Relationships of clay mineralogy to landscape stability in Western Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xg94hs74h

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  • Soil erosion by mass wasting is the major problem on forest lands of the Pacific Northwest, The clay fractions of soils from a large number of sites in Oregon's Western Cascades were characterized in order to determine the relationships of various clay materials to mass movements. Each site was either designated as stable or assigned to one or more of the following categories' debris avalanche, debris flow, slump, earthflow, creep. All clay samples were analyzed by X-ray diffraction, and certain selected samples were analyzed by differential thermal analysis and/or electron microscopy. The most significant findings related amorphous gel and hydrated halloysite to flowage-type failures. These occurred in deposits of weathered basaltic oolluvium and volcanic ash which overlie highly weathered, smectite-rich pyroclastic tuffs and breccias. These conditions are most notable near the contacts of the Sardine Formation and the Little Butte Volcanic Series. The smectite clays, which are highly cohesive and slowly permeable, support perched water tables which keep much of the overlying soil saturated, or nearly saturated, throughout the year. Furthermore, the amorphous clays and hydrated halloysite help to maintain the saturated conditions due to their very high water holding capacities. The amorphous gels are thought to form microscopic "water balloons" which release their contents upon disturbance. Smectites, primarily montmorillonite, formed in pyroclastic tuffs and breccias are the most important clays of the well defined rotational slumps and of the areas of deep seated soil creep--in which the soil mass undergoes gradual deformation rather than abrupt failure. Contrastingly, the shallow cohesionless soils which are prone to failure by debris avalanche tend to have clay materials which are typically of large size, low charge and low water holding capacity. These include chloritic intergrades or hydroxy interlayered smectites, and microaggregates of halloysite and other materials bound by amorphous films and strands (i.e., imogolite). Associations of gibbeite, zeolite, mica, and kaolinite are also common. The more stable sites occur either at high elevations, with poorly formed soils having minimal clay development; or at low elevations, with relatively well drained soils containing kaolinite, dehydrated halloysite, ohloritic intergrades, and microaggregates bound by amorphous materials.
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