A MID-NEOPROTEROZOIC WESTERN INTERIOR SEAWAY: A 766 MA TRANSGRESSION RECORDED AT THE UTAH-COLORADO BORDER, U.S.A

The formation of Outlaw Trail (fOT), Uinta Mountain Group (UMG), eastern Uinta Mountains represents marine deposition, yields a <766 Ma age, and shows stratigraphic evidence for synextensional deposition. These data suggest an intracratonic seaway transgressed eastward to beyond the Utah-Colorado border during the evolution of a nonmagmatic rift basin just prior to the Sturtian.

The fOT is 30-100+ m thick and is one of few shale-dominated intervals in the ~7km-thick sandstone-dominated eastern UMG. This unit contains variegated shale intercalated with thin to thick-bedded arkose and quartz arenite. Sedimentary structures include crossbedding, plane beds, symmetric and asymmetric ripplemarks, mudcracks, gypsum casts, and soft-sediment deformation. Paleoflow directions (n=130) are radially distributed with dominant westerly and northerly flow directions. The shale contains simple acritarchs and carbonaceous filaments (Nagy and Porter, 2005). The lateral extent of this unit, in combination with fossils, sedimentary structures, and regional correlations with the western UMG and the Big Cottonwood Formation, suggests marine transgression across an extensive braidplain. Paleoflow directions suggest west- and north-directed longshore currents; northerly flow could also reflect hanging-wall tilt.

Geologic mapping shows that this unit can be traced for 10's of km eastward into Colorado, along the southwestern margin of Browns Park and is <2 km from the base of the UMG. The unit thickens northward from <30 m to greater than 100 m across a northwest-southeast-trending Neogene fault system, suggesting this fault system was active in mid-Neoproterozoic time. This northward thickening could be controlled, in part, by mid-Neoproterozoic movement ancestral to the Laramide-age Uinta-Sparks reverse-fault system to the north.

SHRIMP U-Pb analysis of detrital zircons from this unit indicate Archean, Paleoproterozoic, Mesoproterozoic, and Neoproterozoic grains; the youngest population of 766.4 ± 4.8 Ma indicates the maximum depositional age (Fanning and Dehler, 2005). Nearly identical detrital zircon populations are found in the Jesse Ewing Canyon Fm ~400 m above the base of the UMG, suggesting similar source areas and possibly a <766 Ma age for the basal UMG.