Assessing the Origin and Age of the Buried Sediment-Bedrock Interface in Unaweep Canyon

Abstract

Unaweep Canyon is a large, enigmatic landform in westernmost Colorado, hosted on the Uncompahgre Plateau. Fluvial gravels of inferred ancestral Gunnison River provenance found near both the eastern and western outlets to the canyon suggest this river occupied Unaweep Canyon during the late Cenozoic (late Miocene-Pleistocene). But glacial-like geomorphic attributes of the Precambrian-hosted western canyon have inspired the hypothesis of a glacial origin, potentially dating from the late Paleozoic. A new core drilled into the inferred deepest part of the western canyon reveals, beneath a Pleistocene lacustrine section, ~23 m of loose gravels of ancestral Gunnison River provenance lying atop a ~5 m lithified interval of heavily fractured and hematized Proterozoic basement directly atop intact Proterozoic basement. Both macro- and microscopic analysis of this 5 m interval reveal pervasive cataclastic textures. Catastrophic mass wasting provides the simplest explanation for the origin of the cataclasis, given that the paleorelief in western Unaweep Canyon exceeded 1000 m. Clasts of the fractured and hematized basement occur in the fluvial gravels; additionally, geochemical data record modest weathering of the fractured and hematized interval and basement contact zone. Accordingly, the deposition, fracturing and hematization, and weathering of the lithified study zone and basement interface occurred before initial occupation of Unaweep Canyon by the ancestral Gunnison River in the late Miocene. This implies the existence of a paleovalley, exhumed by the ancestral Gunnison River. Given that the Uncompahgre Plateau was, prior to its late Cenozoic exhumation, most recently uplifted sufficiently to expose basement in the late Paleozoic, these data suggest that the Precambrian-hosted part of Unaweep Canyon dates from the pre-Miocene and may date from the late Paleozoic.