First Advisor

Martin Streck

Term of Graduation

Summer 2021

Date of Publication


Document Type


Degree Name

Master of Science (M.S.) in Geology






Rhyolite -- Oregon -- Mahogany Mountain (Lake County), Calderas -- Oregon -- Mahogany Mountain (Lake County), Volcanology -- Oregon -- Mahogany Mountain (Lake County), Stratigraphic geology



Physical Description

1 online resource (xi, 155 pages)


The Mahogany Mountain and Three Fingers calderas with their associated tuffs, the tuff of Leslie Gulch and tuff of Spring Creek, respectively, were the centerpiece of a larger rhyolite center that developed in response to Columbia River Basalt volcanism as numerous other mid Miocene rhyolite centers in a corridor from Baker City in the north to northern Nevada. Previous studies suggest a two caldera model, while others advocated for a single large caldera producing solely the tuff of Leslie Gulch. This study refines the eruptive stratigraphy along the northeastern margin of this rhyolite field with important implications for the entire field. Several distinct rhyolitic units are identifiable, these are (from oldest to youngest) the tuff of Leslie Gulch, the Old McIntyre rhyolite, the newly named tuff of Succor Creek, the Young McIntyre rhyolite, and a sequence of thin, non-welded ignimbrites. In addition, intermediate to mafic lavas under- and overlie rhyolites. Stratigraphy in this study area indicates the tuff of Leslie Gulch varies texturally throughout and has an eruptive history that includes multiple phases, with a new 40Ar/39Ar age of 15.98±0.05 Ma. This study also uses geochemical and stratigraphic data to distinguish between the Old and Young McIntyre Rhyolite units, providing two new ages for the Old McIntyre, 16.02±0.02 and 15.95±0.03 Ma. A newly named unit, the tuff and rhyolite of Succor Creek have also been described by this study and based on work by Marcy (2013), has an age of 15.74±0.09 Ma. High precision yet overlapping ages and stratigraphic field relationships highlight the explosive history of a 250 ky lasting, prolific explosive silicic rhyolite field.


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