A NEOPROTEROZOIC (<770 MA) SHORELINE IN THE HIGH UINTAS WILDERNESS, UINTA MOUNTAIN GROUP, NORTH EASTERN UTAH: EVIDENCE FOR A PRE-STURTIAN WESTERN INTERIOR SEAWAY

New 1:24,000 scale mapping and sequence stratigraphy of the Neoproterozoic Uinta Mountain Group (UMG), in the Kings Peak quadrangle of the High Uintas Wilderness, reveals details of a marine shoreline fed by tributaries sourced from the north and east. Near Gilbert Peak, the coarse-grained formation of Red Castle, likely derived from the Wyoming craton via fluvial systems, transitions to shoreline sandstone and mudstone of the formation of Deadhorse Pass. A shoreface to fluvial-plain stratigraphic interface is suggested by alternating, laterally-continuous intervals of sandstone and shale, spectacularly displayed in cirque walls. Sequence boundaries are suggested by incised valleys, one of which cuts tens of meters of relief into a marine (?) sandstone of the formation of Deadhorse Pass and is filled with onlapping mudstone of the Gilbert Peak shale member.

SHRIMP U-Pb analyses on detrital zircons from a sandstone from the basal western UMG (formation of Moosehorn Lake; west of the map area) indicate that the UMG in the western Uinta Mountains is <770 Ma. This is consistent with a recently reported U-Pb age of 765 Ma for the lower-middle UMG in the eastern Uinta Mountains. The <770 Ma age constraint corroborates with microfossil assemblage ages from the overlying Red Pine Shale, UMG, and Chuar Group of Grand Canyon. This sample also contains Archean-, Paleoproterozoic-, Mesoproterozoic- and Grenville-age detrital zircons suggesting a mixing of terrigenous and marine sources, and shares all but the youngest (~770 Ma) grain population with a sandstone sample from the overlying and laterally extensive formation of Hades Pass. Its grain populations are also identical to grain populations from the formation of Outlaw Trail sandstone in the eastern Uinta Mountains, suggesting the same provenance, and a possible east-west correlation.

The recognition of a ~770 Ma shoreline agrees with Wallace and Crittenden's (1969) original model of axial and transverse drainages interfacing with a marine depositional system. A marine interpretation also suggests an interior seaway that exceeded the areal extent of the modern UMG outcrop.