Tertiary Sediments of the Big Hole Valley and Pioneer Mountains, Southwestern Montana: Age, Provenance, and Tectonic Implications

Tertiary terrestrial sedimentary rocks of the Big Hole basin and Pioneer Mountains of southwestern Montana provide a record of regional extensional tectonism. Detailed observations of stratigraphy and sedimentology at widely scattered outcrops indicate the presence of paleosols, fine-grained debris flows, small alluvial channels, and rare fluvial deposits. U-Pb geochronology of detrital zircons and air-fall tuffs indicates the presence of Oligocene to Middle Miocene sedimentary rocks outcropping at the surface. Though the presence of detrital muscovite in several outcrops indicates derivation from the nearby 2-mica Chief Joseph pluton, a predicted ~75 Ma zircon population is not present: instead, a persistent peak of 70-72 Ma zircons is found throughout the basin, indicating the Chief Joseph pluton may have a younger emplacement age than is currently recognized. Simple physical models developed from Bouguer anomalies indicate the basin deepens and widens toward the south, which agrees with prior work suggesting Eocene initiation of extension in the Big Hole and places it in a class of extensional basins that formed in the early Tertiary. Late Cretaceous, Eocene, and Mesoproterozoic detrital zircon populations, along with consistently immature lithic sands, all indicate sediments were derived from local bedrock sources found around the rim of the modern Big Hole Valley. While these results indicate the Tertiary Big Hole basin resembled the modern basin, some later deformation may have cut off and reversed a paleodrainage flowing out of the southeastern portion of the basin, possibly leading to the modern configuration of the Big Hole River and the deep gorge it carves across the eastern Pioneer Mountains.

Name thesis:degree_name

Master of Science (MS)

Grantor dc:publisher

University of Montana

Year

2010

Author dc:creator

• Roe, Warren Paul

• 2-mica
• Anaconda metamorphic core complex
• Bouguer
• Chief Joseph pluton
• detachment
• Eocene
• extension
• geochronology
• graben
• gravity
• half-graben
• Late Cretaceous
• Miocene
• Neogene
• Oligocene
• Paleogene
• Pioneer Mountains
• U-Pb
• zircon

Repository record dc:identifier

https://scholarworks.umt.edu/etd/1318

OAI identifier oai:identifier

oai:scholarworks.umt.edu:etd-2337