GEOLOGICAL MAPPING OF QUATERNARY COLORADO AND GUNNISON RIVER TERRACES IN THE GRAND VALLEY, WESTERN COLORADO

Quaternary strath terraces of the Colorado and Gunnison Rivers in the Grand Valley of western Colorado record alternating episodes of lateral planation and bedrock incision. Hypotheses to explain the terraces range from late Cenozoic rock uplift related to erosional isostasy and/or recent faulting to changes in river hydrology caused by glacial cycles. New mapping and data compilation show that there are at least 6 major terraces with straths that range from ~ 5 to 160 m in height above the Colorado River. Terrace deposits consist of 3 to 5 m of rounded cobbles overlying a bedrock strath. Young terraces (Qt2-4) have 1 to 3 m of sand, silt, and clay with moderately developed calcic soils (Stage II or III) overlying the gravel. Colorado River gravels are distinguished from Gunnison River gravels by their relatively high percentages of sedimentary (30-40%), felsic plutonic (~ 15%), and metamorphic (~ 20%) clasts whereas Gunnison River gravels are dominated by intermediate volcanic clasts (60-70%).

Terrace remnants are concentrated south of the Colorado and Gunnison Rivers and clearly show that these rivers have migrated northward down gentle dipslopes originating from the Uncompahgre Plateau. The geomorphic position of the terraces shows that they post-date the abandonment of Unaweep Canyon. The heights of the older terraces (Qt2-6) suggest Pleistocene ages. A Colorado River tributary gravel located in Ruby-Horsethief Canyon has a correlated age of 640 ka based on the occurrence of the Lava Creek B ash in Prairie Canyon near the CO-UT border. The gravel strath is located ~ 103 m above the Colorado River (similar to the Qt4 terrace) and provides an incision rate of 16.2 cm/ky. Assuming a constant incision rate, the Qt2-Qt6 terraces may range from ~ 100,000 to a little more than 1 million years in age.

The elevation of the Qt6 strath indicates that the rivers have incised a minimum of 160 m through bedrock. This amount of bedrock incision probably reflects regional rock uplift, driven by erosional isostasy. We speculate that alternating episodes of lateral planation and incision are driven by changes in hydrology associated with glacial-interglacial cycles, which are superimposed upon this long-term trend of erosional-isostasy driven rock uplift.