Helena and the North Border Zone of the Boulder Batholith, Montana

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Route

Start point

Helena, Montana

End point

Mount Belmont, MT

Roads

I-12, Colorado Gulch Road, I-15

Total distance

55.4 miles

Geology

Summary

The geologic descriptions for this road log roughly follow the scalloped north edge of the Boulder batholith. The will be ample opportunity to examine the intrusive rocks, and to the north, the broad border zone of contact-metamorphosed Paleozoic and Mesozoic sedimentary rocks.

The route will follow the highway eastward, with a stop in granodiorite of the batholith. The log then detours to the south into the mountains along Colorado Gulch to view skarn and hornfels developed in the Madison Group and Three Forks Formation in a "septum" (or "screen") of country rock caught between two lobes of the batholith. The log returns to Highway 12 and continues east to Helena's Reeder's Alley and Grizzly Gulch. After a visit to a talc vein and teh unconformity between Precambrian (Belt) and Cambrian formations, the trip leaves Helena southbound on Interstate 15. Near Montana City are stops at an outcrop of hornfelsed, stromatolitic Helena Formation (Precambrian Belt Supergroup) and at the contact with the Boulder batholith. The final stop is to examine the complex of intrusions, wall- or roof-rock inclusions and structures in the batholith's border zone. In this area, Precambrian, Paleozoic and Mesozoic sedimentary rocks dip southwest and south toward the intrusion. Within about one to two miles from the batholith, the sedimentary sequence is contact-metamorphosed, yet in most places sedimentary structures are remarkably well preserved.

Key Lithologic Features

  • Cretaceous granitoids of the Boulder batholith, and late-stage alteration
  • andesite dikes cutting Boulder batholith granodiorite
  • contact metamorphic rocks (dolomitic and calcitic marbles, hornfelses, calc-silicate gneisses, and skarns) adjacent to the Boulder batholith
  • various intrusive phases of the Boulder batholith
  • Paleozoic sedimentary rocks
  • talc vein in Cambrian Harmark Formation

Structures

  • joint networks in Boulder batholith granodiorite
  • emplacement-related structures at the margin of the Boulder batholith
  • various faults, veins, and folds in Paleozoic sedimentary rocks

Landforms

  • McDonald Pass (Continental Divide)
  • pediment underlain by granodiorite

Other Features

  • Helena Water Works

Reference

Breuninger, R.H., and Childs, J.F., 1987, Road Log No. 4, Helena and the North Border Zone of the Boulder Batholith, Montana, in Berg, R.B., and Breuniger, R.H., eds., Guidebook of the Helena Area, West-Central Montana: Guidebook for the 12th Annual Field Conference , Montana Bureau of Mines and Geology Special Publication 95, p. 49-52.

Availability

Order from the Tobacco Root Geological Society c/o the Montana Bureau of Mines and Geology. Go to the TRGS publications page for ordering information.