EVIDENCE FOR LATE CENOZOIC UPLIFT ON THE UNCOMPAHGRE PLATEAU, NORTHEASTERN COLORADO PLATEAU

The Uncompahgre Plateau near Unaweep Canyon 30 km south of Grand Junction records a complex history of late Cenozoic deformation superimposed on Laramide uplift. Antecedent Unaweep Canyon was incised 840 m into tilted Mesozoic and Proterozoic rocks before being stranded atop the rising plateau. Before uplift, the ancestral stream system was part of a regional system of low gradient streams that meandered across landscape underlain largely by Mancos Shale. About 3-4 Ma, the Mancos was largely stripped (Fleming, 1994; palynological data), and meanders were deeply incised during Pliocene and younger regional and local uplifts, including rise of parts of the Uncompahgre Plateau. Near Cactus Park where the northeast end of Unaweep Canyon forms the junction of ancestral Colorado River with the combined Gunnison and Uncompahgre Rivers, streambed bedrock is now at least 80 m below streambed bedrock downstream toward the crest of the uplift to the southeast. Old river gravels to the southeast in Cactus Park contain clasts typical of the provenance of both the Gunnison and Uncompahgre Rivers; these gravels are overlain by lakebeds that are 475 m above the modern Gunnison River to the northeast in Grand Valley. We conclude that a tributary of the Colorado River probably followed the Mancos Shale around the northwestern end of the Uncompahgre uplift to capture the Colorado River in Grand Valley upstream of Cactus Park. Continued uplift of the plateau defeated and dammed the smaller Gunnison/Uncompahgre river system. Lakebeds were deposited in the ponded waters only to be captured by a Colorado River tributary that migrated southeastward in the ancestral Grand Valley. Damming took place by about 1.8 Ma, based on incision rates calculated from occurrences of the 640-ka Lava Creek B ash in pediment deposits at Petrie and Deer Creek Mesas on the south side of Grand Mesa north of the Gunnison River. The depth of incision of Unaweep Canyon is approximately the same as the structural offset of the Dakota Sandstone across the southwest side of the Uncompahgre monocline, indicating that structural uplift and antecedent erosion were concurrent, and both took place between about 4 and 1.8 Ma.