GEOMORPHOLOGY AND TECTONISM IN COLORADO’S SOUTHERN FRONT RANGE: IMPLICATIONS OF BOULDER AND VOLCANIC DEPOSITS ON THE LATE EOCENE EROSION SURFACE

One of the most prominent geologic features of central Colorado is the Late Eocene Erosion Surface (LEES), a widespread area of gentle topography stretching across the frontal ranges of the Southern Rocky Mountains at an average elevation of ∼2600 m. While extensive research has been done to characterize the origin and history of the LEES, this study provides a new approach, focusing on an E - W transect of the surface across its widest point at approximately 39◦ north. We use prominent deposits of rounded boulders (some up to 5 m in diameter) and pyroclastic flow remnants that rest directly upon the surface to better understand the evolution of its topography, drainage networks, and tectonic modification. Determining the provenance of the boulders provides an approximation of their transport distance, raising the question of how these massive boulders were moved over a surface of allegedly low relief. New analyses of boulder chemistry indicate lithologic sources up to 15 km from the deposits, primarily to the northwest. Given the size of the boulders and the lack of fluvial sedimentary structures, we propose debris flows along channels as a potential transport mechanism from source to deposit. Uplift of the Puma Hills (10 - 15 km to the west), temporally constrained by new correlations of the 37 Ma Wall Mountain Tuff mantling the surface on both sides, is proposed as a potential stimulus for elevated slopes necessary for debris flow inception.