INCISION HISTORY OF THE COLORADO RIVER IN WESTERN COLORADO AND ITS IMPLICATIONS FOR CLIMATE VS. TECTONIC DRIVEN INCISION

River terraces, debris fans, and basalt flows between Rifle and Dotsero, CO were examined to determine the incision history of the Colorado River. Using geological maps, aerial photos, pebble counts and GPS mapping, Colorado River incision history scenarios were created. To determine the recent incision history of the Colorado River, low terraces and corresponding debris fans in the Rifle area were also studied.

Debris fans near Rifle are sourced from the northern flank of Battlement Mesa and flowed into the Colorado River. Based on elevations of the fans and terraces, 6 separate ages of debris fans, and 5 separate ages of Colorado River terraces have been differentiated. The oldest Colorado River terrace is 577 m above the Colorado River on Flatiron Mesa.

Debris fans, ranging in thickness from 5 to 86 m, consist chiefly of basalt cobbles and boulders in a sandy matrix and are typically capped by a layer of loess. Colorado River gravels commonly underlie debris flow deposits, and have an average thickness of 9 to 11 m.

Ancient river gravels near Carbondale contain ~70% quartzite clasts, but there are also significant percentages of Precambrian granite, diorite and intermediate volcanic clasts, averaging ~15% of the clasts at each location. These clasts, found 837 m above the modern Colorado River at Spruce Ridge and Lookout Mt., where the gravel is associated with a 10 Ma basalt flow, indicate that a westward flowing Colorado River existed at least 10 Mya.

Prior studies suggest the Colorado River experienced an incision rate of 16 m/Ma between 7.8 Mya and 3 Mya, followed by an increased rate of 254 m/Ma in the past 3 Ma. Reevaluation of sites such as Spruce Ridge, Gobblers Knob and Triangle Peak suggests two alternative incision scenarios. A rapid incision scenario that is consistent with prior studies proposes an incision rate of 16 m/Ma between 7.8 Mya and 3 Mya, and a rate of 287 m/Ma from 3 Mya to the present when the Lava Creek B ash at Dotsero is included. A second slower scenario proposes a constant incision rate of 110 m/Ma for the last 7.8 Ma, with an increase to 132 m/Ma in the last 640 ka. This slow alternative scenario brings into question post-3 Ma rapid incision and argues against climate change as an incision catalyst. The slower scenario argues instead for long-term tectonic and/or isostatic rock uplift.