Stratigraphy and genesis of Durorthids and Haplargids on dissected alluvial fans, western Mojave Desert, California

Abstract

Presumed slow weathering rates in arid-region soils have given rise to the hypotheses that (1) the major carbonate source in most Aridisols is from eolian deposition, and (2) soil horizons cemented by carbonates and opaline silica are indicators of geomorphic surfaces of mid-Pleistocene age. To test these hypotheses we studied the stratigraphy and genesis of 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. The study area has a rolling topography with the calcareous Alko soil occurring on narrow summits and on the edges of broad summits, Neuralia (calcareous) on broad summits and on sideslopes, Garlock (noncalcareous) on sideslopes, and Cajon in the drainage ways. Soil stratigraphy in an excavated trench indicates that these soils are forming on deposits of differing ages. Paleomagnetic data indicate that at least four aggradational events have occurred in the past 783 ka (thousand years). The sequence of alluvial units in the area is similar to other sequences observed in the Mojave Desert and eastern San Joaquin Valley. 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. Rodents are active in the profiles of Alko and Neuralia soils. Bioturbation of the upper soil mantle brings calcareous soil and duripan fragments to the surface, thus creating a pedogenically rejuvenated geomorphic surface on which profile development has been retarded. The occurrence of calcareous Haplargids adjacent to Haplargids that are noncalcareous to a depth of 65 cm, indicates that the atmospheric input of CaCO 3 in the study area has been minimal during the Holocene. The rate of CaCO 3 accumulation in soils of the study area is estimated to be 0.03 to 0.05 g cm −2 1000 yr −1 over the past 200 ka. A rate of accumulation of 0.27 g cm −2 1000 yr −1 is required to form the 2-cm-thick laminar cap of the upper Alko duripan during the Holocene. This amount could easily be added to the surface by bioturbation from calcareous duripan fragments in the profile. Thus, much of the laminar part of the duripan is probably a Holocene phenomenon.