Tracking Pleistocene transient incision using Colorado River terraces across the Canyonlands district of the Colorado Plateau, Utah

Abstract:

Transient incision along river networks is an important complexity in landscape evolution. Although frequently modeled and interpreted from topography, it has rarely been quantified at the regional, Pleistocene scale. Recent thermochronology across the central Colorado Plateau indicates a distinct switch from Miocene quiescence to rapid erosion of >2 km of rock in the Plio-Pleistocene driven by integration of the Colorado River. In seeming contrast, research in Glen Canyon and the Moab area has identified a lengthy episode (~1 Ma) of landscape stability in the Early to Mid-Pleistocene followed by <150 m of rapid Colorado River incision in the Mid-Pleistocene. Sparse age control and incision rates across the central Colorado Plateau seem to indicate a pattern of rejuvenated canyon cutting and faster rates going upstream across a landscape of variable bedrock strength, consistent with a propagating wave of incision. We present Colorado River incision histories using new terrace chronostratigraphic data along both Meander Canyon (downstream of Moab, Utah) and in upper Glen Canyon (150 km downstream) to test the hypothesis of young, transient bedrock incision in the central Colorado Plateau and to determine its timing and celerity.

Incision histories and spatial patterns are being quantified by combining surveyed terrace stratigraphy with luminescence and cosmogenic isochron-burial dating techniques on Mid-to-Late Pleistocene fluvial deposits. Nineteen luminescence and three cosmogenic burial samples have been collected from terraces ranging from 8 m to 150 m in height above modern river level. Tracing of terraces in Meander Canyon are also being used to parse out regional incision signals and patterns of river response to the active salt tectonics downstream in Cataract Canyon. Preliminary luminescence ages in upper Glen Canyon, combined with results from prior workers indicate the transition to rapid bedrock incision began earlier (>350ka) than documented in the Moab area (~200 ka). This is consistent with a wave of rejuvenated incision having a celerity of ~1m/yr. There is no evidence for local baselevel change that could initiate this recent transient incision, suggesting that the pulse originated farther downstream in the Grand Canyon region in the Mid-Pleistocene.