LATE CENOZOIC GLACIAL AND FLUVIAL INCISION OF LARAMIDE RANGES

While the crystalline cores of the Laramide ranges are characterized by smooth high surfaces interrupted by glacial canyons, the floors of the bounding sedimentary basins show evidence of broad exhumation. We focus here on the Front Range of Colorado, in which the glacial footprint occupies only a small fraction of the range width, allowing exploration of the fluvial response to baselevel drop due to basin exhumation. Stream profiles show distinctive stepped departures from smooth fluvial profiles in the glaciated headwaters, and strong convexities beyond the glacial limit represent the response to baselevel fall. We place this in a quantitative context using a numerical model of stream profile evolution. We first summarize the history of basin exhumation as revealed in rangefront gravel-covered surfaces that serves as the boundary condition for the range-draining streams. Our numerical model takes into account the actual distribution of drainage area with distance downstream, and an orographically defensible distribution of effective moisture to drive river incision. The fluvial response demonstrates strong transience, with most rivers presently in the midst of their response to basin lowering. Rivers with small drainage areas and no access to glacial headwaters show only modest lowering at the mountain front. Large remnants of the basin piedmont tend to be preserved at these spots (e.g., Rocky Flats surface). Glacial erosion of the headwaters results in lowering and flattening of valley floors, generation of steps, and up-valley erosion of the headwall. By simulating a flexural isostatic response to erosion, we explore the conditions under which the exhumation history of the basins can influence both the orographic precipitation pattern and the glaciation of the range crest.