PETROLOGY OF THE LIBBY FORMATION AND COMPARISON TO THE GARNET RANGE AND PILCHER FORMATIONS OF THE BELT SUPERGROUP: THE CASE FOR STRATIGRAPHIC EQUIVALENCY

The Belt Supergroup is a sequence of sedimentary and meta-sedimentary rock ~20km thick that underpins much of western Montana, with equivalents extending into adjacent parts of Idaho and Canada. The Missoula Group forms the uppermost portion of the Belt Supergroup and is best expressed and best exposed around the Missoula Valley. Within the Missoula Group are six separate formations, all of which are siliciclastic. Our investigation focuses on the Missoula Group’s youngest two formations, the Garnet Range and Pilcher, along with a purportedly equivalent stratigraphic unit, the Libby Formation, that occurs in several structurally isolated fault blocks ~200 km NW of Missoula. In particular, we have sought to compare the petrology and detrital zircon geochronology of the Garnet Range and Pilcher Formations with that documented from the Libby Formation. To this end we examined and sampled outcrops of the Libby Formation at Flagstaff Mountain and along Fishtrap Creek in October 2015. The coarsest samples were cut into thin-sections for petrographic analysis, and we isolated the datable mineral zircon from a subset of these samples using facilities in the UM mineral separation laboratory. Initial results indicate that the coarsest samples collected from the Libby Formation are coarse siltstone. Framework grains include abundant angular quartz, alkali feldspar, and fine micas. Detrital muscovite is notably common. Chert is a common constituent of the finer-grained siltstone samples. Zircon separates are currently being evaluated by scientists at the Boise State University geochronology lab and appear to be at the lower size limit for radiometric dating via La-ICPMS. Along with zircon, other dense minerals were recovered during the separation process. To further test whether the Libby Formation is equivalent to the Garnet Range and Pilcher Formations, these dense minerals will be analyzed and compared with those separated from samples of the Garnet Range and Pilcher Formations.