TEMPORAL CONSTRAINTS ON THE EXTENSIONAL COLLAPSE OF THE EASTERN UINTA MOUNTAINS AND DRAINAGE CAPTURE OF THE GREEN RIVER BY THE COLORADO RIVER, WYOMING, UTAH, AND COLORADO

The Eocene growth of the Uinta Mountains created a major topographic barrier that separated the greater Green River Basin (north) and Colorado River drainage network (south). Throughout the Oligocene, the Green River flowed southward from the Wind River Mountains to Green River, Wyoming, and then eastward through the Green River Basin. The Yampa River likely flowed west near its present course south of the Uintas, and may have been part of the headwaters for the Colorado/White River. Extensional collapse of the eastern Uintas created a half graben (Browns Park) that was subsequently filled with Browns Park Formation by the newly captured and rerouted Green River from the north.

Four samples of the Browns Park Formation from northeastern Utah and northwestern Colorado in Browns Park syncline as well as one from south-central Wyoming were collected for U-Pb detrital zircon age dating by LA-ICPMS. The radiometric ages, and their diversity, provide insight into the timing of diversion of the Green River drainage to the Browns Park depression. The youngest detrital zircon ages from near the base of the Browns Park Formation date the maximum age of the Browns Park Formation at 9.8 ± 2.8 Ma. A sample from near the top of the Browns Park Formation (northeastern Colorado) contains a zircon of 8.4 + 2 Ma. The detrital age populations also include Jurassic and Cretaceous ages found in the strata of the greater Green River Basin. The ages suggest that the Browns Park syncline, and collapse of the eastern Uintas, formed ~10 Ma.

Integration of the Green and Colorado River systems occurred at the intersection of Lodore Canyon with Browns Park, possibly coincident with the end of Browns Park Formation deposition. Hypotheses proposed by previous workers suggest river integration by spillover of the Green River from Browns Park or by headward erosion by a tributary of the Yampa River.

The eastern Uintas is one of several ranges, including the Granite, southern Wind River, and Owl Creek mountains, where WNW-striking contractional basement structures faltered in SSW regional tension at ~6–10 Ma. SSW-trending regional tension is coeval with basaltic magmatism in the Colorado Plateau attributed to continental mantle lithospheric delamination, suggesting a genetic linkage between “passive” lithospheric instabilities and landscape evolution.