The Quaternary Stratigraphy of the Henrys Fork and Western Browns Park, Northeastern Uinta Mountains, Utah and Wyoming

The landscape evolution of the northeastern Uinta Mountains and the manner in which climatic and tectonic forcing have influenced it are not well constrained. Surficial deposits covering ~325 km2 below the glacial termini in the Henrys Fork and ~50 km2 along the Green River in western Browns Park were mapped at 1:24,000 scale to develop a Quaternary stratigraphic framework for the northeastern Uinta Mountains. The Henrys Fork mapping area spans from late Wisconsinan moraines to Flaming Gorge Reservoir. The Henrys Fork stratigraphy contains 10 mainstem gravels, six piedmont gravels, and landslide deposits. Terraces preserved along the Henrys Fork converge downstream and are strath terraces underlain by clast-supported, cobble gravel derived from the Uinta Mountain Group and Paleozoic limestone units. The Henrys Fork terrace stratigraphy was correlated to the Wind River terrace stratigraphy for age control, and incision rates were estimated at 80-110 m/m.y. The Browns Park mapping area includes Little Hole and continues through lower Red Canyon into westernmost Browns Park, ending at the Warren Draw-Swallow Canyon quadrangle boundary. The Browns Park stratigraphy includes eight mainstem gravels, five piedmont gravels, and various landslide, colluvial, and eolian deposits. A tuffaceous bed with Lava Creek Bash (640 ka) was identified near the top of a deposit at Little Hole that was previously mapped as Miocene basin fill. Minimum Green River incision rates were estimated between 90 and 115 m/m.y. using the Lava Creek Bash for age control. These rates are comparable to estimates for the Henrys Fork, but are about half of the rates reported for the south flank of the Uintas and other central Rocky Mountain ranges. A series of three distinct deposits in western Browns Park are interpreted as evidence for the landslide impoundment and subsequent outburst flooding of the Green River. These include slackwater deposits at Little Hole, an outburst flood deposit in western Browns Park, and a large paleolandslide deposit that lies between them. Estimates of sediment accumulation rates behind the paleolandslide dam suggest it was stable for ~605 years. Peak discharge estimates from impounded water volume estimates and paleoflow competence indicators suggest that the resulting outburst flood was ~22,000 m3/s