Paper No. 11
Presentation Time: 11:00 AM
Soil Formation in the Mojave Desert: New TCN Information Suggests Rates of Soil Formation May Exceed Previously Established Rates
New dating techniques are advancing knowledge regarding rates of landscape evolution and soil formation. We present results that have important implications for the use of soils in Quaternary geology of deserts. First, new in-situ terrestrial cosmogenic geochronology indicates that the ages of regionally correlative Pleistocene alluvial fan surfaces and associated soils, may be considerably younger than previously thought. Soils were examined on five alluvial fan surfaces Qf6 (~4ka), Qf5 (~8-12ka), Qf4 (~20-40ka), Qf3 (~60-80ka) and Qf2 (~120-150ka). Previous research in the Providence Mountains (CA) and soil stratigraphic correlations to nearby well studied areas suggested that the Qf4 fan was ~75ka, the Qf3 fan ~150ka, and the Qf2 fan ~500-700ka. The new geochronology for the Providence Mountain alluvial fans is consistent with recent geochronology for alluvial surfaces across other areas of the SW US. Second, results allow direct comparison of rates of soil formation among four lithologically distinct chronosequences: quartz monzonite (QM), mixed plutonic (PM), mixed volcanic (VX), and limestone/marble (LS). Soils on the VX and LS sequences have considerably faster rates of pavement formation and slower rates of dust accumulation relative to soils on the QM and PM sequences. Soil Bt horizons are strongest (texture, rubification) on the PM and QM sequences and first form on Qf5 and Qf4 fan deposits, with first appearance of weak Bt horizons on the Qf3 VX surfaces. Soils formed in the LS fan deposits lack Bt horizons. In contrast, the development of Bk and Bkm horizons is faster and stronger in the LS and VX soils relative to the PM and QM soils due to the abundance of limestone detritus in the VX and LS deposits. Results will enhance use of soil stratigraphy in deciphering the Quaternary tectonic and climatic record contained in alluvial fan deposits that are common across desert regions.