Field Guide; Little Rocky Mountains

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Route

Start point

U.S. Highway 191, miles post 109.1

End point

Zortman, Montana

Roads

U.S. 91, Montana 236

Total distance

25.2 miles

Geology

Summary

This fieldtrip examines deformed Paleozoic sedimentary rocks on the margins of the Little Rocky Mountains, Tertiary intrusive rocks (porphyrys, magmatic-hydrothermal breccias, and dikes), and associated mineralization.

Key Lithologic Features

  • Mississippian limestone
  • Mission Canyon Formation
  • Lodgepole Formation
  • Archean gneisses
  • Tertiary syenite porphyry
  • Sullivan Park porphyry
  • Antoine porphyry
  • Precambrian amphibolites
  • Archean schist
  • Eocene volcanics
  • hypabysal intrusions
  • Emerson Formation

Structures

  • Great Falls tectonic zone

Landforms

Other Features

  • Zortman Mining, Inc.

Reference

Russell, C.W., and Gabelman, J., 1991, Field Guide; Little Rocky Mountains, in Baker, D.W., and Berg, R.B., eds., Guidebook of the central Montana alkalic province: Geology, ore deposits and origin , Montana Bureau of Mines and Geology Special Publication 100, p. 185-194.

Availability

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