EXAMINING THE TIMING OF LARGE LANDSLIDES IN THE SOUTHEASTERN SAN JUAN MOUNTAINS OF COLORADO: IMPLICATIONS FOR POST-GLACIAL LANDSCAPE EVOLUTION IN THE REGION

Large landslide deposits are surprisingly common in the southeastern San Juan Mountains. A combination of historical mapping by Atwood and Mather (1932) and our more recent mapping has identified no fewer than five landslide deposits measuring 4 km² or larger in the southeastern San Juan Mountains. Determining the timing of the landslide events and comparing the timing with local climate records could provide important information about the triggers for landslides in the region and the likelihood of modern mass movements. Over the past two field seasons, we have investigated the timing of landslides in the region via a three-prong approach. Our approach for dating the landslides includes: 1) coring sag ponds and bogs on the surfaces of landslides and dating the basal materials from the cores (CB); 2) determining the relative age of soils in the area by comparison with an established chronosequence (SC); and 3) cosmogenically dating large boulders deposited by the landslides (CD). For each of the large landslides that we examined, we completed as many of these methods as possible based on the presence of bogs, quartz-bearing rocks, and suitable soil pit locations. The landslides examined included the Trujillo Meadow Landslide (CB, SC, CD), the Silver Lakes Landslide (CB, SC, CD), the Silver Mountain Landslide (CB, CD), the Beaver Lakes Landslide (none), and the Elk Meadows Landslide (CB, SC, CD). Preliminary dating suggests that most of the landslides in the region occurred at the Pleistocene-Holocene transition indicating that the mass movements were the result of valley wall instability after Last Glacial Maximum (LGM) deglaciation. One exception may be the Silver Mountain Landslide (SML) which appears to have occurred during the Holocene on the basis of low sediment accumulations in bogs. This is noteworthy since it is the only landslide located outside of the LGM ice extent as mapped by Atwood and Mather (1932). If radiocarbon dating of the cores, and cosmogenic dating of the boulders, support the preliminary Holocene age of SML then the precise timing can be compared to local climate records. Thus, it may be possible to determine whether additional landslides are likely under modern climate regimes.