LATE CENOZOIC EVOLUTION OF UNAWEEP CANYON (COLORADO PLATEAU) FROM NEW SEDIMENTOLOGIC AND GEOCHRONOLOGIC RESULTS

Unaweep Canyon is a deep valley incised into Precambrian and Mesozoic units, and drained by two underfit creeks emanating from a divide in the canyon. It is an enigma, and hypotheses for its origin range from Plio-Pleistocene glaciation to fluvial incision by the Gunnison and/or Colorado Rivers to late Paleozoic glaciation and Plio-Pleistocene exhumation. The provenance, composition and age of the sediment fill sheds light on canyon genesis, and yields clues to the Late Cenozoic tectonic, drainage, and climatic evolution of the Uncompahgre Plateau. Outcrops near the southern terminus of the canyon and core (329 m) recovered from the inner gorge reveals four units: 1) a lower diamictite containing only Precambrian basement clasts, 2) a middle clayey/sandy, upwardly coarsening lacustrine unit (150+ m), 3) a paleosol interval with alternating Calcisols and Protosols (8+ m), and 4) a conglomerate containing locally derived Precambrian basement and Mesozoic sandstone clasts (160+ m). Paleomagnetic analysis on the lowermost diamictite reveals a low paleolatitudinal signature which, together with Paleozoic spores within the lower core and corresponding lower canyon outcrops, indicates a Late Paleozoic age for canyon genesis. The remaining infill strata are of Plio-Pleistocene age, based on the presence of Cenozoic pollen and a 5.5 cm ash (inferred to be the Lava Creek B ash; 0.62-0.64 Ma) within the upper conglomerate. The lacustrine unit exhibits a Gunnison provenance, as do gravels present at the western canyon mouth, thus indicating occupation by the Gunnison River. Abandonment of Unaweep Canyon by the Gunnison River likely occurred in response to a landslide near the narrow southern end of the canyon ~0.8 Ma, as inferred from the transition to paleosol development prior to 0.62 Ma and approximate incision rates calculated by Kaplan (2006). Paleosol development and the transition to conglomeratic fill within the canyon may record wet-dry oscillations and associated regional drainage integration. To improve dating of the canyon fill, several samples of core, as well as excavated outcrop samples of the Gunnison River gravels, are being analyzed for cosmogenic burial ages. Results will enable further refinement of incision rates, and of inferences regarding timing of regional tectonic, geomorphic, and climatic events.