SEDIMENT RECORD FROM A BOG ON THE TRUJILLO MEADOWS LANDSLIDE (>10.6 KA), SOUTHEASTERN SAN JUAN MOUNTAINS, SOUTHERN COLORADO

As part of our research into the extent and timing of glaciation and landsliding in the southeastern San Juan Mountains of south-central Colorado, we began an investigation of the 1.75 km² Trujillo Meadows Landslide (TML; N 37.051°, W 106.446°, ~3050-3350 m a.s.l.) in 2011. The TML is located at the southern edge of the Red Lake Plateau (~3500-3600 m a.s.l.), which 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 valley at the modern spillway of the Trujillo Meadows Reservoir dam. We extracted a 3.07-m sediment core (to 4.2 m depth) from a bog located in the southern part of the TML, ~300 m from the approximate location of the toe of the landslide. The bog is elongate northwest-southeast (~100 m by ~50 m), with an area of ~2500 m². The uppermost 3 m of the TML 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. Magnetic susceptibility (MS) was low throughout the core (-8.8-0 SI units), except in the bottom 6 cm (0.8-39.3 SI units); the highest MS reading came from the basal gravel layer. Calibrated radiocarbon dates of 10,658 ± 49, 10,596 ± 57, and 9,410 ± 65 yrs BP from macro-organics near the bottom of the core suggest that the landslide occurred no later than the early Holocene, which was reportedly a time of enhanced monsoonal activity in the region. Ongoing work on the TML includes further analysis of the core, surface-exposure dating of boulders, and analysis of soil samples from three soil pits. Recent landslides in the area, along with the continuing construction of homes in the Rio de los Piños 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.