Abstract

Surficial deposits below the glacial termini of the Henrys Fork drainage have been mapped at 1:24,000 scale to develop a nonglacial Quaternary stratigraphic framework for the northeastern Uinta Mountains. This study area spans from Pleistocene glacial moraines, approximately 6 km south of the Utah-Wyoming border, to the termination of Henrys Fork at Flaming Gorge Reservoir near Manila, Utah. The Henrys Fork nonglacial stratigraphy contains nine distinct mainstem gravels, six piedmont gravels, and landslide deposits. Gravels on the Henrys Fork are grouped as older, high remnant gravels that cannot be directly linked to glacial units and younger gravels that can be traced from glacial till, through outwash plains, to stream-valley gravels with terraces formed upon them. Henrys Fork gravels are clast-supported, cobble gravel derived from the Uinta Mountain Group and Paleozoic limestone units. Near moraines, gravels are thicker but they quickly thin downstream and lie on planar bedrock straths, and so form strath terraces that converge downstream. No absolute age control currently exists for any of the Henrys Fork gravels or terraces.

Henrys Fork terraces Qag2 and Qag3 can be correlated to relatively well-dated Wind River terraces and tentative incision rates for the Henrys Fork are estimated at 80-110 m/my over the late Pleistocene. These rates are similar to rates estimated for the Green River on the north slope of the Uintas in western Browns Park, but are significantly less than reported rates in other central Rocky Mountain ranges and are two to three times lower than incision rates, estimated without direct age control, for the south flank of the Uinta Mountains. Extrapolating a linear incision-rate suggests that the oldest gravels on the Henrys Fork were deposited in the early Pleistocene.