QUATERNARY ABANDONMENT AND SEDIMENTARY FILL HISTORY OF CACTUS PARK AND UNAWEEP CANYON, UNCOMPAHGRE PLATEAU, COLORADO

Unaweep Canyon, an abandoned windgap cutting across the structural grain of the Uncompahgre Plateau, is of controversial origin. Like the modern Black Canyon of the Gunnison, Unaweep was incised into crystalline Precambrian rock by the Gunnison River. Unaweep is larger, with a central drainage divide between East and West Creeks, and contains a thick sediment fill, which suggests that the Black Canyon may be a suitable analog for comparison.

River gravel composition data was collected from a variety of locations and elevations from both canyons, demonstrating how the composition of river gravels will change as a river passes through an incised canyon. A recently discovered Gunnison River gravel site by U. of Oklahoma researchers as well as Cactus Park gravel locations allow for reconstruction of the gradient of the Gunnison River in Unaweep Canyon and the depth of the sediment fill.

Reverse motion on the fault bounding the Ute Creek Graben is suggested by the projected gradients of the Gunnison River, which cannot account for the Precambrian rock cropping out downstream of Unaweep Seep. Based on known sediment depth and river gradient projections, it can be determined that Unaweep Seep does not represent the lowest point of sediment fill in the canyon. Fill at lower elevations may have been dammed by the fault bounding the Ute Creek fault block.

Finely laminated deposits overlying Gunnison River gravels in Cactus Park suggest that one or several lakes formed in the area following incision. Geochemical and pollen analysis of lake deposits from Cactus Park indicates that the lake deposit material resembles Mancos Shale, and is probably locally derived, indicating that the Gunnison River had abandoned the canyon before formation of the Cactus Park lake. The Ute Creek fault block may have served as a dam bounding the western end of the lake system.