LATE CENOZOIC RIVER INCISION RATES FROM THE COLORADO ROCKIES AND IMPLICATIONS FOR NEOGENE DIFFERENTIAL UPLIFT

Existing data on late Cenozoic river incision rates, augmented by new age estimates of basalt flows and ancient river gravels, is providing an improved basis for interpreting Neogene landscape evolution in the Colorado Rockies. We hypothesize that regional differences in incision rates between sub-basins of western Colorado and parts of Utah and Wyoming, reflect up to 1.5 km of differential uplift during the Neogene. Age estimates involve cosmogenic, ⁴⁰Ar/³⁹Ar, U/Th, and optically stimulated luminescence (OSL) dating of fluvial and volcanic deposits, which range in age from 25 to 0.06 Ma.

Incision rates show significant spatial and temporal variations. Long-term (ca. 10 Ma to the present) incision rates based primarily on basalt flows, illustrate the general pattern of spatial variability. Maximum rates (100-150 m/Ma) and magnitudes (1000-1500 m) of river incision in western Colorado are recorded between Grand Mesa and the Flat Tops. Minimum rates (<50 m/Ma) and magnitudes (<250 m) of river incision are associated with the Browns Park-Sand Wash Basin in northwestern Colorado, Middle Park of northern Colorado, and in salt collapse regions of central Colorado. Intermediate rates (50-100 m/Ma) and magnitudes (250-1000 m) are represented by the Elkhead Mountains and the Gunnison region of northern and western Colorado.

Stratigraphic relations show that between 25 and 10 Ma, Rocky Mountain basins were filling, and an integrated Colorado River system did not yet exist. Following the onset of regional exhumation ca. 10-8 Ma and the advent of the Colorado River, incision rates remained semi-steady through most of the Neogene. This interpretation is based on similarities in long- and short-term incision rates calculated using ca. 10 Ma basalt flows and the ca. 640 ka Lava Creek B ash as a datum. For instance, incision rates are 100-150 m/Ma near Grand Junction using both ca. 10 Ma basalt flows of Grand Mesa and the Lava Creek B ash as a datum. However, shorter-term incision rates calculated for the past 0.4 Ma in the region, are 200-400 m/Ma, roughly twice the long-tem incision rate. Post-0.4 Ma changes in incision rate could reflect the effects of climate superimposed on long-term (Neogene) regional uplift of the Colorado Rockies driven by a combination of erosional isostasy and mantle flow and buoyancy.