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Abstract

The Black Canyon of the Gunnison and Unaweep Canyon in western Colorado have long been viewed as classic examples of post-Laramide Plio-Pleistocene uplift, which in the case of Unaweep, is thought to have forced the Gunnison River to abandon the canyon. Ongoing field studies of the incision histories of these canyons and their surrounding regions, however, suggest that post-Laramide rock uplift has been regional, rather than local in nature. River incision rates calculated using ca. 10 Ma basaltic lava flows as a late Miocene datum suggest that long-term incision rates range from 61 to 142 m/m.y. with rates decreasing eastward towards the central Rocky Mountains. Incision rates calculated using the ca. 640 ka Lava Creek B ash range from 95 to 162 m/m.y., decrease eastward towards the mountains, and are broadly similar in magnitude to the longer-term incision rates. Locally, incision rates are as high as 500–600 m/m.y. along the lower reaches of the Black Canyon of the Gunnison, and these anomalously high values reflect transient knickpoint migration upvalley. Knickpoint migration was controlled, in part, by downvalley base-level changes related to stream piracy. For example, abandonment of Unaweep Canyon by the Gunnison River could have led to rapid incision through erodible Mancos Shale as the Gunnison River joined the Colorado River on its course around the northern end of the Uncompahgre Plateau.

Geophysical data show that abandonment of Unaweep Canyon was not caused by differential uplift of the crest of Unaweep Canyon relative to the surrounding basins. Instead, the ancestral (Plio-Pleistocene?) Gunnison River flowed through Cactus Park, a major paleovalley that feeds into Unaweep Canyon, and continued downvalley to its juncture with the Dolores River near present-day Gateway, Colorado. The average gradient of the ancestral Gunnison River through the canyon prior to abandonment was ~7.5–7.6 m/km. Lithological and mineralogical considerations suggest that the Colorado River also flowed through and helped to carve Unaweep Canyon, although the Colorado River probably exited Unaweep Canyon prior to abandonment by the Gunnison River. The ancestral Gunnison River remained in its course and incised through bedrock for a long enough period of time to produce terrace remnants in the Cactus Park region that range in elevation from 2000 to 1880 m. Abandonment of the canyon by the Gunnison River was followed by formation of a natural dam that probably led to deposition upvalley of ~50 m of lacustrine sediments in Cactus Park. Recent mapping in the lower reaches of Unaweep Canyon indicate that a landslide could have led to damming of Unaweep Canyon, perhaps while it was occupied by underfit streams.

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