Characteristics and Genesis of Cascade and Powell Soils of Northwestern Oregon

Abstract

Abstract Soils of the Cascade and Powell series of northwestern Oregon were studied to establish the identity of their parent materials, their mode of formation, and some of their physical and chemical properties. Conclusions were based upon integrated mechanical analysis, exchangeable cation, and mineralogical data coupled with field observation. Common to each of the soils is one or more profile discontinuities marking zones of relatively recent deposition over pre‐existing soil horizons. A similar and relatively high proportion of unweathered feldspar, hornblende, glass, and other such weatherable minerals is present in the sand fractions of the upper zones of each profile. Within the clay fractions, interstratified vermiculite‐chlorite is a dominant constituent of these upper horizons and the ratio of expandable layer silicates to chlorite and the concentration of kaolin increases with depth. In both soils, relative concentrations of these clay minerals change abruptly between adjacent horizons. The abrupt differences in mineral concentrations are usually accompanied by a notable shift in particle size distribution and, in one case, by a marked difference in content of free iron oxide. Each of the profiles contains a considerable amount of relatively unstable alumino‐silicate clay material. As much as 20% of the clay fraction of some horizons is dissolved as SiO 2 and Al 2 O 3 by digestion in 0.5 N NaOH. Absence of gibbsite and lack of wide variation in ratio of dissolved SiO 2 to dissolved Al 2 O 3 indicates the presence of allophane which has probably formed as a residue after leaching of bases from the volcanic glass present in the sand.