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

We present new cosmogenic nuclide ages for the Central City Gravel, a sedimentary deposit previously inferred to be a Miocene-aged high-elevation gravel, but which we interpret as a remnant of one of the earliest Pleistocene glaciations of the southern Rocky Mountains. The Central City Gravel has some fluvial components near the base, but the center and top of the deposit is an undifferentiated till, with large boulders peppering the surface. Perched ~250 m above Clear Creek and surrounding valleys atop the Rocky Mountain peneplain, the age of this deposit is significant for both paleoclimatic and geomorphic studies in the region. 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 379.5 +/- 405.6 ka for deposition of the till; one other burial sample was inconclusive due to a negative burial age. This preliminary age allows for new interpretation of the timing of Denver Basin exhumation and abandonment of the Rocky Flats surface. Additional samples from the till-bedrock contact and erratics atop the deposit are currently being processed.