The Geology of the Southern Bull Mountain Area, Jefferson County, Montana

Western Michigan University ScholarWorks at WMU

Master's Theses Graduate College

12-1983

Michael Edward Streeter

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Recommended Citation Streeter, Michael Edward, "The Geology of the Southern Bull Mountain Area, Jefferson County, Montana" (1983). Master's Theses. 1653. https://scholarworks.wmich.edu/masters_theses/1653

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]. THE GEOLOGY OF THE SOUTHERN BULL MOUNTAIN AREA, JEFFERSON COUNTY, MONTANA

Michael Edward Streeter

A Thesis Submitted to the Faculty of The Graduate College in partial fulfillm ent of the requirements for the Degree o f Master o f Science Department o f Geology

Western Michigan University Kalamazoo, Michigan December 1983

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THE GEOLOGY OF THE SOUTHERN BULL MOUNTAIN AREA, JEFFERSON COUNTY, MONTANA

Michael Edward S tre e te r, M.S.

Western Michigan University, 1983

The Late Cretaceous-Early Tertiary Laramide orogeny was

responsible for the northward tiltin g of the pre-Eocene rocks of

the area. The Late Cretaceous deposition of the Elkhorn Mountains

Volcanics resulted in the volcanic-Madison Group unconformity. Late

Cretaceous faulting took place probably in response to caldera sub

sidence. Laramide u p li f t erosion and downstream tra n sp ort resulted

in the deposition of the Conrow Creek conglomerate and debris flows

over the Lodgepole Limestone. Block faulting took place in response

to regional extension probably during Late Oligocene-Early Miocene

time resulting in the formation of the St. Paul's Gulch, Golden

S u n lig h t, and Boulder basin fa u lts . The Oligocene Dunbar Creek

Member (Renova Formation) and the Late Miocene-Pliocene Sixmile Creek

Formation were deposited as stream channel and mudflow deposits in

the North Boulder River basin. The sandstone units are composed of

particles derived from the Boulder batholith and Elkhorn Mountains

Volcanics.

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

I would like to thank the following people who made the

completion of this work possible. I have Dr. Christopher J.

Schmidt to thank for the idea of this study, his guidance in the

field, and his useful criticism as my thesis advisor. Thanks also

go to Dr. William B. Harrison III and Dr. W. Thomas Straw for their

advice. I would also like to thank the Graduate College for their

financial support, William G. Gierke for his companionship and

transportation to Montana, and Robert Havira (Western Michigan

University) for help in technical areas and with photography.

I would especially like to thank Lori G riffin and family for their

warm hospitality during my stay in Whitehall, Montana.

Heartfelt thanks go to Kim, my wife, for her moral support

and love.

Michael Edward Streeter

i i

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ProQuest Information and Learning 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 800-521-0600

STREETER, MICHAEL EDWARD

THE GEOLOGY OF THE SOUTHERN BULL MOUNTAIN AREA, JEFFERSON COUNTY, MONTANA

WESTERN MICHIGAN UNIVERSITY M.S. 1983

University Microfilms internstionsi 300N.ZtebRoad,AnnAibor,MI48106

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72-74

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ACKNOWLEDGEMENTS ...... \ i i

LIST OF TABLES...... ' ...... v

LIST OF FIGURES...... vi

LIST OF MAPS vi i i

INTRODUCTION ...... 1

PREVIOUS WORK...... 5

REGIONAL SETTING ...... 6

PRECAMBRIAN THROUGH CRETACEOUS STRATIGRAPHY ...... 14

Introduction ...... 14

"Mississippian" Karst and Solution Breccia Development . . . 14

Elkhorn Mountains Volcanics-Madison Group Unconformity . . . 19

Late Cretaceous Igneous Intrusion ...... 21

TERTIARY TO RECENT STRATIGRAPHY - BOZEMAN GROUP ...... 22

Depositional Sequence and S trati graphic Nomenclature .... 22

Petrographic A nalysis: Dunbar Creek Member, Renova Formation ...... 23

Petrographic Analysis: Sixm ile Creek Formation ...... 37

CONROW CREEK CONGLOMERATE ...... 45

LATE CRETACEOUS TO RECENT GEOLOGIC HISTORY ...... 50

STRUCTURE...... 53

Introduction ...... 53

Major Late Cretaceous-Early Tertiary Faults ...... 55

Major Cenozoic F aults ...... 57

i i i

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Minor F a u lts ...... 61

Structural History ...... 62

SUMMARY OF GEOLOGIC HISTORY ...... 64

Precambrian ...... 64

C a m b ria n ...... 66

Devonian-Pennsylvanian ...... 66

Permian-Middle Cretaceous ...... 67

Late Cretaceous-Early Tertiary ...... 67

Eocene-Recent ...... 68

APPENDIX: MEASURED SECTIONS ...... 70

BIBLIOGRAPHY...... : ...... 82

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

TABLE

1. Modal Analysis o f Sandstone U nit from the Dunbar Creek Member, Renova Formation, Montana ...... 34

2. Modal Analysis of Sandstone Unit from the Sixm ile Creek Formation ...... 42

3. Modal Analysis o f the Conrow Creek Conglomerate M atrix ...... 45

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

FIGURE

1. Southwestern Montana showing Cenozoic basins and valleys ...... 2

2. Index map and generalized geologic map of the southern flank of Bull Mountain and vicinity ...... 3

3. Regional structural setting of this author's fie ld a r e a ...... 7

4. Generalized strati graphic column of the study area . . . 8

5. Coincidence of the Eastern Margins of the Belt basin and the Fold and Thrust B e lt ...... 10

6. Limestone-boulder conglomerate amidst Mississippian Lodgepole Formation strata in SE%, sec. 8, T2N, R3W . . . 15

7. Elkhorn Mountains Volcanics basal conglomerate in SE%, sec. 8, T2N, R3W ...... 20

8. Rock-stratigraphic correlation and age of the Bozeman Group in the Townsend, C larkston, Three Forks, Jefferson, and Ruby b a s in s ...... 23

9. Mineralogic classification of the Renova and Sixmile Creek Formations sandstone units described in this report ...... 26

10. Typical appearance o f Dunbar Creek sandstone in thin section ...... 27

11. Cumulative curves (probability ordinate) for sand-size particles and calculated sediment parameters of mean grain size (Mz) and sorting or standard deviation ( a \ ) o f sandstone units G, B, and A o f the Dunbar Creek Member o f the Renova Formation ...... 28

12. Cumulative curves (probability ordinate) for sand-size particles and calculated sediment parameters of mean grain size (Mz) and sorting or standard deviation (ai) o f sandstone un its K, I , and F o f the Dunbar Creek Member o f the Renova Formation ...... 29

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

13. Mechanical analysis of sandstone units A and B from the Dunbar Creek Member o f the Renova Formation ...... 31

14. Mechanical analysis of sandstone units F and G from the Dunbar Creek Member o f the Renova Formation ...... 32

15. Mechanical analysis of sandstone units I and K from the Dunbar Creek Member o f the Renova Formation ...... 33

16. Typical appearance o f Sixm ile Creek sandstone in thin section ...... 39

17. Cumulative curve (p ro b a b ility ordinate) and calculated sediment parameters of mean grain size (Mz) and sorting or standard deviation (ax) of a sandstone unit from the Sixm ile Creek Formation ...... 40

18. Mechanical analysis of a sandstone unit from the Sixm ile Creek Formation ...... 41

19. Conrow Creek conglomerate in NE%, sec. 17, T2N, R3W . . . 47

20. Generalized block diagram illustrating the proposed accumulation o f the Conrow Creek conglomerate ...... 48

21. Geologic cross sections along the southeastern flank of Bull Mountain extending into the North Boulder River basin ...... 54

22. Diagram illu s tr a tin g the re la tio n s h ip between the major faults which cut the southern flank of Bull Mountain ...... 58

23. A - Isopach map of Lower Cambrian, Ediacaran, and Hadrynian rocks with the areas of occurrence of Belt Group and equivalent rocks superposed; B - Major lower Paleozoic strati graphic belts within the Cordilleran fold belt of North America; C - Diagrammatic reconstruction of the island arc and basin of early Paleozoic time in the C o rd ille ra ...... 65

vi i

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

MAP

1. Geology of the Southeast Flank of Bull Mountain, M ontana ...... 89

vi i i

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

The project which led to this report involved geologic field

mapping (1:24,000 scale) o f approximately sixteen square m iles in

southwestern Montana (Figures 1 and 2) and a subsequent petrologic

and statistical analysis of collected rock samples. The area contains

a great variety of rock types including: (a) Late Precambrian Belt

Supergroup sandstones and shales (Greyson Formation); .(b) folded and

faulted rock of Paleozoic age; (c) Late Cretaceous andesitic volcanic

rocks (Elkhorn Mountain Volcanics); (d) Cenozoic mudstones, sandstones

and conglomerates (Renova and Sixmile Creek Formations); (e) limestone

boulder conglomerates o f uncertain age and o rig in (Conrow Creek

Conglomerate).

The purpose of this study is to investigate four problems

related to the tectonic evolution of western Montana. These problems

can be framed as a series of questions: (a) What is the nature and

origin of the contact between the volcanic rocks and older rocks

(fault or unconformity)? (b) What are the ages and mechanisms of the

various structural elements contained within the area? (c) What is

the age, origin, and significance of the unusual limestone boulder

conglomerates in the area? (d) What is the provenance of the Tertiary

conglomerates, sandstones, and mudstones?

Two months were spent in the f ie ld during the summer o f 1981.

During this time a geologic map (Map 1) was prepared on topographic

sheets supplied by the U.S. Geological Survey (Black Butte l h

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2

oGreat Falls

V.4 'Townsend

K i 6 B u tt*

iFtman Ancei

s'. :

- >

114'

112‘

Figure 1. Southwestern Montana showing Cenozoic basins and valleys (shaded). The study area is the heavily outlined area. Key to basins and valleys: 1, Jefferson River basin; 2, Three Forks basin; 3, Clarkston basin; 4, Townsend V a lle y; 5, Beaverhead V a lley; 6, Upper Ruby River basin; 7, Blacktail Valley; 8, Lima Valley; 9, Deerlodge Valley; 10, F lin t Creek basin; 11, B itte rro o t V a lle y; N. Boulder River basin (after Kuenzi, 1966).

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

But*e^ * TKb <■ u ± u

Cenozoic basin deposits

$ Cretaceous-Tertiary batheliths •Shor

Cretaceous Elkhorn Mts. Volcanics

Paleozoic/'Mesozoic sedimentary rocks

Prccambrion sodimontory (Belt) rock*

Procambri&n mstamorpbic rocks I------10 Mi.------I

Figure 2. Index map and generalized geoloaic map of the southern flank of Bull Mountain and vicinity (after Kuenzi, 1966). That part of Montana covered in this reoort is heavily outlined.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4

quadrangle, eastern half) and the U.S. Department of the Interior,

Bureau of Reclamation (Missouri Basin sheet, eastern half) at a scale

of 1:24,000. The map data of Alexander (1955) was used extensively

in mapping the extreme western p o rtion o f the map area because of the

dense tree cover. Four selected sections of the Tertiary rocks in

the North Boulder River basin were measured and every d is tin c t change

in lithology was sampled.

Laboratory work during the 1981-1982 academic year included:

binocular study o f samples from measured se ctio ns, study o f 30 th in

sections, and sieve analysis of 7 samples.

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

The firs t geological work known to have been done in the southern

Bull Mountain area was by Hayden (1872, 1873, 1876) who b rie fly men

tioned the area as part of the Three Forks Folio (U.S.G.S. Folio 24).

This work is mainly a study of the general stratigraphy of the region

between W h ite h all, Montana and Yellowstone Park. Weed (1912) refers

to the southern end of Bull Mountain in his report dealing with the

geology and ore deposits of the Butte district. In studying the

eastern portion of the Boulder batholith, Billingsley (1915) made

reference to the area around Whitehall. The firs t known detailed

map of the southern Bull Mountain area was produced by Rambosek

(1946) as part of his master's thesis for the Montana School of Mines.

The most extensive work performed in the area was by Alexander (1955)

who mapped the geology of the Whitehall area and described the

Precambrian and Phanerozoic rocks of the area. Satoskar (1971)

studied the Elkhorn Mountains Volcanics on Bull Mountain as part of

his master's thesis and produced a crude map of the area.

Numerous reports pertaining to the areas surrounding the study

area have been produced. The most notable o f these are the Three

Forks quadrangle to the east (Robinson, 1963); the southern Elkhorn

Mountains to the northeast (Klepper et a l., 1957); and the Jefferson

River basin to the east, south, and southwest (Kuenzi, 1966). The

Laramide tectonic history of the region has most recently been

discussed by Schmidt and O'Neill (1982).

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

The Precambrian and Paleozoic tectonic framework of Montana

consists primarily of three major features: (a) the Cordilleran geo-

syncline to the west, (b) the cratonic area or foreland comprising

the stable shelf to the east, and (c) a west-trending trough comprising

a thick wedge of Precambrian rocks which transects the shelf and part

of the geosyncline. The present site of the Sevier fold and thrust

belt closely approximates the boundary between the craton and the

geosyncline (Figure 3). The west-trending trough is marked by a

pronounced deflection of the thrust belt, called the Helena salient.

The southern margin of the trough is thought to be a fault zone which

was active during Late Precambrian time. This is evidenced by coarse

clastic debris deposited along the southern margin as a northward-

thinning wedge (Figure 4) of conglomeratic arkose which makes up the

LaHood Formation o f the B e lt Supergroup (Alexander, 1955; McMannis,

1963; Schmidt & Garihan, 1984).

South o f the trough Precambrian metamorphic basement rocks are

exposed as broad, fault-bounded arches in the Wyoming structural

province (Rocky Mountain foreland). To the north of the trough,

Mesozoic rocks are widely exposed as the trough is inferred to lie

a t a greater depth w ith a th ic k sequence o f infolded Late Precambrian,

Paleozoic, and Mesozoic strata. The western part of the trough (west

of the craton) is dominated by Belt rocks (Figure 5). This suggests

that the area to the west has been tectonically thickened and younger

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .Boulder .Boulder I Batholith If FORELAND MOUNTAIN V o o ROCKY 4 THRUST LOMBARD

Batholith Idaho \ AREA FIELD < i .HELENA 2 I k 2 Whitehall

BOULDER b a t h o l it h

\ ' £ L ' BATHOLITH ROOT TOBACCO \

1 PLUTONS FLINT CREEK 3 ) BLOCK Elkhorn Mountain Volcanics Thrust Faults Precambrian metamorphicrocks Early Tertiary volcanic rocks ’ Strike-Slip’ Faults (mqjor) SAPPHIRE IDAHO ,v< ,v< ' Granitic rocks (Cretaceous) Figure 3. Regional structural setting of this author's field area (after & Smedes Schmidt, 1979). , BATHOLITH

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GENERALIZED STRATIGRAPH1C

COLUMN OF FIELD AREA

AND IMMEDIATE VICINITY Z DUAL VERTCAl SCALE______

Each m c i represents 200 tee:

Alluvium

Sixm ile Creek Fm

Bozeman

Group

Ckmetng Arro* ■ 0* Mbr Dl.000'

Mm! 550* I650*

Elkhorn Mountains Volumes

Ui 11

.* 'o' 'o • O ' Three Forks Fm. 250- 250* LocanGwien mo»

z Jefferson Fm. 550*- 650* Amsoen Fm 350*- 650*

Figure 4. Generalized strati graphic column of the study area (a fte r McLane, 1971).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9

EXPLANATION

Metambmnc rocts id.—-T-n

Artose or conglomerate

SanCitone

Dry Creet Fm Crossoeooee sanostone

VW) R.OO

Park Fm.

Crossocoocd limestone

Meagner Fm. Snaiey or tnm. becoeo limestone

0o*>t< limestone fEcn ! * * * » Cne'tv limestone Silver Mrti Mbr. I Wolsey Fm.

Limestone Dreccu

Flameaa Fm

p«€bir Beit Ser

Pony ana Cherry Cr Ser V oort« conglomerate rm m m m or anar eeoovts

Figure 4 - Continued

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CRETACEOUS-TERTIARY INTRUSIVES 7771 PRECAMBRIAN METAMORPHICS

*** thrust fault

_ (H ELEN A ) > < EMBAYMENT

PIONEERyi TOBACCO BATHOLITH ROOT BATHOLITH

0 10 2,0 3,0 40 50Miles 1 AO RO Kilometers

Figure 5. Coincidence of the Eastern Margins of the Belt basin and the Fold and Thrust belt (after Smedes & Schmidt, 1979).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. rocks have been removed by erosion or that the crust rises westward

within the trough (Smedes & Schmidt, 1979).

Isopach and lithofacies data (Sloss, 1950) suggest that the

trough has been in existence since at least Middle Proterzoic time

and has exerted a control upon deposition and subsequent deformation

(Figure 5). The trends and the positions of many axes of troughs of

the Belt, Paleozoic, and Mesozoic seas display marked sim ilarity with

the trends of the boundaries of the crustal blocks.

Montana's Late Mesozoic and Early Cenozoic regional geology was

dominated by the Late Cretaceous-Early Tertiary Laramide orogeny.

Laramide regional compression was responsible for a zone of convex-

eastward th ru s t fa u lts and folds which combine to make up the Helena

salient of the Sievier fold and thrust belt in west-central Montana.

The Helena salient may be underlain by a decollmont near the base of

the Belt Supergroup (Schmidt & O 'N eill, 1982; Woodward, 1981). Such

a decollmont could have allowed the rocks above i t to deform indepen

dently of the Precambrian basement rocks. The deformation above the

decollmont took place by thrusting and folding which decreased in

horizontal movement and intensity eastward. Thrusts and related

overturned tight folds are common in the western portion of the sali

ent whereas open up rig ht fo ld s dominate the eastern po rtion (Woodward,

1981). The southern margin of the Helena salient (southwestern Mon

tana transverse zone, Schmidt & O 'Neill, 1982) consists of an alloch-

thonous panel of rocks. Here the decollmont surfaces as a transverse

ramp (in the terms of Dahlstrom, 1970), a zone characterized by an

im bricated sequence o f e a st-to -n o rth east-tre nd ing fa u lts th a t display

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dextral and reverse movement (Schmidt & O 'N eill, 1982). The Laramide

orogeny in western Montana was also attended by volcanism. This is

evidence by widespread flows, tuffs, and volcanoclastics of the Elk-

horn Mountains Volcanics. Volcanism was associated with the intrusion

of the Late Cretaceous Boulder batholith (Klepper et a l., 1974).

Erosion which accompanied Laramide u p lif t continued in to Early

Tertiary time forming broad, shallow basins throughout western Montana

(Pardee, 1950; Reynolds, 1979). After complete cessation of Laramide

compressional forces, probably during Eocene time, the basins became

the sites of sediment depositions as erosion of the highlands contin

ued (Kuenzi, 1966; Kuenzi & F ie ld s, 1971).

In the Oligocene, block faulting (basin and range faulting) began

to take place resulting in further basin development throughout west

ern Montana (Kuenzi & Fields, 1971; Reynolds, 1979). Rhyolitic vol

canism ranging in age from 38 m.y. to 30 m.y. was characteristic of

northwestern Montana near Helena (Chadwick, 1978). Reynolds (1979)

suggested that the volcanism and block faulting took place in response

to crustal extension.

Since the Late Cenozoic, western Montana has been dominated by

erosion of the highland areas and with the intermontane basins exper

iencing alternating periods of erosion and deposition. This deposi

tion has, in many cases, concealed the structural character of the

boundaries between highland blocks and basins.

Within this framework, the present study area lies within the

boundaries of the Helena embayment near its southern edge and on the

northern edge of the southwestern Montana transverse zone. It lies

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. north o f the Tobacco Root b a th o lith and to the east o f the southern-

half of the Boulder batholith on the southern portion of Bull Moun

tain that extends into the North Boulder River basin (Figure 5).

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

Introduction

Numerous stratigraphic descriptions of the Cenozoic, Paleo

zoic and Precambrian sequence of strata exist (see for example:

Alexander, 1955; Klepper et a l., 1957; Robinson, 1963). Therefore

no detailed description of these rocks is presented here. Special

attention w ill be focused on Karst development within the Mississip

pi an Madison Group and the unconformity th a t may e x is t between the

Madison Group and the overlying Elkhorn Mountains Volcanics on the

southern end of Bull Mountain. A generalized stratigraphic column

representing the rock units which are found in the study area is

shown in Figure 5. General statements regarding the stratigraphic

column are presented with the summary of the geologic history in a

later section of this report.

"M ississippian" Karst and S olution Breccia Development

Outcrops of limestone-boulder conglomerates were found scattered

amid Lodgepole u n its a t SE%, sec. 8, T2N, R3W and NE%, sec. 17, T2N,

R3W. The very poorly so rte d , mostly m atrix supported, muddy-sandy

conglomeratic (Figure 6) consists of subangular to subrounded, si 11-

to boulder-size limestone fragments, the largest measuring approxi

mately 50 cm across. The matrix, when present, is a friable, gray

weathering to light gray, silty-sandy mudstone. Although this author

was unable to rule out the possibility that the conglomerate was 14

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15 sec. 8, T2N, R3W. S E k , (above (above in left view) in Figure Figure 6. Liniestone-boulder conglomerate amidst Mississippian Lodgepole Formation stra ta

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. formed as the result of solution-collapse within the Lodgepole Forma

tion in (on) which it is contained, the following five factors lead

to the conclusion that the rock is the product of the transport and

deposition of material from the Mission Canyon Limestone:

1. The subangular to subrounded nature of the clasts indicate a

relatively short transport distance. The clasts which make up

solution-collapse breccias are almost exclusively very angular to

angular (Middleton, 1960).

2. The outcrops consist of lobate masses of very poorly sorted,

mostly matrix supported clasts. Such features are characteristic of

debris flows (Selley, 1976).

3. The poorly defined lower boundaries, which characterize the

lobes of the conglomerate, are uncharacteristic of solution-collapse

breccias known elsewhere in the Mission Canyon Limestone. Middleton

(1960), in discussing Mission Canyon evaporite solution-collapse

breccias, stated that "the lower boundary of almost every breccia

is well defined" (p. 191).

4. A very isolated outcrop of limestone-boulder breccia was

found by this author within the Mission Canyon Limestone near its

base in SE%, sec. 6, T3N, R3W. The exposure is approximately 20 ft

thick and consists of very angular to subangular s ilt- to boulder-

size limestone clasts, the largest measuring approximately two meters

across. The breccia is almost entirely clast supported. The matrix,

when present, is gray weathering to light gray colored and friable.

This author believes that this breccia represents solution-collapse

brecciation near the base of the Mission Canyon Limestone which

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. could provide an ample source of material for the formation of the

conglomerate.

5. The w riter has found no documentation of solution-collapse

brecciation within the Lodgepole Limestone, whereas numerous reports

document b re ccia tio n w ith in the Mission Canyon Limestone.

A more detailed examination of the conglomerate is needed to

ascertain whether the clasts are derived from the Mission Canyon or

Lodgepole. Because no samples were taken from e ith e r the conglomerate

or solution-collapse breccia and due to the very isolated nature of

the solution-collapse breccia outcrop, the following discussion is,

for the most part, drawn from pertinent literature.

Solution-collapse brecciation within the Mission Canyon Lime

stone, as documented by numerous other authors (Andrichuck, 1955;

Blackstone, 1940; Campbell, 1977; E llio t, 1963; Henbest, 1958;

Klepper et a l., 1957; Laudon & Severson, 1953; Mann, 1954; Maughan

e t a l. , 1967; McCaleb & Wayhan, 1969; McMannis, 1955; M iddleton,

1960; N ordquist, 1953; Roberts, 1966; Robinson, 1963; Sando, 1967,

1976; SIoss, 1952; Thom et a l., 1935; Tourtelot & Thompson, 1948),

could provide an ample source of rock for the conglomerate contained

within the study area. Brecciation in the Mission Canyon Limestone

is thought to be the result of fragmentation and collapse of rela

tively insoluble rocks after the solution by groundwater of beds of

evaporite (Middleton, 1960). Although the mechanism by which these

breccias were formed (solution-collapse) is agreed upon by most

authors, the time at which this solution-collapse took place has

been debated. Some authors (Andrichuck, 1955; Rae, 1963; Roberts,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18

1966) have held that solution breccias formed during Late Cretaceous-

Early T e rtia ry time whereas others (McCaleb & Wayhan, 1969) have

contended that the breccias were formed during Late Mississippian-

Early Pennsylvanian time. This w riter believes that solution breccia

formation most likely originated during Late Mississippian-Early

Pennsylvanian time during the well-documented period of subaerial

erosion. The circulation of groundwater that produced the karst

features which characterize the top o f the Madison Group could also

have removed the interbedded evaporites (McCaleb & Wayhan, 1969).

During very Late Mississippi an time, when the top of the Madison

Group was resubmerged, groundwater circulating through cavities

created by earlier solution could have continued minor solution of

the Mission Canyon units. During the Laramide orogeny, the s tra ti

graphic units in the area were folded and erosion exposed the

Madison Group. Madison Group limestone clasts contained w ith in the

Late Cretaceous Elkhorn Mountains Volcanics clearly demonstrate that

the Madison Group was being eroded during Late Cretaceous time. This

u p lif t and erosion may have caused the continuation o f the s o lu tio n -

collapse brecciation within the Mission Canyon Limestone. This

u p lift and erosion could have produced enough local re lie f to provide

the slope required for the colluvial transport of debris which orig

inally formed by solution-collapse within the Mission Canyon Limestone.

This downslope transport could have resulted in the deposition of the

conglomerate as scattered debris flows over an eroded surface on the

Lodgepole. Because the conglomerate contains no volcanic rock

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19

fragments, this transport and deposition must have occurred prior to

the extrusion of the Elkhorn Mountains Volcanics.

Elkhom Mountains Volcanics-Madison Group Unconformity

The Elkhorn Mountains Volcanics within the study area have been

well described by Alexander (1955) and Satoskar (1971). Therefore

only the basal volcanic u n it w ill be b r ie fly described because deposi

tion of this unit was responsible for the angular unconformity between

i t and the underlying Madison Group (see Map 1).

The Elkhorn Mountains Volcanics basal unit, located along the

unconformity in SEV, sec. 8, T2N, R3W, is a very poorly sorted, dark

gray weathering to greenish-gray, volcanic conglomerate. This highly

indurated conglomerate (Figure 7) contains particles ranging from

s ilt to boulders of volcanic rock that measure up to 50 cm across.

The conglomerate contains mostly volcanic rock fragments with some

smaller (maximum diameter less than 10 cm) limestone clasts derived

from the Madison Group.

The basal Elkhorn Mountains conglomerate was deposited on an

irre g u la r erosion surface on the Madison Group (Map 1). S trik e and

dip measurements taken across the unconformity show an angular

unconformity of from 25 to 35 degrees. When restored the attitude

of the unconformity is approximately N50W, 32NE. The presence of

Madison Limestone cla sts c le a rly demonstrates th a t the Madison Group

was being eroded at the time of Elkhorn Mountains Volcanics deposi

tio n . Alexander (1955) also located Madison clasts in the basal

conglomerate on the west side of Bull Mountain. The map pattern

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 7. Elkhorn Mountains Volcanics basal conglomerate in SE%, sec. 8, T2N, R3W. Note: limestone clast at the point of the hammer.

(Map 1) shows that the unconformity occupies approximately the same

stratigraphic position to the northeast. The local relief that

existed on Bull Mountain prior to the extrusion of the volcanics

probably played a key role in the location at which the volcanics

were deposited. The Madison Group is at a higher elevation (7500 ft)

in the northwestern portion of the map area than in the southeastern

(5800 ft) part of the area. The greater elevation is probably due

to the fact that the Lodgepole Limestone was not eroded as deeply

in the northeastern sector of the map area. Cenozoic faulting makes

tenuous the calculation of the exact local re lie f that was present

prior to the deposition of the volcanics, but the local relief may

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 v have been as much as 500 f t . This is based upon the v a ria tio n o f

Lodgepole thickness. Because the basal conglomerate was apparently

deposited as a mudflow (Alexander, 1955), it probably firs t covered

the lower portions of the eroded Lodgepole Limestone. The very

limited exposure and the extreme lateral discontinuity of the basal

volcanic unit, a common characteristic of all the volcanic units

w ith in the study area, made i t too time consuming to measure la te ra l

thickness variation and lateral changes in clast size composition.

Late Cretaceous Igneous Intrusion

An augite-biotite diorite is present as a s ill intruded into

the Three Forks shale in secs. 21, 22, T2N, R3W (Map 1). Due to the

highly weathered and friable nature of the rock, thin section analysis

was made d iffic u lt so that actual percentages of the mineral consti

tuents making up the rock could not be accurately determined. Thin

section analysis reveals that this green-colored rock contains brown

subhedral biotite, altered subhedral augite, minor anhedral magnetite,

minor altered anhedral hornblende, a serpentinous alteration product,

and a groundmass of plagioclase laths with minor anhedral quartz.

The limited exposure of the s ill permits only a wide range in age

determination. The s ill is clearly younger than the Devonian Three

Forks shale and older than the Oligocene Renova Formation. But,

considering the possible regional mechanisms concerning igneous

activity in southwestern Montana, it is probable that the s ill was

intruded during the Late Cretaceous in association with the extrusion

of the Elkhorn Mountains Volcanics.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TERTIARY TO RECENT STRATIGRAPHY - BOZEMAN GROUP

Depositional Sequence and Stratigraphic Nomenclature

The Bozeman Group was described by Kuenzi and Fields (1971) as

"continental Tertiary basin-fill deposits" (p. 3373). The Bozeman Group

is made up of two unconformable and lithologically distinct units and

has been subdivided into two formations: (a) Renova Formation

(Kuenzi & F ie ld s , 1971) and (b) Sixm ile Creek Formation (Robinson,

1967). The Early and Middle Oligocene Renova Formation consists

predominately of fine-grained flu v ia tile sediments with minor amounts

o f conglomerate. The Late Miocene and Middle Pliocene Sixm ile Creek

Formation is mainly coarse-grained with a significant amount of

conglomerate.

The Renova Formation was subdivided by Kuenzi and Fields (1971)

into three distinct units: (a) the Early Oligocene Bone Basin

Member, (b) the Early Oligocene Climbing Arrow Member, and (c) the

Middle to Late Oligocene Dunbar Creek Member (Figure 8). Only the

Dunbar Creek Member and the unconformably overlyin g Sixm ile Creek

Formation have been identified within the confines of the study area

in the North Boulder River basin. Four measured sections were taken

across this strata and are described in the appendix section of this

re p o rt.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23

AGE RUBY BASIN JEFFERSON BASIN THREE FORKS S. TOWNSEND (DsrM’W h n lcr, (Ku«nzi4 Field*, BASIN (Robinson> CLARKSTON BA- 1964) 1971) 1963; Dorr.1956) 5IN5(RnlwnonJ90 Alluvium QUATERNARY

Blancon

KenfMion

Sixmile "kxmkiiiiin Madiiori Valley Creek Formation equivalent Sixmile Creek «on>Uley Fm Borstovian Formation

Arikareen

Whitney on

Orellan DunbcrCr F rassamari Dunbar Creek Fm Formation Bone Climbing Arrc N|Chadrontan Basin Arrow Climbing Arrow Fm Fm

EOCENE ?Phin, Conci 1 /

Figure 8. Rock-stratigraphic correlation and age of the Bozeman Group in the Townsend, C larkston, Three Forks, Je fferso n , and Ruby basins (m odified a fte r Kuenzi & Fields, 1971).

Petrographic A nalysis: Dunbar Creek Member, Renova Formation

Sample Identification

The sandstone u n it o f the Dunbar Creek Member outcrops in parts

of southwestern Montana, especially in the Jefferson basin (Kuenzi &

F ie ld s, 1971) and in the South Townsend and Clarkston basins (Robinson,

1967). The description is based on a composite from samDles collected

from outcrops in the North Boulder River basin.

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

The maximum thickness o f the Dunbar Creek Member is 1000 f t in

the Jefferson basin (Kuenzi & Fields, 1971). According to Kuenzi and

Fields (1971), "the Dunbar Creek Member consists o f 75% immature and

submature v itric siltstone, 9% volcanic glass-bearing montmori11onite

mudstone, 13% immature to submature, very fine arkose to granular

very coarse arkose, and 3% conglomerate" (p. 3381). Strati graphically

the member rests conformably on the Climbing Arrow Member. Dunbar

Creeic strata are-unconfor-mably overlain by the Miocene-Pliocene

Sixm ile Creek Formation (Kuenzi & F ie ld s, 1971).

In the North Boulder River basin the Dunbar Creek Member is

approximately 60 ft thick. It is characterized by alternating sand

stones, siltstones, and mudstones. It is unconformably overlain by

pebble-boulder conglomerates belonging to the Sixmile Creek Forma

tion. On the outcrop, crude small-scale crossbedding and upward-

fining graded bedding sequences are characteristic throughout.

Approximate orientations taken from several crossbeds indicate a

southwesterly direction of flow. The units dip at angles of 10-30

degrees to the southeast.

Hand-Specimen D escription o f Sandstone U nit

The sandstone, in outcrop, is reddish gray to red-brownish gray.

It is fine to coarse grained, contains scattered pebbles, and is

poorly sorted. It can readily be identified as a feldspathic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. litharenite by the relative abundances of quartz, sedimentary rock

fragments, and feldspar (Figure 9).

Cementation is moderate (can easily be disaggregated without

breaking grains) but some specimens show extensive cementation (dis

aggregation d iffic u lt and breaks grains). Grains appear subangular,

and c a lc ite cement crysta ls hide some d e tr ita l o u tlin e s . Small-scale

crossbedding and graded bedding is present in some specimens.

This Section Description

Thin Section Abstract

The sandstone is typically a poorly sorted feldspathic lithare

nite sim ilar to many Tertiary sandstones within the Bozeman Group in

southwestern Montana (Figure 10). Monocrystalline and polycrystal

line quartz grains are subangular and comprise 24% of the rock,

feldspar comprises 14%, and rock fragments 12%. Mica, magnetite,

calcite, and iron oxide make up the rest of the rock. The framework

is loosely bound together due to the effects of minor compaction.

The sandstone was derived primarily from the Boulder batholith with

minor components from the Elkhorn Mountains Volcanics. The outcrop

is an isolated stream channel deposit.

Texture

1. The fabric is also typical of the feldspathic litharenites

of the area. Most of the rock is supported structurally by calcite

cement within the quartz-feldspar-rock fragment framework. Grain

contacts are slightly more numerous (1.02 contacts/grain) as compared

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

Quartzarenite

Subiitharenit*

Lithic arkose Feldspathic litharenite

Feldspar : Rock fragment ratio

______KEY______

Renova Formation Dunbar Creek Member

Sample units 1 © Composite Sixmile Creek Formation ______J______© Sample unit ______

Figure 9. Mineralogic classification of the Renova and Sixmile Creek Formations sandstone units described in this report (after Folk, 1974). Abbreviations used are described as follows: Q - all tyoes of quartz including chert; F - all single feldspar qrains including granite and gneiss fragments; RF - all other rock fragments, such as slate, schist, sandstone, volcanic rock fragments, etc.

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

Figure 10. Typical appearance o f Dunbar Creek sandstone in thin section; crossed nicols, X 43.75

to those for modern river sands (0.85 contacts/grain) indicating a

very minor amount of compaction (Pryor, 1973).

The sandstone is cemented almost to ta lly by calcite with very

minor amounts of hematite and mica. Cement averages approximately

31% and this reduces porosity to about 15% from an original porosity

that must have been about 40% to 45% (Pettijohn et a l., 1972).

2. Grain size distribution is that of a poorly sorted, pebbly,

fine-to-coarse-grained sand (Figures 11 and 12). Size distributions

are skewed to fin e sizes because o f the abundance o f clay.

3. The average visual roundness o f the medium-sized quartz

grains is 3.04 (subrounded). Coarser grains tend to be better

rounded.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ro oo s r PHI of sandstone 7 / ( 6 , ) - 1 0 1 2 3 16 95 50 84 0.1 99.8 n c 2 c < ■n m m 70 n m Z (0.30mm) ♦ 1.69

B / i t * Mt*L774 Mt*L774 PHI / • / f < 0 / / •

/ -1

-2 99.8 95 16 0.1 84 50

.... ♦ »♦ 0.19mm) »♦ * ts i / G • 6 \ M i*2 .4 : PHI Cumulative curves parameters (probability of grainmean size ordinate) (Mz) and sorting for sand-size or standard particles deviation and calculated sediment units G, cumulative B, and A o curve f the represents Dunbar Creek o Member f a determined the weight Renova Formation. percentage. Each point (See on appendix the for s tra ti graphic position of samples.) (Cumulative curves represent only sand-size distribution.) / / / • / • / Figure 11.

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

3 k t . l / M *>2.87*6,4 7mn^ *>2.87*6,4 M - f PHI 1 1 i 3 ~ i i / - 1 0 1 2 3 5 16 95 0.1 84 99.8 n c TO m Z - l ^ 50 c n ? 4

8 ’0.30 mm) ’0.30 sand-size particles and calculated sediment

UI* f o r 1 l / A *1.82 A M PHI / / o o 1 i 3 i / * / / • / ■2 ■2 -1 0.1 50 5 84 16 95 99,8 / A* A* u»* M j i 06 34(B 27m m ) PHI units K, I , and F o f the Dunbar Creek o Member f the Renova Formation. Each point on the cumulative curve graphic represents position a determined of weight samples.) percentage. (Cumulative (See appendix curves for represent s only tra sand-size ti distribution.) parameters of grain mean size (Mz) and sorting or standard deviation (A ) of sandstone / r / -2-1812 34 -2-1812 TO u m m Z n 5 n c c Figure 12. Cumulative curves (probability ordinate)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4. The sandstone is texturally immature as indicated by the

clay content. The mechanical (sieve) analysis fo r each sample u n it

is summarized in Figures 13, 14, and 15.

Mineralogy - Terrigenous Detritus (Table 1)

1. Quartz is separable into monocrystalline quartz and poly

crystalline quartz, and chert. The absence of undulatory grains and

significant amounts of flu id inclusions suggest a predominate plutonic

igneous source.

2. Feldspars present are sodic varieties of plagioclase and

undifferentiated potassium feldspar. The ratio of K-feldspar to

plagioclase is 10.6 based upon thin-section analysis. Most K-feldspar

is untwinned.

3. Rock fragments are dominately sedimentary with a relatively

low percentage of volcanic and gneiss fragments. The volcanic frag

ments are mostly altered diabases. The sedimentary fragments are

mudstone, siltstone, sandstone, and chert which are probably of local

o rig in .

4. Mica is nearly all muscovite with small amounts of biotite

and chlorite. It occurs in shreds and plates, many fragments are

bent and broken, and many are deformed around the edges of other

grains.

5. Heavy minerals are not abundant but include hematite and

magnetite.

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

u c o

o o -o > u c o o • v 1 7 c ■o o E e M 3 -D O C _ Mo -5 b o o — o E o w c o _aj o M _® 3 o -Q c I % f l S P~ 7 y _Q % 2 I M Y /Y ,777A 2 0 0 0 03 8.7 41 2133 3922 PERCENTPERCENT

Figure 13. Mechanical analysis of sandstone units A and B from the Dunbar Creek Member o f the Renova Formation. (See appendix for stratigraphic position of samples.)

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

uj 4 0 -

2 V s V,

PERCENT PERCENT

Figure 14. Mechanical analysis of sandstone units F and G from the Dunbar Creek Member o f the Renova Formation. (See appendix for stratigraphic position of samples.)

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

lu 4 0 -

“ 3 0 -

p e r c e n t PERCENT

Figure 15. Mechanical analysis of sandstone units I and K from the Dunbar Creek Member o f the Renova Formation. (See appendix for stratigraphic position of samples.)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CO 4* 1.9 3.7 1.0 0.4 0.3 14.6 11.4 30.9 10.6 21.6 5 3 0.2 8 10 35 66 46 2.6 262 582 205 190 388 1900 99.2 COMPOSITE Point Ind. count X ” “ “ 1.3 0.7 1.3 14.0 25.7 25.3 99.9 25.3 4 4 2 UNIT X 76 77 42 300 Point Ind. count X “ 1.7 1.3 -- -- 4.0 6 2.0 X 12.7 23.3 76 23.7 13 4.3 11.3 I Ind. 20.3 5 4 5 1.7 UNIT 38 71 34 Point count -- 0.7 0.3 “ 0.3 6.6 12 15.7 16.7 70 99.8 300 100.0 20.3 26.0 61 I 1 UNIT G — 47 40 13.2 20 61 78 Point Ind. count X Table Table 1 8.0 18.6 36.0 50 -- 2 3,6 1 4.6 X 100.0 300 Ind. F UNIT 24 -- 33-- 11.0 14 55 18.2 300 Point count 1.0 11 1.0 — 0.3 56 2.7 6.0 12.7 11.7 — 40.7 108 i 1 3 8 UNIT B — — 122 300 99.8 Point Ind. count X 4.3 35 0.7 3.6 3 7.0 18 -- 0.7 6 2.0 13.0 38 52.0 17.3 65 21.7 4 1.3 UNIT A 13 39 —— -- 156 300 99.9 count count X Point Ind. Point Ind. TOTALS sed. vole. gneiss. k-sparplag. 21 2 poly- mono- 52 MINERALS Iron oxides Iron Pore space Calcite Rock Rock Frags, Magnetite 2 Feldspar Mica 11 Quartz Modal Modal a n alysis o f sandstone u n it from the Dunbar Creek Member, Renova Formation, Montana

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chemical C onstituents

1. Calcite is present as a cloudy mostly-untwinned mosaic of

interlocking crystals making up a significant proportion of the rock

(31%).

Interpretation

Kuenzi and Fields (1971) stated the following:

Strata of the Renova Formation were deposited in floodplain, pond, and stream channel environments, and to a lesser degree as primary eolian deposits. The Dunbar Creek Member, dominated by v itric s ilt and subordinate arkosic and ande- s itic sand and gravel, accumulated primarily in floodplain and channel environments, (p. 3382)

The composition and texture of this sandstone unit are consistent

with the general interpretation of Kuenzi and Fields (1971). The

outcrop is characterized by crossbedding, graded bedding, alternating

coarse-to-fine-grain size, and lateral discontinuity, all of which

are indicative of an abandoned meandering stream channel deposit.

The predominance of "common" monocrystalline quartz is indicative of

a plutonic igneous source and therefore suggests that the Boulder

batholith was the original source of the sediment. The abundance

of sedimentary rock fragments and the red clay matrix suggest rework

ing of pre-existing sedimentary rocks probably from earlier Renova

strata. The presence of small amounts of volcanic rock fragments

suggests a minor influx of sediment from the Elkhorn Mountains

Volcanics.

The paleogeographic direction of the source lands is derived

from paleocurrent channel studies that indicate a major source to the

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

west in the Late Cretaceous Boulder batholith and a lesser immediate

source to the north and west from the Elkhorn Mountains Volcanics

(Kuenzi & Fields, 1971).

The abundance o f moderately alte re d fe ld spa r grains and the

subrounded nature of the medium-sized grains suggest a warm-temperate

climate as well as a moderate-to-high source area. The warm-temperate

interpretation is consistent with work done by R. F. Flint (1971) in

which average temperatures of North America were inferred from fossil

plants. By studying the patterns of erosion and basin fillin g ,

paleosol mineralogy, vertebrate faunal assemblages, sedimentary struc

tures, and provenance of Renova strata in central and western Montana,

Thompson et a l. (1981) have demonstrated that an arid to semi-arid

climate existed during the time of Renova deposition. The abundance

of rock fragments, poor sorting and subrounded nature of the sandstone

indicates a transport distance on the order of tens of miles. This

further supports the interpretation of the Boulder batholith that

outcrops approximately 20 miles west o f the sample lo ca tio n as a

major source.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Petrographic Analysis: Sixm ile Creek Formation

Sample Identification

The sandstone u n it o f the T e rtia ry Sixm ile Creek Formation

outcrops in parts of southwestern Montana, especially in the Jefferson

basin (Kuenzi & Fields, 1971) and in the southern Townsend and

Clarkston basins (Robinson, 1967). The description is from an outcrop

in the North Boulder River basin.

Field Relations

The sandstone has a maximum thickness of 2,400 f t (Robinson,

1967) and thins to extinction laterally. The formation is typically

coarse grained. Conglomerate is c h a ra c te ris tic o f the Sixm ile Creek

Formation. Stratigraphically, the formation rests unconformably on

Renova and older prebasin rocks. Sixmile Creek strata are truncated,

in turn, by an erosion surface that is commonly veneered by a thin

layer of Quaternary deposits.

The North Boulder River basin composite outcrop of the Sixmile

Creek Formation that is contained within the field area is approxi

mately 100 ft thick. It is characterized by sandstone and conglomer

ate. On the outcrop, crude small-scale crossbedding and upward-

fining graded bedding are characteristic throughout. Dips are low

and d iffic u lt to read but appear to be 0 to 5 degrees to the south

east.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hand-Specimen D escription o f Sandstone U nit

The sandstone, in outcrop, is yellowish light-gray. It is

coarse grained and contains scattered pebbles. It can easily be

identified as a lith ic arkose by the relative abundance of feldspar

and rock fragments (Figure 9). Cementation is lacking and the rock

may be easily disaggregated. Grains appear subangular and moderately

sorted.

Thin Section Description

Thin Section Abstract

The sandstone is typically a moderately sorted lith ic arkose

sim ilar to many Tertiary sandstones within the Bozeman Group in

southwestern Montana (Figure 16). Monocrystalline and polycrystalline

quartz are subangular to rounded and comprise 29% of the rock, feld

spar comprises 15%, and rock fragments 31%. The remaining percentage

of the rock is made, up of mica (1%) and magnetite (1%). The framework

is loosely bound together (25% porosity) due to minor compaction.

The sandstone was mostly derived from the Boulder batholith with minor

components from the Elkhorn Mountains Volcanics. The outcrop is an

isolated stream channel deposit.

Texture

1. The fabric is atypical of the arkoses of the area in that

it displays a complete lack of cement, although the rock does display

a typical quartz-feldspar-rock fragment framework of which it is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39

Figure 16. Typical appearance o f Sixm ile Creek sandstone in thin section; crossed nicols, X 43.75.

supported. Grain contacts (1.92 contacts/grain) indicate some minor

compaction which reduced porosity to about 25% from an original

porosity that must have been 40% to 45% (Pettijohn et a !., 1972).

2. Grain-size distribution is that of a moderately sorted

pebbly coarse sand (Figure 17).

3. The average visual roundness of the median-sized quartz is

2.5 (subangular). Coarser grains tended to be better rounded.

4.. The sandstone is texturally submature as indicated by sort

ing and roundness. The mechanical analysis of the sandstone unit

is summarized in Figure 18.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40 99.8 R /

n r1 I 1 0 c t— / > r • < * ■o m 1 3 0 s n 0 m Z / 0 MziO-Ol t(0S5mm) f a 0.9( >♦

0.1 I 2 - i i> 1 i 3 . • PHI

Figure 17. Cumulative curve (probability ordinate) and calculated sediment parameters of mean grain size (Mz) and sorting or standard deviation (<>,) of a sandstone unit from the Sixmile Creek Formation. Each point on the cumulative curve represents a determined weight percentage.

Mineralogy - Terrigenous Detritus (Table 2)

1. Quartz is separable into monocrystalline and polycrystalline

grains. The absence o f undulatory grains and s ig n ific a n t amounts o f

flu id inclusions suggest a predominate plutonic igneous source.

2. Feldspars present are sodic varieties of plagioclase and

undifferentiated potassium .feldspar. Some grains are kaolinized, but

it is not certain whether this alteration occurred in place or during

transport. More K-spar grains are altered than plagioclase.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41 80

70- ■o c 60- o

®•• TJ— 50- :O M8 u ■o c ui40- Q)k bO B M u BC > W 0 tc « a a! 30-j 3 ■o C 0 1 1 0 I E “o 2 *5 c 20-1 O ® "O M C C © mT D 10 > > , JQ .5 _Q ® -2 0 u Q. 0 I16X 33.C 30.(19.0 5.0 PERCENT Figure 18. Mechanical analysis of a sandstone unit from the Sixm ile Creek Formation.

Most K-spar is untwinned. Plagioclase exhibits albite, carlsbad,

and pericline twinning.

3. Rock fragments are dominately granitic with a relatively

low percentage of volcanic, gneissic, and sedimentary fragments. The

volcanic fragments are highly altered diabases. The sedimentary

fragments include mudstone and si Itstone which are probably of local

o rig in .

4. Mica is almost all biotite, with small amounts of muscovite

and chlorite. It occurs in shreds and Dlates, many bent and broken,

and many deformed around the edges of other grains.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2 Modal Analysis o f Sandstone U nit from the Sixm ile Creek Formation

Point % Mi nerals counts

Quartz

Monocrystalline 65 21.7

Polycrystalline 21 7.0

Feldspar

K-spar 18 6.0

Plagioclase 26 8.7

Mica 2 0.7

Rock fragments

Gneissic 3 1.0

Sedimentary 10 3.3

Volcanic 19 6.3

G ra n itic 59 19.6

Magneti te 2 0.7

Pore space 75 25.0

TOTALS 300 100.0

5. Heavy minerals are few but include zircon, hematite, magne

tite , and apatite, most occurring within the granitic rock fragments.

Chemical Constituents

The rock is almost entirely devoid of chemical constituents.

There is a trace amount of authigenic kaolinite occurring inpore spaces.

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

Interpretation

According to Kuenzi and Fields (1971):

The Sixmile Creek Formation was deposited in a complex of ephemeral and perennial stream channel and subordinate overbank deposits. Coarse angular detritus along the southeast flank of Bull Mountain was deposited with reworked sand by ephemeral streams and mudflows. Vi t r ie s ilts to n e associated with arkose accumulated as point bar, overbank and occasionally as direct eolian deposits, (p. 3384)

The composition and texture of the Sixmile Creek sandstone unit and

associated conglomerate are consistent with the general interpreta

tion by Kuenzi and Fields (1971). In outcrop, the sandstone unit

displays crossbedding, graded bedding, and lateral discontinuity,

all of which are indicative of stream-channel deposits. The associ

ated conglomerate, which contains limestone boulders up to 5 ft in

diameter, is interpreted as a mudflow deposit with clasts of very

local origin. The predominance of granitic rock fragments and

monocrystalline "common" quartz reflect the Boulder batholith as

the original source of sediment. The presence of small amounts of

volcanic rock fragments suggest a minor influx of sediment from the

Elkhorn Mountains Volcanics.

According to Kuenzi and Fields (1971), "relatively high energy

ephemeral and stream channel environments were more important during

Sixmile Creek deposition than during Renova deposition, and textural

and mineralogical data suggest less equable and more arid climate

conditions" (p. 3384). The coarser-grained nature of the Sixmile

Creek rocks as compared to Renova rocks tends to bear out a higher

energy interpretation. A complete lack of calcite cement within

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44

the Sixmile Creek unit studied by this author may be evidence that

a somewhat more arid climatic condition existed during Sixmile Creek

time, although it is possible that the cement, if it did once exist,

may have been leached out sometime after deposition of the rock. By

studying the patterns of erosion and basin fillin g , paleosol miner

alogy, vertebrate faunal assemblages, sedimentary structures, and

provenance o f Sixm ile Creek s tra ta in central and western Montana,

Thompson et al. (1981) have demonstrated that an arid to semi-arid

climate existed during the time of Sixmile Creek deposition. The

abundance o f rock fragments, moderate s o rtin g , and subrounded nature

of the grains indicates a distance of transport on the order of tens

of miles with locally derived mudflows as indicated previously.

The paleogeographic direction of the source lands comes from

paleocurrent channel studies that indicate a major source to the

west in the Late Cretaceous Boulder batholith and a lesser immedi

ate source to the north and west from the Elkhorn Mountains

Volcanics (Kuenzi & Fields, 1971).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONROW CREEK CONGLOMERATE

A very poorly sorted limestone boulder conglomerate of ques

tionable age and origin is located at NE%, sec. 17, T2N, R3W. This

m atrix supported, muddy-sandy conglomerate consists o f subrounded

to angular s ilt to boulder size limestone, shale and andesite rock

fragments. The matrix is friable, rust-gray weathering to rust

colored s ilty -s a n d y mudstone consisting o f very poorly sorted

subangular clasts. The FeO-rich matrix is cemented by sparry

c a lc ite . A modal analysis o f the m atrix is summarized in Table 3.

Table 3 Modal Analysis o f the Conrow Creek Conglomerate M atrix

Point Minerals % Counts

Quartz 16 3.2

Chert 22 4.4

Rock fragments

Shale 41 8.2

S p a rrite 51 10.2

Mi c ri te 62 12.4

S p a rrite cement 189 37.8

FeO 116 23.3

Chalcedony 2 0.4

Dolomite 1 0.2

TOTALS 500 100.0

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The conglomerate (Figure 19) appears to occupy a stra ti graphic

position above the Mississippian Mission Canyon Formation and below

the Late Cretaceous Elkhorn Mountains Volcanics. As was firs t

reported by Smedes and Schmidt (1979), i f the conglomerate does in

fa c t lie s tr a ti g ra p h ica lly between the Madison Group and the Vol

canics it probably documents unroofing of Paleozoic rocks from the

region of the Boulder batholith prior to the extrusion of the

Elkhorn Mountains Volcanics. The conglomerate, which consists for

the most part of limestone fragments primarily derived from the

Madison Group and Jefferson Formation and shale fragments derived

from the Three Forks Formation, also contains a few andesite rock

fragments that were probably derived from the Elkhorn Mountains

Volcanics. This suggests that the conglomerate does not predate

the extrusion o f the volcanics. The main body o f conglomerate,

which appears to occupy a position in a narrow gorge j’ust upstream

from the volcanics, may have accumulated as the result of the

extrusion of the volcanics (Figure 20). The Madison Group was

being eroded during Late Cretaceous time and therefore the afore

mentioned gorge was probably cut during this time (Figure 20, A)..

It is possible that the volcanics, upon their extrusion and deposi-

’ tion, blocked the lower reaches of this narrow gorge which drained

an area bounded by the Jefferson, Three Forks, Lodgepole, and

Mission Canyon Formations and thereby resulted in the accumulation

of the conglomerate as debris was transported from the upper reaches

of the drainage area during periodic flash flood events (Figure 20, B)..

The deposition of the Conrow Creek Conglomerate therefore took place

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 19. Conrow Creek Conglomerate in NE%, sec. 17, T2N, R3W. Top: fu ll view showing main outcrop (facing west). Bottom: closeup view (facing west).

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

ELKHORN MOUNTAINS PALEOZOIC VOLCANICS STRATA

PALEOZOIC STRATA ELKHORN MOUNTAINS 71 VOLCANICS

Figure 20. Generalized block diagram illustrating the proposed accumulation o f the Conrow Creek Conglomerate. A - Late Cretaceous time, B - Early tertiary time, C - present time.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. over an undetermined period during Late Cretaceous and Early Tertiary

time. The narrow gorge has been partially exhumed and enlarged

around the main body of conglomerate by post Elkhorn Mountains Vol

canics erosion which has caused some downstream sca tte rin g o f debris

from the Conrow Creek Conglomerate (Figure 20, C).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LATE CRETACEOUS TO RECENT GEOLOGIC HISTORY

The purpose of this section is to place the Tertiary to Recent

geologic events in to a chronologic sequence. Conditions are based

upon data discussed in earlier sections of this report. Because

this study area occupies such a small portion of the North Boulder

River basin, the geologic history of the Jefferson basin, as deduced

by Kuenzi (1966), was re lie d on to a great degree in in te rp re tin g

the history of the North Boulder River basin. The author also gained

helpful information about the geology of the North Boulder River

basin to the north o f the fie ld area when he accompanied the Indiana

University Geologic Field course (G-429) to that area. The following

Late Cretaceous to Recent history is suggested for that small part of

the North Boulder River basin within the confines of the study area.

1. U plift associated with the Laramide orogeny probably in iti

ated erosion in Middle to Late Cretaceous time and continued until

Eocene time. Denudation associated with this orogeny developed an

erosion surface across the area now occupied by North Boulder River

basin. Laramide u p lif t also caused erosion deep enough to expose the

Boulder batholith to the west (Kuenzi, 1966).

2. After studying the Jefferson and other basins in southwest

ern Montana, Kuenzi (1966) stated:

By the Early Oligocene drainage was either slowed or impaired and deposits of alternating microcrystalline calcium carbonate mud, oolite montmorillonitic mud, and v it r ic and arkosic sand, accompanied by minor q u a n titie s of gravel accumulated in shallow pond, floodplain, and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. stream channel environments (Bone Basin Member), and mont- m orillonitic mud and arkosic sand accumulated in floodplain and channel environments (Climbing Arrow Member), (p. 177)

3. Basin deposition continued into Middle Oligocene as the

Dunbar Creek Member accumulated as a stream channel deposit.

4. Following Oligocene basin fillin g , probably sometime during

Late Oligocene or Early Miocene, block faulting occurred along the

western margin of Bull Mountain. Earlier Tertiary deposits (Dunbar

Creek Member w ith in the confines o f the fie ld area) were t ilt e d as

much as 30 degrees into the southeasterly dipping normal fault.

This faulting took place as the result of regional extension

(Hamilton & Myers, 1966; Reynolds, 1979).

5. Contemporaneous w ith o r fo llo w in g Late Oligocene o r Early

Miocene block faulting a period of erosion took place. This is

evidenced by the unconformity which separates the Oligocene Dunbar

Creek s tra ta from the overlying Late Miocene-Pliocene Sixm ile Creek

s tra ta . A q u a n tita tiv e ly unknown amount o f Dunbar Creek s tra ta was

removed during th is erosional period.

6. In the Late Miocene the Sixm ile Creek Formation began to

accumulate as stream channel and mudflow deposits.

7. If rocks younger than Late Miocene-Pliocene Sixmile Creek

rocks were deposited they have since been removed by erosion.

8. Late Tertiary rejuvenation along the fault mentioned in

#4 (above) probably took place causing displacement of an undeter

mined amount. This fa u lt is postulated to explain the occurrence

of Sixmile Creek strata which appear to be in a lower stratigraphic

p o sitio n than surrounding Dunbar Creek s tra ta .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9. Downcutting and excavation of the basin resulting in the

development of a series of pediment surfaces has continued interm it

tently to the present.

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

Introduction

The prebasin rocks were tilte d and faulted during Laramide

deformation. Tilting began prior to the extrusion of the Elkhorn

Mountains Volcanics (Alexander, 1955; Barnes, 1954; Klepper et a l.,

1957, 1974; Robinson e t a l . , 1969; Smedes & Schmidt, 1979). During

the later stages of Elkhorn Mountains Volcanics extrusion the Boulder

batholith began to intrude 78 m.y. ago. Regional folding continued

after the final stages of the intrusion of the Boulder batholith

68 m.y. ago (B illin g s le y , 1915; Klepper e t a l . , 1957, 1974; Robinson

et a l., 1969; Smedes & Schmidt, 1979). The angular unconformity

(discussed on p. 19-21 of this report) in which northward-dipping

(32-55 degrees) Elkhorn Mountains Volcanics are in contact with

northward-dipping (58-75 degrees) M ississippi an Madison Group rocks

is evidence that tiltin g began prior to the extrusion of the vol

canics and persisted until after their accumulation. The u p lift

responsible for this tiltin g may have been continuous or may have

occurred in stages. Klepper et a l. (1957, 1974) have demonstrated

that the youngest rocks of the Boulder batholith crosscut tilte d and

folded strata of the Elkhorn Mountains Volcanics (see Figure 21 for

geologic cross sections).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. cn 4* -45 00 ' “ 5500 6500 ? -5500 - 7500' T b r fl\ Mmm 1 mile 1 - 4 5 0 0 “ 5500 - 6500 Mml \Mm m ' Mml T b r. Mml Ke T br Mml (no (no vertical exaggeration) SCALE SCALE 1*24000 Mml \ Mmm \VN\ \ \ . l\. \ \ fa P € g the North Boulder River basin. (See Map 1 for locations.) 5500- 45 00 45 - 6 5 0 0 “ P€g - Tbr, 6 5 0 0 - 5 5 0 0 7500 OO'- 5 5 0 0 - 45 45 Figure 21. Geologic cross sections along the southeastern flank of Bull Mountain extending into

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

Major Late Cretaceous-Early Tertiary Faults

A northwest-southeast-trending (N25W) normal fault (Tebay Spring

fa u lt, new name) extends from NW%, sec. 20, T2N, R3W to NE%, sec. 1,

T2N, R4W. The fa u lt has brought Precambrian through Mississippi an

rocks on the southwest into contact with Late Cretaceous Elkhorn

Mountains Volcanics on the northeast. Because the stratigraphy of

the Elkhorn Mountains Volcanics was not determined by this author

only minimum values for throw (3100 ft) and vertical separation

(2700 f t ) could be measured along the fa u lt. The map pattern o f the

fa u lt and d r i l l hole data obtained from R. G. Garwood, Regional

Geologist with Placer Amex, Inc., Spokane, Washington (personal

communication, 1982), lead to the conclusion that the fault dips

approximately 70 degrees to the northeast. According to Garwood,

Amex drilled one rotary d rill hole approximately 330 ft deep at a

location which is approximately 150 ft northeast of the Elkhorn

Mountains Volcanics-Greyson Shale contact (SW*s, sec. 17, T2N, R3W)

and encountered Elkhorn Mountains Volcanics (andesite) its entire

length. Therefore the fault must dip at an angle greater than 60

degrees to the northeast. Solution of a three-point problem along

the trace of the fault in SW%, sec. 17, T2N, R3W yields a dip of

70 degrees to the northeast. The d rill hole data lessens the lik e li

hood that the contact is an unconformity, an interpretation once

favored by Smedes & Schmidt (1979). If the contact is indeed an

unconform ity, the Greyson Shale u nits should have been encountered

at a drilling depth of approximately 150 ft, given that the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. unconformable surface was not tilte d very steeply after it formed.

This fa u lt appears to be crosscut by both the Golden S unlight fa u lt

(Alexander, 1955) (NW%, sec. 20, T2N, R3W) and the St. Paul's Gulch

fa u lt (Kuenzi, 1966) (NE%, sec. 1, T2N, R4W) in d ic a tin g th a t fa u ltin g

took place between Late Cretaceous and Late Oligocene tim e. (The

S t. Paul's Gulch and Golden S unlight fa u lts are discussed in the

Cenozoic faults section of this report.) The author speculates that

the Tebay Spring fa u lt and the fault which extends from SE%, sec. 17,

T2N, R3W to SE%, sec. 25, T3N, R4W (Mud Spring fa u lt, new name;

further discussed in the next paragraph of this report) formed at

the same time as the result of calderon subsidence producing a graben

which comprises the central portion of the map area. This collapse

caldera may have formed as the result of very late Elkhorn Mountains

Volcanics eruptions which caused a withdrawal of underlying support

in this area. Klepper et a l. (1974) suggested similar (but larger

scale) faulting while discussing the emplacement of the Boulder

batholith. They speculated that caldera subsidence, which was guided

by northeast and northwest structural trends, resulted as a conse

quence o f "ra p id expulsion o f the large volume o f t u f f la te in the

volcanic episode" (p. 1959). The upper sequences of Elkhorn Mountains

Volcanics, which are located just to the northeast of the study area,

consist mostly of clastic material (Alexander, 1955; Satoskar, 1971).

The Tebay Spring and Mud Spring faults follow the northwest structural

trends c h a ra c te ris tic o f th is area in southwestern Montana. These

structural trends were originally established in Late Precambrian

time (Schmidt & Garihan, 1984; Schmidt & Hendrix, 1981). It is well

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. known that ash can be erupted well away from the point of actual

collapse as was probably the case in this area. For example, the

form ation o f a caldera on Mt. Katmai in Alaska during the 1912

eruption was the result of an eruption from Novarupta, 7 mi to the

west (Curtis, 1968). The present writer speculates that tiltin g ,

which as stated previously persisted until after the accumulation of

the Elkhorn Mountains Volcanics, tilte d the Tebay Spring fault plane

from a more normal angle (60 degrees NE) to the proposed present

70 degrees NE dip.

As was the case fo r the Tebay Spring fa u lt, only a minimum throw

(1800 ft) and vertical separation (2700 ft) could be determined for

the Mud Spring fault. The dip of this normal fault, which strikes

N25W, is speculated to be approximately 50 degrees to the southwest.

Because only minimum values for the throw and vertical separation

could be determined for the two graben-bounding faults, this author

was unable to reach any conclusions based upon comparisons between

the faults. Late Cenozoic movement of an undetermined amount along

the southeastward extension of the Mud Spring fault is postulated

to explain the Cambrian-Tertiary contact (sec. 21, T2N, R3W). The

re la tio n sh ip s between the Golden S u n lig h t, S t. Paul's Gulch, Mud

Spring, and Tebay Spring faults is illustrated in Figure 22.

Major Cenozoic Faults

The St. Paul's Gulch fault (Alexander, 1955; Kuenzi, 1966) is

located on the west side of Bull Mountain (SW%, sec. 13, T2N, R4W to

SW%, sec. 25, T3N, R4W. The fa u lt was interpreted by Alexander (1955)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. g r a b e n

Figure 22. Diagram illu s tr a tin g the re la tio n s h ip between the major faults which cut the southern flank of Bull Mountain. (See Map 1 for locations; diagram not to scale.)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to represent "Late Tertiary rejuvenation of the original major

movement which took place in Pal eocene to Eocene (?) time and caused

displacement o f approximately 3000 fe e t" (p. 93). However, Kuenzi

(1966) suggested that "the trace of the fault might not extend south

ward from sec. 12, T2N, R4W along the base o f B ull Mountain where i t

is veneered by Late Tertiary and (or) Quaternary deposits" (p. 21).

Regardless of the fault's exact location, it is inferred to explain

the contact between T e rtia ry Renova and e a rlie r s tra ta . Alexander

(1955) interpreted it to be a normal fault dipping (70-75 degrees)

to the west. The St. Paul's Gulch fault appears to crosscut and le ft

separate the Tebay Spring fault (sec. 1, T2N, R4W). From the separa

tion cross-cutting relationship of these two faults, a vertical

displacement of approximately 5600 ft is inferred for the St. Paul's

Gulch fault given that there has been no strike-slip motion along

this fault. The major movement along the St. Paul's Gulch fault

probably occurred during Late Oligocene to Early Miocene time as

the result of regional extension which followed the cessation of the

compressional Laramide orogenic episode (Reynolds, 1979). This

north-south trending fault follows the same structural trends and

character of regional extensional basin-range faulting as set forth

by Reynolds (1979).

The northeast-southwest-trending fault, located on the east side

of the southeast flank of Bull Mountain (sec. 20, T2N, R3W), cuts

the la t it e in tru s iv e o f the Golden S unlight mine (Alexander, 1955)

located just to the south of the field area (sec. 19, T2N, R3W). The fa u lt was named the Golden S unlight fa u lt by Alexander (1955). The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60

present w riter agrees with the interpretation of Alexander (1955)

that the topographic surface on the east side of the southeast flank

of Bull Mountain (sec. 1, T2N, R3W) represents the fault scarp.

Slickensides with an approximate attitude of 48 degrees, S45E were

noted on the fa u lt scarp which is on Greyson shale. The a ttitu d e o f

the slickensides seems to indicate pure dip-slip motion. The occur

rence of Precambrian units to the west of the fault trace and the

Tertiary Renova Formation units to the east of the fault leads to

the conclusion th a t the east side was downdropped re la tiv e to the

west side and therefore defines a normal fault. Major movement of

an unknown amount along this fault probably occurred during Late

Oligocene to Early Miocene as a re s u lt o f the previously discussed

regional extension (basin-range faulting) which followed the cessa

tion of the compressional Laramide orogenic episode (Reynolds, 1979).

The fa u lt extending from NE%, sec. 27, T2N, R3W to NE%, sec. 23,

T2N, R3W (Boulder Basin fa u lt, new name) is thought to be a high

angle normal fault which dips to the southeast. The vertical separa

tion cannot be determined due to lack of outcrop. The fault is postu

lated to explain the occurrence of Late Miocene-Pliocene Sixmile

Creek strata outcrops which appear to be in a lower strati graphic

position than surrounding Oligocene Renova strata (SEls, sec. 22,

T2N, R3W). It is also postulated to explain the location of the

North Boulder River basin. The location of the fault is further

indicated by a silty-calcareous tufa outcrop (NW^s, sec. 23, T2N,

R3W) which may have been formed by a Tertiary fault-related spring.

The author thinks the fault represents rejuvenation of a fault which

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. originally developed in Late Oligocene-Early Miocene time. This

conclusion is based upon the observation th a t the Dunbar Creek s tra ta

dips in to the proposed fa u lt plane a t angles o f 20-30 degrees whereas

the Sixmile Creek strata dips at angles of no greater than 5 degrees

into the proposed fault plane. Thus the original movement along the

fault must have taken place prior to the deposition of the Late

Miocene-Pliocene Sixm ile Creek Formation. Moreover, because the

Sixm ile Creek is cut by the same fa u lt, a second post-S ixm ile Creek

movement must have taken place. This interpretation is consistent

with work done by Reynolds (1979) in which he demonstrated sim ilar

fault-related structural situations in southwestern Montana. It

seems reasonable to conclude that the original Late Oligocene-Early

Miocene movement is also the re s u lt o f regional extension.

Minor Faults

Numerous small, less significant faults are scattered throughout

the area. These faults all display vertical displacements in the

tens of feet. The fault located in sec. 25, T3N, R4W which splays

from the St. Paul's Gulch fault explains the abrupt, near linear

termination of the northwestward-striking Paleozoic rocks. The pair

of normal faults located in S%, sec. 8, T2N, R3W are thought to be

eastward-dipping, high angle faults which are responsible for a

minor repetition of beds within the Elkhorn Mountains Volcanics and

Paleozoic beds. This faulting probably took place at the same time

(Late Cretaceous) as the two previously discussed graben-bounding

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. faults. The minor fault located in SE%, sec. 17, T2N, R3W is

thought to be a high angle normal fa u lt which caused some s lig h t

displacement o f the Paleozoic rock i t cuts. This fa u lt is marked by

a spring. The southeastward-dipping, high angle normal faults

located in SE%, sec. 21, T2N, R3W are responsible for the repetition

o f several members w ith in the Dry Creek Formation which has caused

an apparent slight thickening of the formation in this small area.

The fault located in NE%, sec. 23, T2N, R3W is responsible for the

displacement which has uplifted Devonian Three Forks Shale on the

western side of the fault into contact with the Mississippi an Madison

Limestone on the western side of the fault. Field evidence suggests

that the fault dips at a high angle to the northwest and therefore

defines a reverse fault which probably took place during Laramide

tim e.

Structural History

The purpose here is to place known and inferred conditions and

events deduced from the structural elements within the field area

into their chronologic sequence. The following structural history

is suggested to explain the folding and faulting within the area.

1. Early regional arching in response to the Late Cretaceous

beginning of the Laramide orogenic episode resulted in the northward

tiltin g of the Paleozoic and Precambrian rocks. The tiltin g was

perhaps due to the emplacement a t depth o f the Boulder b a th o lith .

2. As regional folding continued the Elkhorn Mountains

Volcanics began to be extruded.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63

3. Very closely following the extrusion of the middle units of

the Elkhorn Mountains Volcanics faulting took place in Late Creta

ceous time as the re s u lt o f caldera subsidence. High-angle normal

faulting formed a graben in the central portion of the map area.

4. Continued uplift resulted in the northeastward-tilting of

the normal fault planes and the Elkhorn Mountains Volcanics as well

as a further tiltin g of the Paleozoic and Precambrian units.

5. Following complete cessation of Laramide compressional

forces probably in Eocene time, block faulting took place in Late

Oligocene-Early Miocene time re s u ltin g in the form ation o f the St.

Paul's Gulch, Golden S u n lig h t, and Boulder basin fa u lts .

6. Late Tertiary rejuvenation along the faults mentioned in

#5 probably took place.

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

The geologic history of the area is a synthesis of work done by

numerous others throughout the region and information obtained in the

field. The Precambrian to Recent history is subdivided into several

individual periods each of which relates to a particular event or

closely related events.

Precambrian

During Precambrian time (1400-1500 m.y. ago) riftin g of the

older Precambrian mass took place in itia tin g the development of a

Cordilleran-type continental margin (Burchfield & Davis, 1975).

This branching r ift system with aulogens (Belt and Uinta basins)

separated the Siberian platform from the North American craton.

By Late Precambrian time the Cordilleran geosyncline was well

developed in the West as evidenced by isopach maps of rocks which

clearly show the position of the shelf edge as do lithofacies

w ith in the Cambrian system (Figure 23) (Sloss, 1950). The southern

edge o f the B e lt basin (Helena embayment; Harrison e t a l. , 1974) was

an active zone at times during this period as indicated by coarse

detritus deposited along it as a northward-thinning wedge of Belt

Supergroup conglomerate (LaHood Formation) and arkose (Alexander,

1955; McMannis, 1963; Schmidt & Garihan, 1984). W ithin the study

area the B e lt Supergroup is represented by the Greyson Shale which

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65

B A / \ r \

x s M ' t A - S ”*

i *i North American craton

13 Is 'fry - 1 1Carbonate bank and . quartiite belt Continental i t ijinto Mt. 13 yvf'.:A’; W g shelf and upper continental /jM V 'r ? : . slope deposits

/ W \ t==l Grontolitic thole and chert «*X/ f t-'v :'; belt,Marginal ocean basin J

• n m Volcanic rock and \ - V greytMocke belt, Volcanic Group rXsf | fix'-'-' arc deposits v S f e * •* 388 L k m ^ 1

c Vblconic arc Shale Miogeocline . basin

( -Loww Poleazok

W ------______, ____ 7

Figure 23. A - Isopach mao of Lower Cambrian, Ediacaran, and Hadryn- ian rocks with the areas of occurrence of Belt Group and equivalent rocks superposed (modified after Stewart, 1978). B - Major lower Paleozoic stratiqraphic belts within the Cordilleran fold belt of North America (modi fied after Churkin, 1974). C - Diagrammatic reconstruc tion of the island arc and basin of early Paleozoic time in the Cordillera (based on diagrams of Churkin, 1974).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. was deposited in the sea (Helena embayment) which occupied much of

southwestern Montana during Precambrian time.

Cambrian

W ithin the study area the Cambrian sequence o f s tra ta is

represented by the basal Flathead Sandstone and conformably overlying

Wolsey, Meagher, Park, Pilgrim, and Dry Creek Formations. This

a lte rn a tin g sequence o f sandstones, lim estones, and shales represents

deposition in a shallow marine environment on a shelf undergoing

slight, recurrent subsidence (Alexander, 1955; Sloss, 1950). The

absence o f O rdovician, S ilu ria n , and Early Devonian rocks indicates

a long period of nondeposition and erosion which separates the

Cambrian u n its from the overlying Devonian Jefferson Dolomite.

Devoni an-Pennsylvani an

Deposition continued during Middle Devonian time as evidenced

by the accumulation of the Jefferson Formation and conformably over-

lying Three Forks, Lodgepole, and Mission Canyon Formations. The

M ississippian Lodgepole and Mission Canyon Formations (Madison Group)

represent a massive carbonate platform which accumulated on the

western s h e lf. Culmination o f deposition o f the Madison Group

occurred with the regional u p lift and exposure at the end of Meramec

or beginning of Chester time (Gutschick et a l., 1976; Roberts, 1979).

Exposure o f the Madison Limestone perm itted development o f lo ca lized

karst topography and solution brecciation (McCaleb & Wayhan, 1976).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Following th is , resubmergence made possible the deposition o f the

Late Mississippian-Pennsylvanian Amsden Formation and conformably

overlying Upper Pennsylvanian Quadrant Formation.

Permian-Middle Cretaceous

The Permian-Middle Cretaceous sequence o f rocks present else

where in southwestern Montana is not represented within the study

area. This area is located in the area that Imlay (1945) and others

referred to as the Belt Island and that was later termed the Boulder

high by Gwinn (1960) and Schwartz (1972). The Belt Island was a

complex of shallow marine emergent areas which explain variations in

thickness patterns of Late Paleozoic through Middle Cretaceous units

in southwestern Montana. The Boulder high, which formed as a part

of the Belt Island system, is thought to have influenced thickness

patterns in this area since Cambrian time. This feature would

u ltim a te ly become the s ite o f emplacement o f the Boulder b a th o lith

(Peterson, 1981). Therefore either deposition did not occur or the

units that may have been deposited were later removed by erosion.

Late Cretaceous-Early Tertiary

Late Cretaceous time marked the beginning of the Laramide

orogenic episode. Laramide regional compression, which continued

until Eocene time, was responsible for a zone of convex-eastward

th ru s t fa u lts and fo ld s which combine to make up the Disturbed B elt

of west-central Montana (Cohee, 1962; Woodward, 1981). The beginning

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the Laramide was closely followed by the extrusion of the Elkhorn

Mountains Volcanics. The existence of an unconformity (Map 1; NW%,

sec. 14, T2N, R3W to SE%, sec. 25, T3N, R4W) in which northward-

dipping (32-55 degrees) Elkhorn Mountains Volcanics are in contact

with northward-dipping (58-75 degrees) erosionally truncated Paleo

zoic rocks is evidence that folding began prior to and lasted until

after the accumulation of the volcanics. To the west of the study

area during the later stages of Elkhorn Mountains Volcanics extru

sion, the Boulder batholith began to intrude (78 m.y. ago) as grano-

diorite. Mafic magma penetrated into steep-bounding fractures in

the volcanics (Klepper et a l., 1974). According to Klepper et a l.

(1974), a much larger volume of quartz monzonite (Butte Quartz

Monzonite) then followed fillin g the central part of a large (100 km

by 60 km) depression in the volcanics which was created by collapse.

As the uppermost Elkhorn Mountains Volcanics members were extruded

caldera subsidence resulting in normal faulting produced a graben in

the central portion of the study area (see Map 1). After the upper

member of the volcanics was deposited the last event of Boulder

batholith intrusion took place (68 m.y. ago) as Butte Quartz Monzonite

found its way upward intruding folded and faulted volcanic and pre-

volcanic rocks (Klepper et a l., 1974). The graben-bounding fault

planes were later tilte d to the north by subsequent Laramide folding.

Eocene-Recent

Erosion which accompanied the Laramide orogeny continued in to

Eocene time developing an erosion surface across the area now occupied

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. by the North Boulder River basin. During Oligocene time alternating

periods of basin deposition resulted in the accumulation of the vari

ous members o f the Renova Formation (only the Dunbar Creek Member is

represented within the confines of this w riter's field area). Follow

ing Oligocene basin fillin g , probably sometime during Late Oligocene

or Early Miocene time, basin and range style block faulting took

place along the eastern margin of Bull Mountain. This faulting

occurred as the result of regional extension which followed the cessa

tion of Laramide compressional forces (Reynolds, 1979). This regional

extension also resulted in the formation of the St. Paul's Gulch and

Golden Sunlight faults. Following a short period of erosion after

Renova deposition, basin fillin g continued with the accumulation of

the Late Miocene-Pliocene Sixmile Creek Formation. After deposition

of the Sixmile Creek sediments the Boulder basin fault was probably

reactivated. This reoccurrence is postulated to explain the occur

rence of Sixmile Creek strata which appear to be in a lower stra ti-

graphic position than surrounding Renova strata. Post-Sixmile Creek

downcutting and fan deposition within the basin resulted in the

development of a series of pediment surfaces that are currently

being eroded.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX - MEASURED SECTIONS

Measured Section I

Measured Section II

Measured Section III

Measured Secti on IV

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Measured Section I (Renova Formation, Dunbar Creek Member)

Location (see Map 1): SE%, sec. 22, T2N, R3W

,, . _ . .. Thickness U nit______Description ______(in fe e t)

F Muddy medium sandstone: calcite cemented, hematitic, 10 immature feldspathic litharenite; reddish-brown, poorly sorted, subangular, friable, high permeability, graded bedding, crude small scale cross-bedding. Microscopic examination: monocrystalline quartz mainly "common" quartz, 18%; moderately altered potas sium feldspar, 5%; sedimentary rock fragments including siltstone, chert and mudstone, 11%; sparry calcite cement, 36%; scattered highly weathered mica cement, 4%; hematitic cement, 19%; pore space, 8%.

E Medium-silty mudstone: reddish-brown, moderately indur- 3 ated, high permeability, massive, laterally discontinuous- interfingers with sandstone units. Microscopic examination: silt-size particles include quartz, feldspar, magnetite and mica (muscovite and biotite).

D Muddy fine sandstone: calcite cemented immature felds- 5 pathic litharenite; reddish-brown, poorly sorted, subangular, friable, high permeability, crude cross stratification, graded bedding, laterally discontinuous- in te rfin g e rs w ith mudstone. Microscopic examination: monocrystalline "common" quartz, 11%; moderately altered potassium feldspar, 3%; sparry calcite cement, 41%; mica, 1%; sedimentary rock fragments, 4%; magnetite, 1%; pore space, 39%.

C Very coarse to fine-silty mudstone: moderately indur- 20 ated, highly permeable, massive, contains scattered gypsum mineralization veins. Microscopic examination: sand and silt-size particles include common quartz, 80%; potassium feld spar, 10%; sedimentary rock fragments (chert, sandstone- siltstone), 10%; gneissic rock fragments, t r . ; limestone clasts, tr .; and magnetite, tr.

B Muddy coarse sandstone: c a lc ite cemented, immature 19 feldspathic litharenite; orangish-brownish-white, weathers to orangish-brown, poorly sorted, subrounded, friable,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thickness Unit Description (in fe e t)

G Pebble-conglomeratic muddy medium sandstone: calcite 4 cemented, immature lith ic arkose; pale reddish-white, weathers to orangish-brownish-red, poorly sorted, subrounded, friable, high permeability, mostly massive with some small-scale cross stratification. Microscopic examination: monocrystalline "common" quartz, 26%; polycrystalline quartz, 7%; moderately altered k-feldspar, 20%, sedimentary rock fragments (c a lc ite cemented sandstone, s ilts to n e , mudstone, chert) 13%; magnetite, 1%; biotite, t r . ; volcanic rock fragments, t r . ; sparry calcite cement, 17%; pore space, 16%.

VERTICAL ROCK SCALE UNIT

60 f t

Figure A-3. Strati graphic column representing Section II.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. cr» « , , sec. 22, T2N, R3W. 4 wSB&EE&k Left view: units K fuand J. ll section Note: looking cross-bedding northwest. in sandstone Right unit view: K. close-up of contact between Figure A-4. Section I I exposure (Dunbar Creek Member, Renova Formation) in SEI

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 77

Measured Section I I I (Jefferson and Sixm ile Creek Formations)

Location (see Map 1): SW^a, sec. 22, T2N, R3W

Thickness Unit Description (in fe e t)

Jefferson Fm. Limestone-cobble breccia: light gray matrix 60 (Devoni an spotted w ith w hite s ilt - s iz e specs (k a o lin ite ? ) intraforma- and sand to cobble-size dark gray limestone tio n a l clasts, matrix weathers to yellowish-light gray, breccia) very poorly sorted, angular, highly indurated, low permeability. Microscopic examination: m atrix (medium- s ilty mudstone) consists of silt-size fragments of calcite, limestone, quartz, feldspar, magne tite , hematite, chert and muscovite. Scattered sand-size fragments are dominately monocrystalline quartz. Larger clasts (sand to cobble- size) are limestone of sparry calcite.

111-A— 1 Medium-silty mudstone: 25 to grading to

1 1 1 — A — 5 Muddy cobble conglomerate: lig h t gray sprinkled with dark gray clasts, weathers to yellowish- light gray, very poorly sorted, subrounded, moderately indurated, high permeability, upward-fining graded bedding (III-A -1 down to III-A-5 sampled every 5 feet). Microscopic examination: clay matrix consists of s ilt and fine sand-size quartz, feldspar, mica, calcite, magnetite and limestone. Coarser material (sand to cobble- size) are predominately sparry calcite lime stone fragments.

VERTICAL ROCK SCALE .UNIT

^P^-fN III-A -l- 80 f t . III-A-5

Figure A-5. S tra tig ra p h ic column representing Section I I I .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CO i* £*£& «*■j t # 2 S f S * . i ?>> ■* /*•« i t i Wet’- ■* i?>> AL." r * 'H * Left view: form ational full breccia section o f the looking Devonian Jefferson north. Formation. Right view: close-up of the intra-

» . • •■ >’ -7 4/.** -7 >’ • . •■ » 4 4 1 *'/ ifKW

Figure A-6. Section III exposure (Sixmile--below in section--and Jefferson Formations).

f) C/) C/) < o o

■O ight owner. Further reproduction prohibited without permission. 79

Z' - v d L ^ ^ -S ^ ’-'

Figure A-7. Close-up view of unit III-A -I (muddy cobble conglomerate) from Section III (see Figure A-6).

Measured Section IV (Sixm ile Creek Formation)

Location (see Map 1): SE%, sec. 21, T2N, R3W

Thickness Unit Description (in fe e t)

IV-C Muddy limestone-cobble conglomerate: light yellowish- 30 gray with dark gray clasts, weathers to orangish- light gray, very poorly sorted, subangular, moderately indurated, high permeability. Microscopic examination: matrix (very fine-sandy mudstone) consists of s ilt and sand-size fragments of calcite, quartz, feldspar, chert, hematite and magnetite. The cobble-size fragments are limestone of sparry calcite.

IV-3 Muddy limestone-cobble conglomerate: rust weathering 15 to yellow-orange with dark gray limestone clasts, very poorly sorted, subangular, moderately indurated, high permeability.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thickness U nit Description (in fe e t)

IV-B Microscopic examination: matrix (medium si 11- (cont.) stone) consists of silt-size particles of calcite, limestone, quartz, feldspar, magnetite, and hematite. There are scattered siltstone and sandstone rock fragments. Cobble-size fragments are limestone of sparry calcite.

IV-A Muddy limestone-boulder conglomerate: grayish-white 40 with scattered dary gray limestone clasts, weathers to yellowish-gray, very poorly sorted, angular, moder ately indurated, high permeability. Limestone clasts up to 5 ft across. Microscopic examination: matrix (s ilty mudstone) consists of silt-size particles of calcite, limestone, quartz, feldspar, magnetite, hematite and chert. There are scattered siltstone and sandstone rock fragments. Cobble-size fragments are limestone of sparry calcite. The matrix shows good evidence of soft sediment deforma tion due to gravitational slumping which is indicative of debris flows.

VERTICAL ROCK 0 SCALE

IV-C

.'I TV-A

80 ft-?

Figure A-8. Stratigraphic column representing Section IV.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 00 Right view: foreground.) close-up of Unit A from Section IV (Note: limestone boulder in Figure A-9. Section IV exposure (Sixmile Creek Formation). Left view: fu ll section looking north.

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

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