Presentation Type

Poster

Abstract

The Belt Supergroup is a sequence of sedimentary and meta-sedimentary rock ~20 km 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, which 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. Results indicate that the coarsest samples collected from the Libby Formation are coarse siltstone. Sedimentary structures and hand samples of Libby indicate similarities between the three formations. Framework grains include abundant angular quartz and rare alkali feldspar. Detrital muscovite is notably common, as is diagenetic chert. Zircon separates evaluated by scientists at the Boise State University geochronology lab are at the lower size limit for radiometric dating via laser ablation-inductively coupled plasma mass spectrometry (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 to seperates from the Garnet Range and Pilcher Formations.

Category

Physical Sciences

Share

COinS
 
Apr 15th, 11:00 AM Apr 15th, 12:00 PM

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 ~20 km 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, which 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. Results indicate that the coarsest samples collected from the Libby Formation are coarse siltstone. Sedimentary structures and hand samples of Libby indicate similarities between the three formations. Framework grains include abundant angular quartz and rare alkali feldspar. Detrital muscovite is notably common, as is diagenetic chert. Zircon separates evaluated by scientists at the Boise State University geochronology lab are at the lower size limit for radiometric dating via laser ablation-inductively coupled plasma mass spectrometry (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 to seperates from the Garnet Range and Pilcher Formations.