LARAMIDE RANGE MORPHOLOGY AND GEOMORPHIC IMPLICATIONS

Researchers of the Laramide ranges have long noted the mountains' distinctive topographic features, including "subsummit" surfaces of low local relief, steep-walled canyons dissecting such surfaces, and high-standing bedrock summit spines. The nature and distribution of these features have important implications for interpretations of regional geologic history. For example, one argument for the origin of subsummit surfaces asserts that they reflect long periods of elevated local base level, when the sediment fill surfaces in range-adjacent basins were relatively stable and higher than today's. If this hypothesis is true, one might expect subsummit surfaces in ranges bordering a particular intermontane basin to express a common range of elevations. If, on the other hand, subsummit surfaces reflect Cenozoic tectonic uplift of formerly low elevation surfaces, as early Laramide workers argued, no such relationship would be expected. A systematic, regional scale analysis of Laramide range topography is required to address questions like these. We have launched such an analysis, focusing on the Bighorn, Wind River, Medicine Bow, Laramie, Front, Black Hill, and Uinta Ranges. Using 30 m USGS digital elevation models, we initially focus on: 1) identification and mapping of subsummit surfaces; and 2) documentation of longitudinal stream profile morphology of range-draining rivers, since the latter forms an important link between basin exhumation history and montane morphologic evolution. Analyses of range hypsometry, range-perpendicular topographic swaths, and stream profiles show that most Laramide ranges exhibit morphologic asymmetry. This assymmetry apparently does not systematically reflect range-scale structural features and may instead reflect the geomorphic history of the adjacent basins.