PALEOFLOODING AND INTEGRATION OF THE GREEN RIVER OVER THE UINTA MOUNTAINS—INFORMATION FROM NEW SURFICIAL MAPPING

Longstanding questions about the Cenozoic landscape history of the interior western U.S. include the timing and mechanisms of uplift, incision, drainage integration, and canyon cutting. A groundwork is being built for addressing these questions through surficial mapping of the Green River corridor in the northeastern Uinta Mountains, where we are revising and adding detail to the record explored by Wally Hansen.

Our mapping has unexpectedly highlighted two anomalous and correlative deposits that dominate the stratigraphy along the Green River corridor in Red Canyon and western Browns Park. Upstream of a narrow reach of canyon with multiple landslide scars is a 60-m thick, coarsening-upward, lenticular-bedded, sandy deposit with a combined Green River and local provenance. Downstream, an expansive, fan-shaped deposit covers much of western Browns Park. It thins downstream, has 50-300 cm, subangular boulders of local bedrock at its base, and grades upward to well rounded, pebbly gravel of diverse Green River provenance. These two deposits are hypothesized to be the result of middle-Pleistocene landslide impoundment of the river and a resultant outburst-flood, respectively.

Relating to drainage integration, J.W. Powell formally introduced the term “antecedent” in an attempt to explain why the Green River turns abruptly from gentle Browns Park and cuts through the Uinta uplift in Canyon of Lodore. S.F. Emmons disagreed and argued for superposition of the river along the top of the Green River Formation. These early ideas were abandoned with further geologic investigation, and instead W.H. Bradley thought headward erosion of a stream through the Uinta uplift captured the Green River, whereas Hansen suggested superposition on the Neogene Browns Park Fm. Our mapping thus far indicates the Green River locally postdates deposition of the Browns Park Fm. Furthermore, it is unlikely that the Browns Park graben was filled with ~700 m of now-eroded sediment or that it underwent ~700 m of Plio-Pleistocene subsidence with no net deposition, one of which would be necessary to allow superposition of the river over the Uintas. A remaining possibility is that headward erosion of a Yampa River tributary captured the Green as it flowed eastward through the graben, diverting it south through Canyon of Lodore.