Rock weathering in the central western Cascades

Rock weathering pathways of the three major bedrock formations in the central western Cascade Mountains of Oregon are distinctive
as a result of chemical and mineralogical differences in parent material and variations in climate found along an elevation gradient. These influences are expressed in secondary mineralogy, cation retention, elemental distribution in soils and runoff waters and rates of soil development. Parent materials include welded to ashflow rhyodacitic tuffs, unwelded airfall ash breccias, andesite and basalt flows and flow breccias.
Total chemical analyses of soil/bedrock profiles augmented by evaluations of secondary mineralogy and analyses of runoff water chemistry indicate that podzolization is the primary pedogenic process above 1,150-1,333 M. Continuous leaching of mobile rock forging cations(Si, Ca, Mg, K and Na) occurs below this elevation, resulting in the accumulation of Fe, Al, Ti, P and Mn towards the surface of soils which are not forming under the influence of a substantial snowpack.
The importance of lithic textures, exclusive of bedrock chemistry is illustrated by the significant differences in soil mineralogy and rates of formation between soils forming on Oligocene or Miocene rocks. At the Andrews Forest study site, Miocene pyroclastic rocks were deposited primarily as an airfall ash while Oligocene pytoclastic rocks included several welded ashflow tuffs. The contrast in textures due to the different modes of emplacement appears to be responsible for differences in weathering rates and pathways since initial chemistry is very similar for the two rock types.
Chemical evidence of buried profiles demonstrates the longevity of biological influences on the chemical composition of soils. The results of this study generally emphasize the importance of biological processes in the control of ionic distributions of biologically essential cations in the soil profile and the regulation of soil activity which influences inorganic weathering reactions.
Chemical and mineralogical differences noted in two profiles,developed on the same parent material in different geomorphic positions illustrate the importance of drainage and soil mantle stability as they related to oxidation of Fe, Ti and Al the development of biologically cycled ion distributions in soil profiles.