YOUNG STRATH TERRACES ON WESTERN HIGH PLAINS RECORD CLIMATE-PACED VARIATIONS IN SEDIMENT SUPPLY FROM COLORADO FRONT RANGE RIVERS

Along the Colorado Front Range, the western High Plains record a long period of post-Laramide deposition followed by exhumation in the last few million years. The formation of strath terraces adjacent to the Front Range records this exhumation as rivers incise vertically and laterally leaving thinly mantled gravel-capped surfaces behind. Approximately 6 alluvial units have been mapped along 300 km of the western High Plains based on soil development and elevation; each unit was thought to represent a fairly consistent elevation of the Denver basin during various stages of exhumation, driven by base-level fall of the South Platte River. Absolute dates, however, that can be compared to the existing relative age chronology exist at only a few locations so far.

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.