Quaternary climate change and hillslope processes: What can we learn from alluvial fans?

Examining the timing of sediment deposition on alluvial fans may clarify relationships among Quaternary changes in climate, sediment production, and sediment removal from uplifted mountain blocks. Deposition on fans indicates that (1) ample sediment is available for transport within contributing basins and (2) that stream power is adequate to move that sediment to the fan environment. Dating alluvial fan deposition clarifies relationships among climatically controlled factors (e.g. precipitation, vegetation, temperature), and hydrologic and geomorphic responses (e.g. weathering rates, frost action, glaciation, stream power) that influence landscape evolution. Numerous 2-5 km radius, low gradient alluvial fans head along the western side of the Lost River Range (LRR) in east-central Idaho. Timing of deposition on these fans is based on optically stimulated luminescence dating (OSL). In addition we described general deposit characteristics and mapped different aged fan surfaces to explore how fan deposition has changed over time. OSL results indicate that evacuation of sediment from contributing basins and deposition on fans was enhanced ~10-14 ka and ~40-50 ka. The younger episode is more robust in this record, with deposition recorded on all five studied fans despite differences in Quaternary glacial extent in contributing basins that varied from ~0-80%. Glacial chronologies from the nearby Sawtooth Range (Thackray, 2008) and Yellowstone-Teton region (Licciardi and Pierce, 2008; Gosse et al, 1995) suggest that this time period may have coincided with and followed the last glacial maxima in the northern Rocky Mountains. Deposition during the ~40-50 ka episode is recorded on the two largest studied fans, both with <10% glaciation in basin areas, as well as a ~40 m terrace of the East Fork Big Lost River that drains the Pioneer Range west of the LRR. A ~60-65 ka moraine in the northern LRR dated by U-series on pedogenic carbonate, an extensive glacio-fluvial terrace in the Wind River Basin (~200 km east of the LRR) dated to >55 ± 8.6 ka (Sharp et al, 2003), and substantial loess accumulation near the Teton Range at ~46-54 ka (Pierce et al, personal comm.) suggest that this episode of fan deposition was late- to post-glacial with respect to the MIS 4 glaciation and associated climatic conditions. Deposition on alluvial fans generally results from (1) increases in the sediment supply and/or (2) changes in stream power that alter the relative balance between sediment supply and stream power. In this region, sediment supply could increase following glacial conditions if reduced effective moisture decreases hillslope vegetation cover, freeing accumulated regolith for transport to fans, or greater temperature fluctuations increase the effectiveness of frost weathering. Stream power following a glacial maxima may have decreased in response to effectively drier climate, but may still have been adequate to transport sediment to fans. It’s also possible that stream power increased following glacial maxima with greater frequency of stochastic events such as rain on snow.