A Traverse Across the Eastern Belt Basin From Neihart to Townsend, Montana

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Route

Start point

Niehart, Montana

End point

Townsend, Montana

Roads

U.S. 89, Jefferson Creek Road, Montana 360, U.S. 12, and minor gravel roads

Total distance

131.5 miles

Geology

Summary

This road log focuses on Precambrian (Proterozoic) sedimentary rock deposited in the Helena embayment of the eastern Belt basin. It also includes a general description of the geology between exposures of Proterozoic strata, briefly describing occurrences of Precambrian igneous and metamorphic rocks, Paleozoic sedimentary rocks, and Tertiary igneous rocks. A stratigraphic column is provided for refeference.

Key Lithologic Features

  • Proterozoic sedimentary rocks of the Belt Supergroup
    • Neihart Quartzite
    • Chamberlain Shale (and "molar-tooth" dolomite)
    • Newland Limestone
    • Greyson Shale (and interbedded sandstones)
    • Spokane Formation
  • Paleozoic sedimentary rocks
    • Flathead Sandstone (Cambrian)
    • Wolsey Shale (Cambrian)
    • Meagher Limestone (Cambrian)
    • Park Shale (Cambrian)
    • Pilgrim Limestone (Cambrian)
    • Jefferson Formation (Devonian)
  • Tertiary igneous rocks
  • Tertiary and Quaternary sedimentary rocks, including pediment gravels

Structures

  • Helena Embayment (Salient)
  • angular unconformity between Paleozoic sedimentary rocks and folded Belt Supergroup rocks
  • soft sediment folds in the Newland Limestone
  • sedimentary structures in Belt Supergroup rocks
  • various thrust faults and related folds (Laramide)
  • Volcano Valley fault (reactivated Precambrian normal fault)

Landforms

  • Kings Hill Pass
  • Black Butte
  • Big Belt Mountains
  • Boulder Baldy
  • Castle Mountains
  • Deep Creek Canyon
  • Deep Creek valley flood plain
  • Missouri River terraces

Other Features

  • fossiliferous Tertiary sediments containing camels, deer, land tortoises, mammoths, and oreodonts
  • various old mining towns and camps

Reference

Zieg, G.A., Godlewski, D.W., 1986, Road Log No. 1, A Traverse Across the Eastern Belt Basin From Neihart to Townsend, Montana, in Roberts, S.M., ed., Belt Supergroup: A Guide to Proterozoic Rocks of Western Montana and Adjacent Areas , Montana Bureau of Mines and Geology Special Publication 94, p. 1-16.

Availability

Out of print; can be photocopied for a fee. For more information, see this publication's record at the MBMG .