A MAPPING, PETROGRAPHY, AND GEOCHEMISTRY-BASED STUDY OF VOLCANIC UNITS IN THE SOUTHERN OREGON CASCADES

In-depth geologic mapping and petrological studies have been completed on many of the large stratovolcanoes throughout the Cascades, such as Mts. Rainier, Mazama, and Shasta. Only limited analysis has been completed, however, on the smaller flows between these cones, particularly in the dominantly-mafic southern Oregon Cascades. This study is a continuation of mapping by the lead author of volcanic features in the 1:24,000 USGS quadrangles near Mt. McLoughlin, Oregon. Prior, completed mapping of the Willow Lake and Rustler Peak quadrangles is here complemented by the completion of the Big Butte Springs quadrangle, and the preliminary sampling and mapping of the Prospect South and Imnaha Creek quadrangles.

The definition of geologic unit extents was jointly based upon macroscopic descriptions, and the geochemistry and petrography of samples collected over multiple field seasons. Formal maps were drafted using ArcGIS, allowing for a clearer determination of relative flow ages and edifice volumes. Through an application of previously-published regional eruption ages, this in turn has allowed for a first order determination of minimum cone-building rates for several of the smaller, peripheral vents within this field area. Crystallization textures observed in some of the more-expansive of these flows have also suggested a potential correlation with distance from eruptive centers, with patterns relating post-eruptive crystal growth to flow volumes and distance traveled.