THE POST-LARAMIDE ROCKY MOUNTAIN SURFACE ON THE FRONT RANGES OF COLORADO – ITS CHARACTER AND POSSIBLE CAUSES OF ITS DEFORMATION

The Rocky Mountain Surface is a low-relief post-Laramide surface that has long been used as a marker in studies of Cenozoic deformation in the Colorado Front Range and adjacent ranges. This study employs geomorphic and stratigraphic analysis to provide information on the original relief of the surface, the post-formation erosional lowering of its remaining low-relief portions, the distribution and magnitude of focused incision of the surface along river valleys, and the location, character, and magnitude of Neogene/ Quaternary deformation of the surface. Much of the surface has experienced less than 50 m of erosion during the Neogene, although locally incision along drainages approaches 1 km. This focused incision is greater in the south, along the Arkansas River and its tributaries, than in the north, along the South Platte River and its tributaries, a pattern consistent with that observed on the plains to the east where piedmont incision in the Arkansas drainage is much greater than that in the South Platte drainage.

In the northern Front Range, the Rocky Mountain Surface has a consistent eastward slope, presumably reflecting its initial formational slope. To the south, its geometry is more complex. The southern portion of the surface generally dips eastward or southeastward, but its eastern edge is elevated several hundred meters above its central portions. Traditionally, deformation of the southern portion of the surface has been attributed to Rio Grande Rift-related stresses, and some of the complexity of the southern portion of the surface may reflect its original geometry. A possible alternative interpretation is that some or all of the apparent relative uplift of the eastern margin of the surface is an isostatic response to the long-wavelength, km-scale erosion of the Arkansas drainage on the plains to the east, in contrast to the areally much more limited, short-wavelength erosion by mountain streams to the west. Initial flexural modeling suggests that erosion-driven flexure would produce a westward tilt of the eastern portion of the Rocky Mountain surface consistent with the observed pattern of relative uplift. However, the observed magnitude of relative uplift is several times that predicted as a flexural response to differential erosion alone.