Nyberg's Thesis Abstract

The Magnetostratigraphy of the Crescent Basalts, Olympic Mountains, Washington

Emilie Nyberg

Ten sites in the southern portion of the Crescent basalts, Olympic Mountains, Washington, were sampled for paleomagnetic analysis. This data is part of a project to develop a regional magnetostratigraphy and reversal sequence for the Crescent basalts. The magnetic directions found in this study show a large amount of scatter. Demagnetization results from nine of the ten sites yielded high alpha 95s and low k values. The alpha 95 values are between 12° and 180°. Seven of the sites have values above 50°. These confidence ovals are much larger than those generally accepted for paleomagnetic studies. Seven of the k values are below 10, the others being 23, 44, and 213. Nine of the ten sites showed abnormally low initial intensities. The scatter in the data is due to weak primary signals and overprints that were not removed. These weak primary signals and overprints are probably caused by post-eruptive metamorphism and alteration. Outcrops from which the samples were taken are also heavily fractured, possibly increasing the scatter in the data. In order to further investigate the causes of the scatter, samples from selected sites were analyzed for mineralogical and textural properties using the scanning electron microscope and reflected light microscopy. Scanning electron microscope analysis showed areas of alteration, secondary mineralization, and a lack of iron oxides. The reflected light analysis revealed a low content of primary oxides and alteration of these oxides to goethite and maghemite.

University of Puget Sound