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Geologic map of the Strawberry Butte 7.5’ quadrangle, Meagher County, Montana

Scientific Investigations Map 3379
By:  and 
https://doi.org/10.3133/sim3379

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Abstract

The 7.5′ Strawberry Butte quadrangle in Meagher County, Montana near the southwest margin of the Little Belt Mountains, encompasses two sharply different geologic terranes.  The northern three-quarters of the quadrangle are underlain mainly by Paleoproterozoic granite gneiss, across which Middle Cambrian sedimentary rocks rest unconformably.  An ancestral valley of probable late Eocene age, eroded northwest across the granite gneiss terrane, is filled with Oligocene basalt and overlying Miocene and Oligocene sandstone, siltstone, tuffaceous siltstone, and conglomerate.  The southern quarter of the quadrangle is underlain principally by deformed Mesoproterozoic sedimentary rocks of the Newland Formation, which are intruded by Eocene biotite hornblende dacite dikes.  In this southern terrane, Tertiary strata are exposed only in a limited area near the southeast margin of the quadrangle.  The distinct terranes are juxtaposed along the Volcano Valley fault zone—a zone of recurrent crustal movement beginning possibly in Mesoproterozoic time and certainly established from Neoproterozoic–Early Cambrian to late Tertiary time.  Movement along the fault zone has included normal faulting, the southern terrane faulted down relative to the northern terrane, some reverse faulting as the southern terrane later moved up against the northern terrane, and lateral movement during which the southern terrane likely moved west relative to the northern terrane.  Near the eastern margin of the quadrangle, the Newland Formation is locally the host of stratabound sulfide mineralization adjacent to the fault zone; west along the fault zone across the remainder of the quadrangle are significant areas and bands of hematite and iron-silicate mineral concentrations related to apparent alteration of iron sulfides.  The map defines the distribution of a variety of surficial deposits, including the distribution of hematite-rich colluvium and iron-silicate boulders.  The southeast corner of the quadrangle is the site of active exploration and potential development for copper from the sulfide-bearing strata of the Newland Formation.

Suggested Citation

Reynolds, M.W., and Brandt, T.R., 2017, Geologic map of the Strawberry Butte 7.5’ quadrangle, Meagher County, Montana: U.S. Geological Survey Scientific Investigations Map 3379, pamphlet 16 p., 2 sheets, scale 1:24,000, https://doi.org/10.3133/sim3379.

ISSN: 2329-132X (online)

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Table of Contents

  • Abstract
  • Geologic Setting
  • Stratigraphy
  • Structure
  • Summary
  • Acknowledgments
  • Description of Map Units
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Geologic map of the Strawberry Butte 7.5’ quadrangle, Meagher County, Montana
Series title Scientific Investigations Map
Series number 3379
DOI 10.3133/sim3379
Year Published 2017
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Geosciences and Environmental Change Science Center
Description Report: iii, 16 p.; 3 Sheets: 37.53 x 33.04 inches or smaller; Data Release; Read Me
Country United States
State Montana
County Meagher County
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
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