Graduate Thesis Or Dissertation

Willamette and associated soils on late Pleistocene geomorphic surfaces, Polk County, Oregon

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  • The morphology and genesis of soils of the Willamette series and of some Willamette-associated soils and their relationships to geomorphic surfaces were studied in an area near McCoy, Polk County, Oregon. Influences of regional extent, such as climate and parent material, were related to the soils and landforms. The soils were mapped by methods proscribed in the Soil Survey Manual, and the geomorphic surfaces were recognized and delimited using descriptions previously published. Interpretations of the soil and geomorphic relationships were extended to other portions of the northern Willamette Valley, particularly the Yamhill River Valley. Of four geomorphic surfaces mapped in the study area, three were previously recognized, and a new one, the Bethel surface, was proposed. It was described and compared to the Quad surface in the Willamette Valley, and is presumably of similar age. The other geomorphic surfaces in the area were mapped as Calapooyia, Senecal, and Ingram units. Stratigraphic investigations identified the nature of the materials under the surfaces. Chemical and physical analyses were made of samples from horizons of representative soil profiles. The laboratory data were used to compare and contrast soils of the different surfaces. The Willamette "group" of soils, which occur on three of the surfaces, were found to be similar. They are members of fine-silty, mixed, mesic families of Pachic Ultic Haploxerolls, Ultic Argixerolls, Pachic Ultic Argixerolls, and Aquultic Argixerolls, a topo-chronolitho (perhaps bio-) sequence. Soils of the subgroups Argiaquic Xeric Argialbolls, Typic Ochraqualfs, and Typic Albaqualfs are limited to two geomorphic surfaces, and one soil, a member of Fluventic Haplaquolls, is on only one surface, the youngest mapped in the study area. Litho logic discontinuities, correlated with stratigraphic evidence, were observed in and recorded for each soil profile described. Each of the soils that was recognized and analyzed is comprised of more than one deposit. The upper limit of the older material in the sola is marked by brown and reddish brown clay films on ped faces, probably the result of clay illuviation prior to the deposition of the next major stratum. Modification of the strata by additions of organic matter, eluviation of bases, reduction of iron and manganese under wet conditions, local erosion and deposition of alluvium, and possibly alteration under different kinds of vegetation produced different morphologies of the soils, depending on the topographic locations and nature of the strata. Soils on the hilltops, the Bethel surface, have thinner mollic epipedons than do other Willamette "group" soils on the main valley terrace, the Senecal surface. The clayey strata under the Calapooyia and Ingram surfaces, although deposited at different times, affected the soils on those surfaces similarly by restricting the downward movement of water. Reducing conditions in all of the soils sometime during the rainy season was considered. Cool, moist winters and warm, dry summers affected the kinds and rates of chemical reactions in the soils. The observations made in the study area are applicable to similar areas in other parts of the Willamette Valley. During the progress of the Polk County soil survey, careful examination of the soils associated with the Bethel surface is important.
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