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Title: The Beaverhead impact structure, SW Montana and Idaho: Implications for the regional geology of the western U.S.

Abstract

The Beaverhead impact structure in SW Montana and Idaho is an allochthonous fragment of a large impact structure ({approximately} 100 km diameter) that was transported some distance eastward during the Cretaceous Sevier orogeny. It is the first tectonic fragment of a large impact structure identified in the geologic record. The present evidence for impact consists of shatter cones, pseudotachylites, and planar deformation features in quartz. The age of the impact is not well constrained but is estimated to be Neoproterozoic to Cambrian (1000-500 Ma). The Beaverhead impact event must have created other features that may be preserved, elsewhere in western Montana and Idaho. These include proximal and distal ejecta (which may be misinterpreted as diamictites and/or tuff horizons) and other fragments of the crater floor containing shatter cones and pseudotachylite. A large circular gravity, magnetic and topographic anomaly, which could be the root of the impact structure, has been identified near Challis, Idaho. An enigmatic lithic tuff, identified in drill cores from the Challis area and an intraformational quartzite breccia in the Leaton Gulch area may be impact-related deposits, but no definitive evidence of shock metamorphism has been observed in these materials. The discovery of more pieces of the Beaverheadmore » puzzle, as well as the recognition of other large impacts in the geologic record, are likely once the regional geologic community grows to accept the incidence of such events and becomes more familiar with the features of shock metamorphism in the field. To that end, the community of geologists in this area should integrate the Beaverhead structure into their research and teaching curriculum.« less

Authors:
;
Publication Date:
Research Org.:
Lawrence Livermore National Lab., CA (United States)
Sponsoring Org.:
USDOE, Washington, DC (United States); National Science Foundation, Washington, DC (United States); Geological Survey, Reston, VA (United States)
OSTI Identifier:
86889
Report Number(s):
UCRL-JC-116524; CONF-9308269-1
ON: DE95014513; CNN: Grant EAR 92-05975; TRN: 95:005542
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Conference
Resource Relation:
Conference: 3. the belt symposium, Whitefish, MT (United States), 14-21 Aug 1993; Other Information: PBD: 17 Mar 1994
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; METEORITES; IMPACT SHOCK; CRATERS; STRATIGRAPHY; GRAVITY SURVEYS; MAGNETIC SURVEYS; MONTANA; IDAHO

Citation Formats

Fiske, P.S., and Hargaves, R.B. The Beaverhead impact structure, SW Montana and Idaho: Implications for the regional geology of the western U.S.. United States: N. p., 1994. Web.
Fiske, P.S., & Hargaves, R.B. The Beaverhead impact structure, SW Montana and Idaho: Implications for the regional geology of the western U.S.. United States.
Fiske, P.S., and Hargaves, R.B. Thu . "The Beaverhead impact structure, SW Montana and Idaho: Implications for the regional geology of the western U.S.". United States. doi:. https://www.osti.gov/servlets/purl/86889.
@article{osti_86889,
title = {The Beaverhead impact structure, SW Montana and Idaho: Implications for the regional geology of the western U.S.},
author = {Fiske, P.S. and Hargaves, R.B.},
abstractNote = {The Beaverhead impact structure in SW Montana and Idaho is an allochthonous fragment of a large impact structure ({approximately} 100 km diameter) that was transported some distance eastward during the Cretaceous Sevier orogeny. It is the first tectonic fragment of a large impact structure identified in the geologic record. The present evidence for impact consists of shatter cones, pseudotachylites, and planar deformation features in quartz. The age of the impact is not well constrained but is estimated to be Neoproterozoic to Cambrian (1000-500 Ma). The Beaverhead impact event must have created other features that may be preserved, elsewhere in western Montana and Idaho. These include proximal and distal ejecta (which may be misinterpreted as diamictites and/or tuff horizons) and other fragments of the crater floor containing shatter cones and pseudotachylite. A large circular gravity, magnetic and topographic anomaly, which could be the root of the impact structure, has been identified near Challis, Idaho. An enigmatic lithic tuff, identified in drill cores from the Challis area and an intraformational quartzite breccia in the Leaton Gulch area may be impact-related deposits, but no definitive evidence of shock metamorphism has been observed in these materials. The discovery of more pieces of the Beaverhead puzzle, as well as the recognition of other large impacts in the geologic record, are likely once the regional geologic community grows to accept the incidence of such events and becomes more familiar with the features of shock metamorphism in the field. To that end, the community of geologists in this area should integrate the Beaverhead structure into their research and teaching curriculum.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Mar 17 00:00:00 EST 1994},
month = {Thu Mar 17 00:00:00 EST 1994}
}

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  • The north end of the Pioneer Mountains is underlain by the following rocks: lower Proterozoic gneiss and amphibolite; middle proterozoic clastic rocks of the Missoula Group (Mount Shields Formation.); basal Cambrian clastic unit; Cambrian-Devonian-Carboniferous shelf sequence; Permian Phosphoria Formation; and Lower Triassic Dinwoody Formation. Jurassic rocks are missing. The Lower Cretaceous Kootenai Formation is of lagoonal to fluviatile facies, overlain by a thick (approx. = 2 km, 1 mi) section of fluviatile Colorado Group. The sedimentary rocks are cut by calc-alkalic plutonic rocks (80 to 65 m.y.B.P.), the oldest of which are synchronous with upper Colorado sedimentation. The youngest pre-quaternarymore » rocks are Eocene and Oligocene calc-alkalic lavas and Oligocene pumiceous tuff. The Missoula Group is entirely in thrust sheets that postdate the Colorado, so the thrusting is no older than Campanian, but the thrusts are cut by 72 to 74 m.y.B.P. plutons. In addition to the thrust sheets two families of high-angle faults dominate. The eastward projection of the mountain front at Maiden Rock, just south of Divide, resulted from block displacement along two strands of this fault. The second family of high-angle faults trends north-northeast. Wise River valley is interpreted to be a graben in this system. The west-northwest fault system began at least before the Eocene lava flows, but complex field relations between the two high-angle fault systems indicate their growth must have overlapped in age, possibly through the late Tertiary.« less
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