Using OSL to Assess the Role of Climate and Glaciation in Sediment Delivery to Alluvial Fans of the Lost River Range, Idaho

In the Lost River Range of east-central Idaho, numerous large (>5 km radius), sheetflood-dominated alluvial fans extend from the mountain front. During the late-Pleistocene, glacial extent varied from 80% in the north, to no evidence of glaciation in the southern catchments. These low gradient (2-3°) fans are relict features of Quaternary climates, as modern deposition is generally restricted to inset, debris-flow dominated fans. Post-fan offsets of the Lost River fault are generally small (2-3 m) and have minimal impact on overall fan gradient. Past research suggests that climate and glaciation are major controls on the timing of deposition and incision on alluvial fans. However, indequate age control has made it difficult to test this idea in many settings.

For this study, we have used OSL to date the timing of deposition on alluvial fans in the LRR. Numerous surfaces from both glaciated and unglaciated basins have been sampled to assess the role of climate and glaciation on fan deposition. Complimentary dating of fan surface abandonment and stabilization will be obtained through U-series dating of pedogenic carbonate coats. U-series dating will also be applied to carbonate coats from inferred Pinedale and Bull Lake moraine surfaces.

Multiple geomorphic surfaces have been identified on the five fans included in this study. Preliminary OSL ages have been obtained from the Upper Cedar Creek Fan in the central portion of the LRR (10% glaciated). These results indicate that one of the higher Pleistocene surfaces is ~50 ka, and the lowest non-active surface is ~3-4 ka. Further age control will be obtained to determine the timing of sediment delivery to LRR fans and its relationship to regional climate and catchment glaciation.