EVIDENCE OF A MIOCENE ANCESTRAL COLORADO RIVER, GRAND MESA, WESTERN COLORADO

Grand Mesa, a flat-top mountain capped by 9.2 to 10.8 Ma basaltic lava flows, is one of the most prominent landforms in western Colorado. Previous studies have suggested that Grand Mesa represents a former location of the ancestral Colorado River, but until recently, no definitive evidence for this claim has been provided. Newly discovered river gravels, present in a landslide scar beneath the lava flows on the northwestern flank of Grand Mesa, however, strongly suggest the ancestral Colorado River occupied this area ca. 10.8 Ma.

The river gravels lie at an elevation of 2813 m and mantle an eroding hillslope. They are not in situ, but can be visually traced more than 100 m up the precipitous hillslope to a stratigraphic unit that underlies ~60 m of the basalt flows. There is evidence of ground water seepage at the base of the basalt, which might suggest that the river gravels directly underlie the basalt. Examination of basalt outcrops elsewhere and Bureau of Reclamation cores show that comparable gravels are absent from sedimentary units interbedded with the basalt flows.

The gravels are rounded and consist of pebbles to large cobbles. A pebble count shows that the gravels consist of 38% quartzite, 33% sandstone, 13% granite, 7% vein quartz, 5% basalt, 3% diorite, and 1% chert clasts. The presence of significant quantities of granite is particularly interesting because the nearest granite outcrops at comparable or higher elevations are 130 km to 300 km away in the Sawatch, Gore or Park Ranges. If the granite is from the Gore or Park Ranges, then the river gravels were probably deposited by the ancestral Colorado River. If the granite is from the Sawatch Range, then the ancestral Roaring Fork River could have delivered the clasts to an ancestral Colorado River as it flowed west. Chemical analyses of the basalt clasts will be used to help determine potential source areas for this rock type.

The significance of the river gravels is two fold. First, they provide information on the Miocene paleogeography of the region, and indicate that there was a major west-flowing river in the Rocky Mountain region by 10.8 Ma. Second, the gravels and ages of the overlying basalt flows permit calculation of a long-term Colorado River incision rate of ~140 m/My, which agrees well with previous estimates for western Colorado.