The Structural, Volcanic, and Hydrothermal Geology (maps)
by Thomas J Callmeyer
A thesis submitted in partial fulfillment of the requirements of the degree of Master of Science in Earth
Sciences
Montana State University
© Copyright by Thomas J Callmeyer (1984)
Abstract:
The eastern Garnet Range of western Montana is composed of folded and faulted Precambrian Y
through Mesozoic sedimentary quartzites, carbonates and clastics unconformably overlain by nearly
horizontal, normal faulted Eocene volcanic rocks. Minor Cenozoic travertine, sinter, and alluvium form
local thin surficial deposits.
Laramide structures in the Warm Springs Creek area include northwest-striking thrust and reverse
faults, northeast-striking extensional faults, and southeast-plunging, northwest-striking en echelon folds
affecting Precambrian through Mesozoic strata. Local folding and faulting developed due to a left
lateral shear couple in the northern edge of the relatively eastward thrust Sapphire Plate.
Northwest-striking Laramide structures created northwest trending topographic troughs in which a once
more extensive cover of Eocene volcanic rocks is preserved. These volcanics include alkalic-calcic
porphyritic dacites and siliceous tuffs and aphanitic andesites which represent lava flows, dikes and
air-fall deposits. Since they are similar in age and composition to nearby volcanic members of the
Idaho-Montana Porphyry Belt, these volcanics may be a distal facies of the belt. Eocene volcanism
probably originated as lavas generated by a late phase of arc magmatism associated with the Laramide
orogeny were erupted after the cessation of Laramide folding and faulting. Northwest-trending post
volcanic normal faults cut the volcanic and older bedrock both along and across older
northwest-striking Laramide structures.
Large eroded pre-Holocene travertine and sinter deposits on the east side of the range represent a past
episode of extensive hydrothermal activity which followed Eocene volcanism. Minor modern warm
springs activity and associated travertine deposition along Warm Springs Creek is the result of the
circulation of meteoric waters through carbonate bedrock solution channels where they are heated by
the regional geothermal gradient before their re-emergence as warm springs. THE STRUCTURAL, VOLCANIC, AND HYDROTHERMAL GEOLOGY
OF THE WARM SPRINGS CREEK AREA, EASTERN
GARNET RANGE, POWELL COUNTY MONTANA
by
Thomas J . Callmeyer
A t h e s i s s u b m it te d i n p a r t i a l f u l f i l l m e n t
o f t h e r e q u i r e m e n t s o f t h e degre e
of
M ast er o f Scie nc e
in
E a r t h Sc ie nce s
MONTANA STATE UNIVERSITY
Bozeman, Montana
August 1984
APPROVAL
o f a t h e s i s su bm itt ed by
Thomas J . C al lmeyer
This t h e s i s has been r e a d by each member of t h e t h e s i s committee
and h a s b e e n f o u n d t o be s a t i s f a c t o r y r e g a r d i n g c o n t e n t , E n g l i s h
usage, fo rm at , c i t a t i o n s , b i b l i o g r a p h i c s t y l e , and c o n s i s t e n c y , and i s
read y f o r submission t o t h e C o lle ge o f Graduate S t u d i e s .
— = — -----;
Date
C h ai r p e r s o n , G r i d u a t e Committee
Approved f o r th e Major Department
13_!
Date
Head, Qlajor Department
Approved f o r t h e College o f Graduate S t u d i e s
Graduate Dean
iii
STATEMENT OF PERMISSION TO USE
In
presenting
th is
thesis
in
p artial
fu lfillm en t
of
the
r e q u i r e m e n t s f o r a m a s t e r ' s d e g r e e a t Montan a S t a t e U n i v e r s i t y ,
I
a g r e e t h a t t h e L i b r a r y s h a l l make i t a v a i l a b l e t o b o r r o w e r s u n d e r
r u l e s of t h e L i b r a r y .
B r i e f q u o t a t i o n s from t h i s t h e s i s a r e a l l o w a b l e
without s p ec ia l permission,
p r o v i d e d t h a t a c c u r a t e acknowledgment of
so urc e i s made.
.
-
-
P e r m i s s io n f o r e x t e n s i v e q u o t a t i o n from or r e p r o d u c t i o n of t h i s
t h e s i s may be g r a n t e d by my major p r o f e s s o r , o r i n h i s / h e r a bse n c e , by
the D i r e c t o r of L i b r a r i e s when, i n t h e o p i n i o n o f e i t h e r , t h e proposed
use of t h e m a t e r i a l i s f o r s c h o l a r l y pu rp o s e s.
th e m a t e r i a l
i n t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l no t be a l l o w e d
w i t h o u t my w r i t t e n p e r m i s s i o n .
Signature
Date
Any copying or us e o f
U ltisr
z
/ f
Z
V
iv
ACKNOWLEDGMENTS
I w i s h t o t h a n k Dr. D a v i d R. L a g e s on (C o m m i t t e e C h a i r m a n ) , t h e
late
Dr.
D o n a l d L. S m i t h ,
Dr. R o b e r t A. C h ad w ic k,
and Dr.
John
Montagne o f t h e r e a d i n g committee f o r t h e i r s u g g e s t i o n s , g u id a nce , and
c r i t i c i s m during the p re p a ra tio n of t h i s t h e s i s .
F u r t h e r tha nks i s e xte nded t o :
W.
Mogk,
professor
at
P e t e r M e j s t r i c k , g e o l o g i s t ; David
M o n ta n a S t a t e U n i v e r s i t y ,
Hugh M.
More,
g e o l o g i s t , and M a r s h a l l M. C ol e , f i e l d a s s i s t a n t and g r a d u a t e s t u d e n t
a t Montan a S t a t e U n i v e r s i t y , f o r t h e i r c o n t r i b u t i o n s d u r in g mapping
and r e s e a r c h pha ses of t h i s t h e s i s .
T h i s t h e s i s was p a r t l y
f u n d e d by M e r i d i a n Land and M i n e r a l
Company ( f o r m e r l y B u r l i n g t o n N o r t h e r n Energy and M i n e r a l s Department).
A p p r e c i a t i o n i s extended t o B u r l i n g t o n N o rt h e rn , and to Dan H. Vice,
g e o l o g i s t . Dr. L a g e s o n , Dr. S m i t h , and Dr. C hadwick f o r t h e i r h e l p i n
securing t h i s aid.
F i n a l l y I would l i k e t o th an k my w if e Rose M. B l a z i c e v i c h f o r h e r
s u p p o r t and f o r c o n t r i b u t i n g h e r a r t i s t i c s k i l l s i n p r e p a r i n g t h e
i l l u s t r a t i o n s f o r t h e f i n a l m a n u s c r ip t .
V
TABLE OF CONTENTS
Page
1.
LIST OF TABLES.......................................
2.
LIST OF FIGURES..........................
3.
LIST OF PLATES..............................................................................................
x
4.
ABSTRACT................................................................................................................
xi
5.
INTRODUCTION.......................................................................................................
j
6.
7.
vi i i
w I-
I
Ul
Purpose of I n v e s t i g a t i o n .................
L o c a t i o n and A c c e s s i b i l i t y o f F i e l d Area
F i e l d and Lab P r o c e d u r e s . ..............
P r e v io u s Ge ologic I n v e s t i g a t i o n s ......... ..
S t r a t i g r a p h y ............................................................
v ii
STRUCTURAL GEOLOGY.........................................................................................
7
F o l d s . . . ......... .............................
F a u l t s . . . . ...................................................................................................
S t r u c t u r a l Sequence...............
L i n e a r s ...........................................................................................................
T e c t o n i c s .....................................
7
15
23
24
28
EASTERN GARNET RANGE VOLCANIC FIELD....................................................
36
Regi onal V o l c a n i s m . . . . . ..........................
Local Volcanism..........................
P e t r o g r a p h y ....................................................
Chemical A n a l y s i s .............................................................
O r i g i n o f Volcanism.....................................................
36
40
49
57
52
8.
HYDROTHERMAL GEOLOGY.....................................................................................
65
9.
ECONOMIC POTENTIAL.........................................................................................
71
Min eral R e s o u r c e s . ..........................................................................
Geothermal F l u i d s ..........................................
71
72
10.
CONCLUSIONS.........................................................................................................
73
11.
REFERENCES CITED..............................................................................................
75
vi
TABLE OF CONTENTS—Cont inued
Page
12.
APPENDICES................................. ........................................................................
80
Appendix A.........................................................................................................
R ad io m et r ic Dates (K-Ar T e c h n i q u e ) ...................................
Appendix B......... ...............................................................
R ad io m etr ic Dates ( C ^ T e c h n i q u e ) .....................................
gj
82
33
84
vii
LIST OF TABLES
Ialle
1.
Psge
Micr osc opi c P e tr o g r a p h y o f e a s t e r n Garnet Range D a c i t e
P o r p h y r y . . ...........................................................................................................
51
2.
M icroscopic P e t ro g r a p h y o f e a s t e r n Garnet Range D a c i t e .........
52
3.
Micr osc opi c P e tr o g r a p h y o f e a s t e r n Garnet Range
A n d e s i t e ............................ ..................................................................................
52
4.
Mi croscopic Pe t r o g r a p h y o f e a s t e r n Garnet Range T u f f s ............
54
5.
Chemical a n a l y s i s o f e i g h t v o l c a n i c rock samples t a k e n
from th e e a s t e r n Garnet Range......................
53
Normative m i n e r a l p e r c e n t a g e s f o r f i v e e a s t e r n Garnet
Range v o l c a n i c ro c k s a m p l e s.................................
59
6.
viii
LIST OF FIGURES
n S-"
Page
1.
L oc a ti on o f t h e e a s t e r n Garnet Range f i e l d a r e a .........................
2
2.
S t r a t i g r a p h i c column of t h e e a s t e r n Ga rnet Range........................
g
3.
S po le diagram o f t h e G a r r i s o n
a n t i c l i n e .......................................
g
4.
Tangent diagram o f th e Warm S p r in g s Creek a r e a g i v i n g
di p d i r e c t i o n s on t h e s c a l e a t i t s c i r c u m t e r e n c e and
di p v a l u e s on t h e c o n c e n t r i c s c a l e . . .................................................
9
5.
S t r u c t u r a l c r o s s s e c t i o n A - A ' .................................................................
10
6.
S tru ctu ral cross
s e c t i o n B - B ' ..................
n
7.
S tru c tu ra l cross
s e c t i o n C - C ................................................................
12
8.
S t r u c t u r a l c r o s s s e c t i o n D -D ' .....................................................
13
9.
S t r u c t u r a l c r o s s s e c t i o n E - E ' ................ .....................................
14
10.
S t r u c t u r a l c r o s s s e c t i o n F - F ' ................ ................................................
15
11.
Map o f w e s t e r n Montana, showing o r i e n t a t i o n s o f major
f o l d s and f a u l t s i n t h e a r e a c o n t a i n i n g Pr ecambrian B el t
Supergroup r o c k s .....................................................
17
H a lf r o s e diagram o f l i n e a t i o n s i d e n t i f i e d by computer
enhancement o f a Lands at image o f t h e Warm S p r in g s
Creek a r e a ...................
25
13.
Map o f l i n e a t i o n s
i n t h e Warm S p r i n g s Creek a r e a ........................
26
14.
Map of t h e Precam brian Y B e l t B a s i n ....................................................
30
15.
Map o f Lat e Cre ta c e ous t o M i d - T e r t i a r y igneous ro c ks i n
and n e a r t h e e a s t e r n Ga rnet R a n g e . . . .................................................
37
The e a s t e r n Garnet Range v o l c a n i c f i e l d i n r e l a t i o n s h i p
t o o t h e r igneous r o c k s and s t r u c t u r a l t r e n d s i n and n e a r
w e s t e r n Montana........... ..................................................................................
39
12.
16.
Ix
LIST OF FIGURES— Cont inued
F ig ure
17.
Map of T e r t i a r y v o l c a n i c ro c k s i n and n e a r t h e e a s t e r n
Garnet Range................ ...........................................................
Page
42
18.
Volcanic c r o s s s e c t i o n VA-VA1.................................................
19.
Volcanic c r o s s s e c t i o n VB-VB'.............................................
20.
Volcanic c r o s s s e c t i o n VC-VC1.................. ...........
21.
The I . U . G . S . c l a s s i f i c a t i o n f o r s i l i c a s a t u r a t e d a p h a n i t i c
ro c ks s i m i l a r t o t h o s e in t h e e a s t e r n G arnet Range................
60
Graph o f CaO and Na20 + KgO a g a i n s t SiOg f o r f i v e samples
of e a s t e r n Ga rnet Range v o l c a n i c r o c k s ......................... .................
61
22.
23.
24.
Thi s AFM diagram o f v o l c a n i c r o c k s from t h e e a s t e r n
Ga rnet Range d e m o n s t r a t e s th e c a l c - a l k a l i n e n a t u r e of
t h e s e r o c k s ........................... ............................................
D ia gra m ati c c r o s s s e c t i o n o f t h e G a r r i s o n Warm Sp r in g s
w a te r c i r c u l a t i o n s y s te m ...................................... ............................
43
44
45
63
66
X
LIST OF PLATES
Plates
1.
Topographic map o f upper Warm S p r in g s Creek and a d j a c e n t a r e a s
2.
Geologic map o f upper Warm S p r in g s Creek and a d j a c e n t a r e a s .
xi
ABSTRACT
!■ S S li
IiElliliimii
gimmmm
I
INTRODUCTION
Purpose o f I n v e s t i g a t i o n
The o b j e c t i v e o f t h i s
thesis
is to e v a l u a t e the s t r u c t u r a l ,
v o l c a n i c and hyd ro th e rm a l g e o lo g y o f t h e Warm S p r i n g s Creek a r e a ,
the
eastern
Garnet
Range
of
w estern
c o n tr ib u tio n s of the study in c lu d e :
M on ta na
(Fig.
I).
in
Major
I ) a g e o l o g i c map o f t h e Warm
Sp r in gs Creek a r e a ; 2) d e t a i l e d d e s c r i p t i o n s o f p r e v i o u s l y u n e v a l u a t e d
E a r l y T e r t i a r y v o l c a n i c r o c k s ; 3) doc ume nt atio n o f a lon g h i s t o r y o f
h yd ro th e rm a l a c t i v i t y i n t h e e a s t e r n Garnet Range; and 4) a summary o f
the
stru ctu ral
and t e c t o n i c
evolution
of
the
Garnet
Range and
su rr o u n d in g a r e a s .
B urlington
N orthern,
provided p a r t i a l
Inc.,
E n e r g y and M i n e r a l s
Departm ent,
fun di ng toward t h i s s tu dy i n o r d e r t o e v a l u a t e t h e
economic p o t e n t i a l o f t h e a r e a s g e o lo g y .
The p r e s e n c e of warm s p r i n g s
i n t h e a r e a i n d i c a t e d a p o s s i b i l i t y of a ge oth erma l energy s o u r c e . A
section ev alu atin g
t h e a r e a ' s economic p o t e n t i a l
is
included at the
end o f t h e t h e s i s .
L o c a t i o n and A c c e s s i b i l i t y o f F i e l d Area
The G a r n e t Range i s l o c a t e d n o r t h o f t h e C l a r k F o r k R i v e r i n
p a r t s of P o w e l l ,
I).
G r a n i t e and M i s s o u l a C o u n t i e s , w e s t e r n Montana (Fig.
The a r e a mapped i n c l u d e s t h e y p p e r d r a i n a g e s o f Warm S p r i n g s
C r e e k , G a l l a g h e r C r e e k , B ro c k C r e e k , and L i m e s t o n e Canyon ( P l a t e I )
L e w i s a nd Cl a r k
County
FIELD
AREA
Wa r m
Springs
Creek
Gr anite County
Powell
Kilometers
Helena
County
Garnet
Range
Butte
50
Figure I .
100
L o c a t i o n o f t h e e a s t e r n G a rn e t Range f i e l d a r e a . The Ga rnet Range i s l o c a t e d
i n w e s t e r n Montana, i n p a r t s o f P ow e ll , G r a n i t e and Miss oula C o u n t i e s .
3
c o v e r i n g a p p r o x i m a t e l y 120 sq u a r e k i l o m e t e r s c e n t e r e d on s e c t i o n 30,
T . 11 N ., R. 9W.
A c c e s s t o t h e a r e a i s by u n i m p r o v e d d i r t r o a d s a l o n g s t r e a m s
d r a in in g south to th e C la rk Fork R i v e r , or ran ch ro ad s e x te n d in g to
th e e a s t e r n ba se o f t h e range i n t h e Avon V a l l e y .
r o a d s , un m a in ta in ed i n
Mining and l e g g i n g
v a rio u s s t a t e s of d is r e p a i r , provide access to
t h e i n t e r i o r o f t h e range.
F i e l d and Lab P r oce dure s
F i e l d w ork was done d u r i n g t h e summers o f 1981 and 1982.
Base
maps a t s c a l e s o f 1 : 2 4 , 0 0 0 and 1 : 4 1 , 7 0 0 w e re p r e p a r e d by e n l a r g i n g
p a r t s of the 1.62,500 s c a l e U ni te d S t a t e s G e o l o g i c a l Survey Avon and
Garrison quadrangles.
both q u a d ra n g le s ,
The f i n a l b a se map O pia te I ) i n c l u d e s p a r t s of
j o i n e d a t t h e i r common b o r d e r a nd e n l a r g e d t o
I :41,700 s c a l e .
V o l c a n i c u n i t s a m e n a b l e f o r f i e l d m a p p in g w e r e d i f f e r e n t i a t e d
according to c o lo r ,
relatio n sh ip s.
tex tu re,
m acroscopic m in e ra lo g y ,
T h e s e a p h a n i t i c and p o r p h y r i t i c v o l c a n i c r o c k s were
re-exam ined u sin g th e p e tr o g r a p h ic m icroscope.
flu o rescen ce
and f i e l d
s p e c t r o s c o p y was
done on s e l e c t e d
l a b o r a t o r y o f W ashington S t a t e U n i v e r s i t y .
Whole r o c k x - r a y
sam ples
at
the
Age d a t i n g o f s e l e c t e d
s a m p l e s i n c l u d e d t h e K/Ar r a t i o t e c h n i q u e f o r v o l c a n i c s a m p l e s and
c a r b o n - 1 4 d a t i n g o f a h y d r o t h e r m a l d e p o s i t by T e l e d y n e I s o t o p e s ,
Westwood, New J e r s e y .
S t r u c t u r a l m a p p in g was a u g m e n t e d by a l i n e a t i o n s t u d y t o h e l p
i d e n t i f y s t r u c t u r e s t h r o u g h t h e i n t e r p r e t a t i o n o f t o p o g r a p h y and
4
v e g e ta tiv e trends.
A Landsat f a l s e c o l o r f i l m p o s i t i v e was examined
u s i n g a S p a t i a l D a t a S y s t e m s , I n c . c o m p u t e r w i t h a n "Eye Com" c am e ra
and a l i g h t
tab le.
The p r o g r a m u s e d was an " e d g e e n h a n c e m e n t "
program, which d i s t i n g u i s h e s t h e b o u n d a r i e s o f a r e a s w i t h d i f f e r i n g
a lb ed os as p e r c e i v e d by Lands at s c a n n e r s .
Pr e v io u s Ge ologic I n v e s t i g a t i o n s
An e a r l y g e o l o g i c s t u d y o f t h e e a s t e r n G a r n e t Range ( P a r d e e ,
1917) i n c l u d e s a g e o l o g i c map and s t r a t i g r a p h i c d e s c r i p t i o n s f o r t h e
s o u t h e r n and e a s t e r n p o r t i o n s o f t h e t h e s i s a r e a .
S ev eral other
papers deal w ith the
(Gwinn,
s t r a t i g r a p h y of nearby a re as
1961;
Kauffman, 1963; K rause , 1963; Weidman, 1965; and Kauffman, 1965).
D a t a on v a r i o u s a s p e c t s o f l o c a l h y d r o t h e r m a l
activ ity
are
i n c l u d e d i n r e p o r t s by W i l l i a m s (1975), Chadwick and Kaczmarek (1975),
and
Sonderegger
and
B ergantino
(1981).
Inform ation
concerning
v o l c a n i c r o c k s o f t h e G a r n e t Range a r e r e p o r t e d by Gwinn and Mutch
( 1 9 6 5 ) , C hadwick ( 1 9 8 1 ) and C a r t e r ( 1 9 8 2 ) .
A map o f t h e T e r t i a r y and
Q u a t e r n a r y g e o l o g y o f t h e e a s t e r n s i d e o f t h e a r e a was p r o d u c e d by
Weber and Witkind (1979).
An e v a l u a t i o n o f t h e Idaho-Montana t h r u s t b e l t (Ruppel, W a l l a c e ,
S c h m i d t , and L o p e z , 1981) i n c l u d e s a t e c t o n i c map o f s o u t h w e s t e r n
Montana and p r o v i d e s a model f o r r e g i o n a l t e c t o n i c s ,
t h e e a s t e r n G a r n e t Range.
which i n c l u d e s
Maps o f t h e a r e a i l l u s t r a t e t h e o v e r a l l
g e o l o g i c s t y l e ( C l a p p , 1932; W a l l a c e and o t h e r s , 1981; and Ross and
o t h e r s , 195 5) .
5
Stratigraphy
The o l d e s t
rocks
in th e
a re a are P recam brian ( B e lt)
s e d i m e n t a r y q u a r t z i t e s , w i t h m i n o r a r g i l l i t e s and c a r b o n a t e s .
age
They
r e p r e s e n t s e d i m e n t s d e p o s i t e d i n an embayment o r r e e n t r a n t on t h e
w e s t e r n edg e o f t h e P r e c a m b r i a n c r a t o n .
carbonates,
representing a s ta b le
P a le o z o ic rocks are m ostly
s h e l f environm ent.
U ppe rm os t
P a l e o z o i c and Mesozoic fo r m a t i o n s c o n s i s t o f a complex i n t e r s t r a t i f i e d
s e r i e s o f e l a s t i c s and c a r b o n a t e s , which r e p r e s e n t i n c r e a s i n g e r o g e n i c
effects
and s e d i m e n t
o v e r l a i n by E o c e n e
av ailab ility
during
that
tim e.
These a re
d a c i t e s and a n d e s i t e s w h i c h a r e t h e m s e l v e s
p a r t l y o v e r l a i n by T e r t i a r y t r a v e r t i n e ,
s i n t e r , and a l l u v i u m .
A
summary o f Pr ecambrian t h ro u g h Cenozoic roc k u n i t s i s p r e s e n t e d i n t h e
f o l l o w i n g s t r a t i g r a p h i c c o l u m n ( F i g . 2) The r e a d e r i s r e f e r r e d t o
Kauffman (1963) f o r f u r t h e r i n f o r m a t i o n c once rn in g l o c a l s t r a t i g r a p h y .
6
tK A
h tK IU )
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aluam cm
» TO 10
BASIN FILL DEPOSITS
oLD
T W .'
CENE H Y D R O S * * .
DEPOSITS
CENQZDlC
'
10 TO IlO
Te r t ia r y
UAClTE POKPHYRT
_
--------------------- — —
<5U
EOCEie VOLCANI CS
ICU
bO
UACITE AUTDeRECCIA------------------------;------------------- ----------Colorado ( k u >
A m EQUIVALENT
ROCKS
H-AOC SHALE L IfC STOfC ANO SILTSTOfC
JURASSIC
ItRMlAN
HS
b lL I S I U C . SANDSTOC. SHALE. A m LIfCSTONE
(S etaceous
IESOZOIC
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L
Sw if t
l is
(f.
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em .
SB
—
I L au CONUE SANOSTOfC-------------------------- -------------------------- ■
R ieroon Fm .
L x i T I C LIfCSTW C A m S H A L E ---------------------------------
Sawtooth Fm.
U tL A N tO S SHALE, S IL T S T O t,
ISO
AtC L IK S T O C
FjHo s p h c r ia Torm ation
kj U
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-
U u iA K tlA S 6 ILTSTOfC, SHALE, AND MINOR CARBONATES --------- ------PALEOZOIC
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Canyon Fm .
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r_ _ .
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iieE E Eorcs L
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LaLCAICOCS SHALE A m MIfCR AfKYDRUE
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LUCSTOtC, ARGILLACEOUS SHALE, DOLOMITE. SILTSTWC
no
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410
---------------------------------------------
L ifc s T O fc A m
Fla teea o Ao r v it io n
SB----------------
UcwiTZlTE
—
sh ale
----------— -
Fbx EXPOSED LOCALLY
-------- --------------------------------------------------
RlLO CR FM.
Ga r ic t
Be l t
FFECAreRIAN
Fi g u r e 2.
FtECAfeRIAN Y
it z u e
Am a r g il l it e
Qu a r t z it e
w o w g iu it e
Hw
range
FbT EXPOSED LOCALLY
,
Su per
i^ w c r a
„
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Ho r c r Fm .
KCD PETERS
Fm .
Qu a r t z it e
O x c m it e
S il ic e o u s L i t c s r w c A m
F b l EXPOSED LOCALLY
Am a r g il l it e
dolomite
?
S t r a t i g r a p h i c column of t h e e a s t e r n Garnet Range (m odified
from Kauffman, 1 9 65 ),
7
STRUCTURAL GEOLOGY
Folds
The G a r r i s o n a n t i c l i n e i s t h e s i n g l e l a r g e s t s t r u c t u r a l e lem ent
of t h e Warm S p r i n g s Creek a r e a .
I t s a x i s as det ermi ne d by c o n to u r and
" T a n g e n t d i a g r a m s " ( B e n g t s o n , 1980) ( F i g s . 3 a nd 4) i s o r i e n t e d N.
40°W.,
and i t s
southeast
in
plunge
the
is
v ariab le,
northw est
part
of
ranging
fr om a few d e g r e e s
t h e map a r e a ,
s o u t h e a s t n e a r t h e s o u t h e a s t c o r n e r o f t h e map a r e a .
to
about
Figures
30°
5, 6 ,
7, 8, and 9 a r e c r o s s s e c t i o n s t r a n s v e r s e t o t h e t r e n d o f t h e G a r r i s o n
a n t i c l i n e , w h i l e F i g u r e 10 i s a l o n g i t u d i n a l c r o s s s e c t i o n o f t h e
fold.
The G a r r i s o n a n t i c l i n e i s asy m m et ri ca l w i t h southwest v e r g e n c e .
I t s n o r t h e a s t l i m b i s s h a r e d w i t h an a d j a c e n t , s o u t h e a s t —p l u n g i n g
syncline.
Minor f o l d s , whose a x i a l
surfaces strik e p a r a l l e l
to the
a x i a l s u r f a c e s o f t h e two m a j o r f o l d s , c o m p l i c a t e t h e for m o f t h e
G a r r i s o n a n t i c l i n e but do n o t s i g n i f i c a n t l y change t h e o v e r a l l form of
the f o l d .
Development o f t h e s e open, c o n c e n t r i c f o l d s was acc o m p li s h e d by
f l e x u r a l f o l d i n g and w i t h s i g n i f i c a n t c a t a c l a s i s .
B r i t t l e f a i l u r e of
t h e P a l e o z o i c c a r b o n a t e s and t h e Q u a d r a n t F o r m a t i o n q u a r t z i t e s i s
e s p e c i a l l y e v i d e n t n e a r t h e n o s e o f t h e G a r r i s o n a n t i c l i n e w h e re
p e r v a s i v e b r e c c i a t i o n and t h e d e v e l o p m e n t o f s I i c k e n s i d e s i n t h e
Q u a d r a n t F o r m a t i o n i s a c c o m p a n i e d by nu m e ro us s m a l l - s c a l e f a u l t s
8
Fi g u r e 3 .
S p o l e diagram o f t h e G a r r i s o n a n t i c l i n e . The g r e a t c i r c l e
(dashed l i n e ) which b e s t f i t s th e a p p a r e n t g i r d l e o f
c o n t o u r d e n s i t y ( s o l i d c u r v e ) i n d i c a t e s an a x i a l p l a n e
o r i e n t e d s o u t h e a s t 140" w i t h an a x i s p l u n g i n g a b o u t 7°.
The s o u t h e a s t p l u n g e o f t h e G a r r i s o n a n t i c l i n e i n c r e a s e s
and t r e n d s more s o u t h e r l y n e a r Luke M o u n t a i n i n t h e
s o u t h e a s t p a r t of t h e map a r e a .
9
N
,6 0
270
280
360
001
F ig u r e 4.
Oiv
Tangent diagram of t h e Warm Sp r in gs Creek a r e a g i v i n g d ip
d i r e c t i o n s on t h e s c a l e a t i t s c i r c u m f e r e n c e , and d i p
v a l u e s on t h e c o n c e n t r i c s c a l e ( a f t e r B e n g t s o n , 1 9 8 0 ).
D ata p o i n t s r e p r e s e n t th e a c t u a l d ip v a l u e s o f bedding
p l a n e s , n o t p o l e s t o t h e s e p l a n e s . The p r e d o m i n a n c e o f
s o u t h w e s t - and n o r t h e a s t - d i p p i n g beds i n t h e f l a n k s o f t h e
G a r r i s o n a n t i c l i n e i s a p p a r e n t . Most p o i n t s d e s c r i b e a
n o r t h e a s t t r e n d i n g g i r d l e (dashed l i n e ) which i n d i c a t e s a
p l u n g i n g c y l i n d r i c a l f o l d whose a x i s t r e n d s N 40® W
p l u n g i n g l e s s t h a n 10® s o u t h e a s t . S e v e r a l d a t a p o i n t s from
t h e s o u t h e a s t end o f t h e s o u t h e a s t p l u n g i n g G a r r i s o n
A n t i c l i n e l i e in the s o u th e a s t q u ad ran t of the graph,
r e f l e c t i n g t h e i n c r e a s e d p l u n g e o f t h e f o l d n e a r Luke
Mountain.
A SW
Buckskin
-2000
M eters
FCY
—1 0 0 0 M e t e r e
F i g u r e 5.
S t r u c t u r a l c r o s s s e c t i o n A-A' s h o w i n g t h e G a r r i s o n a n t i c l i n e u n c o n f o r m a b l y
o v e r l a i n by Eocene v o l c a n i c s , and t h e n o r t h e a s t t h r u s t S a l t Gulch f a u l t .
The
l o w - a n g l e t h r u s t i n g c u t s t h e s t e e p l y f o l d e d beds a t t h e southwest f l a n k o f t h e
G a r r i s o n a n t i c l i n e a t a lower a n g l e t h a n t h e d i p o f t h e bed s, r e s u l t i n g i n t h e
S a l t Gulch f a u l t s c u t t i n g down s e c t i o n . (See P l a t e 2 f o r k e y . )
B SW
Pp
BNE
Pq
Avon Valley
Normal Fault
\
C
-2000M etera
-IOOOMeters
Gar r i s on Anticline
Fi g u r e 6 .
S t r u c t u r a l c r o s s s e c t i o n B-B1 showing t h e bre ach ed G a r r i s o n a n t i c l i n e , t h e E a s t
Brock Creek t h r u s t f a u l t , and t h e e a s t e r n range f r o n t normal f a u l t . (See P l a t e
2 f o r key.)
CNE
OThI
- 2 0 0 0 M e ters
-IOOOMetere
G errle o n Anticline
C r e e e l y Ninel Feult S y ste m
F i g u r e 7.
S t r u c t u r a l c r o s s s e c t i o n C-C' s h o w i n g t h e G a r r i s o n a n t i c l i n e and r e l a t e d
f o l d i n g t o t h e n o r t h e a s t . The G r a v e l y Mine f a u l t system i s o v e r l a i n by Eocene
v o l c a n i c r o c k s i n t h i s l i n e o f s e c t i o n , h e l p i n g d a t e i t a s p r e - E o c e n e . (S ee
P l a t e 2 f o r key.)
DSW
DNE
Pq
■ 2 0 0 0 M e i e rs
KJ
OThI
TOeI
IOOOMeters
Ga r r i s on Ant i cl i ne
mi nor I Luka
Mou n t a i n Iault
Figure 8 .
Gr a v e l y Mine
S t r u c t u r a l c r o s s s e c t i o n D-D' s h o w i n g t h e G a r r i s o n a n t i c l i n e a nd n e a r b y
f o l d i n g . The G r a v e l y f a u l t s y s t e m i s n o t c o v e r e d by v o l c a n i c r o c k s i n t h i s
l i n e o f s e c t i o n . (See P l a t e 2 f o r key.)
E
W
E'E
Pq
Ra
Tt
Pq
Avon V a l l e y
N or m a l Fa u l t
■2000 M e t e r s
-1 0 0 0 Meters
PCY
Luke M ou nt ain
Figure 9.
lNorma
PC Y
Faults
S t r u c t u r a l c r o s s s e c t i o n E-E1 showing t h e normal f a u l t s e t i n t h e nose o f th e
G a r r i s o n A n t i c l i n e a t Luke M o u n t a i n , and t h e Luke M o u n t a i n r e v e r s e f a u l t .
These two m u t u a l l y p e r p e n d i c u l a r f a u l t systems e v o l v e d as s p a t i a l a d j u s t m e n t s
th ro u g h f a u l t i n g became n e c e s s a r y i n t h e nose o f t h e G a r r i s o n a n t i c l i n e d u r i n g
l a t e s t a g e s o f t h e f o l d ' s d e f o r m a t i o n . (See P l a t e 2 f o r k e y . )
L u k e M o u n ta in N o r m a l F a u l t s
2000
M eters
Fault S y s te m
M M1
F i g u r e 10*
S t r u c t u r a l c r o s s s e c t i o n F-F1 showing t h e l o n g i t u d i n a l p r o f i l e o f t h e G a r r i s o n
an ticlin e.
The p l u n g e o f t h e f o l d i n c r e a s e s t o t h e s o u t h e a s t n e a r Luke
M o u n t a i n , a s t r a i n w h i c h was p a r t l y t a k e n up i n t h e Luke M o u n t a i n n o r m a l
faults.
(See P l a t e 2 f o r key.)
16
(P late
2,
and F i g s .
9 and 10).
D e f o r m a t i o n o f t h e more d u c t i l e
Mesozoic s e c t i o n a ppea rs t o have i n v o l v e d f l e x u r a l - f l o w f o l d i n g .
The l a r g e f o l d s o f t h e e a s t e r n G a r n e t Range a r e t y p i c a l o f a
s e r i e s of en e c h e l o n ,
s o u t h e a s t - p l u n g i n g f o l d s which occu r a l o n g t h e
n o r t h e r n s i d e of t h e C l a r k Fork and L i t t l e B l a c k f o o t R i v e r s between
Bearmouth and E l l i s t o n (F ig. 11).
South of t h e Ga rnet Range, t h e axes
o f t h e s e s o u t h w e s t - p l u n g i n g f o l d s bend t o t h e s o u th .
S e v e r a l of t h e s e
f o l d s c o n ti n u e s o u th i n t o a group of f o l d s d e v e l o p e d on t h e n o r t h e r n
p a r t of t h e F l i n t Creek Range,
crossing a s tr u c t u r a l
low, t h e " C l a r k
Fo r k Sag", t o w a r d s w h i c h f o l d a x e s p l u n g e fr om t h e n o r t h and s o u t h
( F i g . 11) (Wiedman, 19 61; and B ak e n, 1 9 81).
The C l a r k F o r k Sag i s
c o n s id e re d p a r t o f t h e Montana Lineament (Lewis and C l a r k Li ne) and i s
d i s c u s s e d i n t h e s e c t i o n on t e c t o n i c s .
Faults
F a u l t s i n t h e G a r n e t Range f a l l
i n t o two g e n e r a l c a t e g o r i e s ;
Late C retaceous to P a le o c e n e (Laramide) f a u l t s ,
Recent e x t e n s i o n a l f a u l t s .
a nd O l i g o c e n e t o
Laramide f a u l t s a r e t y p i c a l l y p a r t l y
c ov ered a l o n g t h e i r t r a c e by o v e r l y i n g Eocene v o l c a n i c s , which a r e not
o f f s e t by Laramide f a u l t s .
Eocene v o l c a n i c s .
O l i g o c e n e to Recent normal f a u l t s o f f s e t
Laramide f a u l t s w i l l be d i s c u s s e d f i r s t .
S a l t GuI c h F a u l t .
The S a l t G u l c h f a u l t , n e a r t h e n o r t h w e s t
c o r n e r o f t h e a r e a mapped ( S e c t i o n s 6 , 7 , and 8, T. 11 N., R. 10 W.),
strik es
n o r t h w e s t (N.
4 0 eW) and d i p s 29° s o u t h w e s t .
It
contains
M i s s i s s i p p i a n ro c ks i n t h e h a ng in g w a l l and Precam brian r o c k s i n t h e
f ootwal I ( P l a t e 2, and Fig. 5).
The s o u t h e a s t p a r t o f t h e f a u l t t r a c e
17
-I__=£:-----a— a —
Fm ul lm -ball a n d b a r on d o w n t h r o w n
KEY
atrlke
------- $----------
ellp
motion,
teeth
on
aide,
arrowa Indicate
upper
plate.
Ant I c l l n e
Autochthonoua
\
F i g u r e 11.
PCY
H trx r e a l
Belt
^ i
etrata
Map o f w e s t e r n M o n t a n a , s h o w i n g o r i e n t a t i o n s o f m a j o r
f o l d s and f a u l t s i n t h e a r e a c o n t a i n i n g P r e c a m b r i a n B e l t
S u p e r g r o u p r o c k s , i n c l u d i n g t h e M ontana L in e a m e n t and
S a p p h i r e T h r u s t P l a t e . The i n s e t c o n t a i n s s t r u c t u r a l d a t a
f o r t h e a r e a n e a r Warm S p r i n g s C r e e k , i n c l u d i n g t h e
n o r t h w e s t t r e n d i n g C l a r k Fork Sag s t r u c t u r a l t r o u g h .
The
n o r t h w e s t bend o f f a u l t s and f o l d ax es n e a r t h e C l a r k Fork
Sag i s a p p a r e n t ( m o d i f i e d a f t e r H a r r i s o n and o t h e r s , 1974;
R u p p e l and o t h e r s , 1 9 8 1 ; B a k e n , 1 9 8 1 ; Wiedman, 1 9 6 5 ;
Kauffman, 1963; W a l la c e and o t h e r s , 1981 and t h i s a u t h o r ) .
18
18 c o v e r e d by T e r t i a r y v o l c a n i c s ( P l a t e 2 ) .
This f a u l t has been
mapped a s a " y o u n g e r - o v e r - o l d e r " t h r u s t f a u l t i n a s e r i e s o f t h r u s t f a u l t s a t t h e n o r t h e r n e dge o f t h e S a p p h i r e t h r u s t p l a t e by W a l l a c e
and o t h e r s (1981) and Ruppel and o t h e r s (1981).
Based on t h i s
s tu d y ,
t h e S a l t G u l c h f a u l t d o e s a p p e a r t o be a y o u n g e r - o v e r - o l d e r t h r u s t
f a u l t w i t h r e s p e c t to i t s f i e l d r e l a t i o n s , o r i e n t a t i o n ,
age, and t h e
p r e s e n c e of nearby n o r t h w e s t s t r i k i n g Laramide t h r u s t f a u l t s .
IM t B r ^ C r e e k fault.
the
east
n o r th w e s t.
fork of East
The E a s t Brock Creek f a u l t i s l o c a t e d on
B r o c k C r e e k and i n a d j a c e n t a r e a s
to the
This l o w - a n g l e f a u l t has a geometry s i m i l a r t o t h a t o f t h e
S a l t Gulch f a u l t i n t h a t i t a l s o d i p s southwest and s t r i k e s n o r t h w e s t .
I t c o n t a i n s M i s s i s s i p p i a n r o c k s i n t h e han ging w a l l and a c o m p l i c a t e d
s e c t i o n o f Cambrian, Devonian and M i s s i s s i p p i a n u n i t s i n t h e f o o t w a l l .
The t r a c e o f t h e f a u l t i s c o v e r e d a l o n g s t r i k e t o t h e n o r t h w e s t by
Eoce ne v o l c a n i c r o c k s .
T h i s f a u l t may be c o n t i n u o u s w i t h t h e S a l t
G u l c h f a u l t s i n c e t h e t r a c e s o f t h e two a p p e a r t o l i n e up a t e i t h e r
s i d e o f t h e i n t e r v e n i n g v o l c a n i c c o v e r ( P l a t e 2).
Creek f a u l t
The E a s t B ro c k
i s a l s o mapped as a y o u n g e r - o v e r - o l d e r t h r u s t by Ruppel
and o t h e r s (1981) and W a l l a c e and o t h e r s (1981).
This a u t h o r s u p p o r t s
t h e younger o v e r o l d e r t h r u s t f a u l t i n t e r p r e t a t i o n .
G r a v e l y Mine f a u l t
system .
On t h e
east
s id e o f th e range
( S e c t i o n s 3 2, 33, and 3 4, T. 11 N., R. 4 W., and S e c t i o n s 2 and 3, T.
10 N.,
R.
9 W.),
two n o r t h w e s t - o r i e n t e d
fau lts
fo r m a g r a b e n i n
P a l e o z o i c and Mesozoic sed im en ta ry r o c k s ( P l a t e 2 and F i g s . 7 and 8).
F a u l t p l a n e s a p p ea r h i g h - a n g l e ,
n o t be d e t e r m i n e d .
a l t h o u g h t h e i r e x a c t a t t i t u d e c o u ld
T h e s e f a u l t s a r e a l s o p a r t l y c o v e r e d by Eo ce ne
19
v o l c a n i c r o c k s , b u t a p p e a r t o be c o n t i n u o u s w i t h a p a i r o f f a u l t
t r a c e s to th e n o r t h w e s t .
These a r e t r a c e a b l e westward f o r a l m o s t two
k i l o m e t e r s beyond th e v o l c a n i c f i e l d b e f o r e bending t o t h e s o u th w e s t,
m e r g i n g and d i s a p p e a r i n g b e n e a t h T e r t i a r y
trav ertin e
v o l c a n i c r o c k s ( S e c t i o n 3 2 , T. 11 N., H. 9 W.).
mapped t h e s e f a u l t s a s t h r u s t f a u l t s .
fau lt
planes
were
not
determ ined,
c h a l l e n g e d by t h i s a u t h o r ,
and E oc e ne
W a lla c e (1981) has
Since the a t t i t u d e s of th e se
th at
in terp retatio n
is
not
a l t h o u g h t h e a p p a r e n t h ig h a n g l e s o f t h e
f a u l t p l a n e s and younger s t r a t a exposed between t h e f a u l t s s u g g e s t s a
graben structure.
Discussion.
A l l f a u l t s thus f a r discussed, w ith the exception of
t h e G r a v e l l y Mine f a u l t s y s t e m , h a v e some i m p o r t a n t common t r a i t s .
They h a v e
all
been p r e v i o u s l y
mapped a s t h r u s t
fau lts
dipping
s o u t h w e s t , a l t h o u g h i n m o s t c a s e s h a n g i n g w a l l a nd f o o t w a l I ag e
r e l a t i o n s h i p s a r e i n d i c a t i v e o f normal f a u l t s .
I f c o n s i d e r e d as lo w -
a n g l e normal f a u l t s t h e y may hav e e v o l v e d i n r e s p o n s e t o t e n s i o n a l
s t r e s s e s n e a r t h e convex c r e s t o f t h e G a r r i s o n a n t i c l i n e ( B i l l i n g s ,
1972), a s e t t i n g which would g e n e r a t e t e n s i o n a l f a u l t i n g p a r a l l e l t o
the axis of the fo ld .
A l t e r n a t e l y , c o n s i d e r i n g t h e f a c t t h a t t h e s e l o w - a n g l e f a u l t s d ip
in t h e same d i r e c t i o n as l o c a l s t r a t a and t h a t d i p s f o r l o c a l s t r a t a
a r e g e n e r a l l y s t e e p e r th a n t h e f a u l t p l a n e which c u t s them,
p o s s ib ility is evident.
another
S t r a t a p r e v i o u s l y f o l d e d t o s t e e p d i p s would
be d i s p l a c e d by a t h r u s t f a u l t which would c u t up s t r u c t u r a l l y , w h i l e
c u t t i n g down s e c t i o n s t r a t i g r a p h i c a l l y .
This e f f e c t would be enhanced
i f th e f a u l t s were l i s t r i c and i f t h e s t r a t a were t r a n s p o r t e d from an
20
area
s t e e p dip t o t h a t o f lower d i p , as i s t h e ca se i n t h e map a r e a
(Fig. 5).
This h y p o t h e s i s i s f a v o r e d by th e a u t h o r be cause t h e f a u l t s
a r e p r e - E o c e n e and t h u s c o r r e l a t e w i t h o t h e r L a r a m i d e t h r u s t - s l i p
f a u l t s i n th e r e g i o n .
Luke Mountain r e v e r s e f a u l t .
The Luke Mountain r e v e r s e f a u l t i s
l o c a t e d n e a r t h e nose o f t h e G a r r i s o n a n t i c l i n e ( P l a t e 2, and F i g . 9).
The f a u l t p l a n e s t r i k e s N. 3O6W, and d i p s n o r t h e a s t as o b s e r v e d i n th e
Luke P h o s p h a t e Mine (Hugh Mo ore , P e r s o n a l C o m m u n i c a t i o n , 1 9 8 2 ).
At
t h e s o u t h e a s t c o r n e r o f t h e map, P e r m i a n r o c k s a r e d i s p l a c e d i n t h e
hanging w a l l
against
M esozoic s t r a t a
in the footw al I.
At t h i s
l o c a t i o n th e f a u l t i s n e a r t h e h in g e l i n e o f t h e G a r r i s o n a n t i c l i n e .
F u r t h e r to t h e n o r t h w e s t t h e f a u l t t r a c e i s a l o n g t h e n o r t h e a s t
limb
o f t h e G a r r i s o n a n t i c l i n e and be com es a l o w - a n g l e b e d d i n g f a u l t ,
r e s u l t i n g in l i t t l e
s t r a t i g r a p h i c displacement.
A lt h o u g h t h i s f a u l t
i s n o t c o v e r e d by E o c e n e v o l c a n i c s , i t i s c u t by f a u l t s w h i c h a r e
c o v e r e d by v o l c a n i c s .
W a l l a c e ( 1 9 8 1 ) a l s o shows t h i s f a u l t a s a
southw est d ip p in g younger o v e r o l d e r t h r u s t
fau lt.
This is not
c o n s is te n t w ith the evidence th a t i t is a n o rth e a s t dipping re v e rs e
fault.
lnkf. M ountain norm al f a u l t s .
On Luke M o u n t a i n s e v e r a l n o r m a l
f a u l t s cut t h e Luke Mountain r e v e r s e f a u l t .
of n o r t h e a s t
s t r i k i n g (N. 2 5 - 5 5 eE.), h i g h - a n g l e normal
t r a c e s a r e p a r t l y o b s c u r e d by v o l c a n i c s .
tig h tly
These f a u l t s form a s e t
folded
nose
of
the
G arrison
faults,
whose
These f a u l t s o c c u r a t th e
an ticlin e
and
are
nearly
p e r p e n d i c u l a r t o t h e a x i s o f t h e f o l d ( P l a t e 2, a nd F i g s . 9 a nd 10).
T his group o f f a u l t s o c c u r s where t h e G a r r i s o n a n t i c l i n e b e g i n s to
21
s tee p e n in p lu n g e to th e s o u t h e a s t .
Thus t h e y p r o b a b l y r e p r e s e n t
b r i t t l e f a i l u r e o f t h e P a l e o z o i c q u a r t z i t e s and c a r b o n a t e s a t t h i s
stru ctu ral
p o sitio n .
Th e
larg est
of
th ese
fau lts
is
the
no rth w es ter nm ost and i t o f f s e t t h e b l o c k t o t h e s o u t h e a s t d o w n - t o - t h e s o u t h e a s t , r e s u l t i n g i n d i s p l a c e m e n t o f t h e c r e s t o f t h e f o l d and
c au si ng s t r a t a t o t h e s o u t h e a s t t o be d i s p l a c e d toward t h e f o l d c r e s t .
In g e n e r a l t h e n o r th w e s t s i d e o f t h e o t h e r f a u l t s a r e d i s p l a c e d down
r e l a t i v e t o t h e s o u t h e a s t s i d e , d i m i n i s h i n g t h e e f f e c t of t h e l a r g e r
normal f a u l t to t h e n o r t h w e s t .
Ot h e r La r a m i d e
fau lts.
F au ltin g of p re -T e rtia ry
strata
e v i d e n t w i t h i n a window i n t h e v o l c a n i c s a t G a l l a g h e r C r e e k .
is
Here,
P r o t e r o z o i c r o c k s h a v e b e e n o f f s e t a l o n g a f a u l t s t r i k i n g N. 2 5 eE.
( S e c t i o n 2, T. 11 N., R. 10 W.).
M in o r f a u l t s
in P aleozoic u n its
a l o n g Warm S p r i n g s C r e e k ( S e c t i o n 5 T. 10 N., R. 9 W.) a r e r e l a t e d t o
t h e Luke M o u n t a i n r e v e r s e
fau lt
by a l i n e
of breccias
(in
the
c a r b o n a t e s ) and t i g h t s m a l l - s c a l e f o l d s ( i n a r g i l l a c e o u s and s h a l y
b e d s ) t r a c e a b l e i n t h e M i s s i s s i p p i a n s t r a t a b e t w e e n t h e two f a u l t s .
T e r t i a r y normal f a u l t s a r e t h e most r e c e n t
larg e sca le geologic
s t r u c t u r e s i n t h e a r e a , and a r e d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n .
Ajton range, f r o n t, f a u l t , ,syatem., The west s i d e o f t h e Avon V a l l e y
i s p a r t o f a g r a b e n c r e a t e d by a s e r i e s o f r a n g e f r o n t n o r m a l f a u l t s
on t h e e a s t e r n e d g e o f t h e G a r n e t Range ( P l a t e 2).
B o th s e d i m e n t a r y
and v o l c a n i c r o c k s a r e c u t by t h e s e f a u l t s , t h u s e x p o s i n g t r u n c a t e d
s t r a t i g r a p h i c and s t r u c t u r a l
Range.
This
is
evident
t r e n d s on t h e e a s t e r n s l o p e s o f Garnet
in Section
2 (T.
10 N.,
R.
9 W.) w h e r e
h o g b a c k s o f f o l d e d s t r a t a and n o r t h w e s t s t r i k i n g f a u l t t r a c e s end
22
a b r u p t l y a t the range f r o n t normal f a u l t .
e a s t e r n G a r n e t Range v o l c a n i c
field ,
The t r u n c a t i o n o f t h e
whose a t t i t u d e s
and f i e l d
r e l a t i o n s su gge st t h a t i t once exten de d e as tw a rd i n t o t h e Avon V a l l e y
a re a , is the r e s u l t o f range fr o n t f a u l t i n g .
The Avon V a l l e y i s
f i l l e d w i t h T e r t i a r y sed im ent s which e n t i r e l y c o v e r and f i l l - i n
the
hanging w a l l b l o c k .
T e r t i a r y normal
fau lts
are
oriented
northw est
(N. 30°W) i n
S e c t i o n s 11, 13, a nd 24 (T. 10 N., R, 9 W) and i n S e c t i o n s 2 2, 1 5, 9,
4,
and
5 (T.
11 N.,
R.
9 W.).
Normal
fau ltin g
occurred
along
n o r t h e a s t and e a s t - w e s t s t r i k i n g z o n e s n e a r t h e G r a v e l y Mine i n
S e c t i o n 2 (T. 10 N., R. 9 W.) a nd S e c t i o n s 3 6 , 25, a nd 26 (T. 11 N.,
R. 9 W.).
Range
front
faulting
is
responsible
for
poorly
developed
t r i a n g u l a r f a c e t i n g on t h e e a s t e r n s l o p e s o f t h e Garnet Range a s w e l l
as a s t r a i g h t a lig n m e n t of th e e a s t e r n s l o p e s o f th e ran g e.
This
s t r a i g h t to p o g r a p h i c t r e n d i s s u g g e s t i v e of a f a u l t - l i n e s c a r p .
P-PTma I, f a u l t s .
A n o r t h w e s t - s t r i k i n g normal
fau lt
in
Buckskin Gulch n e a r S a l t Gulch has downdropped and p r e s e r v e d Eocene
v o l c a n i c s in the hanging w a l l ( n o r t h e a s t ) , w ith Precam brian B e l t
s t r a t a i n th e f o o t w a l l .
Salt
Gulch
fau lt,
Thi s f a u l t a p p e a r s t o merge w i t h t h e Laramide
indicating
in teractio n
between
the
two a n d ,
t h e r e f o r e , p o s s i b l e po s t- L a ra m id e m o t i o n .
A norm al
fau lt
in S e c tio n
6 (T . 10 N ., R. 9 W.) c o n t a i n s
P a l e o z o i c s t r a t a i n t h e f o o t w a l l a nd P a l e o z o i c s t r a t a a n d Eoce ne
v o l c a n i c s i n t h e hanging w a l l .
Thi s f a u l t s t r i k e s N. 28'E and a ppea rs
23
t o have a id ed i n th e p r e s e r v a t i o n o f a p o r t i o n o f t h e e a s t e r n Ga rnet
Range v o l c a n i c s i n t h e hanging w a l l .
,In G a l l a g h e r Creek,
S e c t i o n s 2 and 12 (T. 11 N., R. 10 W.) a
normal f a u l t , s t r i k i n g n o r t h w e s t , o f f s e t s v o l c a n i c and s e d i m e n t a r y
s tra ta there.
The f a u l t a p p e a rs t o cu t a Laramide f a u l t .
S t r u c t u r a l Sequence
The e a r l i e s t t e c t o n i s m e v i d e n t i n t h e Warm S p r i n g s C r e e k a r e a
produced l a r g e ,
open f o l d s d u r in g t h e
others,
This is
1981) .
youngest rocks
latest
s u p p o r t e d by l o c a l
in v olved
in
folding
are
Cre ta c e ous (Ruppel and
fie ld relatio n s.
lower
Upper
(S an to ni an ?) age, and r e p r e s e n t a maximum age f o r f o l d i n g .
The
C retaceous
The f o l d s
a r e unconformabIy o v e r l a i n by u n f o l d e d Eocene v o l c a n i c r o c k s (d a te d by
t h e P o t a s s i u m A rg on m e th o d a s 43 t o 45 m .y .b .p .).
rocks
w ere
erupted
onto
an e r o s i o n
surface
These v o l c a n i c
which
breached
the
Pha nerozoic and P r o t e r o z o i c s e c t i o n s and which i s e v i d e n t i n canyons
and
at
the
ero sio n al
m argins
of
the
v o lcan ic
field .
unconf ormab I y o v e r l a i n by t h e s e v o l c a n i c s a r e t h e S a l t G ulc h,
A lso
Ea st
Brock Creek, and Luke Mountain f a u l t s , which a r e t h e r e f o r e a l s o p r e Eocene.
However, a t l e a s t two e p i s o d e s o f f a u l t i n g a r e r e p r e s e n t e d by th e
pre-Eocene f a u l t s .
The r e v e r s e f a u l t a t Luke M o u n t a i n i s c u t by
normal f a u l t s , which a r e t h e m s e l v e s o v e r l a i n by Eocene v o l c a n i c r o c k s .
The normal f a u l t s which o f f s e t t h e Eocene v o l c a n i c s r e p r e s e n t t h e
most r e c e n t p e r i o d o f f a u l t i n g ,
V a lle y graben.
and a r e r e s p o n s i b l e f o r t h e Avon
These f a u l t s a r e t y p i c a l o f r e g i o n a l e x t e n s i o n a l
24
t e c t o n i c s w h i c h b e g a n i n t h e m i d - T e r t i a r y and r e m a i n s a c t i v e i n t h e
H o l o c e n e ( H a r r i s o n and o t h e r s , 1974 ; S t i c k n e y , 1978; a nd S m i t h and
S b a r , 197 4).
Lineapd^A/
L i n e a r s i n t h e Warm S p r i n g s C r e e k a r e a w e r e i d e n t i f i e d u s i n g a
c o m p u t e r - e n h a n c e d L a n d s a t f i l m p o s i t i v e image o f p a r t o f w e s t e r n
M onta na .
The r e s u l t s o f t h i s s t u d y a r e s u m m a r iz e d on a h a l f r o s e
d i a g r a m ( F i g . 1 2 ) , and a map ( F i g . 13) o f l o c a l
l i n e a r topographic
trends.
E x a m i n a t i o n o f F i g u r e s 12 a nd 13 i n d i c a t e t h a t two p r e f e r r e d
o rie n ta tio n s e x is t fo r these l in e a r s .
30° and 70° e a s t
of n o rth ,
w hile
One d i f f u s e s e t l i e s b e t w e e n
a second,
a p p r o x i m a t e l y 30° t o 5 0 ° w e s t o f n o r t h .
narrow er se t tre n d s
A m inor group i s o r i e n t e d a
few d e g re e s e a s t of n o r t h .
N o rth w e st-tre n d in g l i n e a r s r e f l e c t bedrock s t r u c t u r e s .
Some
f a u l t s a r e o b v i o u s l i n e a r s , s u c h a s a t B u c k s k i n G u l c h and t h e Avon
V a lle y graben.
The s o u t h w e s t s l o p e s o f t h e r a n g e a l s o p r o d u c e d
lin e a r s r e l a t e d to n o rth w e s t- s tr ik in g ,
southw est-dipping s t r a t a ,
c a u s i n g r e s i s t a n t c a r b o n a t e s on t h e s ou th w es t f l a n k o f t h e G a r r i s o n
a n t i c l i n e to s ta n d w ith r e l i e f above l e s s r e s i s t a n t s t r a t a .
O ther
n o rth w e s t l i n e a r s were l e s s e a s i l y a ss ig n e d to id e n tif ie d geologic
features.
Some n o r t h w e s t - t r e n d i n g d r a i n a g e s a r e l i n e a r a n d may be
r e l a t e d to j o i n t s .
The n o r t h w e s t o r i e n t e d l i n e a r b o r d e r i n g t h e Avon V a l l e y n e a r
G i m l e t C r e e k i n S e c t i o n s 11 and 13 (T. 10 N., R. 9 W) i s r e l a t e d t o
25
N
10
I*
F ig u r e 12.
10
6
«>
io
6
10
16
H a l f r o s e d i a g r a m o f l i n e a t i o n s i d e n t i f i e d by c o m p u t e r
e n h a n c e m e n t o f a L a n d s a t image o f t h e Warm S p r i n g s C r e e k
a r e a . Azimuths o f t h e s e l i n e a t i o n s h a v e be en a v e r a g e d t o
t e n d e g re e i n c re m e n t s . Northwest l i n e a t i o n s dominate the
area, although a d iffu s e n o rth e a st p a tte rn is a ls o
a p p a r e n t . The v a l u e s r e p r e s e n t e d on t h e c o n c e n t r i c s c a l e
a r e t h e sum o f l i n e a t i o n s r e c o g n i z e d d u r i n g t e n s e p a r a t e
e x a m i n a ti o n s o f t h e L an ds at image.
26
R . 10W.
—
R.9W.
#'
l1
Ki l ometer
R. 9W. \
Fi g u r e 13.
Map o f l i n e a t i o n s i n t h e Warm S p r i n g s Creek a r e a , showing
m ajor n o rth w e s t s t r i k i n g s u b p a r a l l e l l i n e a t i o n s
r e p r e s e n t i n g l a n d f o r m s c r e a t e d by n o r t h w e s t s t r i k i n g
s t r u c t u r a l tr e n d s . N o rth e a st tre n d in g lin e a rs represent
e i t h e r d r a i n a g e s fo r m e d by n o r t h e a s t s t r i k i n g f a u l t s , o r
con sequent d r a i n a g e s on l a n d s l o p e s c o n t r o l l e d by no rt h w e s t
s t r i k i n g f o l d s and f a u l t s (compare w i t h P l a t e 2).
27
range f r o n t f a u l t i n g
fo r only a short d ista n ce .
ob vio us a l o n g t h e n o r t h e a s t
Avon V a l l e y
This
lin ear
is
s i d e o f t h e d r a i n a g e d i v i d e between t h e
and Warm S p r i n g s
Creek
(Fig.
13 ).
Further
to
the
n or th w e s t a normal f a u l t which c r o s s e s G a l l a g h e r Creek p a r a l l e l s t h e
p ro je c te d s t r i k e of the l i n e a r .
southeast,
A lthough t h i s f a u l t d ie s to th e
t h e p r e s e n c e o f t h e l i n e a r between two f a u l t s may i n d i c a t e
s t r u c t u r a l c o n t r o l o f t h e e a s t e r n ra nge c r e s t .
N o r t h e a s t t r e n d i n g l i n e a r s may be r e l a t e d t o s e v e r a l c a u s e s .
The
l i n e a r along the n o r t h e a s t - s t r i k i n g s lo p e s of th e range n o rth of
G i m l e t C r e e k , S e c t i o n 36 (T. 11 N., R. 9 W.) a n d S e c t i o n I (T. 10 N.,
R. 9 W.), i s
related
to range f r o n t f a u l t i n g .
The
large
lin ear
t r e n d i n g n o r t h e a s t b e t w e e n E a s t B r o c k C r e e k and Warm S p r i n g s C r e e k
S e c t i o n I (T. 10 N., R. 11 W.) and S e c t i o n 6 (T. 10 N., R. 10 W.) i s a
normal f a u l t .
Most n o r t h e a s t t r e n d i n g l i n e a r s r e p r e s e n t d r a i n a g e s f l o w i n g
n o r t h e a s t i n t o t h e Avon V a l l e y o r south wes t i n t o t h e C l a r k Fork R i v e r .
These a r e con sequent d r a i n a g e s f l o w i n g down s l o p e s formed by f o l d s and
fau lts.
E x a m p l e s i n c l u d e F i n n C r e e k , D a v i s C r e e k , a nd L i m e s t o n e
Canyon ( L o s t Creek).
O t h e r l i n e a t i o n s seem u n r e l a t e d t o s t r u c t u r e , a l t h o u g h t h o s e
which p a r a l l e l r e c o g n i z e d s t r u c t u r a l t r e n d s a r e s u s p e c t e d t o r e f l e c t
j o i n t p a t t e r n s or minor f a u l t i n g e x p l o i t e d by e r o s i o n .
For example,
t h i s i s b e l i e v e d t o be t h e c a s e f o r t h e l o w e r Warm S p r i n g s C r e e k
:
d r a i n a g e , w h i c h p a r a l l e l s t h e t r e n d o f t h e Luke M o u n t a i n n o r m a l
faults.
28
Tectonics
L a te C re ta c e ous t o E a r l y T e r t i a r y (Laramide) compression r e s u l t e d
i n c r u s t a l s h o r t e n i n g i n t h e N o r t h e r n Rocky Mountains,
pro d u c in g p r e -
Eoce ne f o l d s and f a u l t s t h e e a s t e r n G a r n e t Range (K a u f f m a n , 1 9 6 3 ) .
The Laramide orogeny d e v e l o p e d i n r e s p o n s e t o B e n i o f f s u b d u c t i o n o f
o c e a n ic c r u s t b e n e a th th e w e s t e r n edge of th e N o rth Am erican p l a t e
( B u r c h f i e 1 , 1 981) .
S tr u c tu re s t y p i c a l of t h i s te c t o n ic episode in
w e s t e r n M o n ta n a i n c l u d e n o r t h - s o u t h and n o r t h w e s t o r i e n t e d t h r u s t
f a u l t s w ith g e n e r a lly w e st-d ip p in g ,
low -angle f a u l t
eastw ard t r a n s p o r t o f t h r u s t p l a t e s (Ruppel and o t h e r s ,
p l a n e s and
1981).
Post-
Laramide t e c t o n i s m i s c h a r a c t e r i z e d by e x t e n s i o n and t h e d e vel opm en t
of normal
f a u l t s which d i s r u p t e d b u t a l s o
exploited
stru ctu ral
p a t t e r n s e s t a b l i s h e d d u r i n g e a r l i e r t e c t o n i s m ( H a r r i s o n a nd o t h e r s ,
1974).
Three
tecto n ic
elem ents
had
sig n ifican t
in flu en ce
on t h e
s t r u c t u r a l s t y l e o f t h e e a s t e r n G a r n e t Range d u r i n g t h e L a r a m i d e
orogeny.
These i n c l u d e :
I ) t h e B e l t B as in ; 2) t h e Montana Lineament;
and 3) t h e S a p p h i r e P l a t e .
These f e a t u r e s a r e , r e s p e c t i v e l y :
I) a
P r e c a m b r i a n s e d i m e n t a r y b a s i n w h i c h o n c e e n c l o s e d much o f w e s t e r n
Montana and whose s t r u c t u r a l e l e m e n t s a r e b e l i e v e d t o h a v e i n f l u e n c e d
l a t e r s t r u c t u r e s ; 2) a n o r t h w e s t s t r i k i n g s t r u c t u r a l t r e n d w h i c h i s
t r a n s v e r s e t o more n o r t h e r l y t r e n d s i n t h e r e g i o n ; and 3) an e a s tw a rd
t r a n s p o r t e d t h r u s t p l a t e , whose n o r t h e r n edge l i e s i n t h e a r e a mapped.
B e l t Basin.
The Pr eca mb ria n Y B e l t B as in ,
a roughly t r i a n g u l a r
embayment i n t h e P r o t e r o z o i c s h o r e l i n e o f t h e C o r d i l l e r a n g e o c l i n e
29
(F ig.
14),
is
extensional
b eliev ed
tectonism
to h ave d e v e lo p e d d u rin g
(H arrison
and o t h e r s ,
a period of
1974).
i l l u s t r a t e s p a r t s o f two m o d e l s f o r t h e B e l t B a s i n .
Figure
14
These m o d e ls
d i f f e r as t o t h e p o s i t i o n s a nd o r i e n t a t i o n s o f d e p o s i t i o n s ! t r o u g h s
w ith in the b a sin ,
but each su g g est th e p re s e n c e o f e a s t- w e s t o r
northw est d e p o s i t i o n s ! b a s in s (or grabens) w ith in the B e lt Basin
( H a r r i s o n and o t h e r s , 1974; Winston, P e r s o n a l Communication, 1982) .
A ccording
Communication,
to
stratig rap h ic
work
by
W inston
(Personal
1982) t h e r e i s e v i d e n c e f o r Pr ec a mb ria n growth f a u l t s
in B e l t s e d im e n ta ry ro c k s a lo n g a l i n e which c r o s s e s t h e n o r t h e r n
t h i r d o f t h e Warm S p r in g s Creek a r e a map.
represents
Thi s e a s t - w e s t growth f a u l t
t h e s o u t h e r n e d g e o f t h e Ovando B l o c k ( F i g . 14 ).
The
Ovando Block i s t h e d e e p e s t o f s e v e r a l f a u l t - b o u n d s e d im e n ta ry b a s i n s
d e v e lo p e d w i t h i n th e B e l t B eein d u rin g th e Preoem brie= (W inston,
P e r s o n a l Communication, 1982).
The s t r u c t u r e and s t r a t i g r a p h y o f th e B e l t Basin has had a s t r o n g
i n f l u e n c e on l a t e r Laramide s t r u c t u r a l
M onta na .
fe a tu re s throughout w estern
The B e l t B a s i n i n f l u e n c e d s u c h f e a t u r e s a s t h e D i s t u r b e d
B e l t , t h e He len a S a l i e n t , t h e "Montana Lineament" and t h e l o c a t i o n o f
t h e t h r u s t b e l t and f o r e l a n d boundary in south wes t Montana ( H a r r i s o n
and o t h e r s , 1974, McMannis, 1965; and Winston, P e r s o n a l Communication,
1982).
Mon t a n a Li n e a m e n t .
The " M ont an a L i n e a m e n t " (L ew is and C l a r k
Line) i s composed o f a s e r i e s o f n o r t h w e s t - s t r i k i n g s t r u c t u r e s which
t r e n d n o r t h w e s t from C o e u r d ' A l e n e , I d a h o t o s o u t h c e n t r a l M on ta na
30
IOO 2 0 0
Kilometers
North
American
Cor dl l l er an
Qeocllne
Craton
Canada
Washington
Mont a na
Craton
Rrecambrlan
Qeocllne >
Idaho
F ig u r e 14.
WyoJ
Map o f t h e P r e c a m b r i a n Y B e l t B a s i n e x t e n s i o n a l b a s i n
d u r i n g d e p o s i t i o n o f B e l t Supergroup sedim ent s.
Several
proposed i n t e r n a l f e a tu r e s of th e B e lt B asin are
i d e n t i f i e d on t h i s f i g u r e , i n c l u d i n g t h e H e le n a Embayment,
a d e e p ly su b sid e d p o r tio n of th e B e lt B asin.
An
a s s o c i a t e d d e e p l y s u b s i d e d e a s t - w e s t o r n o r th w e s t t r e n d i n g
t r o u g h n o r t h w e s t o f t h e He len a Embayment ha s been termed
t h e C oeu r d ' A l e n e t r o u g h ( H a r r i s o n and o t h e r s , 1974) o r
t h e Ovando B loc k (Winston, P e r s o n a l Communication, 1981).
This deep t r o u g h r e c e i v e d e s p e c i a l l y t h i c k a cc u m u la ti o n s
o f s e d i m e n t d u r i n g t h e P r e c a m b r i a n . The s t r u c t u r e and
s t r a t i g r a p h y d e v e lo p e d in the B e lt Basin during the
P r e c a m b r i a n i n f l u e n c e d l a t e r ( L a r a m i d e and C e n o z o i c )
t e c t o n i c s (c o m p a r e w i t h F i g u r e 1 1 ) . The x i n d i c a t e s t h e
l o c a t i o n o f t h e s t u d y a r e a ( m o d i f i e d a f t e r H a r r i s o n and
o t h e r s , 1974; and Winston, P e r s o n a l Communication, 1981).
31
(Fig.
11) (K a u ff m a n ,
1 963; Weidman,
1 965;
and W i n s t o n ,
Personal
Communication, 1982 ).
In
the
Warm S p r i n g s
Creek a r e a ,
the
M o n ta n a L i n e a m e n t
is
r e p r e s e n t e d by t h e " C l a r k F o r k S a g" , a s t r u c t u r a l and t o p o g r a p h i c
d e p r e s s i o n t r e n d i n g n o r th w e s t between t h e e a s t e r n G arnet Range and t h e
F l i n t Creek Range (Fig. 11).
F o l d s on e i t h e r s i d e o f t h i s d e p r e s s i o n
p lu n g e toward i t s
low a x i s ,
l o c a t e d a few k i l o m e t e r s s o u t h o f t h e
C la r k Fork R i v e r .
The i n f l u e n c e o f t h e C l a r k F o r k Sag i n t h e Warm
S p r in g s Creek a r e a i s e v i d e n t b o th in th e i n c r e a s i n g p l u n g e o f th e
G arrison a n t i c l i n e to the s o u th e a s t,
and t h e G a r r i s o n a n t i c l i n e s
ve rg en ce t o t h e s o u t h w e s t .
F o l d s w h i c h a r e c o n t i n u o u s a c r o s s t h e C l a r k F o r k Sag r e v e r s e
t h e i r pl unge and t h e i r ax es change t r e n d , j F o l d s (and t h r u s t f a u l t s )
sou th of t h e C l a r k Fork Sag t r e n d n o r t h - s o u t h , b u t t h o s e n o r t h o f t h e
Sag t r e n d n o r t h w e s t (F ig. 11) (Weidman,
1965).
T his westward bending
of s t r u c t u r a l tr e n d s has been i n t e r p r e t e d as i n d i c a t i v e of l e f t l a t e r a l s i m p l e s h e a r f o r t h e a r e a (Weidman, 1 9 6 5 ).
The p a t t e r n o f
L a r a m i d e s t r u c t u r e s i n t h e Warm S p r i n g s C r e e k a r e a , w i t h n o r t h w e s t
s t r i k i n g f o l d a x e s and t h r u s t f a u l t s and n o r t h e a s t s t r i k i n g n o r m a l
f a u l t s , w o u l d s u p p o r t i n t e r p r e t a t i o n s of e i t h e r n o r t h e a s t - s o u t h w e s t
compression o r a l e f t - l a t e r a l sim ple s h e a r c o u p l e .
S o u t h e a s t p l u n g i n g Laramide f o l d s such as t h e G a r r i s o n a n t i c l i n e
oc cur a l o n g a n o r t h w e s t l i n e a r t r e n d which e x t e n d s from M i s s o u l a on
t h e w e s t t o E l l i s t o n on t h e e a s t ( H a r r i s o n and o t h e r s , 1 9 7 4 ) .
This
t r e n d r e p r e s e n t s an im p o r ta n t p a r t o f t h e Montana Lineament, e x te n d i n g
along p a rt of i t s
s o u t h e r n b o r d e r (Fig. 11).
R ig h t-la te ra l stra in is
32
e v i d e n t a l o n g most o t h e r s t r u c t u r e s on th e Montana Lineament.
T his i s
e s p e c i a l l y t r u e a t t h e major n o r t h w e s t - s t r i k i n g , r i g h t - l a t e r a l o b l i q u e
s l i p f a u l t s ( s o u t h s i d e down), which ex te nd f o r hundreds o f k i l o m e t e r s
a l o n g t h e Montana Lineament and which l a r g e l y d e f i n e i t s p o s i t i o n and
e x t e n t (Fig. 11) ( H a r r i s o n and o t h e r s , 1974; Ruppel and o t h e r s , 1981).
T h e r e f o r e , th e l e f t - l a t e r a l s t r a i n b e l i e v e d i n d i c a t e d by t h e n o r t h w e s t
bend i n f o l d s c r o s s i n g t h e C l a r k F o r k S a g , r e p r e s e n t s a s t y l e o f
s t r u c t u r e s which a r e e x c e p t i o n a l t o t h e r i g h t - l a t e r a l s t r a i n e l s e w h e r e
al ong t h e Montana Lineament.
Sa p j h i r e t h r u s t g l a t e .
This t h r u s t p l a t e i s b e l i e v e d to have
been t r a n s p o r t e d r e l a t i v e l y e a s t w a r d o v e r 60 k i l o m e t e r s d u r i n g t h e
Laramide orogeny (Hyndman, 1979).
A lt hough t h e F l i n t Creek Range has
t y p i c a l l y been i d e n t i f i e d as t h e n o r t h - e a s t e r n c o r n e r o f t h e S a p p h ir e
p late,
a d etailed
stratig rap h ic
and t e c t o n i c
study o f southw est
Montana (Ruppel and o t h e r s , 1981) i n d i c a t e s t h a t t h e f o l d e d P a l e o z o i c
ro c ks o f t h e e a s t e r n G arnet Range a r e p a r t o f t h e n o r t h e r n edge o f t h e
t h r u s t p l a t e , which a p p a r e n t l y e x te n d s e a s t o f Warm S p r in g s Creek f o r
a n o t h e r 20 k i l o m e t e r s .
North o f t h e S a pphir e P l a t e a p a ra u to c h th o n o u s
b lo c k of Precam brian B e l t sedim ents ap p ears to have a c te d as a west
and no rth w es t t r e n d i n g b u t t r e s s which r e s t r i c t e d t h e n o r t h e r n edge o f
th e t h r u s t p l a t e (F ig. 10) (Ruppel and o t h e r s ,
1981).
B uttressing of
t h e S a p p h i r e P l a t e by t h e G a r n e t Range h a s b e e n p r o p o s e d by o t h e r
g e o lo g is ts attem pting to e x p la in the s t r u c t u r a l p a tte r n of the C lark
Fork Sag ( P o u l t e r , 1954; McGill, 1959; and Baken, 1981) .
D iscussion.
The n o r t h e r n e d g e o f t h e L a r a m i d e S a p p h i r e t h r u s t
p l a t e and th e s o u t h e r n edge o f t h e Pr eca mb ria n Ovando Bloc k b o th c r o s s
33
t h e n o r t h e r n Warm S p r i n g s Creek a r e a n e a r t h e S a l t Gulch t h r u s t f a u l t
( W i n s t o n , P e r s o n a l C o m m u n i c a t i o n , 19 82; R u p p e l and o t h e r s , 1 9 8 1 ).
Pa rauto ch tho nous B e l t s t r a t a n o r t h of t h e S a p p h ir e P l a t e r e p r e s e n t t h e
t h i c k Precambrian d e p o s i t s on t h e down-dropped Ovando B loc k (Winston,
P e r s o n a l Communication, 1982).
According
t o W inston (1982),
P r e c a m b r i an b l o c k s
created
Laramide co m p re ssio n u p l i f t e d
by B e l t i a n
ex ten sio n
in
am ounts
p r o p o r tio n a l to the degree of subsidence ( i.e ., th ic k n e s s of s t r a t a )
d u r in g B e l t Basin e x t e n s i o n .
I f t h i s were t r u e ,
u p l i f t o f B e l t ro c k s
w i t h i n t h e Ovando B lo c k t o t h e n o r t h would ha ve o c c u r r e d t o a g r e a t e r
degree th a n t o t h e s out h.
T hi s u p l i f t c o u l d h a v e c r e a t e d t h e w est and
no rth w es t t r e n d i n g b u t t r e s s o f B e l t ro c k s which c on fi n e d t h e n o r t h e r n
e dg e o f t h e S a p p h i r e P l a t e .
The w e s t w a r d bend i n f o l d s a p p r o a c h i n g
th e n o r t h e r n edge of t h e S a p p h ir e P l a t e c o u l d ha ve d e v e l o p e d by l e f t l a t e r a l dra g a t t h e edge of th e c o n f i n e d ,
eastw ard t h r u s t p l a t e .
However, l e f t —l a t e r a l s t r a i n i s n o t common a l o n g t h e Montana Lineament
e xc e pt a l o n g i t s s o u t h e r n boundary ( H a r r i s o n and o t h e r s ,
lateral
1974).
Left-
s t r a i n a l o n g t h e M o n ta n a L i n e a m e n t a t t h e C l a r k F o r k Sag
o c c u r r e d w i t h i n t h e S a p p h ir e P l a t e where t h e Ovando Block c o n f i n e d and
deformed i t s n o r t h e r n ed ge.
C o r r o b o r a t i n g e v i d e n c e f o r a ma jor c r u s t a l weakness c r o s s i n g t h e
n o r t h e r n p a r t of t h e Warm S p r i n g s Creek a r e a i s p r o v i d e d by S t i c k n e y
(1978).
H is work on c o n t e m p o r a r y s e i s m i c i t y
i n w e s t e r n M o n ta n a
i n d i c a t e s th e p r e s e n c e o f a deep (15-26 k i l o m e t e r s ) , a c t i v e s e is m ic
zone w h i c h t r e n d s e a s t - w e s t a c r o s s t h e Avon V a l l e y .
T h i s z one i s
a l i g n e d w i t h t h e n o r t h e r n edge o f t h e S a p p h ir e P l a t e and t h e s o u t h e r n
34
edge o f t h e Ovando B l o c k , and i s a p p a r e n t l y r e s p o n d i n g t o a r i g h t lateral
shear
couple.
R ig h t-lateral
strain
along
t h e M o n ta n a
Lineament d i d n o t end w i t h t h e Laramide orogeny, bu t c o n t i n u e s th ro u g h
t h e Cenozoic ( H a r r i s o n and o t h e r s , 1981) .
Cenozoic t e c t o n i s m .
C o m p r e s s i v e t e c t o n i c s i n t h e Warm S p r i n g s
Creek a r e a ended b e f o r e t h e e x t r u s i o n of t h e Eocene v o l c a n i c r o c k s
which unconformably o v e r l i e Laramide s t r u c t u r e s .
and M i o c e n e ,
extensional
During t h e O li g o c e n e
te c to n ism began th ro u g h o u t th e re g io n ,
b e co m i ng t h e d o m i n a n t p r o c e s s s h a p i n g M on ta na g e o l o g y by t h e m i d Miocene ( B u r c h f i e 1 , 1981; Chadwick, 1981).
A c t i v e e x t e n s i o n may have
be gu n a s a r e s u l t o f d e v e l o p m e n t o f a t r a n s f o r m b o u n d a r y a t
the
s o u th w e s te rn edge o f t h e North American c r a t o n when N orth America came
in c o n ta c t w ith th e P a c i f i c - F a r a l l o n sp re ad in g c e n te r ( B u r c h f i e l ,
1981).
N o r t h w e s t - s t r i k i n g normal f a u l t s i n t h e H e l m s v i l l e and Avon
V a l l e y s w h i c h d e f i n e t h e n o r t h e a s t c o r n e r o f t h e r a n g e may h a v e
e x p l o i t e d p r e - e x i s t i n g n o r t h w e s t - s t r i k i n g s t r u c t u r a l weaknesses.
Alth ough t h e i n f l u e n c e o f o l d e r , n o r t h w e s t —o r i e n t e d s t r u c t u r e s on
Cenozoic f a u l t p a t t e r n s c o m p l i c a t e s th e i n t e r p r e t a t i o n of s tr e s s e s
i n v o l v e d i n Cenozoic f a u l t i n g ,
S t i c k n e y (1978) r e p o r t s t h a t a n a l y s i s
of seism ic d ata in the re g io n su g g ests e i t h e r n o rth e a s t- s o u th w e s t
tension or r i g h t - l a t e r a l shear are re s p o n sib le fo r re cent seism ic ity .
S e i s m i c i t y i n t h e Avon V a l l e y i n d i c a t e s r i g h t - l a t e r a l motion a l o n g a
15 t o 26 k i l o m e t e r d e e p z o n e t r e n d i n g e a s t - w e s t
a c r o s s t h e Avon
V a l l e y , and n o r t h w e s t - s t r i k i n g normal f a u l t s p ro duc in g s ei s m ic e v e n t s
from 6 t o 15 k i l o m e t e r s d e p t h ( S t i c k n e y , 1978).
35
T e r t i a r y motion may have e x a g g e r a te d Laramide o f f s e t i n th e C l a r k
Fork Sag.
M o n o c li n a l f o l d i n g o f T e r t i a r y sediments i n t h e C l a r k Fork
Sag on th e sou th f l a n k o f t h e Ga rnet Range n e a r Drummond (Gwinn, 1961)
and s u s p e c t e d n o r m a l ( s o u t h s i d e down) f a u l t i n g i n C r e t a c e o u s u n i t s
j u s t .s outh o f t h e a r e a mapped ( H a r r i s o n and o t h e r s , 1976), r e s u l t e d i n
f u r t h e r s t r u c t u r a l d i s p l a c e m e n t between t h e e a s t e r n G a rn et Range and
the
C l a r k F o r k Sag.
T ertiary
normal
fau ltin g
is
the
dominant
s t r u c t u r a l p r o c e s s sh ap in g t h e p h y s i o g r a p h y o f t h e m ode rn r a n g e s i n
th e a r e a and d e f i n e s t h e n o r t h e r n and e a s t e r n b o u n d a r i e s o f th e range.
36
EASTERN GARNET RANGE VOLCANIC FIELD
Regi onal Volcanism
Eocene v o lc a n i s m s i m i l a r t o t h a t o f t h e Ga rnet Range o c c u r r e d i n
w e s t e r n Montana a f t e r t h e c e s s a t i o n o f Laramide s t r u c t u r a l
activity*
but p r o b a b l y c o n s i s t o f arc- ma gma ti c l a v a s e r u p t e d a s a l a t e phase o f
E arly T e rtia ry tectonism .
E a r l y T e r t i a r y v o l canism in th e n o r t h
American Rocky Mountains i s b e l i e v e d r e l a t e d t o t h e
l a t e C re ta c e o u s
and P a l e o c e n e L a r a m i d e o r o g e n y a s p a r t o f a m a g m a t i c a r c c o m p l e x
l o c a t e d i n l a n d fr o m t h e L a r a m i d e s u b d u c t i o n z one t o t h e v e s t .
Eoce ne
volcanics
in
w estern
M o n ta n a h a v e
been
in terp reted
The
as
r e p r e s e n t i n g t h e second o f two maxima o f a r c magmatism (80-60 m.y.b.p.
and 54—45 nuy.b.p.) a s s o c i a t e d w i t h B e m o f f —ty p e s u b d u c t i o n of oc e a n ic
c r u s t b e n e a t h t h e N o r t h A m e r i c a n c r a t o n ( B u r c h f i e l , 1 9 8 1 ; Ch ad w ic k,
1 9 81 ).
N e ar b y i g n e o u s r o c k s r e p r e s e n t i n g t h e f i r s t p e r i o d o f a r c
magmatism i n c l u d e t h e I d a h o b a t h o l i t h , t h e P h i l i p s b u r g s t o c k , t h e
Bo ul der b a t h o l i t h and r e l a t e d E l k h o r n Mountains v o l c a n i c s , and members
o f t h e G o l d e n S p i k e F o r m a t i o n (C h a d w i c k , 1 9 8 1 ).
The G o l d e n S p i k e
Formation i s p r e s e r v e d i n t h e f o l d e d s t r a t a o f th e C l a r k For k Sag west
o f G a r r i s o n and i s b e l i e v e d t o r e p r e s e n t d i s t a l
d e p o s its of the
E lk ho rn Mountains v o l c a n i c f i e l d (Gwinn and Mutch, 1961).
F ig u r e 15 shows t h e
l o c a t i o n s and ages o f s e v e r a l
interm ediate
com pos ition v o l c a n i c f i e l d s s i m i l a r to t h e e a s t e r n Garnet Range f i e l d .
The Lo w la nd C r e e k v o l c a n i c s ( 5 4 - 4 8 m. y.b .p.) a r e l o c a t e d a l o n g t h e
5.
Map o f L a t e C r e t a c e o u s t o M i d - T e r t i a r y i g n e o u s r o c k s i n
and n e a r t h e e a s t e r n G a r n e t R a n g e , i n c l u d i n g a g e d a t e s
re p o rte d for s p e c i f i c d e p o s its of T e r t i a r y v o l c a n i c rocks.
L a t e C r e t a c e o u s i g n e o u s i n t r u s i v e (KI) and e x t r u s i v e (Kv)
r o c k s w e r e p r o d u c e d by L a r a m i d e a r c m a g m a t i s m .
E arly
T e r t i a r y i g n e o u s r o c k s (TV s t i p p l e d p a t t e r n ) r e p r e s e n t
e i t h e r b a s a l t - r h y o l i t e v o lc a n is m r e l a t e d to Cenozoic
e x t e n s i o n a l t e c t o n i c s , o r a second p u l s e o f a r c magmatism
which c r e a t e d i n t e r m e d i a t e v o l c a n i c r o c k s such as t h o s e i n
t h e e a s t e r n G a r n e t Range f i e l d ( m o d i f i e d a f t e r M e j s t r i c k ,
w r i t t e n c o m m u n i c a t i o n , 1 9 8 2 ; R o s s and o t h e r s , 1 9 5 5 ; a n d
t h i s a u t h o r . Age d a t e s a r e f r o m D a n i e l and B e r g , 1 9 8 1 ;
M e j s t r i c k , p e r s o n a l co m mu ni ca tio n, 1981; and t h i s a u t h o r .
38
west edge of t h e B o u ld e r b a t h o l i t h .
Range f i e l d ,
These v o l c a n i c s ,
l i k e t h e Ga rnet
have g e n e r a l l y been c o r r e l a t e d w ith the second a rc
magmatic maximum (Chadwick,
1981).
B a s a l t and r h y o l i t e v o l c a n i c s which a r e commonly a s s o c i a t e d w i t h
e x t e n s i o n s ! t e c t o n i s m (Chadwick, 1981) a r e p r e s e r v e d a l o n g t h e C l a r k
Fork and L i t t l e B l a c k f o o t R i v e r s (Fig. 15).
They range i n age from 29
to 50 nuy.b.p., t h u s g e n e r a l l y p o s t d a t i n g t h e i n t e r m e d i a t e v o l c a n i c s
in
the
G a r n e t Range a nd Lo w la n d C r e e k f i e l d s
(C h a d w i c k ,
1 9 8 1 ).
E x t e n s i o n a l v o l c a n i s m may a l s o be r e p r e s e n t e d by t h e H e le n a f i e l d (3 6 40 m .y .b .p .), c r o p p i n g o u t a s e r o s i o n a l r e m n a n t s e x t e n d i n g e a s t fr o m
Avon t o McDonald Pass w est o f H e le n a (Chadwick, 1981).
The
eastern
G arnet
Range
v o lcan ic
fie ld
i n t e r s e c t i o n o f two l i n e a r g e o l o g i c t r e n d s .
lie s
near
th e
These t r e n d s a r e th e
Montan a L i n e a m e n t " a nd t h e " I d a h o - M o n t a n a P o r p h y r y B e l t " ( F i g . 16)
(Cha dw ick ,
1 9 81 ).
The G a r n e t Range v o l c a n i c
field
is
elongated
n o r t h w e s t , p r o b a b l y due t o s t r u c t u r a l c o n t r o l o f t h e f i e l d by t h e
no rth w es t t r e n d i n g Montana Lineament.
The G a rn et Range v o l c a n i c s l i e
on the n ort hw e s t edge o f t h e Idaho-Montana Porphyry B e l t as d e f i n e d by
Chadwick (1981) (F ig. 16).
T h i s n o r t h e a s t - t r e n d i n g z one o f p l u t o n i c
p o r p h y r y m e t a l d e p o s i t s a nd a s s o c i a t e d i g n e o u s r o c k s r a n g e s i n ag e
from 38 t o 69 m.y.b.p. a t i t s
its
n o rth east
end.
T his
s o u t h w e s t e n d , t o 60 t o 69 m.y.b.p. a t
tren d
is
b eliev ed
to r e p re s e n t
the
e x p l o i t a t i o n o f a n o r t h e a s t - t r e n d i n g z one o r z o n e s o f s t r u c t u r a l
w e a k n e s s by L a t e C r e t a c e o u s and E a r l y T e r t i a r y magmas (C h a d w ic k ,
1981).
39
Montana
/
FrontalXZone
Thruet X Belt
Idaho
IDAHO-MONTANA
PORPHYRY BELT
G a r n e t Range
Volcanlca
•
\ e
y
E lkhorK tL ew le iie C re e k
V e l e e i i le e
Idaho
,
Batholith /
Montana
Wyoming
Idaho
F ig u r e 16.
The e a s t e r n G a r n e t Range v o l c a n i c f i e l d i n r e l a t i o n t o
o t h e r i g n e o u s r o c k s and s t r u c t u r a l t r e n d s i n and n e a r
w e s t e r n M o n t a n a . The M o n ta n a L i n e a m e n t and t h e I d a h o M o n ta n a P o r p h y r y B e l t i n t e r s e c t n e a r t h e G a r n e t R a n g e ,
i n f l u e n c i n g t h e e a s t e r n G arnet Range v o l c a n i c f i e l d . The
M o n ta n a L i n e a m e n t p r o v i d e d s t r u c t u r a l l y c o n t r o l l e d
n o r t h w e s t s t r i k i n g t o p o g r a p h i c t r o u g h s i n which t h e
v o l c a n i c s were p r e s e r v e d , as w e l l a s c r u s t a l weaknesses
e x p o l i t e d by e r u p t e d l a v a s .
The c o m p o s i t i o n s o f
r e c o g n i z e d Idaho-Montana Porphyry B e l t v o l c a n i c ro c k s a r e
s i m i l a r t o t h o s e o f t h e e a s t e r n G a r n e t Range ( a f t e r
Ch ad w ic k, 1981).
40
The G a r n e t Range v o l c a n i c s
a r e o f t h e same g e n e r a l a g e and
co mposition as t h e ne ar by Lowland Creek V o l c a n i c s (F ig . 15), which a r e
c o n s i d e r e d p a r t o f t h e I d a h o - M o n t a n a Porphyry B e l t .
T h i s , however,
does no t c l e a r l y q u a l i f y t h e G arnet Range v o l c a n i c s a s a member o f t h e
porphyry b e l t ,
although
the
sim ilarities
indicate
th at
the
two
v olcanic f i e l d s are r e la te d .
Local Volcanism
- Most o f t h e n o r t h e r n h a l f o f t h e t h e s i s map a r e a i s c o v e r e d by a
sequence o f a l t e r n a t i n g d e p o s i t s o f a n d e s i t e ,
t u f f s ( P l a t e 2).
dacite.
and s i l i c e o u s
These v o l c a n i c s u nconform abIy o v e r l i e Laramide
s t r u c t u r e s d e v e lo p e d i n P r o t e r o z o i c th ro u g h M esozoic s e d im e n ta ry
rocks.
The v o l c a n i c s a r e d * t e d a s E o c e n e ,
a s d e t e r m i n e d by t h e
p o t a s s i u m a r g o n w h o l e r o c k m e th o d ( a n a l y s t , Te le d y n e I s o t o p e s In c .) .
An a n d e s i t e s a m p l e d n e a r Windy Rock ( s o u t h e a s t 1 / 4 s e c t i o n 1 5, T. 11
N., R. 10 W.) y i e l d e d a n a g e o f 44.8 + 2.2 m.y. M e j s t r i c k ( P e r s o n a l
Communication, 1981) r e p o r t s and age o f 47 m i l l i o n y e a r s f o r a d a c i t e
on Warm S p r i n g s C r e e k i n s e c t i o n 3 1 , (T. 11 N., R. 9 W.) a l s o d a t e d by
Teledyne I s o t o p e s Inc. u s i n g t h e wh ole r oc k p o ta s s iu m argon method.
The e a s t e r n G a r n e t Range v o l c a n i c f i e l d
oriented
southeast,
stru ctu rally
w h i c h was
co n tro lled
filled
w ith
valley ,
lie s
in a northw est
deepening
v olcanics
during
toward
the
the
Eocene.
V o l c a n i c s on t h e e a s t e r n margin o f t h e f i e l d e x te n d from 2140 m e t e r s
(7020 f e e t ) a l t i t u d e down t o 1524-1585 m e t e r s (5080 f e e t ) .
Thi s t h i c k
sequence o f v o l c a n i c s i s c u t by normal f a u l t s on t h e Avon V a l l e y range
front,
so t h a t an o b l i q u e c r o s s s e c t i o n th ro u g h t h e v o l c a n i c - f i l l e d
41
Eocene v a l l e y i s exposed.
field,
Along t h e sou thwest t r e n d i n g margin o f th e
a l t i t u d e s range from 1710 t o 2135 m e t e r s (5610 t o 7005 f e e t ) .
The e n t i r e exposed v o l c a n i c sequence r a n g e s in t h i c k n e s s from a
few m et ers f o r t u f f s n e a r Luke Mountain and a t t h e e r o s i o n a l marg ins
o f t h e f i e l d , t o o v e r 600 m e t e r s t h i c k n e a r G a l l a g h e r C r e e k .
The
average t h i c k n e s s i s a bout 200 m e t e r s .
This v o l c a n i c f i e l d i s i n an advanced s t a t e o f e r o s i o n and l a c k s
any r e l i c t v o l c a n i c morphology.
A l a r g e r and p e t r o g r a p h i c a l I y s i m i l a r
v o l c a n i c f i e l d , p r o b a b l y once c on ti g u o u s w i t h t h e e a s t e r n G arnet Range
f i e l d , e x t e n d s w e s t w a r d fr o m t h e e a s t e r n G a r n e t Range ( F i g . 17) a s
i n d i c a t e d by numerous e r o s i o n a l o u t l i e r s .
Some o f t h e s e o u t l i e r s and
p o r t i o n s o f t h e f i e l d n o r t h o f t h e map b o u n d a r y c o n t a i n b a s a l t and
q u a r t z - l a t i t e flow s in a d d i t i o n to th e l i t h o l o g i e s w ith in th e a re a
mapped ( M e j s t r i c k , P e r s o n a l Communication, 1981).
Post-volcanic
normal
fau lts
have o f f s e t
some p a r t s
of the
v o l c a n i c f i e l d . The l a r g e s t o f t h e s e a r e t h e n o r m a l f a u l t s w h i c h
d e f i n e t h e Avon V a l l e y range f r o n t , and t h e E e l m s v i l l e V a l l e y g ra b e n
f a u l t which t o g e t h e r t r u n c a t e t h e n o r t h e a s t b oundary o f t h e f i e l d .
F i g u r e s 18,
19, and 20 i l l u s t r a t e
t h e v o l c a n i c s t r a t i g r a p h y and
fa u ltin g of these volcanic ro ck s.
The d a c i t e s , t u f f s , a nd a n d e s i t e s o f t h e e a s t e r n G a r n e t Range
v o l c a n i c s were s u b d i v i d e d i n t o f i v e mappable u n i t s :
non-porphyritic
dacite,
dacite
autobreccia,
d a c i t e po rp hy ry ,
andesite
and
tu ff.
E r u p ti o n s o f a n d e s i t e and d a c i t e o c c u r r e d p e n e c o n t e m p o r a n e o u s l y , a s
e v id en c e d by th e r e p e t i t i v e and a l t e r n a t i n g sequence o f a n d e s i t e and
d a c ite flow u n i t s .
A l t e r n a t i n g e r u p t i o n s o f a n d e s i t e and d a c i t e onto
NJ
F i g u r e 17.
Map o f T e r t i a r y v o l c a n i c r o c k s i n and n e a r t h e e a s t e r n G a r n e t R an ge ,
i l l u s t r a t i n g t h e p r o x i m i t y o f t h e e a s t e r n Garnet Range v o l c a n i c f i e l d to t h e
s i m i l a r f i e l d t o t h e west .
Both v o l c a n i c f i e l d s were once a s i n g l e d e p o s i t
which has s i n c e be en h i g h l y d i s s e c t e d and eroded (m odified a f t e r M e j s t r i c k ,
w r i t t e n communication, 1982; Ross and o t h e r s , 1955; and t h i s a u t h o r ) .
2250.
2000-
Oth3
1750-
U)
1500-
S e d i m e n t a r y Bedrock
Me te rs
F ig u re 18.
V o l c a n i c c r o s s s e c t i o n VA-VA'.
(See P l a te 2 f o r k e y .)
VB W
VBE
-2250
-
200 0
1750
Sedimentary
Bedrock
1500
k Avon
\ Val l ey
iGr abe n
Metera
F i g u r e 19
V o l c a n i c c r o s s s e c t i o n VB-VB1.
(See P l a t e 2 f o r k e y .)
VC S W
2250
-2000
?
o’ T Ta
Avon
Va ll ey
Graben
-1750
1500
Se dimentary Bedrock
Me te rs
F ig u re 20.
V o l c a n i c c r o s s s e c t i o n VC-VC1.
(S ee P la te 2 f o r k e y .)
46
an i r r e g u l a r s u r f a c e r e s u l t e d i n an a n d e s i t e b a s a l u n i t i n some p l a c e s
and a d a c i t e b a s a l u n i t i n o t h e r s . D a c i t e porphyry i s t h e l o w e s t u n i t
e x p o s e d a t t h e f a u l t bound e a s t e r n m a r g i n o f t h e f i e l d , and i s a l s o
exposed a t th e lo w e st v o l c a n i c - s e d i m e n t a r y bed ro ck c o n t a c t in th e
area,
giving
the
im pression th a t
dacite
v o lc a n i c rock type d e p o s ite d in th e a re a .
p o r p h y r y was t h e
first
This u n i t i s g e n e r a l l y
o v e r l a i n by an e x t e n s i v e a n d e s i t e u n i t , a h i g h e r s e q u e n c e o f d a c i t e
flow s,
a second l a r g e a n d e s i t e u n i t ,
exposed a t t h e to p o f th e p i l e .
a nd a d a c i t e p o r p h y r y u n i t
However, t h i s g e n e r a l seq u e n c e i s
c o m p li c a te d by r a p i d l y t h i n n i n g u n i t s and t h e p r e s e n c e o f minor fl o w s
and t u f f s which acc ou nt f o r m i s s i n g and e x t r a u n i t s i n most l o c a t i o n s
t h a t c o n t a i n good e x p o su r e s o f t h e sequence.
Dacite a u to b re c c ia s cut
a c r o s s m o s t o f t h e s e u n i t s and a r e , t h e r e f o r e y o u n g e r t h a n many o f
them.
I n t h e w e s t e r n p a r t o f t h e map a r e a ( P l a t e 2) a n d e s i t e d i r e c t l y
o v e r l i e s pre -Eocene s e d im e n ta ry r o c k s , e x c e p t n e a r t h e F o u r t h o f J u l y
Ridge where a t h i n d a c i t e b a s a l u n i t i n t e r v e n e s .
Sedimentary bedrock
i n t h e e a s t e r n p a r t o f t h e a r e a i s m o s t l y o v e r l a i n by d a c i t e , a l t h o u g h
a n d e s i t e s and t u f f s i n t h e s o u t h e a s t q u a r t e r o f t h e map a r e a for m
local basal units.
E r o s i o n has exposed P r o t e r o z o i c u n i t s o v e r l a i n by
a n d e s i t e and d a c i t e i n G a l l a g h e r Creek.
No a s s o c i a t e d v o l c a n i c l a s t i c
s ed im en ta ry u n i t s d e r i v e d from eroded v o l c a n i c u n i t s a r e found i n t h e
Warm S p r in g s Creek a r e a o f t h e e a s t e r n Ga rnet Range, a l t h o u g h p o o r l y
i n d u r a t e d v o l c a n i c l a s t i c T e r t i a r y s a n d s t o n e s occu r i n t h e Avon V a l l e y
i n a s s o c i a t i o n w i t h v o l c a n i c c o b b l e s and a l l u v i u m .
47
D a c it e po rp hyr y.
D a c i t e porphyry i s t h e most voluminous v o l c a n i c
l i t h o l o g y in the area.
I t i s composed o f a t l e a s t t h r e e d i s t i n c t f l o w
u n i t s s e p a r a t e d fr o m e a c h o t h e r by a n d e s i t e and o t h e r d a c i t e f l o w s
( F i g s . 18, 1 9, and 20 ).
D a c i t e p o rp h y ry i s an a p h a n i t i c , r e d , g r e e n ,
or gra y ( l o c a l l y banded) u n i t c o n t i n i n g w h i t e p l a g i o c l a s e p h e n o c r y s t s .
Color is i n d i c a t i v e of th e degree of a l t e r a t i o n , grey being l e a s t
a l t e r e d and r e d r e p r e s e n t i n g p e r v a s i v e h e m a t i z a t i o n .
S m a ll b i o t i t e
f l a k e s a r e common, w h i l e h o r n b l e n d e o r a l t e r e d amp hib ole s a l s o occu r
i n some samp le s.
This u n i t w e a t h e r s i n t o b l o c k s o r p l a t e s , w i t h some
o u tc r o p s d e v e l o p i n g i n t o hoodoos and s p i r e s .
Irregular, sub-parallel
j o i n t s w i t h hem atize d s u r f a c e s a p p ea r t o ha ve d e v e l o p e d by e x p l o i t i n g
p l a n a r m i n e r a l f lo w t e x t u r e s .
Flow u n i t s a r e 10 t o 250 m e t e r s t h i c k
and a r e h o r i z o n t a l .
D acite.
v ariatio n
The
of
the
non-porphyritic
d acite
p e tr o g r a p h ic a l Iy s i m i l a r ,
dacite
porphyry
it
unit
appears
u n it.
A lthough
forms d i s t i n c t
flow u n i t s .
to
be
it
a
is
Three
s e p a r a t e d a c i t e f l o w u n i t s seem t o be p r e s e n t .
Owing t o t h e s m a l l
s i z e o f e r o s i o n a l remnants o f t h e s e d a c i t e u n i t s ,
c o r r e l a t i o n between
e x p o s u r e s i s u n c e r t a i n , and t h e e x a c t number o f d a c i t e f l o w u n i t s
c ou ld n o t be e s t a b l i s h e d .
T his n o n - p o r p h y r i t i c , g r e y , a p h a n i t i c rock
commonly c o n t a i n s b i o t i t e
flakes,
and i n p l a c e s amphibole c r y s t a l s .
I t c o n t a i n s l e s s t h a n 10% p l a g i o c l a s e p h e n o c r y s t s , as compared t o more
th a n 10% f o r t h e p o r p h y r i t i c f l o w s .
exploited
groundmass.
flow
planes
delineated
U n d u l a t o r y j o i n t s a p p e a r t o ha ve
by
aligned
m inerals
in
The non p o r p h y r i t i c d a c i t e forms n e a r l y h o r i z o n t a l
u n i t s 5 t o 100 m e t e r s t h i c k .
the
flow
48
Dacite a u to b reccia.
The d a c i t e a u t o b r e c c i a i s p e t r o g r a p h i c a l l y
s i m i l a r to the hem atized, red d a c it e porphyry.
I t is d istinguished
from th e re d porphyry by i t s b r e c c i a t e d t e x t u r e , i t s t h i c k n e s s and i t s
discordant nature.
the a u to b re c c ia
F i g u r e s 18, 19, and 20 i l l u s t r a t e t h e te n d en c y o f
to cut p e r p e n d i c u l a r l y a c ro ss the
h o r i z o n t a l fl o w and t u f f u n i t s .
la y e r s of the
The b r e c c i a s commonly form d i k e —l i k e
b o d ie s w ith a le n g t h to w id th r a t i o o f about t h r e e to one.
bo di e s a r e i r r e g u l a r masses up t o 230 m e t e r s t h i c k ,
These
and u s u a l l y l e s s
t h a n 1500 sq uar e m e t e r s i n map p l a n .
Exposures of d a c i t e porphyry a u t o b r e c c i a a r e composed o f b l o c k s
o f h i g h l y he m at iz e d, r e d d a c i t e porph yr y (from I t o 1,000 c e n t i m e t e r s
maximum d i m e n s i o n ) e n c a s e d i n a m a t r i x o f r e d , c r y s t a l l i n e , d a c i t e
porphyry which c omp ris es ab out 25% o f t h e t o t a l r oc k.
These b r e c c i a s
may h a v e o r i g i n a t e d by l a v a p i c k i n g up f r a g m e n t e d a u t o b r e c c i a t e d
d acite blocks,
form ing a d e p o s it o f d a c i t e frag m en ts in s l i g h t l y
younger c r y s t a l l i n e d a c i t e m a t r i x .
A ndesite.
andesite.
less
The s e c o n d m o s t v o l u m i n o u s v o l c a n i c r o c k t y p e i s
I t i s composed o f two major f l o w u n i t s and s e v e r a l t h i n n e r ,
e x te n s iv e flow u n i t s .
The two m a j o r f l o w u n i t s o c c u p y t h e
e a s t e r n and w e s t e r n marg ins o f t h e f i e l d and a r e s e p a r a t e d by s e v e r a l
d a c i t e and minor a n d e s i t e f l o w u n i t s ( P l a t e 2 and F i g s . 16 and 17).
The a n d e s i t e s a r e a p h a n i t i c and r a n g e i n c o l o r fr o m b l a c k t o
g r e y , and l e s s commonly r e d .
b io tite
are
H y p e r s t h e n e , a u g i t e , h o r n b l e n d e , and
common a c c e s s o r y
m in erals,
but
are
m e g a s c o p i c a l I y v i s i b l e due to t h e s m a l l c r y s t a l s i z e .
not
alw ays
A n d e s i t e fl o w s
v a r y fr om h i g h l y v e s i c u l a r t o m a s s i v e and w e a t h e r i n t o i r r e g u l a r
49
masses, b l o c k , and p l a t e s .
amygdule f i l l i n g , .
Some . c o r i a c e o u s e x po su r es c o n t a i n o p a l i n e
Columnar j o i n t i n g i . r a r e , a l t h o u g h aome o u t c r o p ,
e x h i b i t column, up t o t h r e e m e t e r , i n l e n g t h and t w e l v e c e n t i m e t e r ,
thick.
Flow u n i t e range from 10 t o 225 m e t e r , t h i c k .
Ho a n d e s i t e e x t r u s i v e c e n t e r was f o u n d w i t h i n t h e a r e , ma ppe d,
a l t h o u g h an eroded a n d e . i t e cone i . l o c a t e d ab out 20 k i l o m e t e r , t o th e
west i n a r e l a t e d v o l c a n i c f i e l d which wa, p r o b a b l y once c o n ti g u o u s
w it h t h e e a s t e r n f i e l d ( M e j . t r i c k . P e r s o n a l Communication. 1 9 82).
lu ff.
T u f f , a r e g e n e r a l l y composed o f a c r y s t a l l i n e m a t r i x
surrounding
fractu red
com position.
o r w hole
This v a r i e t y
form,
cry stals,
irreg u larly
and a r e d . c i t i c
shaped,
th in
in
flow
d e p o s i t s in te rb e d d e d between l a v a flo w u n i t , o r f o r , t h i n b a s a l
sheets.
Alth oug h t u f f s r e s e m b l e d a c i t e f l o w , m i n e r . l o g i c a l l y ,
they
a r e commonly w h i t e o r e x h i b i t h i g h l y d e v e l o p e d fl o w banding i n sh a d e ,
o f t a n , r e d . and brown from .1 t o 3 c e n t i m e t e r , t h i c k .
S m a ll b a s a l ash f a l l t u f f d e p o s i t s n e a r Deer P a r k a n d G r a v e l y
Mountain d i s p l a y a wide r a ng e o f c o m p o s i ti o n s and t e x t u r e s .
V arieties
i n c l u d e banded c r y s t a l d a c i t e t u f f , and l a p i l l i t u f f , w i t h minor b l a c k
v o lc a n i c g l a s s shard accumulations.
A l a p i l l i t u f f composed o f 501
w hi te . c o r i a c e o u s l a p i l l i i n a m a t r i x o f y e l l o w unwelded v o l c a n i c ash
and a n g u l a r q u a r t s fr agm en ts domin ate s t h e a r e . s o u th o f Deer P a r k .
Petrography
The f i v e v a r i e t i e s o f v o l c a n i c r o c k s r e c o g n i z e d i n t h e G a r n e t
Range ( a n d e s i t e ,
d a c i t e p o r p h y r y , d a c i t e , d a c i t e a u t o b r e c c i a , and
t u f f ) a r e d e s c r i b e d below.
The r e s u l t s o f m i c r o s c o p i c ex am i n at io n o f
50
t h e s e u n i t s i s s u m m a ri z ed i n T a b l e s I t h r o u g h 4.
c o m p o s itio n s w ere
Rock t y p e s and
d e te r m in e d through m icroscopic p e tro g ra p h ic
t e c h n i q u e s and chemi cal a n a l y s i s #
M ineral p ercen tag es in d ic a te d in
T ables
on
I
through
4 are
based
v isu al
estim ates.
A northite
p e r c e n t a g e s i n p l a g i o c l a s e were d e te r m in e d u s i n g t h e M ic h e l Levy
M ethod
(M oorehouse,
1959)
for
t h e m o s tly unzoned m ic ro s c o p ic
groundmass l a t h s , and by d e t e r m i n i n g t h e r e f r a c t i v e i nd e x o f t h e zoned
macro sco pic p h e n o c r y s t s (Slemmons,
t h e m a pp in g u n i t s e s t a b l i s h e d
1962).
These t e c h n i q u e s su p p o rt e d
b a s e d on f i e l d
relatio n sh ip s
and
ma croscopic m i n e r a l o g y .
The v o l c a n i c u n i t s o f t h e e a s t e r n G arnet Range a r e d e s c r i b e d i n
o r d e r o f r e l a t i v e abundance ( g r e a t e s t t o l e a s t ) i n t h e d e s c r i p t i o n s
below.
D a c i t e a u t o b r e c c i a s a r e s i m i l a r t o o t h e r d a c i t e porphyry u n i t s
m iners l o g i c s I l y ,
m ineralogical
and do n o t m e r i t a s e p a r a t e d i s c u s s i o n .
T heir
c o m p o s i t i o n i s a d e q u a t e l y d e s c r i b e d by t h e d a c i t e
porphyry s e c t i o n below.
D a c it e po rp hyr y.
T hi s r o c k c o n t a i n s 5 0 -7 OZ a p h a n i t i c groundmass.
Most o f t h e groundmass i s composed o f m i c r o s c o p i c p l a g i o c l a s e
(oligoclase-andesine),
but i t
also
laths
in c lu d e s m icroscopic fib ro u s
amp hibo le, b i o t i t e , m a g n e t i t e and i n some samples q u a r t z .
Macroscopic
m i n e r a l o g y i n c l u d e s b i o t i t e , a m p h i b o l e , and e u h e d r a l
to anhedral,
embayed, zoned, and p o l y s y n t h e t i c a l l y - t w i n n e d p l a g i o c l a s e p h e n o c r y s t s
( o l i g o c l a s e - a n d e s i n e ) which r e p r e s e n t an e a r l y c r y s t a l i z a t i o n e v e n t .
B iotite,
tex tu res
amphibole and p l a g i o c l a s e l a t h s g e n e r a l l y e x h i b i t t r a c h y t i c
around
the
larg er
p lag io clase
phen o cry sts.
D acite
T ab le I .
M i c r o s c o p i c P e t r o g r a p h y o f e a s t e r n G a r n e t Range
AMPHIBOLE
D a c ite P o rp h y ry .
QUARTZ
EARLY PLAGIOCLASE
LATE PLAGIOCLASE
P erce n tag e
10-152
30-60%
0-152
0-1 52
.1-22
0-52
S ize
2x3mm-.5x1mm
.lx.5mm to .025x
.25mm
.2xlmm to . l x
.2xlmm t o
.Ix.lm m
0 5 .mm to Imm
• I HHD to
. 25mm
Shape
E uhedral t o anhed r a l & embayed
prism s
Euhedral l a t h s
F ib e ro u s to
euhedral p r i s ­
m a tic
Subhedral
flak es
Euhedral
F ractu red ,
a n g u l a r an
hedral
T r a c h y t ic
T r a c h y t ic
T rsch y tic
F ib e ro u s amphib o le & b i o t i t e
pseudomorphs,
h em atitiz a tio n ,
m a g n e i t i t e rims
H em a tiz atio n
clay m in e r a li­
z a tio n
MINERALS
T ex tu re s
BIOTITE
MAGNETITE
Zoning
Zoning
A lteratio n
Kaol i n i t i z a t i o n ,
c l a y , m a g n e tite
rimming
K ao lin itiz atio n ,
h e m a ti z a tio n
Twinning
P olysynthetic
P olysynthetic
In clusions
M agnetite
A n o rt h ite
P e r c e n t­
ages
26-34% ( Phenoc r y s t p o p u la tio n
24-32% ( l a t h s )
Comments:
E a r l y p l a g i o c l a s e p h e n o c r y s ts r e p r e s e n t an e a r l y c r y s t a l i z a t i o n e v e n t . Red d a c i t e s a r e h i g h l y o x id iz e d
t o h e m a ti te s t a i n e d c l a y m i n e r a l s , k a o l i n i z e d p l a g i o c l a s e and a l t e r e d b i o t i t e .
■
m
T ab le 2 .
M i c r o s c o p i c P e t r o g r a p h y o f e a s t e r n G a r n e t R ange D a c i t e
MINERALS
EARLY
PLAGIOCLASE
LATE
PLAGIOCLASE
AMPHIBOLE
BIOTITE
QUARTZ
FELDSPAR
GLASS
MAGNETITE
Percentage
0-10%
40-7 5%
5-20%
5-20%
0-1%
0-5%
0-15%
5%
Size
2x2mm.25x.75nnn
.25%.25mme025x.25mm
•3 x 2 tcd.Ix .Imm
#25x2mme15x aI5mm
I ,2x1.5mme25x.7 5mm
5x5mm2.2mm
Submicroscopic
#I-a 025mm
Shape
Euhedral to
subhedral em­
bayed prisms
Euhedral lathe
Fiberoue to
euhedral
Euhedral
prismatic
Irregular
blebs
Euhedral to
aub-hedral
priamatic
Amorphous
Euhedral
octohedral
Trachytic
Fractures
Undulatory
extinction
Hematized
Secondary
quartz
rims,
calcite
embayments
Resorption?
Partly al­
tered to
clay &
hematite
Textures
Trachytic
Fracturea
Zoning
Zoning
Alteration
Kaolinization,
sericite,
magnetite,
rimming
.
Twinning
Polyaynthetic
Polyeynthetic
Inclusions
Magnetite
Magnetite
Anorthite
Percentage
3 5 -3 8 %
Andeaine
Zoning rare
Some hematite
taericite;
- mostly along
cleavage
Hematite,
epidote, illmenite, I
actinolite
pseudomerpe of
hornblende,
and augite
Magnetite
hornblende
biotite
Early plagioclaee phenocryets represent an early crystalleation event. Much of the amphibole may represent alteration
of other mafic minerals. Sub-microscopic fractures are found in the plagioclase groundmass of some samples.
T ab le 3 .
M i c r o s c o p i c P e t r o g r a p h y o f e a s t e r n G a r n e t R ange A n d e s i t e
MINERALS
PLAGIOCLASE
AUGITE &
HYPERSTHENE
AMPHI BOLE
GLASS
MAGNETITE
P e r c e n ta g e
50-1OOZ
0-15%
0-1%
0-30%
1-5%
S iz e
.25x1.2m o-.05-.lm m
.25x.25nmi
.lx.25mm
Sub-m icroscopic
. 1 - . 025mm
Shape
E uhedral l a t h s
Mostly e u h e d r a l ,
some i r r e g u l a r
altered c ry sta ls
F iberous
n e e d le s
Amorphous
Euhedral
octohedral
T e x tu re s
Flovage d e r iv e d s u b p a r a l l e l alignm ent
Zoning
O c c a sio n a l zoning
A lte ra tio n
Rot common, c a l c i t e
r e p la c e m e n t, k a o l i n i t e ,
se ric ite
Some h i g h l y a l t e r e d
r e p la c e d by s e c o n d a r y a o p h ib o le and
clays
Replaced by
b i o t i t e and
ca lc ite
K ao lin ite,
h e m a ti te
Twining
P olysnthetic
A u g ite tw inning
In clusions
B i o t i t e , m a g n e ti te
A nortbite
P erc e n ta g e
H ighly v a r i a b l e from flow to flow ; 21-51%; commonly about 31-33%,
O li g o c l a e e , A n d e sin e , low sodium L a b r a d o r i t e
Comments:
S u b -m icroscopic flow s t r u c t u r e s r e a c h h i g h e s t developm ent in t h i s u n i t . Opal and c h a lc ed o n y
f i l l i n g s in some s c o r ia c e o u s o r f r a c t u r e d u n i t e .
S u b -m icro sco p ic g l a s s i s u s u a l l y s e v e r e l y
• I t e r e d o r h e m a ti te s t a i n e d . Two p e r c e n t (.5-5 mm) c r y s t a l l i n e q u a r t s found i n one sa m p le , i s
n o t r e p r e s e n t a t i v e o f e a s t e r n G a r n e t Range a n d e s i t e . One f l o w (S.V. 1 / 4 S e c t . 3 4 T. 11 N., R.
9 W.) i e much r e p l a c e d by c a l c i t e (35% by v o lu m e ) and may r e f l e c t a l t e r a t i o n by l a t e o r p o s t
v o lc a n ic h y d ro th erm a l f l u i d s r e s p o n s i b l e f o r eroded t r a v e r t i n e and m a rl d e p o s i t s n e a r b y .
T ab le 4 .
M i c r o s c o p i c P e t r o g r a p h y o f e a s t e r n G a r n e t Range T u f f s .
MINERALS
FLAGIOCLASE
PHENOCRYSTS
PLAGIOCLASE LATHS
AMPHIBOLE
BIOTITE
GLASS
MAGNETITE
Percentage
5-1OZ
75-85Z
0-5%
5-1OZ
0-5%
O-IZ
Size
2x3nn-.25x.5mo
.lx . 5n-.025x.lm m
.75x.2mm
.7 5 x l n - . 0 2 5 x .l n
Subm icroscopic
.0 2 5 -.5mm
Shape
Euhedral
p rism atic
Euhedral la th s
Euhedral
prisma to
ra d ia tin g
fib ero u a
masses
Subhedral c r y s ta ls
broken fragments
Amorphous
Euhedral
Tezturea
Often highly
fra c tu re d
T rachytic
Zonation
Strongly
developed
A lte ra tio n
A lte ra tio n r i s e ,
e e r ic it e
K a o lin itiz a tio n
Fiberoua
amphibole
pseudomorphous
a f te r hornblende
Bematization
Some epidote
Twinning
P olysynthetic
P o ly sy n th etic
A northite
Percentage
22-25%
O ligoclase
Coonenta:
The above ta b le re p re se n ts c r y s t a l l i n e tu f f s , th e m a jo rity of e a s te rn Carnet Range tu f f s are c r y s t a l l i n e .
Scoriaceous and l a p i l l i tu f f s sre r e s tr ic te d to the southeast p o rtio n of the map. Most are h ig h ly a lte r e d
and now composed e n t ir e ly of replacem ent m a te r ia l, c la y s , fib ero u a amphibole, ep id o te, and some z e o lite s .
Near Deer Park a l a p i l l i tu f f is composed of SOZ w hite sc o ria to pumice l a p i l l i (.3-3an) in a h em atite and
lim e n ite s ta in e d ash m a trix . A n g u lar q u a r ts frag m en ts (.l-.Smm) and red and brown c r y s t a l l i n e h e m a tite
pseudomorphe of b i o t i t e ( . S n ) are c o n o n .
-
•
I
55
au to b reccias
are
sim ilar
to
o th er
d acite
porphyry
u n its
m ineralogically.
Andesite.
The groundmass o f t h e s e a p h a n i t i c u n i t s a r e composed
m o stly of p o l y s y n t h e t i c a l Iy tw inned, e u h e d r a l, p l a g i o c l a s e l a t h s .
They r a n g e i n c o m p o s i t i o n fr o m o l i g o c l a s e t o l a b r a d o r i t e , a l t h o u g h
most
flow s
constituents
contain
andesine
i n c l u d e py ro xe ne s,
p lag io clase.
m agnetite,
O ther
groundm ass
and v o l c a n i c g l a s s .
The
p y r o x e n e s ( w h i c h a r e common i n a n d e s i t e u n i t s ) a r e s o m e t i m e s l a r g e
eno ugh t o be d e t e c t e d w i t h t h e n a k e d e y e .
Most s a m p l e s e x h i b i t a
strong tr a c h y tic te x tu re .
D a c it e .
The a p h a n i t i c groundmass of d a c i t e s comprise 70-90% o f
the e n t i r e rock.
G ro un dm as s m i n e r a l s i n c l u d e p o l y s y n t h e t i c s I I y
t w i n n e d e u h e d r a l p l a g i o c l a s e l a t h s ( a n d e s i n e ) , a m or phou s v o l c a n i c
g l a s s , m a g n e t i t e , b i o t i t e and a m p h i b o l e .
M acroscopic m ineralogy
commonly i n c l u d e s a s m a l l p o p u l a t i o n o f twinned co rro de d p l a g i o c l a s e
p h e n o c r y s t s , p o t a s s i c f e l d s p a r p r i s m s , i r r e g u l a r q u a r t z b l e b s and
sm all
cry stals
of b i o t i t e
a nd a m p h i b o I e .
Ground ma ss m i n e r a l s
g e n e r a lly e x h ib it tr a c h y tic t e x t u r e s around th e l a r g e r p l a g i o c l a s e
crystals.
Tuffs.
T u f f s v a r y i n c o m p o s i t i o n more t h a n any o t h e r map u n i t .
C r y s t a l l i n e t u f f s c o n t a i n groundmass p l a g i o c l a s e l a t h s and p h e n o c r y s t s
o f o l i g o c l a s e , which may account f o r up to 90% o f t h e r oc k by volume.
P l a g i o c l a s e c r y s t a l s a r e m o s t l y e u h e d r a l l a t h s and pris ms up t o t h r e e
m i l l i m e t e r s a c r o s s and many i n d i v i d u a l g r a i n s a r e f r a c t u r e d .
Twinning
56
and z o n i n g i s common, e s p e c i a l l y i n t h e p l a g i o c l a s e p h e n o c r y s t s .
B i o t i t e i s a common a c c e s s o r y m i n e r a l , a l t h o u g h m a g n e t i t e , a m p h ib o le s ,
and v o l c a n i c g l a s s a r e a l s o found i n some sa mp le s.
G lass shards are
uncommon i n t h i s type o f c r y s t a l f l o w t u f f , which sometimes e x h i b i t s
banding in
colors
of re d ,
brown,
and t a n ,
c e n t i m e t e r s t h i c k . ' Less common a sh f a l l
r a n g i n g fr om .1 t o 3
deposits,
containing g la s s
s h a r d s , a n g u l a r q u a r t z fr a g m e n ts , and s c o r i a c e o u s l i t h i c fr ag m e nts a r e
prominent n e a r Deer Pa rk and e l s e w h e r e ne ar by i n t h e s o u t h e a s t p a r t o f
t h e a r e a mapped.
Discussion.
are e v id e n t.
plagioclase
Several s i m i l a r i t i e s w ithin in d iv id u al
lithologies
A n d e site s c o n ta in the h ig h e s t p e rc en ta g e of t o t a l
o f any u n i t ,
subm icroscopic
as w e ll
as th e h ig h e s t
groundmass m a t e r i a l .
percentage
Common a n d e s i t e
of
accessory
m i n e r a l s i n c l u d e a u g i t e and h y p e r s t h e n e , w h i l e d a c i t e u n i t s commonly
c o n t a i n b i o t i t e and a l t e r e d a m p h i b o l e s .
P la g io c la s e phenocrysts in
d a c i t e u n i t s a r e a b o u t h a l f e u h e d r a l , and h a l f s u b h e d r a l p r i s m s .
S ub he dra l p l a g i o c l a s e p h e n o c r y s t s ha ve a l t e r a t i o n r i m s , a r e c o r r o d e d ,
and p r o b a b l y r e p r e s e n t an e a r l i e r c r y s t a l l i z a t i o n e v e n t t h a n t h e
euhedral phenocrysts.
S m aller l a t h s of p l a g io c la s e e x h ib it tr a c h y tic
and f l o w t e x t u r e s a r o u n d t h e l a r g e r p l a g i o c l a s e p h e n o c r y s t s and
therefore
are
a
la te r
generation.
Flo w
stru ctu res
are
common
th r ou gh ou t t h e samples and a r e b e s t d e v e l o p e d i n th e n o n - s c o r i a c e o u s
a n d e s i t e s and i n some d a c i t e s .
Dar k c o l o r e d ( b l a c k and d a r k brown) a p h a n i t i c e a s t e r n G a r n e t
Range v o l c a n i c s were c a l l e d a n d e s i t e f o r f i e l d mapping p u rp o s e s . The
57
l i g h t e r c o l o r e d ( r e d , g r e e n , a nd g r e y ) p o r p h y r i t i c u n i t s a r e more
s i l i c e o u s and were termed l a t i t e d u r in g f i e l d mapping.
However, t h e
s i g n i f i c a n t p r o p o r t i o n s of s u bm ic ro sc opic and e x t r e m e l y f i n e - g r a i n e d
m ic ro s co pi c s c a l e groundmass m i n e r a l s i n t h e s e s am pl es , made t h e e x a c t
d e t e r m i n a t i o n of t h e modal m i n e r a l o g y of t h e s e samples u n c e r t a i n .
The
n o r m a ti v e m i n e r a l p e r c e n t a g e s d e te r m in e d f o r t h e f o l l o w i n g s e c t i o n a re
c o n s i d e r e d more d i a g n o s t i c o f t h e n a t u r e o f t h e s e s a m p l e s , t h a n t h e
r o c k names n o t e d a b o v e .
F i e l d map u n i t s b a s e d on p h y s i c a l r o c k
c h a r a c t e r i s t i c s a r e r e f l e c t e d i n c h e m i c a l c a t e g o r i z a t i o n s , a nd a r e
thus confirm ed as d i s t i n c t rock ty p e s a lth o u g h th e l i g h t e r c o lo r e d
u n i t s p ro ve d t o be d a c i t e s r a t h e r t h a n l a t i t e s .
Chemical A n a l y s i s
S i n c e e a s t e r n G a r n e t Range v o l c a n i c s
contain a s ig n ific a n t
p e r c e n t a g e of s u b - m i c r o s c o p i c to n e a r l y s u b m ic r o s c o p ic groundmass
m i n e r a l s , and be ca us e SiOg p e r c e n t a g e i s a c o n s t r a i n t o f t h e a n d e s i t e b a s a l t c a t e g o r y o f t h e I.U.G.S. system,
c hem ic al d a t a were o b t a i n e d i n
o r d e r t o p r o v i d e a n u n a m b i g u o u s means o f na m in g r o c k t y p e s b a s e d on
n o r m a ti v e m i n e r a l o g y and c hem ic a l t r e n d s .
These d a t a were t h e f i n a l
d e t e r m i n a n t o f l i t h o l o g i c a l c a t e g o r i e s used i n t h i s t h e s i s .
C h e m i c a l d a t a o b t a i n e d fr o m e i g h t e a s t e r n Ga rnet Range samples
(T ab le 5) i n d i c a t e t h a t t h e s e r o c k s ha ve s i l i c o n d i o x i d e p e r c e n t a g e s
t y p i c a l o f a n d e s i t e s and d a c i t e s when compared w i t h s i l i c o n d i o x i d e
p e r c e n t a g e s g e n e r a l l y a c c e p t e d f o r t h e s e r o c k t y p e s ( C a r m i c h a e l and
o t h e r s , 1 9 7 4 ).
The n o r m a t i v e m i n e r a l o g y o f e a c h u n i t ( T a b l e 6 ) was
d e t e r m i n e d by t h e C.I.P.W. m e t h o d ( J o h a n n s e n , 1 9 31).
When p l o t t e d on
T ab le 5.
C h e m ic a l a n a l y s i s o f e i g h t v o l c a n i c r o c k s a m p l e s t a k e n fro m t h e e a s t e r n G a r n e t R a n g e .
SAMPLE
#
SiO2
AI2O3
Ti02
Fe2O3
FeO
MnO
CaO
MgO
K2O
Na2O
P2O5
I
55.34
17.26
1.07
3.15
3.6
1.3
6 .8 8
3.78
4.03
3.78
0.92
2
67.68
16.12
0.5
1.57
1.79
0.06
3.53
1.71
3.38
3.52
0.14
3
71.64
16.45
0.45
0.27
0.31
0 .0 2
2.47
0.28
4.76
3.1
0.24
4
58.91
17.16
1.1
2 .88
3.3
0.08
5.37
4.52
2.74
3.55
0.4
5
70.93
17.48
0.63
1.95
2.23
0 .0 2
0.58
0.24
3.72
2.01
0 .1
6
66.95
17.96
0.62
1.99
2.28
0.01
2.86
0.58
2.99
3.48
0.28
7
65.28
17.42
0.64
1.99
2.28
0 .0 2
3.37
1.24
3.48
3.93
0.34
8
71.23
24.76
0.82
0.84
0.96
0.01
0.38
0.03
0.14
: 0.57
0 .2 2
1.
2.
3.
4.
5.
6.
7.
8.
Black, aphanitic andeeite from aouthweat 1/4 Section 34, T. 11 N., R. I V.
Gray, aphanitic dacite from aouthweat 1/4 Section 32, T. 11 N., R. 9 W.
Gray, elightly porphyritie to aphanitic dacite from aouthweat 1/4 Section 24, T. 11F. R. 10 W.
Black aphanitic andeeite from aouthweat 1/4 Section 29, I. 11 N., R. 9 V.
Red porphyritie dacite from eoutheaet 1/4 Section 25, I. 11 H., R. 10 V.
Green porphyritie dacite from aouthweat 1/4 Section 14, T. 11 N. R. 10 W.
Gray, alightly porphyritie to aphanitic dacite from eoutheaet 1/4 Section 19, T. 11N., R. 9 W.
Brown and tan banded dacite porphyrycryetal tuff from northweet 1/4 eection 16, I. 11 N., R. 9 W.
59
t h e I.D.G.S. c l a s s i f i c a t i o n system (Fig.
21) t h e more s i l i c i c
samples
(which commonly c o n t a i n b i o t i t e ) p l o t a s d a c i t e s , w h i l e t h e l e s s
s i l i c i c samples a r e a n d e s i t e s .
The m a f i c m i n e r a l o g y o f t h e s e samp les
r e f l e c t s t h i s c a t e g o r i z a t i o n s i n c e d a c i t e s commonly c o n t a i n s m a l l
b i o t i t e o r h o r n b l e n d e c r y s t a l s , w h i l e pyroxene b e a r i n g a n d e s i t e s a r e
o f t e n found.
Table 6 .
No rmative m i n e r a l p e r c e n t a g e s f o r f i v e e a s t e r n Ga rnet Range
v o l c a n i c ro c k sam ples.
SAMPLES
Z MINERALS
I
—
23.9
32 .0
18.1
6 .7
Quartz
Orthoclase
A lbite
A northite
W ollastonite
Corundum
Enstatite
Ferrositite
Magn et ite
-
- —
.
-
2
4
6
7
23.1
2 0 .0
29.9
17 .5
0 .2
7.4
16.1
29.9
2 2 .8
1 .6
28.3
17.7
29.3
14.2
19 .0
20 .6
33 .0
16.7
—
—
5.8
4 .0
4 .6
4 .3
2 .0
2 .2
13.6
3.7
4.2
■
■I
3.8
1.4
0 .2
2 .9
1 .1
3 .1
2 .6
2 .9
D e u te r ic and p o s t - v o l c a n i c hyd ro th e rm a l a l t e r a t i o n has p r o b a b l y
r e s u l t e d i n n e t changes i n t h e c h e m i s tr y of some sa mp le s.
s a m p l e s 3 , 5, and 8.
especially
T h e s e m o s t s i l i c i c s a m p l e s , w h i c h c o n t a i n no
v i s i b l e f r e e q u a r t z , h a v e been s e c o n d a r i l y e n r i c h e d i n Si&g r e l a t i v e
to o t h e r o x id es th ro u g h a l t e r a t i o n .
For t h i s re a s o n th e s e sam ples
were not used i n d e t e r m i n i n g norm at iv e m in e ra lo gy o r chemical t r e n d s .
The v o l c a n i c s o f t h e e a s t e r n G a r n e t Range a r e " a l k a l i - c a l c i c "
according to t h e i r " a l k a l i - l i m e
B a rk e r and A r t h ,
1976).
i n d e x " ( F i g . 22) ( P e a c o c k , 1 9 3 1 ) ;
Th e s i l i c a
d io x id e,
a lk alin e
oxide
p e r c e n t a g e s f o r t h e s e v o l c a n i c s p l o t in b o th t h e c a l c - a l k a l i n e and
60
Qu a r t z
Mtfle m in erals l e t s than
Qua r tz -Al kali T r a c h l t * /
z
/ T r a e hl t e
Alkali T r a eh l t a /
Latlte
-Andeslte-Baeel
Trachlte
Alkali F e l d s p a r
Plagloclase
* e l M a f l e e by v o l u m e
Melal A ndesite
SS
Leue i- B e e a lt
— - Andesite
% of 8 10 3 by w e i g h t
F ig u r e 21.
The I.U.G.S. c l a s s i f i c a t i o n f o r s i l i c a s a t u r a t e d a p h a n i t i c
r o c k s s i m i l a r t o t h o s e i n t h e e a s t e r n G a r n e t Range. The
c l a s s i f i c a t i o n t r i a n g l e i s based on t h e modal m i n e r a l o g y
o f a p h a n itic rocks, w h ile the a n d e s i t e - b a s a l t category is
f u r t h e r d i f f e r e n t i a t e d by m i n e r a l o g y and s i l i c a
p e r c e n t a g e . N o r m a t i v e m i n e r a l o g y d e te r m in e d f o r e a s t e r n
G a r n e t Range v o l c a n i c r o c k s p l o t i n t h e d a c i t e and t h e
a n d e s i t e - b a s a l t t r a p e z o i d s (from E h l e r s and B l a t t , 1982) .
10
Alkal ine Field
• I A
Alkaline
Cal c- Al kal i ne
Field
P e r c e n t of
CaO
,
a n d Alkaline
Oxides
T ho l e l l t l c Field
TC eO
A IkeM e-C elcle
C ele-A lkellm e
C elele
P e r c e n t 8102
F i g u r e 22.
G raph o f CaO a nd Na^O + K2O wA" a g a i n s t S iO 2 f o r f i v e s a m p l e s o f e a s t e r n
G a r n e t Range v o l c a n i c r o c k s . The i n t e r s e c t i o n o f t h e a l k a l i n e and c a l c i u m
o x i d e l i n e s d e t e r m i n e s t h e P e a c o c k I n d e x ( P e a c o c k , 1931) o f a s u i t e o f
v o l c a n i c ro c k s . For t h e e a s t e r n G a rn e t Range v o l c a n i c f i e l d t h e Peacock Inde x
i s ab out 55-56% SiO2, i n d i c a t i v e o f an a l k a l i c - c a l c i c s u i t e . A c l a s s i f i c a t i o n
d i s c r i m i n a t i n g between a l k a l i n e , c a l c - a l k a l i n e and t h o l e i i t i c r o c k s (B a r k e r
and A r t h , 1976) and based on K2 + Na2O "A" v s . SiO2 i s i n c l o s e agreement w i t h
t h e Peacock c l a s s i f i c a t i o n f o r e a s t e r n G arnet Range v o l c a n i c ro c k s s i n c e most
e a s t e r n Garnet Range ro c k s p l o t n e a r t h e b o r d e r between th e a l k a l i n e and c a l c a l k a l i n e f i e l d s of t h i s c l a s s i f i c a t i o n .
Both c l a s s i f i c a t i o n s i n d i c a t e t h a t
t h e e a s t e r n G a r n e t Range v o l c a n i c f i e l d c o n t a i n s s u b - a l k a l i n e r o c k s w h o s e
p e r c e n t a g e o f a l k a l i n e o x id e s i s h i g h (compare w i t h F i g u r e 23) ( a f t e r B a r k e r
and A r t h , I 976).
62
alk alin e
fie ld
(F ig.
22)
of
B arker
and A r th
(1976).
c l a s s i f i c a t i o n s i n d i c a t e a somewhat a l k a l i n e s u i t e of v o l c a n i c s .
Both
An
AFM diagram o f t h e s e v o l c a n i c s (F ig. 23) d e n o te s a c a l c - a l k a l i n e s u i t e
o f ro c ks w i t h a somewhat a l k a l i n e t r e n d .
O r i g i n o f Volcanism
These s l i g h t l y a l k a l i c i n t e r m e d i a t e v o l c a n i c s may ha ve d e v e l o p e d
from f u s i o n o f c o n t i n e n t a l c r u s t .
C a l c - a l k a l i n e b a s a l t and r h y o l i t e
v o l c a n i c ro c k s n e a r t h e C l a r k Fork and L i t t l e B U c k f o o t R i v e r s range
from 29 t o 50 m i l l i o n y e a r s i n age.
Since t h e r a d i o m e t r i c ag es o f t h e
e a s t e r n Ga rnet Range f i e l d and t h e s e o t h e r v o l c a n i c s o v e r l a p , t h e y may
be r e l a t e d .
The y o u n g e r o f t h e s e n e a r b y b a s a l t s and r h y o l i t e s a r e
b e l i e v e d t o r e p r e s e n t r h y o l i t e - b a s a l t v o lc a n i s m g e n e r a t e d d u r i n g m id T e r t i a r y t o Recent e x t e n t i o n a l t e c t o n i c s (Chadwick, 1981), r a i s i n g th e
p o s s i b i l i t y t h a t t h e e a s t e r n Ga rnet Range v o l c a n i c s o r i g i n a t e d d u r i n g
extension.
However, t h e s e i n t e r m e d i a t e c o m p o s i t i o n c a l c - a l k a l i n e
v o l c a n i c s a r e to o o l d t o be c o r r e l a t e d w i t h m i d - T e r t i a r y e x t e n s i o n a l
tectonism .
A l t e r n a t e l y , many g e o l o g i s t s r e l a t e E o c e n e v o l c a n i c s i n t h e
n o r t h e r n Rocky Mountains t o a r c ma gmatism p r o d u c e d d u r i n g t h e d y i n g
p h a s e s o f L a t e C r e t a c e o u s and P a l e o c e n e o r o g e n y ( B u r c h f i e l , 19 82;
Lipman and o t h e r s , 1972).
I f t h e e a s t e r n Ga rnet Range v o l c a n i c s h a v e
an a r c - m a g m a t i c o r i g i n ,
t h e i r s l i g h t a l k a l i n i t y and i n t e r m e d i a t e
s i l i c a c o n t e n t may h a v e e v o l v e d t h r o u g h a s s i m i l a t i o n o f c r u s t a l
m a t e r i a l or d i f f e r e n t i a t i o n of th e r i s i n g magma.
The Laramide orogeny
e v i d e n t l y c r e a t e d s t r u c t u r a l and t o p o g r a p h i c s a g s o r g r a b e n s w h i c h
63
Iron O x i d e s
T hol e l l t l c
Calc-Alkaline
Alkallc
Alkaline Oxi des
F ig u r e 23.
Magnesium O x i d e s
This AFM diagram o f v o l c a n i c ro c ks from t h e e a s t e r n Garnet
Range d e m o n s t r a t e s t h e c a l c - a l k a l i n e n a t u r e o f t h e s e
rocks.
A lth o u g h t h e s e r o c k s a r e c a l c - a l k a l i n e , most
s am p les c o n t a i n enough a l k a l i n e o x id e s to p l o t n e a r th e
b o r d e r between t h e a l k a l i c and c a l c - a l k a l i n e f i e l d s .
64
a l l o w e d p r e s e r v a t i o n o f t h e e a s t e r n Garnet Range v o l c a n i c s . I t i s a l s o
l i k e l y t h a t when r i s i n g E o c e n e magmas w e r e e r u p t e d a s l a v a a t t h e
e a rth 's
surface,
they e x p l o i t e d w eaknesses d e v e lo p e d d u rin g th e
Laramide orogeny. Eocene e a s t e r n G a rn et Range v o l c a n i c r o c k p r o b a b l y
o r i g i n a t e d a s magmas p r o d u c e d a t t h e F a r a l l o n p l a t e w h i c h h a d b e e n
s u b d u c t e d b e n e a t h t h e c r a t o n d u r i n g t h e L a t e C e n o z o i c a nd E a r l y
T e r t i a r y d u r i n g t h e P a l e o c e n e Laramide orogeny. A l s o o f a r c magmatic
origin,
t h e c a l c - a l k a l i n e Lowland Creek v o l c a n i c f i e l d i s s i m i l a r i n
age and c o m posi tio n t o t h e Ga rnet Range d e p o s i t s ( F ig . 15).
Si nc e t h e
Lowland Creek v o l c a n i c s a r e c o n s i d e r e d p a r t o f t h e n o r t h e a s t - t r e n d i n g
Idaho-Montana Porphyry B e l t (F ig . 16),
t h i s b e l t may h a v e i n f l u e n c e d
th e place me nt o r t im in g o f t h e Ga rnet Range v o l c a n i c e r u p t i o n s .
65
HYDROTHERMAL GEOLOGY
Hydrothermal d e p o s i t s c o n s i s t i n g o f s i n t e r and t r a v e r t i n e oc cu r
i n s e v e r a l p l a c e s n e a r Warm S p r i n g s C r e e k ( P l a t e 2).
T ravertine is
more common th a n s i n t e r , t h e l a t t e r bei ng r e s t r i c t e d t o s m a l l d e p o s i t s
s c a t t e r e d th ro u g h o u t s e c t i o n s 26, 27, 34, 35, T. 11 N . , R. 9 v .
A modern t r a v e r t i n e d e p o s i t i s
l o c a t e d a t t h e s o u t h edge o f t h e
g e o l o g i c map ( P l a t e 2), where a w a t e r f a l l a l o n g Warm S p r i n g s Creek i s
d e v e lo p e d o v e r a hogback o f C re ta c e o u s san d s to n e .
Warm w a t e r s ( 2 3 -
24°C) emerge a t G a r r i s o n Warm S p r i n g s from bedrock and a l l u v i u m ab out
one k i l o m e t e r up s tre am , b u t do n o t d e p o s i t t r a v e r t i n e u n t i l th e y fl o w
over the f a l l s .
T h i s may be t h e r e s u l t o f a g i t a t i o n o f t h e w a t e r a t
t h e f a l l s , a l l o w i n g CO2 t o e s c a p e , and r e s u l t i n g i n c a l c i u m c a r b o n a t e
d e p o s i t i o n (C ha dw ic k and K a c z m a r e k , 1 9 75).
The v u g g y t o m a s s i v e
m i c r o c r y s t a l l i n e d e p o s it e x h i b i t s h o r iz o n ta l bedding, t r a v e r t i n e
layering
and " s t r o m a t o l i t e
c o n s i s t s of i r r e g u l a r ,
bedding".
discontinuous
This s t r o m a t o l i t e
bedding
l e n s e s and beds a bout t h r e e t o
f i v e c e n t i m e t e r s t h i c k and 50 t o 15 m e t e r s h o r i z o n t a l l y and w h i c h
p in c h ou t l a t e r a l l y o v e r o n l y a few c e n t i m e t e r s d i s t a n c e .
Trappin g o f
c a l c a r e o u s mud by g re e n a l g a e (which t h r i v e i n t h e s tre am ) i s p r o b a b l y
t h e cause of t h e i r r e g u l a r b e d d in g .
The volume o f w a t e r fl o w i n Warm S p r in g s Creek i s n o t i c e a b l y l e s s
i n t h e two k i l o m e t e r s a b o v e t h e s p r i n g s t h a n i t
s e v e r a l k i l o m e t e r s ups tre am (F ig. 24).
is
fo r the next
During summer months, t h e two
Z o n e of y e a r - r o u n d w ar m s p r i n g s
flow In s t r e a m b e d
Travertine depoelted
a t Ialla
Z o n e of s e a s o n a l s u r f a c e w a t e r flow
Springe
Z o n e of
year-round
surface water
flow
Spring R e c h a r g e
-ITSOMetera
- I SOOMetere
U e e a r F a l e e i e l e end M e e e i e l e
-ITSOMetere
M leileeleelee
eeraeeetie
-IO OOMetere
- TSOMetere
F i g u r e 24.
D i a g r a m m a t i c c r o s s s e c t i o n o f t h e G a r r i s o n warm s p r i n g s w a t e r c i r c u l a t i o n
s y s t e m , s h o w i n g t h e p o r t i o n o f Warm S p r i n g s c r e e k a l o n g w h i c h much o f t h e
s tr e a m s f l o w i s underground. Water c i r c u l a t i n g t o 500 m e te rs below t h e l a n d
s u r f a c e c o u l d be h e a t e d t o t h e t e m p e r a t u r e r e c o r d e d a t t h e s p r i n g s ( 2 4 e C)
w i t h o u t n e e d i n g t o r e a c h a s p e c i f i c h e a t s o u r c e due t o h e a t i n g by t h e l o c a l
g e o th e r m a l g r a d i e n t ( l eC/30 m e t e r s ) .
67
k i l o m e t e r s t r e t c h o f s t r e a m bed a b o v e t h e s p r i n g s i s u s u a l l y d r y ,
alth o u g h w ater runs y e ar-ro u n d in the next te n k ilo m e te r s of stream
above t h i s two k i l o m e t e r s t r e t c h .
Stream f l o w s t a r t s t o d i m i n i s h i n
a l l u v i u m i n t h e c re e k bed a t a bout t h e M i s s i s s i p p i a n - D e v o n i a n c o n t a c t
(F ig . 24).
The s p r i n g s a r e l o c a t e d a t t h e t o p o f t h e M i s s i s s i p p i a n
s e c t i o n , presumably b e c a u s e f u r t h e r s u b t e r r a n e a n t r a v e l d o w n s t r e a m
(down d i p ,
up s e c t i o n )
is
restricted
s i l t s t o n e s h ig h er in the s e c tio n .
bedrock and a l l u v i u m .
by t h e
shales,
clays
and
T h e s e s p r i n g s e m er ge fr o m b o t h
T h i s i n d i c a t e s t h a t t h e w a t e r o f Warm S p r i n g s
Creek p a s s e s down th r o u g h t h e s tre am bed a l l u v i u m and i n t o s o l u t i o n
channels
in
the
carbonate
bedrock.
W ater c i r c u l a t e s
deeply
underground and emerges as warm s p r i n g s where t h e s e c o n d u i t s i n t e r s e c t
the surface.
on L o s t
A s i m i l a r s e t t i n g (with l e s s deep c i r c u l a t i o n o f w a t e r s )
Creek
accounts
for
dry
stretch es
of
stream
bed
in
M i s s i s s i p p i a n bed roc k and c o l d s p r i n g s n e a r t h e to p of t h e P a l e o z o i c
section.
W at e r i n t h e Warm S p r i n g s C r e e k s y s t e m i s p r o b a b l y h e a t e d by
circulation
to m oderate
depths
and
is
heated
by t h e
reg io n al,
g e o t h e r m a l g r a d i e n t p r e s e n t i n s o u t h w e s t M o n t a n a , r a t h e r t h a n by
c i r c u l a t i o n w i t h i n a s p e c i f i c h e a t s o u r c e s u c h a s a c o o l i n g magma
chamber.
Water need o n l y c i r c u l a t e from f o u r t o f i v e hundred m e t e r s
depth i n o r d e r t o h e a t from a v e r a g e ground w a t e r t e m p e r a t u r e ( S - I l eC)
t o t h e o b s e r v e d t e m p e r a t u r e ( 2 3 - 2 4 eC), assuming a g e o th er m al g r a d i e n t
of l°C /30 m e te rs,
the
(Chadwick and Kaczmarek,
approxim ate
1975).
geotherm al
gradient
a t B utte
68
There a r e s e v e r a l o t h e r t r a v e r t i n e d e p o s i t s e ls e w h e re in the
a r e a , but none a r e t h e r e s u l t o f modern h y d r o t h e r m a l a c t i v i t y .
These
d e p o s i t s a r e o l d e r t h a n t h e d e p o s i t d e s c r i b e d a b o v e , a nd may h a v e
o r i g i n a t e d from g e o th e r m a l f l u i d s d e r i v e d from t h e Eocene v o l c a n i c s .
For ex am p le, t h e r e i s a c o n s p ic u o u s f o s s i l - r i c h t r a v e r t i n e d e p o s i t
a l o n g Warm S p r i n g s Creek w e st o f G r a v e l y Mountain.
and
ch ao tically
d istu rb ed
bedding
ch aracterize
and beds a v e r a g e 3 t o 50 c e n t i m e t e r s t h i c k .
of
coarsely
cry stallin e
calcium
the d e p o s it,
Thi s d e p o s i t i s composed
carbonate,
unlike
c r y s t a l l i n e c a l c i u m c a r b o n a t e o f t h e modern d e p o s i t .
15 mm d i a m e t e r )
Ga stropod f o s s i l s
the
fin ely
The l a r g e r (5 t o
c r y s t a l s o f t h e o l d e r d e p o s i t may be r e s u l t o f p o s t
d e p o s i t i o n a l r e c r y s t a l l i z a t i o n which a l s o produced i n d i s t i n c t bed ding
i n many p a r t s o f t h e mound.
The d r a i n a g e o f Warm S p r i n g s C r e e k
a pp ea rs t o h a v e been d e f l e c t e d by t h e growing mound,
b e n d s a r o u n d t h e e d g e o f t h e mound ( P l a t e 2 ) .
not a recen t f e a tu re .
since the creek
H o w e v e r , t h e mound i s
No h o t s p r i n g s e m i n a t e fr o m t h e mound, and a
k a r s t t o p o g r a p h y and s o i l h a v e d e v e l o p e d on i t s s u r f a c e .
A sample
t a k e n from t h e d e p o s i t i s o l d e r t h a n t h e 40,000 y e a r age l i m i t o f t h e
C a r b o n - 1 4 d a t i n g t e c h n i q u e ( T e l e d y n e I s o t o p e s I n c . , 1982).
However,
t h e mound i s a t l e a s t s l i g h t l y younger t h a n t h e Eocene v o l c a n i c s i t
overlies.
S e v e r a l c o l d s p r i n g s a t t h e ba se o f t h e mound a r e r e c h a r g e d
i n m a r s h y l a n d and a s i n k h o l e pond a t t h e t o p o f t h e mound.
The
d e p o s i t has been s t r i p mined i n t h e p a s t f o r i t s h i g h - p u r i t y c a l c i u m
c a r b o n a t e used i n p r o c e s s i n g s u g a r b e e t s .
At A n t e l o p e
H ill
in
t h e Avon V a l l e y
(P lates
I and
2 ) an
i n t e r e s t i n g r e l a t i o n s h i p b e t w e e n s i l i c e o u s and c a l c a r e o u s u n i t s i s
69
found.
T r a v e r t i n e caps t h e h i l l ,
o v e r l y i n g a s i l i c i f i e d m a r l which
i t s e l f o v e r l i e s an u n s i l i c i f i e d m a r l .
sedim entary d e p o s i t s .
Thi s i n t u r n o v e r l i e s T e r t i a r y
These d e p o s i t s p r o v i d e a p a r t i a l h i s t o r y o f t h e
n a t u r e and sequence o f h y d ro th e rm a l w a t e r s a t A n t e l o p e H i l l and g i v e s
the o v e r a l l
im pression of a l t e r n a t i n g
c alcareo u s hydrotherm al a c t i v i t y .
recry stallized
(calcite
or
phases
of
siliceo u s
and
T h e s e d e p o s i t s commonly c o n t a i n
silica)
gastropod
sh ells
a nd wood
fr ag m e n ts .
The e a s t s i d e o f t h e r a n g e c o n t a i n s n u m e ro u s s i n k h o l e s p l u s
d e p o sits of th in , g a stro p o d -b e a rin g tu f a T e r t i a r y m a rls . T e r t i a r y
sin ter
d ep o sits
and
g rav el.
D is c o n tin u o u s p a tc h e s o f eroded
t r a v e r t i n e d e p o s i t s s t r e t c h s o u th from D a v is Creek t o t h e s o u t h e r n
e dg e o f t h e a r e a mapped.
S i n t e r d e p o s i t s b e t w e e n D a v i s a nd G i m l e t
Creeks o c cu r as s m a l l o u t c r o p s o f l i g h t g r e y ,
f o r m l e s s masses,
deposits.
fine-grained s i l i c a in
b e l i e v e d t o be t h e eroded rem ai ns o f l a r g e r s i n t e r
T h e s e u n i t s a r e v e r y h a r d and c o n t a i n n u m e r o u s h e m a t i t e
stained, s i l i c a - h e a l e d fra c tu re s.
S i l i c i f i c a t i o n h a s a f f e c t e d z o n e s i n f a u l t b r e c c i a s and a l o n g
p l a n a r v e r t i c a l zones up t o 75 m e t e r s t h i c k and s e v e r a l k i l o m e t e r s i n
length.
Not a l l f a u l t b r e c c i a s a r e s i l i c i f i e d , b u t l on g s t r e t c h e s o f
f a u l t b r e c c i a s i n c a r b o n a t e b e d r o c k h a v e b e e n h i g h l y i n d u r a t e d by
f i n e - g r a i n e d s i l i c a which h as t o t a l l y
r e p la c e d th e fragm ents of
c a r b o n a t e ro ck i n t h e b r e c c i a s and i n f i l l e d
t h e i n t e r s t i c e s between
b r e c c ia fragm ents w ith f i n e - g r a i n e d , dense s i l i c a .
vertical
Some o f t h e s e
s i l i c i f i e d b r e c c i a zones a r e not l o c a t e d where f a u l t i n g i s
c l e a r l y d e m o n s t r a t a b l e , bu t some cause s l i g h t o f f s e t o f a d j a c e n t beds
70
or a r e r e l a t e d t o f o l d s .
These s i l i c i f i e d b r e c c i a s a r e u s u a l l y b r i g h t
red to brown or g r e y - w h i t e i n c o l o r .
The zones o f s i l i c i f i e d b r e c c i a s h o u l d n o t be confused w i t h zones
o f r e d b e d d e d c h e r t a nd r e d c h e r t b r e c c i a s w h i c h o c c u r i n t h e u p p e r
M ississip p ian
M ission
Canyon L im e s to n e
and
M i s s i s s i p p i a n k a r s t f o r m a t i o n (Kauffman, 1963).
represent
la te
The M i s s i s s i p p i a n
c h e r t i s d i s t i n g u i s h e d from s i l i c i f i e d b r e c c i a s by i t s bedded n a t u r e ,
by t h e
lack of a fin e -g ra in e d
M ississippian cherts,
silica
m atrix
in
the
brecciated
and by t h e r e s t r i c t i o n o f M i s s i s s i p p i a n c h e r t s
to l e n t i c u l a r masses l e s s t h a n 50 m e t e r s t h i c k , p a r a l l e l t o b e d d i ng,
a s c o n t r a s t i n g t o b e i n g v e r t i c a l l y e x t e n s i v e , and c u t t i n g a c r o s s
s ed im en ta ry u n i t s .
71
ECONOMIC POTENTIAL
M in er al R eso urc es
Phosphate r i c h r oc k a t t h e ba se o f t h e Ph osp horia Form at ion has
b e e n mined i n t h e a r e a s i n c e t h e b e g i n n i n g o f t h e c e n t u r y ( P a r d e e ,
1 9 16 ).
A m a j o r m in e o p e r a t e d by Cominco A m e r i c a n , I n c . i s l o c a t e d
where Warm S p r in g s Creek c r o s s e s t h e Ph o s p h o ri a Formation. The R e l y a e
Mine o p e r a t e s on t h e s o u t h s i d e o f t h e r a n g e a b o u t t h r e e k i l o m e t e r s
n o r t h w e s t o f t h e Cominco o p e r a t i o n ( s o u t h w e s t 1 / 4 S e c t i o n 1 2, T. 10
N., R. 10 W.).
A d e c r e a s e i n p h o s p h a t e c o n t e n t and an a s s o c i a t e d
c h a n g e t o a more s a n d y f a c i e s o c c u r s i n t h e f o r m a t i o n w e s t o f B ro c k
C r e e k and r e p r e s e n t s a c h a n g e t o
a nearshore
environm ent
and a
d e c r e a s e i n v a l u e as a pho s pha te o re .
The a r e a c o n t a i n s a s m a l l d e p o s i t o f h e m a t i t e , m a l a c h i t e , and
manganese o x i d e , a l t h o u g h i t i s to o s m a l l t o be o f economic u s e .
In
u p p e r E a s t B r o c k C r e e k ( S e c t i o n 20, T. 11 N., R. 10 W.), n e a r f a u l t s
i n t h e Hasmark For mation c a r b o n a t e s , a r e i n t e n s e l y s i l i c i f i e d
zones
which have produced a s i l i c a - r i c h g o s s a n - I ik e f l o a t where w e at he re d.
T h i s f l o a t i s w h a t a t t r a c t e d p r o s p e c t o r s t o d i p two s h a l l o w a d i t s ,
numerous p r o s p e c t p i t s ,
and a s m a l l s h a f t t h e r e .
I n t h e s e w ork ing s,
t h e h o s t d o l o m i t e s can be s e e n t o c o n t a i n co nco rd an t l e n s e s o r v e i n s
of
silica
one
to
tw enty
centim eters
thick,
a nd f i v e
to
th irty
c e n t i m e t e r s i n l e n g t h ( v e i n s may r e a c h s e v e r a l m e t e r s i n l e n g t h ) .
These q u a r t z d e p o s i t s c o n t a i n e i t h e r m a s s i v e m i l k y q u a r t z , o r i n some
72
ex po su re s c l e a r q u a r t z c r y s t a l s up t o t h r e e c e n t i m e t e r s i n l e n g t h .
h e m a t i t e - m a n g a n e s e o x i d e r i n d s u r r o u n d s some q u a r t z b o d i e s .
A
Some
c o n t a i n s m a l l (.5 t o 4 mm2) p o c k e t s o f c u p r i f e r o u s m i n e r a l s s u c h a s
malachite.
P l a c e r g o l d d e p o s i t s a l o n g Gold Creek were mined i n t h e mid 19th
century.
The mouth o f t h i s ' c r e e k i s a b o u t a k i l o m e t e r n o r t h w e s t o f
Warm S p r i n g s C r e e k a l o n g t h e C l a r k F o r k R i v e r ( P l a t e I ) .
I t drains
t h e F l i n t Creek ra nge t o t h e s o u t h and does n o t i n d i c a t e t h e p r e s e n c e
o f g o l d o r e in t h e G a rn e t Range.
Mining ha s been abandoned t h e r e f o r
many d e c a d e s .
Geothermal F l u i d s
S u r f i c i a l t r a v e r t i n e and s i n t e r d e p o s i t s f o u n d t h r o u g h o u t t h e
a re a p ro v id e evidence o f p a st w idespread hydrotherm al a c t i v i t y .
P r e s e n t a c t i v i t y a t t h e s u r f a c e i s r e s t r i c t e d t o t h e few warm s p r i n g s
a l o n g Warm S p r i n g s C r e e k .
T h e i r t e m p e r a t u r e i s o n l y 24"C, n o t h o t
enough t o d e v e l o p f o r h y d r o t h e r m a l en ergy.
young en o u g h t o i n d i c a t e a s t i l l
source.
No v o l c a n i c s nearby a r e
c o o l i n g magma c ham be r a s a h e a t
The s o u r c e o f h e a t i s c o n s i d e r e d t o be t h e c i r c u l a t i o n o f
w a t e r s thro ugh t h e
local
g e o th e r m a l g r a d i e n t o f about I "C/30 m e te rs
(Chad wick and K a c z m a r e k , 1 9 7 5 ).
I t is u n l i k e l y th a t the r e s e v o ir
t e m p e r a t u r e a t t h e base o f t h i s c i r c u l a t i o n system i s any warmer th a n
t h a t o f t h e t e m p e r a t u r e (24"C) e l s e w h e r e i n t h e r e g i o n a t t h a t d e pth
(400-500 meters).
73
CONCLUSIONS
•
The m o s t
obvious
Garrison A n tic lin e ,
structure
n e a r Warm S p r i n g s C r e e k i s
?
the
one o f a s e r i e s o f asymmetric (s out hw e st v e r g e n t )
s o u t h e a s t p l u n g i n g Laramide f o l d s t r e n d i n g n o rt h w e s t from E l l i s t o n t o
Bearmouth.
Laramide n o r t h e a s t - s t r i k i n g
extensional
fau lts,
and
♦
n o r t h w e s t - s t r i k i n g c o m p r e s s io n s I f a u l t s occu r i n a s s o c i a t i o n w i t h th e
folding.
These s t r u c t u r e s d e v e lo p e d in re s p o n s e to a n o r t h e a s t -
o r i e n t e d c o m pre ss iv e a x i s of s t r e s s ( a I ) .
N orthw est-striking fo ld
a x e s a nd f a u l t s
are r e la te d
to the
Montana Lineament, a n o r t h w e s t - s t r i k i n g s t r u c t u r a l t r e n d t r a n s v e r s e t o
other s tr u c tu r a l trends
in the reg io n .
The C l a r k F o r k Sag i s
a
n o r t h w e s t s t r i k i n g s t r u c t u r a l d e p r e s s i o n s o u t h o f t h e G a r n e t Range
r e c o g n i z e d a s p a r t o f t h e M o n ta n a L i n e a m e n t .
Folds c ro ssin g t h i s
d e p r e s s i o n r e v e r s e i n p l u n g e , and t h e i r axes change s t r i k e .
F o l d s and
t h r u s t f a u l t s bend from n o r t h - s o u t h t r e n d s t o n o r t h w e s t t r e n d s as th e y
ap pro ach t h e C l a r k Fork Sag from t h e s o u th .
The westward bend o f t h e
n o r t h e r n ends o f f o l d s and f a u l t s and n o r t h e a s t
<? I a t t h e sag i s t h e
r e s u l t of a l e f t - l a t e r a l s h e a r c o u p le which a c te d a lo n g a w est or
n o rt h w e s t l i n e d u r i n g t h e Laramide orogeny.
These s t r e s s e s d e v e l o p e d
i n re s p o n s e t o b u t t r e s s i n g o f t h e n o r t h e r n edge o f t h e e a s tw a rd t h r u s t
S a p p h i r e P l a t e by a p a r a u t o c h t h o n o u s b l o c k o f B e l t s t r a t a .
This
b u t t r e s s was c r e a t e d by u p l i f t o f a s t r u c t u r a l b l o c k a l o n g e a s t - w e s t
o r n o r t h w e s t t r e n d i n g s t r u c t u r a l we aknesses or f a u l t zones o r i g i n a l l y
74
d e v e l o p e d by t h e Precam brian Y e x t e n s i o n which c r e a t e d t h e B e l t Basin
on t h e w e s t e r n s h o r e o f t h e P r e c a m b r i a n N o r t h A m e r i c a n c r a t o n .
The
n o r t h e r n edge of t h e S a p p h ir e P l a t e , and t h e Precambrian Y f a u l t zone
b o t h c r o s s t h e map a r e a n o r t h o f Warm S p r i n g s C r e e k , m o s t l y b e n e a t h
Eocene v o l c a n i c s .
Laramide d e f o r m a t i o n i n t h e Warm S p r in g s Creek a r e a ended b e f o r e
the
deposition
of
the
44-47 m i l l i o n
year
unconformably o v e r l i e Laramide s t r u c t u r e s t h e r e .
old
volcanics
which
These Eocene a l k a l i -
c a l c i c d a c i t e s , a n d e s i t e s and t u f f s r e p r e s e n t t h e s e c o n d o f two a r c
m a g m a ti c maxima g e n e r a t e d by m e l t i n g o f c r u s t a l m a t e r i a l d u r i n g
s ub duc ti on o f t h e F a r a l l o n P l a t e d u r i n g t h e Laramide orogeny. Because
of t h i s r e l a t i o n s h i p ,
t h e y were p r o b a b l y c r e a t e d by c r u s t a l f u s i o n a t
d e pth a l o n g t h e F a r a l l o n P l a t e , whose s u b d u c ti o n was r e s p o n s i b l e f o r
Laramide f o l d i n g and f a u l t i n g .
They a p p e a r t o be a d i s t a l f a c i e s o f
t h e I d a h o - M o n t a n a P o r p h y r y B e l t , s i n c e t h e y a r e s i m i l a r i n a ge and
co mp os it io n t o t h e nearby Lowland Creek v o l c a n i s ,
along th i s
B elt.
which a r e a l i g n e d
The e a s t e r n G a r n e t Range v o l c a n i c s
owe t h e i r
p r e s e r v a t i o n t o n o r th w e s t o r i e n t e d Eocene to p o g r a p h i c t r o u g h s c r e a t e d
by Montana Lineament t e c t o n i c s .
D e p o s i t i o n o f t r a v e r t i n e and s i n t e r began o v e r 40,000 y e a r s ago,
p r o b a b l y as f a r back as Eocene time when v o lc a n i s m p r o v i d e d s u f f i c i e n t
h e a t and f l u i d s
fo r hydrotherm al a c t i v i t y .
a c tiv ity originates
Modern warm s p r i n g s
from t h e c i r c u l a t i o n o f w a t e r t h ro u g h l i m e s t o n e
bedrock s o l u t i o n c h a n n e l s where i t i s h e a t e d by t h e l o c a l ge o th e r m a l
g r a d i e n t b e f o r e r e - e m e r g i n g as s p r i n g s .
S i l i c i f i e d zones a l o n g f a u l t s
75
in d ic a te th a t r i s i n g hydrotherm al f l u i d s e x p lo ite d o ld e r c r u s t a l
weaknesses d e v e l o p e d by Laramide and Cenozoic f a u l t i n g .
Normal f a u l t i n g began i n t h e m i d - T e r t i a r y i n r e s p o n s e t o e i t h e r
n o r t h e a s t - s o u t h w e s t e x t e n s i o n , o r an e a s t - w e s t r i g h t l a t e r a l s h e a r
couple.
Most n o r m a l f a u l t s s t r i k e n o r t h w e s t , b o t h e x p l o i t i n g and
c u t t i n g a c r o s s Laramide s t r u c t u r e s .
The m ode rn p h y s i o g r a p h y o f t h e
re g io n i s m o s tly th e r e s u l t of t h i s on-going p e rio d of e x te n s io n a l
tectonism.
Economic p o t e n t i a l i s l i m i t e d i n t h i s a r e a .
P h o sp h ate mining
w i l l p r o b a b l y c o n t i n u e f o r some t i m e , b u t m i n i n g w i l l become more
ex pe ns iv e as t h e l a s t e a s i l y a c c e s s i b l e d e p o s i t s a r e e x p l o i t e d .
76
REFERENCES CITED
77
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—'
an<* Kaczmarek, 1975, Geothermal i n v e s t i g a t i o n s o f s e l e c t e d
M ont an a Hot S p r i n g s :
M onta na G e o l o g i c a l S o c i e t y , E n e r g y
r e s o u r c e s o f Montana, 22nd annual p u b i c a t i o n , p . 209-216.
C l a p p , C.H., 1 932, G e o l o g y o f a p o r t i o n o f t h e Rocky M o u n t a i n s o f
n o r t h w e s t Montana: Montana Bureau o f Mines and Geology, Memoir
4 ,
30
p .
D a n i e l , F. and B e r g , R.B., 1 981, R a d i o m e t r i c d a t e s o f r o c k s i n
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. 136
p .
78
E h l e r s 1 E.G. a nd B l a t t 1 H., 1982, P e t r o l o g y : i g n e o u s , s e d i m e n t a r y ,
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732 p.
Gwinn, V.E . , 1 961, G e o l o g i c map o f t h e Drummond a r e a . G r a n i t e and
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P u b l i c a t i o n 21, Map 4 .
____________ s n d M utc h, T.A., 1 9 6 5 , I n t e r t o u n g e d Upper C r e t a c e o u s
v o l c a n i c and n o n v o l c a n i c r o c k s , c e n t r a l w e s t e r n M o n ta n a :
G e o lo g ic a l S o c i e t y o f America B u l l e t i n , v . 76, p . 1125-1144.
H a r r i s o n , J . E . , G r i g g s , A.B., and W e l l s , J . D . , 1 974, T e c t o n i c f e a t u r e s
o f t h e Precam brian B e l t Basin and t h e i r i n f l u e n c e on p o s t - B e l t
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U nited
S t a t e s G e o l o g i c a l Survey Map HF-923.
Hyndman, D.W., 197 9, M a j o r t e c t o n i c e l e m e n t s and t e c t o n i c p r o b l e m s
a l o n g t h e l i n e o f s e c t i o n from n o r t h e a s t e r n O re g o n t o w e s t c e n t r a l Montana: G e o l o g i c a l S o c i e t y o f America, MC-28C.
J o h a n s e n , A., 1 9 3 8 , A d e s c r i p t i v e p e t r o g r a p h y o f t h e i g n e o u s r o c k s :
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S o c i e t y 16th ann ual f i e l d c o n f e r e n c e quidebook, p . 137-143.
W i n s t o n , D., J a c o b , P. , B a l d w i n , D.O., a nd R e i d , J . P . , P r o t e r o z o i c
b l o c k f a u l t i n g i n t h e B e l t Bas in, Montana and Idaho. I t ' s e f f e c t
on Rocky M o u n t a i n s t h r u s t i n g and B a s i n and Range e x t e n s i o n :
u n p u b li s h e d r e p o r t . U n i v e r s i t y o f Montana, 72 p.
/lAPGr.
80
APPENDICES
81
APPENDIX A
RADIOMETRIC DATES (K-Ar TECHNIQUE)
SAMPLE
LOCATION
COLLECTED
TAr40rad
ZK
See Ar40ra d/
gm.xlO-5
Porphyritic
Dacite
(Whole Rock)
S.W. 1 / 4
Section
23,T .ll
N . ,R .1 0 W
76.5
75.9
2.31
2 .3 2
.397
.400
Ae= 4.9 62xl0 -|0y r - l
Xe=O.581x1O-1 y r -1
K 4 0 - 1 . 1 6 7 x l 0 ' 4 atom/
atom o f n a t u r a l
Po ta ssi um
4 3 .7 +2 .2
Aphanitic
Andesite
(Whole Rock)
S.W. 1 / 4
Section
2 3 ,T .l l N.,
R.10 W.
85.6
85 .5
3.02
3.04
.532
.537
X g.4.962xl0-l°yr-l
Xe-O^SlxlO- 10Yr" 1
K40-1.167x10- 4 atom/
atom of n a t u r a l
Po ta ssi um
4 4 .8 +2 .2
Data from Te ldyne I s o t o p e s , Westood, New J e r s e y .
CONSTANTS
AGE ( m . y . )
83
APPENDIX B
RADIOMETRIC DATES ( C l4 TECHNIQUE)
84
SAMPLE
Calcium
Carbonate
(Travertine)
D ata:
LOCATION
COLLECTED
- 6C14
N.W. 1 / 4
Section 5
T.10 N .,R .
.9 W.
933
Bas ed on t o t a l c a r b o n a t e
Westvood New J e r s e y .
carbon,
AGE IN YEARS
>40,000
from T e le d y n e
Isotopes,
N 3 U
Cl 3 *
PLATE 2-Geologic Map of upper Warm Springs Creek and adjacent areas Powell County, Montana.
R. 10W.
R. 9W.
S c a l e - 1 : 4 17 0 0
. I ON.
1 K ilo m e te r
1 Mile
MAP U N I T S
Alluvial
Al l u v i u m
Q T a I
deposits
Q T h S
Travertine
Quaternary
Travertine
and
Tertiary
covering
and
Eocene
Travertine
Cenozoic
sinter
covering
and
partly
Hydrothermal
Q T h 2
Based
on
field
mapping
by
Callmeyrr (1981
deposits
sinter
Porphyritic
dacite
Porphyritic
dacite
carbonates
autobreccia
andesite
Q T h
f
Siliceous
Mesozoic
Cretaceous
and J u r a s s i c
P e r mi an
dacite
and
others(l 982)
Mejstrick
and
(Personal
Pardee
Communication,
1 9 8 2) ,
(1917).
Tdb
MAP S Y M B O L S
Tdp
T a
Vol cani c
Contact
T d
Normal
*
on
Jurassic
undifferentiated
Phosphoria
from
Contact
approximately
^To n t a c t
covered
Contact
inferred
located
Formation
Ititude
A 1X I t o o
Pp
/\
/\
/\
A
ball
Thrust
and
bar
side
fault,
D
on
side
fault,
teeth
on
upper pl at e
breccia
in
angle
downthrown
K J
Silicified
faul t,
downthrown
High
Tt
tuffs
Cretaceous
and
and d a t a
1
rocks
poor
1 9 8 2)
partly
Tertiary
Phenocryst
and
Wal l ace
volcanics
Paleozoic
Aphanitic
R. 9 W.
meters
A
A
1' A
A
Reverse
fault
,
teeth
on
Ra n g i n g wa l l
Spring
Quadrant
Formation
Marshy
Pq
rocks
Paleozoic
Amsden
Formation
Pa
f
■*-------- »
Precambrjan
Devonian
Devonian
Cambrian
Cambrian
Proterozoic
Hi n g e
l i ne
of a n t i c l i n e
Hi nge
l i ne
of
Inferred
Mississippian
Madison
Belt
Group
undifferentiated
undifferentiated
Series
M
/\
/\
/X
Younger
teeth
Sedimentary
Pennsylvanian
/\
area
------------- Ifr--
Hi nge
hinge
l i ne
J.
syncline
4 4 "
P-G Y
I
on u p p e r
and
e
Hor i zont al
J.
C a l l m e y e r ,
J u n e ,
1 9 8 4
thrust
plate
dip,showing
l i ne
of mi n o r f o l d
T h o m a s
older
v a l u e of dip
D
-e
Strike
on
bedding
fault,
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