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