Abstract
Systematic lateral and vertical trends are detectable in proportions of montmorillonite, illite, and chlorite occurring in the lutites of late Pleistocene and Holocene sediments from southern Cascadia Basin and Blanco Fracture Zone. In the Holocene sediments, a lateral pattern radiates outward from the mouth of the Columbia River into the marine environment; the percentage of chlorite and, to a lesser extent, illite, increases with distance from the source. The percentage of montmorillonite presently introduced by the Columbia River to the Pacific Ocean is apparently higher than that of adjacent continental sources. In the late Pleistocene marine sediments, proportionally more illite is present than in the overlying Holocene sediments. Because the change in clay mineral composition is abrupt, and associated with the Pleistocene-Holocene faunal boundary, it is not believed to be the result of gradual marine diagenetic processes. Instead, it probably results from a glacially controlled change in the rate at which clays of varying composition are contributed by the different subbasins of the Columbia River to southern Cascadia Basin.
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