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Master's Theses Graduate College

12-1984

Geology of a Section of the Northwest Tobacco Root Mountains, Madison County, Southwest Montana

Kiff James Samuelson

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Recommended Citation Samuelson, Kiff James, "Geology of a Section of the Northwest Tobacco Root Mountains, Madison County, Southwest Montana" (1984). Master's Theses. 1550. https://scholarworks.wmich.edu/masters_theses/1550

This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. GEOLOGY OF A SECTION OF THE NORTHWEST TOBACCO ROOT MOUNTAINS, MADISON COUNTY, SOUTHWEST MONTANA

by

Kiff James Samuelson

A Thesis Submitted to the Faculty of The Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Geology

Western Michigan University Kalamazoo, Michigan December 1984

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GEOLOGY OF A SECTION OF THE NORTHWEST TOBACCO ROOT MOUNTAINS, MADISON COUNTY, SOUTHWEST MONTANA

Kiff James Samuelson, M.S.

Western Michigan University, 1984

The northwest Tobacco Root Mountains are located on the boundary

between two major tectonic provinces: the fold and thrust belt and

the Rocky Mountain foreland. Deformation typical of the Rocky

Mountain foreland is present in the form of two major northwest-

trending faults with associated folds. Paleozoic rocks were

passively draped over at least one of the elevated Precambrian

blocks. Foliation of the Precambrian rocks also appears to have been

gently folded in the process.

Folds and thrusts occur within the Precambrian and Paleozoic

rocks, on the westernmost flank of the range, appearing to follow the

general trend of the fold and thrust belt. The two major thrusts dip

gently northwest and show dip-slip movement. The Paleozoic thrust

block is developed in thick carbonate units along the attenuated limb

of an eastwardly verging anticline; and the Precambrian thrust block

consists of schists, amphibolites, and quartzites with foliation

trends cut by the thrust plane.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS

The writer is grateful to Dr. Christopher Schmidt of Western

Michigan University. His assistance with the field research and his

counsel is appreciated.

Acknowledgment is made to Dr. Thomas Straw and Dr. Ronald Chase,

Western Michigan University, for their reading and suggestions of the

early draft.

The writer also was ably assisted by Dr. Phillip Kelly, Mankato

State University, Mankato, Minnesota.

The research for this study was partially funded by grants from

the National Science Foundation (NSF Grant EAR 7926380 to C. Schmidt

and J. Garihan), the Geological Society of American (GSA Grant 2715-

80), and the Graduate College of Western Michigan University.

Kiff James Samuelson

ii

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SAMUELSON, KIFF JAMES

GEOLOGY OF A SECTION OF THE NORTHWEST TOBACCO ROOT MOUNTAINS, MADISON COUNTY, SOUTHWEST MONTANA

WESTERN MICHIGAN UNIVERSITY M.S. 1984

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ACKNOWLEDGMENTS ...... ii

LIST OF F I G U R E S ...... v

LIST OF PLATES...... vi

INTRODUCTION ...... 1

Geologic Setting ...... 1

REVIEW OF SELECTED LITERATURE ...... 6

METHODOLOGY...... 8

STRATIGRAPHY ...... 10

General Statement ...... 10

Description of Stratigraphic Column ...... 10

Precambrian Rocks ...... 10

Paleozoic Rocks ...... 13

Cambrian Systems ...... 13

Devonian S y s t e m s ...... ' ...... 16

Mississippian Systems ...... 16

Late Cretaceous-Early Tertiary Intrusive Rock ...... 17

Cenozoic Deposits ...... 21

DEFORMATIONAL FEATURES OF THE ROCKY MOUNTAIN FORELAND .... 23

Pre-Thrust Uplift ...... 23

Northwest-Southeast Faults ...... 24

Late Precambrian Ancestry to Northwest Faults ...... 27

Folding Associated With Northwest Faults ...... 28

DEFORMATIONAL FEATURES OF THE FOLD AND THRUST BELT...... 30

lii

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Pattern and Relative Age of Thrusting...... 30

The Beall Canyon Thrust ...... 33

Hellroaring Thrust ...... 36

SIGNIFICANCE OF THRUSTING IN BASEMENT ROCKS ...... 39

CENOZOIC NORMAL FAULTING ...... 42

GEOLOGIC HISTORY ...... 45

CONCLUSION...... 49

BIBLIOGRAPHY ...... 50

%

iv

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

1. Location Map of the Study A r e a ...... 2

2. Exposures of Pre-Belt Basement and Anticlinorium and Synclinorium of Southwest Montana ...... 4

3. Geologic Map of the Study Area Along the Northwest Flank of the Tobacco Root Mountains...... 9

4. Generalized Stratigraphic Column of the Northern Tobacco Root Mountains ...... 11

5. Paleozoic Rock Locations ...... 14

6. Igneous Intrusive Locations ...... 18

7. Triangular Diagram for the System Quartz-Alkali Feldspar— Plagioclase of the Plutonic Rocks...... 19

8. Major Northwest-Southeast Faults of the Region ...... 25

9. Stereographic Plots, Depicting Fold (Cambrian) Bedding and Flexed (Precambrian) Foliations ...... 29

10. Thrust Faults of the Study Area, With Respect to the Structures North of the Ar e a ...... 31

11. Hellroaring and Beall Canyon Thrust Block Locations . . . 32

12. Stereographic Plots of Structural Data for the Hellroaring and Beall Canyon Thrust Blocks ...... 35

13. Hellroaring Thrust Block, Lithologies and Structure . . . 37

14. Aerial Photograph Mosaic Illustrating the Change of Trend Along the Range Front Fault Trace ...... 43

v

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. L IS T OF PLATES

I. Geologic Map of the Northwestern Flank, Tobacco Root Mountains

II. Cross Sections: A-A', B-B\ C-C'

vi

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION

The purpose of this study is to analyze the structural features

in a segment of the western slopes of the Tobacco Root Mountains, an

area not previously mapped in detail. Mapping on a scale of 1:24000

was done to show the relationship between near-vertical northwest

trending faults and low-angle thrust faults involving Precambrian and

Paleozoic rock. The research involved studying the style and geomet­

rical characteristics of the structures (faults and folds), the

extent of their interrelationships, the fault movements involved, and

the behavior of the basement rocks in each deformation style. This

area is particularly interesting in that it is located on the bound­

ary between the fold and thrust belt and the Rocky Mountain foreland.

The area described in this paper is located in Madison County,

Montana, on the western slopes of the Tobacco Root Mountains,

approximately 40 miles southeast of Butte and 20 miles south of

Whitehall. It covers approximately 35 square miles extending from

Bear Gulch on the south to Brooks Creek on the north and from the

western front of the range into the Precambrian core on the east

(Figures 1 and 3).

Geologic Setting

The west flank of the Tobacco Root Mountains is largely made up

of Paleozoic limestones, minor shales, and sandstones. These rocks

rest uuconformably upon Precambrian gneisses, and both have been

1

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MADISON COUNTY

TOBACCO ROOT MOUNTAINS 45*30 m u Area Mapped

MONTANA

30 MILES

40 KILOMETERS

112°00

Figure 1-Location map of the study area, Tobacco Root Mountains, Madison County, southwest Montana r (modified from Vitaliano, and o t e r s , 1 9 7 9 ) .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3

intruded by igneous rock, folded, and subsequently, block-faulted.

The Precambrian terrain is composed of Archean rock which is

located about 80 kilometers northwest of the boundary between the

Churchill and Wyoming provinces (Figure 2). Major fault trends in

the terrain are northwest, northeast, and east-west. These fault

trends may have been initiated during middle Precambrian time in

response to compressive forces associated with accretion of the

Churchill terrain to the Wyoming craton. This Precambrian

(Churchill) terrain formed part of a southern barrier to late Pre­

cambrian Belt sedimentation, as outlined by Burchfiel and Davis

(1975) and Schmidt (1975). Schmidt and Garihan (1979) have suggested

that this barrier may have developed during late Precambrian rifting,

preventing the deposition of Belt sediments in the area.

The Paleozoic formations which unconformably overlie the Pre­

cambrian rocks are shelf deposits associated with the Cordilleran

geosycline of western North America. These formations represent a

fluctuating marine environment of an Atlantic-type trailing margin

(Stewart, 1976).

Tectonic changes during Mesozoic and Cenezoic time were related

to the emplacement of igneous rocks, deformation during the Sevier

and Laramide orogenies, and Basin and Range type block faulting. The

closest magmatic belt is the Idaho-Montana porphyry belt (Chadwick,

1981). Along the southeastern edge of this belt lies the Boulder

batholith which is exposed 13 kilometers northwest of the study area

(Figure 8).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■t-

(1.7 b.y.o.) (2.7 b.y.o.) (Gilettl, 1966) Wyoming Province Churchill Province Province boundary 20 40 K m TR j ' IDAHO MONT. 113' —45°l (modified from Fountain and Desmarais, 1981; and Schmidt, 1981). Schmidt, and 1981; Desmarais, and Fountain from (modified Greenhorn Range(G), Snowcrest Range(S), Gravelly Range(GR), and the Absaroka Range(A) Absaroka the and Range(GR), Gravelly Range(S), Snowcrest Range(G), Greenhorn Madison Range(M), Gallatin Range(GA), Ruby Range(R), Blacktail Range(B), Tendoy Range(T), Range(T), Tendoy Range(B), Blacktail Range(R), Ruby Range(GA), Gallatin Range(M), Madison Montana, in the following mountain ranges: Highland Mountains(H), Tobacco Root Range(TR), Range(TR), Root Tobacco Mountains(H), Highland following ranges: the in mountain Montana, Figure 2- Exposures of pre-Belt basement with major anticlinorium and synclinorium of southwest of synclinorium and anticlinorium major with Exposures basement 2- Figure pre-Belt of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The Sevier orogeny (late Jurassic-Cretaceous) resulted in a

foreland fold and thrust belt within an arcuate pattern. One section

of this pattern is called the Central Montana Salient (McMannis,

1965). The Tobacco Root Mountains lie on the Southwest edge of this

salient (Figure 10).

Laramide deformations (Late Cretaceous to middle Eocene) gave

rise to basement uplifts of the Wyoming province style of deformation

(Prucha, Graham, & Nickelson, 1965). The Tobacco Root Mountain area

is one of the northwesternmost uplifts. The Precambrian basement

rocks form a large domal uplift with arches radiating outward (Figure

2), extending as individual segments, one plunging to the southwest

and the other south (Scholten, 1967). According to Schmidt and

Garihan (1979), the uplift was facilitated by movement along

northwest-southeast faults spaced roughly 7 to 10 kilometers apart

(Figure 8). These northwest faults trend subparallel to the Lewis

and Clark ("mega-shear") linement (Sales, 1968). Sevier and Laramide

age orogenies resulted in the superposition of two different styles

of deformation; uplifted Precambrian basement rocks with associated

"draped" Paleozoic formations and folded and thrusted Precambrian and

Paleozoic rocks.

The study area also lies within part of the Intermountain

Seismic Belt (R. E. Smith & Sbar, 1974) where elongated blocks were

tilted and elevated relative to down-dropped grabens. This block-

faulted terrain forms the present topographic relief of the Tobacco

Root Mountains.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REVIEW OF SELECTED LITERATURE

The earliest geologic report of the study area was made by

Hayden (1872). Investigations on the general geology and ore de­

posits in the area have since been made by Winchell (1914); Tansley,

Schafer, and Hart (1933); and Reyner (1947). Hayden’s report related

the physiography of the area to descriptions of rock formations and

structures. Winchell's study contains a description of the geology

of the district and its ore deposits; whereas Tansley, Schafer, and

Hart (1933) produced a geologic map of the Tobacco Root Mountains.

Reyner (1947) presented a general geologic appraisal and ore survey

and produced a map showing more specifically the area described in

this paper.

Johns (1961) concluded from his study of the southern Tidal Wave

mining district that three fault groups in the district trend north­

west, easterly, and northeasterly.

Reid (1957) made a general study of the bedrock geology of the

northwestern corner of the Tobacco Root Mountains. This study was

extended and modified by Hanley (1975), who made a detailed study of

the metamorphic petrology of the Precambrian rocks of the northern

and western Tobacco Root Mountains. Schmidt (1975) discussed the

structural geology within the Paleozoic rocks in the northern part of

the mapped area.

Kuenzi and Fields (1971) investigated the Tertiary stratigraphy

and structure of the Jefferson basin, on the west side of the range,

6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7

and Burfiend (1967) conducted a gravity survey of the northern

Tobacco Root Mountain, J. L. Smith (1970). Further studies and maps

have been made by Vitaliano et al. (1979) and Gillmeister (1972).

Their areas of study are either inclusive or adjacent to previous

studies.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. METHODOLOGY

For a period of 12 weeks, the study area was traversed, observa­

tions were made, and structural measurements were taken using a

Brunton compass. Data locations were plotted on aerial photographs.

A structural data map and rock contact map were produced in the

field. Igneous intrusions were sampled as were the various Pre­

cambrian lithologies of the Precambrian Hellroaring thrust block. A

third map was produced in the laboratory using photogeologic proce­

dures employing a mirror stereoscope and transparent overlays. Upon

completion of the three individual maps, they were overlaid, and a

single composite geologic map was produced (Figure 3, Plate I).

Three cross-sections were then constructed (Plate II). The struc­

tural data were plotted on a Schmidt stereographic projection to

facilitate statistical evaluations. Counting was accomplished using

the Kalsbeck method outlined by Ragan (1973). Igneous rocks were cut

into slabs and mineralogically analyzed by staining with sodium

cobanitrate to determine the feldspar content and a petrographic

study was made on the Precambrian rock samples from which a

lithologic map was produced (Figure 13).

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r : ^Ol'ciroon

I&alf Canj^n^-yV^

- - - • / *'.:* ^— ^b£^^r?1 - il :{kM f^.

Bsafc Gulch

9 Figure 3- Geologic map of the study area along the north­ west flank of the Tobacco Root Mountains, (see also Plate I).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STRATIGRAPHY

General Statement

Precambrian metamorphic rocks and Paleozoic sedimentary rocks up

to the Madison Group (Mississippian) are well exposed In the study

area. These have been intruded by Late Cretaceous stocks and sills

similar petrographically to the Tobacco Root batholith, which in­

truded the Precambrian metamorphic rocks of the core of the range

(J. L. Smith, 1970). The Jefferson basin is filled with Oligocene

and Miocene tuffaceous sands, conglomerates, and mudstones of the

Bozeman Group and capped with Quaternary pediment gravels and allu­

vium (Kuenzi & Fields, 1971). A complete generalized stratigraphic

column of the study area, and that area adjacent to it, of the

Tobacco Root Mountains is depicted in Figure 4.

Description of Stratigraphic Column

Precambrian Rocks

The Precambrian rocks of the Tobacco Root Mountains are correla­

tive to the metamorphic rocks of the Churchill province in the

Canadian Shield (1.7 b.y.). They unconformably underlie the un­

metamorphosed, dominantly sedimentary rocks of the area. These Pre­

cambrian rocks are believed to be a stratiform sequence of epi-

clastic, volcan-oclastic, and carbonate sedimentary rocks (Vitaliano

et al., 1979) which have been folded and metamorphosed during at

10

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GENERALIZED STRATIGRAPHIC COLUMN OF NORTHERN TOBACCO ROOT MOUNTAINS. MONTANA

OUM vtttCAl

Elkhom Mouni»int ( J C * t e c * 200 l**1

□

e j

DCi

Figure 4 (from Me Lane, 1974)'

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least three Precambrian episodes of deformation. They were origi­

nally divided by Peale (1896), Tansley et al. (1933), and Reid (1957)

into two distinct mappable units: the Pony series, consisting of a

layered sequence of metasediments, primarily quartzofeldspathic

gneiss, hornblende gneiss, amphibolites, and iron formation; and the

Cherry Creek series, consisting of dolomitic marbles, aluminous

schists, and a variety of gneisses.

A third distinct lithology was interpreted by Gillmeister

(1972), Cordua (1974), and Hanley (1975). The three units have been

termed the Spuhler Peak Formation, the Garnet Gneiss unit, and the

Gneiss of Catarck Mountain, respectively. These lithologic groupings

may be correlative to the Precambrian rock which occurs as an alloch-

thonous block known as the Hellroaring thrust, lying on the western

flank of the Tobacco Root Mountains. The allochthonous Precambrian

rock appears to be divided into two portions by an eastwardly trend­

ing fault. The lithologic sequence of the southern portion consists

of interbedded aluminous schists with varying amounts of quartz,

garnet, and hornblende; amphibolites; and micaceous quartzites.

These units appear to lense out along strike, ranging in thickness

from 0.6 to 6 meters. Mineral assemblages including the biotite-

hornblende-garnet assemblage would be indicative of lower

amphibolite-grade metamorphic conditions. Other major minerals pres­

ent are quartz and plagioclase with minor amounts of kyanite and

epidote.

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Paleozoic Rocks

The Paleozoic section above the Precambrian metamorphlc core is

roughly 1,554 meters thick. Formations from Middle Cambrian age

through Mississippian age are present. The section is roughly 80%

carbonate and 20% sandstone and mudstone (McLane, 1972). Two major

unconformities of regional extent are present. Besides the

Precambrlan-Cambrian unconformity, there is a surface of dis-

conformity between the Maywood Formation (Middle Devonian) and the

Snowy Range Formation (Upper Cambrian), which was formed as a result

of pre-Maywood erosions (Myers, 1952). Sedimentary rocks ranging in

age from Pennsylvanian through Cretaceous are found on the northern

and eastern flanks of the Tobacco Root Mountains, but are absent

from the study area.

The Paleozoic rocks from the Flathead Formation through the

Madison Group are well-exposed in the area (Figure 5). Generally,

the rocks dip toward the west and are exposed in ascending order of

the stratigraphic column from east to west. Locally, due to complex

folding and faulting, the beds are repeated or missing entirely.

Cambrian Systems

Flathead Formations. The Flathead Formation, the basal member

of the middle Cambrian age, is approximately 38 meters thick. It

consists of glauconitic, light red silica-cemented, medium-grained,

well sorted and rounded quartz sandstone which is typically cross­

bedded.

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N k

I_____ 1_____ I 0 I 2 km I______I 0 I mil*

M y j/y

Figure 5 Paleozoic rock locations which tilt to the west

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The sandstone lies nonconformably upon the Precambrian metamor-

phic complex. Interbedded quartzite and green shale beds are present

toward the top of the formation.

Wolsey Formation. The Wolsey Formation, lying conformably upon

the Flathead Formation, is a glauconitic olive to gold-brown,

calcareous and micaceous silty shale. Lenses of siltstone with

burrows occur throughout. The upper portion is interlayered with

shale and limestone. The Wolsey Formation is about 75 meters thick.

Meagher Formation. The Meagher Formation, which lies con­

formably upon the Wolsey Formation, has a mottled appearance and

ranges from dark gray to black in color. At the base is a re­

crystallized biomicrite, with fragmented fossil materials near the

bottom. Towards the middle, it is a blocky, mottled dolomite which

grades into an oolitic grainstone with cross-bedding. The upper

portion is thinly-bedded bio-sparite with algal coated grains. The

Meagher Formation is a cliff former, approximately 137 meters thick.

Park Formation. The Park Formation lies conformably upon the

Meagher Formation. It is a soft, green to black, non-calcareous,

fissile shale. It is a valley former, about 46 meters thick.

Pilgrim Formation. The Pilgrim Formation consists of about 110

meters of dolomite with color ranging from a medium-to-light gray

mottled tone. The Pilgrim is finely laminated limestone at the base

containing crystalline interclasts with massive cross-bedded dolo­

mite near the top.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16

Maywood Formation. The Maywood Formation as described by

Hanson (1952) Is upper Cambrian and lower Devonian age. It has a

thickness of 34 meters and is composed of cream-colored siltstone

and black to greenish-gray shale.

Devonian Systems

Jefferson Formation. The Jefferson Formation is a dark dolo­

mite varying from black to gray. It contains algal structures as

well as black chert nodules. The dolomite emits a distinct hydrogen

sulfide odor when broken. This cliff-forming rock is approximately

305 meters thick.

Three Forks Formation. The Three Forks Formation contains

green shale in the lower layers, a fossiliferous limestone in the

middle, and an upper layer of fine-grained tan sandstone with some

siltstone. The Three Forks Formation conformably overlies the

Jefferson Formation and is approximately 69 meters thick.

Mississippian Systems

Madison Group. The Madison Group is made up of cliff-forming

carbonates. It consists of the Lodgepole and Mission Canyon Forma­

tions. The Lodgepole is a gray limestone which is thinly bedded,

fossiliferous, and is approximately 312 meters thick. It grades

into the Mission Canyon which is a massive white-to-gray limestone

with cert nodules, approximately 366 meters thick.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17

Late Cretaceous-Early Tertiary Intrusive Rock

Igneous rocks have been defined as stocks, sills, and dikes

(Figure 6). Stocks intrude the entire stratigraphic section at the

front of the range near Bear Gulch (profile C-C', Plate II) and Dry

Boulder Gulch. They intrude Precambrian metamorphic rock at the

head of Bear Gulch.

In the lower Bear Gulch, a quartz monzodiorite stock occupies

an area of approximately 1.2 square kilometers. Although the stock

has probably intruded the entire stratigraphic section, it is

visible at the surface in only the Madison carbonates and the Three

Forks Formation. Although the primary composition of the intusion

is quartz monzodiorite (Figure 7), it changes to tonalite as it

approaches the contact with the country rock. The quartz

monzoidiorite is a gray, medium-grained rock consisting of plagio-

clase, orthoclase, hornblende, and quartz. Near the contact with

the country rock, the intrusive contains dark gray xenolith with

compositions tending toward diorite. The inclusions probably repre­

sent incompletely absorbed fragments of the country rock (Winchell,

1914 ).

Another stock is present along the north side of Dry Boulder

Gulch, covering an approximate area of one square kilometer. The

intrusion again is inferred to cut the entire section, and truncates

the southernmost section of the Beall Canyon thrust block. The

coloring and composition of the rock in Dry Boulder Gulch is similar

to that of Bear Gulch, with the compositional range being from

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Figure 6- Igneous intrusive locations,including stocks, sills, and dikes .

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w m m

ALKALI FELDSPAR- ! PLAGIOCLASE------2 • Head of Bear Gulch ■ Mouth of Bear Gulch T Mouth of Dry Boulder Gulch • Sill - Dry Boulder to Beall Canyon

Figure 7- Triangular diagram for the system quartz- alkali fe1dspar-p1agioc1 ase of the ulutonic rocks .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20

quartz monzodiorite to granite. It, too, contains dark gray xeno-

liths.

A third stock lies at the head of Bear Gulch and is eraplaced in

the Pony metamorphic complex. It occupies an area of 0.4 square

kilometer. Its texture and color range from even-grained to porphy-

ritic, white to light gray. Its composition is primarily grano-

diorite.

A quartz monzonite to quartz monzodiorite sill extends almost

continuously along the eastern edge of the Paleozoic formations from

the head of Dry Boulder Gulch northward into Beall Canyon. It is

bounded on the east by Precambrian metamorphic rocks which comprise

the core of the range. The southernmost exposure of the sill is

found on the south side of the Dry Boulder Gulch beneath the Flat­

head Formation. Northward, the sill is exposed between the Pilgrim

Formation and the Pony metamorphics, eliminating the Park, Meagher,

Wolsey, and Flathead Formations. In this area, the sill is about

500 meters thick and appears to have assimilated rocks through upper

Cambrian age. Further north at Hulbert Gulch, the sill engulfs a

block of Cambrian Meagher Formation and appears to split into two

segments. The western part is 330 meters thick and intrudes the

Jefferson Formation (Devonian).

North of Bumby Gulch, the western and eastern sections merge

and begin to intertongue with the sedimentary section. It appears

that the sill is intruded along the major lithologic boundaries,

narrowing within each of the intruded carbonate formations. The

sill is a red-brown weathering rock with phenocrysts of orthoclase

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21

Imbedded in a medium-grained to aphanitic groundmass composed of

orthoclase, plagioclase, hornblende, and biotite. The primary com­

position is quartz monzonite porphyry; however, it ranges in compo­

sition from quartz monzonite to tonalite, probably as a result of

assimilation of the country rock.

The most prominent mafic dike of the area cross-cuts the Bear

Gulch stock and is about 25 meters thick. The rock is a dark brown

to black, aphanitic basalt (Reyner, 1947). A second mafic dike cuts

the stock at the mouth of Dry Boulder Gulch. Other minor silicic

sills and basic dikes occur throughout the study area.

According to Vitaliano et al. (1979), these Late Cretaceous-

Early Tertiary intrusives of the area were emplaced in one major

surge with compositional variation due to assimilation of country

rock and in place differentiation. J. L. Smith (1970), using modal

and chemical analysis, indicated that dioritic rock is restricted to

satellite intrusions along the northwest side, near the Tobacco Root

batholith, with the rock becoming more silicic toward the southeast.

Cenozoic Deposits

Cenozoic deposits exist in the form of basin fill, floodplain

sediments, glatial debris, and colluvial materials. Basin fill of

the fault-bounded Jefferson basin include aoelian silts of the Six-

mile Creek Formation, volcanic debris of the Renova Formation

(Kuenzi & Fields, 1971) and floodplain sediments. The pediment is

covered along the west side of the range by broad alluvial fans

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. radiating out from Dry Boulder Gulch and Bear Gulch. Lateral

moraine glacial deposits occur in Bumby Gulch and Dry Boulder Gulch.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DEFORMATIONAL FEATURES OF THE ROCKY MOUNTAIN FORELAND

Pre-Thrust Uplift

The study area lies on the northwestern flank of a large north­

westerly plunging arch (Figure 2) named the Madison-Gallatin Arch

(Scholten, 1967) and the Tobacco Root Anticlinorium (Schmidt, 1975).

In the northwest Tobacco Root Mountains this broad arch, defined by

the attitude of the lowermost Cambrian strata, is oriented 40 degrees

north, 25 degrees west (Figure 2).

Paleozoic formations are draped over the arch. Excluding the

areas that have been folded and thrusted, these Paleozoic formations

show a gentle dip near the core of the range. The Cambrian rocks

which unconformably overlie the Precambrian metamorphic rock, are

tilted to the west with dips varying from 30 degrees to 40 degrees

west. There is a lowermost portion of the Cambrian formation lying

southeast of the study area, and closer to the crest of the Tobacco

Root Anticlinorium. Existing there are dips of 5 degrees to 10

degrees west. The bedding planes become steeper toward the western

flank of the range, where the Mississipplan formations dip 55 degrees

to 85 degrees west. This can be best seen along cross-section line

C-C* (Plate II) north of Bear Gulch.

The arching in the southwestern Montana area may have been

active by mid-Cretaceous time as indicated by provenance studies

conducted 50 kilometers west, in the Pioneer Mountains (Schwartz,

23

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24

1972). The deltaic sequence of the Blackleaf formation Implies that

an elevated eastern landmass was in existence. Further studies

(Schmidt, 1973; Schmidt, Vitaliano, & Exkerty, 1977; Smedes &

Schmidt, 1979) indicate that arching was active prior to the deposi­

tion of the Elkhorn Mountains Volcanics, based on the regional con­

figuration of conglomerates occurring on the sub-Elkhorn Mountains

Volcanics' erosion surface. Although some foreland uplift continued

during and after thrusting, a significant amount had occurred before­

hand to noticeably deflect the thrusting on the west flank of the

arch (Samuelson & Schmidt, 1981; Schmidt, 1975).

Northwest-Southeast Faults

The uplift of the Tobacco Root Anticlinorium apparently

occurred, not as a single arching, but in a series of northwest-

elongate blocks bounded by prominent northwest-trending faults, which

appear to be the oldest and largest in the Tobacco Root Mountains

(Schmidt & Hendrix, 1981). Two of these faults cross the study area:

the Bismarlc fault and the B and H fault (Figure 8).

Of the two northwesterly faults, the most evident is the Bismark

fault which cuts across Beall Creek north of the Strawn mine. It has

brought Precambrian hornblende gneiss into fault contact with

Cambrian formations. The Bismark fault may be the continuation of

the Spanish Peaks fault trend (Reid, 1957). Study of the Bismark

fault-line on the landsat imagery indicates that the fault is 100 to

150 kilometers long (Samuelson & Schmidt, 1981). The Bismark fault

strikes north 50 degrees west and dips 86 degrees northeast in Beall

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25

CM

E O

oco a S o <

■o in a

<£T Major northwest-southeast faults of the region (from 1981) Schmidt, region (from the of faults Major northwest-southeast Figure Figure 8-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26

Gulch north of the Strawn mine. Reid (1957) postulated a vertical

movement from 6.1 to 9.0 kilometers, to explain the present Cambrian-

Precambrian contact configuration. Hanley (1975) suggested the pos­

sibility of a horizontal displacement with a minimum of 2.8 kilome­

ters left-lateral slip to explain the present outcrop geometry.

Well exposed slickensides along the fault (68 degrees south, 60

degrees east) indicates an oblique movement for the Bismark fault

responsible for the offset of the Cambrian-Precambrian contact. As

measured off the map, the lower Cambrian units have been offset 5.1

kilometers. The apparent oblique-slip, based on slickenside orienta­

tion and net-slip, is 7 kilometers. Thus the Bismark fault is a near

vertical oblique-slip fault, north side up and toward the northwest.

The B and H fault is not well-exposed and exists primarily

within the Precambrian rock at the head of Bear Gulch close to the B

and H mine. Reid (1957) described the fault as having brought the

Precambrian metamorphic rocks into the contact with the Meagher

Formation (Cambrian). However, this cannot be verified with cer­

tainty, due to poor exposure and heavily timbered terrain. The

existence of this fault is clearly indicated by the truncation of

Precambrian amphibolite units. A shear zone within the Precambrian

rocks and mineralization at the B and H, and other mines lying paral­

lel to the fault zone, verify the fault's existence. Another indica­

tion of the presence of a northwest trending fault is an apparent one

kilometer right-lateral separation of a granodioritic intrusion be­

tween the B and H mine and Smelter Mountain.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27

The alignment of mines and prospects along this B and H fault

trend continues across the southern Tobacco Root Mountains and may be

expressed as a northwest-trending anticline (Buck Creek Anticline) in

the Madison Range (Hall, 1960). Compared with the Bismark fault,

there has been a minimal amount of offset of the B and H fault during

the Laramide orogeny.

Study of the basin fill by aerial photography indicates that if

the fault extends to the northwest, then there appears to be a line

separating a dissected pediment from a less dissected pediment of the

Jefferson basin. There is the possibility of late Cenozoic movement

along the northwest extension of the B and H fault, with the southern

side moving up, relative to the northern side movement.

Late Precambrian Ancestry to Northwest Faults

Based on slip and separation data collected on northwest faults,

Schmidt and Garihan (1979) postulated that the faults are related to

a major period of northeast-southwest rifting during the opening of

the Belt basin (Schmidt & Garihan, 1979).

Reid (1957) first suggested that the Bismark fault displayed a

horst movement during Precambrian time, with the northern block

moving upward. Mapping by Hanley (1975) clearly indicates that

Precambrian metamorphic units separated by the fault do not become

aligned when Laramide slip is restored. Hanley found that the total

displacement of the Precambrian units are on the order of three times

the Laramide slip. Determination of the horizontal component of

movement is difficult due to igneous intrusion along the fault.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28

Because of the emplacement of the Tobacco Root batholith, correlation

of similar rock types appearing on opposite sides of the fault is

uncertain. A regional compilation of major Precambrian rock assem­

blages by Vitaliano et al. (1979), however, may permit some estimates

to be made in spite of the presence of the Tobacco Root batholith.

It has been determined that a garnet schist-marble-quartzofeldspathic

gneiss sequence truncated by the fault, has a 15 kilometer left

separation of the Precambrian units (Samuelson & Schmidt, 1981).

This is approximately three times the 5.1 kilometers of Laramide

offset.

Folding Associated With Northwest Faults

The Laramide movement, although one-third the magnitude of Pre­

cambrian displacement, was a result of stresses released along the

same zone of weakness. According to statistical evaluation (Figure

9), the Precambrian foliation trends appear to have been flexed and

have a fold axis orientation of (B) 35 degrees north, 34 degrees

east. Hanley (1975) also noted a gentle folding of the Precambrian

rock adjacent to the Bismark fault. It was not until the Bismark

fault was reactivated, during the Laramide Orogeny, that the over-

lying Paleozoic formations were folded, with a fold axis developing

40 degrees north, 24 degrees west. The northwesterly plunge of the

folded Cambrian rock diverges from the northeast trend of the fold

axis in the folded Precambrian rock, due to the fact that the

Cambrian formations were deposited with moderate angular unconformity

above the foliated Precambrian rocks.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29

POLES TO BEDDING (Cambrian) \ W 90 56 BROOKS CREEK ANTICLINE £$ ' A -P

A . 4 r A A A POLES TO FOLIATION - BROOKS CREEK ANTICLINE 0 P ts. - 2.4•«, 6.0%, 8.4%, 10.8% Maxima 8.0% v 90 37 i *_

Figure 9- Sterographic plots, depicting folded (Cambrian) bedding and flexed (Precambrian) foliations.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DEFORMATIONAL FEATURES OF THE FOLD AND THRUST BELT

Pattern and Relative Age of Thrusting

Thrust faults and associated asymmetrical folds exist along the

western flank of the Tobacco Root Mountains for a distance of about

15 kilometers (Figure 10). Thrusts strike north-northeast and dip 30

to 40 degrees west. The folds associated with the faults are gener­

ally overturned to the east with axial traces paralleling the strike

of the faults.

Two major thrusts, the Hellroaring thrust on the west and the

Beall Canyon thrust, about one kilometer east of the Hellroaring

thrust, are present in the study area (Figure 11). Both are exposed

discontinuously along the western front of the range due to post-

Laramide normal faulting, basin filling, and pedimentation which

obscures their traces. The Beall Canyon thrust is the more continu­

ous of the two faults. In the northern part of the study area, it

truncates the gently arched Brooks Creek anticline and merges with

the northeasterly-trending Mayflower Mine fault near the hinge of the

anticline. Field relationships cannot conclusively prove that the

Beall Canyon thrust cuts the Bismark fault because of heavily tim­

bered west-facing dip slopes on the western flank of the range.

However, aerial photographic interpretation suggests that the Beall

Canyon thrust overrides the Bismark fault. Initial movement along

the Bismark fault, and associated folding of the Paleozoic section,

therefore appears to have been followed by thrusting.

30

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31

TOWNSEND BASIN

.ikoulcier. B'4tholfth*

/ Three Forks

'JEFFERSON BASIN, THREE FORKS BASIN

10M., _ j M iles 10 Ki lometers Fault with significant la te r a l movement Precambrian Metamorphic Rocks Thrust fault Concealed faults C D Precambrian-Mesozoic Sedimentary Rocks A n tic lin e E D Igneous Intrusions Syncline -Gr Overturned anticline I T s I T e rtia ry Sedimentary Rocks Overturned syncline

Figure 10- Thrust faults of the study area with respect to structures north of the area (from Schmidt, 1981) .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32

2km

ImlU

BEALL CANYON THRUST BLOCK

HELLROARING THRUST BLOCK

vOl

Figure 11- Hellroaring and Beall Canyon thrust block locations.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33

The Beall Canyon Thrust

The Beall Canyon thrust at Its northernmost segment was origi­

nally named the Mill Canyon fault by Schmidt (1975), who Interpreted

it as a splay or continuation of the Bismark fault. It now appears

more likely that it is a thrust which developed after the Bismark

fault and which connects with the thrust in Beall Canyon about one

kilometer to the south (Samuelson & Schmidt, 1981).

The Beall Canyon thrust strikes north 20 to 25 degrees west and

dips west at 40 to 50 degrees, and places Cambrian and Devonian

strata over the Precambrian metamorphic complex. The younger-over-

older relationship is a result of asymmetrical folds formed to the

west and subsequently thrusted onto the Precambrian cored uplift and

the hanging wall of the Bismark fault.

The allochthonous Paleozoic rocks are highly folded, with two

distinct fold orientations (Figure 9). One orientation of axial

traces trends slightly northeast while the other trends southwest and

lies perpendicular to the thrust plane. The north-trending fold is

directly related to the compressive stress which resulted in thrust­

ing. The west-trending folds appear to have been caused by secondary

stresses developed parallel to the strike of the thrust on the hang­

ing wall. Analysis of both sets and their relationship to the adja­

cent fault (Schmidt, 1975) indicates nearly pure dip slip for the

thrust.

The thrust splays into two segments at Mill Canyon rejoining at

the mouth of Wyckham Creek. The easternmost splay, which brings

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34

Mission Canyon limestone (Mississippian) over the Precambrian meta­

morphic rock (Schmidt, 1975), again shows a younger-over-older rela­

tionship. The fault dips 30 to 45 degrees west with well-developed

slickensides plunging due west (Samuelson & Schmidt, 1981). At the

mouth of Wyckham Creek the fault bends in a westerly direction and is

covered by the pediment gravel of the Jefferson basin.

The Beall Canyon thrust surfaces north of Beall Canyon near the

trace of the Bismark fault. Gently dipping Mission Canyon limestone

and Lodgepole limestone (Mississippian) are thrust over the Pilgrim

limestone (Cambrian). This portion of the fault strikes north 40 to

45 degrees east and dips gently north 20 to 30 degrees west as

determined by the three point method. The fault trace is exposed for

about two kilometers. It bends to the west and disappears beneath

the unconsolidated pediment gravels north of Bumby Gulch.

The thrust reappears 3 kilometers to the south (Figure 12)

between Bumby Gulch and Hulbert Gulch (T. 2 S., R. 5 W., sec 23). It

strikes north 30 degrees east, dips 30 to 35 degrees west, and trun­

cates folded Devonian and Mississippian rocks with a separation of

about 650 meters (Profile B-B', Plate II). Within the thrusted Beall

Canyon block, Mississippian and Devonian rocks are folded into an

overturned, moderately inclined syncline-anticline pair verging

toward the west. The anticlinal hinge is subparallel to the fault

trace which suggests the predominent movement direction is dip-slip.

The fold axle orientation shifts from north 17 degrees west to north

18 degrees east (Figure 12). The two domains of folding B^ and B£

may reflect diverse stress orientations on the leading edge of a

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35

6 ", N18 E ° 36 • N17 W

] mile

H e ' - * ° i A POLES TO BEDDING BEALL CANYON THRUST ‘'V ' 100 Pts. - 1.0., 2.0V, 4.0"., 6.0% Maxima 8.0% y /V* V ) ) f & s t f J f /* f ^Mmm\ &

Mml il»M

POLES TO FOLIATION - HELLROARING THRUST 72 Pts. - 2 .7 " , 5 .6 1 , 8 .3 1 , ILI% Maxima 15.0%

Figure 12- Stereographic plots of structural data for the Hellroaring and Beall Canyon thrust blocks.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36

thrusted block or the secondary forces applied to the Beall Canyon

thrust block by the overlying Precambrian (Hellroaring) thrust block.

A spaced cleavage which occurs in the Lodgepole limestone (Mississip­

pian) on the overturned limb of the syncline in the Beall Canyon

block is not axial planar, but strikes parallel to the Hellroaring

thrust and dips at nearly right angles to the thrust plane. The

Beall Canyon thrust plane and Paleozoic fold axis extend about 3

kilometers further south, where they are truncated by an igneous

intrusion.

Hellroaring Thrust

The Hellroaring thrust is exposed to the west of the Beall

Canyon thrust (Figure 12). The fault trace is subparallel to the

Beall Canyon thrust (north 30 degrees east) and dips 30 to 35 degrees

west. The lower plate (Beall Canyon) consists of folded Devonian and

Mississippian rocks while the upper plate (Hellroaring) consists of

Precambrian aluminous schists, interlayered with quartzite, marble,

and amphibolite (Figure 13). These rocks are significantly different

in composition from the rocks of the Precambrian metamorphic complex

which occur on the east side of the map area. Similar lithologies

have been described as the Spuhler Peak Formation (Gillmeister,

1972), located 10.3 kilometers to the southeast.

The foliations of the Precambrian rocks on the upper plate of

the Hellroaring thrust change trend from north-south (dip southeast)

as northeast-southwest (dip east). In the northernmost section of

the thrust block, the foliations trend east-west and dip north. In

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37

/\

.-O 1 /4 km

1 /4 m il*

Thrust fault

Quartzite layers

Shear zone

Tear fault

Quartz vein

Metabasite rock

Figure 13- Hellroaring thrust block, lithologies and structure. Light areas, with foliation symbol, consist of inter­ layered biotite schist with varying amounts.of horn­ blende and garnet; and amphibolite. Queried where uncertain.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38

general, foliations are cut obliquely by the Hellroaring thrust

(Profile B-B', Plate II), and the fold axis of the lower plate paral­

lels the thrust (Figure 12). This suggests that movement on the

fault was predominently dip-slip. A minimum dip-slip displacement of

3 kilometers is estimated considering the thickness of the Paleozoic

section cut by the fault. Foliation trends are also cut in places by

at least two shear zones in the Hellroaring block (Figure 13). The

shear zones appear to have been injected by quartz and garnet-rich

fluids, and may have formed during the fold and thrust phase of

deformation.

The Hellroaring thrust block is divided along a possible fault

evidenced by the abrupt change in rock type, the presence of higher

temperature mineral assemblages adjacent to the fault, the shift of

foliation orientation, and a change in topographic expression. The

northern portions of the block has limited outcrop exposure and

consists primarily of metabasite rock.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SIG N IFIC A N C E OF THRUSTING IN BASEMENT ROCKS

Low angle thrusting of Precambrian metamorphic basement rocks,

such as that seen in the Hellroaring thrust, and inferred for the

Beall Canyon thrust, occurs along the frontal portion of the fold and

thrust belt of the Cordillera in the western United States. In most

cases epidermal sheets of miogeoclinal rocks have been transported

eastward along detachment horizons within the sedimentary section.

Ruppel (1978) and Coppinger (1980) cited possible examples of

thrusted Precambrian metamorphic rocks in the Medicine Lodge thrust

system of the Beaverhead Mountains. Another example of thrusted

Precambrian metamorphic rocks in this same thrust system was cited by

Dubois (1981) for the northern Tendoy Range. Occurrence of alloch-

thonous Precambrian metamorphic rocks in the frontal zones of the

fold and thrust belt have been indicated by Schmidt (1975) and

Schmidt and Hendrix (1981) along the Jefferson Canyon-Mayflower mine

fault in the northern Tobacco Root Mountains. Anomalous blocks of

Precambrian rock overlie younger rocks within the Ruby and Blacktail

ranges and may be related to gravity sliding of anisotropic Pre­

cambrian blocks off the crests of these uplifts (Achuff, 1981;

Tysdal, 1970). Low angle thrusts with allochthonous Precambrian

metamorphic rock also occur within the Rocky Mountain foreland of

this region. The Greenhorn and Snowcrest thrusts in the Greenhorn,

Snowcrest, and Gravelly ranges have been described by Beck (1960),

Hadley (1960, 1980), Hall (1960), Mann (1960), Perry (1962), Swanson

39

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40

(1950), and Tilford (1978) describing thrusting of basement rock in

the western Madison Range.

A number of hypotheses have been offered for the involvement of

the Precambrian metamorphic rocks in the thrusts of southwest

Montana. R. L. Armstrong (1975), Ruppel (1978), and Sloss (1954)

suggest that a positive tectonic element, the Lemhi Arch, developed

in late Precambrian time, and was emergent intermittently during

Paleozoic time. This may have acted as a buttress during Cordilleran

folding and thrusting within the Medicine Lodge thrust system

(Samuelson & Schmidt, 1981). This implies that the Precambrian

basement rocks, being considerably higher here than elsewhere in the

fold and thrust belt, can be expected to be involved in thrusting.

Schmidt (1975) and Schmidt and Hendrix (1981) explained the thrusted

pieces of Precambrian basement in the northern Tobacco Root Range

analogously. They indicated that the frontal thrusts of the tear

thrust zone encountered uplifted foreland blocks at depth within the

Belt basin, breaking off pieces along foliation planes. Tilford

(1978) suggested the thrusted blocks of Precambrian in che Greenhorn,

Snowcrest, and Gravelly ranges were folded during upthrusting.

It appears that low angle thrusting, involving Precambrian rock

of the western Tobacco Root Mountains was a result of three sets of

circumstances: (1) the area's ductility was increased due to heating

associated with magmatic arc migration, (2) the Precambrian core of

the Tobacco Root Mountains was elevated enough so that the thrusting

impinged upon the arched rock, and (3) the Precambrian rock consisted

of a layered and foliated sequence which is highly anisotropic. In

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. effect, the anisotropy of these rocks did not permit them to behave

significantly differently from the overlying layered Paleozoic sec­

tion. Therefore, the increase in ductility, the elevated structural

position of these rocks, and their anisotropic nature lead a mechani­

cal behavior similar to the sedimentary rocks of the epidermal

thrusts of the fold and thrust belt (Samuelson & Schmidt, 1981).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42

CENOZOIC NORMAL FAULTING

West of the study area, the Jefferson basin began filling by

early Oligocene time. The Oligocene Renova Formation is inferred to

be the oldest Cenozoic sedimentary unit within the basin (Kuenzi &

Fields, 1971). During this time, it is unlikely that there was much

movement along the normal fault, which would account for the predomi­

nance of mudstones in the Renova Formation.

The occurrence of locally derived fanglomerates in the overlying

Sixmile Creek Formation (late Miocene-early Pliocene) and in the

northern Jefferson basin (Kuenzi & Fields, 1971) suggest that major

normal faulting along the Tobacco Root Mountains range-marginal

fault, began in late Miocene time. Post-middle Pliocene, pre-late

Pleistocene movement on this fault caused the Sixmile Creek strata to

be gently titled eastward toward the range. Estimates of dip separa­

tion range from 910 meters to 1,800 meters (Kuenzi & Fields, 1971;

Pardee, 1950).

The Tobacco Root Mountains range-marginal fault lies several

kilometers from the western front of the range as indicated by a

gravimetric anomaly (Burfiend, 1967). The fault scarp abruptly

changes trend in several places along the western portion of the

study area (Figure 14). Just north of Beall Canyon the trend changes

from north-south to northeast-southwest. Near Hellroaring Gulch

there is a north-south trend, and at Coal Creek Canyon, it changes

again to a northeast-southwest trend. These frequent changes in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43

N

u m m m. m. y, m' s.^...... s ' .:&& i i * i l JBSstfs*

» N r ? *

,:‘ *V ?\1 > hf **» „ t > ‘ *’ i » BEALL C A N Y O N - ^

v-:oV'y 1 \ ' \ f > ' Ar• : ^ ' \ '**''' 4 ^ ^ HEUROARtMGGULCH *.i4tofcfc$V*-v ' V

v* t 4* v 6 ****, \r COAfc-CRgEK CANYON-sK3; > ^ .S ; u

• F \ .-V. V : ' ' • <- ^ S 'V

i V 42 **,

*x<...... %yi*~riS3>'

«£:., ,..'.^ ...j------1 Figure 14 - Aerial photograph mosiac illustrating the change of trend along the range front fault trace.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. strike of the late Cenozoic range-marginal fault scarp are also

typical of the Madison and Bridger Ranges. They occur where major

northwest trending Laramide faults meet the range-marginal fault and

apparently reflect the influence of these previously developed struc­

tures (Samuelson & Schmidt, 1981).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GEOLOGIC HISTORY

The Wyoming-Superior Precambrian province (Fountain & Desmarais,

1980), an aggregate of preexisting Archean proto-continents, had

sufficient tectonic stability to support plate marginal mio-

geosyncline-eugeosyncline development by 2150 m.y. (Goodwin, 1974).

An accretionary sedimentary prism of epicratonic shelf, mio-

geosyncline, and eugeosyncline sedimentation developed out from the

margin of the Wyoming craton. The eugeosynclinal environment may

have evolved into a converging plate margin (Bird, Toksoz, & Sleep,

1975) initiating the accretion of the Churchill province to the

Wyoming continent (Fountain & Desmarais, 1980). The accreted sedi­

mentary prism of the Tobacco Root Mountains developed as a stratiform

sequence of epiclastic, volcano-clastic, and carbonate sedimentary

rock (Vitaliano et al., 1979).

Deformation and regional metamorphism occurred during the

Dillion event (1700-1500 m.y.) (Giletti, 1966), with the closing of

the marginal basin along reverse fault upthrusts possibly correlative

to the east-west zone of weakness of the Mayflower Mine fault-

Jefferson Canyon fault, north of the study area. The Bismark and B

and H faults (northwest) and the northeast trending shear faults may

have also been created at this time. Later stages of continent-

continent collision produced regional uplift (doming) (Bird et al.,

1975), which may have caused the Tobacco Root Mountains area to

become a southern barrier to the accumulating sediments of the Late

45

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46

Precambrian Belt-basin. This belt sediment barrier may have been

further uplifted along northwest faults, as a result of northeast-

southwest extensional rift tectonics, in Late Precambrian time

(Schmidt & Garihan, 1979).

During this time, from 1450 m.y. and apparently until 850 m.y.,

the accreting cratonic shield featured a broad central plateau which

may have provided the necessary provenance and regional transport

slope (Goodwin, 1974) for the accumulation of sediments along the

western margin of the North American craton, in the Belt basin, and

the Cordilleran geosyncline.

Paleozoic sediments of the area accumulated in an unstable shelf

environment, with continually fluctuating sea level. The Cambrian

sediments deposited were sandstone and siltstone overlain by alter­

nating limestones and shales. During Ordovician time, the Klamath-

Sierra island arc formed to the west, in present-day California and

Oregon, beginning subduction tectonics. By Devonian time, the com-

pressional forces acting on the west side of the island arc shifted

to the eastern margin of the Cordilleran marginal sea (Burchfiel and

Davis, 1975). The Tobacco Root Mountain area became a positive

element during Devonian time (Suttner, Schwartz, & James, 1981) with

Ordovician and Silurian sediments having been eroded or not de­

posited. Shales and limestone were deposited on the area of late

Devonian time and were later overlain by the accumulation of exten­

sive carbonates during Mississippian time. During Pennsylvanian

time, the outlying deposits of the study area became terrestrial and

shallow marine.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. During Mesozoic time, subduction tectonics continued to the

west, and resulted in plutonic-volcanic arc activity in the craton

with the development of a batholith complex in central Idaho and

southwest Montana (Burchfiel & Davis, 1972; Hyndman, 1979). In­

creased ductility in conjunction with expanding igneous intrusion and

regional tectonic compressive forces produced crustal shortening by

thrusting during the Sevier orogeny (Late Jurassic-Cretaceous). How­

ever, thrusting of the Hellroaring and Beall Canyon blocks was pre­

ceded by vertical uplift, possibly by the development of a thermal

welt over these rising batholiths (Suttner et al., 1981). As the

subducting plate's rate of movement slowed, the angle of subduction

increased, and the magmatic arc shifted eastward. This may have

caused basement uplifts to occur as the Rocky Mountains foreland

during Laramide time (Late Cretaceous-mid Eocene) along the Bismark

and B and H faults.

The Sevier and Laramide orogenies had dramatically affected the

area by late Cretaceous to Eocene time, with Precambrian fault blocks

becoming reactivated along previous zones of weakness (Schmidt &

Hendrix, 1981). The initial uplifting of the Rocky Mountain foreland

preceded the thrusting and folding of the miogeosyncline and shelf

deposits from west to east. Intrusion of the Boulder batholith, the

Tobacco Root batholith and satellite stock and sills, accompanied the

folding and thrusting (Robinson, Klepper, & Obradovich, 1968).

During mid to late Cenozoic time, extensional (Basin-Range)

tectonics affected the area through uplift of Precambrian-cored base­

ment blocks along normal faults. This fragmentation of the crust may

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48

be attributed to a plastically extending substratum at a depth

(R. E. Smith & Sbar, 1974; Stewart, 1972) produced by magmatic

activity migrating eastward, intraplate weaknesses (R. B. Smith,

1978), and extensional forces created by the formation of transform

fault tectonics to the southwest (Coney, 1978).

The lntermontane Jefferson basin (graben) was formed between

these uplifted blocks and was filled by mudstones during Oligocene

time. Following this was a broad regional uplift during the Miocene

epoch, with block faulting and volcanisra. Major epierogenic uplift,

during the Pliocene epoch, produced broad alluvial fans radiating out

from Dry Boulder Gulch and Bear Gulch. During Pleistocene time,

basin fill was partially eroded due to continued uplift (Kuenzi &

Fields, 1971). Alpine glaciation in the higher valleys cut cirques

and deposited glacial debris in the gulches of the Tobacco Root

Mountains with stream erosion and rockslides occurring in the

gulches.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONCLUSION

The western Tobacco Root Mountains are part of a zone of overlap

in the fold and thrust belt and the Rocky Mountain foreland. The

foreland was elevated by recurrent movement along northwest-trending

faults which originally developed during Late Precambrian time. Up­

lift of the foreland blocks was accompanied by folding of the Paleo­

zoic sedimentary section with the formation of northwesterly plunging

asymmetrical folds. The Precambrian rock adjacent to the northwest

faults also appear to be gently folded.

Initial phases of Laramide faulting were followed by low-angle

thrusting associated with eastward migration of the fold and thrust

belt. The Beall Canyon thrust developed in asymmetrically folded

Paleozoic rocks, along the west flank of the Precambrian-cored fore­

land uplift. Thrusting attenuated the east limb of these asymmetri­

cal folds. The Hellroaring thrust transported layered Precambrian

metamorphic rock over the folded Paleozoic Beall Canyon block.

In both thrust belt and foreland deformation, the Precambrian

rocks are folded adjacent to the faults. The thrust belt folding of

the allochthonous Precambrian rock may be due to their anisotropic

nature, causing them to behave mechanically similar to the overlying

Paleozoic section. The Precambrian folding in the foreland deforma­

tion occurred in late stages of faulting (Schmidt & Garihan, 1980)

along corners of major blocks (Tysdal, 1981).

49

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIBLIOGRAPHY

Achuff, J. M. (1981). Structural analysis of faulting and folding in Small Horn Canyon, Beaverhead County, Montana. Montana Geological Society, Field Conference Southwest Montana, pp. 239- 243.

Andretta, D. B., & Alsup, S. A. (1960). Geology and Cenozoic history of the Norris-Elk Creek area, southwest Montana. Billings Geological Society 11th Annual Field Conference, pp. 185- 190.

Armstrong, F. C., & Oriel, S. S. (1965). Tectonic development of Idaho-Wyoming thrust belt. American Association of Petroleum Geologists Bulletin, 49, 1847-1866.

Armstrong, R. L. (1975). Precambrian (1500 m.y. old) rocks of central Idaho: The Salmon River Arch and its role in Cordilleran sedimentation and tectonics. American Journal of Science, 275-A. 437-467.

Armstrong, R. L. (1978). Cenozoic igneous history of the United States Cordillera from latitude 42 degrees to 49 degrees north. In R. B. Smith & G. P. Eaton (Eds', , Cenozoic tectonics and regional geophysics of the western Cordillera (Memoir 152). Boulder, CO: Geological Society of America.

Beck, F. M. (1960). Geology of the Sphinx Mountain area, Madison and Gallatin Counties, Montana. Billings Geological Society, Eleventh Annual Field Conference, pp. 129-134.

Berg, R. R. (1962). Mountain flank thrusting in Rocky Mountain foreland, Wyoming and Colorado. American Association of Petroleum Geologists Bulletin, 46, 2019-2032.

Bird, P., Toksoz, M. N., & Sleep, N. (1975). Thermal and mechani­ cal models of continent-continent convergence zones. Journal of Geophysical Research, 80, 4405-4416.

Bruhn, R. L., & Beck, S. L. (1981). Mechanics of thrust faulting in crystalline basement, Sevier orogenic belt, Utah. Geology, 9, 200-204.

Burchfiel, B. C., & Davis, G. A. (1972). Structural framework and evolution of the southern part of the Cordilleran orogen. American Journal of Science, 272, 97-118.

50

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51

Burchfiel, B. C., & Davis, G. A. (1975). Nature and controls of Cordilleran orogenesis, western United States: Extension of an earlier synthesis. American Journal of Science, 275A, 363-396.

Burfiend, W. J. (1967). A gravity investigation of the Tobacco Root Mountains, Jefferson Basin, Boulder Batholith, and adjacent areas of southwestern Montana. Unpublished doctoral thesis, Indiana University, Bloomington.

Burger, H. R. (1969). Structural evolution of the southwestern Tobacco Root Mountains, Montana. Geological Society of America Bulletin, 80, 1329-1341.

Carl, J. D. (1969). Block faulting and development of drainage, northern Madison Mountains, Montana. Geological Society of America Bulletin, 80, 1329-1341.

Chadwick, R. A. (1981). Chronology and structural setting of volcanism in southwest and central Montana. Montana Geological Society Field Conference Southwest Montana, pp. 301-310.

Chamberlain, R. T. (1945). Basement control in Rocky Mountain deformation. American Journal of Science, 243A, 98-116.

Compton, R. R. (1962). Manual of field geology. New York: John Wiley & Sons.

Coney, P. J. (1978). Mesozoic-Cenozoic Cordilleran plate tectonics. In R. B. Smith & G. P. Eaton (Eds), Cenozoic tectonics and regional geophysics of the western Cordilleran (Memoir, 152). Boulder, CO: Geological Society of America.

Coppinger, W. (1980). Structural and stratigraphic relationships of an isolated fault bounded basement block, Beaverhead Mountains, southwest Montana. Geological Society of America Abstracts With Programs (Rocky Mountain Section), 12-6, 270-271.

Cordua, W. S. (1974). Structural evolution of the Precambrian rocks of the southern Tobacco Root Mountains, Madison County Montana. Geological Society of America Abstracts and Programs, 6, 436-437.

Couples, G., & Stearns, D. W. (1978). Analytical solutions applied to structures of the Rocky Mountains foreland on local and regional scales. In V. Matthews, III (Ed.), Laramide folding associated with block faulting in the western United States (Memoir 151). Boulder, CO: Geological Society of America.

Crosby, G. W. (1968). Vertical movements and isostacy in the Wyoming overthrust belt. American Association of Petroleum Geologists Bulletin, 52, 2000-2015.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52

Dickinson, W. R., & Snyder, W. S. (1978). Plate tectonics of the Laramide orogeny. In V. Matthews, III (Ed.), Laramide folding associated with block faulting in the western United States (Memoir 151). Boulder, CO: Geological Society of America.

Dubois, D. P. (1981). Basement thrusts and basin range faulting in the northern Tendoy Range, southwest Montana. Geological Society of America Abstracts With Programs, 13, 195.

Fountain, D. M., & Desmarais, N. R. (1980). The Wabowden terrain of Manitoba and pre-Belt basement of southwestern Montana: A com­ parison. Montana Bureau of Mines and Geology Special Publica­ tion, 82, 35-46.

Garihan, J. M. (1979). Geology and structure of the central Ruby Range, Madison County, Montana. Geological Society of America Bulletin, 90, 695-788.

Garihan, J. M., Schmidt, C., & O'Neill, J. M. (1981). Recurrent tectonic activity in the Ruby and Highland ranges, southwest Montana. Geological Society of America Abstracts With Programs, 13, 197.

Giletti, B. J. (1966). Isotopic ages from southwestern Montana. Journal of Geophysical Research, 71, 4029-4036.

Gillmeister, N. M. (1972). Creek-Pony Group relationship in the central Tobacco Root Mountains, Madison County, Montana. Northwest Geology, JL, 21-24.

Goodwin, A. M. (1974). Precambrian belts, plumes, and shield development. American Journal of Science, 274, 987-1028.

Goyzhevskiy, A. A. (1979). The Meso-Cenozoic stage of development of the Ukrainian shield. Geotectonics, 13, 365-369.

Grout, F. F., & Balk, R. (1934). Internal structures in the Boulder batholith. Geological Society of America Bulletin, 45, 877-896.

Hadley, J. B. (1960). Geology of the northern part of the Gravelly Range, Madison County, Montana. Billings Geological Society 11th Annual Field Conference Guidebook, pp. 149-153.

Hadley, J. B. (1980). Geology of the Varney and Cameron Quad­ rangles, Madison County, Montana. Geological Survey Bulletin, 1459, 108.

Hall, W. B. (1960). Geology of the upper Gallatin river basin, southwestern Montana. Unpublished doctoral thesis, University of Wyoming, Laramie.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53

Hanley, T. B. (1975). Structure and petrology of the northwestern Tobacco Root Mountains, Madison County, Montana. Unpublished doctoral thesis, Indiana University, Bloomington.

Hanson, A. M. (1952). Cambrian stratigraphy in southwestern Montana. Montana State Bureau of Mines and Geology, (Memoir 33). Boulder, CO: Geological Society of America.

Harrison, J. E. (1972). Precambrian Belt basin of the north­ western United States: Its geometry, sedimentation, and copper occurrences. Geological Society of America Bulletin, 83, 1215-1240.

Harrison, J. E., Griggs, A. B., & Wells, J. D. (1974). Tectonic features of the Precambrian Belt basin and their influence on post-Belt structures. Geological Survey Professional Paper, 866, 15.

Hayden, F. V. (1872). Preliminary report of the United States Geological Survey of Montana and portions of adjacent territories. Geological Survey of Territories (Fifth Annual Report).

Hodgson, R. A. (1965). Genetic and geometric relations between structures in basement and overlying rocks, with examples from Colorado and Wyoming. American Association of Petroleum Geologists Bulletin, 49, 935-949.

Hoppin, R. A., & Palmquist, J. C. (1965). Basement influence on later deformation: The problem, techniques of investigation, and examples from Bighorn Mountains, Wyoming. American Association of Petroleum Geologists Bulletin, 49, 950-1003.

Houston, R. S. (1971). Regional tectonics of the Precambrian -rocks of the Wyoming province and its relationship to Laramide structure. Wyoming Geological Association Guidebook, pp. 19-27.

Hyndman, C. W. (1979). Major tectonic elements and problems along the line of section from northeast Oregon to west-central Montana: Map-summary. Geological Society of America Bulletin, 90, 715-718.

Johns, W. M. (1961). Geology and ore deposits of the southern Tidal Wave mining district, Madison County, Montana. Montana Bureau of Mines Bulletin, 24, 53.

King, P. B. (1977). The evolution of North America. Princeton, NJ: Princeton University Press.

Kuenzi, W. D., & Fields, J. (1971). Tertiary stratigraphy, struc­ ture, and geologic history, Jefferson Basin, Montana. Geological Society of America Bulletin, 82, 3373-3394.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54

Lowell, J. D. (1974). Plate tectonics and foreland basement deformation. Geology, 2^, 275-278.

Mann, J. A. (1960). Geology of part of the Gravelly Range area. Billings Geological Society 11th Annual Field Conference, pp. 114-127.

Matthews, V., Ill, & Work, D. F. (1978). Laramide folding asso­ ciated with basement block faulting along the northeastern flank of the Front Range, Colorado. In V. Matthews, III (Ed.), Laramide folding associated with block faulting in the western United States (Memoir 151). Boulder, CO: Geological Society of America.

McLane, M. J. (1972). Sandstone, secular trends in lithology in southwestern Montana. Science, 178, 502-504.

McMannis, W. J. (1965). Resume of depositional and structural history of western Montana. American Association of Petroleum Geologists Bulletin, 49, 1801-1823.

Muehlberger, W. R. (1980). The shape of North America during the Precambrian. In National Research Council (Ed.), Continental tectonics. Washington, DC: National Academy of Science.

Myers, W. B. (1952). Geology and mineral deposits of the north­ west quarter of Willis Quadrangle and adjacent Brown's Lake area, Beaverhead County, Montana: A preliminary report. Spokane, WA: United States Geological Survey, Mineral Deposits Branch.

Palmquist, J. C. (1978). Laramide structures and basement block faulting: Two examples from the Big Horn Mountains, Wyoming. In V. Matthews, III (Ed.), Laramide folding associated with block faulting in the western United States (Memoir 151). Boulder, CO: Geological Society of America.

Pardee, J. T. (1950). Late Cenozoic block faulting in western Montana. Geological Society of America Bulletin, 61, 359-406.

Peale, A. C. (1896). Geologic Atlas of the United States: Folio 24. Three Forks, MT: U.S. Geological Survey.

Perry, E. S. (1962). Montana in the geologic past. Montana Bureau of Mines and Geology Bulletin, 26, 78.

Prucha, J. J., Graham, J. A., & Nickelson, R. P. (1965). Basement- controlled deformation in the Wyoming province of Rocky Mountain foreland. American Association of Petroleum Geologists Bulletin, 49, 966-992.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55

Ragan, D. M. (1973). Structural geology; An Introduction to geometrical techniques. New York: John Wiley & Sons.

Ray, R. G. (1960). Aerial photographs in geologic interpretation and mapping. Geological Survey Professional Paper, 373, 230.

Reid, R. R. (1957). Bedrock geology of the north end of the Tobacco Root Mountains, Madison County, Montana. Montana Bureau of Mines Memoir, 36, 25.

Reyner, M. L. (1947). Geology of the Tidal Wave district, Madison County, Montana. Unpublished master's thesis, Montana School of Mines, Butte.

Robinson, G. D., Klepper, G. D., & Obradovich, J. D. (1968). Over­ lapping plutonism, volcanism, and tectonism in the Boulder batholith region, western Montana. In R. R. Coasts, R. L. Hay, & C. A. Anderson (Eds), Studies in Volcanology (Howel Williams volume). Geological Society of America Memoir, 116, 557-576.

Ruppel, E. T. (1978). Medicine Lodge thrust system east-central Idaho and southwest Montana. Geological Survey Professional Paper, 1031, 23.

Sales, J. K. (1968). Crustal mechanics of Cordilleran foreland deformation a regional and scale-model approach. American Association of Petroleum Geologists Bulletin, 52, 2016-2044.

Samuelson, K. J., & Schmidt, C. J. (1981). Structural geology of the western Tobacco Root Mountains, southwest Montana. Montana Geological Society, Field Conference Southwest Montana, pp. 191- 199.

Schmidt, C. J. (1971). Kinematics of Rocky Mountain foreland deformation in the northern Tobacco Root Mountains, Montana. Geological Society of America Abstracts With Programs, _5, 410-411.

Schmidt, C. J. (1973). Late Cretaceous and Paleozene cratonic uplift in the Tobacco Root Mountains, southwestern Montana. Geological Society of America Abstracts With Programs, _5, 796.

Schmidt, C. J. (1975). An analysis of folding and faulting in the northern Tobacco Root Mountains, southwest Montana. Unpublished doctoral thesis, Indiana University, Bloomington.

Schmidt, C. J. (1976). The influence of Precambrian structural trends on Laramide faulting in the northern Tobacco Root Mountains, southwest Montana. Geological Society of America Abstracts With Programs, j}, 627.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 56

Schmidt, C. J. (1979). Mechanical basis for deformation and plate tectonic models for the Wyoming province. In Manual for geological field study of northern Rocky Mountains (pp. 177-189). Bloomington: Indiana University, Department of Geology.

Schmidt, C. J., Brumbaugh, D., & Hendrix, T. E. (1977). Fault movements and origin of the salient-reentrant pattern in the disturbed belt of southwestern Montana. Geological Society of America Abstracts With Programs, j), 760.

Schmidt, C. J., & Garihan, J. (1979). A summary of Laramide base­ ment faulting in the Ruby, Tobacco Root, and Madison ranges and its possible relationship to Precambrian continental rifting. Geological Society of America Abstracts With Programs, 11, 301.

Schmidt, C. J., & Garihan, J. (1980). Upthrust-drape fold relation­ ships along northwest faults in the Rocky Mountain foreland, southwest Montana. Geological Society of America Abstracts With Programs, 12, 303.

Schmidt, C. J., & Hendrix, T. E. (1981). Tectonic controls for thrust belt and Rocky Mountain foreland structures in the northern Tobacco Root Mountains-Jefferson Canyon area, southwest Montana. Montana Geological Society, Field Conference Southwest Montana, pp. 167-180.

Schmidt, C. J., Vitaliano, C. J., & Exkerty, D. G. (1977). Simul­ taneous uplift and volcanism in southwest Montana. Geological Society of America Abstracts With Programs, j), 760-761.

Scholten, R. (1967). Structural framework and oil potential of extreme southwestern Montana. Montana Geological Association, 18th Annual Field Conference Guidebook, pp. 7-19.

Schwartz, R. (1972). Stratigraphic and petrographic analysis of the lower Cretaceous Blackleaf Formation, southwestern Montana. Unpublished doctoral thesis, Indiana University, Bloomington.

Sloss, L. L. (1954). Lemhi arch, a mid-Paleozoic positive element in south-central Idaho. Geological Society of America Bulletin, 65, 365-368.

Smedes, H. W., & Schmidt, C. J. (1979). Regional geologic setting of the Boulder batholith, Montana. In L. J. Suttner (Ed.), Guidebook for Penrose Conference— Granite II (pp. 1-36). Unpublished report and guidebook, Gregson, MT.

Smith, J. G. (1965). Fundamental transcurrent faulting in Northern Rocky Mountains. American Association of Petroleum Geologists Bulletin, 49, 1398-1409.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57

Smith, J. L. (1970). Petrology, mineralogy, and chemistry of the Tobacco Root batholith, Madison County, Montana Unpublished doctoral thesis, Indiana University, Bloomington.

Smith, R. B. (1978). Seismicity, crustal structure, and intraplate tectonics of the interior of the western Cordillera. In R. B. Smith & G. P. Eaton (Eds.), Cenozoic tectonics and regional geophysics of the western Cordillera (Memoir 152). Boulder, CO: Geological Society of America.

Smith, R. B., & Sbar, M. L. (1974). Contemporary tectonics and seismicity of the western United States with emphasis on the Intermountain Seismic Belt. Geological Society of America Bulletin, 85, 1205-1218.

Spencer, E. W., & Kozak, S. J. (1975). Precambrian evolution of the Spanish Peaks area, Montana. Geological Society of America Bulletin, 86, 785-792.

Stearns, D. W. (1971). Mechanism of drape folding in the Wyoming province. Wyoming Geological Association, 23rd Annual Field Conference (Wyoming Tectonics Symposium), Guidebook, pp. 125-143.

Stearns, D. W. (1978). Faulting and forced folding in the Rocky Mountain foreland. In V. Matthews, III (Ed.), Laramide folding associated with block faulting in the western United States (Memoir 151). Boulder, CO: Geological Society of America.

Stewart, J. H. (1972). Late Precambrian evolution of North America: Plate tectonics implication. Geology, j4, 11-15.

Stewart, J. H. (1976). Basin-range structure in western North America: A review. In R. B. Smith & G. P. Eaton (Eds.), Cenozoic tectonics and regional-geophysics of the western Cordillera (Memoir 152). Boulder, CO: Geological Society of America.

Suttner, L. J., Schwartz, R. K., & James, W. C. (1981). Late Mesozoic to Early Cenozoic foreland sedimentation in southwest Montana. Montana Geological Society, Field Conference Southwest Montana, pp. 93-103.

Swanson, R. W. (1950). Geology of a part of the Virginia City and Eldridge Quadrangles, Montana: A summary report. Washington, DC: United States Geological Survey.

Tansley, W., Schafer, P. A., & Hart, L. H. (1933). A geological reconnaissance of the Tobacco Root Mountains, Madison County, Montana. Montana Bureau of Mines and Geology Memoir, j), 57.

Thornbury, W. D. (1969). Regional geomorphology of the United States. New York: John Wiley & Sons.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58

Tilford, M. J. (1978). Structural analysis of the southern and western Greenhorn Range, Madison County, Montana, and magnetic benefaction of Montana talc ores. Master's thesis, Indiana University, Bloomington.

Tysdal, R. G. (1970). Geology of the north end of the Ruby Range, southwestern Montana. Unpublished doctoral thesis, University of Montana, Missoula.

Tysdal, R. G. (1981). Foreland deformation in the northern part of the Ruby range, southwest Montana. Montana Geological Society, Field Conference Southwest Montana, pp. 215-224.

Vitaliano, C. J., Cordua, W. S., Burger, H. R., Haley, T. B., Hess, D. F., & Root, F. K. (1979). Geology and structure of the southern part of the Tobacco Root Mountains, southwestern Montana: Map summary. Geological Society of America Bulletin, 90, 712-715.

Winchell, A. N. (1914). Mining districts of the Dillon quadrangle, Montana and adjacent areas. Geological Survey Bulletin, 574, 191.

Windley, B. F. (1978). The evolving continents. New York: John Wiley & Sons.

Wooden, J. L., Vitaliano, C. J., Koehler, S. W., & Ragland, P. C. (1978). The late Precambrian mafic dikes of the southern Tobacco Root Mountains, Montana: Geochemistry, Rb-Sr geo­ chronology and relationship to belt tectonics. Canadian Journal of Earth Sciences, 15, 467-479.

Woodward, L. A. (1980). Laramide deformation of Rocky Mountain foreland: Geometry and mechanics. New Mexico Geological Society Special Publication, 6^, 11-17.

Woodward, L. A. (1981). Tectonic framework of Disturbed Belt of west-central Montana. American Association of Petroleum Geologists Bulletin, 65, 291-302.

Woodward, N. B. (1979). Stacked thrust sheets and upper Mississip- pian rocks in the Snake River range, Idaho and Wyoming. Geological Society of America Abstracts With Programs, 11, 306.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLATES

Geology of a Section of the Northwest Tobacco Root Mountains, Madison County, Southwest Montana

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