NEW CONSTRAINTS ON HOLOCENE GLACIATION IN THE SAWTOOTH MOUNTAINS, IDAHO

Multiple small, active glacigenic rock glaciers and older moraines occupy northerly facing cirques below Mt. Cramer and Elk Peak in the headwaters of the Redfish Lake Creek drainage, central Idaho. Detailed field mapping and relative age criteria, including vegetation and soil development, moraine crest form (e.g. concave or convex), and moraine frontal slope angle indicate at least two ages of cirque advances, possibly more. Rock glaciers below Mt. Cramer currently produce meltwater that flows directly into the paternoster Cramer Lakes via a small un-named stream. The modern meltwater stream carries suspended sediment (rock flour), attesting to the continued movement of the rock glaciers and their link to “clean” glaciers. The rock flour also provides a means to constrain records of past, basin-wide fluctuations in glacier extent in sediments deposited in the tarns downstream. Because the local lake drainages are dominated by heavily glaciated crystalline bedrock or thin soils, glacial meltwater provides the largest potential source of fine-grained clastic sediment to the lakes. Furthermore, other sources of clastic sediments (e.g., rock fall, avalanche, mass wasting) would likely supply coarser sediments to the lakes.

To evaluate the Holocene rock-flour record of the Sawtooth Mountains, we collected 18 individual sediment cores using a modified Livingston corer, from Upper Cramer Lake (1), Middle Cramer Lake (8), and the Cramer Pass lakes (9). Magnetic susceptibility analysis of the longest Middle Cramer Lakes core yields high susceptibility values in the first few centimeters of brown/gray gyttja, potentially correlating to recent LIA activity of the modern rock glaciers in the headwaters. Stranded lateral moraines and continued thinning of the modern rock glaciers attest to past climate variability, complimenting observed magnetic susceptibility values. Mazama (~6850 yr) and Mt. St. Helen Ye (~3400 yr) tephras have been identified by electron microprobe analysis and are present in most cores. Preliminary results suggest sedimentation rates and events, constrained by core tephras and AMS radiocarbon dates, may reflect variations in glacier activity upstream.