DRAINAGE INTEGRATION AS A FIRST-ORDER CONTROL ON THE EROSIONAL EXHUMATION OF THE INTERIOR WEST—THE EXAMPLE OF THE GREEN RIVER AND THE UINTA MOUNTAINS

The Interior West has undergone a notable change from relatively gentle, broad-wavelength middle Cenozoic topography to a deeply incised and rugged terrain in the late Cenozoic. Epeirogenic uplift has been the traditional explanation for what has driven this change, but climate change also has been implicated, associated with increased glaciation and periglaciation as well as potentially more erosive streams. Finally, lowering of effective baselevel through delayed integration of rivers off the greater Rocky Mountain plateau is potentially a major cause for recent incision. Premier examples are the Colorado River being integrated off the southwestern edge of the Colorado Plateau, the upper and lower Rio Grande joining in northern New Mexico, and the Snake River’s integration in western Idaho. Recent work relating to the long-term history of the Green River in the eastern Uinta Mountains further supports the importance of drainage changes in the incision of the West. Ongoing mapping and geochronologic studies are revisiting the conundrum of why the Green River turns south from Browns Park at the northeast edge of the Uintas and cuts the canyons of Dinosaur National Monument. It was this place that inspired J.W. Powell to introduce the term “antecedent”, but workers later argued for superposition of the river, specifically on the Browns Park Fm. New research suggests the Green River may have first entered Browns Park before ~2 Ma, based on extrapolation using an incision rate from the local position of the Lava Creek B tephra, but the river also arrived after deposition of the Miocene(?) Browns Park Fm., rendering superposition of the river over the Uintas infeasible. The position of the bedrock structural divide, the relict middle Cenozoic drainage divide of the Uinta uplift, and then subsidence of the Browns Park graben are favorable for the capture of the upper Green by a tributary of the paleo-Yampa River around Pliocene time. This change would have driven incision upstream in areas that previously had been sheltered from baselevel fall, and the contrast between the Uinta Basin to the south and the Green River Basin to the north of the Uintas emphasizes the importance of the effective baselevel of regional rivers in landscape dissection, incision rates, and total erosional exhumation.