ANALYSIS OF SEDIMENT CORES FROM THE ELK MEADOWS LANDSLIDE AND THE SILVER LAKES LANDSLIDE: WHAT SEDIMENT CORES TELL US ABOUT THE TIMING AND ENVIRONMENTAL CONDITIONS OF LARGE LANDSLIDES IN THE SOUTHEASTERN SAN JUAN MOUNTAINS

The Silver Lakes Landslide (SLL) and the Elk Meadows Landslide (EML) are located along the Alamosa and Conejos Rivers respectively in the southeastern San Juan Mountains of south-central Colorado. Silver Lakes Landslide (SLL), an estimated 6.1 km², flows northeast down the flank of Red Mountain. Similarly, the 4.5 km² Elk Meadows Landslide (EML) flows north off of the Red Lake Plateau. Radiocarbon dating on the nearby Trujillo Meadows Landslide (TML), suggests that landslides in the region occurred as paraglacial events after LGM deglaciation. This study aims to determine if SSL and EML occurred at roughly the same time as TML and therefore would have had similar paraglacial triggers. To test the age of the landslides, we have used a three prong approach including: 1) coring bogs formed on the surface of the landslides, 2) determining the relative age of soils on the surfaces of the landslides, and 3) using cosmogenic dating on boulders on the surface of the landslides. Here we focus on recoverable data from the cores taken from bogs on SLL and EML using a square piston rod Livingstone corer. Both bogs were relatively dry, with little to no standing water, and grass was removed from the surface to aid in coring. We recovered ~2.5 m of core on the SLL from drives extending 4.5 m below the surface while on the EML we recovered 74 cm of core from drives that extended ~3.5m below the surface. Sediments within the cores lack any sand-sized grains and are generally composed of silt with varying amounts of clay. Increasing Munsell chroma and value with depth indicate a general decrease in organic content. Nonetheless, abundant organic material throughout likely will provide C14 age control for the cores. Soil pits on SLL and EML contained large amounts of angular rocks consistent with the landslide deposit interpretation. Continuing to research these landslides is important as we further our understanding of the cause of the massive slope failures. This is especially true on SLL, where the majority of the landslide deposits are now private property containing vacation homes. Proving that the landslides were caused by the paraglacial instability of deglaciation may offer confirmation that landslides of this sort occur about as often as glaciation and are not a current threat to land owners.