GEOLOGIC MAPPING AND CLASSIFICATION OF MIOCENE LAVAS, SUSANVILLE, CALIFORNIA

Recent sampling and mapping of Miocene volcanic rocks in northern California have produced more detailed description of lithologic variety of lavas, the styles of their emplacement, and their petrogenetic relationships. The field area is centered around Susanville California in Lassen County, on the southern edge of the Modoc Plateau and the western edge of the Great Basin. Areas of study include Bald Mountain, Diamond Mountain, Fredonyer Butte, Little Fredonyer, and the Bizz Johnson National Recreation Trail area. This work has focused on more detailed (1:2400) mapping and classification of distinguishing characteristics of individual lavas, which primarily range from andesite to basalt. We have also begun correlating Susanville–area basalts with others found locally, including the Warner Basalt, Lovejoy Basalt, and the Modoc Basalt using petrography and geochemical analyses. Thin section analyses indicate that lavas have typical intermediate-mafic phase assemblages, dominated by plagioclase, olivine, and clinopyroxene phenocrysts with varying groundmass phase assemblages. Geochemical analyses further constrain lava compositions and petrogenesis. This and ongoing work characterizes lavas previously mapped on a broader scale in order to improve our understanding of volcanism that preceded eruptive products of modern Cascades volcanoes and of the Modoc Plateau.