Mapping of the Bull Lake and Pinedale Glaciations in the Lost River Range, Idaho, Based on Carbonate Coat Thicknesses of Geomorphic Features

We are studying the relationship between climate change and pulses of alluvial fan aggradation during the Late Pleistocene in the northern Basin and Range. The Lost River Range and Valley are separated by the active Lost River normal fault, site of the 1983 Borah Peak earthquake, magnitude Ms7.3. The western front of the Lost River Range in eastern Idaho is marked by glaciated and non-glaciated valleys that drain onto mostly relict alluvial fan surfaces. Both glaciated and non-glaciated drainage basins of comparable size appear to produce similar large alluvial fan deposits during the last (late Pleistocene?) fan-depositing episode. Gaining a better understanding of the relationship between glacial advances and fan aggradation requires determining the extent and timing of glacial advances in the region.

Carbonate coats on the underside of stones increase in thickness with stratigraphic age of fans and moraines, providing a tool for relative age determination and correlation. Ultimately, U/Th dating, generally of the innermost carbonate laminations will be carried out to provide numerical ages for coatings, thereby estimating the ages of landforms. Pinedale and Bull Lake(?) moraines have been identified, and carbonate coats have been sampled and measured, in the drainages of Willow Creek and Cedar Creek. Results from Willow Creek revealed mean thicknesses of 0.57 ± 0.31mm for the possible Pinedale moraine, Cedar Creek revealed mean coat thicknesses for the Pinedale and Bull Lake(?) moraines to be 1.56 ± 0.24 mm and 3.83 ± 1.99 mm respectively. A third moraine was identified at Cedar Creek suggesting a possible intermediate glacial advancement between Pinedale and Bull Lake. Despite geographic proximity, the higher elevation forested site at Willow Creek may account for the thinner Pinedale carbonate coat thicknesses at this site as compared to Cedar Creek.