Pedogenesis and mineral weathering in serpentinitic landslide terrain, Klamath Mountains, California

Due to their geomorphic and hydrologic characteristics, landslide deposits provide an opportunity to observe pedogenic processes and mineral weathering across differing moisture conditions and depositional environments. The objectives were to (1) determine how pedogenic processes and geochemistry vary on differing landscape positions, and (2) investigate mineral weathering and the formation of secondary phyllosilicates in serpentinitic soils. The landscape positions of the landslide were defined as the scarp and north and south flanks, located above the landslide bench, and the foot and bulge, which make up the landslide bench. Several cycles of pedogenesis and landslide events have occurred, as evident by stratigraphic variation in rock fragment content and buried soils in the landslide bench. The scarp and flank positions are subject to episodes of surficial erosion, thereby delivering sediments to the lower landscape positions. The major pedogenic processes on the slopes of the scarp and flanks include mineral weathering and illuviation of clay, resulting in soils with a thick argillic horizon with an abundance of Al and Cr associated with secondary Fe-oxides. Mineral weathering products from the scarp and flanks accumulate in the poorly drained area behind the bulge where they participate in the synthesis of clay minerals. The wet area has relatively high C contents and low metal concentrations due to reducing conditions. Translocation of Ni and Mn through the wet area results in a concentration of these elements as hydrous oxides in the solum of the bulge. Abundance of exchangeable cations throughout surface soils ranked in the following order: Mg>Ca>>K>Mn>Na>Ni. Ca:Mg ratios range from 0.13 to 3.77 (mean 0.43), with the highest ratios found in non-serpentinitic colluvium. Exchangeable cations are concentrated in the wetland relative to surrounding terrain due to poor-drainage conditions.;Serpentine and chlorite are the major primary minerals within these soils. Serpentine weathers incongruently to Mg and Si ions that are abundant in soil solutions. Weathering products concentrate in lower horizons or downslope in poorly drained areas where they precipitate as smectite. Chlorite transforms to vermiculite by loss of the hydroxy-interlayer to form interstratified chlorite/vermiculite. The vermiculite then alters to a high charge smectite, possibly through oxidation of Fe2+ within the octahedral sheet.