Fourier grain-shape analysis of detrital quartz grains in sediments of Bear Creek Valley and adjacent Western Cascade Mountains, southern Oregon

Detrital quartz grains separated from sediments derived from lithologically different source rocks within Bear Creek Valley and adjacent Western Cascade Mountains, southern Oregon, were studied to visually and numerically distinguish between sediments of different provenance. Analytical methods include Fourier grain-shape analysis, scanning electron microscopy, and three-dimensional optical microscopy. A new Fourier grain-shape analytical technique was developed for this study that analyzes the change in angle around a grain's perimeter. Tests using artificial shape data show that this technique is mathematically robust and allows shape features of geological interest to be expressed quantitatively, including reentrants and protuberances.; The outlines of 19,000 medium sand-size grains and 15,299 coarse sand-sized grains were subjected to Fourier transformation. Resulting shape spectral data were reduced using a standard chi-squared feature extraction technique to identify the shape harmonics showing the greatest differences between samples. Q-mode factor analysis was then applied to feature extraction results to identify quartz grains having distinctive shapes that reflect their provenance.; Factor analytical results show that two dominant quartz grain-shape populations exist in the sediments in Bear Creek and Antelope Creek valleys: one characteristic of quartz grains derived from plutonic rocks and one from volcanic rocks. Quartz grains from metamorphic rocks do not have a unique shape but instead exhibit shapes similar to those of either plutonic or volcanic quartz. The wide distribution of volcanic quartz grains in sediments from Bear Creek Valley and neighboring areas suggests that these areas were sites of airfall volcanic ash deposition during the growth of the Western Cascade Mountains. Results from Fourier grain-shape analysis and microscope observations also strongly suggest that the Mount Ashland pluton was already exhumed at the onset of Western Cascade volcanism.; The Fourier grain-shape algorithm has been improved so that highly convoluted grain shapes can now undergo Fourier transformation. Results of this study show that grain shapes are differentiated on the basis of large-scale differences in form, not upon small-scale features such as microtexture.