OSL CHRONOLOGY FOR ALLUVIAL FANS OF THE LOST RIVER RANGE, IDAHO: LARGE-SCALE DEPOSITION DURING OIS 3 AND 4

The western side of the Lost River Range (LRR) of east-central Idaho is characterized by numerous sheetflood-dominated, Quaternary alluvial fans that extend ~2-6 km from the range front. While the active Lost River Fault has created and maintained the uplifted block and accommodation space required for fan formation, deposition and incision of these fans has occurred on shorter timescales (10-10⁴ yrs). Since LRR fans are largely inactive in modern times, past environmental conditions must have greatly enhanced movement and deposition of sediment from contributing basins onto fan surfaces. Optically stimulated luminescence (OSL) dating has been applied to determine the timing of major episodes of deposition on five of the alluvial fans and deposit characteristics such as grain size and slope have been described to provide insight into links between climate and geomorphic response. Results indicate that ~88 % of fan area was deposited during the late Pleistocene (last ~70 ka) and only ~12% of fan area was deposited ~4-10 ka. Clast sizes and average slope of late Pleistocene fan surfaces are greater than Holocene surfaces, indicating greater stream power during the late Pleistocene. While OSL results highlight a major episode of deposition on the majority of LRR alluvial fans during the glacial climate of OIS 2 (dated deposits ~12-25 ka; ~50% total fan area), results also show significant episodes of deposition during OIS 3 and OIS 4 (dated deposits ~26-65 ka ~40% total fan area). These results suggest that while the glacial climate of OIS 2 resulted in widespread deposition on LRR fans, climate conditions during OIS 3 and 4 also favored enhanced deposition in comparison to the Holocene. Climate during OIS 3 and 4 is not as well understood, but records available within the region of the LRR and greater western U.S. indicate glacial advance during OIS 4 and cooler, more variable climate during both OIS 4 and OIS 3. The timing of fan aggradation in the LRR suggests that while the glacial climate of OIS 2 was an important time period for fan building, the cooler climate throughout OIS 3 and 4 also lead to a significant geomorphic response in this region.