Soils on igneous and metavolcanic rocks in the Sonoran Desert of Baja California, Mexico

Soils were examined on the widespread volcanic, plutonic and metavolcanic rocks within the Sonoran Desert of Baja California. In order to focus on in situ weathering and pedogenesis, most of the soils studied were on stable, nearly level summits. The combination of summit landscape positions and the aridic moisture regime tended to yield shallow soils, yet these soils display features of strong morphologic development, including eluvial horizons, subsoil clay increases, strong blocky and prismatic structure, reddish hues, and illuviation and stress argillans. Soils derived from volcanic materials are Paleargids with clayey natric horizons, saline subsoils, and petrocalcic horizons. Soils on granodiorite and metavolcanic rocks are loamy Haplargids that are neither saline nor sodic and are entirely noncalcareous. The granodiorite-derived soils had pH values as low as 4.0, apparently the result of metal sulfide weathering in the hydrothermally altered, ore-bearing rock. Torripsamments have formed in the reworked mantle of grus on an extensive tonalite pediment. Despite the variety of parent materials, smectite and kaolin are ubiquitous in the clay fraction of the soils. The smectites are dioctahedral and are neoformed and/or transformed from biotite or chlorite, depending on the parent material. The kaolin is l largely the result of feldspar weathering. A Paleargid on rhyolite contained abundant palygorskite in the clay and silt fractions, particularly in association with the petrocalcic horizon. In all of the soils, oxalate-extractable Si plus Al was <1% of the fine earth fraction. Most of the soils contained 2–3% dithionite-extractable Fe, but Fed values were as high as 11% in the soil on metavolcanic rock. Goethite and hematite are the dominant Fe oxides, judging from low Feo/Fed ratios and 1OYR-2.5YR soil hues. Only the soils on basalt and rhyolite were calcareous, but the amount of CaCO3 in the soils was unrelated to the Ca content of the rock, since these two rocks contained the most and the least amounts of Ca, respectively, of the five rock types represented in the study. Apparently, CaC03 accumulation in the soils is more dependent on dust contributions, possibly from nearby calcareous plays and fluvial sediments, than on in situ mineral weathering.