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

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.