New Insights into the Timing of Exhumation of the Uinta Basin and Mountain-front Retreat of the Uinta Mountains, Utah

Douglas A. Sprinkel, Warren Sharp, and Steven Schamel
Utah Geological Survey

Timing of Laramide uplift of the Uinta Mountains and coeval deposition of the Uinta Basin is fairly well known. However, new geologic mapping and isotopic dating suggest that the timing of the post-Laramide erosion that profoundly altered the landscape within the Uinta Basin is younger than previously thought. The Uinta Basin is a strongly asymmetric intracratonic depression formed during Late Cretaceous to Paleogene time by crustal loading beneath the rising Uinta Mountains anticlinorium. Pre- and synorogenic strata within the basin dip uniformly northward towards the Uinta Basin boundary fault, which borders the deepest part of the basin and cuts rocks as young as the late Eocene Duchesne River Formation. Crustal stability replaced basin subsidence and Uinta Mountains uplift about 30 million years ago (Oligocene). Consequently, an extensive, gently sloping surface (pediment) called the Gilbert Peak erosion surface developed around the flanks of the highland and extended into at least the northern part of the Uinta Basin, leaving only the higher parts of the range above the surface. Streams flowed away from the highlands in a radial pattern and across the surrounding broad plain. The Oligocene Bishop Conglomerate was deposited on the Gilbert Peak erosion surface and the south-dipping Duchesne River and older formations. Tectonic activity resumed about 15 to 10 million years ago as extension lowered the eastern Uinta Mountains, faulted the Gilbert Peak erosion surface, and altered the drainage pattern. A poorly to unconsolidated gravel deposit unconformably overlies the Bishop Conglomerate. The gravel deposit cuts into the underlying Bishop and locally removes it so that the gravel rests directly on the Gilbert Peak erosion surface and underlying pre-Bishop formations. Present-day outliers of this gravel deposit suggest that much of Ashley Valley was covered by the Bishop Conglomerate and this gravel deposit. A uranium-series age obtained from the innermost, densely laminated calcium-carbonate coatings on cobble-sized clasts collected from the gravel deposits indicates the minimum age of the deposit is about 187 ± 11 ka. This suggests that much of the erosion of sedimentary strata and mountain-front retreat along the south flank of the Uinta Mountains occurred in less than 500,000 years. The rapid erosion and downcutting in the Uinta Basin may be linked to the timing of capture of the Green River near or at Lodore Canyon, Colorado.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013