SEARCHING FOR LANDSLIDE TRIGGERS: EARLY HOLOCENE TRUJILLO MEADOWS LANDSLIDE IN THE SOUTHEASTERN SAN JUAN MOUNTAINS, SOUTHERN COLORADO

The 1.75 km² Trujillo Meadows Landslide (TML; N 37.051°, W 106.446°, ~3050-3350 m a.s.l.) is one of four rotational landslides ranging in age from ~14 ka to ~2.5 ka that we are investigating in the southeastern San Juan Mountains of south-central Colorado. 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. In 2011, 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. 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. 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. The core was sub-sampled at 2-cm intervals and samples were analyzed for grain size and organic material. 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. The >12-kyr range of landslide ages in our study suggests that a modern landslide of similar proportions cannot be ruled out.