INVESTIGATION OF MIOCENE AGE RHYOLITE TUFFS ALONG THE YELLOWSTONE HOT SPOT TRACK

The Cougar Point Tuff (CPT) in the Central Snake River Plain (CSRP) was deposited ~12.67 - 9.50 Ma in southern Idaho and northern Nevada. The purpose of this research is to study the progressive change of composition, modal percent, and size of the crystals in the numerous eruptions and how they compare to the pre-CPT units of Bruneau-Jarbridge (BJ) from early Miocene and the calc-alkaline Bieroth volcanics from the Eocene. This work is a continuation of the investigation of the CPT in Bruneau canyon (NV) and upper Sheep Creek drainage (ID), while expanding the information to include pre-CPT units and samples of indeterminate association. My initial data indicate the major crystals in the CPT to be plagioclase feldspar, alkali feldspar, quartz, and minor pyroxenes with a large variation of crystal percentages even within units i.e., one thin section of CPT VI has an approximated crystal count of 15% with sieve textures prominent, while another section shows a finely crystalline groundmass with significantly fewer sieve textures evident. Additionally, comparing the composition of the CPTs to that of a sample mapped as Bieroth andesite that I estimate to have 70-80% crystals and which contains hydrous minerals like biotite, suggests this latter sample is substantially different and is likely Eocene Bieroth.