Basin fill, erosion surfaces and tilted markers: Evidence of late Cenozoic tectonic uplift of the Rocky Mountain Orogenic Plateau.

摘要

Geologists have long believed that the Rocky Mountain Orogenic Plateau experienced late Cenozoic tectonically-driven uplift. This conclusion is based on observations of post-Miocene widespread deep incision. Recent studies have refocused attention on the causes of long wavelength incision and suggest that climate change can produce the same landscape features as tectonic forces. I present three studies examining if climate or tectonic forcing is the primary cause of this late Cenozoic erosional event. First, I present evidence of the distribution, magnitude, and timing of incision and evaluate the patterns with respect to climatic versus tectonic signals. My results reveal a complex pattern that is controlled to first-order by tectonism, and supports that elevation gain across the entire Rocky Mountains, Colorado Plateau, and western Great Plains began between 4–8 Ma.; Second, I re-evaluate the use of low-relief, sub-summit erosion surfaces as indicators of uplift in the Rocky Mountains. The age and extent of these surfaces are well constrained in the Front Range of Colorado, but not so in ranges to the north and west. Results indicate a positive relationship with the low-relief surfaces and three factors: (1) lithology, (2) mean elevation of the projection of Late Tertiary basin fill, and (3) the mean elevation of Pleistocene glacial features. The most important implication of this work is that only surfaces with known age controls or those clearly associated with the Late Tertiary basin fill can be used as a datum for interpretations of elevation changes and incisional events affecting the region.; Finally, I present an evaluation of the post-depositional tilt of the Cheyenne Tablelands in the western Great Plains, which provides the most robust evidence of late Cenozoic tectonic uplift within the region. Paleohydraulic analysis of preserved gravel channels capping the Tablelands indicates a depositional slope of 10−3 to 10−4. The modern slope of the Ogallala deposits (10−2) is up to an order of magnitude greater than the slope during deposition. The difference translates into 680 m of differential uplift. Climate-induced erosion alone cannot account for the change in slope, suggesting broad wavelength tectonic uplift to the west, centered under the Rocky Mountains.