Pacing the post-last glacial maximum demise of the Animas Valley glacier and the San Juan icecap, Colorado

During the last glacial maximum (LGM), a 5000 km2 icecap covered the San Juan Mountains. The largest valley glacier flowed 90 km and drained the Animas Valley. To characterize the demise of the Animas Valley glacier, I employ cosmogenic 10Be to date an LGM terrace 27 km down-valley of the terminal moraines and a suite of nine glacially-polished bedrock samples. I date the terrace by amalgamating samples at 5 depths. The 10Be depth profile constrains the age of the terrace and the average cosmogenic radionuclide inventory (CRN) that accumulated during the exhumation and transport (inheritance) of terrace sediments. The terrace age suggests that the Animas River abandoned the floodplain at 18.7 +/- 1.6 ka. I argue that as deglaciation began, the ponding of Glacial Lake Durango behind the terminal moraines shut off fluvial sediment supply and caused the river to incise, forming the terrace. (Abstract shortened by UMI.)...