NEW GEOLOGIC MAPPING RESULTS FROM UNAWEEP CANYON, AN ENIGMATIC LANDSCAPE OF THE UNCOMPAHGRE PLATEAU (COLORADO)

Unaweep Canyon is a deep, Precambrian-cored gorge incised through Mesozoic strata of the western Uncompahgre Plateau, and is occupied by two underfit creeks flowing from a divide in its midst. The origin of the canyon has long been enigmatic owing to its anomalous size, orientation, longitudinal profile, and the odd geomorphology, which appears glacial. All recognize that the creeks currently draining the canyon could not have carved it, but hypotheses for its origin range from Plio-Pleistocene fluvial incision and subsequent abandonment by the Gunnison and/or Colorado Rivers to Pleistocene glaciation, to late Paleozoic glaciation and Plio-Pleistocene fluvial exhumation. Assessing the relative merits of these hypotheses remains challenging because the canyon has never been mapped in any detail, save for the far western mouth, which was mapped in the 1940s.

As part of the USGS EDMAP program, we conducted new mapping of the canyon at the 1:12,000 scale, focusing on the Precambrian-Mesozoic nonconformity surface throughout the canyon, and the geology of the (tectonically disturbed) western mouth, where Precambrian basement is onlapped by the Permo-Pennsylvanian Cutler Formation. Among other goals, detailed mapping will enable assessment of a possible Paleozoic age for the canyon by shedding light on (1) the possible presence and form of paleorelief, and (2) the locations of and displacements on post-Paleozoic structures to enable accurate palinspastic reconstruction. New findings include the discovery of a paleotrough carved within Precambrian basement near the western mouth of the canyon, and filled with Permo-Pennsylvanian Cutler Formation. This paleotrough was previously unrecognized and documents a paleo-embayment leading into the main gorge that deviates from the modern canyon mouth; hence, any cross-section of a possible paleocanyon must follow this path. Furthermore, the Cutler onlap onto Precambrian basement in this region buries substantial paleorelief, demonstrating the existence of a paleolandscape here.

The possibility that Unaweep Canyon originated atop the ancestral Uncompahgre uplift during the late Paleozoic has important implications for our understanding of climatic and tectonic behavior of the greater region.