STRATIGRAPHY AND GENESIS OF DURORTHIDS AND HAPLARGIDS ON DISSECTED ALLUVIAL FANS, WESTERN MOJAVE DESERT, CALIFORNIA

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 carbonate s and opaline silica are indicators of geomorphic surfaces of mid-Plei stocene age. To test these hypotheses we studied the stratigraphy and genesis of four soils, a Typic Durorthid (Alko series), two Typic Hapl argids (Neuralia and Garlock series and a Typic Torripsamment (Cajon s eries), found in close association on a dissected alluvial fan in the western Mojave Desert. The study area has a rolling topography with th e calcareous Alko soil occurring on narrow summits and on the edges of broad summits, Neuralia (calcareous) on broad summits and on sideslop es, Garlock (noncalcareous) on sideslopes, and Cajon in the drainage w ays. Soil stratigraphy in an excavated trench indicates that these soi ls are forming on deposits of differing ages. Paleomagnetic data indic ate 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 car bonate-free argillic horizon, occurs on an older geomorphic surface, b ut younger deposit, than the Alko and Neuralia soils. Rodents are acti ve in the profiles of Alko and Neuralia soils. Bioturbation of the upp er soil mantle brings calcareous soil and duripan fragments to the sur face, thus creating a pedogenically rejuvenated geomorphic surface on which profile development has been retarded. The occurrence of calcare ous Haplargids adjacent to Haplargids that are noncalcareous to a dept h of 65 cm, indicates that the atmospheric input of CaCO3 in the study area has been minimal during the Holocene. The rate of CaCO3 accumula tion 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 t he upper Alko duripan during the Holocene. This amount could easily be added to the surface by bioturbation from calcareous duripan fragment s in the profile. Thus, much of the laminar part of the duripan is pro bably a Holocene phenomenon.