ANALYSIS OF A SEDIMENT CORE FROM A LARGE LANDSLIDE IN THE SOUTHEASTERN SAN JUAN MOUNTAINS, SOUTHERN COLORADO

We present initial results from our analysis of a sediment core obtained from the 1.75 km² Trujillo Meadows Landslide (TML; N 37.051°, W 106.446°, ~3050-3350 m a.s.l.) at the southern edge of the Red Lake Plateau (~3500-3600 m a.s.l.) in the southeastern San Juan Mountains of south-central Colorado. The Plateau is bounded by valleys glaciated during the Last Glacial Maximum, including Río de los Piños valley. The TML occurred in the headwaters of the Rio de los Piños and formed a natural dam (now breached) across the entire width of the valley at the modern spillway of the Trujillo Meadows Reservoir dam. As part of ongoing research into the extent and timing of glaciation in the Red Lake Plateau area, we began an investigation of the TML in 2011. Mapping of the landslide identified areas dominated by large clasts (2-5 m in diameter) and other areas with relatively small clasts (<2 m), as well as a primitive, now abandoned drainage system for the landslide. We dug three soil pits on the landslide surface and examined soil profiles, collected the first of a suite of samples for surface-exposure dating of boulders on the landslide, and extracted a 3.07-m sediment core (to 4.2 m depth) from a bog on the landslide. Soil profiles displayed development (A-AB-B horizonation, 10 YR 4/4 to 3/2 coloration, moderately developed A horizons) similar to colluvial deposits in the nearby Conejos Valley that have been dated to the Pleistocene-Holocene transition. Observations during coring indicate that the uppermost 3 m of the bog consist primarily of peaty organic material, with a transition to siltier, less organic-rich sediment between 3 and 4 m depth. The sediment corer met refusal in angular gravel at a depth of 4.18 m, indicating relatively thick deposits for a closed basin. Taken together, the extent of soil development and the >4.2-m depth of the bog suggest that the TML occurred soon after deglaciation of the Río de los Piños Valley. Radiocarbon dating of the core and ¹⁰Be dating of boulder surfaces will greatly improve our understanding of the history of the TML and the Rio de los Piños Valley. Recent landslides in the area, along with the ongoing construction of homes in the valley, make it important that we understand whether a landslide this large could be a modern reality or if it was simply a product of paraglacial instability during deglaciation.