INTEGRATING GEOLOGY AND GEOPHYSICS TO DETERMINE THE ORIGIN OF UNAWEEP CANYON AND LATE CENEZOIC FLUVIAL INCISION IN THE COLORADO PLATEAU-ROCKY MOUNTAIN REGION

Late Cenozoic fluvial incision and the formation of deep canyons in the Colorado Plateau-Rocky Mountain region of the western U.S. remains poorly understood. A preliminary geophysical survey involving high-resolution seismic reflection and refraction was conducted to aid in understanding the incision history of Unaweep Canyon in western Colorado, which was carved by the Gunnison and possibly the Colorado Rivers. A long-standing question has been whether or not the canyon contains a thick sequence of sedimentary fill. Seismic data was acquired in Cactus Park and at Unaweep Divide, the highest point along the floor of Unaweep Canyon, where previous studies indicate the presence of up to 300 m of fill. Preliminary results indicate that three geophysically distinct units exist within the canyon (Cactus Park and Unaweep Divide) and may be continuous. These units consist of a shallow layer (p-wave velocity less than 1200 m/s) generally 10-40 meters thick, a middle layer (about 2600 m/s) typically 20-70 meters thick, and a deep layer (greater than 3600 m/s). The shallow layer is presumed to be bouldery gravel associated with tributary alluvial-fan complexes and the deep layer is interpreted as consolidated Precambrian bedrock. The middle layer is significant, because the velocity is indicative of unconsolidated material, and recent borehole information from researchers at the University of Oklahoma suggest that this unit may correlate with lacustrine deposits located in Cactus Park. The presence of thick fill at Unaweep Divide suggests that abandonment of the canyon by the Gunnison River was not triggered by differential uplift of the Uncompahgre Plateau including Unaweep Divide. An improved understanding of fluvial incision is critical for understanding sediment transport from continental interiors to depositional basins, the interplay between tectonics and fluvial processes, and landscape responses to climate change.