YOUNG STRATH TERRACES ON WESTERN HIGH PLAINS RECORD CLIMATE-PACED VARIATIONS IN SEDIMENT SUPPLY FROM COLORADO FRONT RANGE RIVERS
Recent cosmogenic radionuclide (CRN) dates on terraces north of Boulder, CO, indicate that these surfaces are up to an order of magnitude younger than the correlative alluvial units to the south of Boulder. We present new CRN and optically-stimulated luminescence (OSL) data from alluvial units in the Boulder area that have been mapped as middle to late Pleistocene surfaces. Preliminary CRN data on the middle Pleistocene surface yields a date of ~ 91 ka, far younger than expected based on correlation, but in accord with the 10Be-based age of the nearby surface just above it.
The new dates on strath surfaces in the western High Plains are consistent with a fluvial history marked by long periods of aggradation and lateral planation, punctuated by brief episodes of rapid incision through soft shale underlying the Boulder area. This model supports a “top-down” approach in which fluvial incision and aggradation are driven by variable sediment production from source basins in the adjacent crystalline Front Range: glacial and periglacial climates produce high sediment yields leading to aggradation and lateral planation; extreme interglacial climates correspond with low sediment supply, leading to vertical bedrock incision. Under this model: (1) strath terraces cannot be correlated based on elevation alone, (2) exhumation of the Denver basin is likely spatially and temporally variable due to climatically-driven variations in sediment supply, and (3) Front Range rivers likely experienced a complex and basin-specific history of aggradation and incision over the Quaternary.