INVESTIGATIONS OF THE NEOPROTEROZOIC UINTA MOUNTAIN GROUP: COMBATING “THE CURSE OF THE PROTEROZOIC SANDSTONES”

The eastern Uinta Mountain Group (UMG) is an undivided and extremely thick succession of dominantly sandstone (estimated 7 km thick by Hansen (1965)) with the exception of the basal unit, the coarser-grained Jesse Ewing Canyon Formation (~200 m thick). To better characterize and further subdivide the eastern UMG, stratigraphic facies mapping and petrographic analyses are in progress in the Swallow Canyon Quadrangle (SCQ) and surrounding areas. The SQC, located along the UT-CO border, combined with the Hoy Mountain Quadrangle adjacent to the south, contains exposure of a near-complete and continuous section of the UMG sandstones. Four sedimentary facies are recognized, including a shale unit that may allow correlation with better-known and subdivided UMG west of Flaming Gorge. The measured thickness of the eastern UMG is ~4.6 km, considerably less than the estimated ~7 km. A generalized stratigraphic column of the eastern UMG showing facies and petrographic trends will be presented.

The undivided UMG facies include: thickly-bedded to massive sandstone (Zut); low-angle cross-bedded and trough cross-bedded sandstone (Zul); interbedded Zut and Zul facies (Zui); and fine-grained facies (Zuf). Facies analysis to the south, where the eroded top of the UMG is exposed, shows similar facies trends with predominantly Zut interbedded with intervals of Zuf. The sandstone facies (Zut, Zul, Zui) are fine to coarse-grained quartz arenites (~90-95% quartz, ~1-5% feldspar, ~0.2-5% fine grained micaceous matrix, n=8) and represent a braided river system. The fine-grained facies (Zuf) comprises fine grained subarkosic arenite (~80% quartz, ~18% feldspar, ~2% micaceous matrix, n=2) to shale and represents a lower energy deposit that may have a different sediment source, likely representing delta plain/shoreline deposition.

The lowermost Zuf unit is mapped westward for 10s of kms and allows correlation of the undivided UMG from the SCQ area to the UMG westward in the Flaming Gorge area. Further stratigraphic mapping will facilitate regional correlation of the UMG between the eastern and western Uinta Mountains, and continued characterization may also lead to linkage with Neoproterozoic strata in the Wasatch Mountains and beyond.