Bedrock geology of the southern part of Tom Miner Basin, Park and Gallatin counties, Montana
by Stanley Glenn Todd
A thesis submitted to the Graduate Faculty in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE IN APPLIED SCIENCE Physical Science (Geology)
Montana State University
© Copyright by Stanley Glenn Todd (1969)
Abstract:
In Tom Miner Basin south of Tom Miner Creek, an Eocene volcanic succession consists of well
bedded to crudely stratified epiclastic volcanic beds with intercalated andesite flows of
augite-hypersthene composition in the lower part of the succession and augite-hornblende-hypersthene
flows in the upper part. The entire volcanic sequence is at least 2800 feet thick and rests unconformably
on Precambrian metamorphic rock and in places on Cretaceous strata.
Vents in a north-northeast-trending horst in the Electric Peak-Sepulcher Mountain region of
Yellowstone National Park may have supplied volcanic materials to the lower part and most of the
upper part of the succession in the map area.
Dikes east of Horse Creek may have fed some of the upper andesite flows. The volcanics high in the
succession near Buffalo Horn Pass could have been supplied from sources in either the Northern
Gallatin Range, the Electric Peak-Sepulcher Mountain region, or the dikes east of Horse Creek.
Dacite has intruded the volcanic succession in the form of a stock at Sheep Mountain, a sill southwest
of Sheep Mountain and a plug east of Horse Creek. Seven andesite dikes also intrude the succession
east of Horse Creek. These intrusions are located along the northwest-trending, potassium-poor
Western Absaroka belt of volcanic centers which are in line with the Spanish Peaks Laramide reverse
fault northwest of the map area.
Probable Plio-Pleistocene rhyolitic (?) welded tuff occurs as erosional remnants along Tom Miner
Canyon between Sheep Creek and Horse Creek and also along the Yellowstone Valley east of Wigwam
Creek. The Tom Miner deposits may represent welded ash flows which came from northwest of Tom
Miner Basin.
The northeast-trending Deep Creek normal fault which forms the northwest border of the Beartooth
structural block, cannot be positively traced into the map area. If it continues into the map area, it must
trend along a strip about one-quarter mile wide, closely paralleling Tom Miner Creek, Any
displacement along this projection of the fault must have been small because Precambri an
metamorphic rock units and welded tuff remnants located on both sides of Tom Miner Creek are not
appreciably offset. In presenting this thesis in partial fulfillment of the require­
ments for .an advanced degree at Montana State University, I agree that
the Library shall make it freely available for inspection.
I further
agree that permission for extensive copying of this thesis for scholarly
purposes may be granted, by my,major professor, or, in his absence, by
the Director of Libraries.
It is understood that any copying or publica­
tion of this thesis for financial gain shall not be allowed without my
x
written permission.
Date
S
■f
BEDROCK GEOLOGY OF THE SOUTHERN PART OF TOM MINER
BASIN, PARK AND GALLATIN COUNTIES, MONTANA
by
J
STANLEY GLENN TODD
A t h e s i s s u b m i t t e d t o t h e Gr a d u a t e F a c u l t y 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 f o r t h e degr ee
of
MASTER OF SCIENCE IN APPLIED SCIENCE
Physical
Science
(Geology)
UuL
Head,
M a j o r D&^&r t ment
Ch a i r ma n ,
E x a mi n i n g Commi t t ee
MONTANA STATE UNIVERSITY
Bozeman, Montana
Au g u s t
1969
iii
ACKNOWLEDGMENTS
Sincere
Earth
appreciation
Sciences,
the
use o f i t s
like
to thank
and v a l u a b l e
is
Montana S t a t e
fa c ilitie s .
Dr.
suggestions
Chadwi ck f o r
in
like
the f i e l d
to express
W i l l i a m J . McManni s and Dr.
and e n c o u r a g e m e n t , t o Mr.
problems
L. Christiansen,
States
related
and t o
and Dr.
Geological
to
this
Ma l c o l m f a m i l y
Dr.
for
the
Appreciation
is
t h e a u t h o r woul d
his
ed itoria l
as w e l l
his
indebtedness
Smedes o f t h e
discussion
Special
thanks
use o f t h e i r
t o Dr.
advice
guidance w i th
H a r o l d J . P r o s t k a , Dr .
H a r r y W.
aid
as i n t h e o f f i c e .
John Montagne f o r t h e i r
o f t h e P o i n t o f Rocks Lodge f o r t h e
camp.
Department o f
Bozeman, Montana f o r
M. J . Edi e f o r h i s
Survey f o r
study.
U niversity,
In p a r t i c u l a r
R o b e r t A.
The a u t h o r wou l d a l s o
drafting
extended to the
Robert
United
of various
topics
ar e due t h e
Sandy
c a b i n and Mr.
use o f h i s
sheep h u n t i n g
e x t e n d e d t o t h e numerous
o f t h e mapped ar ea who g r a n t e d u n l i m i t e d
Max Chase
l an d o w n e r s
access.
S . G. T ,
iv
TABLE OF CONTENTS
Chapter
I .
II.
Page
INTRODUCTION .
T
Location
. . . . . . . . . . .
Regional S e t t i n g
. . . . . . .
Previous Geologic In v e s tig a tio n s
Pur pose o f S t u d y . . . . . . . .
a,
I •
5
7
GEOLOGIC UNITS
8
Pr e c a mb r i an M e t a m o r p h i c Rocks
. . . . .
M a j o r Eocene L i t h o l o g i c Types
. . . . .
Lava Fl ows
. . . . . . .
. . . . .
Hypersth ene-Augite Andesites
. . . .
Augite-Hornblende-Hypersthene
Andesites
. . . . . . . . . . . . . .
E p i c l a s t i c V o l c a n i c Rocks
. . . . . .
Crudely S t r a t i f i e d E p i c l a s t i c
Volcanics
. . . . , :
Wel l Bedded E p i c l a s t i c V o l c a n i c s
.
S t r a t i g r a p h i c Succession
. . . . . .
Eocene I n t r u s i o n s
. . . . . . . . . . .
Andesites
. . . . . . . . . . . . . .
Da c i t e s
. . . . . . . ■, . . . . . .
P l i o - P l e i stocene
. . . . . . . . . . .
Wel ded T u f f
. . . . . . . . . . . . .
Quaternary
. . . . . . . . . . . . . .
S u r f i c i a l Deposits
. J . . . . . .
III.
PETROGRAPHIC DESCRIPTIONS
.
.
.
.
.
.
. . . . . . . .
P r e c a m b r i a n Me t a mo r p h i c Rocks
...................
Gn e i s s
. . .
. . . . . .
. . . . . .
S Ch l S t
.
oe o . e o o o . o
Schistose Q u a rtz ite
. . ., . . . T' .
Gr anul i t e
. . . . * . . . © . -. . .
S lllS
. . . . .
.
. .
Eocene Lava Fl ows
. . . . . . . . . . .
Hypersthene-Augite-Ande s ite s
. . . .
Augite-Hornblende-Hypersthene
Andesites
.. . . . . . . . . . . . .
Eocene C r u d e l y S t r a t i f i e d E p i c l a s t i c
V o l c a n i c Beds
. . . . . . . . . . . . .
6
0
0
0
0
.
0
0
8
10
10
10
10
11
13
16
18
21
21
21
24
24
27
27
28
28
28
29
" ""' 30
30
3l
32
32
33
34
.
' ",
Table o f Contents
Continued
Chapter
Page
Eocene I n t r u s i o n s ................................. . . .
A n d e s i t e Di k es
. . . . . . . . . . .
Andesite S i l l s
. . . . . . . . . . .
D a c i t e I n t r u s i o n s .......................
P l i o - P l e i s t o c e n e Welded T u f f
. . . . .
C h a r a c t e r i s t i c s Common t o t h e
Wel ded T u f f Remnants
. . . . . . . .
Wel ded T u f f Remnant E as t o f Sheep
Creek
. . . .
. . . . . . .
. . . . . .
Wel ded T u f f Remnant West o f Horse
Creek
. .
.« . . .
. . .
. .
. . . .
.
Welded T u f f Remnant East o f Wigwam
Creek
® . . . . . . . . . . . . . . .
.
R e f r a c t i v e I n d e x o f Gl as s e s
. . . . . . .
I V.
STRUCTURE
.
. .
.
.
P r e c a mb r i an
. . . . . . .
Eocene E x t r u s i v e Rocks
Eocene A n d e s i t e Di kes
. .
Eocene D a c i t e I n t r u s i o n s
Deep Creek Normal F a u l t
.
S p a n i s h Peaks and G a r d i n e r
V.
VI.
PALEOMAGNETICS
. . . . . . .
. . . . . . .
. . . . . . .
Faults
. .■ .
.
.
.
.
. . . . .
. . . .
. . . .
. . . .
Remnants
34
35
35
35
36
36
37
38
38
38
40
40
,40,
40
- 40
41
42
44
'.......................
49
.
.
.
.
.
. . . . . . . . . . .
Pr e c a mb r i an
. . . . . . . . . . . . .
Paleozoic-Mesozoic . . . . . . . . . .
Cenozoi C
. . . . . . . . . . . . . .
REFERENCES CITED
.
. . . . . . . . . . . .
SOURCE' OF VOLCANICS
GEOLOGIC HISTORY
.
. . . . . . .
P r e - V o l c a n i c T o p og r ap hy
. .
A S o u t h e a s t e r n Sour ce
. . .
A N o r t h e r n Sour ce
. . . . .
A n d e s i t e Di k es
. . . . . .
P T i o - P l e i s t o c e n e Wel ded T u f f
V II.
.
'
49
49
50
51
51
'5,3
.
.
.
53
,55
55
59
vi
LI ST OF TABLES
Table
Page
I
Representative
Section
A1
II
Representative
Section
B
. . . . . . .
III
Representative
Section
C
. . . . . . . .
IV
V
VI
VII
Wel ded T u f f Sequence
Wel ded T u f f
18
...................
Dipole O rie n ta tio n s
. . . .
Data
. .
. . . . . .
19
20
...................
D i p o l e O r i e n t a t i o n s o f C l a s t s f r om
an E p i c l a s t i c V o l c a n i c Bed
. . . .
Empl acement T e m p e r a t u r e
.
■ 24
46
.
48
48
vii
LI ST OF PLATES
Pl a t e
I
II
III
IV
V
Page
Map o f t h e Bedr ock Geol ogy o f t he
S o u t h e r n P a r t o f Tom M i n e r Bas i n
• •
F i g u r e I . Common P r e c a mb r i an Rock
Types i n t h e Map Area
.
F i g u r e 2. S i l l s i n Pr ecambr i an G r a n i t i c
Gn e i s s
* # . . . . . . . . . . » *
Figure I.
T y p i c a l Ex pos ur e
Fl ows Bel ow 8400 Feet
. .
F i g u r e 2.
T y p i c a l Ex pos ur e
Fl ows Above 8400 Feet
. .
of
. .
of
. .
Lava
. .
Lava
. .
Figure I .
Lens o f Wel l Bedded
E p i c l a s t i c Volcan ic in Crudely
S t r a t i f i e d E p i c l a s t i c V o l c a n i c Bed
F i g u r e 2.
T y p i c a l Ex pos ur e o f
Crudely S t r a t i f i e d E p i c l a s t i c
V o l c a n i c Bed
, .
.-. . . .
. . .
.
VII
VIII
F o s s i l M a t e r i a l f r om
V o l c a n i c Beds
. . ._.
9
9
I2
V-,
O •
12
' V1,f ;
14
14
•
Figure I .
Banded Pr e c a mb r i an Fr agment
i n E p i c l a s t i c V o l c a n i c Bed
. . . .
F i g u r e 2.
U p r i g h t O p a l i zed Tree i n
Coarse Epi c l a s t i c V o l c a n i c Bed
. . • •
Figure I .
E piclastic
In
Pocket
I
P
VI
;■
P
'
15
-X
V
15
'
.
•
Figure I .
Vi ew L o o k i n g Sout hwar d Toward
Sheep M o u n t a i n , A D a c i t e I n t r u s i o n
e O
Figure I .
Vi ew o f Tom M i n e r Basi n
L o o k i n g Ea s t
. . . . . . . . . .
F i g u r e 2.
L i t h o l o g i c Sequence o f
Welded T u f f Remnant L o c a t e d East
o f Sheep Creek
. . . . . . . . . .
17
23
‘
',V,
0 •
25
O •
25
.
1
1
I
. 1
;
. 1V V 1' *■
v iii
LI ST OF FIGURES
Figure
Page
1
I n d e x Map
.
. ,......................................- .....................
2
The A b s a r o k a - G a l l a t i n
V o l c a n i c Centers
2
Province
4
V 1-
I X
ABSTRACT
In Tom M i n e r B a s i n s o u t h o f Tom M i n e r C r e e k , an Eocene
v o l c a n i c s u c c e s s i o n c o n s i s t s o f w e l l bedded t o c r u d e l y
s t r a t i f i e d e p i c l a s t i c v o l c a n i c beds w i t h i n t e r c a l a t e d a n d e s i t e
flows o f a u g i te -h y p e rs th e n e com position in the lower pa rt o f
t h e s u c c e s s i o n and a u g i t e - h o r n b l e n d e - h y p e r s t h e n e f l o w s i n t h e
up p e r p a r t .
The e n t i r e v o l c a n i c sequence i s a t l e a s t 2800
f e e t t h i c k and r e s t s u n c o n f o r m a b l y on P r e c a m b r i a n me t a mo r p h i c
r o c k and i n p l a c e s on C r e t a c e o u s s t r a t a .
Vent s i n a n o r t h - n o r t h e a s t - t r e n d i ng h o r s t i n t h e E l e c t r i c
P e a k - S e p q l c h e r M o u n t a i n r e g i o n o f Y e l l o w s t o n e N a t i o n a l Park
may have s u p p l i e d v o l c a n i c m a t e r i a l s t o t h e l o w e r p a r t and
most o f t h e u p p e r p a r t o f t h e s u c c e s s i o n i n t h e map a r e a .
Di k es e a s t o f Horse Creek may have f e d some o f t h e upper
andesite flows.
The v o l c a n i c s h i g h i n t h e s u c c e s s i o n near
B u f f a l o Horn Pass c o u l d have been s u p p l i e d f r o m s o u r c e s i n
e i t h e r t h e N o r t h e r n G a l l a t i n Range, t h e E l e c t r i c PeakS e p u l c h e r M o u n t a i n r e g i o n , o r t h e d i k e s e a s t o f Horse Cr eek.
D a c i t e has i n t r u d e d t h e v o l c a n i c s u c c e s s i o n i n t h e f o r m
o f a s t o c k a t Sheep M o u n t a i n , a s i l l s o u t h w e s t o f Sheep
Mo u n t a i n and a p l u g e a s t o f Horse C r ee k .
Seven a n d e s i t e d i k e s
a l s o i n t r u d e t h e s u c c e s s i o n e a s t o f Horse Cr ee k .
These
i n t r u s i o n s are l o c a t e d al ong t h e n o r t h w e s t - t r e n d i n g , p o ta s s i u m p o o r West er n Abs ar ok a b e l t o f v o l c a n i c c e n t e r s w h i c h ar e i n
l i n e w i t h t h e S p a n i s h Peaks L a r a mi d e r e v e r s e f a u l t n o r t h w e s t
o f t h e map a r e a .
P r o b a b l e P l i o - P l e i s t o c e n e r h y o l i t i c ( ?) we l d e d t u f f
o c c u r s as e r o s i o n a l r emnant s a l o n g Tom M i n e r Canyon between
Sheep Creek and Horse Creek and a l s o a l o n g t h e Y e l l o w s t o n e
V a l l e y e a s t o f Wigwam Cr ee k .
The Tom M i n e r d e p o s i t s may
r e p r e s e n t we l d e d ash f l o w s wh i c h came f r o m n o r t h w e s t o f
Tom M i n e r B a s i n .
The n o r t h e a s t - t r e n d i n g Deep Creek normal f a u l t whi ch
f or ms t h e n o r t h w e s t b o r d e r o f t h e B e a r t o o t h s t r u c t u r a l b l o c k ,
c a n n o t be p o s i t i v e l y t r a c e d i n t o t h e map a r e a .
If it
c o n t i n u e s i n t o t h e map a r e a , i t must t r e n d a l o n g a s t r i p a b o u t
o n e - q u a r t e r m i l e w i d e , c l o s e l y p a r a l l e l i n g Tom M i n e r Cr eek,
Any d i s p l a c e m e n t a l o n g t h i s p r o j e c t i o n o f t h e f a u l t must have
been s ma l l because P r e c a mb r i an m e t a m o r p h i c r o c k u n i t s and
we l d e d t u f f r e mna nt s l o c a t e d on b o t h s i d e s o f Tom M i n e r Creek
are not a p p r e c i a b l y o f f s e t .
I.
INTRODUCTION
Location
The map a r e a , c o n s i s t i n g
m iles,
is
located
in
o f a p p r o x i m a t e l y 40 s qu ar e
the G a l l a t i n
Montana a p p r o x i m a t e l y 35 m i l e s
20 m i l e s
Range o f S o u t h w e s t e r n
southwest o f L i v i n g s t o n
and
northwest o f Gardiner.
The G a l l a t i n
Range i s
n e a r Bozeman 50 m i l e s
a mountai nous
southward i n t o
ar ea e x t e n d i n g f r o m
Yellowstone
National
Park and o c c u p y i n g t h e 20 t o 30 m i l e wi de ar ea bet ween t he
G allatin
and Y e l l o w s t o n e
Rivers.
It
is
Rocky M o u n t a i n p h y s i o g r a p h i c p r o v i n c e
The map ar ea i s
Yellowstone
River,
G a lla tin-Yellowstone
National
divide
extends
is
press).
on t h e w e s t by t he
and on t h e n o r t h
located
This
early
northwestward nearly
Wyoming t o t h e G a l l a t i n
(Figure
Park,
by Tom Mi n e r
Setting
in
the n o rt hw e st
t e r me d t h e A b s a r o k a - G a l l a t i n
in
between
I).
Tom M i n e r B a s i n
Chadwick,
by t h e
on t h e s o u t h by t h e n o r t h e r n
Regional
wh a t i s
(1931).
on t h e s o u t h e a s t by t h e d i v i d e
boundary o f Y e l l o w s t o n e
(Figure
the Northern
o f Fenneman
bounded on t h e n o r t h e a s t
Tom M i n e r and C i n n a b a r B a s i n s ,
Creek
part of
volcanic
Tertiary
150 m i l e s
portion
province
volcanic
of
( ‘R. A.
province
f r om n e a r Thermopol i s ;
R i v e r s o u t h w e s t o f Bozeman,
Montana
2).
f.
ri: /
-2-
INTERSTATE 9 0
Livingston
Mont ana
FRIDLEY
PEAK
R SE.
R. 7
R. 6 E.
POINT
OF ROCKS
Emigrant
e.
EMIGRANT
PEAK
CINNABAR
BASIN
T. 7 S.
SEPULCHER
MTN.
YELLOWSTONE
Gardiner
NATIONAL
PARK
MONTANA
.. — . ■ ' ’
I
• •
••
WYOMING
I I I 0OO1
AREA STUDIED
Figure I - Index Map
T. 9 S.
-3-
Many w o r k e r s
were e x t r u d e d
have n o t e d t h a t
f r o m a number o f v o l c a n i c
northw est-trending
recognized
belts:
Absaroka-Gallatin
zone.
a r e shown i n
The p r i n c i p a l
Figure
belt
( R. A.
are:
(I)
Chadwick,
Kirwin
1964a,
1964b);
19 6 7 ) ;
(3)
(Schultz,
ar e a
(2)
Sylvan
1962,
in
I shawooa ar e a
(5)
lie
centers
thus
far
on a
a l o n g two s u b p a r a l l e l
The West er n b e l t
( H e w e t t , 1914;
1968);
that
and t h e E a s t e r n Absar oka
press).
Pass ar ea
centers
2 and l i e
t h e Wes t er n A b s a r o k a b e l t
volcanics
Rouse,
( Rou s e,
( Casel l a ,
E lectric
1940;
1933,
19 6 7 ) ;
centers
Wilson,
1937;
(4)
Mr.
Fisher,
Washburn
P e a k - S e p u l c h e r Mt.
\
(Hague, Id d in g s
Creek p l u g s
centers
and Weed,
(H all,
are:
(7)
(8)
Hurricane
(9)
Cooke C i t y
1899 ; Smedes, 1 9 6 8 ) ;
1961).
Sunlight
Mesa ar e a
ar e a
centers
(Parsons,
( R o u s e , 1947;
(Covering,
1 929); ,
R u b e l , 1 964,
1968) ;
(A.
1965,
( 1 2)
Easier,
(Chadwick,
G allatin
1966);
ar e g e n e r a l l y
found t h a t
richer
in
West er n A b s a r o k a b e l t
and t h a t
traced northwestward,
seem t o
and S p a n i s h
(Figure
2).
1968);
Krushensky, I960,
1962);
(11)
I n d e p end enc e ar ea
Emigrant
Peak ar ea
P o i n t o f Rocks ar ea
Range d i k e swarm ( C h a d w i c k ,
( 1 968 )
volcanic
1958,
( 10)
1939,
1 965 ; Chadwi c k and McCaskey, 1 968) ; ( 1 3 )
Chadwi ck
centers
Porcupine
The E a s t e r n Abs ar ok a b e l t
( Emmons, 190 8;
L.
(6)
the
1964).
t han t h o s e o f t he
Eastern
and West er n b e l t s ,
coincide with
Peak L a r a mi d e r e v e r s e
,
Eastern Absaroka b e l t
potash
the
Northern
fa u lts,
the
Squaw Creek
respectively
_4 -
BILLINGS
LIVINGSTON
MONTANA
WYOMING
YELLOWSTONE
CODY
VL A K E |
PARK
THERMOPOLIS
M il es
Figure
I .
2.
3.
4.
5.
6.
7.
2.
The A b s a r o k a - G a l l a t i n
Centers
K i r w i n ar ea
Ishawooa ar ea
S y l v a n Pass ar ea
Mt . Washburn
E l e c t r i c P e a k - S e p u l c h e r Mt .
P o r c u p i n e Creek s t o c k s
S u n lig h t centers
---
K_ - K --
Eastern
West er n
" D . C. " Deep Creek f a u l t
"S . C . 11 Squaw Creek f a u l t
Volcanic
8.
9.
10.
11 .
12.
13.
Province
Volcanic
H u r r i c a n e Mesa area
Cooke C i t y a r e a
I nd ep end enc e ar ea
E m i g r a n t Peak ar ea
P o i n t o f Rocks area
G a l l a t i n Range d i k e swarm
potassium r i c h b e l t
p o t a s s i u m po o r b e l t
"S. P . " Spani s h Peaks f a u l t
" G. "
Gardiner f a u l t
-5-
These f r a c t u r e
across
zones may be t r a c e d f u r t h e r
t h e Madi son Range t o - t h e
batholith
( P e a l e , 1896;
and Al s u p ,
Swanson, 1 950 ; R e i d ,
recorded along the
andesites,
orthoclase
gabbros,
as sembl age i s
dacites,- basalts
(Chadwick,
1968).
rocks
volcaniclastic
the
centers
major igneous
that
the
lava
have.in
Eastern
rock
large part
A.
differences
Ch a d wi c k ,
Previous
detailed
c o n d u c t e d by t h e
(Hague, Id d in g s
Geologic
and Weed, 1894,
Folio
and o t h e r
come f r om
bet ween t he
oral
communication,
1968).
Investigations
in the
Geological
1896;
(Hague,
may s u g g e s t
t a p p e d a somewhat d i f f e r ­
g e o l o g i c wor k
United States
Tjie L i v i n g s t o n
sheets,
and Wes t er n A b s a r o k a b e l t s .
E a s t e r n and West er n b e l t s
E arliest
calc-alkalic
breccias
as sembl ages o f - t h e two b e l t s
(R.
The
phaneri t i c e q u i v a l e n t s
volcanic
and m i n e r a l o g i c al
e n t magma s e r i e s
and s y e n i t e s .
and t h e i r
flows,
r o c k s wh i c h
along the
Chemi cal
diorites
monzonit e s ,
t ake the form o f s t o c k s , p l u g s ,
dikes,
19 6 6 ) .
Eastern Absaroka b e l t
p r e d o m i n a n t l y normal
andesites,
la cco lith s,
1 957 ; A n d r e t t a
trachyandesites, la t it e s ,
basalts,
Wes t er n ^ e l t
These
o f t h e Tobacco Root
1 960 ; K a v a n a g h , 1 965 ; B e c r a f t and o t h e r s ,
Rock t y p e s
include
vicin ity
northwestward
Survey,
1883- 93
P e a l e , 1896).
Iddings
)
r e g i o n was
and Weed,
1894)
- 6-
described
meridian
the
g e o l o g y o f Tom M i n e r Bas i n f r o m t h e
to the Y ellowsto ne
classified
together
description
For ks
and s c h i s t were d e s c r i b e d
Weed ( 1 8 9 6 ,
Folio
1899)
breccias
rocks
includes
and-flows".
Hague, I d d i n g s
breccias
hornblende-pyroxene-andesite
intercalated
with
lava
flow s/of sim ilar
Iith o lo g y.
. .
No d e t a i l e d
Mi ner Basin s i n c e
geologic
o f the
(1 950)
investigations
areas,
upper G a l l a t i n
studied
the
H a l T ( 1961)
P r e c a mb r i an e a s t o f Tom M i n e r
in
Yankee J i m Canyon.
north
o f Tom M i n e r B a s i n i s
R. A.
Chadwi ck o f Montana S t a t e
National
Park
mapped a l a r g e
V a l l e y w e s t o f Tom M i n e r B a s i n .
Basin
( Smedes, 1968)
ar e known i n Tom
1893.
In the s u r r o u n d i n g
Yellowstone
and
t o be t h e same u n i t
a sequence o f v o l c a n i c
pyroxene-andesite,
and b a s a l t c l a s t s
Bryden
a general
t h e r e were mapped as
d e s c r i b e what ap pea r s
basic b re c c ia " ,
containing
( P e a l e , 1896)
The i g n e o u s
"basic a n d e s itic
portion
and f l o w s " .
o f t h e g e o l o g y o f Tom M i n e r Bas i n w e s t o f t he
meridian.
as " e a r l y
gneiss
breccias
r o c k s were
b u t mapped as " P r e c a mb r i an u n d i f f e r e n t i a t e d " .
The Thr ee
Tilth
The i g n e o u s
as " b a s i c a n d e s i t i c
Pr ecambr i an g r a n i t e ,
separately
Valley.,
Tilth
is
The Rock C r e e k - B i g Creek area
currently
b e i n g s t u d i e d by Dr.
University.
currently
and C i n n a b a r B a s i n ,
Northwestern
un der i n v e s t i g a t i o n
d ire ctly
s o u t h o f Tom
M i n e r B a s i n 5 has been mapped by E. L . McCaskey
M. S . T h e s i s 5 Montana S t a t e
(Pending
U niversity).
Purpose o f St ud y .
The p u r p o s e o f t h i s
study
g e o l o g i c map o f t h e s o u t h e r n
describe
(3)
rock
the
record
units;
various
rock u n i t s
in
the s t r u c t u r a l
encountered.
to:
(I)
construct a
p a r t . o f Tom M i n e r B a s i n ;
fie ld
exposures
and t h i n
and age r e l a t i o n s h i p s
(4) - d e t e r m i n e , i f
units
is
possible,
( 2)
section;
o f these
t he o r i g i n
o f t he
II.
Evidence
units
w ill
used f o r
GEOLOGIC UNITS
assigning
be m e n t i o n e d l a t e r
ages t o t h e v a r i o u s
in
this
geologic
paper,
P r e c a m b r i a n Me t a m o r p h i c Rocks
P r e c a mb r i an m e t a m o r p h i c r o c k s
two s q u a r e m i l e s
in
crop out over a p p r o x i m a t e l y
t h e n o r t h w e s t c o r n e r o f t h e map a r e a .
These Pr e c a mb r i an r o c k s , u n d i f f e r e n t i a t e d
pocket,
include
pegmatites
gneiss,
and p r o b a b l e
Gnei ss o c c u r s
b u t extends
o f the
is
schistose
across
ty p ica lly
quartzite,
I , in
schist,
granulite,
sills .
p rin cipally
the e n t i r e
intersection
on P l a t e
s o u t h o f t h e Tom M i n e r road
mapped Pr ec ambr i an ar ea s ou t h
o f Tepee and Wigwam Cr ee k s .
a massive,
fa in tly
gram" t i c - f e l d s p a t h i c r o c k .
banded,
The g n e i s s
g r e y and w h i t e
M u s c o v i t e and e p i d o t e
ar e l o c a l l y
abundant.
Interbedded
north
schistose q u a rtz ite
o f t h e Tom M i n e r r oad and s o u t h o f t h e
Tepee Creek be l ow i t s
schistose q u a rtz ite
is
and s c h i s t
predominantly
quartz
schist
is
lig h t
with
r oad wes t o f
Wigwam Cr e e k .
gr een and mi c a c e o u s .
The
The s c h i s t
a d a r k gr een h o r n b l e n d e - p l a g i o c l a s e - b i o t i t e -
(Plate
p i a g i o c l a s e ar e
intersection
crop out
II,
lo ca lly
Figure
absent.
I),
but b i o t i t e
Plagioclase
and ( o r )
porphyroblasts ,
ar e a b u n d a n t l o c a l l y .
A g r e y and w h i t e ,
fa in tly
b i o t i t e - a u g i t e. g r a n u l i t e
is
folia ted,
plagioclase-quartz-
l o c a t e d on Wigwam Creek near t h e
- 9PLATE I I
A
B
C
Figure I .
Common P r e c a m b r i a n r o c k t y p e s i n t h e map a r e a .
A.
Q u a rtz -fe ld s p a r pegmatite.
B.
L i g h t gr een s c h i s t o s e q u a r t z i t e .
C.
P lagio cla se-q u a rtz-b io tite -a u g ite -g ra n u lite .
D.
Dark gr een s c h i s t .
E.
Grey and w h i t e g n e i s s .
Figure
A.
2.
S i l l i n Precambrian
t i o n o f the s i l l is
the g n e i s s .
S ill.
granitic
parallel
gneiss.
The d e f o r m a ­
to the f o l i a t i o n o f
-10-
Precambria n - T e r t i a r y - v o l c a n i c
Smal l
quartz
veins,
contact.
pods y l e n s e s
and s m a l l
quartz-
f e l d s p a r p e g m a t i t e masses ar e - c o mmo n - t h r o u g h o u t t h e met amorphic t e r r a i n .
S ills ,
g n e i s s , are p r e d o m i n a n t l y
metamorphi c f o l i a t i o n
I t o 100 f e e t
thick,
concordant.but
(Plate
II,
Figure
located
loca lly
in the
transect
2).
M a j o r Eocene L i t h o l o g i c
Types
Lava Fl ows
L i g h t grey to bl a ck
thick,
tion
a r e ex posed i n
and h i g h l y
boundaries
bet ween f l o w s
Andesite
miles.
in
Fl ow b a n d i n g ,
brecciated
parts
vesiculaand
a r e common t o most f l o w s .
e a s t o f Horse C r e e k , w i t h
have e x p o s u r e s
They c o n t a i n
hypersthene
zones
u n i t s , 80 t o 1400 f e e t
Andesites
flows
t ha n 8400 f e e t ,
flow
t h e map a r e a .
oxidized
Hypersthene-Augite
andesite
ranging
hypersthene
elevations
f r om 0 . 1 2 5
and a u g i t e
less
t o 2 s qu ar e
phenocrysts with
commonly more a b u n d a n t t h a n a u g i t e
(Plate
III,
Fig ure I ).
Augite-Hornblende-Hypersthene
Andesite
in-the
flows
southeastern
above 8400 f e e t .
higher in
Andesites
t h e sequence o c c u r p r i n c i p a l l y
and s o u t h e r n
portions
o f t he ,map area-
They ar e 80 t o 480 f e e t t h i c k
and have
abundant phe no cry sts
o f aug. i t e and h o r n b l e n d e and l e s s
abundant p h e n o c r y s t s
o f hypersthene
j
E piclastic
Volcanic
older
rocks
III,
Figure
2).
Rocks
The t e r m " e p i c l a s t i c
sedimentary
(Plate
volcanic
consisting
rocks o f v o l c a n i c
rocks"
is
used her e t o d e f i n e
mainly o f m a te ria ls
and ( o r )
intrusive
d e r i v e d from
origin
(Fisher,
1966).
Hall
(1961)
conglomerates,
Fortress
describes
loca lly
in te rla y e re d with
M o u n t a i n member o f t h e
However ,
in
and f i n e r
litho lo gies
variations.
extended i n t o
maximum measur ed t h i c k n e s s
types
crudely
For t h i s
stra tifie d
in
epiclastic
t he
lateral
terminology
units
in
is
and
no t
t h e map
I.
o f 2500 f e e t ,
breccia,
reason,
be d i s t i n g u i s h e d
o f the n o r t h ­
rapid
o f Tom M i n e r B a s i n ,
bet ween v o l c a n i c
and s i l t s t o n e .
types w i l l
H all's
on P l a t e
deposits
on
ar e m a i n l y b r e c c i a s
which e x h i b i t
Therefore
undifferentiated
north
deposits
t h e map a r e a and e p i c l a s t i c
The e p i c l a s t i c
tional
ash and t u f f
Speci men Creek f o r m a t i o n .
Tom M i n e r Bas i n t h e s e
grained
coarse v o l c a n i c
He names t h e s e d e p o s i t s
Fortress
ar e a ar e
volcanic
M o u n t a i n a p p r o x i m a t e l y two m i l e s
w e s t c o r n e r o f t h e map a r e a .
vertical
s tra tifie d
only
ex pos ed t o a
contain
all
conglomerate,
grada­
sandstone
two m a j o r l i t h o l o g i c
the p r e s e n t d i s c u s s i o n :
volcani c s ;
(2) w e l l
bedded
(I)
-12PLATE I I I
Figure
A.
2.
Typical
Lava f l o w s .
exposure o f lava
flows
above 8400 f e e t
-13-
e p icla stic
volcanics
logic-types
contain
composed o f g r a i n s
the
intercalated
(Plate
dark to
lig h t
Figures
secondary s i l i c a
and f r a g m e n t s
andesite
map and n e a r b y a r e a s .
Common c o l o r s ,
IV,
flows
fragments
sim ilar
by o x i d a t i o n
and m i n e r a l
Crudely S t r a t i f i e d
E piclastic
consist of poorly
fine r
grained m a trix.
and w e l l
s tra tifie d
epic la s tic
V esicularity,
clasts
is
V,
ar e c o n s p i c i o u s
state
and t h e
ratio
grey,
beds
feet
in
commonly
angular to
diameter,
in
a
amounts o f c o n g l o m e r a t e s
a r e . interbedded in
the crudely
and a n g u l a r i t y
Noni gneous- c l a s t s
a r e ' rare
I).
Many l e v e l s
these e p i c l a s t i c
e x c e p t near
clasts
o f.fo ssil
deposits
o f t he
(Plate
Crude s t r a t i f i c a t i o n ,
angularity
o f m a t r i x bet ween
of clasts,
fragm entsand
V.,
large
clasts
ar e
forest
2 ).
quantities
of
beds.
Figure
in
of the
ar e i n t e r b e d d e d .
t h e G a l I a t i n - Y e l l o w s t o n e . d i v i d e wher e P r e c a m b r i a n
common ( P l a t e
to
types
volcanic
to e ig h t
s ta te .o f oxidation
variable.
intrusions
c on tainin g mostly
Subordinate
bedded e p i e l a s t i c s
composition
green.
epic la s tic
loca lly,u p
and are
Volcanics
sorted breccias
subangglar c l a s t s ,
Both l i t h o ­
g r a i n s , ar e l i g h t
brown and l o c a l l y
stra tifie d
in
and d a c i t i c
dark g r e y , re d d i s h
The c r u d e l y
and cal c i t e
Both l i t h o l o g i c
controlled
I and 2 ) .
up t o
Figure
-14PLATE
Figure
A.
B.
Figure
IV
I.
Lens o f w e l l bedded e p i c l a s t i c v o l c a n i c i n
s t r a t i f i e d e p i c l a s t i c v o l c a n i c beds.
Wel l bedded e p i c l a s t i c v o l c a n i c l e n s .
C r u d e l y s t r a t i f i e d e p i c l a s t i c v o l c a n i c bed.
2.
T y p i c a l exposure o f c r u d e l y
v o l c a n i c beds.
stra tifie d
crudely
e p i c l as t i c
-15PLATE V
Figure
I.
Banded Pr e c a mb r i an f r a g m e n t
beds .
Figure
2.
U p r i g h t o p a l i zed t r e e
v o lc a n ic bed.
in
in e p i c l a s t i c
c o a r s e e p i c l as t i c
volcanic
-16-
eight
f e e t i n d i a m e t e r s u g g e s t a mu d f l o w o r i g i n
' f
cl a s t i c m a t e r i aI .
P a l e o m a g n e t i c measur ement s
e p ic l astic
volcanic
b r e c c ia , in
random s c a t t e r o f d i p o l e
on a n d e s i t i c
sec.
for
this
clasts
epi-
f r o m an
I S i I . 8 S . , . R . 6 E. show a
o rie n ta tio n s ..T h is
indicates
that
t h e t h e r m o - r e m n a n t m a g n e t i z a t i o n was d i s o r i e n t e d ^ , by movement
o f fragments
ture
be l ow t h e i r
blocking
r a nge i n wh i c h m a g n e t i z a t i o n
temperatures,
is
acquired during coo lin g.
Empl acement by mu d f l o w a t t e m p e r a t u r e s
temperatures
of the clasts
reasoning s tated
McCaskey
Wel l
this
loca lly
suggested,
Further discussion
paper in
Bedded E p i c l a s t i c
The w e l l
a section
prossbedded,
and c o n g l o m e r a t e .
t p hu ndr eds
leaves, plant
VI,
Figure
w ater-laid
ar e l o c a l l y
I).
following
( 1957)
the l i n e
of
and Chadwi ck and
o f p a l eomagnetics w i l l
on p a l e o m a g n e t i c s .
beds are, f a i r l y
and i n c l u d e
o c c u r as beds and d i s c o n t i n u o u s
inches
blocking
Volcanics
bedded e p i c l a s t i c
and somp b r e c c i a
bel ow t h e
by A r a ma k i and A k i mo t o
(1968).
appear i n
is
the tempera­
of feet
volcanic
sandstone, s i lts to n e
These l i t h o l o g i c
lenses,
thick.
we 11 s o r t e d ,
ranging
f r o m a few
The beds c o n t a i n
f r a g m e n t s , and l o c a l l y ,
c r o s s bedded;
(3)
(I)
fossil
an a mb e r o i d r e s i n
These v o l cam" c l a s t i cs ar e p r o b a b l y
because t he b e d s :
types
are w e l l
ar e f a i r l y
prim arily
stra tifie d ;
well
(Plate
sorted;
(2)
(4)
-17PLATE VI
Figure
A.
B.
I.
Fossil
material
from e p i c l a s t i c
volcanic
beds.
Ambe r oi d r e s i n , Eh NW% s e c . 25, I . 8 S . , R.5 E.
Leaves and t y p i c a l p e t r i f i e d wood f r a g m e n t .
•[
P 1i
-18commonly o c c u r as d i s c o n t i n u o u s
channels;
(5)
loca lly,co nta in
S tratigraphic
7 and 18,- 1 . 8 S . ,
Wigwam Creek s o u t h
illu stra te d
lenses,
interpreted
t o be
s t r e a m - r o u n d e d . p e t r i f i e d wood. '
Succession
A representative
6,
„
section.of
R.7-E.)
t h e Eocene v p l c a n i c s
(Plate
I)
along the
(secs.
r i d g e wes t o f
t o t h e b o u n d a r y o f t h e map ar e a i s
below.
TABLE I .
REPRESENTATIVE SECTION A
Feet
Description
280
A u g i t e - h y p e r s t h e n e - h o r n b l ende a n d e s i t e
flows
880
Crudely s t r a t i f i e d
ep icla stic
beds
I 360
Hypersthene^augite
andesite
volcanic
flows
I n t e r c a l a t e d w e l l bedded and c r u d e l y s t r a t i f i e d
e p i c l a s t i c v o l c a n i c beds
60
Hypersthene-augite
160
andesite
flows
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Pr e c a mb r i an me t a m o r p h i c
A representative
R.6 E. )
(Plate
section
I ) along the
(secs.
unconformity
rock
14, 23 and 26 ,
r i d g e west o f G r i z z l y
T .8 S . ,
Creek s o u t h
- 19t o t h e b o u n d a r y o f t h e map ar ea i s
illu stra te d
i n Table
II.
/i
TABLE L I .
REPRESENTATIVE SECTION B
Feet
Description
Top Eroded
[•
I 20
Augite-hornblende-hypersthene
andesite
flows
160
Crudely s t r a t i f i e d
epic la s tic
volcanic
beds
Augite-hornblende-hypersthene
andesite
flows
160
Crudely s t r a t i f i e d
volcanic
beds
160
I n t e r c a l a t e d augi t e ^ - h y p e r s t h e n e - h o r n b l ende
a n d e s i t e f l o w s and e p i c l a s t i c v o l c a n i c beds '
320
A u g i t e - h y p e r s t h e n e - h o r n b l ende a n d e s i t e
720
C r u d e l y s t r a t i f i e d e p i c l a s t i c v o l c a n i c beds ,
l o c a l l y i n t e r c a l a t e d w i t h w e l l bedded e p i c l a s t i c
v o l c a n i c beds and l e n s e s
80
epicla stic
" ,■ •;
'
:
flows
'I.
A u g i t e - h y p e r s t h e n e - h o r n b l ende s i l l s
40
Crudely s t r a t i f i e d e p i c l a s t i c v o l c a n i c beds,
l o c a l l y , i n t e r c a l a t e d w i t h w e l l bedded
e p i c l a s t i c v o l c a n i c beds and l e n s e s
960
Cover ed
"
' A representative
sec.
I,
6,
T.9 S . ,
R.6 E . ,
T . 9 S . , R.5 E. )
and G a l l a t i n
section
secs.
(Plate
Counties
(secs.
30 and 31,
25 and 26 , T . 8 S . ,
T. 8 S.,
R.5 E . ,
R.6 E . ,
sec.
I ) a l o n g t h e b o und ar y between Park
south
f r o m Tom M i n e r Creek t o
t he
,
' y ' '
Yellowstone
National
Park b o u n d a r y
TABLE I I I .
is
illu stra te d
i n Table
III.
REPRESENTATIVE SECTION C-
Feet
Description
Er oded t o p
120
Augite-hornblende-hypersthene
. 400
andesite
flows
I n t e r c a l a t e d c r u d e l y s t r a t i f i e d and w e l l bedded
e p i c l a s t i c v o l c a n i c b e d s , w e l l bedded zones
d e c r e a s e i n abundance upward
80
Augite-hypersthene
andesite
flows
80
In te rc a la te d crudely s t r a t i f i e d
e p i c l a s t i c v o l c a n i c beds
80
Augite-hypersthene
andesite
and w e l l
bedded . ,
flows
I n t e r c a l a t e d w e l l bedded and c r u d e l y s t r a t i f i e d
e p i c l a s t i c v o l c a n i c b e d s , w e l l bedded zones
d e c r e a s e i n abundance upward
1840
Covered
Exami n a t i o n
that
in
thicken
o f Tables
I,
a westward d i r e c t i o n ,
(Plate
I,
interbedded with
and p i n c h
out
sec.
the
R.6 E. )
volcanic
I , sec.
I suggest
36 , T . 8 S . ,
volcanic
(T able.II)
flows
becomi ng
beds n e a r Sheep Mo u n t a i n
Gallatin-Yellowstone
The m i d d l e e p i c l a s t i c
and P l a t e
u p p e r mo s t a n d e s i t e
T .8 S . ,
epiclastic
(Plate
Mountai n near the
26,
11 and I I I
R.5 E. )
w e s t o f Sheep
divide.
beds t h i n
(Plate
I , sec.
,
-21-
13, 1 . 8 S . ,
R.6 E. )
(Table
and become i n c r e a s i n g l y
flows.
(Plate
I,
w e s t w a r d t o w a r d Sheep . Mo u n t a i n -
in te rc a la te d with
These e p i c l a s t i c
Mountain
II)
sec.
beds t h i c k e n
6,
become i n c r e a s i n g l y w e l l
T .9 S . ,
thickening
andesite
a g a i n w e s t o f Sheep
R.5 E. )
(Table
III)
and
bedded t o w a r d t h e G a l l a t i n - Y e l l o w s t o n e
d i v i de.
The l o w e r a n d e s i t e
f l o w s , w e s t w a r d t o w a r d Horse Cr ee k ,
gr ade f r o m a sequence o f ma s s i v e f l o w s
T.8 S .,
R.6 E. )
and e p i c l a s t i c
(Table
sec.
II).
(Table
I)
volcanic
The f l o w s
15, T . 8 S . ,
(Plate
I , sec.
t o a sequence o f i n t e r b e d d e d f l o w s
beds
(Plate
I , s ec .
14,
T . 8 . S.,
p i n c h o u t w e s t o f Horse Creek
R.6 E. )
12,
(Table
R.6 E. )
(Plate
I ,
III).
Eocene I n t r u s i o n s
A n d e s i tes
Seven a n d e s i t e
twelve
in
feet
and s i x
thickness,
dikes,
with
andesite
recorded thicknesses
s ills ,
o f two t o
r a n g i n g f r o m 40 t o 80 f e e t
o c c u r e a s t o f Horse Creek and s o u t h w e s t o f
Wigwam C r ee k .
Four a d d i t i o n a l
s ills ,
20 t o 40 f e e t
thick,
ar e
mapped bet ween S u n l i g h t and Horse C r e e k s .
The d i k e s
of plagioclase,
and s i l l s
augite,
ar e d a r k g r e y and c o n t a i n
hornblende
phenocrysts
and. h y p e rs th e n e .
Dacites
A north-south
elongated d a c i t i c ' s to c k ,
a p p r o x i m a t e l y two
-22-
square m i l e s
in
a r e a , comprises
w e s t end o f S a wt o o t h Mo u n t a i n
I . 8 S.,
R.6 E. )
(Plate
elongated d a c i t i c
exposure, is
(secs.
11,
located
12,
l e a s t 80 f e e t
plug,
VII,
(secs.
Figure
16, 21,
I).
28,
A small
29 ,
32 and 33,
northwest-
a p p r o x i m a t e l y 0. 1 2 5 s q u a r e m i l e
a b o u t one m i l e
13 and 14,
thick
Sheep Mo u n t a i n and t h e n o r t h ­
T. 8 S . ,
caps a r i d g e
s o u t h w e s t o f Sheep Mo u n t a i n
e a s t o f Horse Creek
R.6 E. )
and a d a c i t e
approximately
(secs.
in
5,-31
sill
0.75 miles
and 32 ,
T.8 S .,
R.6 E . ) .
The d a c i t e s
contain
ar e l i g h t
phenocrysts
to dark grey or r e d d i s h
of plagioclase,
bio tite
g r e y and
and h o r n b l e n d e .
at
-23-
PLATE V I I
Figure
A.
I.
View l o o k i n g s o u t h t o w a r d Sheep M o u n t a i n ,
dacite in tru s io n .
Sheep Mo u n t a i n s t o c k , a d a c i t e i n t r u s i o n .
a
-24PI i O - P l e i s t o c e n e
Wel ded T u f f
■ Wel ded t u f f
j^Mountai n] ( s e c .
fee t;
17,
1 . 8 S.,
r e mna nt s o c c u r : ( I )
Rv/
( 2 ) w e s t o f Horse Creek
6300 f e e t ;
at
erosional
about
(3)
E. ) a t an a l t i t u d e
(sec.
e a s t o f Wigwam Creek
7600 f e e t
(Plate
3, T . 8 S . ,
(secs.
6,
I).
resting
The l i t h o l o g i c
V III,
Figure
o f ab o u t 7200
R.6 E . ) at about
7,
1.8 S.,
R. I
o f a b o u t 60 f e e t
on a " p a l e o - s o i I " ( P l a t e
sequence i s
as f o l l o w s
V III,
(Refer to
Figure
Plate
2):
TABLE I V.
WELDED TUFF,SEQUENCE
Feet
D e s c r i p t i on
Er oded t o p
26 "0"
Pink f r i a b l e
23'6"
B l a c k d e n s e l y wel de d t u f f w i t h
c a v i t i e s and s p h e r u l i t e s .
p a rtia lly
6'11"
Black
2'0"
Brown p a r t i a l l y
0'3"
V o l c a n i c ash
glassy
we l d e d t u f f
lithophysal
densely welded t u f f
wel ded t u f f
- unconformity
covered
"paleo
soil"
- unconformity
epic la s tic
E.)
I).
The d e p o s i t e a s t o f Sheep Creek c o n s i s t s
o f welded t u f f ,
e a s t o f Sheep
v o l c a n i c bed
(?)
-25-
Figure
I.
Vi ew o f Tom M i n e r Bas i n l o o k i n g e a s t .
Crudely
s t r a t i f i e d e p i c l a s t i c v o l c a n i c bed i n l e f t
foreground.
Exposur e o f we l d e d t u f f r emnant s o u t h o f Tom M i n e r
Creek and e a s t o f Sheep Cr eek .
Exposur e o f we l d e d t u f f n o r t h o f Tom M i n e r Cr eek.
2.
L i t h o l o g i c sequence o f we l d e d t u f f r e mnant ( s e c .
17, R.6 E . , T . 8 S. ) l o c a t e d e a s t o f Sheep Creek.
P i n k p a r t i a l l y wel ded t u f f .
B l a c k d e n s e l y wel de d t u f f w i t h l i t h o p h y s a l c a v i t i e s
and s p h e r u l i t e s .
B l a c k g l a s s y d e n s e l y we l d e d t u f f .
Brown p a r t i a l l y wel de d t u f f .
Ash.
" Pa I e o s o i l " .
A.
B.
Figure
A.
B.
C.
D.
E.
F.
- 26The r e mna nt l o c a t e d w e s t o f Horse C r ee k ,
to f i v e
feet
thick,
is
a pink or grey,
/
The we l d e d t u f f
thick
in
rock,
with
as a h i g h l y
thin
four
'
r e mnant e a s t o f Wigwam C r e e k ,
e x p os ur e, occurs
brown c r y s t a l l i n e
flow banding..
least
fria b le ,p a rtia lly
we l d e d t u f f .
feet
at
altered,
wavy s t r i n g e r s
ab o u t 80
da r k
that
r e s e mb l e
-27-
Quaternary
S urficjal
Deposits
A llu v ia l,
rocks
glacial
and l a n d s l i d e
material
cover o ld e r
o f t h e map a r e a .
The s u r f i c i a l
glacial
d rift
with
Quaternary
of glacial
o f Tom M i n e r Bas i n ar e m a i n l y
m i n o r amounts o f - a l l u v i u m .
deposits
debris,
and a l l u v i u m .
deposits
along
the
recent la n d s lid e
Yellowstone
material
River consist
( Good,
1964)
III.
PETROGRAPHIC DESCRIPTIONS
P r e c a mb r i an M e t a m o r p h i c Rocks
Gnei ss
The g n e i s s e s
55%),
bi o t i t e
se ric ite ,
contain
( 5-30%)' ,
(0-1%),
quartz
epidote
and t r a c e s
(35-50%),
(0-1%),
plagioclase
muscovite,
of garnet,
( 2 0-
including
mi c r o c l i n e ,
chlorite
and opaques.
Quartz, b io t it e
gneisses
ex a mi n e d .
and p l a g i o c l a s e
ar e a b u n d a n t i n
all
t he
S out hwar d t o w a r d t h e P r e c a m b r i a n - v o l cani
c o n ta c t the percentage o f p l a g i o c l a s e
percentage o f b i o t i t e
decreases.
increases,
Thin s e c t i o n s
and t he
indicate
no
X
consistent
change i n q u a r t z
and t h e m i n o r m i n e r a l s
through
the a r e a .
Quartz occurs
to subhedral
as
(I)
anhedral
masses and ( 2 )
masses o f i n t e r l o c k i n g
maximum d i m e n s i o n .
to the f o l i a t i o n .
grains
anhedral ■
up t o 4 . 0 mm.
in
The q u a r t z , masses ar e e l o n g a t e d p a r a l l e l
Grains
ar e f r e s h ,
e x h i b i t wavy e x t i n c t i o n
and some have r e c r y s t a l I i zed m a r g i n s .
P la g io c la s e occurs
up t o 4 mm.
of quartz
long,
having i r r e g u l a r
and b i o t i t e .
" M i c h e l - L e v y Met hod"
ar e f r e s h
as s u b h e d r a l
is
Composition
Iqth
boundaries
and i n c l u s i o n s
d e t e r m i n e d by t h e
An^g, I a b ra d o ri t e .
b u t some ar e p i t t e d .
shaped c r y s t a l s ,
Most c r y s t a l s
A l b i t e , carlsbad
and
\
-29-
peri d i n e
twinning
Subhedral
b io tite ,
elongated p a r a l l e l
in te rio r
loca lly
up t o 2 mm.
long,
to the f o l i a t i o n .
is
altered
to c h l o r i t e
epidote
in
shades o f p i n k ,
in
aggregates.
Garnets
se ricite
ar e f r e s h ,
up t o 6 mm.
in
thin
magnetite
and
is
s lig h tly
pleochroic
i n maximum d i m e n s i o n and o c c u r
parallel
ar e a n h e d r a l , up t o
pale pink
have f r e s h
and m u s c o v i t e .
crystals
E xtinction
commonly b e n t , and
Most c r y s t a l s
b u t ar e commonly ri mmed by a n h e d r a l
Anhedral
sion,
a r e common.
section
in elongated s ectio ns.
1 . 7 5 mm.
i n maximum d i me n­
and commonly a l t e r e d
to
on t h e i r m a r g i n s .
Mi c r o c l i n e
"gridiron
occurs
structure"
as s u b h e d r a l
crystals
exhibiting
and wavy e x t i n c t i o n .
Schist
The s c h i s t s
(27-55%),
a r e composed o f q u a r t z
pI a g i o cI a s e ( 0- 32%) , b i o t i t e
and e p i d o t e
blasts.
hornblende
(0-7%), magnetite
(1%)
(0-1%).
In t h e s l i d e s
loca lly
(33-50%),
e x a mi n e d ,
b io tite
a b s e n t and p l a g i o c l a s e
Al I m i n e r a l
grains
in
and p l a g i o c l a s e
ar e
may be p r e s e n t as p o r p h y r o the s c h i s t are p a r a l l e l
to
the f o l i a t i o n .
Quartz
is
loca lly
many u n i d e n t i f i a b l e
cataclastic,
c ry s ta llite s .
r e c r y s t a l I i zed and c o n t a i n s
Individual
grains
are
anhedral
to subhedral,
Wavy e x t i n c t i o n
and up t o 0 . 5 mm.
and g r a p h i c
Pal e gr e e n s u b h e d r a l
hornblende
up t o
1 . 0 mm.
s e a m - t w i n n e d and o t h e r s
some g r a i n s
and i s
Plagioclase
to anhedral
included
Some g r a i n s
are z on e d.
O.75 mm.
in
laths
of
Some g r a i n s
A n he dr a l
are
magnetite
r i ms -
others.
as s u b h e d r a l
commonly a l b i t e ,
is
contorted
rim b i o t i t e .
a p pea r s
ar e e v i d e n t .
l o n g ar e p r e s e n t .
l o n g and i s
B iotite
intergrowths
i n maximum d i m e n s i o n .
laths
up t o 0 . 5 mm.
c a r l s b a d and p e r i d i n e
p r e s e n t as de f o r me d s u b h e d r a l
twinned.
flakes
up t o
long.
Epidote occurs
as a n h e d r a l
grains
up t o 0.1
mm.
i n-,
maximum d i mensi on.
Schistose
Quartzite
The gr een s c h i s t o s e q u a r t z i t e
and m u s c o v i t e ,
and q u a r t z
including
ar e e l o n g a t e d p a r a l l e l
Most q u a r t z
grains
and an i n t e r l o c k i n g
parallel
sericite
contains
(2-3%).
to
ar e 1 . 5 mm.
texture.
quartz
(97-98%)
Both m u s c o v i t e
the f o l i a t i o n .
long,
Individual
have wavy e x t i n c t i o n
grains
form sub­
bands.
Granulit e
Based on one t h i n
section,
the g r a n u l i t e
is
composed o f
-31-
plagioclase
magnetite
and t r a c e s
( 45%) ,
augite
and h e m a t i t e
( 45%) ,
( 2%),
graphic
hornblende
occurs
Anhedral
dimension
long,
( 1%) ,
(5%),
b io tite
(1%)
o f mi c r o c l i n e .
P l a g i o c l a s e , commonly z o n e d , b e n t ,
fractured,
intergrowths
as s u b h e d r a l
to subhedral
and s u b h e d r a l
ar e a l t e r e d
Microcline
and m a g n e t i t e
laths
augite,
c o r r o d e d and
up t o 3 . 0 mm.
up t o 2 mm.
books o f b i o t i t e ,
to hornblende
and g r a p h i c
and h e m a t i t e
up t o
in
long.
maximum
1. 2 5 mm.
along t h e i r margins.
intergrowth
are random i n
o c c u r as a l t e r a t i o n
inte nsity
products
of
augite.
S ills
The s i l l s
clase
exhibit
(40-50%),
growths
(0-1%),
augite
diabasic texture
( 40- 50%)
hypersthene
and c o n s i s t o f p l a g i o -
in te rs titia l
( 0- 1%)
graphic
and h o r n b l e n d e
in te r­
(less
t han
I %).
Anhedral
twinned
augite,
and l o c a l l y
Subhedral
a composition
Magnetite
laths
up t o 1. 2 5 mm.
uralized
of plagioclase,
and i s
diameter,
is
seam-
to hornblende.
o f An^g, l a b r a d o r i t e
rims
in
up t o 1. 5 mm.
(Michel-Levy
included
in
long,
have
Me t h o d ) .
hypersthene
and
augite,
Met amor phi sm o f t h e s i l l s
is
suggested by:
(I)
pi a g i o -
-32-
clase
(3)
grains
local
with
r e c r y s t a l I i zed e d g e s ; ( 2 )
cataclastic
structures;
(4)
fractured
alteration
crystals;
of augite
to
gr een a m p h i b o l e .
Han d- s p ec i men s
of pegmatite
c o n s i s t o f coarse
q u a r t z , f e l d s p a r and some m u s c o v i t e
grained
and b i o t i t e .
Eocene Lava Fl ows
Hypersthene-Augite A n d e s ite s These a n d e s i t e s
(1-6%),
hypersthene
secondary s i l i c a
Phenocrysts
contain
(1-2%),
plagioclase
basaltic
and c al c i t e
( 0- 1%)
o f h y p e r s t h e n e ar e
(15-55%),
hornblende
augite
(0-2%),
and gr ound mass
( 3 5 - 8 3 %) .
commonly more abundant
than a u g i t e .
The p l a g i o c l a s e
euhedral
laths
zoned, p i t t e d
commonly f r e s h
occurs
up t o
2 mm.
as u n o r i e n t e d a n h e d r a l
long.
and have c o r r o d e d m a r g i n s .
and a I b i t e ,
is
An^,
Anhedral
dimension,
is
euhedral
dimension.
Smaller c r y s t a ls
d e t e r m i n e d by t h e
to subhedral
lo ca lly
augite,
up t o 1. 5 mm.
s e a m - t w i n n e d • and r a r e l y
pleochroic,
to -anhedral
in
"Michel-Levy
i n maximum
altered.
shades o f l i g h t
and up t o 0 . 5 mm.
ar e
twinned.
I abradori t e ,
Hypersthene.is
pink,
are g e n e r a l l y
c a r l s b a d and p e r i c l i n e
Composition o f the ph en ocrysts
Met hod"
Larger c r y s t a l s
to
gr een t o
i n maximum
-33-
Basaltic
up t o
hornblende occurs
1 . 2 5 mm.
l o n g and i s
plagioclase,
of m icro lite s
anhedral
magnetite
Augite-Hornblende-Hypersthene
Andesites
Augite
in
and h e m a t i t e .
of hypersthene,
and c r y s t a l l i t e s .
and h o r n b l e n d e ar e t h e most a b u n d a n t p h e n o c r y s t s
these a n d e s i t e s .
map a r e a ,
elongated c r y s t a ls
ri mmed by m a g n e t i t e
The g r o u n d mass c o n s i s t s
augite,
as s u b h e d r a l
In. t h e
hornblende
u p p e r mo s t a n d e s i t e
phenocrysts
flows
o f the
ar e n e a r l y as a b u n d a n t as
augite.
These a n d e s i t e s
(1-6%),
hypersthene
hornbl ende. (1 - 2% ),
Plagioclase
carlsbad
contain
(1-3%),
plagioclase
basaltic
and g r oun d mass
lath s,
and p e r i d i n e
hornblende
zoned, p i t t e d
and have c o r r o d e d b o u n d a r i e s .
are g e n e r a l l y
fresh.
by t h e
"Michel-Levy
Augite
hedral
Met hod"
phenocrysts
and up t o
phenocrysts-are
1 . 2 5 mm.
Smaller c ry s ta ls
o f phenocrysts
determined
Anl^ - An^ g , I a b r a d o r i t e .
i n maximum d i m e n s i o n .
to sub­
Seam- t wi nn ed
common.
subhedral
Most c r y s t a l s
ar e commonly
ar e commonly f r e s h , a n h e d r a l
Hornbl ende p h e n o c r y s t s
elongate,
is
( 0-2%) ,
l o n g ar e commonly a l b i t e ,
Larger c r y s t a ls
Composition
augite
( 3 5 - 8 5 %) .
up t o 4 mm.
twinned.
(15-55%),
ar e f r e s h
to euhedral
ar e p l e o c h r o i c
in
to h ig h ly
altered,
and up t o 1. 2 5 mm.' l o n g .
shades o f g r e e n ,
some ar e
seam-twinned.
Hypersthene appears
prism atic
habit,
up t o
as s u b h e d r a l
1. 2 5 mm.
The g r oun d mass c o n s i s t s
augite,
hornblende,
anhedral
Eocene C r u d e l y S t r a t i f i e d
Most c l a s t s
in
flows
and d a c i t e
rare.
in
plagioclase,
and s m a l l
contains
basaltic
andesite
Eocene Wel l
s tra tifie d
ar e v e r y
epic la s tic
sim ilar
rare.
Beds
volcanic
to the andesite
Dacite
clasts
ar e
None were o b s e r v e d
of augite,
hypersthene,
hornblende,
magnetite,
sparse hornblende
pa rticles.
magnetite,
Loc al I y * t h e m a t r i x
and cal c i t e .
Volcanic
bedded e p i c l a s t i c
of augite,
fragments.
crystals
Bedded E p i c l a s t i c
The w e l l
blende,
Volcanic
o f t h e map a r e a .
and d a c i t e
cement ed by s i l i c a
crystals
of plagioclase,
section.
The m a t r i x
is
of
m a g n e t i t e and c r y s t a l l i t e s .
descriptions
Pr e c a mb r i an c l a s t s
thin
of m ic ro lite s
E piclastic
intrusions
crystals
long.
the' c r u d e l y
beds have p e t r o g r a p h i c
to anhedral
Beds
volcanic
hypersthene,
beds a r e composed o f
plagioclase,
basaltic
s p a r s e h o r n b l e n d e and a n d e s i t e
The p l a g i o c l a s e
C e m e n t a t i o n by s i l i c a
is
and d a c i t e
commonly s a u s s u r i t i z e d .
and cal c i t e
is
common.
horn­
Eocene I n t r u s i o n s
Andesite
Di kes
In the s l i d e s
characteristics
andesite
flows
Andesite
S ills
S ills
e x a mi n e d ,
sim ilar
the dikes
to the a u g i te -hornble nde -hy p ers th ene
o f t h e map a r e a .
associated with
a u g ite-hypersthene
h o rn b lende-hypersthene andesite
graphic
Dacite
have s i m i l a r
petro­
Intrusions
in
composition
t h e map ar ea s u g g e s t s
The d a c i t e
(less
flows
or au g i t e t
descriptions.
The s i m i l a r i t y
in
had p e t r o g r a p h i c
than
bodies
t h e y ar e c o n n e c t e d a t d e p t h .
contains
1-4%),
among t h e d a c i t e
plagioclase
b io tite
(less
than
(5-6%),
hornblende
1-2%) and g r oun d mass
( 8 9 - 9 5 %) .
The p l a g i o c l a s e
crystals
up t o
o c c u r s as s u b h e d r a l
3 . 2 5 mm.
long.
The l a r g e r p h e n o c r y s t s
commonly a n h e d r a l , z o n e d , p i t t e d ,
margins.
carlsbad
Smaller c r y s t a l s
twinned.
Levy M e t h o d " ,, i s
unoriented
are
and c o r r o d e d a r o u n d t h e
ar e f r e s h ; some a r e a l b i t e
The c o m p o s i t i o n ,
Anj^
to anhedral
d e t e r m i n e d by t h e
and
"Michel-
t o Anz^2 , a n d e s i n e .
Hornbl ende appears
as p o o r l y
terminated elongate
crystals
-36-
up t o 3 . 0 mm.
long.,
The h o r n b l e n d e
and ri mmed by a n h e d r a l
B iotite
2 . 0 mm.
occurs
as t a b u l a r
and p r i s m a t i c
Crystals
C haracteristics
Common t o
Phenocrysts
crysts
up t o
are commonl y f r e s h ;
Welded T u f f
t h e Welded T u f f Remnants
of quartz,
plagioclase,
o r t h o c l a s e and a u g i t e
2 p e r c e n t o f t h e we l d e d t u f f s .
B iotite
pheno-
are s p a r s e .
Fr agment s o f a n d e s i t i c
loca lly
and d a c i t i c
composition
ar e
abundant.
Vapor-phase m i n e r a l s
cation minerals
occur in
Vapor-phase m i n e r a l s
minerals
Smi t h
crystals
inclusio ns, of magnetite.
P lio-Pleistocene
comprise I to
commonly.seam-twinned
magnetite.
i n maximum d i m e n s i o n .
some c o n t a i n
is
but both
(1960)
cristo b o lite
rh yolite
tridym ite
occur in
D e v itrifi­
glass.
ar e c o a r s e r t ha n d e v i t r i f i c a t i o n
a r e t o o s ma l l
finds
po r e s p a c e s .
to
d e vitrifica tio n
be i d e n t i f i e d
minerals
by m i c r o s c o p e .
ar e c h i e f l y
and f e l d s p a r and v a p o r - p h a s e m i n e r a l s ,
ash f l o w s ,
are p r e d o m i n a n t l y
and c r i s t o b o l i t e .
alka lic
in
feldspar,
Welded T u f f Remnant Ea s t o f Sheep Creek
Zones D e s c r i b e d f r o m Bot t om t o Top
Brown P a r t i a l l y
Welded Zone
Shar ds b e n t a r o u n d c r y s t a l s
fragments
suggest compression.
un def o r me d s h a r d s
Hematitic
suggests
and b a r e l y c o l l a p s e d
Welding i s
pumi ce
n o t i n t e n s e bu t
ape r a r e .
staining,
resulting
in
a dull
brown c o l o r ,
oxidation.
No d e v i t r i f i c a t i o n
or vapor-phase m inera ls
are e x h i b i t e d .
B l a c k G l a s s y D e n s e l y Welded Zone
Dense w e l d i n g
d e f o r me d s h a r d s
lost
all
is
illu stra te d
and f l a t t e n e d
by.flattened
severely
pumi ce f r a g m e n t s
that
have n e a r l y
structure„
Thin
sections
did not d i s p la y
d e vitrifica tio n
or vapor-
phase m i n e r a l s .
B l a c k D e n s e l y Wel ded Zone w i t h L i t h o p h y s a l
Spherulite s
:
Dense w e l d i n g
de f or med s h a r d s
is
illu stra te d
cation
minerals
cation
minerals,
pumi ce f r a g m e n t s
aggregates
by s e v e r e l y
and pumi ce f r a g m e n t s .
d e st ro ye d near l i t h o p h y s a l
wh i c h
cavities
Shard s t r u c t u r e
and l o c a l l y
t o be i d e n t i f i e d ,
than s h a rd s.
in
and
is
by d e v i t r i f i ­
D e v itrifi­
are c o a r s e r i n
D evitrifica tion
spherulites
and
flattened
cut -a c r o s s 1 shard s t r u c t u r e .
too small
of minerals
Cavities
f o r ms
radial
and I o c a l I y , a x i o l i t i c
-38-
structure
Pink
in
shards.
P a rtia lly
Wel ded Zone
Shar d s t r u c t u r e
d e vitrifica tio n
is
a l m o s t c o m p l e t e l y d e s t r o y e d by
and v a p o r - p h a s e m i n e r a l s .
minerals,
t o o s ma l l
t o be i n d e n t i f i e d ,
fragments
and s p h e r u l i t e s
phase m i n e r a l s ,
although
d e vitrifica tio n
minerals.
We l d i n g i s
b u t more i n t e n s e
Wel ded T u f f
and f i n e r
in
as,in
pumice,
Vapor-
ar e c o a r s e r t han
t h e brown p a r t i a l I y we l d e d z o n e .
Remnant West o f Horse Creek
r e s e mb l e s
the pink
we l d e d zone- o f t h e r e mna nt e a s t o f Sheep Cr eek.
Wel ded T u f f
Remnant Eas t o f Wigwam C r e e k -
Shar d s t r u c t u r e
d e vitrifica tio n .
flattened.
is
a l m o s t c o m p l e t e l y d e s t r o y e d by
Where o b s e r v a b l e ,
We l d i n g
Inclusions
is
of andesitic
and d a c i t i c
clasts
six
up t o
inches
s h a r d s ar e de f o r me d and
present but not in te n s e .
phenocrysts)
Refractive
shards.
in
d e n s e l y we l d e d zones
The r o c k exposed p e t r o g r a p h i c a l l y
p a rtia lly
ar e c o a r s e
un identifiable,
n o t as i n t e n s e
than
in
D e vitrifica tion
(plagioclase,
(plagioclase,
in
diameter,
augite,
hornblende
hypersthene
phenocrysts)
ar e p r e s e n t .
I n d e x o f Gl as s e s
The r e f r a c t i v e
i n d e x o f t h e ash a t t he base o f - t h e wel de d
-39
tu ff
r e mn a n t e a s t . o f Sheep Creek was f o u n d t o b e , 1 , 5 0 1 .
refractive
we l d e d t u f f
in d e x - o f glass
r e mna nt s
1. 5 0 0 t o
George's
(1924)
suggests
the percent s i l i c a
amount o f s i l i c a
g r aph
is
shards
1.502.
( p e r c e n t SiOg v s .
suggests
magma as r h y o l i t i c
obtained
(Bowen,
o f the
1928).
from the o t h e r
A comparison w it h
index-of re fra c tio n )
rock is
cla ssifica tio n
The
about 72.5.
o f the o r i g i n a l
This
•IV.
STRUCTURE
Precambrian
P r e c a mb r i an f o l i a t i o n
on t h e e a s t e r n
4 3 - 5 5 NE
and n o r t h e r n m a r g i n s o f t h e P r e c a m b r i an o u t c r b p
a r e a and s t r i k e s
margin.
s t r i k e s . N 3 0 - 6 0 W and d i p s
N50-70E and d i p s
The r e s u l t i n g
28-90NW on t h e w e s t e r n
configuration
is-a
northward plunging
a n t i form.
The d i k e s
N70E and d i p
in
25NW.
Eocene E x t r u s i v e
The s t r i k e
and a n d e s i t e
t h e P r e c a mb r i an me t a mo r p h i c r o c k s t r i k e
Rocks
of-the
flows
is
intercalated
generally
ar e as much as 40 d e g r e e s
Eocene A n d e s i t e
Locally,
northwest.
volcanic
Locally,
beds
dips
b u t most a r e near h o r i z o n t a l .
Di k es
andesite
Recor ded s t r i k e s
epicla s tic
dikes
are i r r e g u l a r
v a r y f r o m N60-85W.
in
a ttitu d e .
Di ps v a r y f r o m v e r t i c a l
t o 65N.
Eocene D a c i t e
Intrusions
The Sheep M o u n t a i n s t o c k
lacks
flow s tru c tu re .
e a s t o f Horse Creek has ne a r v e r t i c a l
margins.
Where e x p o s e d , t h e s i l l
has a b r e c c i a t e d
base.
The p l u g
f l o w banding near i t s
s o u t h e a s t o f Sheep Mountai n-,
-41 A n o r t h w e s t t r e n d i n g near v e r t i c a l
have f o u r t o s i x
feet
head o f Horse Creek
beds by t h e
wh i c h
is
of displacement,
(sec.
34, 1 . 8 S . ,
Sheep M o u n t a i n d a c i t e ,
lo c a te d 0.50 miles
fa u lt,
estimated
to
i s , l o c a t e d west o f the
R. 6 E.,)..
Doming o f
t h e exposed c o n t a c t o f
north,
c o u l d have p r o d u c e d t he
fa u lting.
A vertical
northeast
of stratigraphic
trending
displacement
M i n e r Creek on t h e w e s t f l a n k
I . 9 S.,
feels
R.5 E . ) .
that
related
is
f a u l t with
780 t o 820 f e e t
lo c a t e d at the
head o f Tom
o f Bi g Horn Peak ( s e c .
H. W. Smedes
(oral
communication,
domi ng o f t h e beds on B i g Horn Peak i s
to
a dacite
intrusion.
This
suggests
12,
1968)
probably
the f a u l t
could
have been p r o d u c e d by t h e domi ng.
Doming o f b e d s ,
suggest the
dacites
Deep Creek Normal
The t r a c e
fa u lt,
a fa u lt
block,
is
and t r e n d s
flow:banding
were f o r c i b l y
area.
schists
base
empl ac ed.
Fault
o f the northwest t r e n d i n g
Deep Creek normal
boundi ng the n o r t h w e s t si de o f t h e
Beartooth
exposed a b o u t one m i l e n o r t h e a s t o f t h e map area
t o w a r d t h e map a r e a .
r e c e n t movement a l o n g t h i s
No s u r f a c e
and a b r e c c i a t e d
fault,northw est
expression of th is
P r e c a mb r i an g r a n i t i c
can be t r a c e d w i t h
Displacement o f
fa u lt
gneiss
is
fans
suggests
o f t h e map a r e a .
recognized
i n t h e map
and h o r n b l e n d e - b i o t i t e
no a p p a r e n t o f f s e t
f r o m Dome
-42Mountain,
one m i l e
n o r t h e a s t o f t h e map a r e a ,
t o n e a r t h e mouth o f Tepee Creek
McNei l , o r a l
striking
communication,
( R.
19 6 9 ) ,
K e r n , , R„ Macki n and R0
Hornblenderbiotite
N40W and d i p p i n g 48-50NE a r e
Tom M i n e r Creek n e a r i t s
westward e x t e n s i o n
o f the
to a southwest t r e n d i n g
paralleling
mout h,
is
. These f a c t o r s
If
ment i s
s u g g e s t e d because P r e c a m b r i a n
schists
with
located
attitudes
in
this
strip ,
o f the f a u l t .
l o c a t e d on b o t h
sides
o f Torn M i n e r C r e e k ,
just
off
little
o f ■6300 f e e t . and i n
displace­
sec,
sec,
r emnant s
3, T ,8 S . ,
35, T. 7- S, , R , 6 E, ,
o f 6080 f e e t ,
suggest
any d i s p l a c e m e n t a l o n g Tom M i n e r Creek s i n c e
empl acement o f t h e we l d e d t u f f , .
w e s t w a r d up Wigwam,
Creeks,
T. 7 S . ,
w o u l d be
Welded t u f f
t h e map ar e a a t an a l t i t u d e
if
lim ited
little
and o r i e n t a t i o n s
sides
altitude
is
hornblende-bio tite
l o c a t e d on b o t h
R,6 E,,
any,
secs, 25 , 30 and 36 ,
R , 6 E„
sim ilar
if
s u g g e s t sout h-,
a b o u t o n e - q u a r t e r m i l e wi de
Tom M i n e r Creek i n
the f a u l t
schists,
l o c a t e d on b o t h s i d e s o f
Deep Creek f a u l t
strip
southwest ward
G rizzly,
no a p p r e c i a b l e
epic la s tic
volcanic
If
Horse,
offset
is
the f a u l t
swings
south-
Sheep, Walsh o r Tom M i n e r
recognized
beds and a n d e s i t e
in well-bedded
flows.
S p a n i s h Peaks and G a r d i n e r F a u l t s
The t r a c e
fa u lt
is
The t r a c e
o f the n o r t h w e s t - t r e n d in g
exposed a b o u t f i v e
miles
Span i s h Peaks r e v e r s e
n o r t h w e s t of. t h e map a r e a .
of the n o rth w e s t-tre n d in g
Gardiner reverse
fault
is
-43-
exposed a p p r o x i m a t e l y s i x m i l e s
(Figure
2)„
Gener al
a l i g n m e n t . o f these f a u l t s
may r e p r e s e n t
t h e same f a u l t
and C h a d w i c k ,
1964).
these
faults
surface
ar e a o
southeast o f . t h e
trend
However ,
is
expression
o f these f a u l t s
is
recognized
A l i g n m e n t o f t h e Sp a n i s h Peaks r e v e r s e
n o t an e x t e n s i o n
(secs.
I . 8 S.,
R.6 E. )
as e s t a b l i s h e d
16 ,
21 ,
22,
by Chadwi ck
intrusions
Peaks f a u l t
and p r e - Eocene f a u l t i n g
the an desite
dikes
in
between
and no
t h e map
f a u l t with
centers
and n o r t h w e s t s t r i k i n g
23 ,
24*
28 ,
o f t h e map ar ea a r e a l l
and d a c i t e
and S p a n i s h
McManni s
suggests
o f the G ar dine r f a u l t .
intrusions
13,
volcanic
p o t a s s i u m p o o r West er n Abs ar ok a b e l t ,
dikes
19 34;
has n o t been e s t a b l i s h e d - . ( R u p p e l , . 1 9 6 8 ) ,
The d a c i t e
dikes
suggest they
t h e age r e l a t i o n s h i p
p a r t o f t h e West er n A b s a r o k a b e l t
it
(Wilson,
map ar ea
(1968).
with
suggests
29,
31 and 32,
located
labeled
andesite
along the
K~ on F i g u r e 2,
, Alignment o f the andesite
t h e West er n A b s a r o k a b e l t
that
shearing,
fracturing
c r e a t e d a zone o f weakness a l o n g wh i c h
and d a c i t e
intrusions
were emp l a c e d .
V.
When a l a v a c o o l s ,
moment wh i c h
at
is
PALEOMAGNETICS
magnetic m in e ra ls
aligned p a ra lle l
the time o f c r y s t a l l i z a t i o n .
t a k e on a m a g n e t i c
to the e a r t h ' s
This
a l i g n m e n t o f magneti c
moments causes t h e
r o c k t o become s l i g h t l y
rock fragment w i l l
have a n o r t h
a dipole.
altered
The o r i e n t a t i o n
only
magnetized.
of ,this
magnetic d i p o l e
strike.
is
m a g n e t i c d i p o l e may be r e c o r d e d u s i n g a
f I u x g a t e ma g n e t o m e t e r .
ma g n e t o me t e r i s
the p o s i t i o n
(S)
seeking poles
is
placed in
a line
(the
to
northward
locate
poles.
The n o r t h , (N)
ar e marked on each f r a g m e n t .
orig inal
is
orientation
passing through
measured.
the e a r t h ' s
pole p o i n t i n g
normal.
seeking
dipole)
seeking poles
parallel
used t o
its
O rientations
and r e v e r s e d ,
Differences
t h o u g h t t o be r e f l e c t
of
(N)
and s ou t h
parallel
or nearly
north
(S)
seeking
are c a l l e d
seeking pole p o i n t i n g
ar e
reversed.
"fixed"
in the a t t i t u d e
a reversal
(S)
Then t h e f r a g m e n t
downward,
the south
(N)
and t h e o r i e n t a t i o n
and d i p p i n g downward a r e c a l l e d
o r i e n t a t i o n s , normal
and s o u t h
present magnetic d i p o l e ,
with
The
o f the no rt h
the no rt h
n o r t h w a r d - and d i p p i n g
O rientations
upon c o o l i n g .
usually
O rientation,
B r u n t o n compass and a p o r t a b l e
and s o u t h
Each
s e e k i n g and s o u t h s e e k i n g p o l e ,
by r e h e a t i n g o r l i g h t n i n g
o f a rock f r a g m e n t's
magnetic f i e l d
in
These,
the lava
o f the d ip o le
o f the e a r t h ' s
ar e
magnetic f i e l d .
-45Magnet i c d i p o l e , o r i e n t a t i o n s
r emnant s and a s i l l
m e t a m o r p h i c r o c k ar e l i s t e d
Examination
in.th e
a z i m u t h s o f - 10 - 40 d e g r e e s and d i p s
flows
norm al,orientations..
a t an a l t i t u d e
o f a b o u t 5900 t o 6780 and 9360- t o
flows
found a n d e s i t e
orientations.
flows
This
s o u t h . o f t h e map ar e a
9680 f e e t ,
suggests andesite
o f t h e map a r ea between 5900 t o 6780 and 9460 t o 9680
f e e t . may have r e v e r s e d d i p o l e
The d i p o l e
Table V i s
that
w ith,
E. L . -McCaskey ( o r a l
1969)
reversed d ip o le
P r e c a mb r i a n
o f 37-51 .d e g r e e s , n o r t h
communication,
with
dacite
i n T a b l e V.
o f T a b l e V shows a n d e s i t e
s e e k i n g p o l e down,
flows,
orientation
questionable
has been s t r u c k
of,the
because:
by,lightning
m a g n e t i z e d maki ng t h e d i p o l e
uncertain.
If
o f the e a r t h ' s
orientations.
the d i p o l e
third
(I)
dacite
the d a c i t e
sampl e i n
f or ms a r i d g e
and ( 2 )
the d a c i t e - i s
orientations
reading very,
orie nta tions
magnetic f i e l d
is
ment o f t h e Sheep M o u n t a i n d a c i t e
are, c o r r e c t ,
weakly
a reversal
s u g g e s t e d bet ween t h e e m p l a c e ­
and t h e d a c i t e
s ill
south­
w e s t o f Sheep M o u n t a i n .
A reversal
o f the e a r t h ' s - m a g n e t i c
field
is
al s o suggested
bet ween t h e empl acement o f t h e a n d e s i t e f l o w s w i t h
f r o m 7600 t o 9360 f e e t
A reversal
early
in
Pleistocene
altitudes'
and t h e Sheep Mo u n t a i n d a c i t e .
the e a r th 's
or very
late
magnetic f i e l d
Pliocene
took
place
( Nagata, 1961).
in
If
x/
i n t r u s i o n s , we l d e d t u f f
o f andesite
-46-
A ltitude
Rock t y p e
Inclination
Decl!n a tio n
SW^sec. 3 0 , T .8 S. , R. 6 E.
7600
Andesite
f
I
OW
4 3 ° (N)
NW^sec. 3 0 , T . 8 S . , R. 6 E.
7600
Andesi t e
f
I
OW
10°
SW^sec. 2 4 , T . 8 S. , R . 6 E.
8800
Andes!te
f
I
OW
20°
3 7 ° (N) down
N E ^ s e c . 3 T , T „8 S. , R. 6 E.
9000
Andesi t e
f
I
OW
40°
5 1 ° (N)
down
NE^ s ec . 2 4 , 1 . 8
S . , R . 6 E.
9040
Andesite
f
I
OW
30°
4 7 ° (N)
down
NW^sec. 5 , 1 . 9
S . , R. 6
E.
9360
A n d e s ite
f I OW
20°
4 0 ° (N) down
NW^sec. 2 8 , 1 . 8
S. , R. 6
E.
8400
NE%sec. 2 1 , 1 . 8
S. , R. 6
E.
8640
Sheep Mt.
Dacite
Sheep Mt.
SW%sec. 3 1 , 1 . 8
S . , R. 6
E.
9360
D a c i t e s i l l SW.
o f Sheep Mt.
NW%sec. 3 1 , 1 . 7
S . , R. 7 E.
6000
330° 7 0 ° (N) down
Andesite s i l l
i n P r ec a mbr i an me t a mo r p h i c r o c k
Sh s e c . 1 7 , 1 . 8
S . , R.6 E.
7200
235° I l 0 (N) down
Upper p i n k
p a r t i a l l y wel de d z o n e ,
r e mna nt e a s t o f Sheep Creek
Sh s e c . 1 7 , 1 . 8
S . , R.6 E.
7200
Upper p i n k
220° 6 3 ° (N) down
p a r t i a l l y we l d e d z o n e ,
r e mna nt e a s t o f Sheep Creek
S^ s e c . 1 7 , 1 . 8
S . , R.6 E.
7200
Middle black
185°
3 ° (N) down
g l a s s y d e n s e l y we l d e d z one,
r e mna nt e a s t o f Sheep Creek
Sh s e c . 1 7 , 1 . 8
S. , R. 6
E.
7200
Middle black
,220°
5 ° (N) down
g l a s s y d e n s e l y we l d e d z one,
r e mna nt ;e a s t o f Sheep Creek
Sh s e c . 1 7 , 1 . 8
S . , R. 6
E.
7200
Lower brown
231° I T 0 (N) down
p a r t i a l l y wel de d z o n e ,
r e mna nt e ^ s t o f Sheep Creek
155°
Z
O
Location
CO
WELDED TUFF DIPOLE ORIENTATIONS
PO
O
O
TABLE V.
down
down
2 5 ° (S)
down
D a c i t e 58 ° 1 5 ° ( S )
down
35 ° 57° (N) down
-47-
t h e we l d e d t u f f s
consistent
ar e P l i o - P l e i s t o c e n e
dipole
orie nta tions,
s o u t h w a r d and d i p p i n g
i n a g e , anomal ous b u t
north-seeking
do wnwar d, o b t a i n e d
f o r t h e wel de d t u f f
zones -may s u g g e s t t h e y were d e p o s i t e d d u r i n g
Pleistocene
orientations
of andesitic
breccia,located
in
sec.
p f the c l a s t s .
clasts
Table
b r e c c i a was e s t i m a t e d
where t h e m a g n e t i c
before.heating
VI
t h e cube a t v a r i o u s
upon whi c h
is
and A k i m o t o
below the b l o c k i n g
lists
the o r i e n t a t i o n s
for
compared t o
method i s
(1957).
tempera­
this
obtained.
ep icla stic
(J/Jo
vs.
volcanic
T°C.)
o f a cube sawed f r o m each b l o c k
temperatures
this
volcanic
R . 6 E. s u g g e s t s e m p l a c e ­
using h e a t in g curves
intensity
(Jo)
f r o m an e p i c l a s t i c
16, I . 8 S . ,
The empl ac ement t e m p e r a t u r e
Ar amaki
Pliocener
random s c a t t e r o f d i p o l e ,
ment by mu df l ows a t t e m p e r a t u r e s
theory
the
reversal.
As p r e v i o u s l y m e n t i o n e d ,
tures
pole p o in tin g
the magnetic i n t e n s i t y
during
hqating,
based i s
Table V I I
(J)..
discussed
lists
of
The
in
t h e empl acement
temperature -d a t a .
Examination o f Table
VII
shows an empl acement t e m p e r a t u r e
o f 117° C o r l e s s
is
cooling,
at
75 p e r c e n t o f t h e
acquired
at temperatures
least
o f the b r e c c i a .
above t h i s
ment o f t h e
suggested i f
it
above t h a t
is
breccia
to
disorient
during
ultim a te magnetization
o f t he
The m a g n e t i c d i p o l e s
t e m p e r a t u r e were t h e n
assumed t h a t
final
" frozen
rotated
empl acement
into"
t h e rock"
by c o n t i n u e d move­
the d ip o le s .
is
-48TABLE V I .
Sample Tjumber
DIPOLE ORIENTATIONS OF CLASTS FROM AN
EPICLASTIC VOLCANIC BED
Declination
I
Inclination
down
2
26°
4° (N)
down
3
328°
9° (S)
down
4
48°
TO0 (N)
down
5
10°
8 5 ° (S)
down
6
318°
2 7 ° (N)
down
7
42°
4 2 ° (N)
down
8
330°
5 9 ° (N)
down
9
352°
9° (S)
10
28°
11
29q°
, 5 2 ° (S)
down
12
305°
3°(S)
down
13
'28°
5 6 ° (S)
down
14
89°
4 8 ° (S)
down
15
312°
7 9 ° (N)
down
16
35°
2 8 ° (N)
down
TABLE V I I .
-CO
2° (S)
CO
Ot
O
352°
down down
EMPLACEMENT TEMPERATURE DATA
J/Jo
F irst
Temperature
run ,
°C
Second run
I . 00
25
25
. 90
80
45
.80
- 105
80
. 75
117
100
. 70
130
115
VI.
Pre-Volcanic
SOURCE OF VOLCANICS
To po g r a p h y
P r i o r to d e p o s itio n
o f the
Eocene v o l c a n i c
map ar ea was p a r t o f a t o p o g r a p h i c
by a r i d g e
o f Pr e c a mb r i an t o
by i n t e r m i t t e n t
rock
and on t h e e a s t by a r i d g e
rock t h a t
( Peale,
extended southward a t
1896;
Hag ue, I d d i n g s
McMannis and C h a d w i c k ,
t i o n , 1 968;
Plate
A Southeastern
A source
1964;
I , this
strata,
of
P r e c a m b r i an me t a mo r p h i c
least
t o Mol
and Weed,
1894;
and p i n c h - o u t
flows
thin
at a ltitu d e s
with
sequence b e c a u s e :
in
a northwesterly
e a rlier
bel ow 8800 f e e t ,
more r o unded c l a s t s
(3)
fossil
ward and n o r t h w e s t w a r d ;
as i n d i c a t e d
m aterials
forests
(4)
(I)
direction
v o l c a n i c b e ds ,
become b e t t e r
and c o n t a i n
more f i n e
become more a b u n d a n t w e s t ­
ridges
in
the p r e - v o l c a n i c t o p o ­
a b ov e, w o u l d have p r o b a b l y b l o c k e d any
comi ng f r o m t h e e a s t ,
stages
communi ca­
suggested f o r the
t o w a r d t h e w e s t and n o r t h w e s t , e p i c l a s t i c
graphy,
1961;
Sour ce
s o u t h e a s t o f t h e map a r e a i s
g r a i n e d beds;
H all,
study).
andesite
stra tifie d
Heron Creek
E. L . McCaskey, o r a l
Eocene v o l c a n i c
particularly
on t h e n o r t h
o f P r ec a mbr i an me t a mo r p h i c
g r e a t e r p a r t o f the
(2)
the
b a s i n bounded on t h e wes t
Cretaceous
to p o g ra p h ic highs
sequence,
of deposition
north
before
o r west a t
topographic
least
l ows
in the
fille d
-50Stnedes
(1968)
o f t h e map a r e a ,
suggests
t he
was d e r i v e d
northeast trending
horst
Eocene v o l c a n i c
from vents
in
the
in
E lectric
sequence, south
a large
north-
P e a k - S e p u l c h e r Moun­
tain
region,
s o u t h e a s t o f t h e map ar ea i n Y e l l o w s t o n e N a t i o n a l
Park
(Figure
2).
p a r t o f the
The same o r i g i n
Eocene v o l c a n i c
is
suggested f o r
t he g r e a t e r
sequence o f t he map ar e a because
t h e two Eocene sequences ar e i n
part
coextensive
and p r o b a b l y ,
contemporaneous„
A Northern
Sour ce
A northern
source f o r
v o l c a n i c sequence i n
suggest ed by:
stra tifie d
feet;
(2)
(I)
the northw estern
numerous
epicla s tic
andesite
35,
I . 8 S.,
northern
with
f lo w occurs
R.5 E„ )
G allatin
altitudes
P r e c a mb r i an f r a g m e n t s
northeastward
30 de gr e es s o u t h e a s t w a r d ,
be due t o
whi c h
regional
at
that
is
an a l t i t u d e
closely
( 3)
volcanic
beds w h i c h seem c o n t i n u o u s
an a u g i t e
n o r t h w e s t c o r n e r o f t h e map ar ea
rich
(4)
some a n d e s i t e
located
in
flows
t he
epiclastic
from the n o r t h e r n
a b o u t one m i l e
( R. A.
(sec.
the a u g i t e - r i c h
Range gr ade s o u t h w a r d i n t o
Ramshorn Peak a r e a ,
as much as
o f ab out 9280 f e e t
o f a b o u t 8400 t o 9200 f e e t
G allatin
crudely
g r e a t e r t h a n what c o u l d
r e s e mb l e s
Range a n d e s i t e s ;
in
above 8720
and d i p p i n g
tiltin g ;
northern
Range t o t h e
p a r t o f t h e map area i s
v o l c a n i c beds a t a l t i t u d e s
beds s t r i k i n g
reasonably
some o f t h e up per p a r t o f t he
north
Ch a d wi c k ,
oral
G allatin
o f the
-51 -
communication,
Andesite
1969).
Di kes
Compositional
s im ila rity
bet ween t h e seven a u g i t e -
h o rn b lende-hypersthene andesite
and t h e
in
volcanic
contrast
beds,
to the
as a c o n t i n u a t i o n
Horse C r e e k .
probably
statpd
flows
and e p i c l a s t i c
lower andesite
c o u l d have come i n
S e p u l c h e r Mo u n t a i n
part
region;
(2)
flows
from:
the a n desite
came f r o m t h e n o r t h ,
not a l l
and e p i c l a s t i c
(I)
the s o u t h ­
dikes
Peakwes t o f
o f the m a t e r i a l
as i n d i c a t e d
by t h e e v i d e n c e
above.
Welded T u f f
Remnants
The s o u r c e o f t h e wel de d t u f f
Creek ahd w e s t o f Horse Creek i s
map ar ea b e c a u s e :
(I)
o f Tom M i n e r Creek j u s t
ar e
flows
volcanic
o f vol ca ni sm from E l e c t r i c
Howev er , much i f
Plio-Pleistocene
R.6 E. )
andesite
f e d up p e r f l o w s .
Thus t h e u p p e r a n d e s i t e
east,
e a s t o f Horse C r e e k ,
upper a u g i t e - h o r n b l e n d e - h y p e r s t h e n e
may s u g g e s t t h e d i k e s
beds,
dikes,
t h o u g h t t o be n o r t h w e s t o f t h e
a wel de d t u f f
off
lower a l t i t u d e s
tO' t h e v a l l e y
eastward;
r emnant on t h e n o r t h
t h e map ar e a
has t h e same w e l d i n g z o n e s ;
restricted
remnant s e a s t o f Sheep
( 3)
(2)
(sec.
18,
wel ded t u f f
side
T .8 S. ,
remnant s
o f Tom M i n e r Creek and o c c u r a t
in
e x p o s u r e w e s t o f Horse Creek i s
thin
section,
t h e wel de d t u f f
p e t r o g r a p h i c al I y
s i m i l a r to
-52-
the pin k
Creek,
p a rtia lly
suggesting
we l d e d ash f l o w .
wel de d zone o f t h e o u t c r o p e a s t o f Sheep
b o t h we l d e d t u f f s
ar e r e m n a n t s ■o f t h e same
VII.
GEOLOGIC HISTORY
Pr e c a mb r i an
Intercalated
quartzites
suggest d e p o s i t i o n
of a q u a r t z o - f e l dspathic
sequence as t h e e a r l i e s t
schists
flank
represent:
and h o r n b l e n d e - b i o t i t e
recorded event in
a granitic
f e l dsp ath ic sediments;
intrusion
(2)
into
a granitic
sedimentary
t h e map a r e a :
a q u a r t z - b ip t i t e - p l agioclase
(I)
schists
gneiss
t h a t may
the q u a r t z o -
topographic
high t h a t
was o n - l a p p e d by. t h e q u a r t z o - f e l d s p a t h i c s e d i m e n t s ;
block o f a rk o s ic or g r a n i t i c
surface
by f a u l t i n g ;
Intrusion
clase
gneiss
o f basic
a granitized
s ills
features
in
amphibole;
into
structures;
(5)
(4)
parallelism
These s i l l s
(2)
(I)
alteration
to
However ,
mafic T e r t i a r y
o f the
intrude
t o t he
is
s u g g e s t e d by
p i agioclase
crystals;
of augite
grains
( 3)
local
t o gr een
fo lia tio n .
ar e p r o b a b l y o f P r e c a m b r i an age because o f
s u g g e s t e d met amor phi s m and o r i e n t a t i o n
fo lia tio n .
a
area.
fractured
of s i l l s
( 3)
the q u a r t z - b i o t i t e - p l a g io -
the s i l l s :
r e c r y s t a l I i zed e d g e s ;
cataclastic
composition brought
b e f o r e met amor phi sm and f o l d i n g
the f o l l o w i n g
with
(4)
These
dikes
parallel
a P r ec a mbr i an age i s
ar e l o c a t e d
Pr e c a mb r i an B e a r t o o t h
in
block
to
t he
not.positive
because
t h e Lonesome Mo u n t a i n ar ea
and s i m i l a r m a f i c
Cambri an beds n e a r t h e map ar ea
(Larsen,
dikes
Poldervaart
-54a n d . Ki r c h m a y e r , 1 966 ; W„ J„
Diagnostic
McManni s ,
relationships
fo lia tio n .
o c c u r bo t h
Diagnostic
whether the pegmatites
communication,
196,9)
were n o t o b s e r v e d t o d e t e r m i n e
w h e t h e r f o l d i n g was b e f o r e , d u r i n g
Pegmatites
oral
o r a f t e r met amor phi sm.
parallel
to
and c r o s s c u t t i n g
f e a t u r e s were n o t o b s e r v e d t o e s t a b l i s h
f or med d u r i n g
o r a f t e r met amor phi s m.
From Tom M i n e r Creek s o u t h w a r d a l o n g t h e w e s t s i d e o f
Tepee C r e e k , t h e m e t a m o r p h i c sequence c o n s i s t s
hornblende-pla g ib c lase -bio tite
schists
quartzites;
q u a r tz - b io t i t e - p l agioclase
An i n c r e a s e
i n metamorphi c i n t e n s i t y
by:
(I)
epidote
a p pea r an c e o f b i o t i t e ,
in
feldspar
(3)
the s c h i s t s ;
and q u a r t z
augite,
feldspars,
medium t o
garnet,
in
gneiss
o f quartz
amphibolite
to
A pre-Belt
gr ade r e g i o n a l
granulite
age i s
in
of
the gneiss;
of quartz
and
and f r a c t u r e d
The m i n e r a l
assembl age
suggests
met amor phi sm o f t h e
almandine-
facies.
s u g g e s t e d f o r t h e me t a mo r p h i c r o c k s . o f
t h e map ar e a by age d a t e s o f 1 . 6 t o
mi ned on m e t a m o r p h i c
and
intergrowths
b i o t i t e - h o r n b l ende , g a r n e t and a u g i t e
high
suggested
plagioclase
graphic
intergrowths
the g r a n u l i t e .
and g r a n u l i t e .
southward i s
and r e c r y s t a l I i zed q u a r t z
abundant g r a p h i c
laths
and s c h i s t o s e
hornblende,
re c r y s t a l I i zed g r a i n s
plagioclase
epidote,
(2)
o f interbedded
rocks,
s o u t h e a s t o f t h e map ar e a
north,
3.2 b i l l i o n
northeast,
( D„ G. B r o o k i n s ,
years
deter­
n o r t h w e s t and
1965, w r i t t e n
-55communi c a t i on t o IaL
J c McManni s ; McMannis and Ch a d wi c k ,
1 964;
G i l e t t i , 19 6 6 ) .
Paleozoic-Mesozoic
Following
at
least
folding
10,000 f e e t
and met amor phi sm a s e d i m e n t a r y sequence
thick
was d e p o s i t e d w i t h
form ity
on t h e P r e c a m b r i a n m e t a m o r p h i c r o c k
Wilson,
1934a,
1934b).
sequence was p a r t l y
many p l a c e s ,
At a l a t e r
date,
removed by e r o s i o n
including
t h e map a r e a ,
Precambrian metamorphic
a n g u l a r un c on­
(Iddings,
this
since
1904;
sedimentary
volcanics
rest d ir e c tly
in
on
rock.
Ce n o z o i c
Movements a l o n g t h e Span i s h
place
in
several
Eocene t i m e
phases
from l a t e
epiclastic
were empl aced d u r i n g
and a n d e s i t e
probably
t oo k
up t o p r e - m i d d l e
1964),
volcanic
Eocene t i m e .
beds and a n d e s i t e
Dacite
intrusions
d i k e s were empl aced a l o n g t h e n o r t h w e s t - t r e n d i n g
West er n A b s a r o k a b e l t
The e p i c l a s t i c
also
traceable
Eocene t i m e .
beds and a n d e s i t e
flows
o f t he
as Eocene because t h e y a r e c o n t i n ­
s o u t h w a r d f r o m B i g Horn Peak i n t o
mapped as Eocene by Smedes
The d a c i t e
during
volcanic
map ar ea ar e i n t e r p r e t e d
uously
Cretaceous
(McManni s and C h a d w i c k ,
The i n t e r c a l a t e d
flows
Peaks f a u l t
intrusions
beds
(1968).
o f t h e map ar ea ar e Eocene o r
younger in
age because t h e y
seq uen c e.
The d a c i t e
because s i m i l a r
rich
intrusions
dacite
years
Since
Eocene up t o
recognized
(R.
geologic
events
evidence
from nearby areas.
A . Ch a d wi c k ,
place
in
during
o f t h e map a r e a , o l i v i n e
Hepburn's
Mesa,
s i ltstone
flows
overlie
( H o r b e r g , 1 940)
Eocene e p i c l a s t i c
tuffs
flows.
ar e
a late
National
volcanic
this
period
basalt
However, many
a c co rd in g to
Valley,
flows,
capping
Miocene-early
deposits
This r e l a t i o n s h i p
rest
(Yellowstone
and o x i d a t i o n
Pliocene
un c onf or m, abl y on
indicates
Wel ded t u f f s
P a r k . ' C^r os t k a , o r a l
The wel ded t u f f s
(19,67).
beds and a u g i t e - h y p e r s t h e n e
in
communication
Plateau:
that
t h e wel ded
nearby regions
Yellowstone
1968;
1 968)
Christiansen
and
and o t h e r s
o f t h e map ar ea ar e i n t e r p r e t e d t o
be P l i O - P l e i s t o c e n e b e c a u s e :
glass
e r o s i o n was t h e
t h e map a r e a .
( S e p u l c h e r Mo u n t a i n and n o r t h e r n
P li o - P leistocene
19 6 8 ) .
time,
and g r a v e l .
Eocene o r y o u n g e r .
a r e Eocene
press).
have been d a t e d as P l i o c e n e , b y Bush
One o r more we l d e d t u f f
andesite
in
In t h e Y e l l o w s t o n e
northeast
These b a s a l t
o f 4 9 . 0 and 4 9 . 5 ± 1 .5
P lio-Pleistocene
geologic event
took
and Bi g C r e e k s , n o r t h w e s t
K/Ar dates
respectively
as Eocene
lo c a te d along the potassium-
E a s t e r n A b s a r o k a b e l t on M i l l
m illion
t h e Eocene v o l c a n i c
are i n t e r p r e t e d
intrusions
o f t h e map a r e a , y i e l d e d
only
intrude
of iron
(I)
post-cooling a lte ra tio n
ar e l a c k i n g ;
(2)
tridym ite
and
of
-57cristo balite
quartz.
have n o t been c o n v e r t e d t o o p a l ,
Absence o f t h e s e t y p e s
is tic
o f Pliocene
Smi t h
(I960).
lie
r emnant s
a|bou t 50 t o 480 f e e t
suggests the
valley
t h e wel de d t u f f s
time.
Relative
t i me than
floo r
it
feet
were d e p o s i t e d
altitudes
had a s t e e p e r g r a d i e n t
rocks
in
up t o
at
River,
at
least
d rift,
least
This
higher
Plio-Pleistocene
r e mna nt s
(?)
suggest
to the ea st at t h a t
once d u r i n g
Yankee Ji m Cany on„
because s t r i a t e d
mo r a i n e s
channels,which
in
in
o f 8800
occur
Roches mount onees and
parallel
the present
P r e c a m b r i an me t a mo r p h i c
part,
interpreted
by g l a c i a l
Pr e c a mb r i an m e t a m o r p h i c r o c k s
a b o u t even w i t h
period.
and e r r a t i c s
Yankee Ji m Canyon i s
at least
this
at a l t i t u d e s
7600 f e e t .
have been c u t
as h a v i n g been c a r v e d ,
altitudes
floo r.
does a t p r e s e n t .
ice marginal
Yellowstone
in
o f t h e wel de d t u f f
and above and g l a c i a l
straited
acco rdin g to
l o c a t e d a l o n g Tom M i n e r Creek
i n Tom M i n e r B a s i n have f l o o r s
at a l t i t u d e s
character­
above t h e p r e s e n t v a l l e y
The ar ea was g l a c i a t e d
Cirques
is
f l o o r was a b o u t 50 t o 480 f e e t
before
the v a l l e y
of a lte ra tio n
and y o u n g e r we l d e d t u f f s
The wel de d t u f f
chalcedony or
action
occur at
the presen t Yellowstone
River
channel.
The f o r m a t i o n
the Yellowstone
and d e s t r u c t i o n
River
in
of
a landslide
Yankee Ji m Canyon i s
dam b l o c k i n g
s u g g e s t e d by
-58r emnant s
of material
of s im ila r
broke„
that
composition
This
event
f or med t h e dam and out wash m a t e r i a l
d e p o s i t e d downst r eam when t h e dam
is well
document ed by Good ( 1 9 6 4 ) .
' .
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W i l s o n , W. H . , 1 96 4b, G e o l o g i c Re c o n n a i s s a n c e o f P a r t o f t h e
S o u t h e r n A b s a r o k a M o u n t a i n s , Nort h-west Wyomi ng, P a r t I ,
The Wood R i v e r - G r e y b u l l R i v e r A r e a : Wyoming U n i v e r s i t y
C o n t r . G e o l o g y , V o l . 3, No. 2.
V
f
PLATE I
BEDROCK GEOLOGY OF THE SOUTHERN
PART OF TOM
MINER BASIN, PARK AND GALLATIN
COUNTIES,
MONTANA
by
Stanley G. Todd
presented to the faculty of the Department of Earth Sciences
in portal fulfillment of the requirements for the degree of
Master of Science
at
Montana State University
Bozeman, Montana
1969
>
/
MONTANA
CROWN
Contour
Interval
80
Feet
"
BUTTE
I ndex Mo p
EX PLA NA TI ON
Landslide masses
-Afiuviafirand glacial deposits
Welded t u f f remnants
'Eqi
Dacite intrusions
Epiclastic volcanic beds
Epiclastic volcanic beds
inte rcal ated with andesite
Andesite dikes
E p i c l a s t i c volcanic beds
loca lly in t e r c a l a t e d with
andesite
Andesite f l o w s
C
3
X)
CD
O
U
v_
0)
C
>
ipCs (?)] Sills
Metamorphic rocks
Contact
Fault
r o i l a Vr o n
Vertical
foliation
Boundary of map area
12
Strike and dip of b e d s
(including
R — R Representative section l o c a t i o n
flows)
Horizontal beds
A1
Ill0 05
I
I I I 0 OO'
762 10005235 4