ANALYSIS OF SEDIMENT CORES FROM A BOG ON THE SILVER MOUNTAIN LANDSLIDE, SOUTHEASTERN SAN JUAN MOUNTAINS, SOUTHERN COLORADO

The 6-km² Silver Mountain Landslide (SML; N 37.441°, W 106.412°, ~3100-3600 m a.s.l.) is located on the northeast side of Silver Mountain (3787 m a.s.l.) in the southeastern San Juan Mountains of south-central Colorado. As part of our research into the extent and timing of glaciation and landsliding in the southeastern San Juan Mountains, we began an investigation of the SML in 2012. Silver Mountain is thought to have been unglaciated during the Last Glacial Maximum, and thus the SML offers a potential contrast to post-glacial landsliding elsewhere in the region. We extracted four sediment cores (drives 0.35-0.75 m, recovery 0.25-0.52 m) from a bog on the landslide (N 37.4408°, W 106.4116°, 3395 m a.s.l.) and collected a sample from a felsic boulder for surface-exposure dating. The SML bog is teardrop-shaped (elongate in the east-west direction) and has an area of approximately 3700 m² (~115x45m). At the time of coring (late June 2012), open water was present near the western end of the bog. Cores SMB1 and SMB2 were extracted close to the center of the bog, while cores SMB3 and SMB4 were extracted in the eastern and southeastern portions, respectively, making SMB4 the core closest to the adjacent rising slope to the south. The uppermost ~0.1-0.3 m of the SML bog consists primarily of fibrous peaty organic material, with a transition to siltier, less organic-rich sediment below; the thickest peat layers are found in the cores farthest from the southerly slope. The sediment corer met refusal in gravel in all of the coring locations, suggesting relatively thin deposits overlying landslide debris. Magnetic susceptibility (MS) was low throughout the cores (-8.6 to 1.8 SI units), except in the bottom 3 cm of SMB4 (5.1 to 23 SI units). Radiocarbon dates from macro-organics near the bottom of the cores and a ¹⁰Be age for the boulder will provide chronological control for the landslide. If Silver Mountain did indeed remain ice-free during the most recent glaciation, the SML provides an opportunity to investigate landslide triggers other than paraglacial instability.