Missouri Breaks Diatremes, Montana

Click for route map

Route

Start point

Havre, MT

End point

Lewistown, MT

Roads

U.S. 2, Lloyd Road, unnamed ranch roads, Montana 236, U.S. 191

Total distance

180.7 miles

Geology

Summary

The Missouri Breaks diatremes are some of the best exposed diatremes in the United States, and display remarkable subsidence features. The diatremes and associated intrusions are important in showing (1) genetic connections of alnoitic, kimberlitic, and carbonatitic magmas, (2) xenolith suites representing the upper mantle and lower, middle, and upper crust of the Eocene lithosphere, and (3) mechanisms of eruption and emplacement of volatile-rich magmas. Some of the diatremes are fairly accessible in good weather. However, the Missouri Breaks is a remote area in which most of the roads are unimproved tracks across clay-rich sedimentary rocks, and even a small amount of rain may immobile four-wheel drive vehicles and make steep grades terminally hazardous.

Key Lithologic Features

  • Cretaceous sedimentary rocks
  • layered mafic phonolite flows
  • shallow syenite and latite intrusions
  • Black Butte diatremes: bedded pyroclastics, breccias with igneous and sedimentary fragments, massive alnoite plug, and contact-metamorphosed Cretaceous sedimentary rocks
  • Big Slide and Barnard Ridge diatremes
  • Lone Tree Ridge diatreme: monticellite peridotite, bedded pyroclastics, breccias, and adjacent Cretaceous sedimentary rocks

Structures

  • various faulted fold structures

Landforms

  • Missouri River canyon
  • Missouri Breaks badlands

Other Features

Reference

Hearn, B.C., Jr., 1989, Bearpaw 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. 63-73.

Availability

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