Clay Mineralogy in Relation to Landscape Instability in the Coast Range of Oregon1
- J. D. Istok and
- M. E. Harward2
Field and laboratory data reported in this study 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 is dominated primarily by nonexpanding layer silicates that 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 Tertiary sandstones of the Galice and Lookingglass Formations is dominated by chloritic intergrade, chlorite, mica, and kaolinite. Serpentine, chlorite, and mica were the soil clays associated with debris avalances on serpentinite of the Otter Point Formation. Expandable layer silicates, or those with high charge or water-holding capacity, were not major constituents of debris avalanches although smectite and vermiculite commonly occurred in a thin layer of soil above the underlying bedrock.
The clay fraction of samples from sites undergoing failure by creep and slump 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 was the major constituent of a large rotational slump at the contact between the Nye Mudstone and the sandstones of the Astoria Formation. Smectite, chlorite, and serpentine were identified in sites underlain by 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 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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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