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Eocene to Oligocene Paleodrainage of Southwest Montana: Evidence from Detrital Zircon Populations

Stroup, Caleb N.1, Paul K. Link1, Susanne U. Janecke2, and C. Mark Fanning3
1Idaho State University, Pocatello, ID
2Utah State University, Logan, UT
3Australian National University, Canberra, Australia

     Contrasting models have been proposed for paleodrainage within the north-striking Paleogene rift system in southwest Montana, in the western hanging wall of the Muddy-Grasshopper detachment. Models most notably disagree over whether northern or southern drainage within the rift basins persisted from the Eocene to Oligocene.
     Detrital zircon populations from sandstones in and adjacent to the Paleogene Grasshopper, Anaconda and Nicholia Creek supradetachment basins have diverse provenance. Different sands contain a) Proterozoic (1400-1750 Ma) zircons recycled through the Belt Supergroup, b) mixed cratonally derived Archean, Paleoproterozoic, and Grenville zircons from nearby ranges, and c) Cretaceous magmatic zircons of two prominent populations (65-85 Ma and 90-110 Ma).
     Axial Oligocene two-mica sandstones from the Grasshopper basin contain a 65-85 Ma grain population, likely sourced from local granites. Paleocurrents suggest derivation from the north-northwest. A second 90-110 Ma population is found in sandstones deposited in the intra-rift Grasshopper and Nicholia Creek basins, and in older middle-Eocene “Renova” sandstone from Mantle Ranch, MT, east of the rift zone. No known proximal ca. 100 Ma plutonic source exists in southwest Montana, but little U-Pb geochronology has been done in nearby ranges. These data require that regional fluvial systems drained not only plutons from the north and west, but also unknown ~100 Ma plutons, perhaps in the Pioneer Mountains.
     Oligocene two-mica sandstones in the type Renova Formation near Whitehall, MT contain many 70-80 Ma Boulder batholith grains and confirm persistent eastward drainage from uplifts east of the rift basins, even where detachment vergence changes from top-west to top-east.


AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah