PETROLOGIC CORRELATIONS TO EXPAND VOLUME ESTIMATES OF THE MIOCENE COUGAR POINT TUFF, YELLOWSTONE HOT SPOT TRACK

The Cougar Point Tuff (CPT) is a sequence of rheomorphic rhyolitic ignimbrites, best exposed in SW Idaho along the Idaho–Nevada border in the canyons of the Bruneau and Jarbidge Rivers and their tributaries. They erupted from the central Snake River Plain (CSRP) along the Yellowstone hotspot track (YHST) between ~12.7 and ~10.2 Ma. The CPT sequence is composed of at least ten individual high temperature ignimbrites, with approximate volumes of 10² - 10³ km³ (Ellis et al. 2013).

Bernt, studying with Bonnichsen (1982, U. Idaho, M.S. thesis), suggested that the CPT is also present ~15 km west in an area between Sheep Creek canyon and Cat Creek canyon in Idaho. Bernt characterized 8 potential CPT units in this area (Tc1-8 and some stratigraphically undifferentiated units- Tcu). The Tc sequence displays many similarities to Bonnichsen’s CPT units (Bonnichsen 2008), but correlations remain tenuous nearly 4 decades later. If CPT and Tc units can be correlated, this would have significant impacts on models of magma chamber size and eruption dynamics for this period of YHST magmatism.

In order to address this, we conducted field work in 2016 and 2018, sampling units mapped as CPT and Tc, in order to test Bernt’s and Bonnichsen’s tentative hypotheses that these Tc units near Sheep Creek are in fact CPT rhyolites. Analysis of 61 samples was conducted at Washington State University’s GeoAnalytical Laboratory for whole rock geochemistry by XRF and ICPMS. Quantitative analyses of minerals, as well as qualitative element maps, were also performed on thin sections using WSU’s electron microprobe.

Our results provide preliminary support for the correlation of CPT unit III and Tc1 of Bernt (1982) CPTIII/Tc1 samples have higher Nb/Zr (most >>0.12), while all other samples are <0.12. However, modal phenocryst analysis of CPTIII/Tc1 samples shows variability in proportions of plagioclase, alkali feldspar, and quartz. Also, a few samples from outcrops mapped as CPTIII/Tc1 contain titanite (otherwise not seen in any well-established CPT unit), perhaps suggesting variable fO₂ in the magma and/or a need to more carefully evaluate previous mapping/stratigraphy.