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Middle Proterozoic Belt Supergroup, Western Montana

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Route

Start point

Great Falls, MT

End point

Spokane, WA

Roads

Montana Highways 2, 12, 56, 87, 89, 95, 200 and 287 and Interstates 15 and 90

Total distance

8 day trip over 946.09 miles

Geology

Summary

From Great Falls, to Butte, Helena Glacier National Park and Spokane, this field trip crosses onto the leading edge of the Rocky Mountain thrust belt and proceeds to the stratiform copper-silver deposits in the Revett Formation of the Troy Mine in the main part of the Belt basin. The road log includes sections on the paleontology of the Middle Proterozoic Belt Supergroup, stromatolites of the Belt Supergroup (specific to Glacier National Park), Middle Proterozoic Tectonics of the Belt basin, and a sedimentologic and tectonic interpretation of the Belt Supergroup.

Key Lithologic Features

  • Belt Supergroup
  • Purcell Supergroup (Canadan term for the Belt Supergroup)
  • Proterozoic sedimentary rocks of the Belt Supergroup
    • Windermere Group
    • Prichard Formation
    • Ravalli Group
    • Burke Formation
    • Revett Formation
    • Missoula Group
    • Wallace Formation
    • Neihart Quartzite (with Ventifact pebbles and crossbeds)
    • Newland Limestone (sparry calcite)
    • Spokane Formation
  • Archean gneiss
  • Pre-Belt feldspathic gneiss
  • LaHood conglomerate
  • Helena Formation
  • Cretaceous shale
  • Otter shale
  • Kibby Formation
  • Carboniferous Big Snowy Group
  • Archean terrain metamorphosed and intruded by Pinto Diorite, coinciding with the Trans-Hudson orogeny and the assembly of the North American-Baltic continent
  • Fort Steele Formation
  • Snowslip Formation
  • Silver Hill Formation
  • Shepard Formation
  • Bonner Formation
  • McNamara Formation
  • Roosville Formation
  • Libby Formation
  • Garnet Range Formation
  • Pilcher Formation
  • Grinnell Formation
  • Appekunny Formation
  • Empire Formation
  • Altyn Formation
  • Waterton Formation
  • Tombstone Formation
  • Haig Brook Formation
  • St. Regis Formation
  • Creston Formation (Canada?)
  • Purcell Lava (pillow basalt and pahoehoe flows)
  • Mount Shields Formation
  • Baicalia-Conophyton stromatolite cycle in the Helena Formation

Structures

  • Helena embayment, an autochthonous east-trending graben
  • Perry fault
  • Archean Dillon Block
  • Lewis thrust
  • Volcano Valley fault, with sulfide mineralization along the fault
  • Syndepositional faulting
  • Scout Camp-Willow Creek thrust zone
  • Deer Lodge Block
  • Georgetown thrust
  • Sapphire allochthon
  • Tertiary normal faults
  • Hoadley thrust
  • Eldorado thrust
  • Steinback thrust
  • Sawtooth structural salient
  • Ole Creek fault
  • Rocky Mountain Trench
  • Tertiary Hope fault

Landforms

  • McNamara-Garnet Range
  • Little Belt Mountains
  • Big Belt Mountains
  • Flint Creek Hill
  • Glaciated peaks and valleys
  • Little Prickly Pear Canyon
  • Rogers Pass
  • Jefferson Canyon
  • Volcanic sills and dikes
  • Chamberlain Creek
  • Devil's Chair
  • Boulder batholith
  • Flint Creek Range
  • Anaconda Range
  • Cable Mountain
  • Pioneer Mountains of Montana
  • McDonald Pass
  • Helena Valley
  • Lewis Range
  • Maria's Pass
  • Going-to-the-Sun Mountain
  • Matahpi Peak
  • Little Chief Mountain
  • Citadel Mountain
  • Pollock Mountain
  • Apikuni Falls
  • Apikuni Mountain
  • Two Medicine Lake
  • Haystack Butte
  • Lewis and Clark Range
  • Whitefish Range
  • Lake Koocanusa

Other Features

  • Glacier National Park
  • Troy Mine (stratabound copper-silver orebody)
  • Fossils (arthropods, Baicalia, Beltina danai, Conophyton, Grypania spiralis, Morania antiqua, Newlandia, Proterotainia Montana, stromatolites, sphaeromorphs, Tungusia)
  • Coal mining in the uppermost Jurassic near Belt, MT
  • Bear Bulch fish and soft bodies invertebrate fauna
  • Carbonaceous films
  • Trace fossils
  • Blackfeet Indian Reservation
  • Hungry Horse Dam

Reference

Winston, D., Horodyski, R. J., and Whipple, J. W., 1989, Middle Proterozoic Belt Supergroup, Western Montana, 28th International Geological Congress, Field Trip Guidebook T334: Washington, D.C., American Geophysical Union, p. 71-100. [Winston et al., 1989]

Availability

Order from the American Geophysical Union Bookstore. Go to the AGU on line order form for ordering information.