THE HISTORY OF THE ANCESTRAL GUNNISON RIVER THROUGH WESTERN UNAWEEP CANYON AND GATEWAY (COLORADO)

Unaweep Canyon is drained by East and West Creek, which flow from a divide within the canyon. All recognize that these creeks did not cut the canyon; current hypotheses for canyon genesis include Cenozoic fluvial or glacial action, or late Paleozoic glaciation, burial, and Cenozoic fluvial exhumation. Exposed river gravels are scarce within the canyon, but occur at its western mouth (Gateway, CO). We studied the distribution, relative ages, provenance, sedimentology, and paleocurrents of gravels to address the Cenozoic history of Unaweep.

The oldest gravels comprise well-rounded clasts of Precambrian basement (59.3%), Mesozoic sandstone (30.5%) and volcanic lithics (10.2%). These are clast-supported, <9.0 m thick, and occur atop bedrock straths cut into the Cutler Formation near the valley axis. The best-preserved terrace surface occurs at ~1555 m, with others at 1524 m and 1615 m. Pebble imbrications indicate southwesterly directed flow. The rounding and volcanic component indicates a Gunnison connection. A younger, areally extensive unit overlies these gravels, and comprises angular clasts with 10.5% Precambrian basement, 89.5% Mesozoic sedimentary clasts, and no volcanics. This deposit is the most voluminous (up to 30+ m thick) and forms a heavily dissected yet discernable surface, sloping at ~4 degrees from the Mesozoic escarpment toward West Creek. The distribution, provenance and slope indicate this unit records slope retreat from the surrounding cliffs. The youngest gravel unit rests inset against these widespread gravels as fill terraces and comprises clast-supported, crudely stratified, rounded to angular clasts with 33.0% Precambrian basement, 65.9% sedimentary clasts, and 1.1% volcanics. These occur along the axis of West Creek and its tributaries, and represents aggradation by these recent drainages with material reworked from the older two gravels.

The presence of gravels with a Gunnison signature near Gateway confirms that the ancestral Gunnison River flowed southwesterly through Unaweep Canyon to the Dolores River. We suggest that, following abandonment of Unaweep's inner gorge by the ancestral Gunnison, significant aggradation occurred such that the modern longitudinal profile of Unaweep Canyon, including Unaweep Divide, bears no relation to the ancestral Gunnison drainage.