SUB-SUMMIT SURFACES IN THE ROCKY MOUNTAINS: REEVALUATING PAST IDEAS WITH LANDSCAPE DIMENSIONAL ANALYSIS

Laramide ranges in the Rocky Mountains are characterized by low relief sub-summit surfaces that have been recognized since the Hayden survey of 1872. These surfaces sit at concordant elevations predominantly on crystalline rocks; they truncate Laramide structures, and are locally overlain by Tertiary sedimentary and volcanic units. Their broad flat to rolling topography contrasts sharply with the adjacent more rugged steep walled canyons, mountain fronts, and high peaks within each range. Several geomorphic processes have been proposed for the origin of these surfaces including peneplanation (Davis, 1911), pedimentation (Johnson, 1931), altiplanation (Mackin, 1947), re-exhumation of the Precambrian-Paleozoic contact (Hughes, 1933), and primary denudation (Steidtmann et al., 1989). These processes have proven hard to evaluate due in part, to lack of modern analogs and the difficulty in dating erosional surfaces.

We use GIS and dimensional analysis as tools to reexamine these surfaces and the questions regarding their origin, age, and subsequent modification. Our approach is to identify the low-relief “flat” topography within the ranges using factors including the surface derivative, variance, and fractal dimension. Flat topography is then correlated with elevation, relief, lithology, age constraints, and denudation history to test the following hypotheses: 1) the surfaces are re-exhumed disconformities, 2) the surfaces represent the lower glacial limit within the ranges, 3) the surfaces grade to the top of the late Tertiary basin fill, and 4) the surfaces are lithologically controlled.

Preliminary results indicate that there is a positive relationship between the median elevation, average relief and location of these surfaces. There is no correlation with the timing, history and underlying lithology. This suggests that the surfaces were created by either a time transgressive process or multiple processes. Ultimately, understanding the geomorphic development of these surfaces has significant implications as they have been used repeatedly as a datum for delineating the late Cenozoic elevation and erosion history of the Rocky Mountain orogenic plateau.