Abandonment of Unaweep Canyon ~1 Ma and the Effects of Transient Knickpoint Migration, Western Colorado

The timing of Unaweep Canyon's abandonment in western Colorado has long been debated. Cosmogenic burial-ages and studies of lacustrine sediments in Cactus Park, an ancestral Gunnison River paleovalley that is tributary to Unaweep Canyon, provide new insights on this topic.

Cactus Park contains gravels of the ancestral Gunnison River, which flowed through Cactus Park before entering Unaweep Canyon. These gravels represent terrace remnants the lowest of which is 457 m above the modern Gunnison River. A drill hole in Cactus Park shows that the lowest gravels are overlain by ~50 m of lacustrine mud, which was deposited as a result of a damming event in Unaweep Canyon.

A cosmogenic burial-age of 1.06 ± 0.38 Ma on the buried Gunnison River gravels is interpreted as the time immediately prior to canyon abandonment. Mineralogical and geochemical data for the lake beds indicate a provenance of local sedimentary strata rather than San Juan Mountain volcanic terrains. This observation suggests that damming and lake development occurred after abandonment of Cactus Park and Unaweep Canyon.

The cosmogenic burial-age estimate on the Cactus Park gravels gives an incision rate of 431 m/Ma, which is higher than the longer-term regional incision rate of 142 m/Ma, but is similar to high incision rates (400-500 m/Ma) near Black Canyon of the Gunnison. One scenario is that abandonment of Unaweep Canyon, related to capture of the Gunnison by the Colorado River, created a transient knickpoint that migrated upvalley towards Black Canyon of the Gunnison. Lower incision rates (~150 m/Ma) between Unaweep and Black Canyon indicate that this transient knickpoint passed through the Delta, CO area prior to 640 ka, which is consistent with ca. 1 Ma Unaweep Canyon abandonment. These observations suggest that rapid river incision can reflect stream capture and drainage integration as well as tectonic and climatic forcing.