Landscape evolution and genesis of soils on dissected alluvial fans, western Mojave Desert, California

Soils profiles are records of soil genesis and landscape evolution. Four soils, a Typic Durorthid (Alko series), two Typic Haplargids (Neuralia and Garlock series) and a Typic Torripsamment (Cajon series), found in close association on a dissected alluvial fan in the western Mojave Desert, were studied to determine the landscape evolution and mode and timing of pedogenic events. Paleomagnetic data indicate that at least four aggradational events have occurred in the past 730,000 years. The sequence of alluvial units in the area is similar to other sequences observed in the Mojave Desert and eastern San Joaquin Valley. Based on stratigraphic interpretations, the upper calcareous laminar duripan (10 to 25 cm) in the Alko soil is probably not older than latest Pleistocene. Bioturbation of the upper soil mantle of the Alko and Neuralia soils brings calcareous soil and duripan fragments to the surface, thus creating a pedogenically rejuvenated geomorphic surface on which profile development has been retarded. The Garlock soil, with 5YR hues and a distinct carbonate-free argillic horizon, occurs on an older geomorphic surface, but younger deposit, than the Alko and Neuralia soils. The occurrence of calcareous Haplargids adjacent to Haplargids that are noncalcareous to a depth of 65 cm indicates that the atmospheric input of CaCO{dollar}sb3{dollar} in the study area has been minimal during the Holocene. The rate of CaCO{dollar}sb3{dollar} accumulation in soils of the study area is estimated to be 0.03 to 0.05 g/cm{dollar}sp2{dollar}/1000 yr over the past 200,000 years. Smectite is the major constituent of the clay fraction in the soils of the study area, whereas mica and kaolinite are less abundant. The lack of variation in the clay mineralogy of the soils in the study area may indicate that even during wetter climates of the Pleistocene, leaching has not removed excess silica, thus creating a stable environment for smectite. The micromorphology of pedogenic silica, carbonate and clay are described for duripans, duric horizons (Btqk) and underlying buried Bt horizons in the Alko, Neuralia and Garlock soils in an excavated trench to determine the sequence of pedogenic events. The polygenesis of soils in the study area is explained by fluctuations in climate, with development of argillic horizons during pluvial periods, and calcification and silicification during interpluvial periods.