Crazy Mountains, Montana

Click for route map

Route

Start point

Billings, MT

End point

Junction of US Highways 87 and 89

Roads

Interstate 90, US 89, Shield River Road, Smith Creek Road, East Fork Smith Creek Road, Bennett Creek Road, Montana 294, TG Ranch Road, Cottonwood Creek Road, US 12, and unnamed gravel and logging roads.

Total distance

345.2 miles

Geology

Summary

This trip examines a variety of mid-Eocene alkalic (feldspathoidal) stocks, laccoliths, sills, and dikes emplaced into Cretaceous and Paleozoic sedimentary strata. These rocks include mafic and felsic varieties, and are both texturally and compositionally variable. Most of the mafic alkalic rocks are unique in the Montana Alkalic Province in having Na2O > K2O. They are unusual even among feldspathoidal rocks because they are stongly enriched in incompatible elements, and have Sr, Nd, and Pb isotopic compositions which reflect an ancient source having low Rb/Sr, Sm/Nd, and U/Pb.

Key Lithologic Features

  • sedimentary rocks of the Belt Supergroup (not emphasized)
  • various Paleozoic and Mesozoic sedimentary rocks (not emphasized)
  • "Great Cliffs of Theralite": analcimite, malignite, and felsic alkalic rocks
  • Anticlinal Phacolith: malignite with xenoliths and olivine cumulates, with nearby trachyte and lamprophyres
  • minette with xenoliths
  • quartz latite dikes with xenoliths
  • Comb Creek dike swarm: malignite, phonolite, trachyte, and lamprophyre
  • Comb Butte: nepheline syenite
  • Gordon Butte: differentiated sill of malignite with clinopyroxene cumulates, chilled margins, and pegmatites
  • unnamed xenolith-rich malignite plug
  • lamprophyllite-bearing pegmatite

Structures

  • various anticlines, synclines, monoclines, and domes

Landforms

  • Crazy Mountains
  • various buttes

Other Features

  • rare mineral collecting localities

Reference

Hearn, B.C., Jr., 1989, Crazy Mountains, Montana, in Hearn, B.C., Jr., Dudas, F.O., Eggler, D.H., Hyndman, D.W., O'Brien, H.E., McCallum, I.S., Irving, A.J., and Berg, R.B., Montana High-Potassium Igneous Province : 28th International Geological Congress, Field Trip Guidebook T346: Washington, D.C., American Geophysical Union, p. 7-22.

Availability

Order from the American Geophysical Union Bookstore. Go to the AGU On Line Book Catalog for ordering information.