Quaternary Geology and Faulting of the Helena Valley, Montana

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Route

Start point

Helena, MT

End point

Helena, MT

Roads

Roads within the Helena valley

Total distance

74.7 miles

Geology

Summary

This field trip will give the participants an overview of the Quaternary deposits and late Cenozoic faulting which have shaped the Helena valley. The Helena valley is a NW-trending graben surrounded by bedrock highlands. Most of the western half of the valley is a young alluvial plain formed of coalescing alluvial outwash fans issuing from drainages flowing towards Lake Helena. The western valley is ringed by older pediment surfaces sloping gently down from the valley margins. The eastern half of the valley is underlain primarily by later Tertiary silts, sands and gravels uplifted and segmented by normal faulting. Atop these Tertiary deposits along the southern valley margin, early Quaternary alluvial deposits are preserved as hilltop remnants of a formerly much larger deposit. Late Cenozoic faulting shaped the Helena valley, creating the modern topography and influencing sedimentation patterns; however, only a few faults show evidence of late Quaternary offsets.

Key Lithologic Features

  • well-cemented breccia of siltstone
  • flow-banded rhyolite from Elkhorm Mountainas
  • Belt argillite
  • Mount Mazama ash

Structures

  • E-W-trending fault from late Quaternary, behind Fort Harrison
  • Franklin Mine Road fault
  • Iron Gulch fault
  • Helena valley fault with later Pleistocene offset
  • late Pleistocene alluvial gravels
  • Pinedale(?) alluvial fan deposit from Scratchgravel Hills
  • late Pleistoncen alluvial depostis of Silver Creek
  • Glacial Lake Great Falls deposits near Lake Helena

Landforms

  • alluvial plain near Lake Helena
  • Holocene terraces on Prairie Road
  • Holocene alluvial plain

Other Features

  • Last Chance Gulch-gold in Holocene gravels
  • Scratchgravel Hills-gold bearing gravel
  • Frankline Mine
  • Helena Lake
  • pretrified wood and fossil bones near rhyolite from Elkhorn Mountains
  • Castle sapphire mine

Reference

Stickney, M., 1987, Road Log No. 3, Quaternary Geology and Faulting of the Helena Valley, Montana, in Berg, R.B., and Breuniger, R., 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.