QUATERNARY DRAINAGE DEVELOPMENT AND INCISION HISTORY IN THE BLACK CANYON OF THE GUNNISON AREA, COLORADO

The Black Canyon of the Gunnison River is the most dramatic canyon of the Colorado River system (only 345 m wide at 555 m depth) and has a remarkably steep gradient for a river of its discharge and basin area, suggesting that neotectonic activity is partly responsible. In seminal, comprehensive work, Hanson (1987) used the ~600 ka Lava Creek B ash to infer incision rates of 300 m/Myr in the Black Canyon and 100 m/Myr upstream at Blue Mesa Reservoir. He also inferred that Grizzly Gulch, an underfit tributary of the Black Canyon, underwent drainage reversal because of Quaternary rejuvenation and NE tilting of the Laramide Gunnison uplift. The 2001-2002 UNM field camps conducted new terrace studies and tested Hanson’s hypothesis for Quaternary tectonism.

Terraces and remnant fluvial gravels are well preserved at the confluence of the Black Canyon and North Fork of the Gunnison. Major terraces lie about 6, 24, 37, 43, 61, 70, 130, 165, 190 m above river level, with remnant gravels at ~230, 250, and 305 m. Redlands Mesa, a pediment on the south flank of Grand Mesa, contains the Lava Creek B ash within its gravel cap. Profiles projected from Redlands Mesa to Gunnison River terraces indicate that the gravels were graded to a level between 190 and 250 m above the modern river. These data are compatible with an incision rate of at least 300 m/Myr for the lower Black Canyon.

Clast size, composition, and imbrication of gravels near Grizzly Gulch indicate a southwestward-flowing paleo-drainage from the West Elk Mountains. Reconstructions of possible former stream profiles and analysis of mixed gravels at the mouth of Grizzly Gulch suggest that this paleo-drainage was active during initial incision of the Black Canyon into Proterozoic rocks roughly 2 Ma, but provide no definite evidence of NE tilting. Instead, these relationships are more simply explained by stream capture of middle Grizzly Creek by northward-flowing tributaries of the Smith Fork, which enter the Gunnison River in the lower Black Canyon. Causes for rapid incision of the Black Canyon remain uncertain and may include broad-scale epeirogenic uplift related to buoyant mantle, and/or knickpoint migration following late Pliocene(?) capture and rerouting of the Colorado River from Unaweep Canyon (70 km below the Gunnison confluence) to a course in soft Cretaceous rocks around the Uncompaghre Plateau.