Genetic and stability relationships of four Western Cascade soils

A sequence of soils derived from tuffaceous rocks was sampled on the H.J. Andrews Experimental Forest and adjacent U.S. Plywood land to determine some genetic and stability relationships. The less than two micron size fraction from each sample was characterized by x-ray diffraction with respect to crystalline clay minerals. Electron micrographs were made of selected clay samples to determine the morphology of kaolin minerals. Standard chemical and physical analyses were performed. The coarse-silt mineralogy of each sample was characterized by a randomized count of 300 grains. Data from these procedures showed that kaolin minerals studied were not well crystallized. Mixtures of kaolinite and halloysite resulted in broad rather diffuse, moderately intense diffraction maxima with some degree of asymmetry. These diffraction maxima were characterized by a morphological sequence of kaolin minerals from poorly crystallized platy forms of kaolinite to platy forms with rolled edges and eventually to fairly well crystallized tubular forms of halloysite. The major part of the sand and silt fractions consisted of pseudomorphs of clay that were chemically reactive in cation exchange reactions and moisture retention. The number of pseudomorphs of clay decreased and measured clay increased upward in the soil profile from the C or R horizons into the B horizons. Pseudomorphs of montmorillonite were more resistant to disruption than pseudomorphs of kaolin. Chemical weathering resulted in alteration of plagioclase and ferromagn.esian minerals to heulandite-clinoptilolite and ultimately to pseudomorphs of montmorillonite. Physical and biological disruptive forces reduced these pseudomorphs to discrete montmorillonite. Intergrade minerals formed in well drained weathered horizons as a result of hydroxy interlayering of smectites. Alternate wetting and drying coupled with more intense weathering resulted in the transformation of halloysite to kaolinite upward in the soil. Slipout and Budworm soils had clay fractions that were mainly expanding type clays and buff tuff and McKenzie River soils had clay fractions that were mainly non-expanding clays. As a result, Slipout and Budworm soils were more prone to slope failure than buff tuff and McKenzie River soils. Free iron oxide content of all four soils increased in the same order as relative slope stability. There was no apparent relationship between stability and exchangeable ions. More of the total clay occurred as silt and sand-size pseudomorphs of clay in McKenzie River and buff tuff soils than in Slipout and Budworm soils. These pseudomorphs functioned as individual primary particles increasing the solid-to-solid contact in the soil framework causing McKenzie River and buff tuff soils to have additional resistance to shear failure.