EVIDENCE FOR A LARGE-SCALE PALEOFLOOD IN THE NORTHEASTERN UINTA MOUNTAINS

Geologic mapping of surficial deposits along the Green River corridor in the northeastern Uinta Mountains near the Utah-Colorado border has revealed an 18 m thick deposit that is interpreted to be the result of a single, large-scale paleoflood. The base of the deposit consists of boulders of neoproterozoic Uinta Mountain Group (UMG) orthoquartzite that typically measure 50-90 cm in diameter. It is clast-supported, sub angular, and has well-rounded pebbles, cobbles, and sand within the framework openings. The largest boulder identified within the deposit measured over 3 m in diameter. The deposit fines upward into a clast-supported, well rounded gravel, and its upper surface forms an extensive terrace that dominates the surrounding landscape.

The proposed mechanism for this flood is the failure of a landslide dam that temporarily impounded the Green River. Approximately 3 km upstream from the first appearance of the flood deposit, a landslide scarp is clearly visible in the UMG, and beneath it a black shale bed outcrops along the Green River. The landslide was likely initiated by the Green River as it undercut the shale. Further evidence for an impounded Green River exists upstream in a deposit at Little Hole. This deposit was formerly mapped as the Browns Park formation (Tbp), a Tertiary basin fill, but it is outwardly different than the Tbp exposed downstream. A new interpretation for this deposit is that it is a fill that accumulated in the slackwater behind the dam. It is a 60 m thick, cross bedded to massive sand with channel forms containing clasts derived from the adjacent local catchments. Within the upper 4 m it coarsens upward to well-rounded pebble gravel. The timing of the flood is likely mid-Pleistocene, and the duration of the impoundment is unknown.