Investigation of Miocene Age Rhyolite Tuffs Along the Yellowstone Hotspot Track (2023)

The Cougar Point Tuff (CPT) in the Central Snake River Plain (CSRP) was deposited ~12.67 Ma and continued until ~9.50 Ma in southern Idaho and northern Nevada. This study charts the progressive compositional change of the magma chamber throughout numerous eruptions to determine if pre-CPT units of Bruneau-Jarbridge (BJ) and Bieroth volcanics are related to CPT or an entirely different eruption series. My work builds on existing data from the Snake River Plain, CPT, BJ, Bieroth, and Tokembamby with the addition of pre-CPT units and samples of indeterminate association. Initial results demonstrate that the CPT’s crystals composition to be primarily plagioclase feldspar, quartz, and pyroxenes with considerable percentages of crystal variations even within units (i.e., CPT 6 has a slide with an approximated crystal count of 15% with sieve texture prominent; another slide has an approximated 100% crystal count with significantly fewer crystal textures evident). A comparison sample of Bieroth andesite (estimated at between 70 and 80% crystal count) contains hydrous minerals, including biotite.