LATE CENOZOIC EROSIONAL HISTORY AND MAJOR DRAINAGE CHANGES OF THE COLORADO-GUNNISON RIVER SYSTEMS, WESTERN COLORADO

Miocene to Pleistocene river gravels record the evolution of the Colorado-Gunnison River system in western Colorado. Miocene basalt flows of Grand Mesa provide a starting point for the late Cenozoic history of the region. Basalt flows at the east end of Grand Mesa (>3350 m) overlie river gravels composed of volcanic clasts derived from the West Elk Mountains. The gravels and basalt flows fill paleochannels that slope W-NW towards the modern Colorado River (CR), which suggests that they represent tributaries of the ancestral CR. We speculate that the CR was flowing W-SW through Unaweep Canyon at the time of Grand Mesa basalt extrusion. The long-term incision rate based on the 9.7 Ma basalt present at the west end of Grand Mesa (3000 m) is ~ 16.2 cm/ky.

Ancient river gravels on the Uncompahgre Plateau are concentrated along NW-trending “belts” that parallel the axis of the uplift. The gravels are present at elevations ranging from 3000 m to 1600 m, and document NE migration of the ancient Uncompahgre and Gunnison Rivers down the dipslope of the Uncompahgre Plateau. Gravels comprised chiefly of volcanic clasts are interpreted as ancient Uncompahgre River deposits whereas gravels with significant quantities of granitic clasts are interpreted as sediments of the ancient Gunnison River (GR). Based on the long-term incision rate of Grand Mesa, these gravels range in age from ~ 1.3 to 7.5 Ma, which suggests that these gravels are much older than previously recognized.

The oldest (~ 7.5 Ma) Gunnison River gravels related to Unaweep Canyon and a probable south-flowing CR occur at 2575 m near Keith Creek. Gravels in Cactus Park, a Gunnison paleovalley, represent a more recent (~ 3.8-3 Ma) episode of GR activity through Unaweep Canyon. These gravels are overlain by lake beds that mark the abandonment of Cactus Park and Unaweep Canyon by the GR ~ 3 Ma. We suggest that stream piracy, without tectonic influence, was responsible for abandonment of this GR course.

Rapid incision rates (~ 62.2 cm/ky) for the Black Canyon of the Gunnison exceed regional incision rates (~ 17.1 cm/ky) over the past 640 ky. This difference probably reflects headward migration of a GR knickpoint triggered by abandonment of Unaweep Canyon. This suggestion highlights stream piracy as a potential driving mechanism for river incision in western Colorado.