Shonkin Sag and Square Butte Laccoliths, Montana

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Route

Start point

Geraldine, MT

End point

Havre, MT

Roads

Montana 80, unnamed road into Shonkin Sag, county and ranch roads near Square Butte, Montana 387, U.S. 87, U.S. 2

Total distance

11.9 miles: Geraldine to Shonkin Sag laccolith
7.9 miles: town of Square Butte to Square Butte laccolith
13.9 miles: sidetrip to south and west side of Square Butte
75.6 miles: Fort Benton to Havre

Geology

Summary

Shonkin Sag and Square Butte laccoliths are at the eastern edge of the Highwood Mountains as part of the Central Montana Alkalic Province. Rocks in the Highwood Mountains date to 50-53 Ma. Both laccoliths are internally differentiated and contain a variety of rock types, including shonkinite, syenite, and pegmatite.

Key Lithologic Features

  • Eagle Sandstone
  • rocks of the Shonkin Sag laccolith:
    • basal chill zone
    • main shonkinite
    • pegmatite
    • syenite
  • rocks of the Square Butte laccolith:
    • massive shonkinite: mafic and intermediate varieties
    • syenite dikes, sills, and "patches"
    • layered (flow banded?) syenite

Structures

Landforms

Other Features

  • preglacial valley of the Missouri River
  • glacial tills near Big Sandy
  • Boxelder Butte laccolith

Reference

Hyndman, D.W., Alt, D., and Tureck-Schwartz, K., 1989, Shonkin Sag and Square Butte laccoliths, 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. 37-49.

Availability

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