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GEOLOGY OF THE LOWER CRETACEOUS
CUTBANK CONGLOMERATE

IN NORTHWEST MONTANA

By
Alton V. Gallagher

A THESIS

Submitted to the School of Graduate Studies of Michigan
State University of Agriculture and Applied Science
in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Geology

1957

ACKNOWLEDGEMENTS

The writer wishes to gratefully acknowledge the assis-
tance of Al Hruby, geologist of The California Company, for
suggesting the problem and The California Company, for
furnishing invaluable assistance in the form of base maps,

electric logs and inumerable ways.

An appreciation should be expressed to J. Harms, stu-
dent at the Colorado School of Mines, and Dr. E. L. Tullis,
of The South Dakota School of Mines, for many helpful

suggestions while the field work was in progress.

Many thanks are due Dr. B. T. Sandefur, of Michigan
State University for his guidance and constructive criticism
during the preparation of this paper. Dr. J. W. Trow, of
Michigan State University, for his suggestions in connec-
tion with the oriented sections and lastly to the staff of

Michigan State University for their many helpful suggestions.

ii

GEOLOGY OF THE LOWER CRETACEOUS
CUTBANK CONGLOMERATE

1N NORTHWEST MONTANA
Alton V. Gallagher
ABSTRACT

The continental Cutbank conglomerate is the base of the
lower member of the Cretaceous Kootenai formation in north-
west Montana. It overlies unconformably strata of the
marine Jurassic Ellis Group. it is overlain by the Cut-
bank Sand, a salt and pepper sandstone, which also forms
the matrix and interbeds in the conglomerate. It is
highly lenticular and frequently cross-bedded. The con-
glomerate consists of black and tan chert and white and
black to purplish-black quartzite pebbles with minor amounts

of shale and fossil fragments.

An isopach map of the Cutbank phase and a structure
contour map on the base of the Cutbank phase were con-

structed using information from twenty-two wells in the

area.

Samples were collected for laboratory study in the

Disturbed Belt and in the Sawtooth Range west to the

Lewis Overthrust Fault.

iii

 

The conclusions which have been reached as a result of

this investigation are summarized below:

(1) The isopach map in conjunction with lithologic
evidence strongly suggest that the Cutbank phase in the

Disturbed Belt is a deltaic-type deposit.

(2) Fossil and shale fragments in the conglomerate
indicate that at least part of the sediments forming the

conglomerate are later than Precambrian.

(3) The mineral suite found in the conglomerate is as

follows: pyrite, limonite, hematite, siderite, leucoxene,-

 

tourmaline, barite, zircon, cassiterite, magnetite, calcite,

 

 

kaolinite, chert and quartz. Underlined minerals are detrital.

(h) Oriented samples are highly suggestive of a south-
westerly source area for the gravels forming the conglomerate
and this suggests that the material may have come from the

area of the Coeur D'Alene Mountains.

iv

 

CONTENTS

INTRODUCTION . . . . . . .
STRATIGRAPHY .
TECTONIC HISTORY

SUBSURFACE CUTBANK OF THE DISTURBED BELT
AND SHELF AREA

FIELD STUDY OF THE CUTBANK CONGLOMBRATE
Stratigraphy
Lithology‘

LABORATORY STUDY OF THE CUTBANK CONGLOMERATE
Heavy Mineral Analysis
Thin Section Analysis
Orientation Analysis

SUMMARY AND CONCLUSIONS
BIBLIOGRAPHY

Page

Figure

11.
12.

13.

FIGURES

‘Montana Location Map

Tectonic Map of Thesis Area in North Central
Montana. . . . . . . . . .

Stratigraphy of Northwest Montana.

Structure Contour Map of the Base of the
Cutbank Zone . . . . . .

lsopach Map of the Cutbank Zone

Map of Field Study Area in North Central
Montana. . . . . . . .

Fresh Sample of the Cutbank Conglomerate
Weathered Sample of the Cutbank Conglomerate
Histogram of Sample Number 10..

Histogram of Sample Number 9.

Histogram of Sample Number 20

Histogram of Sample Number 22 .

Possible Source Areas of the Cutbank
Conglomerate

vi

Page

12

13

18
23

3O
3O
31

31

37

INTRODUCTION

The Cutbank phase is the base of the lower member of
the Cretaceous Kootenai formation in northwest Montana. The
Cutbank phase is a continental deposit and unconformably
overlies the marine Jurassic Ellis Group. The basal part
of the Cutbank phase is a chert-quartz conglomerate and will
be the primary concern of this investigation. This research
was directed toward learning the origin of this conglomerate.

The area covered in this study is shown in Figure 1.

The Cutbank Sand from the Cutikufliand Kevin areas was
first recognized and described by Bartram (1935). Feray and

Sloss (19u8), made an examination of thin sections of core

32 i CUT BANK

’GREATFALLS

I“ 0 N T' A N A
‘HELENA

 

samples in the Cut ._1

 

Bank Oil Field.

 

   
 

Cobban (1955), has
also done extensive
work on the Mesozoic

beds in this same

 

area .

 

 

 

This study has Figure l.-Montana Location Map

been divided into a surface and a subsurface investigation.
The surface phase was restricted to the northern part of the

Sawtooth Range. This area is bounded on the north and west

by the Lewis Overthrust. The eastern boundary of the sur-
face phase is formed by what the writer prefers to call the
eastern boundary of the Transition Zone separating the Dis-
turbed Belt from the Front Ranges (Fig. 2). The subsurface
part is bounded on the north by the International Boundary
between Canada and the United States. The eastern boundary
falls approximately along a line drawn from the Canadian
border south along ROW near the town of Cutiknflt. The

southern limit of the area is T27N.

The field work for this paper which consisted of study-

ing outcrops of the conglomerate and collecting of samples
for laboratory work was accomplished during July, August
and early September of 1956. The subsurface investigation
consisted of an examination of electric, radioactivity,

core and sample logs.

 

 

 

 

 

 

     
 

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TECTONIC MAP
OF THESIS AREA IN

NORTH CENTRAL
MONTANA

 

 

fi—SRST ._6.

\ GLACIER £04

REAGAN
OIL FIELD

 
   

CUTBANK
OIL FIELD

 

 

 

  

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35

 

 

 

 

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CUTBANK-
33
32
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FIGURE 2

 

STRATIGRAPHY

The general stratigraphic column of the study area is

shown in Figure 3.

The Cretaceous Kootenai formation is underlain uncon-
formably by the Jurassic Swift formation and to some extent-

in the eastern part of the study area by the Rierdon forma-

tion.

The Rierdon formation is a dark gray, calcareous,
micaceous shale with thin layers of gray, dense, blocky
limestone. The Swift formation consists of an upper thin-
bedded, ripple-marked sandstone unit interspersed with thin
micaceous shale films. The lower unit of the Swift forma-
tion is predominantly shale with a thin glauconitic sand-

stone at its base.

The overlying Kootenai formation is divided into two
members. The lowest member has three phases which are from
oldest to youngest, the Cutbank, Sunburst and Moulton. The
Sunburst and Moulton phases are located by subsurface drill-
ing in the eastern part of the study area. These beds do
not crop out in the Disturbed Belt. The Cutbank phase crops

Out in the western part of the Disturbed Belt as well as

Ilumerous places in the Sawtooth Range.

 

S

STRATIGRAPHY IN NORTHWESTERN MONTANA

 

 

ERA

PERIOD

GROUP

FORMATION

MEMBER

PHASE

 

 

Cret.

Willow Creek
St. Mary River

 

Montana

H6rsethief
Bearpaw

Two Medicine
Virgelle
Telegraph Cr.

 

Colorado

Upper
Blackleaf

 

Kootenai

Upper

 

Lower

Moulton
Sunburst
Cutbank

 

Jur.

Morrison

 

Ellis

Swift
Rierdon
Sawtooth

 

Miss.

Madison

Charles
Mission Canyon
Lodgepole

 

Dev.

Three Forks
Potlatch

 

Duperow

Upper
Lower

 

Souris River

 

Camb.

Devil's Glen
Switchback sh.
Steamboat ls.
Pagoda ls.
Dearborn ls.
Damnation ls.
Gordon sh.
Flathead ss.

 

 

Missoula

Kintla

 

Siyeh

Sheppard dol.
Purcell lava
Upper Siyeh
Spokane sh.
Lower Siyeh

 

Ravalli

Grinnel sh.
Appekunny
Altyn ls.

 

 

 

 

Figure 3

 

 

 

In the western part of the area the base of the Cutbank
phase consists of a chert-quartz conglomerate averaging ten
feet in thickness. A salt and pepper sandstone, which is
absent in the extreme western part of the area thickens
eastward to 130 feet and thins to forty feet in the vicinity
of the town ofChrtBank, overlies the basal conglomerate.

The conglomerate zone thins rapidly to theeasi.and through-
out most of the area it consists only of a few inches of
basal conglomerate. The Cutbank phase is highly lenticular

and cross-bedding is visible in most outcrops.

The Sunburst phase is from ten to ninety feet thick and
consists of variegated mudstone, siltstone and white lentic-
ular sandstones. Near the top of the Sunburst phase is a
limestone bed up to eleven feet thick which contains fresh
water fossils. This limestone weathers bright orange and
tan. The Moulton phase consists of two units; an upper
massive cross-bedded sandstone and variegated mudstone unit,
and a lower lacustrine deposit consisting of dark gray shale
with thin beds of limestone, siltstone and very fine-grained

sandstone (Cobban, 1955).

The upper member of the Kootenai formation is composed
of variegated mudstone, siltstone and greenish-gray sand-
stone. The sandstones are fine to coarse-grained, massive,
cross-bedded and highly lenticular. Cobban states that,
"In contrast to the sandstones of the lower member, those

of the upper member contain more micas and other grains

 

 

derived from freshly weathered igneous rocks." (Cobban, 1955).

The Kootenai formation is overlain by the Colorado for-
mation which consists of a lower member composed of dark
marine shale and considerable sandstone, siltstone, mudstone
and bentonite and an upper member which is primarily a dark-

gray marine shale.

The Colorado is in turn overlain by the Montana Group

which from oldest to youngest is composed of the marine
Telegraph Creek and Virgelle formations, the continental
Two Medicine, marine Bearpaw and the brackish to marine

Horsethief formations.

The St. Mary River and the Willow Creek formations

overlie the Montana Group and are both non-marine in origin.

TECTONIC HISTORY

The area which is discussed in this paper forms the
boundary between two major tectonic provinces, the Central
Stable Platform on the east, and the Cordilleran Geosyncline
on the west. The Stable Shelf Area is represented by a
south-trending arch, which lies just to the east of the
area covered by the Tectonic Map (Fig. 2). This arch extends
from Great Falls, Montana north into Alberta and is composed
of two major elements; the South Arch to the south of the
area and the Kevin Sunburst Dome on the north which lies
directly to the east of the Cutbank Field. The Cordilleran
Geosyncline in north-central Montana and its general regional
geologic history is best summarized by the following from
Alpha.

The Cordilleran Geosyncline of northern United
States extended westward to the Pacific margin
until the Nevadan orogeny in late Jurassic time.
The Nevadan orogeny created a north-south uplift
from southeastern California to northern Idaho
and northward into British Columbia, separating
the geosyncline into east and west troughs. In
late Mesozoic‘and Cenozoic time in what is now
western Montana the east trough became the site
of an orogenic belt commonly known as the Cordill-
eran Genanticline. This geanticline is composed
of large thrust sheets of sedimentary strata pushed
eastward. The Front Ranges of the Rockies lie along
the east margin of the Cordilleran Geosyncline.

The Disturbed Belt is essentially a tectonic transition

zone between the two major provinces and is composed mainly

 

of highly squeezed Mesozoic strata. On the west directly
adjacent to the Sawtooth Range the rocks forming the Dis-
turbed Belt appear as overturned drag folds, small anti-
clinal and synclinal structures and numerous small thrusts,
which have undergone extreme compression from the southwest.
The Cutbank conglomerate zone is one of the most competent
beds in the Cretaceous.phase. As such it is usually found
in the Disturbed Belt as a prominent ridge. Eastward toward
the margin of the Disturbed Belt, the folding and faulting
become less intense and reverse faulting becomes dominant.

On the Tectonic Map the intensely folded part of the

Disturbed Belt has been designated the "Transition Zone."

 

SUBSURFACE CUTBANK OF THE DISTURBED BELT

AND SHELF AREA

The object of studying the subsurface Cutbank phase was
to determine by a critical examination of logs, core and
sample descriptions whether the type of depositional environ—
ment could be determined and the direction from which the
gravels forming the conglomerate of the Cutbank phase came.

A structure contour map on the base and an isopach map of
the Cutbank phase were constructed. The wells that were
used in this study along with their map identification
number and the type of material obtained from them are

listed below.

Electric Log = E Radioactivity Log 2 R Sample Log = S

 

No. Name Location Log Samples Core
1. Carter—Phillips Sec. 28
(Lucy Weaselhead No. l) TSMN R12W X
2. Carter Oil Co. Sec. 12
(Indian Tribal No. l) T36N R7W E
3. Union 011 CO. Sec.
(Tribal 53A No. 1) T36N R8W E
A. Union Oil Co. Sec. 31
(Union Tribal BEE—l) T36N R5W E
5. Wagner & O'Hanlon Sec. 30
(Marceau No. l T35N R6W E
6. Union-Carter' Sec. 7
(Miller-Tribal). T33N R6W ER X X

10

 

11

 

See text for discussion.

N9_ Name Location L99 Sample Core
7. Skelly Oil Co. Sec. 5
(H. U..Amid0n NO. 1) T33N RUW E
8. Stanolind Oil Co. Sec. 1h
(Rutherford No. 1) T3lN ROW E x x
9. Union 011 C0. sec. 18
(Morning Gun No. l) T31N RllW R X X
*10. Union Oil Co. Sec. 12
(Mittens No. I) T29N RllW R
11. Grant Brown Jr. Sec. h
(Brown-Deruwe No. l) T30N R8W E
12. Texas Co. Sec. 8
(Kingsbury No. 1) T29N R7W s x
13. Texas Co. Sec. 11
(Tribal 28h No. 1) TSON R7W E x
1h. Chicago Corp. 8 Republic
Natural Gas Sec. 10
(Nelson No. 1) TSON Raw E
15. Farmers Union Sec. 25
(Mancornerl No. 1) T28N R5W E
10. Northern Ordnance Sec. 13
(Jannusch No. 1) T28N R7W E
17. Northern Ordnance Sec. 27
(Pitchairn No. 1) T28N R8W E
18. Mule Creek Oil Co. Sec. 23
(Rockport No. 1) T27N R7W E
19. George R. Sharman Sec. 7
(Ryan Estate No. 1) T29N ROW E
20. Union Oil Co. Sec. 13
(Dahlquist No. 12) T3uN R5W E
21. D. E. L. Byers Sec. 17
(Tribal No. l) T3uN R7W E
22. Carter Oil Co. Sec. 2
(Carter No. l Coulee T3uN R7W E
Tribal)

12

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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STRUCTURE CONTOUR MAP L E G E N D
or THE BASE or THE * % OIL HELD
CUTBANK ZONE -
SCALE GAS FIELD
0 314115.55 ...---’ PENDROY FAULT

NUMBERS ARE WELL LOCATIONS
BY-.A. GALLAGHER - MAY 1957

\

 

FIGURE 4

 

13

 

 

 

ISOPACH MAP L E G E N D
OF'THE g OIL FIELD
CUTBANK ZONE
SCALE GAS FIELD
0 3mg” '2 ,.~-’ PENDROY FAULT

NUMBERS ARE WELL LOCATIONS
BY- A. GALLAGHER - MAY |957

 

FIGURE 5

 

1h

In working with the well logs it was necessary to work
with the entire Cutbank phase rather than the conglomerate

zone itself.

Up to the time of this investigation published isopachs
and Structure contour maps of the Cretaceous have terminated
at the eastern limit of the Disturbed Belt. The primary
reason for this limitation was lack of control. Union Oil
Company's, Morning Gun (well no. 9) and Mittens Well (well
no. 10) were completed in late 1955. It can readily be
seen from the isopach map (Fig. 5) that these two wells,
particularly well number 9, occupy important positions for
control purposes. Since these two wells are within the
Disturbed Belt a word of explanation is in order to justify

their use as valid control points.

The complex faulting characteristic of the Disturbed
Belt extends from the surface to a depth of approximately
6510 feet. Upper Colorado shale in the faulted zone was
repeated several times. There is a possibility that some
lower Montana Group rocks were also involved. Between the
depth of 6510 feet and 9550 feet where the well "bottoms"
in the Mississippian Misson Canyon formation, the column
appears as an undisturbed sequence. Since the Cutbank
Phase occurs at a depth of 8335 feet it was felt that the
inlf‘tormation pertaining to the Morning Gun well could be
utilized with a good degree of certainty. .A further check

onthe Morning Gun was made by comparing the computed

15

apparent dip measured at the base of the Cutbank phase

between wells 9, 11 and 16. These wells are roughly in line

with each other as can be readily seen from Figure 5. The
apparent dip between wells 11 and 16 is 102III and between
wells 9 and 11 is 1025'. The close correspondence between

the dips supports the premise that the Cutbank phase is

undisturbed in well 9.

Well 10 (Mittens) lies even farther to the west than
well 9 and is approximately midway between the Front Range
and the east edge of the "Transition Zone". It is roughly
one half mile east of the base of Little Plume Peak (Fig. 6).
This well "bottomed" in Kootenai formation at 7500 feet.
There is no way of knowing whether or not the well ended in

a thrust plate or in the underlying normal undisturbed

Sequence. In this well the Kootenai formation is repeated

five times and the Cutbank phase is repeated twice. The
Cutbank sediments in the two thrusts are in both instances
overlain by Kootenai formation and underlain by Ellis Group
sediments. As this is the usual relationship found in the
field and in surrounding wells it is considered that the
thiCknesses represented are true values. The Cutbank phase
13 6h.feet thick in the upper thrust plate and 60 feet thick
In 1the lower.. As there is no way of knowing the original
geographic location of the thicknesses represented in the
thru‘Sts they were not used in the construction of the
ISOpach or structure contour maps. However, even though

t
Tug (jepths in this well and the thicknesses in the thrust

16

plates have no significance by themselves, they can help to

support the overall picture.

The structure contour map fairly well outlines the
southward extension of the Alberta Trough, which has been

thoroughly discussed in the existing literature (Alpha, 1955).

The isopach map of the Cutbank phase shows the rapid
decrease in thickness to the east near the town of Cut Bank.
There is a more rapid decrease in thickness toward the north
and south. This decrease in thickness does not appear to
have been structurally controlled except possibly in a minor
way in the northeast part of the area. An examination of
the map indicates that the sediments forming the Cutbank

phase came from approximately SO5O- 850W.

FIELD STUDY OF THE CUTBANK CONGLOMERATE

The Cutbank conglomerate crops out in scattered locali-
ties just west of the east margin of the Transition Zone.
In the Badger Creek area (Fig. 6) the conglomerate forms
several small ridges across the Transition Zone to the base
of the Sawtooth Range where it is overidden by Mississippian
and Devonian sediments. In the vicinity of South Fork Creek
samples 20 and 21 were collected from the fartherest east
outcrop of the conglomerate. Samples 12 and 1h taken from
the south side of Little Plume Peak are from the most
westerly outcrops in the Badger Creek area. Samples from
the Lee Creek exposure represent the next major line of
outcrop. The intervening area consists mainly of Paleozoic
and small amounts of Jurassic strata. OutcrOps one mile
south of Elkcalf Mountain on the Continental Divide and in
the vicinity of Kip Creek are the last good exposures of the
conglomerate before the Sawtooth Range is terminated by the

Lewis Overthrust Fault.

STRATIGRAPHY

The conglomerate unconformably overlies marine sedi-
ments of the Jurassic Ellis Group. The writer believes it
to lie on the Upper Swift formation described by‘Cobban (19h5).

Within the Upper Swift of the Little Plume and Badger Creek

17

18

 

 

 

  
 
 
  
   
   
 

MAP OF FIELD STUDY
AREA IN NORTH CENTRAL
MONTANA

SHOWING LOCATION OF
POINTS DISCUSSED.

ALTON v. GALLAGHER — MAY I,I957

 

 

 

 

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B ORIENTED SAMPLE

 

 

 

 

 

 

FIGURE 6

 

19

areas numerous outcrOps of a brown—stained sandstone ledge
containing worm(?) burrows or trails were seen. This sup-
ports the premise that the underlying sandstone ledge is

the Swift formation. In most small exposures of the conglom-
erate and the underlying Jurassic form1tions, the conglomer—
ate appears to overlie conformably the Jurassic, however
slight angular discordance was noted at several large out-

crops.

In all localities in which the conglomerate was observed
it is overlain by the gray salt and pepper sandstone char-
acteristic of the Cutbank phase. At the Lee Creek, Little
Plume and Kip Creek outcrops the Cutbank phase is overlain
by a very persistent limestone bed which is from three to
fifteen feet thick. In the Lee Creek and Kip Creek areas
the limestone bed is separated from the conglomerate by
ten to fifteen feet of sandstone. In the Morning Gun well
this same limestone is separated from the Cutbank phase by
seventy-five feet of the Sunburst sandstone. In well 6
the limestone bed is eighty-five feet above the top of
the Cutbank phase and it is seventy feet above in well 13.
This is possibly the same limestone which Cobban describes
as occurring east of Great Falls. As this limestone is an
excellent horizon marker in the Sawtooth Range it is felt
that it is worthy of description. On fresh exposure the

lower part of the limestone bed appears dense, dark black,

finely crystalline and fossiliferous. This comprises

 

20

approximately two-thirds of the bed in most outcrops. A
fresh exposure of the upper part of the bed appears light
gray, fairly dense and non-fossiliferous. On the weathered
surface the lower part is dark gray and vuggy with calcite
stringers and fossils weathering out in relief, the upper
part is orange to buff. The line separating this lower
black limestone from the light gray top of the bed is very

sharp.

At the time the samples from all areas were taken,
the lithology and any other data that might have some
value was recorded. It should be kept in mind while
observing this map (Fig. 6) that sample locations are geo-
graphic only. For example, samples 6 and 7 are farthest
east geographically, however they are in a thrust which
overlies the thrusts from which samples 8 through 10 were
taken, thus their true geographic relationship is west

of samples 8 through 10.

Difficulty was experienced in obtaining good samples.
IVevertheless oriented samples were taken from exposures where
'there was absolutely no question as to the correct orienta-
'tion of the main body of the conglomerate lens and to avoid

t2iking any samples from foreset beds.

L I THOLOGY
The Cutbank conglomerate consists of chert and quartz

pebbles in a matrix of light gray salt and pepper sandstone.

Black, tan, white and very minor amounts of blue and green
chert pebbles comprise from 70 to 90 percent of the clastics.
The remainder of the fragments, with two exceptions, consist
of pure white and black to purplish—black quartzite pebbles.
The fragments vary in size from less than an eight inch to
more than seven inches in diameter. All of the cobbles over
five inches in diameter were composed of white quartzite.
The average size of the pebbles decreases from bottom to

top of the conglomerate zone. The conglomerate is so well
cemented that it fractures across the grains rather than
around them. The salt and pepper sandstone that forms they
matrix is generally light gray although near the top of the
section it is tan at many places. The sandstone consists

of colorless to tan quartz and black and tan chert grains.

The purplish-black to black quartzite pebbles are found
only in samples which were taken from near the Continental
Divide, namely the Lee and Kip Creek areas. These quartzite
pebbles occur only in the top two feet of the conglomerate

zone .

Kaolinite in small varying amounts occurs in all samples.
as part of the cementing material. It is found in consider-

ably larger amounts where the conglomerate is more quartzitic.

The conglomerate is slightly fossiliferous from two to
three feet above the base in the Lee Creek section; identifi-

Cation of the fragments was impossible. This is the only

aJWBa in which fossils were observed.

 

22

On the Continental Divide, about one mile south of Elk
Calf Mountain, several shale fragments were observed in a
series of small thrusts of the conglomerate. The largest of
these measured five by three by one—half inches. These
shale fragments occurred in the middle of a lens in the

lower half of the conglomerate zone.

The conglomerate is exposed in lenses varying from
four inches to over six feet in thickness, and from ten feet

or more in length.

On the weathered surface the conglomerate character-
istically is a dark reddish-brown color although occasionally
some of the sandstone interbeds appear in their normal light
gray or tan color. A fresh and weathered surface views of

the conglomerate are shown in Figures 7 and 8.

Pyrite which is a fairly abundant heavy mineral in the

basal foot of the conglomerate, is not visible macrosc0pi-

cally even with a good hand lens.

 

 

 

— ___ _~-“ —. A

Figure 7.- Fresh sample of the Cutbank conglomerate.

 

I
I

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Figure 8.- Weathered sample of the Cutbank conglomerate.

LABORATORY STUDY OF THE CUTBANK CONGLOMERATE

A laboratory study of the Cutbank conglomerate was made
using the samples that were collected in the field. A
heavy mineral analysis was run on the samples and two thin
sections were made and studied. The oriented samples were

studied to determine if there was any preferred orientation.

HEAVY MINERAL ANALYSIS

All attempts to disaggregate the samples in order that
a. size, shape and heavy mineral analysis could be run were
futile . This was found to be primarily due to secondary
enlargement of the quartz crystals. The size and shape
analysis were abandoned and attention was turned to crush—
ing the rock samples in a diamond mortar so that heavy
mineral separations could be carried out. The crushed
fragments were sieved repeatedly until they would pass

through a thirty—five mesh screen. The standard heavy

mineral separate procedure presented by Krumbein and Petti-

john (1938), was then used. Two heavy mineral separates

were made from each sample in order to provide a check

aQainst possible error. The heavy minerals from the two

separates after being weighed were then combined into one

sample. Part of the heavy minerals were mounted on slides

and part were used for the determination of refractive

indices.

2h

 

25

The heavy mineral analysis did not prove to be signifi-
cant; the heavy minerals varied from .032 to .081 percent.
With an average percentage present in all samples of .065.
There seems to be no apparent pattern of the distribution
of the heavy minerals and it was therefore concluded that
heavy mineral determinations were of little value in the

study of the Cutbank conglomerate.

Actual percentage counts of the heavy minerals found
in each slide were not made for three reasons: (1) the
amount of heavy minerals was insufficient and too erratic,
even when samples were located in close proximity to one
another; (2) fracturing as a result of crushing the con-
glomerate had occurred to some extent; (3) samples from the
sections on Lee and Badger Creeks were examined closely and
the erratic distribution was not found to vary laterally

to any appreciable extent.

.A composite mineral suite covering the average ten foot
thickness of the conglomerate was made. The most common
heavy minerals were, pyrite, limonite, hematite, siderite,
leucoxene , tourmaline, barite, zircon, cassiterite and
magnetite. Minerals identified in minor amounts were,
andradite, green hornblende and a green iron silicate

mineral possibly greenalite.

Pyrite is present almost to the exclusion of all other

minerals in the first foot of the conglomerate, but decreases

considerably in the second foot and is very rare three feet

 

I 26

above the base of the conglomerate. It does not reappear

 

until the top foot of the conglomerate section where it is
found in small amounts. The pyrite always appears to show

varying degrees of alteration to limonite.

Limonite is abundant in the lower foot of the conglome—
rate and is dominant throughout the section. It always

surrounds the pyrite and hematite grains.

Hematite is present in small amounts throughout the
section and is unusually abundant from two to four feet

and from seven to nine feet above the base of the section.

Magnetite and siderite are both found in small amounts

throughout the section.

Leucoxene is common in all except the middle part of

the ten—foot section of the conglomerate where it is scarce.

Tourmaline is found throughout the section in varying

amounts. The variety indicolite was noted from three to five
feet and from seven to ten feet above the base of the section.
Tourmaline increases in abundance in the upper foot of the

typical ten foot conglomerate zone.

Barite was observed in varying small amounts throughout
the section, but increases noticeably in the upper foot of

the ten foot section.

Zircon is found in scattered grains throughout the

5Section. Several euhedral crystals which showed little if

 

   

 

‘Iv- u-v - -

 

 

   

 

 

 

 

27

any rounding were observed notably in the five to Six foot

zone of the typical conglomerate section..

Cassiterite was noted only in the upper three feet of

the section where it is scarce to common.

Andradite and greenalite were noted as one or two
grains per slide throughout the conglomerate section. The
andradite showed a notable increase in the top foot of the
section. Hornblende was observed in only two instances in

the upper foot of the section.

The light minerals found in the conglomerate are:

quartz, chert, kaolinite and calcite.

THIN SECTION ANALYSIS

The thin section analysis was used to aid in determin-
ing which minerals in the conglomerate are detrital and which
are authigenic. .A very thorough study of thin sections from

the Cutbank Sand was made by Sloss and Feray in l9u8.

A study of the thin sections revealed that pyrite,
limonite, siderite, hematite, calcite and kaolinite occur
as cementing or void-filling materials. Most of the quartz
grains exhibit secondary growth by the addition of silica
in optical orientation with the original quartz grain. Many
0f the chert grains are cut by fractures which are filled

with secondary Silica. The hematite and pyrite show'altera-

tion of varying degrees to limonite.

28

On the basis of the thin section analysis, heavy mineral
separates and also information obtained from Milner's, "Sed-
imentary Petrography," the heavy mineral suite was divided

into detrital and authigenic constituents.

 

Detrital Authigenic
Tourmaline Pyrite
Zircon Limonite
Cassiterite Hematite
Andradite Siderite
Leucoxene Barite

ORIENTATION.ANALYSIS

As was mentioned previously all attempts to disaggre-
gate the samples proved futile. Thus a method was devised
for obtaining the azimuths of elongation of the pebbles in

the oriented samples of the conglomerate.

Each sample was first cut into 1/8 to 3/16 inch slabs.
Out of seven original oriented samples only four were com—
petent enough to withstand this operation. The maximum
permissible thickness of the Slabs is dependent upon the
average size of the pebbles in the conglomerate. If the
average size of the pebbles is around 1/2 inch in diameter,
a 3/16 inch Slab will give satisfactory results. If the
pebbles average from l/h to 3/8 inch in diameter then an
attempt should be made to cut the slabs to 1/8 inch or
thinner. Care had to be taken in cutting the slabs to
prevent too much pressure from being applied by the saw to

the rock causing the saw to veer or the conglomerate to

 

29

disintegrate. AS each slab was being cut the width of the
saw cut was measured. After each cut the slab was oreinted
and so marked. The pebbles were then traced very carefully
with India ink on clear tracing vellum. Each tracing was
oriented and a north-south line was also placed on each
tracing. The use of the tracing vellum permitted the
superposition of the slabs, represented by the tracings,
which gives a three-dimensional effect which is necessary

to determine the true axis of elongation in each pebble.

The azimuths of the axes were then plotted on polar coordinate
paper to which had been added several closely spaced north—
south lines which greatly aidedin orienting the tracings on
the coordinate paper. Each pebble was numbered, the number
being placed on the tracing and also on the polar coordinate
Paper next to the azimuth of the pebble. Thus if there is

any variation from the bottom to the top of the sample it can

readily be determined. The amount of material removed by

the saw (usually 1/16 of an inch) must be given due con-

sideration in determining the correct position of the

long axis in each pebble.

Since this is, to my knowledge, a new method, the

azimuths of the samples were carefully determined three

- 8.
times. The average error was found to be + or two degree

The histograms (Figs. 9 — 12) were constructed from these

measurements.

According to Twenhofel (l9h8), ellipsoidal particles

 

=r

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12‘ ~>§Y;<’
-I \Y K ‘A‘ /\l
m ,fi/ x \z {x >\
CIO'I \/\X\(‘</ K/ >/\/ ( \/ /\/
K/\ I/ \/\> Y
«4 . f / X \I Y ‘3. ‘(\r )g
m ,<>>»fl<>>)§VV> <
a . >ANINA\ QKVx
0.3- {‘9/M K<>A%<
§<§XKA4\,'\ >A/AIX/\ < \A/
(8 ‘~ 3' >1 UV». \/\/1<><\/\/\>3/><\
6 )1 > \ V< V\ V 5/ 3x
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Z 4 . '\ ’</\)
I’ /\/I
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2
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I80 '90 200 20 40 360
Azimuths of axes of elongation
Figure 9.—Histogram of sample No. 10. The blackened
area represents observed frequency count in 50 groups.
The lined areas represent 200 frequency groups used in com-
puting the mean and standard deviation.
-I
x‘yxrx'
€‘~ xK
. \>K‘?V)
310. <)\/ X)
"—g (x \ /
(U 4 / Y<‘<(\/
8’87 z,<afv ”<O>>”A
'4 X / Y \/)\ >\\/\ v v
t... K / )K/((\<\/\/ ) Y
05. .‘/ «(v / /\/\/ y\)
. /\ {‘</ /\/(\/>.)‘ N/
O ’V A/ ‘ ,(t/\, / IK/VV/
Z4.<‘)’\(/’(> h/\ /Vz/\\/ YYYV\/y)r) TXX)‘.
‘0 V3 ///\/\ 9 I‘K/VV‘ I < "f./
/\ y} ): \/\/\ 4 }
21 K YY ‘K A
A >\ )‘ a
\ A A
180 '90 200 20 40 so so 300 20 40 360

Azimuths of axes of elongation

Figure 10.« Histogram of sample No. 9. The blackened
area represents observed frequency count in 50 groups.
The lined areas represent 200 frequency groups used in com-
puting the mean and standard deviation.

 

 

 

 

 

 

 

 

--

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2‘ 2.
. a
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31

 

 

of grains

9

X

I

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No.
A

 

 

 

 

 

 

  

 

I80 I90 200 20 40
Azimuths of axes of elongation

Figure ll.—Histogram of sample No. 20.' The blackened
area represents observed frequency count in 50 groups.
The lined areas represent 200 frequency groups used in com-
puting the mean and standard deviation.

 

 

 

 

 

 

   
 

 

 

     
     
 

 

 
  

 

 

 

 

 

 

  

   
   

       
 
 

  

 

   

 

IDIO- /\I‘(
C.‘ \ )/\/\/:
o... q V
II /\X / Y\
L. 8" \/\/\I\’ \ \(I/\’\/
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> \ > \I \I \/ <\,\, /\< <
.. I/XV 5' VY /V> \/ // /\’\/‘
(H I) AI<X/\(VX\I/\\/>/)‘ <\/>’
06. ~’\/\/ ,‘LVVV<)\Y> \r)\/)I\
. . \”>‘\"/\/)>‘ )KA,/I>/\x/\A/\A \Pl‘
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I80 ‘ .0 300 360
Azimuths of axes of elongation

Figure l2.-Histogram Of sample No. 22. The blackened
aJrea represents observed frequency count in 50 groups.
Tdue lined areas represent 20 frequency groups used in com-
Pllting the mean and standard deviation.

32

tend to align themselves perpendicular to the current direc-
tion. Allowance for this was not made in the determination
of the azimuths. This was not done because; (I) ellipsoidal
pebbles were small in number, (2) the amount of matrix is
small, which tends to reduce the alignment of pebbles
perpendicular to the current, and (3) it was felt if this
were true then a second high would appear in the histograms
approximately 900 from the major high, if such highs were

to appear.

After each of the histograms was completed the chi-
square test was used to determine whether or not the fabric
was isotropic. Using a 300 grouping all of the fabrics fell
within the .05 Sigificance level suggesting that the parent

:fabrics are anisotropic (Fairbairn).

The mean and the standard deviation were computed for

each of the samples and are listed in the table below.

 

Sample Mean Standard Deviation
1o zus.7° IID.DO
9 262.70 51.60
20 261.60 nu.8°
22 260.20 h8.6°

Sample ten consisted of pebbles which were not too
closely packed and which averaged approximately 5/8 inch

in diameter. Truly ellipsoidal pebbles were few.

Sample nine consisted ofpebbles many of which were in

juxtaposition and of which the average size range is l/h to

 

 

 

33

3/8 inch in diameter. Several of the pebbles in this sampka
were truly ellipsoidal, and all of these were oriented so
that they fell between 310° and 355° with one exception

which was aligned in the direction of 1950.

Sample 20 consists of pebbles which are in the size
range from 1/2 to 5/8 inch diameter. The pebbles are not
too closely packed, but there are more ellipsoidal fragments
in this sample than in any other. Four pebbles in the

O to 3600 group have a definite ellipsoidal shape and

300
several others in this same group are close to being

ellipsoidal.

Sample 22 consists of fairly closely packed pebbles
which have an average size range of l/u to 3/8 inch in
diameter. This sample is highly quartzitic and contains

few ellipsoidal particles.

It would appear from the histograms that the gravels

that formed the Cutbank conglomerate came from somewhere to

the southwest, probably in the range from SMOOW to S8OOW.

 

SUMMARY AND CONCLUSIONS

The main objective of this investigation was to learn
something of the origin of the Cutbank conglomerate by means

of a thorough study of its geology.

The field work, which consisted of examining numerous
outcrops and samples of the Cutbank conglomerate indicated
that this conglomerate contains materials other than its
usual chert and quartz pebbles in a salt and pepper sand-
stone matrix. Fossil fragments, shale slabs and abundant
black to purplish-black quartzite pebbles which have not

been reported elsewhere in the conglomerate were found.

From various types of well logs a structure contour
map on the base and an isopach map of the Cutbank phase
were constructed. The writer believes that the Cutbank
phase is a deltaic—type deposit laid down in a fresh water
lake. This is inferred from data shown on the isopach map
and also from the decrease in the size of the pebbles from

bottom to top and from west to east.

Heavy mineral analyses were made of the mineral suite
to determine if there was any uniform variation in the
amount of heavy minerals present. The heavy mineral suite
Consists of; pyrite, limonite, hematite, siderite, leucoxene

tourmaline, barite, zircon, cassiterite and magnetite.

3’4

35

Minor amounts of andradite, green hornblende and possibly
greenalite were also observed. There is no appreciable
lateral variability of the heavy minerals. A vertical

variation of the heavy minerals does exist.

A method was devised for determining the orientation of
the jpebbles contained in the conglomerate. The azimuths
proved highly suggestive of a SHOOW to S800w orientation.
Though the number of pebbles per sample was small the close

correspondence between individual samples cannot be overlooked.

The conclusions which have been reached as a result of

this investigation are summarized below:

(I) The isopach map explains the rapid fluctuations
in thickness over short distances, experienced in drilling
through the Cutbank phase. The isopach map in conjunction
with lithologic evidence strongly suggest that the Cutbank

phase is a deltaic-type deposit in the Disturbed Belt area.

(2) The presence of fossil fragments in the conglome-
rate indicates that at least part if not all of the sedi-
ments forming the conglomerate are later than Precambrian.
That part of the conglomerate which contained shale frag-
ments cound indicate that some of the rocks forming the

conglomerate are Mesozoic.

(3) Heavy mineral separates other than aiding in the

determination of the mineral suite were of little value.

L»)
O\

(h) The oriented samples are highly suggestive of a
southwesterly source area for the gravels forming the con-

glomerate.

In Figure 13, the writer has plotted the suggested
orientation on a regional map in order to show the possible
source area. The source area would appear to be the southern
half of the Coeur D'Alene Mountains. If this is true then
it is possible that the Cutbank conglomerate represents the
the first folding preceding the intrusion of the Idaho

batholith (Anderson, l9h8).

It is the writer's opinion that the Cutbank conglomerate
is composed of lower Cambrian sediments interspersed with
a small amount of Jurassic material in the lower half of the
conglomerate zone and with some Precambrian Belt strata
represented in the top two feet of the conglomerate section.
This opinion is based entirely on a close examination of the
literature in view of the material presented in this investi-
gation.

An interesting sidelight that the study revealed con-
cerns the fresh water limestone which overlies the Cutbank
phase. As was pointed out previously, the Cutbank phase
and the limestone are widely separated in the east part of
the Disturbed Belt, whereas in the western part of the Saw-
tooth Range they are almost in contact with one another.

From this it might be inferred that the western edge of

the Sawtooth Range is not too distant from the western edge

 

 

 

 

\\

\

\ SAWTOOTH RANGE

\
‘I

 

\
I b- I
) a V H k I
”I V ._ , BITTEROOT RANGE
\ .1 .. '
\\ a p O h ’:r ‘
I I: I
I ' t‘ "h ’4'
%' V fl” q - ‘ /\‘\
w ‘-"\ g,
0/ k v- m u- ’; ‘
«(la 5’ V ’s
Q! V’ V V b ,, I
Ojfi’ v- u- L' \
,.
’) V l/ O a .1
I .9 - .x- r
7 V V V w -\‘ ~ I
5" V ’~’~
”" o v - K,/' ‘V
w L— I V ..
L— V b |
Q P V
,_ .. 1. Q a .,
e r - g .2:
V A» (9 SCALE 4':
w. V-F'F' D 50 IM) 532
V' “'Lv MHES 3

 

 

 

Figure 13.-Possible source areas of the Cutbank conglomerate.

 

 

 

38

of the shallow trough which existed at the beginning of

deposition of the upper member of the Kootenai formation.

 

 

BIBLIOGRAPHY

Anderson, Alfred L. (l9h8) Role of the Idaho Batholith

during the Laramide Orogeny; Econ. Geol., Vol.Fh3,
No. 2, p. 8h.

 

 

Alpha, Andrew G. (1955) Tectonic History of North Central
Montana; Billings Geol. Soc. Guidebook, Sixth Annual
Field Conference, p. 129.

Bartram, J. G. (1935) Geology of Natural Gas; Bull. Amer.
Assoc. Petrol. Geol., p. 257 .

Cobban, W. A. (l9h5) Marine Jurassic Formations of the
Sweet rass Arch; BuIT} Amer. Assoc. Petrol.’GEOl.,
Vol. 9, No. 9, p. 1262

 

 

(1955) Cretaceous Rocks of Northwestern
Montana; Billings Geol. Soc. GuideBOOk, Sixth Annual
Field Conference, p. 107.

 

 

Deiss, Charles (l9h3) Stratigraphy and Structure of South-

west Saypo Quadrangle, MOntana; Bull. Geol. Soc.
Amer., VOI. 5h, p. 205

 

Dobbin, C. E. and Erdman, C. E. (l9h6) Structure Contour
Map of the Montana Plains; U. S. GeoIZ Survey.

 

 

Fairbairn, H. W. (l95h) Structural Petrology of Deformed
Rocks; Addison-Wesley Publishing Co., Cambridge,
Mass.

 

Krumbein, W. C. (1939) Preferred Orientation of Pebbles in
Sedimentary Deposits; Jour. of Geol., WOT} M7, p. 673

 

 

, and Pettijohn, F. J. (1938) Manual of Sedimen-

tary Petro ra h ; Appleton-Century-Crofts, Inc., New
YoFk, p. 383

 

 

 

, and Sloss, L. L. (1955) Stratigraphy and
Sedimentation; W. H. Freeman and Company, San
FranCTSCo,TCalif.

 

 

 

Milner, Henry B. (19h0) Sedimentary Petrography; Nordeman
Publishing Co., New York, p. h90

 

39

 

 

ho

L. L. and Feray, D. E. (l9h8) Microstylolites in
Pet., Vol. 18, No. l, p. 3

 

 

 

Sloss,
Sandstone; Jour. Sed.
Theodosis, Steven D. (1955) Belt Series of Northwestern
Billings Geol. Soc. Guidebook, Sixth Annual

Montana;

EieId Conference, p. 58
(1955) Cambrian System in Northwestern Montana;
Sixth Annual Field

‘Biriings Geol.
Conference, p. 6b.

(19MB)

Jour.
Principles of Sedimentation;

. (I950)
HilI’Book Co. Inc., New York

 

Soc. Guidebook,

 

Environmental Significance of
Sed. Pet., VOI. 17, No. 3, p. 119

Twenhofel, W. H.

Conglomerates;
McGraw-

 

~=uiq

 

Q‘-
j 1*. m s. .’ U SE. III LY
Sakd'idiii In

Date Due

 

 

 

Demco-293

 

 

 

 

 

 

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III III III III I