Unsteady late Pleistocene incision of streams bounding the Colorado Front Range from measurements of meteoric and in situ ¹⁰Be

Abstract

Dating of gravel‐capped strath terraces in basins adjacent to western U.S. Laramide Ranges is one approach to document the history of late Cenozoic fluvial exhumation. We use in situ 10Be measurements to date the broad surfaces adjacent to the eastern edge of the Rocky Mountains in Colorado, 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 ± 129 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. 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 even moderate 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 the High Plains occurred relatively recently and at an unsteady pace.

Interpreting climate‐modulated processes of terrace development along the Colorado Front Range using a landscape evolution model

“…Three of the four models, glacial sediment flux, rainfall intensity, and combined rainfall intensity/hillslope diffusion, match the observation that periods of channel aggradation correspond to glacial intervals, while periods of vertical channel incision correspond with interglacial intervals. Only one of the model scenarios, the combined rainfall intensity/hillslope diffusion model, also shows increased denudation rates in the mountains during periods of aggradation, which is necessary to agree with observed higher paleodenudation rates during the deposition of terrace‐capping sediment [ Fuller et al , ; Foster et al , ; Dühnforth et al , ].…”

“…Dühnforth et al [] used both in situ and meteoric 10 Be to date the abandonment of several terraces, including Table Mountain, which is also ∼100 m higher than local base level (Figure , A‐A' transect). They found an abandonment age for Table Mountain of 95 kyr, which is younger by an order of magnitude than previously inferred from elevation relationships of the few dated terraces in the basin [ Madole , ].…”

“…They found an abandonment age for Table Mountain of 95 kyr, which is younger by an order of magnitude than previously inferred from elevation relationships of the few dated terraces in the basin [ Madole , ]. These young dates suggest that terrace treads were occupied and beveled for long periods before being abandoned following bursts of rapid incision, at rates of up to several mm/yr [ Dühnforth et al , ]. Foster et al [] used 10 Be to date a lower elevation terrace near Table Mountain and report a preliminary abandonment age for the lower terrace that is close to the abandonment age of 95 kyr for Table Mountain reported by Dühnforth et al [].…”

“…These young dates suggest that terrace treads were occupied and beveled for long periods before being abandoned following bursts of rapid incision, at rates of up to several mm/yr [ Dühnforth et al , ]. Foster et al [] used 10 Be to date a lower elevation terrace near Table Mountain and report a preliminary abandonment age for the lower terrace that is close to the abandonment age of 95 kyr for Table Mountain reported by Dühnforth et al []. Foster et al [] also calculated paleodenudation rates from cosmogenic radionuclide inheritance in the sediments capping Table Mountain.…”

The Role of Critical Zone Observatories in Critical Zone Science

“…ages of strath terrace sediment covers on the Plains suggest that terrace planation occurs during glacial episodes, and river incision during deep interglacial episodes (Dühnforth et al, 2012). This further indicates that change in sediment delivery from hill slopes (Anderson et al, 2013) is a key driver of river down cutting, which itself incites transient adjustment of hill slopes.…”