FINGERPRINTING SEDIMENT SOURCES AND EOLIAN PROCESSES IN THE FORMATION OF DESERT AV HORIZONS

Av horizons form beneath desert pavements from the accumulation of eolian sediments thought to be largely derived from playas. Particle size distributions of Av horizon sediments are typically bimodal, with modes in clay or fine silt and coarse silt or sand. The poorly sorted and bimodal character of Av horizons suggests the sediments are derived from multiple sources. We hypothesize that the coarser sand modes of Av horizons are derived from local sources by saltation processes whereas finer silts and clays are derived from a mix of distal sources by suspension fallout. Av horizons and potential source sediments (washes, distal alluvial fans, sand dunes, playas) were collected on and upwind of alluvial fans from the Providence Mts. in the Mojave National Preserve, CA and from the Yuma Proving Ground (YPG) in AZ. Alluvial fans were chosen with different parent materials (limestone vs. quartz monzonite in the Providence Mts., and gneiss vs. rhyolite at YPG) so that local sources such as washes that drain the fans would reflect one source area and be easily distinguishable from each other. Playas and distal alluvial fans should reflect a mixing pot of source areas. Bulk chemical analyses of Av horizons may mask geochemical signatures from diverse sources. To better fingerprint source areas and sediment sizes diagnostic of eolian transport processes, Av horizons and source sediments were separated into fine sand (125 to 250 µm) and fine silt (2 to 32 µm) and analyzed by x-ray fluorescence. Preliminary major and trace element data show that sand components of Av horizons are dominated by local sources, whereas the finer components likely represent a mix of distally derived sources. The fingerprinting of multiple sources for Av horizons suggests that saltation and suspension fallout sediment both contribute to their formation, with the bulk of Av horizon material derived from suspension fallout. While lacustrine and fluvial sources appear to be dominant dust producers near the Providence Mts., YPG lacks lacustrine sediments, suggesting dominant dust sources may be linked to fluvial deposits instead. Continued study of dust provenance will identify geomorphic units that are responsible for dust generation and Av horizon development.