AN 18-KA GLACIAL LAKE OUTBURST FLOOD FROM GLACIAL LAKE DURANGO: SOUTHERN SAN JUAN MOUNTAINS, COLORADO

Cosmogenic nuclide (¹⁰Be) surface exposure ages were obtained on granite and sandstone boulders from a boulder field south of Durango, Colorado. The rounded boulders in the boulder field: (1) measure 2-4 meters in diameter; (2) occur at elevations between 2006 and 2028 meters; (3) are in a remote location approximately 96 meters above the modern Animas River (~246,132mE, 4,122,500mN, Z13); (4) are located approximately 9.4 km down-gradient from the southernmost, Last Glacial Maximum (LGM) terminal moraine; and, 5) are located approximately 22 km down-gradient from the closest granite outcrop. The boulder field was thought to have been emplaced by a glacial lake outburst based on the size of the boulders, elevation, location, local geology and geomorphology. Two boulders from the boulder field, a granite and a sandstone, yielded exposure ages of 18.2 ± 2.5 and 18.1 ± 2.2 ka, respectively. The exposure age dates indicate that the glacial lake outburst flood occurred close to the LGM for this area (19.4 ± 1.5 ka; Guido, et. al, 2007).

Another granite boulder, resting at an elevation of 2012 meters, from a different boulder field, was also exposure age dated and yielded an anomalously young exposure age of 2.5 ± 0.4 ka. The second boulder field: (1) is located approximately 4.5 km down-gradient from the LGM terminal moraine; and, (2) contains a cluster of boulders that form a linear array in a gully along the north face of Smelter Mountain (~243,870mE, 4,128,160mN, Z13). The exposure age date of the granite boulder likely represents the age of the last downslope movement of the boulder in the north-facing gully. It is reasonable to suggest that the downslope movement of 2-4 meter diameter boulders occurred as a result of a significant (regional or local) precipitation event. Remarkably, the 2.5 ± 0.4 ka exposure age on the granite boulder correlates well with the onset of regional climate change recorded in the nearby La Plata Mountains. A palynology study by Petersen and Mehringer (1976) indicates that an upward advance in the position of timberline in the La Plata Mountains, began around 2.55 ± 0.075 ka, and peaked at a mean of about 2.0 ka.