COSMOGENIC NUCLIDE DATING OF THE CENTRAL CITY GRAVEL, A MID-PLEISTOCENE PERCHED GLACIAL TILL IN THE FRONT RANGE, COLORADO

We present new cosmogenic nuclide ages for the Central City Gravel, a sedimentary deposit perched ~250 m above Clear Creek and surrounding valleys atop the Rocky Mountain peneplain. Abandonment of the Rocky Mountain peneplain surface likely did not occur until after deposition of the Central City Gravel; therefore, the age of this unit is significant for both paleoclimatic and geomorphic studies in the region. Previously, the unit was inferred to be a Miocene-aged nonspecific high-elevation gravel, suggesting that incision into the underlying Rocky Mountain peneplain happened at a slow, steady rate. However, we interpret the Central City Gravel as a remnant of one of the earliest Pleistocene glaciations of the southern Rocky Mountains and infer that the till is, at most, 2.6 Myr old, correlating to the initiation of the Laurentide glaciation. Our new interpretation of this unit as a glacial till signifies that incision occurred at a much faster rate than previously thought. Estimates of incision rates into the Rocky Mountain peneplain have been calculated to be at least 300 m/Myr since Lava Creek B deposition ~0.63 Ma. This rate and the timeline for the exhumation of the Denver Basin are not currently based upon direct dating of the peneplain surface. We use cosmogenic nuclide burial dating of the bedrock surface underlying the Central City Gravel to determine the maximum age of glaciation. A single ²⁶Al/¹⁰Be cosmogenic nuclide burial age from the bedrock surface yields an age of 232.4 +/- 378.9 ka for deposition of the till; one other burial sample was inconclusive due to a negative burial age. Additional samples from the till-bedrock contact are currently being processed. Surface exposure dating of boulders peppering the top of the deposit indicates a minimum age of 90 ka for the till. These preliminary ages suggest that the Rocky Mountain peneplain was abandoned far more recently than previously assumed, and the incision rates into this surface and timing of Denver Basin exhumation will need to be reevaluated.