Clay mineralogy in relation to landscape instability in the coast range of Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/fx719r82r

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  • The mineralogy of soils involved in mass movement in Oregon's coast range was examined to determine relationships between clay mineralogy and landscape instability. The objectives were: 1) to determine what kind of materials constitute the less than 2μ fraction of soils involved in different categories of mass movement, and 2) to determine how the composition of the mineral suite varies; at an individual site as a function of depth or slope position, and between sites as a function of parent material. Field evidence was used to assign each site to one of the following categories: debris avalanche, creep and slump, and earthflow. Although no sites were specifically identified as being stable, a number of samples were taken at varying distances laterally away from actively failing sites. The clay fractions were characterized by X-ray diffractions, selected samples were analyzed by differential thermal analysis, and transmission electron microscopy. Field and laboratory data indicate that the kind of mass movement and the mineralogy of the materials involved vary with the parent material. The clay fraction of debris avalanches consisted primarily of nonexpanding layer silicates that characteristically have large particle sizes and small water holding capacities. Dehydrated halloysite, chloritic intergrade, and mica were the common minerals in those areas underlain by sandstones and siltstones of the Tyee Formation as well as the massive basalt flowrock of the Siletz River Volcanic Series. The clay fraction of soils derived from other Tertiary sandstones (Galice and Lookingglass Formations) consisted of chloritic intergrade, chlorite, mica, and kaolinite. Serpentine, chlorite, and mica were the soil clays associated with debris avalanches on serpentinite of the Otter Point Formation. Expandable layer silicates, or those with high charge or water holding capacity were not major constituents although smectite and vermiculite commonly occurred in a thin layer of soil above the underlying support material. The clay fraction of samples from sites undergoing failure by creep and slump did not vary with depth and consisted primarily of smectite. Smectite, chloritic intergrade, dehydrated and hydrated halloysite, and mica were the minerals commonly associated with soil creep and slump on slopes underlain by siltstones of the Tyee Formation. Montmorillonite (smectite) was the major constituent of a large rotational slump at the contact between the Nye Mudstone and Astoria Formation. Hydroxy interlayered smectite, chlorite, and serpentine were identified in sites undergoing creep and slump which are underlain by the serpentinite of the Otter Point Formation. The mineralogy of soils involved in earthflow consisted predominantly of hydrated and dehydrated halloysite, amorphous material, and chloritic intergrade. No difference in mineralogy could be detected between sites underlain by siltstones of the Tyee and Nestucca Formations and tuffaceous siltstones and tuff of the Siletz River Volcanic Series. Surface samples were more poorly crystallized than samples taken at greater depths. Hydrated halloysite, however, was more evident in lower horizons. Electron micrographs reveal an abundance of amorphous gels, and "coatings" on the surface of mineral grains. The abundance of "pores" may account for the fluid behavior of these materials during failure. On all bedrock units, the mineralogy did not vary between actively failing sites and sites on the same slope considered to be "stable." The kind of mass movement and the mineralogy of sites underlain by serpentinite did depend on slope position. Creep and slump were common in poorly drained toeslope positions and contained smectite; debris avalanches were common in well-drained uplands, these soils contained serpentine and chlorite.
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