THE NEOPROTEROZOIC UINTA MOUNTAIN GROUP REVISITED: A SYNTHESIS OF RECENT WORK ON THE RED PINE SHALE AND RELATED UNDIVIDED CLASTIC STRATA, NORTHEASTERN UTAH

Current studies of the Neoproterozoic Uinta Mountain Group (UMG) focus on the understudied upper formation in the western Uinta Mountains (Red Pine Shale) and the undivided clastic strata in the eastern Uinta Mountains. Results thus far indicate that: the Red Pine Shale records marine deposition; the eastern clastic strata record fluvial deposition; these units are genetically related; and the exposed Red Pine Shale post-dates the undivided clastic strata.

Measured sections and stratigraphic mapping of the Red Pine Shale show that it is ~550-1200 m thick in the western part of the range, thins to <300 m in the central-eastern range, and is missing in the eastern range. The western sections comprise organic-rich shales interbedded with medium- to coarse-grained sandstones. Sedimentary structures include graded beds, hummocky-cross stratification, parallel to ripple laminations, tabular crossbeds, and slump folds. Fossils include vase-shaped microfossils and acritarchs. Eastward, the strata comprise variegated laminated mudrock with colonial bacteria and interbedded fine-grained sandstone beds with symmetric ripplemarks. d¹³C_org analyses of organic-rich shales reveal 9‰ (PDB) variability (values range from -21.7 to –30.7‰ PDB) and TOC values range from 0.32 to 5.9 %. Combined data suggest deposition below and near fairweather wave base in a marine deltaic system, and correlation with the >742 Ma Chuar Group, Grand Canyon.

The underlying, and in part correlative (?), undivided strata of the UMG, eastern Uinta Mountains, are dominated by trough- and tabular-crossbedded sandstones showing south-southwesterly paleocurrent flow. At least four, laterally continuous (km-scale) ~50 m-thick intervals of gray-green organic-bearing shale beds have been mapped amongst these sandstones and contain symmetric ripplemarks, mudcrack casts, and ripple troughs. Fossils from shale in the middle (?) of the undivided strata include simple sphaeromorphs and colonial bacteria. These strata represent a sandy braidplain with drowning events that were likely marine incursions from the west. Geologic mapping, TAI analyses, and burial-history modeling suggest the undivided eastern UMG is likely 4-5 km thick instead of >7 km thick, as previously estimated.