MAGNITUDE OF HOLOCENE GLACIATION IN THE SAWTOOTH MOUNTAINS, CENTRAL IDAHO

Constraints on Holocene glaciation in Idaho are limited, with a tentative correlation to the Wind River model based on poorly constrained morphologic evidence of high cirque deposits. To better define and constrain Holocene advances in Idaho, we look to glacigenic rock glacier deposits in the headwaters of the Redfish Lake Creek drainage, Sawtooth Mountains, central Idaho. The northerly facing cirque below Mt. Cramer contains both extensive glacigenic rock glacier deposits and numerous downstream tarns that record past rock glacier activity in lacustrine sediments. Detailed field mapping indicates two distinct cirque advances informally named the Elk Peak (younger) and Cramer Lakes (older) advances. Regional mapping indicates these two advances occur throughout the Sawtooth Mountains, extending < 1 km from cirque headwalls. The Cramer Lakes advance is consistently larger, covering areas ~2x greater and lengths ~1.5x greater than those of the younger Elk Peak advance. Sedimentation related to cirque glacier activity is identified in two cores from the Upper Cramer Lake (UCL06 and Glew-1) by a multi-proxy approach utilizing magnetic susceptibility (MS), loss on ignition (LOI), and grain size analysis of core sediments. Six macrofossils analyzed by ¹⁴C AMS provide an age model for UCL06, while five Glew-1 ¹⁴C dates are forthcoming. A distinctive increase in clastic silts in the lower quarter of UCL06 documents an interval of glacial expansion between ~12,900-12,200 cal yr B.P. This advance correlates to the early stages of the European Younger Dryas advance (12,900 – 11,500 cal yr. B.P.) and is broadly correlative to the Titcomb advance in the Wind River Range, (Gosse et al., 1995) and the Satanta Peak advance in the Rocky Mountains (Menounos and Reasoner, 1997). Preliminary results from the Glew-1 core suggest Neoglaciation began ~3,000 cal yr B.P. based on trends in organics. Rapid reduction in organics and an increase in MS values in the near-surface sediments, overlain by a rapid increase in organics at the core top, appear to represent late-Holocene growth and historic decay of the Elk Peak rock glaciers in the headwaters. Together, the moraine and lake sediment records indicate the Cramer Lakes advance represents a late-glacial event during the Younger Dryas chronozone, whereas the Elk Peak advance encompasses Neoglaciation, culminating late in the Little Ice Age.