OSL AGE ESTIMATES FOR ALLUVIAL FAN DEPOSITION IN THE LOST RIVER RANGE, ID: UNDERSTANDING LINKS BETWEEN CLIMATE CHANGE AND HILLSLOPE PROCESSES

Recent advances in luminescence dating methods have greatly improved age estimates for depositional timing of fluvial sediments (Rittenour, 2008), also improving the ability to make inferences about links between changes in climate driven processes (e.g. mean temperature, precipitation, storm frequency and intensity, and vegetation ) and the resulting geomorphic response (e.g. weathering, sediment production and transport capacity). Alluvial fans provide excellent records of these geomorphic responses and processes. For example, the aggradation and incision cycles that characterize many Quaternary fans may be linked to changes in climate that alter the balance between sediment supply and transport capacity. To examine this hypothesis, optically stimulated luminescence (OSL) dating has been used to determine the timing of sediment deposition on five alluvial fans issuing from the western catchments of the Lost River Range in east-central Idaho. These fans are all low gradient (~3-4º; Patterson, 2006), sheetflood-dominated fans extending 2-5 km from the limestone-dominated mountain front. Offsets on the Lost River fault are generally small (~2-3 m) and thus unlikely to cause deposition or incision over the entire length of such large fans. Quaternary glacial extent within contributing basins varies from ~80% in the north to none in the south, allowing this study to also examine the direct effects of glaciation on sediment delivery to fans versus the role of glacial-interglacial changes in climate. Approximately 22 preliminary OSL ages, ranging from ~3-95 ka, have been obtained from fans included in this study, and one age from a flood deposit on a Lost River terrace. Comparison of these preliminary ages with global and regional climate records indicates that removal of material from hillslopes and deposition on fans has been enhanced by climatic conditions of glacial intervals during MIS 2 and possibly 4. Pierce and Scott (1982) hypothesized that later and more rapid melting of deeper winter snowpacks greatly increased sediment transport capacity within basins, resulting in increased rates of sediment deposition on the fans. Final ages from this study will aid in refinement of conceptual models concerning the role of climate change in transport of material from hillslopes and landscape evolution.