detailed structural geology of the central part of the ... · contact metamorphism. Єhs middle...
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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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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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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
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Anaconda footwal
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Anaconda complehangingwal
detachmen
Town of
Valle
Big Hole
2002 EDMA 7.5’ quadranglePprojec
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Anaconda footwal
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Anaconda complehangingwal
detachmen
Town of
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Anaconda complexfootwall
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Anaconda complex
hanging wall
Detachmentfault
West Valley 7.5’ quadrangle
Deer Lodge Valley
Town of Anaconda
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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
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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.
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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
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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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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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