EVIDENCE FOR A LATE MIOCENE COLORADO-GUNNISON RIVER SYSTEM IN WESTERN COLORADO

River gravels of a late Miocene Colorado-Gunnison River system are present at two important locations in western Colorado: 1) Grand Mesa and 2) the Uncompahgre Plateau. Newly recognized river gravels (elev. 2935 m) located beneath radiometrically dated basalt flows of Grand Mesa confirm the presence of the ancestral Colorado River in western Colorado by ca. 11 Ma. The river gravels are represented by 4 to 5 m of rounded pebbles and cobbles consisting of abundant quartzite, well cemented sandstone, and significant (~5%) quantities of granite. The nearest granitic outcrops at comparable elevations are located a minimum of ~150 km to the east in the Sawatch, Park, and Gore Ranges. The gravel is overlain by ~1 m of coarse sand and a 0.25 m-thick paleosol.

Ancient river gravels representing the ancestral Gunnison and Uncompahgre Rivers are present on the Uncompahgre Plateau. The river gravels are represented by rounded pebble to cobble gravel deposits scattered along the crest of the northwest-trending Uncompahgre Plateau at elevations ranging from 2500 to 3000 m. The gravels are dominated by intermediate volcanic clasts representing Tertiary rocks of the San Juan and West Elk Mountains. North of Goddard Creek, however, there are small but significant percentages of granitic clasts in the river gravels. There is no obvious source for these granitic rocks in the Uncompahgre River basin or nearby San Juan Mountains. There are, however, abundant sources of granite in the Gunnison River basin to the east. Based on these observations, it is plausible that these granite-bearing river gravels represent deposits of an ancestral Gunnison River, which flowed west from present-day Gunnison, Colorado to the Uncompahgre Plateau. The ancestral Uncompahgre River probably joined the ancestral Gunnison River near Goddard Creek, and the combined rivers then flowed northwest to Unaweep Canyon where they joined the ancestral Colorado River. Similarities in the elevations of the ancestral Colorado River gravels on Grand Mesa, the ancestral Gunnison-Uncompahgre River gravels on the Uncompahgre Plateau, and the bedrock rim of Unaweep Canyon suggest that these ancient river systems collectively flowed west out onto the Colorado Plateau ca. 11 Ma.