WHAT CLIMATIC FACTORS CONTROL INTERVALS OF DEPOSITION AND STABILITY ON ALLUVIAL FANS?

Studies indicate that large, low-gradient alluvial fans in the Lost River, Lemhi, and Beaverhead Ranges of southeastern Idaho and western Montana are largely relict surfaces that have developed on the actively downdropping faults of these ranges. While extensive older fan deposits show evidence of large sheetflooding events and other streamflow-dominated processes, little historic deposition has occurred on these fans. If intervals of deposition on Lost River fans are controlled by greater snowmelt-driven stream discharge and increased weathering due to frost action during glacial intervals (the ‘Northern Rockies' model), deposition should correspond with late Quaternary glacial maxima (MIS 2, 4, and 6). If intervals of deposition on Lost River fans are controlled by increased availability of sediment due to post-glacial reduction in vegetation (the southwestern ‘Bull hypotheses'), deposition should follow late Quaternary glacial maxima. Historically, interpreting the record of watershed-scale responses to climatic change preserved in alluvial fans has been restricted by lack of adequate numerical age control. In particular, despite widespread assumptions that Quaternary intervals of fan aggradation and incision are linked to changes in Quaternary climate causality between climate change and geomorphic response has not been clearly established. Optically Stimulated Luminescence (OSL) dating of fan sediments, and ²³⁰ Th/U dating of pedogenic carbonate coats will provide ages for both intervals of deposition (OSL) and surface abandonment and stabilization ( ²³⁰ Th/U). We focus on assessing the role of climate on fan development during and after the penultimate glaciation (~180 ka). Preliminary results from the proposed study area indicate carbonate coats are geochemically suitable for precise ²³⁰ Th/U dating. U-series analysis from pedogenic carbonate coats from a fan on the southern (Arco) segment of the Lost River fault provide an age of 178±8 ka for the oldest fan paleo-surface. This suggests an interval of surface stability, likely due to incision and fan surface abandonment in early MIS 6. At this same site, an age of 69±6 ka on a younger incised and faulted fan surface likely represents active alluviation at the beginning of MIS 4.