USING GIS TO ASSESS CONTROLS ON THE EVOLUTION OF SUB-SUMMIT SURFACES IN THE ROCKY MOUNTAIN FRONT RANGE

The Rocky Mountain Front Range is characterized by a sub-summit zone of broadly flat- to-rolling topography. This zone of low relief, known as the Rocky Mountain surface or Eocene erosion surface, contrasts sharply with the adjacent foothills at the mountain fronts and high peaks at the crest of the range. Now dissected remnants of the surface truncate Laramide structures, sit at concordant elevations on crystalline rocks, and are locally overlain by Tertiary sedimentary and volcanic units. The surface has served as a datum from which both tectonic and climatic events have been inferred. For example, assuming the surface formed at an elevation near sea level, late Cenozoic uplift is invoked as the cause of fluvial dissection and the present elevation of the surface. Alternatively, if the surface formed at similar-to-modern elevations, late Cenozoic climate change is invoked. Debate regarding the surface evolution continues due to the fact that sub-summit erosional surfaces lack modern analogs and are difficult to date.

We use GIS and spatial analysis as tools to examine these surface remnants with regard to their origin, age, and modification through time. Our approach is to digitize the previously mapped extent (Scott and Taylor, 1986) of the sub-summit surface and to overlay topography, lithology, structure, and climate gradient to correlate low relief with tectonic and/or climatic indicators.

Preliminary results indicate that the surface was created by multiple and possibly time transgressive processes. A positive relationship exists between the median elevation of the sub-summit surface and the location of Tertiary fluvial basin fill remnants and some Quaternary glacial deposits. Results from this analysis are important not only for the evolution of the Front Range but also for other Laramide-age ranges within the Rocky Mountains that exhibit similar low relief sub-summit morphology.