Monday, 6 October 2008: 10:30 AM
George R. Brown Convention Center, 350DEF
Despite decades of studies focused on the tectonics, geomorphology and bedrock geology of the Rocky Mountains, the tectonic history of the range through the late Cenozoic remains enigmatic. One of the most fundamental arguments about the range centers around whether the Rockies and High Plains were uplifted (relative to sea level) during the Late Cenozoic, or whether they had already achieved their present elevations by the end of the Laramide Orogeny. The strongest arguments put forth to support a tectonic rejuvenation focus on the deposition and incision of the Ogallala Group, a thin but regionally extensive suite of fluvial and eolian sedimentary rocks that was shed from the central Rockies eastward to the High Plains during the Miocene-Pliocene. However, the possibility that regional incision of the Ogallala carapace may be a signal of Late Cenozoic climate change, rather than tectonic rejuvenation of the Rockies, has been explored in considerably less detail. We summarize the results of new topographic analyses from the western United States along with simple geomorphic models describing sediment transport and fluvial incision in High Plains river systems. Our results allow us to contrast the patterns of incision driven by climatically-induced changes in hydrology and sediment flux with those driven by tectonically-induced changes in baselevel. Based on these results, we suggest that the post-Ogallala incision of the western High Plains may be more compatible with changes in the hydrology of High Plains river systems than with changes in tectonics. While this incision would have given rise to a modest isostatic response from the lithosphere, there may be no need to invoke independent Late Cenozoic tectonic changes to explain the topography we see today.