COASTAL LOESS DEPOSITS: POST GLACIAL INTERIOR VERSUS MARINE SHELF SOURCES?

The objective of this study was to determine if the clay mineralogy of the coastal loess at the Holocene/ Pleistocene boundary can be used as a mineral marker for whether the origin is from a post glacial continental or marine shelf origin. The study area extends from the north coast of Oregon at Newport south to Monterey, California, and it is part of a larger study that extends to Baja, Mexico.

X-ray Diffraction (XRD) was used to determine clay mineralogy using both oriented and random powder methods. Ethylene glycol saturated oriented XRD patterns of the Oregon and northern California samples showed vermiculite with broad peaks at ~14.4Å and 4.25Å; a gibbsite peak at 4.8Å; and crystobalite. The XRD patterns for Monterey had a broad illite peak at 10.0Å and vermiculite. Literature states that the post glacial continental loess forming the Portland Hills Silts contains less than five percent kaolinite, illite and montmorillonite clay minerals. These minerals are absent in the less than 63-micron random powder patterns of the northern Oregon coast samples.

Our preliminary conclusions are: 1) the clay mineralogy of the samples do not reflect the distinct differences between the mafic extrusive provenance of the Coast Range in northern and central Oregon, and the metamorphic provenance of the Klamath Mountains in southern Oregon and Northern California; 2)the loess is from a marine shelf origin rather than a continental source.