Geologic map of the central Beaverhead Mountains, Lemhi County, Idaho, and Beaverhead County, Montana

Scientific Investigations Map 3413
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Abstract

This geologic map of the central Beaverhead Mountains portrays a complex geologic history of depositional basin development interspersed with deformational events. Generalized geology for young basins, compiled from sources on both sides of the range, is combined with newly mapped bedrock geology to better integrate geologic development of the map area.

Successive extensional basins were obliquely oriented across deformed strata of each preceding basin and of the Paleoproterozoic basement. Strata deposited in these basins include (1) thick fine-grained arkosic strata of the Mesoproterozoic Lemhi basin deposited on Paleoproterozoic basement with shoreline exposed on the east side of the map, (2) siliciclastic and carbonate strata of the Late Neoproterozoic-early Paleozoic miogeocline that were deposited in deeper environments to the west and interfingered with cratonal basin deposits to the east, and (3) generally coarse deposits in several nested, fault-bounded Eocene to Holocene basins.

Syndepositional structural disruption including tilting and angular unconformities is present within strata and between stratigraphic packages formed during the different basin-filling events. Cretaceous, east-northeast-directed thrust faults inverted Mesoproterozoic and Neoproterozoic-Paleozoic basins and stacked strata from diverse stratigraphic packages and different depositional settings. The thrust plates rotated as they impinged on the Paleoproterozoic arch on the east side of the map, resulting in complex fault geometries that present as thrust faults to oblique reverse and tear (or ramp) fault along different fault segments. Cenozoic extension caused successive normal-fault basins of several orientations. Eocene volcanic rocks are preserved in fault-bounded depositional basins formed during the onset of Cenozoic extension. Eocene basins were obliquely overprinted by Oligocene-Miocene normal-fault basins. Holocene basins developed during steep normal faulting that formed the present Basin and Range topography.

This geologic map of the central Beaverhead Mountains is mapped at 1:24,000 scale and printable at 1:50,000 scale. These data were collected between 1997 and 2017 and synthesized to provide significant new stratigraphic and structural data and interpretations.

Suggested Citation

Lund, K., 2018, Geologic map of the central Beaverhead Mountains, Lemhi County, Idaho, and Beaverhead County, Montana: U.S. Geological Survey Scientific Investigations Map 3413, pamphlet 27 p., scale 1:50,000, https://doi.org/10.3133/sim3413.

ISSN: 2329-132X (online)

Study Area

Table of Contents

  • Introduction
  • Depositional Settings of Mesoproterozoic and Paleozoic Rocks
  • Settings of Cenozoic Deposits
  • Deformation History
  • Description of Map Units
  • References Cited

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Geologic map of the central Beaverhead Mountains, Lemhi County, Idaho, and Beaverhead County, Montana
Series title Scientific Investigations Map
Series number 3413
DOI 10.3133/sim3413
Year Published 2018
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Central Mineral and Environmental Resources Science Center
Description Sheet: 50.0 x 46.0 inches; Report: iv, 27 p.; Read Me; Data Release
Country United States
State Idaho, Montana
County Beaverhead County, Lemhi County
Other Geospatial Central Beaverhead Mountains
Online Only (Y/N) Y