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

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. 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.

1 113°15' 113°00' 112°45' 112°30' 46°30' R14WR13W R12W R11W R10W R9W R8W

T8N

Deer Lodge

T7N Philipsburg Granite

T6N 46°15'

Galen Georgetown

T5N Lost Creek Warm Springs West Valley

Anaconda Opportunity

T4N

Butte T3N Fairmont Ramsay Rocker Hot Springs 46°00' 113°00' 5 0 5 10 Miles

113°15' 113° N Fred Burr Pozega Mount Location Map 116° Lakes Powell 114° 104° Lake 112° 110° 108° 106° 49°

Kalispell MONTANA 46°15' 46°15' 48°

15 Great Falls

90 Silver West Anaconda Missoula 47° Lake Valley North Helena

94 Butte 46° Bozeman Billings 90 90

15 45° Mount Mount Anaconda Evans Haggin South 100 0 100 Miles

46° 46° 113°15' 113°

Figure 1. Location of the West Valley and adjacent 7.5' quadrangles.

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

Detachmentdetachmen Deer Lodge fault Valley V all 2002West EDMAValley e projec7.5’ quadranglePAnaconda quadrangle complex ? Anaconda hanging wall comple ? hangingwal

Town of Town of AAnacondanaconda complex Anaconda footwallfootwal

Big Hole 10

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).

Qal Qat Holocene Quaternary Qgm Pleistocene

Unconformity

Tdio Intrusive Tg Rocks Tertiary

Unconformity

Kcg Cretaceous

Unconformity

Psh

Pp Permian

&Ma Pennsylvanian

Mm Mississippian Unconformity

^hs

^h Cambrian ^sh

Unconformity

Ybe Middle Proterozoic Yc

Figure 3. Correlation of map units on West Valley 7.5' quadrangle.

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

5 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. 6

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 7 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.

8 Map Symbols

34 Strike and dip of bedding

34 Strike and dip of bedding, tops confirmed by primary structures

Depositional contact; dashed where approximately located, dotted where concealed

Fault contact; dashed where approximately located; dotted where concealed

Steep normal fault, symbol on hanging wall; dashed where approximately located; dotted where concealed

Low-angle detachment (normal) fault, symbol on hanging wall; dashed where approximately located; dotted where concealed

Thrust fault, symbol on hanging wall; dashed where approximately located; dotted where concealed

Strike-slip fault, symbol on hanging wall; dashed where approximately located; dotted where concealed

34 34 Axial plane foliation; overprinting generation with two ticks

34 Mylonitic foliation

34 Fracture cleavage (S5)

15 Lineation trend and plunge

Fold axial trace, generation marked where known F4

Breccia

Foliation (cross section only)

Mylonite (cross section only)

9 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.

10 MBMG Open File 560; Plate 1 of 1 MONTANA BUREAU OF MINES AND GEOLOGY Geologic Map of West Valley 7.5' Quadrangle, 2007 A Department of Montana Tech of The University of Montana

113°15' 113°

Fred Burr Pozega Mount Lake Lakes Powell Location Map 116° 114° 104° 112° 110° 108° 106° 49° 46°15' 46°15'

Kalispell MONTANA 48° Silver West Anaconda 15 Lake Valley North Great Falls

90 Missoula 47° Helena SW NE 94 Butte 46° Bozeman Billings Mount Mount Anaconda 90 Evans Haggin South 90

15 45°

100 0 100 Miles &Ma 46° 46° &q 113°15' 113° Pp Mm Psh N

Ybe MBMG Open File 560 Mm ^h Detailed Structural Geology of the Central Part of the West Valley 7.5' Ye Quadrangle, Southwest Montana

Aaron Berger and Colleen Elliott

Maps may be obtained from: Publications Office 2007 Montana Bureau of Mines and Geology 1300 West Park Street Partial support has been provided by the EDMAP component of the National Cooperative Geologic Butte, Montana 59701-8997 Mapping Program of the U.S.Geological Survey under EDMAP award # 03HQAG0063. Phone: (406) 496-4167 Fax: (406) 496-4451 GIS production: Ken Sandau and Paul Thale, MBMG. Map layout: Susan Smith, MBMG. http://www.mbmg.mtech.edu