NEW CONSTRAINTS ON THE TIMING OF GREEN RIVER INTEGRATION, SOUTHWESTERN WYOMING AND NORTHWESTERN COLORADO
A spectacular series of down-stepping strath terraces ranging from 190 to 5 m in height above the modern Green River are present west of Green River, WY. Quartzite samples from the 120 m terrace yielded a cosmogenic burial-isochron age of 1.2 +/- 0.3 Ma, and samples were acquired from the 150 m and 190 m terraces. The 1.2 Ma cosmogenic burial age establishes the minimum age of the Green River in southwestern Wyoming. Extrapolation of incision rates (~100 m/Ma) calculated using data for the 120 m terrace suggests that the Green River is as old as 1.5-1.9 Ma in this region.
Previous investigations show that the Leucite Hills lava flows in Wyoming range in age from ~0.9-3.0 Ma. Lava flows <2 Ma are present at successively lower elevations, which we interpret to reflect progressive denudation of the region beginning ca. 2 Ma. At Zirkel Mesa (ca. 0.95 Ma), lava flows overlie stream gravel consisting of sandstone clasts derived from local Cretaceous bedrock. Zirkel Mesa suggests that Leucite Hills flows were extruded into tributaries of the ancestral Green River.
Scenarios depicting the establishment of the Green River’s course across the eastern Uinta Mountains imply the prior existence of a Yampa River. Basalt flows that overlie Browns Park sediments in the headwaters of the Yampa Valley range in age from 7-6 Ma. The youngest dated Browns Park deposits in the region are ~9 Ma. Collectively, these observations suggest that the Yampa River was established <9 Ma, and may have begun to incise as late as <6 Ma. Assuming that the Yampa River pre-dates Green River integration across the Uinta Mountains, this event must have occurred <9 Ma and may have occurred <6 Ma.
In summary, the Green River originated after 8-9 Ma, and perhaps as recently as < 6 Ma in northwestern Colorado. The minimum age for Green River integration in southwestern Wyoming is 1.2 to ~2 Ma.