Drainage Basin Influences on Alluvial Fan Processes in the Lost River Range, Idaho

The Lost River Range in central Idaho is characterized by many large (radius > 5 km) low angle (2-3°) alluvial fans developed across and beyond the ~140 km active Lost River normal fault (Crone et al., 1987). Stratigraphic exposures of fan sediments reveal these large fans are sheetflood-dominated (Patterson, 2006). Preliminary field observations indicate ~20-30 cm thick couplets of alternating ~5-15 cm and ~2-5 cm sub-angular, imbricated limestone-dominated clasts. Extensive sheetflood deposition on fan surfaces, however, is not occurring in modern times, and these fans are largely relict features. Optically Stimulated Luminescence (OSL) dating of fan deposits and U-series dating of pedogenic carbonates on fan surfaces is under way: preliminary and previous work suggests major intervals of sheetflood deposition ~15-180 ka. More recent debris-flow deposition on small-radius (<1 km), steep (8-17°), fans is confined to the mountain front (Patterson, 2006; Pierce and Scott, 1982).

A fundamental question concerning alluvial fan evolution is determining what factors control depositional processes (e.g. sheetfloods or debris flows) on fan surfaces. While basin characteristics and fault activity (e.g. basin area and ruggedness) play a large role in fan development, preliminary observations suggest that Quaternary climate change punctuates the timing of depositional process through controls on sediment production, storage, and transport. We investigate the influence of basin characteristics on depositional processes in the Lost River Range through stratigraphic analysis of fan sediments and quantification of basin and fan characteristics including drainage basin area, fan slope, basin ruggedness, and sediment volumes. Additionally, relict surfaces in the area may be used to estimate the amount of sediment made available for subsequent fan formation by calculating the volume of material removed during incision and dissection. The results will be calibrated with ongoing geochronological studies (OSL and U-series) to assess how depositional processes have changed over time.