Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 6
Presentation Time: 11:45 AM

UNSTEADY LATE PLEISTOCENE INCISION OF STREAMS BOUNDING THE COLORADO FRONT RANGE FROM MEASUREMENTS OF METEORIC AND IN SITU 10BE


DUHNFORTH, Miriam, Department of Earth and Environmental Sciences, Ludwig-Maximilians University (LMU) Munich, Munich, 80333, Germany, ANDERSON, Robert, Department of Geological Sciences and INSTAAR, University of Colorado, UCB 450, 1560 30th Street, Boulder, CO 80309, WARD, Dylan J., Department of Geology, University of Cincinnati, 500 Geology/Physics Bldg, ML 0013, Cincinnati, OH 45221-0013 and BLUM, Alex E., US Geological Survey, WRD, 3215 Marine St, Marine Street Science Center, Boulder, CO 80303, dylan.ward@uc.edu

We date gravel-capped strath terraces to document the history of late Cenozoic fluvial exhumation in basins adjacent to the Rocky Mouintains in Colorado. We use in situ 10Be measurements to date the broad surfaces adjacent to eastern edge of the Colorado Front Range, and compare these calculated ages with results from meteoric 10Be measurements. We analyze three sites near Boulder, Colorado (Gunbarrel Hill, Table Mountain, and Pioneer) that have been mapped as the oldest terrace surfaces with suggested ages ranging from 640 ka to the Plio-Pleistocene transition. Our in situ 10Be results reveal abandonment ages of 95 ± 12 ka at Table Mountain, 175 ± 27 ka at Pioneer, and ages of 251 ± 10 ka and 307 ± 15 ka at Gunbarrel Hill. All are far younger than previously thought. Soil development indicators and inventories of meteoric 10Be support this interpretation, yielding ages that are comparable to Table Mountain and ~20% lower than Pioneer in situ ages. We argue that lateral beveling by rivers dominated during protracted times of glacial climate, and that vertical incision rates of several mm/yr likely occurred during times of very low sediment supply during the few interglacials that were characterized by particularly warm climate conditions. In contrast to the traditional age chronology in the area, our ages suggest that the deep exhumation of the western edge of the High Plains occurred relatively recently and at an unsteady pace.