COMPARATIVE LANDSCAPE EVOLUTION IN THE WESTERN GRAND CANYON REGION, ARIZONA

Geologic relationships in the western Grand Canyon region permit a comparison of long- and short-term landscape evolution from the perspective of scarp recession throughout Cenozoic time. The stripping of the Hualapai Plateau down to Paleozoic carbonates by recession of the Shivwits Plateau scarp occurred over the last 65 Ma, as shown by the presence of Laramide paleocanyons, a 65 Ma hypabyssal intrusive, and the position of the pre-Basin and Range boundary fault. Immediately north of the Colorado River along the Grand Wash Cliffs the same stratigraphic sequence is in an earlier stage of forming a similar landscape that began in middle Miocene time. Scarp recession is defined here as the result of mass wasting processes, distinct from the concentrated effect of headward erosion by obsequent tributaries. The average recession rate for the two scarps is the same within qualitative limits, approximately 600 m/Ma. This rate is broadly similar to rates of scarp recession measured in Paleozoic terranes on other continents in cases where the recession of sandstone or carbonate strata is controlled by weaker underlying and/or intervening shales. Previously published studies of Colorado Plateau scarp recession rates give estimates of 450 and 500 m/Ma. Rates of scarp recession in pre-Mesozoic terranes in the geologic literature generally range from 200 to 600 m/Ma, a surprisingly narrow interval, given the range of lithologies and climatic variation through time. This translates to 0.2 to 0.6 mm/yr, a rate that is similar to findings in studies of bedrock and tombstone weathering. The similar scarp recession rates along both the Grand Wash Cliffs and across the Hualapai Plateau lend credence to the proposed antiquity of the western Grand Canyon landscape, disregarding the younger age of the western Grand Canyon. The contrasting degree of fault scarp dissection along the bordering faults is also a crude measure of relative landscape age. The different distances of scarp recession in these two similar stratigraphic settings indicate that such comparisons are a useful qualitative measure of landscape antiquity. In the present case, the multi-stage history and antiquity of the western Grand Canyon is supported by the relatively lower degree of cliff recession and fault scarp dissection in adjacent areas with similar stratigraphy.