QUATERNARY STRATIGRAPHY AND GEOCHRONOLOGY ASSOCIATED WITH THE GREEN RIVER IN THE UINTA MOUNTAINS: INCISION IN THE HEADWATERS OF THE COLORADO RIVER

The late Cenozoic drainage and erosion history associated with the Green River in the Uinta Mountains of Utah and Colorado has puzzled geologists for decades, yet is still largely unknown. Previous mapping in the area has focused mainly on bedrock and Tertiary rocks, with little detail given to surficial deposits. Numerous, well-preserved, strath and fill terraces provide excellent opportunity to study the history of the Green River in the Browns Park area of the northeastern Uintas. This is an important area for studying the development of the Green River across the Uinta Mountains, a key event in the integration of the greater Colorado River drainage. Research has produced a detailed, surficial map of western Browns Park, and current efforts are dating the Quaternary stratigraphy of mainstem and tributary terraces through luminescence, cosmogenic-exposure, and U-series dating of soil carbonate.

Initial mapping of the area has identified at least eight mainstem gravel levels, the highest being a wind gap ~250 m above modern river level. Polished and striated bedrock associated with a series of windgaps, and an abandoned channel, indicate the river has changed course many times during its history. The most extensive mainstem gravel unit in this area correlates with a previously recognized paleoflood deposit to the west. Seven tributary terraces converge upstream into three main levels near the basin edge on the north side of Browns Park, and incision has been much less in the tributary headwaters than in the mainstem Green River over the same time interval. Upstream convergence of tributary terraces near the basin edge indicates local incision driven by baselevel along the Green River rather than climate or tectonics along the basin-edge faults. Reduced incision between the tributary mouths and the basin edge also indicates tributaries are somewhat buffered from the overall baselevel signal of the Green River.