Graduate Thesis Or Dissertation
 

Soil solution chemistry, profile development, and mineral authigenesis in several western Oregon soils

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  • Clay mineral genesis was studied in soils representative of several different geomorphic surfaces in western Oregon, ranging in age from Pliocene-early Pleistocene to late Pleistocene. Soil solution studies, clay mineralogy, and soil raicromorphology were employed to provide evidence of clay mineral synthesis and interpret soil genesis. Soils at each study area were characterized by distinct differences in soil solution chemistry, clay mineralogy, plasmic fabric, and genetic history, although parent material compositions between sites were often similar. Soil solution studies suggested that clay mineral synthesis from solution does not occur in soils representing the oldest geomorphic surface. The cxic soil properties at this location were developed during a prior weathering cycle and the soils are at a genetic endpoint in the present environment. Soil solutions from Ultisols on remnants of Pleistocene surfaces in the Oregon Coast Range were in equilibrium with respect to kaolinite in the solum, but stable with respect to Mg-montmorillonite in the zone characterizing active bedrock weathering. Microraorphological, clay mineralogical, and chemical evidence suggested that alteration of Mgchlorite in sedimentary rocks leads to smectite genesis in the Cr horizon of the Ultisols, followed by conversion of smectite to halloysite and chloritic intergrade in the solum. Alteration of basalt in these soils also leads to the formation of smectite, which is unstable with respect to halloysite in well-drained microenvironments within soil profiles. Soil solutions from soils characteristic of silt-mantled late Pleistocene surfaces on the western margins of the Willamette Valley showed compositional variation from pedon to pedon and horizon to horizon within individual pedons, reflecting the influence of soil parent material compositional variation. Soil solutions were generally in equilibrium with respect to kaolinite, although solutions from horizons developed in tuffaceous sediments were in equilibrium with Mg-montmorillonite. Both kaolinite and smectite were observed as products of mineral authigenesis in these soils. Authigenic clay showed delicate honeycomb or hexagonal morphology in contrast to the parallel oriented appearance of illuvial clays. Andesite alteration in wet, unstable soils of Oregon's western Cascades also resulted in the formation of smectite. The sequence of mineral alteration in andesites is similar to that described for basalts; however, the final products of mineral authigenesis are distinctly microenvironment dependent. Morphologic evidence suggests that chemical properties of bulk soil solutions can not always be considered to represent the solution conditions at the site of mineral authigenesis. In the absence of reliable in situ micro-chemical analytical techniques, detailed micromorphological studies can aid'in interpreting soil microchemical properties at weathering surfaces by revealing the identity of neoformed secondary phases.
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