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Detailed Structural Geology of the Central Part of the West Valley 7.5’ Quadrangle, Southwest Montana by Aaron Berger and Colleen Elliott Montana Bureau of Mines and Geology Open-File Report 560 2006 This report has had preliminary reviews for conformity with Montana Bureau of Mines and geology’s technical and editorial standards. Partial support has been provided by the EDMAP component of the National Cooperative Geologic Mapping program of the U.S. Geological Survey under contract Number 03HQAG0063.

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  • Detailed Structural Geology of the Central Part of the West Valley 7.5’ Quadrangle, Southwest Montana

    by

    Aaron Berger and Colleen Elliott

    Montana Bureau of Mines and Geology Open-File Report 560

    2006

    This report has had preliminary reviews for conformity with Montana Bureau of Mines and geology’s technical and editorial standards. Partial support has been provided by the EDMAP component of the National Cooperative Geologic Mapping program of the U.S. Geological Survey under contract Number 03HQAG0063.

  • 1

    Introduction Detailed geological mapping of the Anaconda metamorphic core complex within the West Valley 7.5’ quadrangle, southwest Montana, during the summers of 2003 and 2004 spanned a region that contained both footwall amphibolite and hanging wall greenschist facies rocks. The footwall contains a locally mylonitic package of rocks within a tectonically thinned stratigraphic sequence that hosts a confusing array of structures and as many as four foliations. The footwall experienced amphibolite-grade metamorphic conditions followed by post-tectonic retrograde hydration that entirely replaced most porphyroblasts with mica. This hydrating event was probably responsible for sulfide mineralization seen within the West Valley 7.5’ quadrangle. The hanging wall of the Anaconda complex locally contains greenschist facies assemblages that exhibit bedding-subparallel breccias within resistant rock types and ductile deformation within aluminous carbonate lithologies. Brecciation was tectonic rather than depositional. Many large faults cut the hanging wall, including a large, possibly syndetachment thrust that placed lower Mississippian carbonates over Upper Cretaceous clastic and carbonate rocks. The deformational history, as determined by detailed mapping and observation of crosscutting relationships, is summarized as follows, where S designates a foliation and F designates a fold generation:

    1) Transposition of bedding into parallelism with an early tectonic fabric (S1), and isoclinal folding (F1);

    2) Boudinage at all scales and emplacement of numerous granitic intrusions; 3) Eastward transport along a subhorizontal mylonitic detachment (mylonitic

    foliation Sm); 4) Two generations of asymmetric folding and creation of related foliations (S2, F2

    and S3, F3). The relative timing of these folds and Sm is not clear; 5) Northeast-southwest folds with no axial plane foliation (F4) 6) Brittle deformation associated with normal and strike-slip faulting and the

    formation of a widespread, north-striking, vertical fracture cleavage (S5). The timing of specific events is not known, but crosscutting relationships and preliminary radiometric dating by other researchers suggest that deformation began after deposition of the Upper Cretaceous Colorado Group and continued perhaps into Eocene time.

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  • 3

    Figure 2a. Location of the West Valley 7.5’ quadrangle in southwest Montana.

    Figure 2b: Bedrock geologic map showing location of West Valley 7.5’ quadrangle in relation to the Anaconda metamorphic core complex. Regional geology of Butte 1° x 2° quadrangle after Lewis (1998).

    2002 projec

    ?

    Anaconda footwal

    10

    Anaconda complehangingwal

    detachmen

    Town of

    Valle

    Big Hole

    2002 EDMA 7.5’ quadranglePprojec

    ?

    Anaconda footwal

    10

    Anaconda complehangingwal

    detachmen

    Town of

    ?

    Anaconda complexfootwall

    10

    Anaconda complex

    hanging wall

    Detachmentfault

    West Valley 7.5’ quadrangle

    Deer Lodge Valley

    Town of Anaconda

    ?

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  • 5

    Unit Descriptions

    Qal Alluvium (Holocene) - Gravel, sand, silt, and clay along active channels of

    modern rivers and creeks.

    Qaf Alluvial fan deposit (Holocene and Pleistocene) - Angular gravel, sand, and silt

    deposited along the margins of major valleys by entering streams.

    Qgm Glacial moraine deposit (Holocene and Pleistocene) - Unsorted mixture of clay

    to boulder size material transported and deposited by alpine glaciers. Deposits

    typically have a characteristic hummocky surface form.

    Tdio Diorite sill (?Tertiary) – Medium-grained diorite sill with phenocrysts of

    plagioclase, hornblende, pyroxene, and biotite. The sill intruded the amphibolite-

    grade Silver Hill and Flathead Formations, and the Upper Belt Supergroup during

    deformation and locally contains two generations of foliations. Sill is as much as

    90 ft (30 m) thick.

    Tg Granite (Tertiary?) – Medium-grained granite both in the amphibolite-grade

    metamorphic rocks and in the overlying lower-grade units. The granite is in some

    cases syntectonic and is weakly foliated.

    Kcg Colorado Group (Lower and Upper Cretaceous) - The upper Colorado Group

    was not previously recognized in the West Valley quadrangle. It consists of dark-

    gray to black shale that contains interbedded gray siltstone, fine-grained

    sandstone, and limestone units. These interbedded units become more abundant

    toward the gradational contact with the underlying Kootenai Formation.

    Kk Kootenai Formation (Lower Cretaceous) – The upper Kootenai Formation

    locally includes very deformed and thinned, gray, gastropod-bearing limestone

    underlain by reddish interbedded siltstone and sandstone. The lower Kootenai

    includes a ridge-forming, medium-grained feldspathic to quartzose sandstone that

  • 6

    exhibits extensive brecciation within those beds subjected to bedding-subparallel

    faults.

    Psh Shedhorn Formation (Permian) - The Shedhorn Formation is a tan to light-red,

    greenschist-grade subarkose that is a local ridge-former. Like the Kootenai

    Formation, the Shedhorn exhibits widespread brecciation. However, the

    brecciation occurs throughout the formation rather than within individual beds.

    Many brecciated clasts have slickensides.

    Pp Phosphoria Formation (Permian) - The Phosphoria Formation consists of a

    ridge-forming meta-chert that is now a dark-gray quartzite and a valley-forming

    phosphatic shale. The quartzite is locally brecciated but deformation is mostly

    confined to the phosphatic shale that has in places been thinned to only a few

    inches.

    q Quadrant Formation (Pennsylvanian) - The Quadrant Formation is a cliff-

    forming massive, white quartzite with little to no bedding observed. The

    formation is extensively brecciated throughout the study area and clasts are

    cemented with silica. Where the formation is intact it, is capped by an inches- to

    several-feet-thick, tan-colored, silty limestone layer.

    Ma Amsden Formation (Pennsylvanian and Upper Mississippian) - The Amsden

    Formation, where present, consists of siltstone that has been metamorphosed to

    greenschist grade. In a few places, a silty dolomite is also present in the Amsden

    Formation, but typically this unit is missing. Thickness is about 10 ft.

    Mm Madison Group (Upper and Lower Mississippian) - The Madison Group

    consists of an upper unit of cherty limestone that is underlain by a more massive

    limestone with less chert. The Madison Group has been regionally and contact

    metamorphosed to foliated folded marble with abundant calc-silicate

    porphyroblasts. Deformation was concentrated in the group and the ~ 2,100-ft-

    (700-m-) thick Madison Group that is locally thinned to 30-60 ft within the West

    Valley quadrangle. Where the Madison Group is strongly thinned it is mylonitic,

    and bedding and foliation are transposed.

  • 7

    Єh Hasmark Formation (Upper Cambrian) - The Hasmark Formation is a

    massive, gray, metadolomite (Єhd) with a ~ 98-ft- (~30-m-) thick shale unit in the

    middle (Єhs) that has been metamorphosed to amphibolite grade. The formation

    is highly folded and some folds are overturned. At least three generations of folds

    are present in the Hasmark and the formation is locally mylonitic. All bedding in

    the Hasmark is transposed with S1. The formation also exhibits a well-defined

    mineral lineation except where this lineation has been obliterated by subsequent

    contact metamorphism.

    Єhs Middle shale unit of Hasmark Formation, where mapped separately

    within the formation.

    Єsh Silver Hill Formation (Middle Cambrian) - The Silver Hill Formation consists

    mainly of a metamorphosed silt- and clay-rich limestone that contains up to three

    generations of foliations. Locally, the Silver Hill contains a metamorphosed lithic

    sandstone that is now a green quartzite containing >5% pyrrhotite (FeS).

    Throughout the quadrangle the formation is a high strain zone, and the regionally

    328-ft- (100-m-) thick unit is thinned to only a few decimeters if it is present at

    all. The Silver Hill may have accommodated a few degrees of rotation of the

    overlying Hasmark Formation. As in the overlying Hasmark Formation, bedding

    in the Silver Hill is transposed with S1.

    Єf Flathead Formation (Middle Cambrian) - The Flathead is an extremely pure,

    ridge-forming quartzite with well-defined cross beds. The formation exhibits a

    weakly developed foliation in zones that contain more aluminous minerals.

    Locally, the Flathead is folded but regionally it shows boudinage at all scales.

    This boudinage reflects the same extension that thinned the overlying Silver Hill

    Formation. Porphyroblasts are common within the Flathead, but were replaced by

    biotite and muscovite during a hydrating retrograde metamorphic event.

    Υbe Upper Belt Supergroup (Middle Proterozoic) - The upper Belt Supergroup

    consists of highly deformed siliciclastic rocks exhibiting amphibolite facies

    metamorphism. Bedding within the upper Belt is transposed with S1 and

  • 8

    boudinage in compositional layers is common. Like the overlying Flathead

    Formation, porphyroblasts contain a retrograde metamorphic assemblage of

    biotite and muscovite. Like the Silver Hill Formation, the upper Belt exhibits up

    to three generations of foliations.

    Υc Middle Belt Supergroup carbonate (Middle Proterozoic) - The middle Belt

    carbonate is an amphibolite grade unit of impure limestone including a few clastic

    units. All bedding within the carbonate is transposed with S1. Also, like the

    overlying Flathead Formation and upper Belt, porphyroblasts that were not calc-

    silicates have undergone retrograde metamorphism and are replaced with mica.

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  • 10

    Source of Previous Mapping Lewis, R.S., 1998, Geologic Map of the Butte 1°x2°quadrangle: Montana Bureau of

    Mines and Geology Open-File Report 363, scale 1:250,000.

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