Stratigraphy and depositional environment of the Upper Mississippian Big Snowy Group in the Bridger
Range, southwest Montana
by Gary Eich Guthrie
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Earth Science
Montana State University
© Copyright by Gary Eich Guthrie (1984)
Abstract:
The Big Snowy Group is restricted to a trough which extends from southwest Montana into the
Williston basin. Thickness variation of the group on the southern margin of the trough reflects
movement of structural elements along an ancient structural weakness located in the central Bridget
Range. Field and petrographic data are integrated to determine the depositional environment and
stratigraphy of the group and to document the tectonic influence on sedimentation along this zone.
The group is divided into the Kibbey Formation, with two informal members, and the Lombard facies
of the Heath Formation. The lower Kibbey is supratidal algal laminated dolostone with dessication
features and evaporite solution breccias were deposited at the leading edge of the transgressing Big
Snowy sea. Siliciclastic intertidal channels on the sabkha are restricted to the central range where
subsidence was greatest. The upper Kibbey and Lombard facies provide further evidence of a trough in
the central Bridgers. The upper Kibbey is a regressive shoreface deposit composed of fine grained
sandstone at the northern and southern ends of the range. Mudstone and siltstone dominate in the center
of the range where deeper water and lower energy conditions prevailed. Ultimately, the Kibbey
shoreface transgressed out of the area and the Lombard facies was deposited in a partially restricted
shelf lagoon. Shale and lime mudstone accumulated in the center of the range, to the north and south
bioclastic wackestones, packstones, and grainstones were deposited in shoaling, higher energy
conditions.
All three units of the group are thickest in the central Bridgers and thin north and south onto the
Lombard arch and Wyoming shelf.
Deeper water lithologies occur with the thick sections indicating that the central part of the range
subsided relative to the Lombard arch and Wyoming shelf during sedimentation. A small positive trend
in the central range trough is a local exception to the pattern. This paleotectonic pattern also operated
during pre-Amsden uplift and erosion and thinned the group on the positive elements more. Evidence
of similar paleotectonic influence on sedimentation along the structural weakness in the Bridgers is
also found in Proterozoic and other Paleozoic rocks. STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF THE UPPER
MISSISSIPPIAN BIG SNOWY GROUP IN THE
BRIDGER RANGE, SOUTHWEST MONTANA
by
Gary E ich G u th rie
\
A t h e s i s s u b m itte d in p a r t i a l f u l f i l l m e n t
o f th e re q u ire m e n ts f o r th e d e g re e
of
M aster o f S c ie n c e
in
E a r th S c ie n c e
MONTANA STATE UNIVERSITY
Bozeman, Montana
J u l y , 1984
\t
i
APPROVAL
o f a t h e s i s s u b m itte d by
Gary E ich G u th rie
T h is t h e s i s h as been re a d by each member o f th e t h e s i s com m ittee
and h as been found to be s a t i s f a c t o r y r e g a r d in g c o n t e n t , E n g lis h
u s a g e , f o r m a t, 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 , / a n d c o n s is te n c y , and
i s re a d y f o r su b m issio n to th e C o lle g e o f Gradulate S tu d ie s .
D ate
C h a irp e rs o n , G ra d u a te C om m ittee
A pproved f o r th e M ajor Dep
D ate
H ead, Major D epartm ent
Approved f o r th e C o lle g e o f G rad u ate S tu d ie s
Date
Graduate Dean
ill
STATEMENT OF PERMISSION TO USE
In p r e s e n ti n g t h i s t h e s i s in p a r t i a l f u l f i l l m e n t o f th e
re q u ire m e n ts f o r a m a s t e r 's d e g re e a t M ontana S ta te U n iv e r s it y , I
a g re e t h a t th 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 to b o rro w e rs u n d er th e
r u l e s o f th e l i b r a r y .
B r ie f q u o ta tio n s from t h i s t h e s i s a r e
a llo w a b le w ith o u t s p e c i a l p e r m is s io n , p ro v id e d t h a t a c c u r a te
acknow ledgem ent o f th e so u rc e i s made.
P e rm iss io n f o r e x te n s iv e q u o ta tio n s from o r r e p r o d u c tio n of t h i s
t h e s i s may be g r a n te d from my m ajo r p r o f e s s o r , o r in h i s a b s e n c e , by
th e D ir e c to r o f L i b r a r i e s w hen, in th e o p in io n o f e i t h e r , th e
p ro p o sed use o f th e m a t e r i a l i s f o r s c h o la r l y p u r p o s e s .
Any copying
o r u se o f th e m a t e r i a l s in t h i s t h e s i s f o r f i n a n c i a l g a in s h a l l n o t
be a llo w Ga,w ith o u
S ig n a tu r
t t e n p e r m is s io n .
iv
ACKNOWLEDGEMENTS
Most n o t a b l y , th e a u th o r i s in d e b te d to th e l a t e D r.
D onald L . Sm ith f o r in tr o d u c in g th e problem to me and f o r h is
g u id a n c e and en couragem ent d u rin g th e e a r l y s ta g e s o f
re se a rc h .
D r. S te v e C u s te r p ro v id e d m ost o f th e c o n s t r u c t i v e
c r i t i c i s m r e q u ir e d to d ev elo p th e r e s e a r c h by r e a d in g s e v e r a l
d r a f t s o f th e t h e s i s .
A s s is ta n c e from U n ite d S ta t e s G e o lo g ic a l Survey g e o l o g is ts
w o rk in g w ith th e Big Snowy Group i s a l s o a p p r e c i a t e d .
Ed
Maughan s p e n t s e v e r a l days in th e f i e l d w ith me and c o n tin u e d
to p ro v id e an im p o rta n t s o u rc e o f i n s i g h t and s u p p o rt d u rin g
th e developm ent o f th e r e s e a r c h .
B ruce Wardlaw k in d ly
i n t e r p r e t e d conodont sam ples p r e p a r e d by th e U n ite d S ta te s
G e o lo g ic a l S u rv ey .
The a u th o r i s a l s o g r a t e f u l to D r. Johnny Moore and D r.
James P e te r s o n o f th e U n iv e r s ity o f Montana D ep artm en t o f
G eology f o r t h e i r a s s i s t a n c e and w ill in g n e s s to h e lp and to
D r. R o b ert Chadwick and D r. K aren V in to n o f th e r e a d in g
com m ittee f o r t h e i r c o n t r i b u t i o n .
F i n a l l y , th e w r i t e r i s
e s p e c i a l l y g r a t e f u l f o r th e u n en d in g encouragem ent and su p p o rt
from h i s m o th er and f a t h e r . .
V
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS..................................................................................
iv
LIST OF FIGURES................................................ ................................................. ............... v i i
LIST OF PLATES....... ........................ ............. ...................................... ............................. v i i i
A BSTRACT........................ ......................................................... ...........................................ix
INTRODUCTION.....................................................................................
P a le o te c t o n ic and S t r a t i g r a p h i c S e t t i n g o f th e
Big Snowy G roup............ ...................... ........................................... ...............
R e g io n a l S e t t i n g ..................................................
B rid g e r Range S e t t i n g .............................................................
P u rp o s e ....................................................................................
Study A r e a . ............................................................
M ethods...........................
I
i
5
6
g
PREVIOUS INTERPRETATIONS OF THE BIG SNOWY GROUP............................................10
I n t r o d u c t i o n ....................
IQ
R e la tio n o f th e Big Snowy Group to th e U n d erly in g
M issio n Canyon F o r m a tio n ...................
„10
Big Snowy G roup................... ................................................................. ........................ 14
H is to r y o f N o m e n c la tu re ..............................................
14
Big Snowy T ro u g h ................................
„15
Snow crest T ro u g h ................................................
17
R e la tio n o f th e B ig Snowy Group to th e O v e rly in g
Amsden F o r m a tio n ..........................
.20
INTERPRETATION OF THE BIG SNOWY GROUP IN THE BRIDGER RANGE.............. ..2 2
I n t r o d u c t i o n .......................
22
Lower C o n ta c t and th e M issio n Canyon F o rm a tio n .........................
24
D e s c r i p t i o n .............................
24
I n t e r p r e t a t i o n ......................
26
Lower K ibbey Member................................
27
L i t h o f a c i e s ....................................................................
27
L am inated D o lo sto n e L i t h o f a c i e s .............................................................28
S a n d sto n e C hannel L i t h o f a c i e s ....................................
33
vi
TABLE OF CONTENTS— Cont inued
Page
D e p o s itio n a l E nvironm ent I n t e r p r e t a t i o n ................................................. 35
L am inated D o lo sto n e L i t h o f a c i e s ................................................. . . . . . 3 5
S an d sto n e C hannel L i t h o f a c i e s ..................
38
S t r a t i g r a p h i c I n t e r p r e t a t i o n ...........................
39
Upper K ibbey Member.......................................
42
L i t h o f a c i e s ....................................................................
.42
S an d sto n e L i t h o f a c i e s ............................ .................................................... „44
M u d sto n e -D o lo sto n e . L i th o f a c i e ................................
45
P e tr o g r a p h y ..................................
.....4 6
D e p o s itio n a l E nvironm ent I n t e r p r e t a t i o n ................................................. 48
I n t r o d u c t i o n . ..............................
48
S an d sto n e L i t h o f a c i e s .........................
51
M u d sto n e-D o lo sto n e L i t h o f a c i e ..................................
53
S t r a t i g r a p h i c I n t e r p r e t a t i o n . ................................
. .. 5 3
Lombard F a c i e s ..............................
57
L i t h o f a c i e s .......................................................
57
I n t r o d u c t i o n ................
57
S h ale L i t h o f a c i e s .........................
60
Lime B io-W ackestone L i t h o f a c i e s ....................................
60
Lime M udstone L i t h o f a c i e s .........................................
......6 1
D e p o s itio n a l E nvironm ent I n t e r p r e t a t i o n . ...............
.61
I n t r o d u c t i o n ...........................
61
S h ale L i t h o f a c i e s ..............................
...6 5
Lime B io-W ackestone L i t h o f a c i e s ..................
67
Lime M udstone L i t h o f a c i e s ..................................
68
S t r a t i g r a p h i c I n t e r p r e t a t i o n . .............................................
69
Upper C o n ta c t and th e Amsden F o rm a tio n ....................
.73
D e s c r i p t i o n . .......................................................
73
I n t e r p r e t a t i o n ............ ..............
.74
IMPLICATIONS: TECTONIC INFLUENCE ON SEDIMENTATION.......................................75
CONCLUSION.......................................................
...77
REFERENCES..................
...8 2
APPENDIX
87
vii
LIST OF FIGURES
F ig u re
'
page
1.
R e g io n a l P a le o te c t o n ic F e a t u r e s ...........................
2
2.
L o cal P a le o te c t o n ic F e a t u r e s ...........................
4
3.
P h o to g ra p h o f Big Snowy Group O u tc ro p .........................
7
4.
S r a t i g r a p h i c C h a r t.........................................................
5.
T im e-S ynchronous F a c ie s D iagram ..................................................................19
6.
G e n e ra liz e d l i t h o l o g i c column o f th e Big Snowy G roup................. 23
7.
Lower K ibbey Member L i th o f a c i e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 9
8.
Is o p a c h Map o f th e Lower K ibbey M em ber................................................ 41
9.
Upper K ibbey Member L i t h o f a c i e s .......................................
43
10 .
Iso p a c h Map o f th e Upper K ibbey Member.........................
55
11 .
Lombard F a c ie s L i t h o f a c i e s . . . . . .................................
12 .
Is o p a c h Map o f th e Lombard F a c i e s . ..................
11
.59
70
viii
LIST OF PLATES
F ig u re
page
1 . S t r a t i g r a p h i c C ro ss S e c t io n ................................... .........................in p o ck et
2 . P h o to g rap h o f Lower K ibbey Member L a m in a tio n s ................................... 30
3 . P h o to m icro g rap h o f Lower K ibbey Member L a m in a tio n s .......................30
4 . P h o to g rap h o f Lower K ibbey Member B r e c c i a . . . . , ............ .............. . . . 3 2
5 . P h o to m icro g rap h o f Lower K ibbey Member S a n d s to n e .
.............. ...32
6 . P h o to m icro g rap h o f Upper K ibbey Member S a n d s to n e ...........................47
7 . P h o to m icro g rap h o f Lombard F a c ie s F o r a m in if e r a l
Lime W ack esto n e............................................ ...........................................................62
ix
ABSTRACT
The Big Snowy Group i s r e s t r i c t e d to a tro u g h w h ich e x te n d s from
s o u th w e s t M ontana in t o th e W i l l i s t o n b a s in . T h ic k n e ss v a r i a t i o n o f th e
group on th e s o u th e r n m a rg in o f th e tr o u g h r e f l e c t s movement o f
s t r u c t u r a l e le m e n ts a lo n g ^n a n c ie n t s t r u c t u r a l w eakness lo c a te d in th e
c e n t r a l B rid g e r Range. F ie l d and p e tr o g r a p h ic d a ta a r e in t e g r a t e d to
d e te rm in e th e d e p o s i t i o n a l en v iro n m e n t and s t r a t i g r a p h y o f th e group
and to docum ent th e t e c t o n i c in f lu e n c e on s e d im e n ta tio n a lo n g t h i s
zone.
The g ro u p i s d iv id e d i n t o th e K ibbey F o rm a tio n , w ith two in fo rm a l
m em bers, and th e Lombard f a c i e s o f th e H eath F o rm a tio n . The lo w er
K ibbey i s s u p r a t i d a l a l g a l la m in a te d d o lo s to n e w ith d e s s i c a t i o n
f e a t u r e s and e v a p o r ite s o l u t i o n b r e c c ia s w ere d e p o s ite d a t th e le a d in g
edge o f th e t r a n s g r e s s i n g B ig Snowy s e a . S i l i c i c l a s t i c i n t e r t i d a l
c h a n n e ls on th e sabkha a r e r e s t r i c t e d to th e c e n t r a l ra n g e w here
s u b s id e n c e was g r e a t e s t . The u p p er K ibbey and Lombard f a c i e s p ro v id e
f u r t h e r e v id e n c e o f a tr o u g h in th e c e n t r a l B r id g e rs . The u p p er
K ibbey i s a r e g r e s s i v e s h o r e f a c e d e p o s it composed o f f i n e g ra in e d
s a n d s to n e a t th e n o r th e r n and s o u th e r n ends o f th e ra n g e . M udstone
and s i l t s t o n e d o m in ate in th e c e n t e r o f th e ra n g e w here d e e p e r w a te r
and lo w e r en erg y c o n d itio n s p r e v a i le d . U lti m a t e ly , th e K ibbey
s h o re fa c e t r a n s g r e s s e d o u t o f th e a r e a and th e Lombard f a c i e s was
d e p o s ite d i n a p a r t i a l l y r e s t r i c t e d s h e l f lag o o n . S h a le and lim e
m udstone a c c u m u la te d in th e c e n t e r o f th e ra n g e , to th e n o r th and
s o u th b i o c l a s t i c w a c k e s to n e s, p a c k s to n e s , and g r a in s t o n e s w ere
d e p o s ite d in s h o a lin g , h ig h e r en erg y c o n d itio n s .
A ll t h r e e u n i t s o f th e group a r e t h i c k e s t in th e c e n t r a l B rid g e rs
and t h i n n o r th and s o u th o n to th e Lombard a rc h and Wyoming s h e lf .
D eeper w a te r l i t h o l o g i e s o c c u r w ith th e t h i c k s e c ti o n s i n d i c a t i n g t h a t
th e c e n t r a l p a r t o f th e ra n g e s u b sid e d r e l a t i v e to th e Lombard a rc h
and Wyoming s h e l f d u rin g s e d im e n ta tio n . A s m a ll p o s i t i v e tr e n d in th e
c e n t r a l ra n g e tro u g h i s a l o c a l e x c e p tio n to th e p a t t e r n . T h is
p a l e o t e c t o n i c p a t t e r n a ls o o p e r a te d d u r in g pre-A m sden u p l i f t and
e r o s io n and th in n e d th e gro u p on th e p o s i t i v e e le m e n ts m ore. E vidence
o f s i m i l a r p a l e o t e c t o n i c in f lu e n c e on s e d im e n ta tio n a lo n g th e
s t r u c t u r a l w eakness in th e B r id g e rs i s a l s o found in P r o te r o z o ic and
o th e r P a le o z o ic ro c k s .
I
INTRODUCTION
P a le o te c t o n ic and S t r a t i g r a p h i c S e tt in g
o f th e Big Snowy Group
R e g io n a l S e tt in g
R e g io n a lly , th e s t r a t i g r a p h y o f th e u p p er M is s is s i p p ia n Big Snowy
Group r e f l e c t s th e in f lu e n c e o f m a jo r t e c t o n i c e le m e n ts on
s e d im e n ta tio n ( F ig u re I ) .
I t i s r e a s o n a b le to assume t h i s
r e l a t i o n s h i p i s a l s o t r u e on a l o c a l s c a l e .
T h is stu d y a tte m p ts to
document th e in f lu e n c e o f p a l e o t e c t o n i c a c t i v i t y on s e d im e n ta tio n o f
th e Big Snowy Group in th e B rid g e r R ange.
Most o f M ontana was th e s i t e o f i n t e r m i t t e n t s h e l f o r p la tfo rm
s e d im e n ta tio n on th e C o r d i l le r a n p la tf o r m th ro u g h o u t P a le o z o ic tim e
( Sando, Gordon, and D u tr o , 1 9 7 5 ).
The p la tf o r m was bounded on th e
e a s t by th e t r a n s c o n t i n e n t a l a r c h in e a s t e r n N orth D akota and South
D akota and on th e n o r t h e a s t by th e C anadian s h i e l d .
T hese w ere
c o n t in u a ll y em ergent a r e a s w hich p ro v id e d sed im en t f o r c l a s t i c
d e p o s it io n on th e p la tf o r m .
The C o r d i l le r a n m io g e o c lin e b o rd e re d th e
C o r d i l le r a n p la tf o r m on th e w est in ex trem e s o u th w e s te rn M ontana,
Wyoming, and e a s t e r n Id a h o .
Huh (1 9 6 7 ) lo c a te d and d e s c r ib e d th e
c r a to n -m io g e o c lin e t r a n s i t i o n w here Big Snowy Group s h e l f s t r a t a
g ra d e in t o e q u iv a le n t m io g e o c lin a l s t r a t a in extrem e s o u th w e s te rn
M ontana.
The m io g e o c lin e r e c e iv e d se d im e n ts th ro u g h o u t B ig Snowy
tim e (G u ts c h ic k , S an d b erg , and Sando, 1 9 8 0 ).
Figure I.
Late Mississippian t e c t o n i c e l e m e n t s
the northern R o c k y M o u n t a i n s .
on t h e
Cordilleran
platform in
3
S t r a t i g r a p h i c s t u d i e s in M ontana i n d i c a t e t h a t s e v e r a l s m a lle r
p a l e o t e c t o n i c e le m e n ts a c t i v e w ith in th e C o r d il le r a n p la tf o r m i t s e l f
w ere p a r t i c u l a r l y i n f l u e n t i a l on l a t e M is s is s ip p ia n and e a r l y
P e n n s y lv a n ia n s e d im e n ta tio n ( Sando, 1976; P e te r s o n , 1 9 8 1 ).
An
u n s ta b l e tro u g h c o n n e c te d th e m io g e o c lin e w ith th e W i l l i s t o n b a s in
and d iv id e d th e p la tf o r m in t o s e v e r a l in d e p e n d e n t s h e lv e s (F ig u re I ) .
The e a s t —w est p a r t o f t h i s u n s ta b le tr e n d h as been v a r io u s l y
d e s c r ib e d as th e Big Snowy b a s in ( E a r d le y , 1 9 6 2 ), C e n tr a l Montana
tro u g h ( R o b e r ts , 1 9 7 5 ), and B ig Snowy tro u g h ( P e te r s o n , 1 9 8 1 ).
Maughan and P e r r y (1 9 6 7 ) named th e so u th w e st e x te n s io n o f t h i s
u n s ta b le tr e n d th e S now crest tro u g h (Ruby tro u g h o f P e te r s o n , 1 9 8 0 ).
The m a rg in s o f b o th tro u g h s c o rre s p o n d c l o s e l y to th e p r e s e n t z e ro
edge o f Big Snowy Group s e d im e n ts ( F ig u r e 2 ) .
P re v io u s a u th o r s have
n o t alw ays r e s t r i c t e d u sag e o f th e te rm s S now crest and Big Snowy
tro u g h to l a t e M is s is s ip p ia n ro c k s ( e . g . Maughan, 1984; R o b e r ts ,
1 9 7 9 ).
In th e p r e s e n t r e p o r t th e te rm s S now crest and B ig Snowy
tro u g h s a r e u sed to i n d i c a t e a r e a s o f a c t i v e l a t e M is s is s ip p ia n and
e a r l y P e n n s y lv a n ia n s u b s id e n c e in w hich Big Snowy s e d im e n ts w ere
d e p o s ite d and p r e s e r v e d .
N o rth o f th e S n o w c rest-B ig Snowy t r o u g h s , th e A lb e r ta s h e l f was
n e u t r a l r e l a t i v e to th e a c t i v e l y s u b s id in g t r o u g h s .
a r e n o t p r e s e r v e d th e r e (S m ith , 1 9 7 2 ).
Big Snowy ro c k s
S outhw ard, e q u i v a le n t u p p er
M is s is s ip p ia n ro c k s a r e p r e s e r v e d on th e Wyoming s h e l f .
The ro c k s
h e re a r e t h i n , h ow ever, i n d i c a t i n g an in te r m e d ia te d e g re e o f t e c t o n i c
s t a b i l i t y b etw een th e u n s ta b le B ig Snowy and S now crest tro u g h s and
th e A lb e r ta s h e l f ( Sando, Gordon, and D u tro , 1 9 7 5 ).
Great Falls
Helena
IOmiIes
North
Montana
Wyoming
Figure 2 .
I s o p a c h m ap of Big S n o w y Group sh ow ing p a l e o f e c t o n i c e l e m e n t s , contours
in f e e t .
S e e Plate I for s e c t i o n A - A 11 B r i d g e r R a n g e dotted, ( m o d i f i e d
fro m H a r r i s , 1 9 7 2 ) .
5
B rid g e r Range S e tt in g
A sm a ll p o s i t i v e elem en t s e p a r a te s th e S now crest tro u g h from th e
Big Snowy tro u g h ( F ig u r e 2 ) .
Big Snowy s t r a t a t h i n a c r o s s t h i s
a r c h —l i k e f e a t u r e as th e y a r e tr a c e d from th e S now crest tr o u g h to th e
Big Snowy tro u g h (B la k e , 1959; H a r r i s , 1 9 7 2 ).
The te rm Lombard a r c h
i s a p p lie d in t h i s r e p o r t to th e r e g io n betw een th e S n o w crest and Big
Snowy tro u g h s w here B ig Snowy ro c k s a r e t h i n .
T h is r e g io n e x te n d s
a p p ro x im a te ly from H elen a t o L iv in g s to n , M ontana.
Big Snowy Group ro c k s w ere f i r s t re c o g n iz e d in th e B r id g e t Range
by McMannis in 1951.
He re c o g n iz e d ex trem e th ic k n e s s changes in th e
group alo n g th e le n g th o f th e B r id g e t R ange.
From a z e ro edge on th e
Wyoming s h e l f , w hich McMannis lo c a te d in th e s o u th e rn B r id g e ts , th e
Big Snowy Group r e a c h e s a maximum l o c a l th ic k n e s s in th e c e n t r a l p a r t
o f th e r a n g e .
The gro u p th e n t h i n s a c r o s s th e Lombard a r c h in th e
n o r th e r n r a n g e .
P u rp o se
G e o lo g is ts re c o g n iz e t h a t th e s e th ic k n e s s v a r i a t i o n s r e p r e s e n t
l a t e M is s is s ip p ia n t e c t o n i c e le m e n ts w ith e i t h e r d i f f e r e n t i a l
s u b s id e n c e d u rin g d e p o s i t i o n , p o s t- B ig Snowy d i f f e r e n t i a l u p l i f t and
e r o s io n , o r b o th ( H a r r i s , 1972; Sando, 1976; M au g h an ,1 9 8 4 ).
U n fo rtu n a te ly e v id e n c e o f d e p o s i t i o n a l th i n n i n g , su ch a s g r a d u a lly
s h o a lin g l i t h o f a c i e s , i s m is s in g a t p r e v io u s ly s tu d ie d l o c a t i o n s due
to p o s t r d e p o s i t i o n a l e r o s io n .
T h u s, th e o r i g i n a l e x te n t o f th e group
■and th e r e l a t i v e im p o rta n c e o f d i f f e r e n t i a l s u b s id e n c e d u rin g
6
d e p o s it io n and p o s t - d e p o s i t i o n a l e r o s io n i s u n c l e a r .
The p r e s e n t stu d y was u n d e rta k e n to d e te rm in e i f th ic k n e s s
v a r i a t i o n s o f th e B ig Snowy Group in th e B r id g e r Range a r e due to
s y n d e p o s itio n a l movement o f p a l e o t e c t o n i c e le m e n ts .
Study A rea
The n o r th - s o u th tr e n d o f B r id g e r Range e x p o su re s p r o v id e s a li n e
o f s t r a t i g r a p h i c s e c t i o n a c r o s s a l a t e M is s is s i p p ia n s t r u c t u r a l
tro u g h a p p ro x im a te ly p e r p e n d ic u la r to d e p o s i t i o n a l s t r i k e .
A ls o , th e
ra n g e i s s i t u a t e d n e a r th e i n t e r s e c t i o n o f th e S n o w crest and Big
Snowy tro u g h s w h ich a llo w s e x a m in a tio n o f th e r e l a t i o n s h i p betw een
s t r a t a o f b o th tro u g h s as th e y t h i n and i n t e r f i n g e r a c r o s s th e
Lombard a r c h .
P a le o z o ic and M esozoic ro c k s in th e B rid g e r Range a r e u p l i f t e d
a p p ro x im a te ly 1500 m e te rs above s u rro u n d in g v a l l e y s .
They a r e a ls o
t h r u s t e d s e v e r a l k ilo m e te r s e a s t (Dave L ag eso n , p e r s o n a l
co m m u n ica tio n ).
The ro c k s o f th e Big Snowy Group a r e w e ll exposed
j u s t e a s t o f th e ra n g e c r e s t w hich i s composed o f n e a r v e r t i c a l
M adison Group lim e s to n e ( F ig u r e 3 ) .
F o u rte e n e x p o su re s w ere a n a ly z e d a lo n g a l i n e from j u s t so u th to
j u s t n o r th o f th e B r id g e r Range ( P l a t e I ) .
The e x p o s u re s ex ten d ed
from Rocky Canyon on th e s o u th end o f th e stu d y a r e a to a canyon on
th e M iddle F o rk o f S ix te e n m ile C reek in th e s o u th e rn Big B e lt
M ountains on th e n o r th en d .
T h ir te e n e x p o su re s in th e n e ig h b o rin g
H o rsesh o e H i l l s , S o u th e rn Big B e l t and C a s tle M o u n ta in s, and
n o rth w e s te r n B e a r to o th M ountains w ere a n a ly z e d in l e s s d e t a i l .
7
F ig u re 3 . E xposure o f u p p er P a le o z o ic ro c k s
a t s o u th e a s t S acajaw ea s e c t i o n ( f o r lo c a tio n
se e P l a t e I ) . R idge on r i g h t i s M is s is s ip p ia n
M issio n Canyon F o rm a tio n , n e x t l e f t low
i n t e r v a l w ith t r e e s i s Big Snowy Group,
n o n - v e g e ta te d i n t e r v a l i s Amsden F o rm a tio n ,
c l i f f on l e f t i s P e n n s y lv a n ia n Q uadrant
F o rm a tio n .
8
M ethods
D uring th e f i e l d s e a s o n o f 1982, e x p o s u re s o f th e B ig Snowy Group
in th e H o rsesh o e H i l l s , s o u th e r n Big B e lt and C a s tle M o u n ta in s,
n o r t h e a s t e r n B e a rto o th M o u n ta in s, and B rid g e r Range w ere lo c a te d and
b r i e f l y d e s c r ib e d .
lite ra tu re .
Many e x p o su re s w ere p r e v io u s ly c i t e d in th e
O th e rs w ere lo c a te d w ith a e r i a l p h o to g ra p h s and
r e c o n n a is s a n c e .
L i t e r a t u r e r e s e a r c h and c o rre s p o n d e n c e w ith
g e o l o g i s t s w orking on t h i s s t r a t i g r a p h i c i n t e r v a l fo llo w e d .
D uring
th e f i e l d s e a s o n o f 1983 f o u r te e n c o m p lete e x p o su re s o f th e Big Snowy
Group in th e B r id g e t Range w ere m easu red w ith a Ja c o b s s t a f f and
d e s c r ib e d in d e t a i l .
S e v e r a l o th e r in c o m p le te o r p o o rly exposed
s e c ti o n s in th e B r id g e ts w ere b r i e f l y d e s c r ib e d .
M easurem ents and
l i t h o l o g i c d e s c r i p t i o n s o f eac h e x p o su re a r e in c lu d e d in th e
a p p e n d ix .
Sam ples w ere c o l l e c t e d a t ea c h s i g n i f i c a n t l i t h o l o g i c a l
change.
L ith o lo g ic sam ples w ere s la b b e d and s tu d ie d u s in g a b in o c u la r
m ic ro s c o p e .
A ls o , o v e r 100 t h i n s e c t i o n s w ere p r e p a r e d and a n a ly z e d
w ith a b in o c u la r p e tr o g r a p h ic m ic ro s c o p e .
A p p ro x im ately te n p e r c e n t
o f th e t h i n s e c ti o n s and s la b s w ere s t a i n e d w ith a l i z a r i n re d
s o l u t i o n to d e te rm in e d o lo m ite c o n t e n t.
U sing th e s e d a t a , l i t h o f a c i e s w ere d e f in e d and i n t e r p r e t e d .
The
l i t h o f a c i e s w ere th e n c o r r e l a t e d b etw een s t r a t i g r a p h i c s e c ti o n s
p l o t t e d on a s t r a t i g r a p h i c c r o s s s e c t i o n .
L o c a tio n s o f m easured
s e c ti o n s d is c u s s e d in th e t e x t a r e p l o t t e d on th e l o c a t i o n map on
P la te I .
The s e c ti o n s w ere p l o t t e d on th e c r o s s s e c ti o n shown on P l a t e I
9
u s in g th e Big Snowy-Amsden c o n ta c t as a datum .
The c o n ta c t was
i n i t i a l l y b e lie v e d u n co n fo rm ab le u s in g f i e l d e v id e n c e .
Conodont d a ta
l a t e r s u p p o rte d t h i s c o n c lu s io n and showed t h a t t h i s c o n ta c t
c o r r e l a t e s w ith th e r e g io n a l Big Snowy-Amsden u n c o n fo rm ity .
A lth o u g h th e s u r f a c e i s n o t n e c e s s a r i l y a tim e l i n e and may have
had r e l i e f p r i o r to Amsden d e p o s i t i o n , i t i s th e m ost a p p r o p r ia te
datum f o r t h i s s t r a t i g r a p h i c i n t e r v a l .
A l i n e o f sy n ch ro n o u s
d e p o s it io n w ith i n th e Big Snowy Group can n o t be e s t a b l i s h e d u n le s s
s i g n i f i c a n t l y more b i o s t r a t ! g r a p h ic work i s d o n e.
u n c o n fo rm ity r e p r e s e n t s th e n e x t b e s t a l t e r n a t i v e .
T h e r e f o r e , an
The Big
Snowy-Amsden u n c o n fo rm ity shows th e geom etry o f th e u n d e r ly in g Big
Snowy Group b e s t .
A ls o , t h i s datum r e p r e s e n t s an e s t a b l i s h e d
r e g io n a l u n c o n fo rm ity .
Samples f o r co nodont age d e te r m in a tio n s w ere c o l l e c t e d from
c a rb o n a te ro c k s in th e B ig Snowy Group and o v e r ly in g lo w er Amsden
F o rm a tio n .
Sample l o c a t i o n s w ere ch o sen in o r d e r to d e f in e th e age
r e l a t i o n s o f th e B ig Snowy Group and to a id in i n t e r p r e t i n g th e
c o n ta c t w ith th e Amsden F o rm a tio n .
Sam ples w ere r e s t r i c t e d to l e s s
th a n o n e - t h i r d m e te r t h i c k i n t e r v a l s o f s t r a t a c ro p p in g o u t in
r e l a t i v e l y r i c h b i o c l a s t i c lim e s to n e and d o lo m ite .
E ig h te e n
o n e -k ilo g ra m sam ples w ere s e n t to th e U n ite d S ta t e s G e o lo g ic a l Survey
in D enver f o r p r e p a r a t i o n and a n a l y s i s by B ruce W ardlaw .
Sample
lo c a tio n s and i n t e r p r e t a t i o n s a r e in c lu d e d in th e a p p e n d ix w ith
d e s c r i p t i o n s o f m easu red s e c t i o n s .
10
PREVIOUS INTERPRETATIONS OF BIG SNOWY GROUP STRATIGRAPHY .
I n tr o d u c t io n
T hree ro c k p ack ag es w ere d e p o s ite d by th r e e
t r a n s g r e s s i v e - r e g r e s s i v e c y c le s d u rin g th e M is s is s i p p ia n and e a r ly
P e n n s y lv a n ia n .
I n a s c e n d in g o r d e r , th e s e in c lu d e th e M adison Group,
Big Snowy G roup, and th e Amsden F o rm a tio n (F ig u re 4 ) .
The d u r a tio n
o f th e h i a t u s e s s e p a r a t in g eac h seq u en ce i s v a r i a b l e .
In g e n e r a l,
h o w ev er, th e f a r t h e r from th e u n s ta b le tro u g h a x i s , th e g r e a t e r th e
d u r a ti o n (S m ith and G ilm o u r, 1 9 7 9 ).
A lth o u g h th e g ro u p s a r e
s e p a r a te d by r e g io n a l u n c o n f o r m itie s o r h i a t u s e s , th e y a r e in t im a te ly
r e l a t e d by common s e d im e n to lo g ic and t e c t o n i c c o n t r o l s .
Some
d is c u s s io n o f th e M issio n Canyon F o rm a tio n below and Amsden F o rm atio n
above th e Big Snowy Group i s n e c e s s a r y to u n d e rs ta n d th e e x t e r n a l
t e c t o n i c and s e d im e n to lo g ic in f lu e n c e s on d e p o s it io n o f th e Big Snowy
Group and b o unding c o n t a c t s .
R e la tio n s o f th e B ig Snowy Group to th e U n d e rly in g
M issio n Canyon F o rm atio n
The M issio n Canyon F o rm atio n ra n g e s in age from m id d le Osage to
m id d le Meramec ( Sando, G ordon, and D u tr o , 1 9 7 5 ).
The fo rm a tio n
e x te n d s th ro u g h o u t M ontana e x c e p t w here removed by T e r t i a r y e r o s io n .
The d iv e r s e ro c k ty p e s o f th e Big Snowy Group c o n t r a s t s h a r p ly w ith
th e u n d e r ly in g r e l a t i v e l y u n ifo rm M issio n Canyon c a r b o n a te s .
The
PENNSYLVANIAN
SERIES
BIG
SNOWY TROUGH
SNOWCREST TROUGH
Northeast
Morrowon
A m sd en
Group (part)
Southwest
^ Amsden Formation (part)
TTT I
HIATU
C he ste ria n
Heat
Otter
Formation
FormatToTT-
MISSISSIPPIAN
Lombard
Meramecian
HIATUS
Mission Conyon
Figure 4 .
Facies
Formation
Stratigraphic cha rt showing temporal rel ations and nomenclature of the
Big Snowy Group in M o n t a n a ( m o d i f i e d from Sando, Gordon, a n d
Dutro, 1 9 7 5 ) .
12
c o n t r a s t i s n o t i c e a b l e in o u tc ro p s in c e n t r a l and so u th w e st Montana
a s a change from m a s s iv e , g ra y M issio n Canyon lim e s to n e r id g e s or
c l i f f s to co v e re d o r p a r t i a l l y co v e re d re d Big Snowy s l o p e s .
Where
s o l u t i o n b r e c c ia s and k a r s t d e p o s it s a r e n o t d ev elo p ed n e a r th e to p
o f th e M issio n Canyon F o rm a tio n , th e c o n ta c t w ith th e B ig Snowy Group
i s p la c e d a t th e change from m a s s iv e , medium g ray -b ro w n lim e o r
d o lo -b io -w a c k e s t one to p a s t e l muddy and - s i l t y d o l o s t o n e , s i l t s t o n e ,
o r s a n d s to n e .
Is o p a c h maps i n d i c a t e t h a t th ic k n e s s t r e n d s o f th e
M issio n Canyon F o rm a tio n c o in c id e c l o s e l y w ith th o s e o f th e Big Snowy
s t r a t a ; maximum M issio n Canyon s e c t i o n s in th e Big Snowy and
S now crest tro u g h s t h i n away from th e tro u g h ax es tow ard b o th th e
A lb e r ta and Wyoming s h e l v e s .
D o lo sto n e and lim e s to n e a r e c y c l i c a l l y in te r b e d d e d in th e u p p er
h a l f o f th e M is s io n Canyon F o rm a tio n w ith two o r more m a jo r e v a p o r ite
beds.
Two o r more s o l u t i o n b r e c c i a zones r e p la c e th e e v a p o r ite s in
e x p o s u re s in th e ra n g e s o f so u th w e st M ontana.
The low erm ost b r e c c ia
zone can be tr a c e d th ro u g h o u t m ost o f th e ra n g e s in s o u t h - c e n t r a l
M ontana ( R o b e r ts , 1966; S ando, 1 9 7 4 ).
T hese s o l u t i o n b r e c c i a zones
a r e composed o f v a r ia b ly ro u n d ed d o lo s to n e c l a s t s w ith a n g u la r c h e r t
c l a s t s in a r e d - y e llo w , muddy, s i l t y c a r b o n a te m a tr ix .
E v a p o r ite
b r e c c ia zones a r e l a t e r a l l y c o n tin u o u s b u t v a ry in t h i c k n e s s .
The
low er b o u n d ary i s g e n e r a lly a d i s t i n c t , c o n tin u o u s re d c la y b e d .
C la s ts become l e s s ab u n d an t u p s e c tio n th ro u g h th e z o n e .
F i n a l l y , th e
b r e c c ia g iv e s way to f r a c t u r e d M issio n Canyon lim e s to n e a t th e to p o f
th e s o l u t i o n b r e c c i a zones a lo n g a p o o rly d e f in e d , i r r e g u l a r
b o u n d a ry .
13
R e se a rc h has shown t h a t k a r s t - l i k e f e a t u r e s a r e d ev e lo p e d alo n g
th e M issio n C anyon-Big Snowy c o n ta c t ( R o b e r ts , 1966; S ando, 1974;
M aughan, 1 9 8 4 ).
R ed d ish b r e c c ia d e p o s it s f i l l c o lla p s e d c a v e rn s
d e v e lo p e d a lo n g j o i n t s and b ed d in g p la n e s on to p o f th e M issio n
Canyon F o rm a tio n .
The k a r s t d e p o s it s v a ry e x tre m e ly in th ic k n e s s and
a r e d is c o n tin u o u s l a t e r a l l y .
E v a p o r ite b r e c c ia s a r e l a t e r a l l y
c o n tin u o u s and v a ry l e s s in t h i c k n e s s .
The r e l a t i o n o f k a r s t fo rm a tio n to th e o v e r ly in g B ig Snowy Group
i s d is p u te d .
Sando (1976) b e l i e v e s t h a t th e M issio n Canyon-B ig Snowy
Group c o n ta c t i s r e g i o n a l l y u n c o n fo rm a b le ; betw een e a r l y Meramec tim e
when M issio n Canyon d e p o s it io n ended and l a t e s t Meramec tim e when Big
Snowy d e p o s it io n b e g a n , th e M issio n Canyon F o rm atio n was exposed
a c r o s s M ontana.
I n t h i s m odel th e k a r s t d e p o s its w ere p ro d u ced by
s u b a e r i a l e r o s io n .
E v id en ce f o r th e u n c o n fo rm ity in c lu d e s t h r e e , and
p o s s ib ly f o u r m is s in g f a u n a l zones a t th e c o n ta c t ( F ig u r e 4 ) .
In c o n t r a s t , Maughan and R o b e rts (1967) and H a r r is (1972) arg u e
t h a t th e M adison Group and Big Snowy Group a r e co n fo rm a b le in th e Big
Snowy tr o u g h .
They c i t e i n t e r f i n g e r i n g o f l i t h o l o g i e s a c r o s s th e
c o n ta c t as e v id e n c e .
F u r t h e r , Maughan (1984) s t a t e s t h a t s in c e
e v id e n c e o f p a le o s o l and o th e r s u b a e r i a l e r o s io n f e a t u r e s on th e
c o n ta c t a r e m is s in g in c e n t r a l M ontana, a s i g n i f i c a n t h i a t u s i s n o t
r e p r e s e n te d in th e Big Snowy tr o u g h .
He i n t e r p r e t s th e k a r s t
f e a t u r e s to be a r e s u l t o f s u b s u r f a c e p o s t - d e p o s i t i o n a l c o l la p s e of
b a s a l Big Snowy se d im e n ts in t o s o l u t i o n zones in th e M is s io n Canyon
r a t h e r th a n s u b a e r i a l e r o s io n .
A lth o u g h th e M issio n Canyon F o rm a tio n and Big Snowy Group may be
co n fo rm a b le in th e B ig Snowy tr o u g h , p r e v io u s work in th e S now crest
tro u g h i n d i c a t e s t h a t t h i s p a r t o f M ontana was em ergent p r i o r to th e
Big Snowy t r a n s g r e s s i o n .
S c o tt (1 9 3 3 ), W alton (1 9 4 6 ), S ev erso n
(1 9 5 2 ), and M i l l e r (1959) a l l r e p o r te d e r o s i o n a l th in n in g o f M issio n
Canyon lim e s to n e and developm ent o f e r o s i o n a l r e l i e f a lo n g th e
c o n ta c t a t d i f f e r e n t lo c a tio n s in s o u th w e s te rn M ontana.■
In summary, a com plex c o m b in a tio n o f p r o c e s s e s a r e th o u g h t to be
r e s p o n s ib le f o r dev elo p m en t o f s o l u t i o n b r e c c i a s , k a r s t d e p o s i t s , and
r e l i e f on th e M issio n Canyon-B ig Snowy c o n ta c t a t d i f f e r e n t lo c a tio n s
in M ontana.
T h ere i s d is a g re e m e n t on w h e th e r th e c o n ta c t i s
co n fo rm ab le in th e Big Snowy tr o u g h .
H owever, th e c o n ta c t in th e
S now crest tro u g h and B r id g e r Range r e p r e s e n t s on uniconform ity w ith
some form o f e r o s io n .
Big Snowy Group
H is to r y o f N om en clatu re
Big Snowy ro c k s w ere i n i t i a l l y a s s ig n e d to th e Q u ad ran t F o rm atio n
w hich in c lu d e d s t r a t a betw een th e M issio n Canyon and E l l i s F o rm atio n s
( P e a le , 1893; Weed, 1896; Id d in g s and Weed, 1 8 9 9 ).
Weed (1900)
s u b d iv id e d th e Q u ad ran t F o rm a tio n i n t o th e K ibbey S a n d s to n e , O tte r
S h a le , and an unnamed u p p e r member and r a i s e d i t to g ro u p s t a t u s .
He
d e s ig n a te d e x p o su re s in th e L i t t l e B e lt M ountains o f c e n t r a l Montana
a s ty p e s e c ti o n s f o r th e s e f o r m a tio n s .
S c o tt (1935) e s t a b l i s h e d th e
Big Snowy Group in c e n t r a l M ontana and in c lu d e d in i t th e K ib b ey ,
O t t e r , and H eath F o rm a tio n s (F ig u r e 4 ) .
The H eath was named f o r
e x p o su re s in th e Big Snowy M ountains o f c e n t r a l M ontana.
S in c e th e n
15
.
th e Big Snowy Group o f c e n t r a l M ontana h as been expanded to in c lu d e
th e to p o f th e M issio n Canyon F o rm atio n ( S e a g e r, 1942) and b o tto m o f
th e Amsden F o rm a tio n (G a rd n e r, 1 9 5 9 ).
However, S c o t t 's o r i g i n a l
te rm in o lo g y h as been r e i n s t a t e d in s u b se q u e n t r e p o r t s (Maughan and
R o b e rts ; 1967; S ando, Mamet, and D u tr o , 1969; J e n s e n and C a rls o n ,
1972; Sm ith and G ilm o u r, 1979) .
B ecause members o f th e gro u p w ere
n o t fo r m a lly d e l i n e a t e d in p a r t s o f th e S now crest tr o u g h , th e Big
Snowy was d e s ig n a te d a s a fo rm a tio n t h e r e ( Maughan and R o b e r ts , 1967;
Sm ith and G ilm o u r, 1979) .
The o n ly d e t a i l e d r e g io n a l stu d y o f th e Big Snowy ro c k s i s an
u n p u b lis h e d d o c t o r a te d i s s e r t a t i o n by H a r r is (1 9 7 2 ).
Most o f th e
fo llo w in g b r i e f d e s c r i p t i o n o f th e K ib b e y , O t t e r , and H eath
F o rm a tio n s i s c r e d i t e d to H a r r i s ' r e g i o n a l stu d y o f u p p er
M is s is s ip p ia n and low er P e n n s y lv a n ia n r o c k s .
H is s tu d y was fo cu sed
p r im a r ily on ro c k s in th e c e n t r a l M ontana a r e a .
B ig Snowy Trough
H a r r is d iv id e d th e K ibbey F o rm a tio n in t o th r e e in f o r m a l members.
The lo w e st member c o n s i s t s o f re d s h a le w ith le n s e s and beds of
s a n d s to n e and gypsum; th e m id d le member c o n s i s t s o f d o lo m iti c
lim e s to n e w ith gypsum b e d s ; and th e u p p e r member c o n s i s t s o f
s a n d s to n e w ith in te r b e d d e d s h a le and le n s e s o f d o lo m ite .
S h ale is
th e dom inant ro c k ty p e in a l l t h r e e members a lo n g th e n o r th e r n m argin
o f th e Big Snowy tr o u g h and s a n d s to n e d o m in ates a lo n g th e s o u th e rn
and s o u th w e s te rn m a rg in s and in th e S n o w crest tr o u g h .
A lth o u g h th e
m id d le member i s r e s t r i c t e d to th e B ig Snowy tr o u g h , th e low er and
16
u p p e r members t h i n a c r o s s th e Lombard a r c h and c o n tin u e i n t o th e
S now crest tro u g h w ith s l i g h t l i t h o l o g i c ch a n g e .
The low er member o f th e K ibbey F o rm a tio n i s i n t e r p r e t e d as a h ig h
en erg y s h o r e li n e d e p o s it w ith a s s o c i a t e d f i n e g r a in e d i n t e r t i d a l and
s u b t i d a l d e p o s its ( B a l l a r d , 1964; S ando, 1 9 7 6 ).
R e s tric te d
c i r c u l a t i o n o f th e w a te r cau sed by e i t h e r r e l i e f on th e M issio n
Canyon F o rm a tio n , o r m inor se a l e v e l f l u c t u a t i o n s , o r b o th , i s
in d i c a t e d by e v a p o r i te b e d s .
The e n v iro n m e n ta l c o n d itio n s rem ain ed
th e same d u rin g d e p o s it io n o f m id d le K ibbey d o lo m itic lim e s to n e and
e v a p o r i t e s , how ever th e sand su p p ly from th e c r a to n was d im in is h e d
( B a l l a r d , 1 9 6 4 ).
The u p p er member o f th e K ibbey i s i n t e r p r e t e d as
s a n d s to n e d e p o s ite d in s i m i l a r h ig h en erg y i n t e r t i d a l and s h o r e lin e
en v iro n m e n ts ( B a l l a r d , 1 9 6 4 ).
In th e B ig Snowy tro u g h l i t h o l o g i e s o f th e u p p er member o f th e
K ibbey F o rm a tio n g ra d e i n t o and in t e r to n g u e s w ith l i t h o l o g i e s o f
O tte r F o rm atio n o v e r a t h r e e to f i v e m e te r i n t e r v a l .
T h is
r e l a t i o n s h i p i s seen w here th e c o n ta c t i s exposed a t D u rfee C reek
dome in th e B ig Snowy M o u n ta in s.
L enses o f K ibbey s a n d s to n e
in t e r to n g u e w ith O tte r s h a le t h r e e to f i v e m e te rs above th e Kibbey
s a n d s to n e b eds ( H a r r i s , 1 9 7 2 ).
The O tte r c o n s i s t s o f a low er g re e n and g ra y s h a le w ith lim e s to n e
and d o lo m ite and an u p p e r g ra y s h a l e .
T y p ic a l O tte r l i t h o l o g i e s t h i n
and p in c h o u t a c r o s s th e Lombard a r c h .
W ith m inor e x c e p tio n s , th e y
a r e n o t found in th e S n o w crest tr o u g h .
The low er c a r b o n a te was
d e p o s ite d in a more norm al m a rin e , san d f r e e , i n t e r t i d a l and s h a llo w
s u b t i d a l en v iro n m en t o f f s h o r e from th e K ibbey ( H a r r i s , 1 9 7 2 ).
17
F u r th e r o f f s h o r e in r e l a t i v e l y calm , s u b t i d a l w a te r , s h a le s
a c c u m u la te d .
These c o n d itio n s p r e v a i le d d u rin g th e t r a n s i t i o n in t o
th e o v e r ly in g H eath F o rm a tio n s h a le and lim e s to n e .
The H eath F o rm atio n i s th e l e a s t a e r i a l l y e x t e n s iv e fo rm a tio n o f
th e B ig Snowy G roup.
D ark , p e t r o l i f e r o u s s h a le s a r e th e p red o m in an t
ro c k ty p e in th e a x is o f th e Big Snowy tr o u g h .
These g ra d e so u th and
so u th w e st in t o s p a r s e ly f o s s i l i f e r o u s , c h e r t y , lim e-m u d sto n e w ith
some q u a r tz - s a n d g r a in s on th e
Lombard a r c h .
The s h a le s accu m u lated
in q u i e t , oxygen p oor c o n d itio n s in th e c e n t r a l tro u g h ( H a r r i s ,
1 9 7 2 ).
A long th e s o u th e r n m a rg in and Lombard a r c h , m ore a g i t a t e d
m a rin e c o n d itio n s p r e v a i le d p ro x im a l to a so u rc e o f c r a t o n i c san d .
S now crest Trough
F a r t h e r so u th w e st in th e S n o w crest tr o u g h , s t r a t i g r a p h i c
r e l a t i o n s o f ro c k s e q u iv a le n t to th e B ig Snowy Group o f th e c e n t r a l
M ontana a r e n o t w e ll u n d e rs to o d due to s e v e r a l f a c t o r s .
Most
s i g n i f i c a n t l y , p e tro le u m e x p l o r a tio n h as la g g e d f a r b e h in d a c t i v i t y
in c e n t r a l M ontana w here s u b s u r f a c e d a ta i s an i n t e g r a l p a r t o f
s t r a t i g r a p h i c u n d e r s ta n d in g .
A ls o , f a c i e s changes in th e
m io g eo cli n e - p l a t f o r m t r a n s i t i o n and te le s c o p in g o f s t r a t a by e a stw a rd
m o tio n o f t h r u s t p l a t e s c o m p lic a te s i n t e r p r e t a t i o n .
G e o lo g is ts o f
th e U n ite d S ta t e s G e o lo g ic a l Survey a r e b e g in n in g to p ie c e to g e th e r
d a ta in th e S n o w crest tro u g h b u t m ost o f th e id e a s a r e a s y e t
u n p u b lis h e d .
S t r a t i g r a p h i c stu d y o f Big Snowy age ro c k s in th e S n o w crest
tro u g h began when B lak e (1959) showed t h a t th e O tte r F o rm atio n and
18
H eath F o rm atio n in t e r to n g u e a c r o s s th e Lombard a rc h and become a
s i n g l e u n i t composed p r im a r ily o f lim e s to n e .
He t e n t a t i v e l y p ro p o sed
th e term Lombard f a c i e s o f th e H eath F o rm atio n to ap p ly to t h i s u n i t .
The name was chosen b e c a u se th e i n t e r v a l i s r e a d i l y a c c e s s i b l e and
w e ll exposed n e a r th e abandoned Lombard r a i l r o a d s t a t i o n 17 m ile s
w e s t- n o r th - w e s t o f th e B fid g e r R ange.
McMannis (1951) f i r s t d e s c r ib e d t h i s lim e s to n e in th e B rid g e r
Range and c o r r e l a t e d i t to th e H eath F o rm a tio n in th e B ig Snowy
tro u g h and to th e s e c t i o n a t Lom bard.
U n ite d S ta t e s G e o lo g ic a l
Survey g e o l o g i s t s w o rk in g in th e S n o w crest tro u g h c a r r y th e Lombard
f a c i e s and K ibbey F o rm atio n i n t o th e ex trem e so u th w e st p a r t o f th e
S now crest tr o u g h .
Both u n i t s a r e p r e s e n t l y c o n s id e re d m appable as
fo rm a tio n s e q u iv a le n t to th e K ib b e y , O t t e r , and H eath F o rm a tio n s o f
th e B ig Showy Group ( p e r s o n a l co m m u n icatio n , Maughan, W ardlaw, 1 9 8 3 ).
The a u th o r a l s o r e c o g n iz e s th e K ibbey and Lombard s t r a t a a s m appable
fo rm a tio n s in th e B r id g e t Range e q u iv a le n t to fo rm a tio n s in th e Big
Snowy Group o f c e n t r a l M ontana.
S e v e ra l l i n e s o f re a s o n in g s u p p o rt th e c o n c lu s io n t h a t th e
fo rm a tio n s o f th e Big Snowy Group a r e in p a r t , b u t n o t c o m p le te ly ,
tim e e q u iv a le n t f a c i e s o f eac h o th e r ( F ig u r e 5 ) .
As m en tio n ed
p r e v io u s l y , c o n ta c ts b etw een fo rm a tio n s a r e g r a d a t io n a l o v e r
c o n s id e r a b le v e r t i c a l d i s t a n c e and fo r m a tio n l i t h o l o g i e s i n t e r f i n g e r .
A ls o , th e lo w e st n e a r s h o r e K ibbey F o rm a tio n s a n d s to n e f a c i e s i s th e
m ost a e r i a l l y e x t e n s i v e , w h ereas th e Lombard and e q u iv a le n t O tte r and
H eath F o rm atio n s a r e r e s t r i c t e d to th e tro u g h a x i s .
S i m i l a r l y , th e
K ibbey F o rm atio n i s t h i c k e s t a t th e tro u g h m a rg in s , and th e Lombard
North
South
----- ------------------ Line of time-synchronous
deposition
Figure 5.
Cross secti on a c r o s s B ig
(from H a r r i s , 1 9 7 2 ) .
S n o w y trough= t i m e - s y n c h r o n o u s f a c i e s d i a g r a m
20
and H eath F o rm a tio n s a r e t h i c k e s t a t th e tro u g h a x is (Maughan and
R o b e r ts , 1967; H a r r i s , 1 9 7 2 ).
U sing b i o s t r a t i g r a p h i c d a t a , Sando (1974) d e te rm in e d t h a t th e
K ibbey F o rm atio n i s y o u n g e st n e a r th e m io g e o c lin e in th e
southw .este rn m o s t p a r t o f th e S now crest tro u g h ( l a t e Merameci a n ) , and
o l d e s t in th e e a s te rn m o s t p a r t o f th e Big Snowy tro u g h and th e
W i l l i s t o n b a s in (m id d le Chest e r ia n ) (F ig u r e 4 ) .
He c o n clu d ed t h a t
th e Big Snowy Group r e p r e s e n t s a c l a s s i c d ia c h ro n o u s t r a n s g r e s s i v e
s e rie s .
The se a i n i t i a l l y tr a n s g r e s s e d i n t o th e S n o w crest tro u g h
from th e m io g e o c lin e and d e p o s ite d th e K ibbey n e a r s h o r e s a n d s to n e
d u rin g l a t e Meramec tim e .
As th e t r a n s g r e s s i o n c o n tin u e d , th e Kibbey
n e a r s h o r e f a c i e s m ig r a te d r e g i o n a l l y e a s tw a rd and l o c a l l y t o th e
tro u g h m a rg in s u n t i l m id d le C h e s te r tim e .
S im u lta n e o u s ly , th e
Lombard and e q u iv a le n t O tte r and H eath o f f s h o r e f a c i e s fo llo w e d
b e h in d th e e a s tw a rd t r a n s g r e s s i n g s h o r e l i n e b u t w ere r e s t r i c t e d to
th e tro u g h a x i s .
R e la tio n s o f th e Upper B ig Snowy Group to th e
O v e rly in g Amsden F o rm atio n
The Amsden F o rm a tio n u nconform ably b la n k e ts th e B ig Snowy Group
s e r i e s (Maughan and R o b e r ts , 1967; Sando, Gordon, and D u tr o , 1975)
(F ig u re 4 ) .
I n th e c e n t r a l p a r t o f th e S now crest and B ig Snowy
tro u g h s th e Amsden o v e r l i e s Lombard f a c i e s and H eath F o rm a tio n
re s p e c tiv e ly .
Towards th e tro u g h m a rg in s , i t r e s t s on th e K ib b e y .
On m ost o f th e A lb e r ta and Wyoming s h e lv e s i t r e s t s on th e M issio n
Canyon F o rm a tio n .
E x te n s iv e e r o s io n p r i o r to Amsden o n la p in e a r ly
21
Morrow tim e i s in d i c a t e d by f o r t y - f i v e m e te rs o f r e l i e f betw een th e
H eath and T y le r F o rm a tio n s in c e n t r a l M ontana (S m ith , 1 9 7 9 ).
The Amsden in th e S n o w crest tro u g h c o n s i s t s o f a lo w er re d
m udstone and s i l t s t o n e w ith some d is c o n tin u o u s s a n d s to n e , lim e s to n e ,
and d o lo m ite b e d s .
The re d i n t e r v a l g ra d e s up i n t o c a r b o n a te ro c k s
w ith m inor s a n d s to n e s and e v a p o r i t e s .
The fo rm er r e p r e s e n t s
n e a r s h o r e la g o o n a l and swamp d e p o s i t s , th e l a t t e r r e s t r i c t e d m arin e
o f f s h o r e banks in a s h a llo w se a ( S ando, Gordon, and D u tr o , 1 9 7 5 ).
22
INTERPRETATION OF THE BIG SNOWY GROUP
IN THE BRIDGER RANGE
I n t r o d u c t io n
I n t h i s c h a p te r l i t h o l o g i c and s t r a t i g r a p h i c d a ta and
i n t e r p r e t a t i o n s from th e stu d y a r e a a r e i n t e g r a t e d i n t o th e r e g io n a l
r e l a t i o n s p r e v io u s ly d i s c u s s e d .
F o rm a tio n s and members a r e d e sc rib e d ,
in seq u en ce from o l d e s t to y o u n g e s t.
A b r i e f d e s c r i p t i o n o f th e
c o n ta c t w ith s t r a t a above and below th e g ro u p i s in c lu d e d .
Each
fo rm a tio n and member h as an i n d i v id u a l s e c t i o n in th e t e x t f o r
l i t h o f a c i e s , d e p o s i t i o n a l e n v iro n m e n t, and s t r a t i g r a p h i c r e l a t i o n s .
The B ig Snowy Group in th e s tu d y a r e a i s d iv id e d i n t o th e K ibbey
F o rm a tio n and Lombard f a c i e s ( F ig u re 6 ) .
The K ibbey i s f u r t h e r
s u b d iv id e d in f o r m a lly i n t o a low er p re d o m in a n tly d o lo s to n e member,
and an u p p er s a n d s to n e and s i l t s t o n e member.
These a r e a p p ro x im a te ly •
e q u iv a le n t to th e u p p er and lo w er members o f th e K ibbey F o rm atio n s in
H a r r is
(1 9 7 2 ) s ta te w id e s tu d y .
The Lombard f a c i e s o v e r l i e s th e
K ibbey and i s e q u iv a le n t to th e O tte r and H eath F o rm a tio n s o f c e n t r a l
M ontana.
23
Amsden Formation
Lombard Facies
(0 to 4 3 meters)
Z tt/
Kibbey Formation
Upper Member
(2 to 6 6 meters)
/
/ •• / •
ZZZZZ
ZZZZ
Lower Member
(4 to 33 meters)
Mission
Canyon
For m at io n
( solution breccia
os for os 200 meters below top)
Rgure 6 .
Generalized
Snowy
lithologic column of the
Group.
Big
24
Lower B ig Snowy Group C o n ta c t and th e U n d e rly in g
M issio n Canyon F o rm atio n
D e s c r ip tio n
One o r more s o l u t i o n b r e c c i a zones a r e p r e s e n t in th e M issio n
Canyon F o rm atio n w ith i n 200 m e te rs o f th e c o n ta c t w ith th e Big Snowy
G roup.
(1 9 6 6 ).
The b r e c c ia zones a r e s i m i l a r to th o s e d e s c r ib e d by R o b e rts
The b r e c c ia s c h a r a c t e r i s t i c a l l y form l e s s r e s i s t a n t zones
w ith i n th e M issio n Canyon lim e s to n e and w e a th e r r e d d is h - y e llo w .
A p p ro x im ately one m e te r o f re d c la y d e f in e s th e b a se o f th e b r e c c ia
zones on a d i s t i n c t c o n ta c t w ith u n d e r ly in g lim e s to n e .
The b r e c c ia s
a r e composed o f v a r io u s s i z e s o f v a r i a b l y rounded d o lo s to n e c l a s t s
and m in o r brown a n g u la r c h e r t in a s i l t y , muddy, r e d d is h - y e llo w
c a rb o n a te m a tr ix .
The b r e c c ia s g ra d e up i n t o f r a c t u r e d lim e s to n e a t
th e to p o f th e z o n e s .
A t s e v e r a l e x p o s u r e s , an i n t e r v a l o f
u n d is tu r b e d lim e s to n e s e p a r a t e s two o r more b r e c c i a z o n e s .
The
th ic k n e s s o f th e zones i s v a r i a b l e a t d i f f e r e n t e x p o s u r e s .
D is ta n c e
from th e c o n ta c t w ith th e Big Snowy Group a l s o v a r i e s .
A b r e c c ia
zone e x te n d s to w ith i n te n m e te rs o f th e M issio n C anyon-S now crest
c o n ta c t a t s t a t i o n 191.
A k a r s t s in k h o le i s d ev elo p ed a lo n g th e c o n ta c t a t th e B rid g e r
Canyon s e c t i o n on th e s o u th end o f th e r a n g e .
.U n b re c c ia te d M issio n
Canyon lim e s to n e e n c lo s e s th e s i x m e te r deep by tw e lv e m e te r wide
c a v i ty w hich c o n s t r i c t s to a p p ro x im a te ly one m e te r a t th e to p .
S i l t s t o n e and d o lo s to n e from th e o v e r ly in g K ibbey F o rm a tio n f i l l th e
c a v ity .
V ario u s s iz e d d o lo s to n e c l a s t s have i n d i s t i n c t b o u n d a rie s
and a p p e a r c o n to r te d and s q u a sh e d .
A r e d muddy, s i l t y d o lo s to n e
25
m a tr ix s u p p o r ts th e c l a s t s in th e c a v i t y .
The b r e c c ia s in th e c a v i ty
g ra d e up i n t o u n d is tu r b e d K ibbey muddy d o lo s to n e j u s t above th e to p
o f th e c a v i t y .
At l e a s t th e low er f i v e m e te rs o f K ibbey F o rm atio n i s
bowed down to w ard s th e k a r s t s in k in th e u p p er M issio n Canyon
F o rm a tio n .
I d e n t i f y i n g th e c o n ta c t o f th e M is s io n Canyon F o rm a tio n and Big
Snowy Group i s s t r a i g h t f o r w a r d a t th e e x p o su re s w here s o l u t i o n
b r e c c i a s o r k a r s t d e p o s it s do n o t o c c u r n e a r th e c o n t a c t .
The to p o f
th e M issio n Canyon F o rm a tio n i s v e ry s i m i l a r to th e ty p e s e c t i o n a t
Logan, M ontana and to th e e q u iv a le n t B u ll R idge member in th e
B e a r t o o th s .
The p rim a ry ro c k ty p e i s f i n e to c o a r s e ly c r y s t a l l i n e ,
medium b ro w n -g ra y , o o l i t i c , d o lo m itic , lim e b io -w a c k e s to n e w ith
c h e rt.
O v e rly in g th e M is s io n Canyon F o rm a tio n a r e muddy, s i l t y ,
la m in a te d , p a s t e l c o lo re d d o lo s to n e s w ith le n s e s o f q u a r tz s i l t s t o n e
and s a n d s to n e t y p i c a l o f th e low er member o f th e K ibbey F o rm a tio n .
At th e ex p o su re a t s t a t i o n 191 in th e c e n t r a l B rid g e r Range a fo u r
m e te r t h i c k co n g lo m e ra te r e s t s on th e M issio n Canyon F o rm a tio n .
The
c o n g lo m e ra te i s composed o f su b -ro u n d e d p a s t e l muddy d o lo s to n e c l a s t s
s i m i l a r to th e low er K ibbey F o rm a tio n in a re d muddy s a n d s to n e
m a tr ix .
The M issio n C anyon-B ig Snowy c o n ta c t i s n o t g r a d a t i o n a l
v e r t i c a l l y o r l a t e r a l l y and ro c k ty p e s do n o t i n t e r f i n g e r .
At
s e v e r a l e x p o s u re s , b etw een one and two m e te rs o f r e l i e f i s d ev elo p ed
a lo n g s e v e r a l te n s o f m e te rs o f th e c o n ta c t and d o lo s to n e , d o lo m itic
s i l t s t o n e , and s a n d s to n e c h ip s s i m i l a r to th e e n c lo s in g K ibbey
F o rm atio n f i l l low a r e a s on th e c o n t a c t .
26
In te rp re ta tio n
S o lu tio n b r e c c i a s , k a r s t d e p o s i t s , and ro c k ty p e s a s s o c i a t e d w ith
th e M is s io n Canyon-Big Snowy c o n ta c t in th e B rid g e r Range pose
i n t e r p r e t i v e problem s s i m i l a r to th o s e w here th e c o n ta c t i s exposed
a t o th e r ra n g e s in s o u t h - c e n t r a l M ontana.
However, a t no s e c t i o n in
th e stu d y a r e a was e v id e n c e f o r c o n tin u o u s d e p o s it io n a c r o s s th e
c o n ta c t fo u n d .
R a th e r , t h e r e i s e v id e n c e f o r a p e r io d o f e r o s io n and
change in d e p o s i t i o n a l e n v iro n m e n t.
N early a l l th e e x p o s u re s o b se rv e d have some d e g re e o f r e l i e f on
th e c o n t a c t .
A ls o , th e th ic k n e s s o f th e M issio n Canyon. F o rm atio n
v a r i e s up to f i f t y m e te rs a c r o s s th e ra n g e (McMannis, 1 9 5 1 ).
U n f o r tu n a te ly , i n s u f f i c i e n t e v id e n c e i s a v a i l a b l e to d e te rm in e
w h e th e r s u b a e r i a l e r o s io n o r s u b s u r f a c e s o l u t i o n o f th e M issio n
Canyon lim e s to n e i s th e m ost im p o rta n t p r o c e s s r e s p o n s ib le f o r
th ic k n e s s v a r i a t i o n a n d . r e l i e f .
However, two r e l a t i o n s h i p s s u g g e s t
t h a t a t l e a s t some th in n in g o f th e M is s io n Canyon F o rm a tio n was
pro d u ced by e r o s io n .
F i r s t , w here th e M issio n Canyon i s t h i n , th e
s o l u t i o n b r e c c ia zones w ith i n i t a r e found c l o s e r to th e fo rm a tio n
to p th a n i s u s u a ll y th e c a s e .
F or exam ple s o l u t i o n b r e c c i a s , w hich
a r e g e n e r a lly lo c a te d 100 m e te rs o r m ore below th e M is s io n Canyon-Big
Snowy c o n t a c t , o c c u r w ith i n te n m e te rs o f th e c o n ta c t a t s t a t i o n 191
w here th e M issio n Canyon F o rm atio n i s f o r t y m e te rs t h i n n e r th a n a t
n e ig h b o rin g s e c ti o n s (McMannis, 1 9 5 5 ).
The e la v a te d o c c u r r e n c e of
th e e v a p o r i te s o l u t i o n zo n es i s b e s t e x p la in e d by e r o s io n o f th e
lim e s to n e o v e r ly in g th e e v a p o r ite o r e v a p o r i te b r e c c i a zones in an
27
a r e a o f s l i g h t p o s i t i v e r e l i e f d u rin g a p e r io d o f e x p o s u r e .
S e c o n d ly , one o f th e t h i n n e s t s e c ti o n s o f th e low er member o f th e
K ibbey F o rm a tio n in th e B rid g e r Range c o i n c i d e n t a l l y i s lo c a te d a t
s t a t i o n 191 ( P l a t e I ) s u g g e s tin g t h a t a p o s i t i v e te n d en cy cau sed t h i s
e r o s io n and in f lu e n c e d s e d im e n ta tio n d u rin g and f o llo w in g th e K ibbey
tra n s g re s s io n .
D i f f e r e n t e n v iro n m e n ta l c o n d itio n s below and above th e M issio n
Canyon-Big Snowy c o n ta c t a r e in d i c a t e d by th e sh a rp change from
c le a n , o o l i t i c ,
lim e b io -w a c k e sto n e to u n f o s s i l i f e r o u s , s i l t y , muddy,
la m in a te d d o lo s to n e w ith i n t e r m i t t e n t b a s a l l i t h o c l a s t c o n g lo m e ra te .
Lack o f i n t e r f i n g e r i n g o f th e l i t h o l o g i e s im p lie s t h a t a s in g le
e x te n s iv e s u r f a c e o f d i s c o n fo rm ity e x i s t e d .
In summary, e v id e n c e o f e r o s io n and change o f d e p o s i t i o n a l
en v iro n m en t a c r o s s th e c o n ta c t s u p p o rt a c o n c lu s io n t h a t th e M issio n
Canyon F o rm atio n was exposed in th e B r id g e r Range a r e a p r i o r to
d e p o s it io n o f th e low er member o f th e K ibbey F o rm a tio n .
The d u r a tio n
o f tim e t h a t th e d is c o n f o r m ity betw een th e M issio n Canyon and K ibbey
F o rm atio n r e p r e s e n t s , h ow ever, can o n ly be i n f e r r e d s in c e n e i t h e r th e
u p p er M issio n Canyon o r low er K ibbey F o rm a tio n s a r e d a t e d .
Lower K ibbey Member
L i th o f a c i e s
In tro d u c tio n .
The low er member o f th e K ibbey F o rm a tio n i s
d iv id e d i n t o two l i t h o f a c i e s .
A la m in a te d d o lo s to n e l i t h o f a c i e s
e x te n d s th ro u g h o u t th e stu d y a r e a .
The se c o n d , a s a n d s to n e ch an n e l
l i t h o f a c i e s i s d ev elo p ed w ith i n th e d o lo s to n e l i t h o f a c i e s in th e
28
c e n t r a l B rid g e r Range (F ig u r e 7 ) .
The c h a n n e ls i n t e r f i n g e r w ith and
g ra d e i n t o th e la m in a te d d o lo s to n e away from th e c e n t r a l p a r t o f th e
ra n g e ( P l a t e I ) .
The c o n ta c t w ith th e o v e r ly in g u p p e r member o f th e
K ibbey F o rm atio n i s p la c e d below th e f i r s t c o n tin u o u s m udstone o r
s i l t s t o n e bed w hich c o r r e l a t e s betw een e x p o s u r e s .
L am inated D o lo sto n e L i t h o f a c i e s .
S i l i c i c l a s t i c and d o lo m iti c mud
and s i l t c o n te n t w ith i n th e la m in a te d d o lo s to n e l i t h o f a c i e s in c r e a s e s
up from a r e l a t i v e l y c le a n d o lo s to n e on th e low er c o n ta c t w ith th e
M issio n Canyon F o rm a tio n to a d o lo m itic s i l t s t o n e in th e u p p er p a r t
o f th e member.
Shades o f m aroon, p in k , and y e llo w a r e common, w ith
t a n , g r e e n , and re d sh ad es l e s s common.
C o lo r a tio n g e n e r a lly
r e f l e c t s v a r i a b l e am ounts o f c l a s t i c mud and s i l t , b u t a l t e r n a t i n g
o x id a tio n c o n d itio n s a r e s u g g e s te d by l i g h t e r and d a r k e r zones w hich
o c c a s i o n a ll y c r o s s b ed d in g b u t g e n e r a lly a r e r e s t r i c t e d to in d iv id u a l
h o riz o n s .
L a t e r a l l y p e r s i s t e n t m i ll i m e t e r to c e n tim e te r t h i c k la m in a tio n s
a r e p r e s e n t th ro u g h o u t th e d o lo s to n e l i t h o f a c i e s ( P l a t e 2 ) .
The
la m in a tio n s a r e v a r ia b ly r e s i s t a n t and w e a th e rin g p ro d u c e s a seq u en ce
o f a l t e r n a t i n g smooth and g r a in y s u r f a c e s .
L o c a lly th e y a r e
c o n tin u o u s o v e r a s e r i e s o f humps ab o u t two c e n tim e te r s h ig h and f o u r
c e n tim e te r s lo n g .
I n some p la c e s th e y a r e c o n to r te d i n t o s te e p ,
sm a ll s c a l e f o ld s betw een r e l a t i v e l y f l a t la m in a e .
D e s s ic a tio n
c r a c k s , te e p e e s t r u c t u r e s , and c a l c i t e f i l l e d f e n e s t r a l vugs and
v e in s d i s r u p t th e la m in a tio n s ( P l a t e 3 ) .
29
Upper Member
Kibbey Formation
/
/
/-
Z-ZZZ-Z
Lower Member
Kibbey Formation
(4 to 33 meters)
ZZZZZ-Z
ZZZZZ
ZZZZZ
/ •/
/
/
Laminated Dolostone Lithofacies
/
/ - /
/ '' /
/
Sandstone Channel Lithofacies
r / ~i
7
~ /
/
Laminated Dolostone Lithofacies
Mission Canyon
Formation
Figure
7.
G e n e r a l i z e d lithologic column of the low er m em b er
tI-A6U Ktbbey Formation.
S e e text for d es cri p ti on
of li t h o f a c ie s .
I
30
P l a t e 2 . P h o to g rap h o f la m in a te d d o lo s to n e in
th e low er member o f th e K ibbey F o rm a tio n . See
te x t fo r d is c u s s io n .
P l a t e 3 . P h o to m icro g rap h o f la m in a tio n s , n o te
th e c a l c i t e f i l l e d f e n e s t r a l f r a c t u r e
(a p p ro x im a te ly 2 .5 mm a c r o s s f i e l d ) .
31
A lso w ith in t h i s l i t h o f a c i e s a r e d o lo s to n e b r e c c ia s w h ich g rad e
l a t e r a l l y and v e r t i c a l l y i n t o u n d is tu r b e d la m in a te d d o lo s to n e .
A lth o u g h beds o f b r e c c i a a r e up to two m e te rs t h i c k , c e n tim e te r to
m e te r th i c k beds a r e t y p i c a l .
The m i l l i m e t e r to s e v e r a l c e n tim e te r
d ia m e te r d o lo s to n e c l a s t s a r e n o t g ra d e d o r s o r t e d .
The b o u n d a rie s
o f th e c l a s t s a r e i n d i s t i n c t and g ra d e i n t o th e m a tr ix ( P l a t e 4 ) .
Upper and lo w er c o n ta c ts o f th e b r e c c i a zones a r e a l s o i n d i s t i n c t ,
and bed th ic k n e s s v a r i e s a lo n g s t r i k e .
I n d iv i d u a l b r e c c i a beds do
n o t e x te n d betw een e x p o s u re s .
F iv e t h i n s e c ti o n s r e p r e s e n t in g b o th th e la m in a te d d o lo s to n e and
d o lo s to n e b r e c c i a s w ere a n a ly z e d .
s iz e d c r y s t a l s .
D o lo m ite a p p e a rs as m ic ro s p a r
In t h i n s e c t i o n la m in a tio n s a r e p r e s e r v e d as
m i lli m e t e r to c e n tim e te r i n t e r l a y e r i n g o f zones r i c h in q u a r t z - s i l t
(P la te 3 ) .
By e s ti m a t io n , q u a r tz s i l t c o n te n t a v e ra g e s te n p e r c e n t
in th e t h i n s e c t i o n s ; in d i v i d u a l la m in a tio n s a r e s i g n i f i c a n t l y
ric h e r.
The la m in a te d d o lo s to n e l i t h o f a c i e s i s th e m ost p e r s i s t e n t f a c i e s
o f th e Big Snowy Group in th e B r id g e r Range and a p p e a rs to c o n tin u e
o u t o f th e stu d y a r e a n o r th and s o u th .
I t i s th e o n ly l i t h o f a c i e s
p r e s e n t in th e n o r th e r n and s o u th e r n p a r t s o f th e stu d y a r e a .
In th e
c e n t r a l B rid g e r R ange, w here th e c h a n n e l l i t h o f a c i e s i s w e ll
d e v e lo p e d , o n ly t h i n beds o f la m in a te d d o lo s to n e a r e p r e s e r v e d above
and below th e c h a n n e l l i t h o f a c i e s ( P l a t e I ) .
An e x c e p tio n to t h i s
p a t t e r n o c c u rs a t th e ex p o su re a t s t a t i o n I 91 w here th e low er member
o f th e K ibbey F o rm atio n i s e x c e p tio n a lly t h i n ; th e c h a n n e l
l i t h o f a c i e s i s a b s e n t and a f o u r m e te r t h i c k b a s a l c o n g lo m e ra te r e s t s
32
P l a t e 5 . P h o to m icro g rap h o f s a n d s to n e in th e
s a n d s to n e c h a n n e l l i t h o f a c i e s (a p p ro x im a te ly
2 .5 mm a c r o s s f i e l d ) .
33
d i r e c t l y on th e M issio n Canyon F o rm a tio n .
The c o n g lo m e ra te i s
composed o f su b -ro u n d e d c l a s t s o f s a n d s to n e , s i l t s t o n e , and d o lo s to n e
in a s i l t y d o lo s to n e m a tr ix .
The c o n g lo m e ra te g ra d e s i n t o th e
la m in a te d d o lo s to n e and s a n d s to n e c h a n n e l l i t h o f a c i e s to th e n o r th
and s o u th .
S an d sto n e C hannel L i t h o f a c i e s .
W ith in th e d o lo s to n e l i t h o f a c i e s
in th e c e n t r a l B r id g e t R ange, le n s - s h a p e d c h a n n e ls o f s a n d s to n e ,
s i l t s t o n e , and m udstone a r e d ev elo p ed ( F ig u r e 7 ) .
T h is second
l i t h o f a c i e s o c c u rs a s a s e r i e s o f up t o s i x d i s c r e t e u p w a rd -fin in g
c h a n n e ls o v er a te n to tw en ty m e te r i n t e r v a l .
I n d iv i d u a l c h a n n e ls
a r e n o t t r a c e a b l e betw een e x p o s u re s , how ever th e p ack ag e i s .
The s a n d s to n e s , s i l t s t o n e s , and m udstones in th e c h a n n e ls a r e a
deep r e d d is h c o l o r , t y p i c a l l y b r i c k r e d .
I d e a l l y an i n d i v id u a l
c h a n n e l g ra d e s up from s a n d s to n e on a s h a rp c o n ta c t c u t i n t o th e
u n d e rly in g c h a n n e l i n t o s i l t s t o n e and m udstone o v e r a one to two
m e te r i n t e r v a l .
At some e x p o s u r e s , th e c h a n n e ls f i n e up fjcom a
c o a rs e s i l t s t o n e r a t h e r th a n a f i n e s a n d s to n e a t th e b a s e .
A ls o ,
d o lo s to n e l i t h o c l a s t c o n g lo m e ra te s may be p r e s e n t a t th e b a s e .
If
th e c h a n n e ls a r e n o t d i r e c t l y s u p e rp o s e d , a bed o f la m in a te d
d o lo s to n e in te r v e n e s betw een them .
I n t h i s in s t a n c e , th e o v e r ly in g
c h a n n e l i s c u t in t o th e d o lo s to n e .
O th e r th a n g r a d in g ,, few b ed d in g
fe a tu re s a re p re se rv e d .
F in e s i l t s t o n e and m udstone a t th e to p o f
c h a n n e ls w e a th e rs to a b lo c k y r u b b le f r a c t u r e d a lo n g s u b - p a r a l l e l
s u r f a c e s one to f o u r c e n tim e te r s a p a r t .
E ig h t t h i n s e c ti o n s w ere a n a ly z e d to d e te rm in e m in e r a l
34
c o m p o sitio n and t e x t u r e o f th e ro c k s in th e s a n d s to n e c h a n n e l
lith o fa c ie s .
O v e r a ll , th e sand and s i l t i s f o r t y - f i v e p e r c e n t
q u a r t z , f i f t y p e r c e n t d o lo m ite , t h r e e p e r c e n t c h e r t , and two p e rc e n t
p la g io c la s e .
M inor h e m a tite s t a i n s rim s on th e g r a in s and im p a rts
much o f th e c h a r a c t e r i s t i c re d c o l o r .
The d o lo m ite o c c u rs as
v a r ia b ly rounded d e t r i t a l c r y s t a l s d i s t r i b u t e d ev e n ly th ro u g h o u t th e
s i l t s t o n e and s a n d s to n e and as medium s i z e l i t h o c l a s t s c o n c e n tr a te d
a t th e b a se o f c h a n n e ls .
V a r ia b le ro u n d in g o f th e d o lo m ite c r y s t a l s
and i n t e g r a t i o n o f th e c r y s t a l s in th e g r a in - s u p p o r te d s a n d s to n e
fram ew ork a t t e s t to t h e i r d e t r i t a l o r i g i n .
In te r g r a n u la r m a te ria l is
p r im a r ily s i l i c i c l a s t i c mud w ith sm a ll am ounts o f c a r b o n a te w hich
a p p e a rs as p a tc h e s o f d o lo - m ic r o s p a r .
The s a n d s to n e and s i l t s t o n e e x h i b i t bim odal d i s t r i b u t i o n o f v e ry
w e ll ro u n d e d , low medium s a n d - s iz e d q u a r tz g r a in s ( .2 5 to .35
m i lli m e t e r d ia m e te r ) in a m a tr ix o f f i n e san d to s i l t - s i z e d subrouded
q u a r tz g r a in s and d e t r i t a l d o lo m ite c r y s t a l s ( P l a t e 5 ) .
The r a t i o o f
th e f i n e sand and s i l t f r a c t i o n to th e medium sand f r a c t i o n i s f o u r
to o n e.
G rain s u p p o rt w ith o u t p r e f e r r e d o r i e n t a t i o n i s t y p i c a l .
However, mud and m a tr ix s u p p o rt become more abu n d an t to w ard s th e top
o f th e c h a n n e l w here th e s a n d s to n e and s i l t s t o n e g ra d e i n t o m u d sto n e.
Mud a t th e to p o f a c h a n n e l i s composed o f ab o u t e q u a l p a r t s
s i l i c i c l a s t i c and c a r b o n a te com ponents.
I
35
D e p o s itio n a l E nvironm ent I n t e r p r e t a t i o n
I n tro d u c tio n .
The two l i t h o f a c i e s in th e low er member o f th e
K ibbey F o rm atio n r e p r e s e n t f a c i e s w ith i n a s i n g l e d e p o s i t i o n a l
e n v iro n m e n t.
The member i s composed p r im a r ily o f s u p r a t i d a l and h ig h
i n t e r t i d a l d o lo s to n e s d e p o s ite d on a s a b k h a .
The o c c u rre n c e o f
e v a p o r i te b r e c c ia s in th e d o lo s to n e s i n d i c a t e s t h a t th e p r e v a i l i n g
c lim a te was h o t and a r i d .
The s a n d s to n e c h a n n e l l i t h o f a c i e s
r e p r e s e n t s i n t e r t i d a l c h a n n e ls ( t i d a l c r e e k s ) in th e n e a r s h o r e m arin e
en v iro n m en t m a rg in a l to th e sa b k h a .
S i l i c i c l a s t i c g r a i n s d e p o s ite d
in th e t i d a l c h a n n e ls and tr a p p e d by a l g a l f ilm s w ere d e r iv e d from
th e n e ig h b o rin g s h o r e f a c e en v iro n m en t w here th e u p p er K ibbey was
c o n tem p o ran eo u sIy d e p o s it e d .
S u p r a ti d a l s e d im e n ta tio n d o m in ated a t
th e n o r th and s o u th ends o f th e ra n g e w here th e M issio n Canyon
s u r f a c e was r e l a t i v e l y e l e v a t e d , w h ereas i n t e r t i d a l c h a n n e l
d e p o s it io n was m ost im p o rta n t in th e c e n t r a l ra n g e w hich was
d e p re sse d .
S t a t i o n 191, w hich was a p o s i t i v e a r e a w ith i n th e c e n t r a l
r a n g e , i s an e x c e p tio n to t h i s t r e n d .
L am in ated D o lo sto n e L i t h o f a c i e s .
Work on modern and a n c ie n t
sabkha e n v iro n m en ts has g e n e r a te d an a s s o c i a t i o n o f s e d im e n to lo g ic
f e a t u r e s w hich a r e u sed to i d e n t i f y t h i s en v iro n m en t in th e r e c o r d .
Most im p o rta n t o f th e s e a r e d e s s i c a t i o n f e a t u r e s , a l g a l m a ts ,
f e n e s t r a l f a b r i c , l i t h o c l a s t c o n g lo m e ra te s , e v a p o r ite s ( o r s o lu ti o n
b r e c c i a s ) , and la c k o f m a rin e o rg an ism s ( L u c ia , 1 9 7 2 ).
The
i n t e r p r e t a t i o n o f th e d e p o s i t i o n a l e n v iro n m en t o f th e lo w er K ibbey i s
b a s e d on o c c u rre n c e o f th e s e i n d i c a t o r s .
36
C l a s t i c t e x t u r e , p re s e n c e o f a n t i g r a v i t y f e a t u r e s such a s
s te e p l y s lo p in g la m in a e , and c o n s ta n t th ic k n e s s o f lam in ae o v er
s u b s t r a t e i r r e g u l a r i t i e s r a t h e r th a n i n f i l l i n g in d e p r e s s io n s and
th in n in g o v e r h ig h s a r e r e p o r te d as im p o rta n t c r i t e r i a f o r
r e c o g n i tio n o f m e c h a n ic a l b in d in g o f sed im en t by a l g a l f il m ( G insburg
e t a l , 1954; D a v ie s , 1 9 7 0 ).
C l a s t i c t e x t u r e i s p r e s e r v e d in th e
K ibbey d o lo s to n e s a s a l t e r n a t i o n o f la m in a tio n s r i c h in c o a r s e and
f i n e s i l i c i c and c a r b o n a te g r a i n s .
L am in a tio n s w hich a r e c o n to r te d
in t o s t e e p , s m a l l- s c a le f o ld s in t h i s l i t h o f a c i e s a r e exam ples o f
sed im en t bound by m a t e r i a l w hich i s c o h e s iv e enough to r e s i s t
g ra v ity .
F i n a l l y , c o n s ta n t th ic k n e s s o f lam in ae o v e r s u b s t r a t e
i r r e g u l a r i t i e s i s common in th e la m in a te d d o lo s to n e l i t h o f a c i e s o f
th e low er member o f th e K ibbey F o rm a tio n .
D e t r i t a l sed im en t tr a p p e d by a l g a l f ilm s on m odern t i d a l f l a t s i s
m o s tly d e r iv e d and t r a n s p o r t e d from n e a rb y m a rin e en v iro n m en t by
t i d a l and sto rm p r o c e s s e s ( P a r k , 1 9 7 6 ).
T h is a p p e a rs to be th e c a se
f o r th e s i l i c i c com ponent o f d e t r i t u s tr a p p e d in th e K ibbey
d o lo s to n e s s in c e th e abundance o f s i l i c i c l a s t i c m a t e r i a l in c r e a s e s
u p s e c tio n a s th e m a rin e en v iro n m en t s im u lta n e o u s ly s h i f t s landw ard
o v e r th e t i d a l f l a t by t r a n s g r e s s i o n .
T rapped c a r b o n a te d e t r i t u s i s
p r im a r ily d e r iv e d from th e sabkha c r u s t (P a r k , 1 9 7 6 ).
R e c o g n itio n o f e v a p o r i te s in o u tc ro p i s c o m p lic a te d b e c a u se th e y
can be le a c h e d to form s o l u t i o n b r e c c i a s .
L u cia (1 9 7 2 ) s t a t e s t h a t
c o r r e l a t i o n w ith s u b s u r f a c e e v a p o r i te s and a s s o c i a t i o n w ith sabkha
c a rb o n a te s a r e th e b e s t c r i t e r i a f o r c o n f ir m a tio n t h a t b r e c c ia s a r e
th e p ro d u c t o f e v a p o r i te s o l u t i o n .
B oth r e l a t i o n s h i p s a r e t r u e f o r
37
th e K ibbey F o rm a tio n ; e v a p o r i te s a r e p r e s e r v e d in th e low er member o f
th e K ibbey F o rm atio n in c e n t r a l M ontana ( H a r r i s , 1 9 7 2 ), and th e
e n c lo s in g d o lo s to n e s a r e i n t e r p r e t e d a s a sabkha d e p o s it in th e
B rid g e r R ange.
L u cia (1 9 7 2 ), a l s o p o in t s o u t t h a t d o lo s to n e i s o f te n
p a t i a I Iy le a c h e d d u rin g th e e v a p o r ite s o l u t i o n p r o c e s s .
T h is i s
e v id e n t in th e K ibbey d o lo s to n e c l a s t s in s o l u t i o n b r e c c i a s .
The
c o n ta c t b etw een b r e c c i a c l a s t s and m a tr ix i s g r a d a t i o n a l and
in d is tin c t.
F i n a l l y , th e d e p o s its ca n n o t be m ista k e n f o r
co n g lo m e ra te s in c e e v id e n c e f o r f l u i d t r a n s p o r t su ch a s s o r t i n g o r
g ra d in g i s m is s in g .
E v a p o r ite m in e r a ls in a sabkha a r e m ost l i k e l y p r e s e r v e d in th e
s u p r a t i d a l zone and in th e t r a n s i t i o n b etw een s u p r a t i d a l and
i n t e r t i d a l zones (S h in n , L lo y d , and G in s b u rg , 1969; L u c ia , 1 9 7 2 ).
F u r t h e r , e x te n s iv e bedded and la m in a te d e v a p o r ite s p r e c i p i t a t e o u t o f
s ta n d in g w a te r on th e sab k h a w h ereas n o d u la r and re p la c e m e n t
e v a p o r i te s p r e c i p i t a t e o u t o f i n t e r s t i t i a l w a te r b e n e a th th e
d e p o s i t i o n a l s u r f a c e (L u c ia 1 9 7 2 ).
T h ic k n e ss and l a t e r a l p e r s i s t e n c e
o f e v a p o r i te s o l u t i o n b r e c c i a s a r e m ore l i k e l y th e r e s u l t o f th e
s o l u t i o n o f e v a p o r ite b eds r a t h e r th a n n o d u la r e v a p o r i te z o n e s .
N o d u lar e v a p o r i te zones u s u a lly le a c h to form vugs w hich may
s u b s e q u e n tly be f i l l e d w ith c a l c i t e (L u c ia , 1 9 7 2 ).
th e s e a r e m is s in g .
F e a tu r e s such as
L e n s-sh a p e d beds o f e v a p o r ite p r e c i p i t a t e d o u t o f
w a te r ponded on th e K ibbey sabkha a s th e w a te r e v a p o r a te d .
S tan d in g
w a te r was p ro b a b ly ponded in d e p r e s s io n s in th e s u p r a t i d a l zone by
sto rm s and s p r in g t i d e s .
M udcracks and f e n e s t r a l vugs and v e in s d i s r u p t th e a l g a l
38
la m in a tio n s in th e la m in a te d d o lo s to n e l i t h o f a c i e s o f th e K ibbey
F o rm a tio n .
S h in n , L lo y d , and G in sb u rg (1 9 6 9 ) r e p o r te d t h a t m udcracks
on modern sab k h as a r e p ro d u ced and m ost l i k e l y p r e s e r v e d in th e
s u p r a t i d a l and h ig h i n t e r t i d a l z o n e s .
S hinn (1968) and o th e r s have
docum ented t h a t f e n e s t r a l vugs and v e in s a r e a l s o p ro d u ced p r im a r ily
on s u p r a t i d a l f l a t s by s h r in k a g e and e x p a n s io n o f s e d im e n ts , gas
bubb b I e s from a i r e sc a p e d u rin g f lo o d i n g , and w r in k le s in a l g a l m a ts .
These d a ta s u p p o rt th e c o n c lu s io n t h a t th e la m in a te d d o lo s to n e
l i t h o f a c i e s r e p r e s e n t s a s u p r a t i d a l and h ig h i n t e r t i d a l sabkha
e n v iro n m e n t.
S an d sto n e C hannel L i t h o f a c i e s .
r e l a t e d to th e s a b k h a .
T id a l c h a n n e ls a r e in t im a te ly
They g e n e r a lly r e f l e c t a seaw ard , i n t e r t i d a l
component o f th e e n v iro n m e n t.
D evelopm ent o f th e c h a n n e ls in low er
K ibbey a l g a l d o lo s to n e s s i g n a l s th e o n la p o f more m a rin e , i n t e r t i d a l
en v iro n m en ts and se d im e n ts o n to th e low a r e a s on th e sab k h a in th e
B rid g e r R ange.
The so u rc e o f sed im en t in t i d a l c h a n n e ls i s th e
s h o r e f a c e en v iro n m en t and sabkha c r u s t (S h in n , L lo y d , and G in s b u rg ,
1 9 6 9 ).
S e v e ra l d e t a i l e d m odels o f c h a n n e l developm ent in t i d a l f l a t and
sabkha e n v iro n m en ts f i t th e low er K ibbey ch a n n e l l i t h o f a c i e s .
S h in n ,
L lo y d , and G in s b u rg , (1 9 6 9 ) docum ented t h a t t i d a l c h a n n e ls on th e
t i d a l f l a t s o f A ndros I s l a n d m ig r a te l a t e r a l l y by p o in t b a r a c c r e tio n
in a s i m i l a r m anner to f l u v i a l s y s te m s .
The c h a n n e ls f i n e up from a
b a s a l c h a n n e l la g d e r iv e d from u n d e r c u t tin g sabkha c r u s t s , i n t o san d ,
s i l t , and f i n a l l y s u p r a t i d a l s e d im e n t.
Most o f th e c ro s s b e d d in g
39
c h a r a c t e r i s t i c to th e f l u v i a l c h a n n e ls i s l o s t to b u rro w in g in th e
t i d a l e n v iro n m e n t.
c h a n n e ls .
A ll th e s e i n d i c a t o r s a r e p r e s e n t in th e s a n d s to n e
The c h a n n e ls can n o t be c o n fu se d w ith f l u v i a l c h a n n e ls due
to th e in tim a te a s s o c i a t i o n w ith th e la m in a te d d o lo s to n e .
S tra tig ra p h ic I n te r p r e ta tio n
The low er member o f th e K ibbey F o rm atio n a l g a l d o lo s to n e s and
i n t e r t i d a l c h a n n e ls r e c o r d th e i n i t i a l t r a n s g r e s s i o n o f th e se a on to
th e M issio n Canyon F o rm a tio n .
The s e a flo o d e d in t o th e S now crest
tro u g h and B r id g e t Range a r e a from th e C o r d il le r a n m io g e o c lin e d u rin g
e a r l i e s t C h e s te r tim e ( Sando, 1 9 7 4 ).
T id e s and sto rm s w ashed enough
w a te r from th e m a rin e en v iro n m en t o n to th e sabkha f l a t s to e s t a b l i s h
a l g a l m ats w hich in t u r n tr a p p e d d e t r i t u s d e r iv e d from th e
s i l i c i c l a t i c n e a r s h o r e en v iro n m en t and c a rb o n a te sabkha c r u s t .
E v a p o rite s p r e c i p i t a t e d from w a te r i n t e r m i t t e n t l y ponded in
d e p r e s s io n s .
T id a l c h a n n e ls r e p r e s e n t an i n t e r t i d a l , seaw ard com ponent o f th e
member.
S u p r a tid a l s e d im e n ta tio n d o m in ated on th e n o r th and so u th
ends o f th e ra n g e and i n t e r t i d a l c h a n n e l d e p o s it io n was m ost
im p o rta n t in th e c e n t r a l r a n g e .
L o c a liz a tio n o f i n t e r t i d a l
d e p o s it io n in th e c e n t r a l B rid g e r Range ( P l a t e I ) i n d i c a t e s t h a t t h i s
a r e a was d e p re s s e d r e l a t i v e to a r e a s s o u th and n o r th e i t h e r p r i o r to
o r d u rin g d e p o s it io n o f th e low er K ibbey F o rm atio n o r b o th . Two
e x p la n a tio n s a r e p o s s i b l e , b o th a r e b a se d on s u b sid e n c e l o c a l i z e d in
th e c e n t r a l B r id g e t R ange.
F i r s t , a d e p r e s s io n d ev elo p ed on th e
M issio n Canyon F o rm a tio n by t e c t o n i c i n s t a b i l i t y in th e c e n t r a l
40
B rid g e r Range p r i o r to o n la p o f th e sabkha f a c i e s .
A l t e r n a t e l y , th e
a r e a was d e p re s s e d by d i f f e r e n t i a l s u b s id e n c e d u rin g d e p o s it io n o f
th e low er member o f th e K ibbey F o rm a tio n .
The o n ly d i s t i n c t i o n
betw een th e two m echanism s .is th e tim in g o f th e s u b s id e n c e r e l a t i v e
to th e K ibbey t r a n s g r e s s i o n .
T h e r e f o r e , i t i s r e a s o n a b le to assume
t h a t b o th w ere p a r t i a l l y r e s p o n s i b l e .
S tr a t ig r a p h y and f a c i e s o f th e low er member o f th e K ibbey
F o rm atio n n e a r s t a t i o n 191 p r e s e n ts an e x c e p tio n to th e
i n t e r p r e t a t i o n o f a s i n g l e u n s ta b le a r e a in th e c e n t r a l B rid g e r
R ange.
Deep e r o s io n o f th e M issio n Canyon F o rm atio n and t h i n
c o n g lo m e ra tic d e p o s it io n o f th e low er member o f th e K ibbey F o rm atio n
a t s t a t i o n 191 i n d i c a t e s t h a t t h i s was an a r e a o f p o s i t i v e r e l i e f
fo llo w in g and p o s s ib ly d u rin g M issio n Canyon tim e , d u rin g d e p o s itio n
o f th e K ibbey F o rm a tio n , and f o llo w in g .
T h u s, d u rin g p a r t o f Big
Snowy tim e , th e c e n t r a l B r id g e r Range was segm ented i n t o two u n s ta b le
e le m e n ts n o r th and s o u th o f th e s t a t i o n 191 a r e a .
F a c ie s and
th ic k n e s s tr e n d s s u g g e s t t h a t th e c o n d itio n s e x i s t e d d u r in g M issio n
Canyon tim e ( l a t e Meramec) , and p e r s i s t e d th ro u g h B ig Snowy tim e
( l a t e C h e s te r ) , and p o s s ib ly in t o Amsden tim e ( e a r l y M orrow ).
P ronounced th in n in g o f th e low er member o f th e K ibbey F o rm atio n
in th e s o u th e r n B r id g e r Range (F ig u re 8 ; P l a t e I ) i n d i c a t e s t h a t th e
a r e a was r e l a t i v e l y p o s i t i v e .
F u rth e rm o re , s u p r a t i d a l and h ig h
i n t e r t i d a l c o n d itio n s p e r s i s t e d th ro u g h o u t th e low er K ibbey tim e
th e re .
T h is th in n in g and s h o a lin g m arks th e t r a n s i t i o n from th e
r e l a t i v e l y u n s ta b l e S n o w crest tro u g h o n to th e Wyoming s h e l f .
The
low er member o f th e K ibbey F o rm atio n e x te n d s o u t o f th e stu d y a r e a to
41
o - - -
O
F i gu r e 8 .
Isopach
Formation.
Contours in f e e t , d a s h e d
Section
5
IO
m iles
north
m a p of t h e lower m e m b e r of t h e Kibbey
A-A' d e p i c t e d on Pl at e I.
( a d a p t e d from Harris, 1 9 7 2 )
whe re co n t ro l is
Bridger
Range
poor.
dashed,
42
th e s o u th .
A s i m i l a r th in n in g tr e n d i s seen to th e n o r th o f th e c e n t r a l
B rid g e r R ange.
An is o p a c h map o f th e low er member o f th e K ibbey
F o rm atio n ( F ig u re 8 ) shows t h a t th e th in n in g tr e n d c o n tin u e s n o r th
o u t o f th e stu d y a r e a a c r o s s th e Lombard a r c h .
U pper K ibbey Member
L i th o f a c i e s
In tro d u c tio n .
The u p p er member o f th e K ibbey F o rm a tio n e x te n d s
a c r o s s th e e n t i r e stu d y a r e a , b u t l i k e th e low er member, t h i n s n o r th
and so u th from a maximum th ic k n e s s in th e c e n t r a l B rid g e r Range
(P la te I ) .
S a n d sto n e s and s i l t s t o n e s in b o th members a l s o have v e ry
s i m i l a r g r a i n t e x t u r e and m in e r a lo g y .
The u p p er K ibbey member cro p s
o u t a s a r e l a t i v e l y , r e s i s t a n t t a b u l a r body and i s d iv id e d i n t o two
l i t h o f a c i e s ( F ig u r e 9 ) .
T y p ic a lly a t h i n b a s a l c o n g lo m e ra te g ra d e s
up in t o m u d sto n e, s i l t s t o n e , and s a n d s to n e th ro u g h th e low er two
t h i r d s o f th e member w hich i s term ed th e s a n d s to n e l i t h o f a c i e s .
H e m a tite im p a rts a re d c o lo r to a l l b u t th e m id d le s a n d s to n e w hich i s
c h a r a c t e r i s t i c a l l y p a le r e d d is h c o lo re d and c ro s s b e d d e d .
The to p one
t h i r d becomes s i l t y and muddy w ith b ed s o f d o lo s to n e w hich w e a th e r
r e d d is h .
At s e v e r a l s e c t i o n s a n o th e r co n g lo m e ra te a t th e to p o f th e
K ibbey F o rm atio n m arks th e c o n ta c t w ith th e o v e r ly in g Lombard f a c i e s .
D is c u s s io n o f th e u p p er member o f th e K ibbey F o rm atio n i s d iv id e d
i n t o a s e c t i o n on th e low er s a n d s to n e l i t h o f a c i e s and one bn th e
u p p er m u d s to n e -d o lo s to n e l i t h o f a c i e s .
43
Lombard
Facies
Mudstone- Dolostone Lithofacies
Upper Member
Kibbey Formation
(2 to 66 meters)
Sandstone Lithofacies
Lower Membe r
Kibbey Formation
7w
Figure 9. Generalized lithologic column of the upper m e m b e r of the
Kibbey Formation.
S ee text for details of lithofacies.
44
S an d sto n e L i t h o f a c i e s .
At s e v e r a l e x p o su re s a r e d d is h
co n g lo m e ra te a t th e b a se o f th e s a n d s to n e l i t h o f a c i e s o v e r l i e s a
s h a rp c o n ta c t on th e lo w er K ibbey muddy, s i l t y , d o lo s to n e .
F in e and
medium g r a in e d sand s u p p o r ts su b -ro u n d e d s a n d s to n e , d o lo s to n e , and
s i l t s t o n e c l a s t s up to te n c e n tim e te r s in d ia m e te r in th e
c o n g lo m e ra te .
At o th e r e x p o su re s muddy re d s i l t s t o n e a t th e b ase o f
th e u p p er member o f th e K ibbey F o rm a tio n a p p e a rs g r a d a t i o n a l w ith
u n d e rly in g lo w er K ibbey muddy, s i l t y , d o lo s to n e .
Mud c o n te n t d e c r e a s e s u p s e C tio n in th e s a n d s to n e l i t h o f a c i e s as
r e d s i l t y m udstone g ra d e s i n t o s i l t s t o n e and f i n a l l y i n t o a f i n e re d
s a n d s to n e .
Mud c o n te n t a l s o d e c r e a s e s l a t e r a l l y away from th e
c e n t r a l B r id g e t Range s e c t i o n s .
T a b u la r b ed d in g i n th e s a n d s to n e
l i t h o f a c i e s v a r i e s from te n c e n tim e te r s to two m e te rs t h i c k .
A ls o , a
few r i p p l e la m in a tio n s a r e p o o rly p r e s e r v e d in t h i s i n t e r v a l .
The
v e ry f i n e s a n d s to n e c o n tin u e s to c o a rs e n up in t o f i n e o r medium
s a n d s to n e and becomes p a le y e llo w -o r a n g e , g r e e n , o r w h ite to w ard s th e
to p o f th e s a n d s to n e l i t h o f a c i e s .
A t m ost e x p o su re s a c r o s s bedded
seq u en ce i s p r e s e r v e d w ith i n t h i s i n t e r v a l a t th e to p o f th e
s a n d s to n e l i t h o f a c i e s .
The N o rth A n g ler ex p o su re e x e m p lif ie s t h i s s e q u e n c e ^
Lowest in
th e seq en ce i s t h r e e to f o u r m e te rs o f la r g e s c a le tr o u g h c r o s s
bedded s a n d s to n e .
m e te rs a c r o s s .
I n d iv i d u a l tro u g h s a r e te n in c h e s deep and th r e e
F o r s e t lam in ae a r e concave and t a n g e n t i a l .
a x i s tr e n d a p p ro x im a te ly e a s t - w e s t .
to th e s o u th a t f i f t e e n d e g r e e s .
n e x t tr o u g h .
Trough
Maximum d ip o f th e c r o s s beds i s
T roughs a r e tr u n c a te d on to p by th e
O c c a s io n a lly , p o o rly p r e s e r v e d r i p p l e c r o s s
45
s t r a t i f i c a t i o n a r e su p e rp o se d on th e tro u g h c r o s s s t r a t a .
Above th e
tro u g h c r o s s bedded i n t e r v a l i s a le n s shaped s a n d s to n e body w ith
s l i g h t l y c o a r s e r low er medium s a n d s to n e ( .2 5 to .35 m i l l i m e t e r ) f i f t y
m e te rs a c r o s s by t h r e e m e te rs t h i c k .
P a r a l l e l f o r s e t s d ip p in g tw enty
f i v e d e g re e s to th e s o u th in th e low er h a l f o f th e tro u g h a r e
tr u n c a te d on to p by p a r a l l e l la m in a te d s a n d s to n e .
F i n a l l y , two and
one h a l f m e te rs o f s a n d s to n e w ith f o u r c e n tim e te r t h i c k t a n g e n t i a l
p la n a r c ro s s b e d s d ip p in g f i f t e e n d e g re e s to th e so u th cap s th e
sequence.
At m ost e x p o s u re s o n ly one o f th e s e c ro s s b e d ty p e s i s
p r e s e r v e d a t th e to p o f th e s a n d s to n e l i t h o f a c i e s .
M u d sto n e-D o lo sto n e L i t h o f a c i e s .
The s a n d s to n e l i t h o f a c i e s
g ra d e s up i n t o a second m u d s to n e -d o lo s to n e l i t h o f a c i e s in th e u p p er
member o f th e K ibbey F o rm a tio n th ro u g h an i n t e r v a l o f in c r e a s in g mud,
s i l t , and d o lo m ite c o n te n t w ith a c o rre s p o n d in g d e c r e a s e in g r a in
s iz e .
T h is u p p er l i t h o f a c i e s i s a m ix tu r e (a b o u t e q u a l am ounts) o f
red -m aro o n s a n d s to n e , s i l t s t o n e , and m u d sto n e , w ith some d o lo s to n e
beds d ev e lo p e d to w ard s th e to p a t s e c t i o n s in th e S outh A n g le r a r e a .
In c o n t r a s t to th e s a n d s to n e l i t h o f a c i e s , l a t e r a l th in n in g o f
i n d i v id u a l b eds i n d i c a t e s th e y a r e d is c o n tin u o u s .
The d o lo s to n e i s
p a le r e d d is h , la v e n d e r , o r p in k and i s o c a a s io n a lly m o ttle d p a le
g re e n .
I r r e g u l a r lumpy b ed d in g and t h i n , w avey, c r in k l y la m in a tio n s
a re c h a r a c te r is tic .
The c o n ta c t w ith Lombard f a c i e s lim e s to n e i s
p la c e d below th e f i r s t d a rk g ra y lim e s to n e o r s h a le .
At s e v e ra l
s e c ti o n s a t h i n mud c h ip co n g lo m e ra te a p p e a rs j u s t below th e c o n t a c t.
46
P e tr o g r a p h y .
A n a ly s is o f t h i r t y - f i v e t h i n s e c ti o n s o f th e two
l i t h o f a c i e s i n d i c a t e s th e s i l i c i c I a s t i c s a n d s to n e and s i l t s t o n e
g r a in s a r e m i n e r a l o g i c a l Iy and p e t r o l o g i c a l Iy u n ifo rm th ro u g h o u t th e
u p p er member o f th e K ibbey F o rm a tio n ,
However g r a in s i z e ,
i n t e r g r a n u l a r mud c o n t e n t, and d e t r i t a l d o lo m ite c o n te n t v a r i e s
re g u la rly .
G rain s i z e in c r e a s e s u p s e c tio n from b a s a l s i l t y m udstone
to f i n e o r medium s a n d s to n e in th e u p p e r sa n d s to n e l i t h o f a c i e s , th e n
d e c r e a s e s to s i l t s t o n e w ith some m udstone and d o lo s to n e in th e
m u d sto n e -d o I o s t one l i t h o f a c i e s .
I n v e r s e l y , i n t e r g r a n u l a r mud
d e c re a s e s up th ro u g h th e low er l i t h o f a c i e s and in c r e a s e s u p s e c tio n in
th e u p p er l i t h o f a c i e s .
D e t r i t a l d o lo m ite c r y s t a l c o n te n t fo llo w s
t h i s same t r e n d .
Bim odal t e x t u r e i s u b iq u ito u s in c o a r s e s i l t s t o n e and sa n d sto n e
in th e u p p er member o f th e K ibbey F o rm a tio n ( P l a t e 6 ) .
S im ila r to
th e i n t e r t i d a l s i l t s t o n e and s a n d s to n e o f th e low er member o f th e
K ibbey F o rm a tio n , a sm a ll com ponent (a b o u t tw en ty f i v e p e r c e n t) o f
r e l a t i v e l y c o a r s e r q u a r tz g r a in s a r e ev e n ly d i s t r i b u t e d th ro u g h th e
f i n e sand and s i l t .
G ra in s a r e su b -ro u n d e d e x c e p t f o r th e l a r g e r
f r a c t i o n w hich i s v e ry w e ll ro u n d e d .
I n t e r g r a n u l a r m a tr ix s u p p o rt i s
common e x c e p t in th e to p o f th e s a n d s to n e l i t h o f a c i e s w here c ro s s
b ed d in g o c c u r s .
I n t e r l a y e r i n g on a m i l l i m e t e r to s i n g l e c e n tim e te r
s c a le o f c o a r s e r and f i n e r f r a c t i o n s o f sand i s a p p a re n t on ab o u t
h a l f th e s l i d e s ( P l a t e 6 ) .
In some c a s e s t h i s s t r a t i f i c a t i o n r e f l e c t s a l t e r n a t i o n o f g r a in
r i c h and i n t e r g r a n u l a r mud r i c h l a y e r s .
A ls o , b o th ty p e s o f
P l a t e 6 . P h o to m icro g rap h o f s a n d s to n e in th e
u p p er K ibbey s a n d s to n e l i t h o f a c i e s , see t e x t
f o r d is c u s s io n (a p p ro x im a te ly 2 .5 mm a c r o s s
fie ld ).
48
i n t e r l a y e r i n g a r e g ra d e d in p a r t .
S tro n g p a r a l l e l o r i e n t a t i o n o f
e lo n g a te g r a in s i s a p p a r e n t on ab o u t h a l f th e s l i d e s .
I
M in e r a lo g ic a lly th e u p p er K ibbey sand g r a in s v a ry l i t t l e
and a r e
much l i k e th e i n t e r t i d a l s a n d s to n e c h a n n e l and s i l t s t o n e below ex ce p t
t h a t d e t r i t a l d o lo m ite c r y s t a l s a r e s i g n i f i c a n t a t th e to p and bottom
o f th e u p p er member o f th e K ibbey F o rm a tio n .
T y p ic a lly g r a i n s a r e
n in e ty two p e r c e n t q u a r t z , f i v e p e r c e n t c h e r t , and t h r e e p e r c e n t
u n a l t e r e d p la g i o c l a s e p lu s m ic r o c lin e in e q u a l p o r t i o n s .
D e trita l
d o lo m ite c r y s t a l s r e a c h f i f t y p e r c e n t a t th e b a se o f th e sa n d sto n e
l i t h o f a c i e s and in th e d e t r i t a l b ed s o f th e d o lo s to n e l i t h o f a c i e s .
The d o lo m ite i s p r e s e n t a s v a r i a b l y ro u n d ed d o lo m ite c r y s t a l s o r
rhombs and s p a r s e l i t h o c l a s t s .
Z irc o n and m a fic m in e r a ls c o n s t i t u t e
a t r a c e to one p e r c e n t o f th e u p p er member o f th e K ibbey F o rm a tio n .
D e p o s itio n a l E nvironm ent I n t e r p r e t a t i o n
In tro d u c tio n .
The u p p er member o f th e K ibbey F o rm a tio n o v e r l i e s
s u p r a t i d a l s e d im e n ts o f th e low er K ibbey and u n d e r li e s s u b t i d a l
o f f s h o r e m a rin e se d im e n ts o f th e Lombard f a c i e s .
The u p p e r member
r e p r e s e n t s a s h o r e f a c e t r a n s i t i o n b etw een th e s e en v iro n m e n ts in th e
t r a n s g r e s s i v e Big Snowy G roup.
The u p p er member s a n d s to n e i s u n d e r la in and o v e r l a i n by
m u d sto n e .
T h is l i t h o l o g i c seq u en ce r e f l e c t s ch an g in g hydrodynam ic
c o n d itio n s w ith i n th e s h o r e f a c e - s h e l f en v iro n m en t in re s p o n s e to
f a c i e s t r a n s g r e s s i o n and r e g r e s s i o n .
M udstone i s d e p o s ite d by s u s p e n s io n in calm , low en erg y
49
c o n d i tio n s , and s a n d s to n e p r i m a r i l y by t r a c t i o n in h ig h e r en erg y
c o n d itio n s .
Low en erg y c o n d itio n s in th e s h o r e f ace e n v iro n m e n t a r e
lo c a te d e i t h e r in th e s u b t i d a l lo w e r s h o r e f a c e - o f f s h o r e s h e l f
e n v iro n m en t o r n e a r s h o r e t i d a l f l a t s and la g o o n a l en v iro n m en t
(R ein ech and S in g h , 1 9 7 5 ).
Thus, th e u p p e r K ibbey s h o r e f a c e s a n d s to n e s can be c l a s s i f i e d as
e ith e r tra n s g re s s io n a l or re g re s s io n a l.
The te rm s t r a n s g r e s s i o n and
r e g r e s s i o n r e f e r to m ig r a tio n o f a s h o r e li n e in a lan d w ard and
seaw ard d i r e c t i o n r e s p e c t i v e l y (C u rra y , 1964).
T r a n s g r e s s io n a l
s h o r e li n e s le a v e a n e a r s h o r e to o f f s h o r e s e d im e n ta ry r e c o r d w h ereas
th e r e v e r s e i s t r u e f o r r e g r e s s i o n a l s h o r e li n e s .
D e te r m in a tio n o f
w h e th e r th e u p p e r member o f th e K ibbey F o rm a tio n r e p r e s e n t s a
t r a n s g r e s s i v e o r r e g r e s s i v e d e p o s it depends on th e i n t e r p r e t a t i o n o f
th e d e p o s i t i o n a l e n v iro n m e n t o f th e m u d sto n es w hich l i e above and
below th e s a n d s to n e .
F e a tu r e s o f th e lo w e r m udstone in th e u p p er member o f th e Kibbey
F o rm a tio n s a n d s to n e l i t h o f a c i e s s u g g e s t an o f f s h o r e o r i g i n .
The
m udstones a r e l a t e r a l l y c o n tin u o u s as i s e x p e c te d in th e e x te n s iv e
o ffsh o re s e ttin g .
A lso , r e g u l a r l y la y e r e d and g rad ed s i l t s t o n e s
t y p i c a l o f t h i s i n t e r v a l in th e u p p er member o f th e K ibbey F o rm a tio n
a r e r e p o r te d p r i m a r i l y fro m th e o f f s h o r e s h e l f and lo w e r s h o re fa c e
w here t i d a l c u r r e n t s a r e a c t i v e (R ein ech and W underluch, 1 9 6 8 ).
F e a tu r e s o f th e m u d s to n e -d o lo s to n e l i t h o f a c i e s above th e
s a n d s to n e s u g g e s t a n e a r s h o r e t i d a l f l a t and la g o o n a l o r i g i n .
M udstone, s i l t s t o n e , and d o lo s to n e b ed s a r e l a t e r a l l y d is c o n tin u o u s
and i n t e r f i n g e r w ith eac h o th e r as would be e x p e c te d from a
50
n e a rsh o re d e p o s it.
B ased on th e same r e a s o n in g vised f o r s i m i l a r
d o lo s to n e in th e low er K ibbey member, t h i n la m in a tio n s and i r r e g u l a r
b ed d in g in d o lo s to n e s above th e s a n d s to n e r e p r e s e n t i n t e r t i d a l and
s u p r a t i d a l c o n d itio n s s i m i l a r to th o s e o f th e low er K ibbey member.
These r e l a t i o n s h i p s s u g g e s t t h a t an o f f s h o r e - n e a r s h o r e seq u en ce i s
re p re s e n te d .
F u r th e r e v id e n c e s u p p o r ts th e c o n c lu s io n t h a t th e u p p e r member
o f th e K ibbey F o rm a tio n i s a r e g r e s s i o n a l d e p o s it w ith i n th e
t r a n s g r e s s i v e Big Snowy Group s e r i e s .
F i r s t , th e g r a i n s iz e tr e n d
and se d im e n ta ry s t r u c t u r e seq u en ce in t h i s member f i t th e r e g r e s s i v e
o f f s h o r e - n e a r s h o r e f a c i e s m odel a s p ro p o se d by R ein so n (1 9 8 3 ).
A lso ,
K le in (1974) s t a t e s t h a t r e g r e s s i v e seq u en ce s have a h ig h e r
p r e s e r v a t i o n p o t e n t i a l r e l a t i v e to t r a n s g r e s s i v e s e q u e n c e s .
As
e v id e n c e he p o in ts o u t t h a t a lo n g m odern t r a n s g r e s s i v e c o a s t l i n e s ,
o n ly a t h i n b a s a l t r a n s g r e s s i v e i n t e r v a l , commonly w ith a
c o n g lo m e ra te , i s p r e s e r v e d b e n e a th r e g r e s s i v e s e d im e n ts .
O th er
s t u d i e s o f modern t r a n s g r e s s i v e s h o r e li n e s i n d i c a t e t h a t few f a c i e s
o f th e s h o r e li n e a r e p r e s e r v e d d u rin g tr a n s g r e s s i o n ( S w i f t, 1975;
S c h w a rtz , 1 9 6 7 ).
They a r e ero d ed in th e h ig h en erg y u p p e r s h o re fa c e
and b each zones a s th e f a c i e s s h i f t la n d w a rd .
The t r a n s g r e s s i o n i s
re c o rd e d as a d is c o n f o r m ity u n d e r ly in g s h o r e f a c e o r s h e l f s e d im e n ts .
C l i f t o n (1973) r e p o r te d t h a t th e s e e r o s i o n a l s u r f a c e s have v e ry low
r e l i e f b u t can be re c o g n iz e d by t h i n c o n g lo m e ra te s w ith c l a s t s
d e r iv e d from u n d e r ly in g s e d im e n ts .
The amount o f e r o s io n i s v a r i a b l e
and depends on p r o x im ity to zones o f e r o s io n alo n g th e d is c o n f o r m ity .
51
In sum m ary, m o st e v id e n c e s u g g e s ts t h a t th e u p p e r K ibbey
s a n d s to n e s a r e r e g r e s s i o n a l .
The c o n g lo m e ra te above and belo w a r e
i n t e r p r e t e d a s e r o s i o n a l d is c o n f o r m i ti e s g e n e r a te d d u rin g th e
landw ard s h i f t o f f a c i e s .
Where c o n g lo m e ra te s a r e m is s in g , m inor
t r a n s g r e s s i v e seq u en ce s may be p r e s e r v e d .
S an d sto n e l i t h o f a c i e s .
The c o n g lo m e ra te a t th e b a s e o f th e
lo w e r member s a n d s to n e l i t h o f a c i e s r e p r e s e n t s a t h i n t r a n s i t i o n
b e tw e e n th e lo w e r K ibbey i n t e r t i d a l - s u p r a t i d a l a l g a l d o lo s to n e
en v iro n m en t and an o f f s h o r e to lo w er s h o r e f ace s u b t i d a l s i l t y
m udstone e n v iro n m e n t.
As th e K ibbey s t r a n d l i n e e n v iro n m e n ts s h i f t e d
o v e r th e lo w e r K ibbey s u p r a t i d a l sab k h a and i n t e r t i d a l c h a n n e ls ,
s i l t s t o n e , s a n d s to n e , and d o lo s to n e w ere r ip p e d up and i n t e g r a t e d
i n t o th e lo w e r s a n d s to n e l i t h o f a c i e s a s c o n g lo m e ra te c l a s t s and
s a n d - s iz e d g ra in s .
M udstones w ith i n t e r l a y e r e d muddy s i l t s t o n e and s a n d s to n e
o v e r ly in g th e c o n g lo m e ra te w ere d e p o s ite d in an e n v iro n m e n t w hich was
f a r enough o f f s h o r e t h a t s u s p e n s io n s e d im e n ta tio n d o m in a te d .
Sand
and s i l t a r e d e p o s ite d d u rin g ebb and f lo o d c u r r e n t s and th e mud
s e t t l e s o u t d u rin g s la c k w a te r ( R ein ech and W underluch, 1 9 6 8 ).
The
o f f s h o r e mud and s i l t t h i n s n o r th and s o u th from a maximum in th e
c e n t r a l B rid g e r Range i n d i c a t i n g t h a t q u i e t o f f s h o r e c o n d itio n s
p e r s i s t e d lo n g e r in th e m id d le o f th e ra n g e r e l a t i v e to a r e a s n o r th
and s o u th .
U p s e c tio n in th e s a n d s to n e l i t h o f a c i e s , th e change from s i l t and
mud dom in ated r o c k s b ack to san d and s i l t d om inated r o c k s s ig n a l s a
52
change from s u s p e n s io n to t r a c t i o n s e d im e n ta tio n .
I n c r e a s e in g r a in
s i z e and c o rre s p o n d in g d e c r e a s e in mud c o n te n t u p s e c tio n in th e
s a n d s to n e l i t h o f a c i e s i s e v id e n c e o f th e in c r e a s in g hydrodynam ic
regim e a s s o c ia t e d w ith th e s h o r e f ace e n v iro n m e n ts .
The tr e n d
c o n tin u e s as m id d le and u p p er s h o r e f a c e zones p ro g ra d e o v e r th e low er
sh o re fa c e .
L arge s c a le tro u g h c r o s s s t r a t i f i c a t i o n w ith su p e rp o se d r i p p l e s ,
th e lo w est o c c u rre n c e o f c r o s s b ed d in g in th e s e q u e n c e , i s commonly
a t t r i b u t e d to th e m id d le and u p p er s h o r e f a c e zone w here lo n g sh o re
b a r s and tro u g h s d ev e lo p in th e s u r f zone (C am pbell 1971; Howard,
1972; Land 1972; R ein ech and S in g h , 1 9 7 5 ).
C o n tin u ed in c r e a s e in
hydrodynam ic en erg y u p s e c tio n i s e v id e n c e d by co m p lete w innow ing o f
f i n e g r a in e d m a t e r i a l from s a n d s to n e and a l s o in c r e a s in g g r a in s iz e
n e a r th e to p o f th e s a n d s to n e l i t h o f a c i e s .
Lens shaped b o d ie s w ith
p a r a l l e l h ig h a n g le f o r e s e t c r o s s b ed d in g and p la n a r la m in a te d sands
a r e d e v elo p ed a t th e to p o f th e s a n d s to n e l i t h o f a c i e s .
D av id so n ,
A r n o l t , and Greenwood (1 9 7 6 ) d e s c r ib e d s i m i l a r b o d ie s on modern b a r
c r e s t s in th e s u r f zone o f th e u p p er s h o r e f a c e .
F i n a l l y , s m a lle r
s c a le and low er a n g le t a n g e n t i a l p la n a r c r o s s beds a t th e to p o f th e
s a n d s to n e l i t h o f a c i e s a r e i n t e r p r e t e d a s a c o n t in u a ti o n o f th e
re g re s s io n a l tre n d .
Howard and R ein ech (1979) r e p o r t t h a t s im i la r
c r o s s b ed d in g i s p ro d u ced in th e s h a llo w , h ig h en erg y u p p e r s h o re fa c e
a lo n g m odern c o a s t l i n e s .
53
M u d sto n e -d o lo sto n e L i t h o f a c i e s .
The m udstone—doI o s to n e
l i t h o f a c i e s above th e s a n d s to n e l i t h o f a c i e s i s a c o n t in u a ti o n o f th e
g r a d u a l r e g r e s s i o n a l s h o a lin g t r e n d .
The l a t e r a l l y d is c o n tin u o u s
s i l t y m udstones w ere d e p o s ite d by s u s p e n s io n in q u i e t w a te r in
la g o o n s o r p o n d s, h e m a t i t i c s i l t s and s a n d s to n e le n s e s in i n t e r t i d a l
and t i d a l c h a n n e ls
fla ts .
„
and d o lo s to n e b ed s on i n t e r t i d a l and s u p r a t i d a l
Wavey, c r in k ly la m in a to n s and i r r e g u l a r lumpy b e d d in g in th e
d o lo s to n e may be a l g a l m ats and r o o te d zones r e s p e c t i v I y .
Maughan
( p e r s o n a l co m m u n icatio n , 1984) i n t e r p r e t s s i m i l a r s t r u c t u r e s a t th e
to p o f th e K ibbey F o rm a tio n a t th e Lombard s e c t i o n a s a r o o te d zone
in p l u v i a l s e d im e n ts .
The mud c h ip c o n g lo m e ra te a t th e c o n ta c t w ith th e Lombard f a c i e s
re c o rd s a landw ard s h i f t o f th e s h o r e l i n e .
In t h i s c a s e th e
t r a n s g r e s s i o n su p e rp o se d Lombard F a c ie s mud and lim e s to n e on th e
m udstone and d o lo s to n e a t th e to p o f th e K ibbey F o rm a tio n .
T r a n s g r e s s iv e d e p o s its o th e r th a n ' th e co n g lo m e ra te a r e n o t re c o g n iz e d
betw een th e n e a r s h o r e ro c k s a t th e to p o f th e K ibbey F o rm a tio n and .
th e o f f s h o r e Lombard f a c i e s r o c k s .
S tra tig ra p h ic I n te r p r e ta tio n
The u p p er member o f th e K ibbey F o rm a tio n r e p r e s e n t s a
p re d o m in a n tly r e g r e s s i v e o f f s h o r e - n e a r s h o r e seq u en ce w ith i n th e
t r a n s g r e s s i v e B ig Snowy G roup.
Below and above th e member a t s e v e r a l
l o c a t i o n s a r e m inor d i s c o n f o r m itie s g e n e r a te d d u rin g e p is o d ic
landw ard s h i f t s o f f a c i e s by s h o r e f a c e e r o s io n .
T r a n s g r e s s iv e
54
seq u en ce s may be p r e s e n t , p a r t i c u l a r l y w here an e r o s io n a l
d is c o n f o r m ity i s n o t r e c o g n iz e d , how ever th e y a r e i n s i g n i f i c a n t in
r e l a t i o n to th e u p p er K ibbey s a n d s to n e .
W hile in d e t a i l s e v e r a l t h i n s t r a t i g r a p h i c s e c t i o n s do n o t f i t
th e g e n e r a l t r e n d , th e u p p er member o f th e K ibbey F o rm a tio n i s
t h i c k e s t in th e c e n t r a l B rid g e r Range ( F ig u r e 10; P l a t e I ) .
The
member t h i n s n o r th and s o u th o n to th e Lombard a r c h and Wyoming s h e l f .
As p r e v io u s ly n o te d , th e u p p e r M issio n Canyon F o rm a tio n and low er
member o f th e K ibbey F o rm a tio n a l s o fo llo w t h i s t r e n d .
As in d i c a te d
by l o c a l i z a t i o n o f th e low er K ibbey i n t e r t i d a l c h a n n e l l i t h o f a c i e s ,
d e e p e r e n v iro n m en ts p e r s i s t e d lo n g e r in th e c e n t r a l B r id g e r Range
r e l a t i v e to a r e a s n o r th and s o u th d u rin g u p p e r K ibbey tim e .
These
c o in c id e n t r e l a t i o n s h i p s i n d i c a t e t h a t th e c e n t r a l B r id g e r Range
c o n tin u e d to be d e p re s s e d r e l a t i v e to a r e a s n o r th a n d .s o u th .
S u b sid en ce fo c u s e d in th e c e n t r a l B r id g e t Range d u rin g u p p er K ibbey
tim e r e s u l t e d in th i c k n e s s e s o f t h i s member f o u r tim e s t h a t o f
e q u i v a le n t s on th e Wyoming S h e lf in th e s o u th e r n map a r e a and tw ic e
t h a t to th e n o r th n e a r th e Lombard a r c h .
F or exam ple, th e member i s
a p p ro x im a te ly s i x t y m e te rs t h i c k a t th e S outh A n g le r s e c t i o n in th e
c e n t r a l ra n g e a r e a .
I t i s t h i r t y m e te rs and te n m e te rs t h i c k
r e s p e c t i v e l y a t th e M id d le fo rk S ix te e n m ile and Rocky Canyon s e c t i o n s .
To th e s o u th o f th e c e n t r a l B r id g e r Range th e member t h i n s to
n e a r ly z e ro u n d er th e Lombard f a c i e s a t Menard C reek ( P l a t e I ) .
If
th in n in g h e r e r e s u l t e d from pre-A m sden e r o s io n , Lombard f a c i e s ro c k s
would n e c c e s s a r i l y be c o m p le te ly rem oved.
P r e s e r v a ti o n o f th e
Lombard f a c i e s lim e s to n e i n d i c a t e s t h a t t h i s i s n o t th e c a s e .
55
I'
/
O
5 IO
miles
Figure 10.
Isopach map of Lombard f a c i e s and Heath
Formation.
Contours
Section
(adopted
in fe e t ,
A - A 1 d e p i c t e d on
d a s h e d where control is
Plate
from Harris, 1972)
I.
Bridger
Range
north
poor.
dashed,
56
R a th e r , th e e x c e p tio n a l th in n in g a t Menard C reek i s a t t r i b u t e d to
e i t h e r a lo c a l i z e d p o s i t i v e te n d en cy d u rin g u p p er K ibbey tim e o r
d e e p e r s h o r e f ace e r o s io n d u rin g th e n e x t t r a n s g r e s s i o n o r b o th .
The
u p p e r K ibbey e x te n d s s o u th o f th e Menard C reek s e c t i o n a t
a p p ro x im a te ly s i x m e te rs t h i c k ( F ig u r e 1 0 ) .
P re v io u s s t u d i e s have n o t e x te n d e d th e K ibbey F o rm a tio n so u th o f
B rid g e r Canyon (McMannis, 1951; H a r r i s , 1972; Sando, 1 9 7 5 ).
S e v e ra l
l i n e s o f e v id e n c e i n d i c a t e , how ever, t h a t a t h i n s t r a t i g r a p h i c
s e c t i o n o f K ibbey F o rm a tio n i s in f a c t p r e s e r v e d u n d e r th e Amsden
F o rm a tio n a t th e so u th e rn m o st two e x p o s u re s in th e stu d y a r e a .
F i r s t , a t b o th th e B r id g e t Canyon and Rocky Canyon s e c t i o n s two re d
s i l t y , sandy i n t e r v a l s a r e s e p a r a te d by a t h i n Lombard f a c i e s
lim e s to n e and w h ite d o lo s to n e r e s p e c t i v e l y , s u g g e s tin g t h a t b o th th e
K ibbey F o rm a tio n and and Amsden F o rm a tio n a r e p r e s e n t .
At b o th
s e c t i o n s a t h i n r e d c la y zone i s p r e s e n t a t th e b a se o f th e second
re d o f th e Amsden F o rm a tio n s u g g e s tin g t h a t an u n c o n fo rm ity i s
d e v e lo p e d .
A ls o , th e lo w er re d s a n d s to n e s and s i l t s t o n e s in th e
s o u th a r e p e t r o g r a p h i c a l Iy i d e n t i c a l to th e K ibbey F o rm a tio n in th e
c e n t r a l B r id g e t Range.
N o rth from th e c e n t r a l B r id g e t R ange, th e member t h i n s to about
t h i r t y m e te rs a t H orse M ountain and M iddle F ork S ix te e n m ile and
c o n tin u e s o u t o f th e stu d y a r e a .
I s o p a c h s f o r th e u p p e r member of
th e K ibbey F o rm atio n show t h a t i t re m a in s l e s s th a n f i f t y m e te rs
t h i c k a c r o s s th e Lombard A rch and r a p i d l y th ic k e n s a g a in i n t o th e Big
Snowy tro u g h ( F ig u r e 1 0 ) .
Most o f t h i s v a r i a t i o n in th ic k n e s s was cau sed by c o n tin u e d
57
s u b s id e n c e in th e c e n t r a l B rid g e r Range r e l a t i v e to th e Wyoming s h e l f
and Lombard a rc h d u rin g u p p e r K ibbey tim e as p r e v io u s ly d is c u s s e d .
The c e n t r a l B rid g e r Range s u b s id e n c e i s a l o c a l m a n i f e s t a t i o n o f th e
r e g i o n a l t e c t o n i c developm ent o f th e S n o w c rest-B ig Snowy u n s ta b le
tre n d .
A p p aren t e p i s o d i c i t y o f t r a n s g r e s s i o n a t th e to p and b ottom of
th e s a n d s to n e l i t h o f a c i e s w ith i n th e B ig Snowy Group t r a n s g r e s s i v e
s e r i e s may be a d i r e c t r e s u l t o f e p is o d ic s u b s id e n c e .
A lte rn a tiv e ly ,
e p i s o d i c i t y may r e f l e c t ch an g in g r a t e s o f in p u t o f c l a s t i c d e t r i t u s
w ith i n a fram ew ork o f c o n tin u o u s s e a l e v e l r i s e as H a r r i s (1972)
s u g g e s te d f o r th e u p p e r member o f th e K ibbey F o rm atio n in th e Big
Snowy tr o u g h .
Most l i k e l y , e p is o d ic s tr a n d l i n e s h i f t i n g was
p ro d u ced by a c o m b in a tio n o f r e g io n a l s e a l e v e l r i s e and s u b s id e n c e
( Sando, 1 9 7 4 ), c h an g in g r a t e s o f d e t r i t a l sed im en t in p u t ( H a r r is ,
1972; B a l l a r d , 1 9 6 4 ), and l o c a l s u b s id e n c e in th e c e n t r a l B rid g e r
Range w hich was p o s s ib ly e p i s o d i c .
Lombard F a c ie s
L i th o f a c i e s
In tro d u c tio n .
M udstone and lim e s to n e o f th e Lombard f a c i e s
o v e r ly a m inor d is c o n f o r m ity on th e u p p e r member o f th e K ibbey
F o rm atio n p ro d u ced d u rin g t r a n s g r e s s i o n o f th e Big Snowy Group s e a .
An e x c e l l e n t ex p o su re o f th e Lombard f a c i e s n e a r Ross Peak i s th e
t h i c k e s t ( f i f t y - t h r e e m e te r s ) and th e m ost co m p lete s e c t i o n in th e
B rid g e r R ange.
T h ic k n e s s o f th e Lombard f a c i e s i s a g a in c o in c id e n t
w ith th ic k n e s s tr e n d s in th e u p p er M issio n Canyon and K ibbey
58
F o rm a tio n s „
At s e c ti o n s n o r th and s o u th from th e c e n t r a l B rid g e r
R ange> th e Lombard i s m is s in g o r t h i n due to e i t h e r p r e —Amsden
e r o s io n o r d e p o s i t i o n a l th in n in g w ith a s s o c ia t e d f a c i e s changes on
th e Lombard a r c h and Wyoming s h e l f .
The Ross Peak s e c t i o n i s v e ry
s i m i l a r to th e p ro p o sed ty p e s e c t i o n o f th e Lombard f a c i e s a t th e
Lombard r a i l r o a d s t a t i o n , a lth o u g h th e ty p e s e c t i o n i s s l i g h t Iy
t h i n n e r (45 m e t e r s ) .
At th e ex p o su re n e a r th e Lombard s t a t i o n , th e low er one t h i r d o f
th e Lombard f a c i e s i s composed o f b la c k f i s s i l e s h a le w ith a few
lim e s to n e b e d s .
T h is g ra d e s up in t o a m id d le lim e b io - p a c k s to n e and
w ack esto n e i n t e r v a l and u p p e r s p a r s e ly f o s s i l i f e r o u s lim e m udstone
in te rv a l.
At Ross P eak , th e s e c t i o n i s d iv id e d s i m i l a r l y i n t o th r e e
l i t h o f a c i e s ; a lo w er s h a le l i t h o f a c i e s w ith some beds o f lim e
m u d sto n e, b io -w a c k e s to n e , and p a c k s to n e , a m id d le
b io -w a c k e s to n e -p a c k s to n e l i t h o f a c i e s w ith m inor d a rk s h a le and lim e
m udstone in t e r b e d s , and an u p p e r lim e m udstone l i t h o f a c i e s w ith a few
b io -w a c k e sto n e beds (F ig u r e 1 1 ) .
A p p ro x im ately t h i r t y t h i n s e c ti o n s r e p r e s e n t in g a l l th r e e
l i t h o f a c i e s o f th e Lombard f a c i e s a t d i f f e r e n t lo c a t i o n s in th e
B r id g e t Range w ere a n a ly z e d .
The p e tr o g r a p h ic d a ta and f i e l d d a ta
a r e i n t e g r a t e d in th e f o llo w in g d is c u s s io n o f th e s e t h r e e
lith o fa c ie s .
59
Amsden Formation
I
I
_
I
I
I
,1
I
I A I
I
I
I
I
Lombard Facies
(O to 4 3 meters)
1 1
,.,!A
I
,1
I
I l l l
Lime Mudstone Lithofacies
I
T
I
a
I
I
I
I
I
I
I
I
I
I
I
I
Lv
I
IH
T
I
I •• I
o
I
I
l
I I
I )'x I
I
I A I
" I) I
I
I - I
I
u I
I •• I O
l
Lime Wackestone-Packstone Lithofacies
l
I
I
Shale Lithofacies
I
l
l
l
I
I •• I
I- I
I o I .. I O
U - I - i vi
o <5fc-P
/
Figure II.
—
/
/ -
/
Upper Member
Kibbey Formation
Generalized
Circles denote crinoid
spicule w o c k e s t o n e .
lithologic column of the Lombard facies.
p ack sto ne , vertical d a s h e s d e n o t e s
S e e text f o r d eta ils of lith o fa cies.
60
S h ale L i t h o f a c i e s .
The s h a le l i t h o f a c i e s c ro p s o u t a s t h r e e o r
few er lim e s to n e b ed s b etw een co v ere d zo n es a t th e b a se o f th e Lombard
fa c ie s .
B lack-brow n s o i l c o v e r w ith g r a s s and m inor b la c k s i l t y
s h a le f l o a t a r e t y p i c a l o f d a rk s h a le e x p o su re s in th e s tu d y a r e a .
The lo w est lim e s to n e o u tc ro p c o n s i s t s o f t h i c k beds o f d a rk g ra y to
brown lim e b io -w a c k e s to n e , lim e b io - p a c k s to n e and lim e m u d sto n e .
P a c k sto n e s a r e composed p r im a r ily o f c r i n o i d o s s i c l e s , s h e l l
fra g m e n ts , u n i d e n t i f i e d s p ic u l e s o r b ra c h io p o d s p in e s , and q u a r tz
s i l t and sand in m in o r q u a n t i t i e s .
W ackestones d i f f e r in
c o m p o s itio n ; th e s p ic u l e s o r s p in e s a r e dom inant w ith c r i n o i d
o s s i c l e s and o th e r c l a s t s in m in o r am o u n ts.
A llochem s a r e w ith o u t
p r e f e r r e d o r i e n t a t i o n , s o r t i n g , g r a d in g , o r r h y th m ic i ty .
Above t h i s ,
one o r two o th e r o u tc ro p s w ith i n th e s h a le l i t h o f a c i e s a r e medium
bedded g ra y and brown lim e m udstone w hich w e a th e rs o r a n g e - y e llo w .
A
p e c u l i a r m o ttle d t e x t u r e and two c e n tim e te r h o r i z o n t a l tu b e s on
b ed d in g s u r f a c e s s u g g e s t b i o t u r b a t i o n .
B io -w a c k e s to n e -p a c k s to n e L i t h o f a c i e s .
The s h a le g ra d e s in to
lim e m udstone a t th e b a se o f th e m id d le b io -w a c k e s to n e -p a c k s to n e
lith o fa c ie s .
About f o u r m e te rs in t o t h i s l i t h o f a c i e s a r e s e v e r a l one
m e te r t h i c k m a ss iv e b la c k -b ro w n lim e b io - p a c k s to n e b e d s .
each bed i s s h a rp on u n d e r ly in g lim e -m u d sto n e .
The b a se o f
The p a c k s to n e i s
s i m i l a r to th o s e in th e s h a le l i t h o f a c i e s e x c e p t t h a t t h e r e i s s tro n g
p a r a l l e l o r i e n t a t i o n o f e lo n g a te g r a i n s and some cru d e m i ll i m e t e r and
c e n tim e te r s c a le g r a d in g .
U p se c tio n in th e b io -w a c k e s to n e -p a c k s to n e
l i t h o f a c i e s i s an i n t e r v a l o f t h i n l y in te r b e d d e d b la c k s h a le and
61
s p a r s e ly f o s s i l i f e r o u s lim e m udstone w hich g ra d e s i n t o a seq u en ce o f
b io —w ack esto n e s and p a c k s to n e s in te r b e d d e d w th lim e m udstone a t th e
to p o f th e l i t h o f a c i e s .
A g ain , w ack esto n e s a r e composed p r im a r ily o f
s p ic u l e s o r s p in e s and p a c k s to n e s p r im a r ily o f c r in o i d o s s i c l e s .
Lime M udstone L i t h o f a c i e s .
At th e to p o f th e Lombard f a c i e s i s
ab o u t te n m e te rs o f t h i n to medium bedded lim e m udstone in th e u p p er
lim e m udstone l i t h o f a c i e s .
The lim e m udstone i s c re a m -p in k and
o liv e -b ro w n , w ith pods and sm a ll le n s e s o f brown c h e r t .
S o lita ry
c o r a l s w ere c o l l e c t e d a t s e v e r a l e x p o s u re s o f t h i s l i t h o f a c i e s .
A
one m e te r t h i c k f o r a m i n i f e r a l bed w ith a p a c k s to n e t e x t u r e o c c u rre d
two m e te rs from th e to p a t Ross Peak ( P l a t e 7 ) .
U n is e ria l, b i s e r i a l ,
and c o i le d form s o f foram s c o n s t i t u t e s i x t y p e r c e n t o f th e c l a s t s ,
w ith s p in e s o r s p ic u l e s as m inor co m p o n en ts.
No g ra d in g o r
p a r a l l e l i s m o f g r a in s was n o te d .
D e p o s itio n s ! E nviro n m en t I n t e r p r e t a t i o n
In tro d u c tio n .
S u b tle changes in c l a s t i c in p u t and c a rb o n a te
d e p o s it io n o f th e Lombard f a c i e s s i g n a l s s l i g h t changes in th e
d e p o s i t i o n s ! en v iro n m en t u p s e c tio n , how ever a l l t h r e e l i t h o f a c i e s
w ere d e p o s ite d in s i m i l a r e n v iro n m e n ta l c o n d i tio n s .
F e a tu r e s o f th e
s h e I f - la g o o n s ta n d a r d f a c i e s b e l t (W ilso n , 1975) a r e p r e s e n t in th e
Lombard f a c i e s .
D ark lim e m udstone w ith s p a r s e b io -w a c k e s to n e and
p a c k s to n e a r e i n d i c a t i v e o f a q u i e t , s u b t i d a l en v iro n m en t in w hich
b i o c l a s t i c d e b r is i s w ashed in by i n t e r m i t t e n t c u r r e n t s to pro d u ce
th e c h a r a c t e r i s t i c t e x t u r a l i n v e r s i o n .
T id a l p r o c e s s e s may be
62
P l a t e 7 . P h o to m icro g rap h o f f o r a m i n if e r a l
p a c k s to n e in th e Lombard f a c i e s (a p p ro x im a te ly
2 .5 mm a c r o s s f i e l d ) .
63
r e s p o n s ib le f o r some t r a n s p o r t o f d e b r i s , a lth o u g h few o b v io u s
c h a n n e l f e a t u r e s a r e re c o g n iz e d and th e m u d - b io c la s t r i c h
in te r b e d d in g i s n o t even o r r e g u l a r as m ig h t be e x p e c te d i f t i d a l
c u r r e n t s w ere r e s p o n s i b l e .
More l i k e l y , sto rm c u r r e n t s , p o s s ib ly
com bined w ith t i d a l c u r r e n t s a r e r e s p o n s i b l e .
As d e f in e d by W ilson (1 9 7 5 ), th e s h e I f - la g o o n l i e s w ith i n a
c a rb o n a te p la tf o r m b e h in d a f r i n g i n g b a r r i e r on th e p la tf o r m edge
w hich p a r t i a l l y r e s t r i c t s w a te r c i r c u l a t i o n in th e la g o o n .
R eg io n al
s t u d i e s show t h a t th e S n o w crest and B ig Snowy tro u g h s w ere a la g o o n a l
c o n n e c tio n betw een th e C o r d il le r a n m io g e o c lin e and th e W ill i s t e n
b a s in e p e i r i c se a d u rin g Lombard tim e (S an d o , D u tro , and G ordon,
1 9 7 5 ).
E n v iro n m en ta l c o n d itio n s became i n c r e a s in g ly r e s t r i c t e d
e a s tw a rd from th e m io g e o c lin e in th e S now crest and B ig Snowy t r o u g h s .
Based on is o p a c h d a t a , Rose (1976) c o n clu d ed t h a t a s h e l f edge
to p o g ra p h ic h ig h in ex trem e s o u th w e s te rn Montana p a r t i a l l y r e s t r i c t e d
c i r c u l a t i o n o f th e Lombard f a c i e s se a in th e Snow crest tr o u g h .
The
s h e l f edge i s c h a r a c t e r i z e d by h ig h c a r b o n a te p r o d u c t i v i t y (W ilso n ,
1975) and p ro b a b ly p ro v id e d a s i g n i f i c a n t so u rc e f o r d e t r i t u s washed
i n t o th e Lombard f a c i e s s h e l f la g o o n .
F u r th e r e a s tw a rd in th e Big Snowy tr o u g h , w a te r c i r c u l a t i o n was
even more r e s t r i c t e d and c a rb o n a te in p u t d im in is h e d ( H a r r i s , 1 9 7 2 ).
S tu d ie s in m odern e n v iro n m en ts show t h a t some d e g re e o f re d u c in g
c o n d itio n s and w a te r r e s t r i c t i o n can be p ro d u ced by s i z e and geom etry
o f e p i c o n t i n e n t a l la g o o n s (Logan and C e b u ls k i, 1 9 7 0 ).
W ater in lo n g ,
l i n e a r lag o o n s a r e n o t m ixed by c i r c u l a t i n g c u r r e n t s n o rm a lly found
in l a r g e b o d ie s o f w a te r .
Lack o f w a te r c i r c u l a t i o n le a d s to th e
64
d ev elo p m en t o f an a n o x ic lo w e r la y e r and r e l a t i v e l y n o rm a lly
o x y g en ated u p p er la y e r .
W illia m s (1983) s u g g e s te d t h a t th e H eath
F o rm a tio n s h a le s w ere d e p o s ite d in a n o x ic c o n d itio n s b elo w a
s t r a t i f i e d w a te r colum n in su ch a lag o o n .
The Lombard a r c h p ro b a b ly
a ls o c o n t r i b u te d to r e s t r i c t e d c i r c u l a t i o n o f m a rin e w a te r s w ith
c a rb o n a te d e t r i t u s e a s t o f th e S n o w crest tr o u g h .
The t h i c k e s t and m o st c o m p le te s t r a t i g r a p h i c s e c t i o n s o f th e
Lombard f a c i e s in th e c e n t r a l B r id g e t Range p ro v id e a fram ew o rk f o r
i n t e r p r e t a t i o n o f th e e n v iro n m e n ta l c o n d itio n s d u rin g d e p o s it io n o f
th e t h r e e l i t h o f a c i e s o f t h i s u n i t .
lag o o n c o n d itio n s p e r s i s t e d h e r e .
Q u ie t w a te r c a r b o n a te s h e lf
In c o m p le te s e c ti o n s n o r th and
s o u th w here two o r fe w e r l i t h o f a c i e s a r e d e v elo p ed r e f l e c t th e
p o s i t i v e te n d e n c y o f th e Lombard a rc h and Wyoming s h e l f d u rin g and
im m e d ia te ly f o llo w in g Lombard tim e .
The t h r e e l i t h o f a c i e s a r e
in c o m p le te ly d ev elo p ed o r m is s in g n o r th and s o u th o f th e c e n t r a l
B rid g e r Range e i t h e r b e c a u se o f s h o a lin g , m ore a g i t a t e d e n v iro n m e n ta l
c o n d itio n s o r pre-A m sden e r o s io n rem oved th e u p p er o n es.
The c o n ta c t b etw ee n th e s h o r e li n e s a n d s to n e , m u d sto n e , and
d o lo s to n e a t th e to p o f th e K ibbey F o rm a tio n and th e Lombard f a c i e s
s h a le s and lim e s to n e s i s th e s h a r p e s t l i t h o l o g i c change in th e Big
Snowy Group.
S e d im e n ta tio n changed fro m u p p e r K ibbey n e a r s h o r e
c l a s t i c to a q u i e t , s u b t i d a l , c a r b o n a te lag o o n e n v iro n m e n t w ith v e ry
little
c o a r s e c l a s t i c in p u t.
b etw ee n th e s e e n v iro n m e n ts .
c o n g lo m e ra te i s p r e s e n t h e r e .
S h o re fa c e s a n d s to n e s a r e n o t re c o g n iz e d
H owever, a t some e x p o su re s a
The c o n g lo m e ra te i s i n t e r p r e t e d as an
e r o s io n a l d is c o n f o r m ity d e v elo p ed d u rin g th e t r a n s g r e s s i o n .
Where
65
th e c o n g lo m e ra te i s m is s in g from t h i s i n t e r v a l , m in o r t r a n s g r e s s i o n a l
d e p o s it s may r e p r e s e n t t h i s t r a n s g r e s s i o n a l f a c i e s s h i f t .
The change
i s a t t r i b u t e d to a s t r a n d l i n e s h i f t b ro u g h t ab o u t by r e g i o n a l sea
l e v e l r i s e and r e g i o n a l s u b s id e n c e com bined w ith s u b s id e n c e l o c a liz e d
i n t h e c e n t r a l B rid g e r Range.
The d i f f e r e n c e b etw ee n th e Lombard f a c i e s tr a n s g r e s s i o n and th e
lo w e r K ib b e y -u p p e r member o f th e K ibbey F o rm a tio n t r a n s g r e s s i o n i s t h a t
th e o f f s h o r e en v iro n m en t o f th e Lombard f a c i e s was c h a r a c te r i z e d
by m udstone and lim e m udstone d e p o s it io n r a t h e r th a n by s i l t y
m udstone d e p o s i t i o n in th e K ibbey F o rm a tio n o f f s h o r e e n v iro n m e n t.
B oth ro c k ty p e s a r e i n t e r p r e t e d a s a p ro d u c t o f s u s p e n s io n
s e d im e n ta tio n in q u i e t la g o o n a l w a te r .
T h is s u g g e s ts e i t h e r t h a t th e
h ig h en e rg y c l a s t i c s h o r e f a c e was s h i f t e d f a r t h e r away fro m th e
B rid g e r Range a r e a d u rin g th e Lombard tr a n s g r e s s i o n r e s u l t i n g in
f a r t h e r o f f s h o r e e n v iro n m e n ts t h e r e o r t h a t th e o f f s h o r e Lombard
e n v iro n m e n t was d e e p e r and q u i e t e r due to more a c u te s u b s id e n c e in
th e B rid g e t Range.
A ls o , c a r b o n a te d e p o s it io n was e s t a b l i s h e d in th e
o f f s h o r e e n v iro n m en t a t t h i s p o i n t .
T h is i s a t t r i b u t e d to tr a p p in g
o r m a n tlin g o f s i l i c i c l a s t i c d e t r i t u s n e a r th e s h o r e li n e and
i n t r o d u c t i o n o f c a r b o n a te d e t r i t u s and c a rb o n a te p ro d u c in g o rg a n ism s.
S h a le L i t h o f a c i e s .
The d a rk s h a le and lim e s to n e o f th e Lombard
s h a le l i t h o f a c i e s a r e an i n d i c a t i o n o f a much low er en e rg y
e n v iro n m en t o f f s h o r e from th e K ibbey s h o r e f a c e s a n d s to n e s .
R eg io n al
s e a l e v e l r i s e and s u b s id e n c e c a u se d t r a n s g r e s s i o n o f th e K ibbey
F o rm a tio n s h o r e f a c e e n v iro n m en t r e g i o n a l l y e a s tw a rd in th e
66
S n o w c rest-B ig Snowy tro u g h s and l o c a l l y to w ard s th e tro u g h m a rg in s .
At th e same tim e th e m udstone l i t h o f a c i e s o f th e Lombard f a c i e s
fo llo w e d b e h in d b u t was r e s t r i c t e d to th e d e e p e r w a te r in th e
s u b s id in g tro u g h a x is ( Sando, G ordon, and D u tro , 1 9 7 5 ).
I n th e
B rid g e r Range th e Ross Peak and S outh A n g le r a r e a s w ere c h a r a c te r i z e d
by th e g r e a t e s t amount o f s u b s id e n c e d u rin g t h i s tim e a s e v id e n c e d by
maximum th ic k n e s s o f o f f s h o r e m udstone ( P l a t e I ) .
As th e t r a n s g r e s s i o n c o n tin u e d , c a r b o n a te d e p o s it io n s p re a d o u t
a c r o s s th e B r id g e r Range a r e a fo llo w in g b eh in d th e t r a n s g r e s s i n g
K ibbey F o rm atio n s h o r e f a c e e n v iro n m e n t.
M inor am ounts o f q u a r tz s i l t
and sand in th e low er Lombard f a c i e s m udstone l i t h o f a c i e s w ere
d e r iv e d from th e K ibbey F o rm atio n lo w er s h o r e f a c e c l a s t i c en v iro n m en t
and t r a n s p o r te d o f f s h o r e i n t o th e la g o o n a l en v iro n m en t soon a f t e r
tra n s g re s s io n .
T h is r e l a t i o n s h i p s u g g e s ts t h a t th e c l a s t i c s h o r e f a c e was n o t
m a n tle d d u rin g lo w er Lombard tim e .
R a th e r , th e Lombard la g o o n a l
en v iro n m en t and K ibbey s h o r e f a c e c o e x is te d l a t e r a l l y .
The la c k o f
c a rb o n a te allo c h e m s t y p i c a l l y p ro d u ced in i n t e r t i d a l c a r b o n a te
e n v iro n m e n ts , su ch as o o l i t e s and l i t h o c l a s t s , s u p p o r ts th e
c o n c lu s io n t h a t th e Lombard f a c i e s s h o r e li n e was s i l i c i c l a s t i c , one
in w hich c a r b o n a te p r o d u c tio n was p r e c lu d e d .
The s p ic u l e s o r b ra c h io p o d s p in e s w h ich a r e th e dom inant c l a s t in
w ack esto n e s w ere e i t h e r in d ig e n o u s o r n e a r ly in d ig e n o u s to th e
Lombard f a c i e s en v iro n m en t o r w ere c a r r i e d i n t o th e en v iro n m en t by
re g u la r c u r r e n ts .
I n c o n t r a s t , c r i n o i d o s s i c l e s a r e p r e s e n t in
s i g n i f i c a n t q u a n t i t i e s o n ly in th e l e s s commonly o c c u r r in g p a c k s to n e s
67
w here th e y a r e th e dom inant c l a s t .
T h u s, th e p a c k s to n e s w ere e i t h e r
g e n e ra te d by more s e v e re c u r r e n t s o r w ere p ro x im al to t i d a l c h a n n e ls
o r s h o a ls w here c u r r e n t s w ere s tr o n g enough to d e p o s it b i o - c l a s t s
w ith o u t s i g n i f i c a n t q u a n t i t i e s o f lim e mud.
D ir e c t e v id e n c e o f b i o t u r b a t i o n was o b se rv e d a t s e v e r a l s e c ti o n s
in th e low er s h a le l i t h o f a c i e s as worm tu b e s o r b urrow s and m o t tli n g .
I n d i r e c t e v id e n c e f o r b i o t u r b a t i o n i s p ro v id e d by la c k o f p r e f e r r e d
o r i e n t a t i o n o r g r a d in g o f c l a s t s in th e low er l i t h o f a c i e s .
B io t u r b a ti o n i n d i c a t e s t h a t b e n th o n ic o rg an ism s w ere e s t a b l i s h e d in .
r e l a t i v e l y n o rm a lly o x y g en ated w a te r a t t h i s tim e ( Je n k y n s , 1 9 7 8 ).
B io -w a c k e s to n e -p a c k s to n e l i t h o f a c i e s .
The b io -w a c k e s to n e and
p a c k s to n e l i t h o f a c i e s in th e m id d le o f th e Lombard f a c i e s i s s i m i l a r
to th e s h a le l i t h o f a c i e s below e x c e p t t h a t s h a le and s i l t c o n te n t i s
d e c re a s e d and lim e s to n e c o n te n t i n c r e a s e d .
T h is s i g n a l s th e
e s ta b lis h m e n t o f w id e s p re a d c a rb o n a te d e p o s it io n in an en v iro n m en t
w ith l i t t l e
s i l i c i c l a s t i c in p u t.
A p p a re n tly th e K ibbey s h o r e f ace
c o n tin u e d to s h i f t f a r t h e r landw ard o r i t was f i n a l l y m a n tle d by
e x te n s iv e c a r b o n a te s e a s .
C arb o n ate g r a in s and mud d e p o s ite d in s h e l f la g o o n s a r e
g e n e ra te d p r im a r ily in somewhat a g i t a t e d , more norm al m a rin e a re a s
m a rg in a l to th e lag o o n and c a r r i e d in by t i d a l and sto rm c u r r e n t s
(W ilso n , 1 9 7 5 ).
In a d d i t i o n to th e d i s t a l s h e l f edge s o u r c e , th e
Lombard a rc h and Wyoming s h e l f p ro b a b ly p ro v id e d a l o c a l s o u rc e o f
c a rb o n a te d e t r i t u s .
B i o c l a s t i c com ponents in p a c k s to n e s and
w ac k e sto n e s i n d i c a t e t h a t s p ic u l e s o r s p in e s w ere r e l a t i v e l y common,
68
in d ig e n o u s com ponents o f t h i s l i t h o f a c i e s e n v iro n m e n t, and c r in o i d
d e b r is was w ashed in and d e p o s ite d o n ly a s p a c k s tb n e s by in f r e q u e n t
sto rm o r t i d a l c u r r e n t s .
D ir e c t e v id e n c e o f b i o t u r b a t i o n i s m is s in g in th e b io -w a c k e sto n e
and p a c k s to n e l i t h o f a c i e s .
P rim a ry d e p o s i t i o n a l t e x t u r e o f
w ack esto n e s su ch as g ra d in g and s o r t i n g and s tr o n g p a r a l l e l i s m o f
e lo n g a te g r a in s a l s o i n d i r e c t l y s u g g e s ts t h a t b i o t u r b a t i o n was
p re c lu d e d in t h i s e n v iro n m e n t.
T h is i n d i c a t e s t h a t th e lag o o n had
become a t l e a s t i n t e r m i t t e n t l y a n o x ic s in c e b e n th ic o rg an ism s w ere
p r e v io u s ly e s t a b l i s h e d in th e la g o o n .
L ik e th e H eath F o rm a tio n s h a l e s , o th e r P a le o z o ic b la c k s h a le s in
th e U n ite d S t a t e s a r e i n t e r p r e t e d to have been d e p o s ite d u n d e r a
s t r a t i f i e d w a te r column ( P o t t e r , M aynard, and P r y o r , 1 9 8 2 ).
By
an a lo g y w ith o th e r b la c k s h a le s and c o r r e l a t i o n w ith th e H eath
F o rm a tio n , th e low er Lombard f a c i e s may a l s o be i n t e r p r e t e d to have
been d e p o s ite d u n d er an i n t e r m i t t e n t l y s t r a t i f i e d w a te r colum n.
The
d e p th , s i z e , and geo m etry o f th e s h e l f lag o o n a l s o was p a r t l y
r e s p o n s ib le f o r r e s t r i c t e d w a te r c i r c u l a t i o n and a n o x ic c o n d i tio n s .
Lime M udstone L i t h o f a c i e s .
C o n tin u ed se a l e v e l r i s e combined
w ith b a s in s u b s id e n c e r e s u l t e d in d e p o s it io n o f th e lim e m udstone
l i t h o f a c i e s a t th e to p o f the-L om bard f a c i e s in ah even q u i e t e r ,
f a r t h e r o f f s h o r e o r d e e p e r w a te r e n v iro n m e n t.
A m arked d e c r e a s e in
th e number o f w ack esto n e and p a c k s to n e b ed s and c o rre s p o n d in g
in c r e a s e in lim e m udstone r e f l e c t s d e c r e a s in g i n t e n s i t y and fre q u e n c y
o f sto rm and p o s s ib ly t i d a l c u r r e n t s due to e i t h e r d is ta n c e from
69
sh o re o r a deepened la g o o n a l s e t t i n g .
B i o c l a s t s a r e s i m i l a r to
th o s e in th e lo w e r l i t h o f a c i e s e x c e p t f o r th e one f o r a m i n i f e r a
p a c k s to n e n e a r th e to p o f th e Lombard f a c i e s a t Ross Peak.
w a te r had become s e a s o n a lly o r c o n t in u a ll y s t r a t i f i e d .
Q u ie t
Upper la y e r s
c o u ld s u p p o rt p e la g ic fo ra m s , b u t th e b o tto m la y e r was som ew hat
re d u c e d as e v id e n c e d by la c k o f b e t h ic f a u n a .
S tra tig ra p h ic I n te r p r e ta tio n
As th e K ibbey F o rm a tio n n e a r s h o r e e n v iro n m e n t o n la p p ed e a s tw a rd
in th e S n o w c rest tro u g h , th e o f f s h o r e Lombard f a c i e s mud and
lim e s to n e e n v iro n m e n t fo llo w e d .
T r a n s g r e s s io n was cau sed by
c o n tin u e d r e g io n a l s u b s id e n c e and se a l e v e l r i s e .
I n th e stu d y a r e a
th e Ross Peak and S outh A n g ler a r e a s in th e c e n t r a l B rid g e r Range
e x p e rie n c e d th e g r e a t e s t am ount o f s u b s id e n c e d u rin g Lombard tim e
(F ig u re 12).
The lo w e s t Lombard s h a le l i t h o f a c i e s s ig n a l s
in t r o d u c t i o n o f la g o o n a l c o n d itio n s i n t o th e l o c a l a x is o f th e
S n o w c rest tro u g h .
D uring d e p o s it io n o f th e s h a le l i t h o f a c i e s
s h a llo w e r e n v iro n m e n ts e x i s t e d to th e n o r th and s o u th o f th e c e n t r a l
B rid g e r Range.
As t r a n s g r e s s i o n c o n tin u e d , th e la g o o n a l e n v iro n m en t
s p re a d a c r o s s th e ra n g e .
As a r e s u l t , d e e p e r e n v iro n m e n ts p e r s i s t e d
and th i c k e r seq u en ce s a r e p re s e r v e d in th e c e n t r a l B r id g e t Range.
Most conodont sa m p le s c o l l e c t e d fro m th e Lombard f a c i e s in th e
B r id g e t Range y ie ld e d a l a t e C h e s te r ia n age ( w r i t t e n com m uncation,
W ardlaw , 1983) (s e e A ppendix f o r l o c a t i o n and i n t e r p r e t a t i o n o f
conodont s a m p le s ).
Sam ples from th e b a se o f th e u n i t a t th e
70
Z/
'
O
5 IO
miles
Figure 12.
Isopach map of Lombard f a c i e s and Heath
Formation.
Contours
Section
(adopted
in fe e t ,
A-A' d e p i c t e d on
d a s h e d where control is
Plate
from Harris, 1972)
I.
Bridger
Range
north
poor.
dashed,
71
Ross Peak and M aynard C reek s e c ti o n s h ow ever, w ere d e s ig n a te d o n ly as
C h e s te r i a n .
T h is i n d i c a t e s t h a t th e Lombard f a c i e s in th e B rid g e r
Range i s p r im a r ily l a t e C h e s te r i a n .
However, one can s p e c u la te t h a t
th e b a se may be s l i g h t Iy o ld e r a t th e Ross Peak and M aynard C reek
a re a s.
The th ic k n e s s v a r i a t i o n o f th e Lombard f a c i e s in th e B r id g e r
Range i s th e r e s u l t o f a c o m b in a tio n o f d i f f e r e n t i a l s u b s id e n c e
d u rin g d e p o s it io n and e r o s io n a f t e r d e p o s i t i o n .
As p r e v io u s ly
d is c u s s e d , th e c e n t r a l p a r t o f th e ra n g e e x p e rie n c e d m ore su b sid e n c e
th a n th e Lombard a r c h and Wyoming s h e l f d u rin g d e p o s it io n o f th e
K ibbey F o rm a tio n .
I n g e n e r a l t h i s tr e n d c o n tin u e d i n t o Lombard
f a c i e s tim e .
The e x c e p tio n to t h i s g e n e r a l p a t t e r n i s seen betw een Ross. Peak
and B ig h o rn Lake ( P l a t e I ) .
H ere th e Lombard f a c i e s i s m is s in g and
th e K ibbey i s r e l a t i v e l y t h i c k .
e x p la in t h i s d i f f e r e n c e .
Two h y p o th e s e s may be p o s t u l a t e d to
F i r s t , th e e n t i r e c e n t r a l B r id g e r Range
s u b s id e d a s a tro u g h d u rin g K ibbey F o rm a tio n tim e , b u t d u rin g Lombard
f a c i e s tim e th e a r e a betw een Ross Peak and B ig h o rn Lake became
p o s i t i v e and Lombard ro c k s a r e m is s in g due to non—d e p o s i t i o n .
In
t h i s c a se th e p r e s e n t c o n f ig u r a tio n r e f l e c t s developm ent o f two
d e p o s i t i o n a l lows d u rin g Lombard tim e .
A second p o s s i b i l i t y i s t h a t
th e c e n t r a l B r id g e r Range was an i n t e g r a t e d d e p o s ito n a l tro u g h
th ro u g h o u t K ibbey and Lombard d e p o s i t i o n .
L a t e r , d u rin g p r e —Amsden
e r o s io n , th e a r e a b etw een th e Ross Peak and B ig h o rn Lake became
u p l i f t e d r e l a t i v e l y more th a n th e r e s t o f th e c e n t r a l B r id g e r Range.
In t h i s c a s e , th e c o n f ig u r a tio n r e f l e c t s p r e —Amsden u p l i f t and
72
e r o s io n .
Is o p a c h s o f th e H eath F o rm atio n and Lombard f a c i e s i n d i c a t e t h a t
th in n in g n o r th o f th e c e n t r a l B rid g e r Range c o n tin u e s o n to th e
Lombard a r c h ( F ig u re 1 2 ) .
A lth o u g h t h i s i s p a r t l y a t t r i b u t e d to
p r e —Amsden e r o s io n (Klemme, 1949, H a r r i s , 1 9 7 2 ), e v id e n c e s u g g e s ts
t h a t th e a r c h c o n tin u e d to be p o s i t i v e r e l a t i v e to th e S now crest and
Big Snowy tr o u g h s d u rin g Lombard f a c i e s tim e .
As th e H eath and O tte r
F o rm atio n s h a le s t h i n o n to th e a r c h from th e Big Snowy tro u g h th e y
g ra d e i n t o th e b i o c l a s t i c lim e s to n e o f th e Lombard f a c i e s (B la k e ,
1 9 5 9 ).
S i m i l a r l y , from th e c e n t r a l B r id g e r Range, th e Lombard f a c i e s
t h i n s and g ra d e s in t o a lim e b i o - g r a i n s t o n e a t th e M iddle F o rk o f
S ix te e n m ile C reek s e c t i o n on th e Lombard a r c h , s u g g e s tin g t h a t
r e l a t i v e l y s h a llo w e r , a g i t a t e d c o n d itio n s e x i s t e d on th e Lombard
a rc h .
T h u s, d e p o s it io n c o n tin u e d on th e a r c h b u t r e s u l t e d in a t h i n
s e d im e n ta ry s e c t i o n due to la c k o f s i g n i f i c a n t s u b s id e n c e .
S outh o f th e c e n t r a l B r id g e r Range th e Lombard f a c i e s t h i n s to
z e ro a t th e B r id g e t P eak s e c t i o n ( P l a t e I ) .
F a r th e r s o u th , Lombard
f a c i e s i s n o t re c o g n iz e d e x c e p t a t B r id g e t Canyon w here t h r e e m e te rs
o f d a rk g ra y to b la c k lim e m udstone w ith t h i n in t e r b e d s o f b la c k
s h a le o v e r l i e s th e K ibbey F o rm a tio n .
E v id en ce o f s h o a lin g c o n d itio n s
a s seen on th e Lombard a r c h i s a b s e n t h e r e .
How f a r th e Lombard
f a c i e s e x te n d e d s o u th o f B r id g e t Canyon and how t h i c k i t was i s
c o n j e c t u r a l s in c e e v id e n c e was removed by pre-A m sden e r o s io n .
73.
Upper Big Snowy Group C o n ta c t and th e Amsden F o rm atio n
D e s c r ip ti o n .
The c o n ta c t betw een th e Big Snowy Group and th e
Amsden F o rm a tio n i s e a s i l y lo c a te d w here Lombard f a c i e s lim e s to n e i s
p re s e n t.
At su ch l o c a l i t i e s re d m udstone and s i l t s t o n e o f th e Amsden
F o rm atio n o v e r l i e an i r r e g u l a r c a l c i t e —d o lo m ite —q u a r tz d u r a c r u s t
( c a l i c h e ) up to one m e te r t h i c k on th e Lombard f a c i e s .
At lo c a tio n s
w here th e Amsden F o rm a tio n o v e r l i e s th e K ibbey F o rm atio n th e c o n ta c t
i s l e s s e a s i l y d is c e r n e d b eca u se o f s i m i l a r i t y in l i t h o l i g i e s .
In
th e B rid g e t R ange, t h i s c o n ta c t i s p la c e d above th e t a b u l a r u p p er
K ibbey s a n d s to n e o r below th e lo w er Amsden re d m u dstone—s i l t s t o n e
in te rv a l.
D if f e r e n c e s in bed l i t h o l o g y and g eom etry o f th e K ibbey
and Amsden F o rm a tio n s a r e th e m ost r e l i a b l e i n d i c a t o r s a t th e s e
lo c a tio n s .
The low er Amsden F o rm a tio n c o n s i s t s o f re d m udstone and
s i l t s t o n e w ith le n s e s and beds o f v a r i a b l y c o lo re d s a n d s to n e , lim e
b io -w a c k e sto n e and p a c k s to n e and f i n e g r a in e d m a ss iv e d o lo s to n e .
Conodont d a te s e s t a b l i s h an e a r l y Morrowan age f o r th e Amsden
F o rm atio n j u s t above th e B ig Snowy Group c o n ta c t ( P l a t e I ; A ppendix)
( w r i t t e n co m m u n icatio n , W ardlaw, 1 9 8 3 ).
The sam pled i n t e r v a l v a r ie s
from t h i r t y - t w o m e te rs on th e s o u th a t Rocky Canyon, to f i f t y - s e v e n
m e te rs a t S outh A n g le r, to z e ro a t M iddle F o rk S ix te e n m ile canyon on
th e n o r th end o f th e stu d y a r e a .
74
In te rp re ta tio n .
D ir e c t and i n d i r e c t l i t h o l o g i c and
s t r a t i g r a p h i c e v id e n c e i n d i c a t e s an u n c o n fo rm ity i s p r e s e n t betw een
th e B ig Snowy Group and Amsden F o rm a tio n in th e B r id g e r R ange.
The
d u r a c r u s t d ev elo p ed a lo n g th e c o n ta c t w here th e Lombard f a c i e s
o v e r l i e s th e Amsden F o rm a tio n i n d i c a t e s a p e r io d o f e x p o su re to
w e a th e r in g .
A ls o , a sh a rp l i t h o l o g i c change from b la c k lim e s to n e to
re d m udstone and s i l t s t o n e i n d i c a t e s a s h a rp change in e n v iro n m e n ts .
The n e g a t iv e t e c t o n i c b e h a v io r o f th e c e n t r a l B r id g e ts d u rin g
Kibbey and Lombard tim e a l s o o p e r a te d d u rin g u p l i f t and e r o s io n o f
th e Big Snowy Group b e f o r e th e Amsden t r a n s g r e s s i o n .
The g r e a t e s t
e r o s io n and t h e r e f o r e r e l a t i v e u p l i f t a s ev id e n c e d by rem oval o f th e
. I
e n t i r e Lombard f a c i e s , o c c u r r e d w here K ibbey se d im e n ts a r e t h i n n e s t .
Where th e Lombard i s t h i c k e s t , th e K ibbey i s a l s o t h i c k .
As
p r e v io u s ly d is c u s s e d , th e e x c e p tio n to t h i s g e n e r a l p a t t e r n i s seen
betw een Ross Peak and B ig h o rn Lake ( P l a t e I ) .
IMPLICATIONS: TECTONIC INFLUENCE ON SEDIMENTATION
The l a t e M is s is s i p p ia n p a l e o t e c t o n i c fram ew ork in th e B rid g e r
Range i s a lo c a l m a n if e s ta t io n o f th e p a l e o t e c t o n i c fram ew ork o f
M ontana.
R e g io n a l is o p a c h and f a c i e s s t u d i e s o f P r o te r o z o i c and
P a le o z o ic ro c k s in M ontana i n d i c a t e t h a t p a l e o t e c t o n i c e le m e n ts w hich
w ere i n f l u e n t i a l on l a t e M is s is s ip p ia n s e d im e n ta tio n ( F ig u r e I ) were
s i m i l a r l y i n f l u e n t i a l p r i o r to and f o llo w in g Big Snowy tim e .
For
ex am p le, in c e n t r a l M ontana th e l a t e M is s is s ip p ia n Big Snowy tro u g h
c o in c id e s c l o s e l y w ith th e P r o te r o z o ic B e lt b a s in and P a le o z o ic
C e n tr a l M ontana tro u g h in w hich t h i c k s e c ti o n s o f P r o te r o z o i c and
P a le o z o ic ro c k s a r e p r e s e r v e d ( P e te r s o n , 1 9 8 1 ).
I n s o u th w e s t Montana
th e S now crest tro u g h c o in c id e s w ith th e Ruby tro u g h ( P e te r s o n , 1982)
w hich had a s i m i l a r lo n g - liv e d h i s t o r y o f r e l a t i v e s u b s id e n c e .
G e o lo g is ts s tu d y in g P r o te r o z o ic and P a le o z o ic ro c k s on a
r e g io n a l s c a le co n c lu d e t h a t th e m a rg in s o f th e s e p a l e o t e c t o n i c
e lem en ts a r e c o i n c id e n t a l w ith , and c o n t r o l l e d by zones o f basem ent
w eak n ess.
F or exam ple, u s in g s t r a t i g r a p h i c and s t r u c t u r a l e v id e n c e ,
Maughan and P e rry (1 9 8 2 ) o b se rv e d t h a t th e S n o w c rest-G re en h o rn l i n e
w hich form s th e s o u th e a s t e r n m a rg in o f th e S now crest tro u g h fo llo w s a
zone o f basem ent w e ak n ess.
The b oundary betw een th e B ig Snowy tro u g h
and Wyoming s h e l f i s s i m i l a r l y b e lie v e d to be lo c a te d on an e a s t- w e s t
zone o f basem ent w eakness (Maughan, 1 9 8 4 ).
The B rid g e r Range l i e s n e a r w here th e s e two s t r u c t u r a l
76
lin e a m e n ts m e et.
P r o te r o z o ic and P a le o z o ic ro c k s have a maximum
th ic k n e s s som ew here n o r th o f Ross P ass in th e u n s ta b le tro u g h and
t h i n o n to th e Wyoming s h e l f s o u th o f Ross P a s s .
Ross P ass h as had an im p o r ta n t and lo n g - l iv e d s t r u c t u r a l
h is to ry .
The p a s s a r e a was th e s o u th e r n b oundary o f th e P r o te r o z o ic
B e lt b a s in .
A t h i c k , e x tre m e ly c o a r s e c o n g lo m e ra te f a c i e s o f th e
B e lt a b u ts a g a i n s t A rchean ro c k s a t Ross P a s s.
As McMannis (1963)
p o in te d o u t, th e s e r e l a t i o n s h i p s s tr o n g ly s u g g e s t a P r o te r o z o ic
f a u l t bo u n d in g th e s o u th e r n B e lt b a s in .
The s o u th e r n f a u l t e d m a rg in o f th e P r o te r o z o ic B e lt b a s in
c o in c id e s ro u g h ly w ith th e boundary b etw ee n th e P a le o z o ic Wyoming
s t a b l e p ro v in c e and C e n tr a l M ontana u n s ta b l e p ro v in c e .
Thus, in th e
B rid g e t R anger we have d i r e c t e v id e n c e o f c o in c id e n c e b e tw e e n a
P r o te r o z o i c zone o f w eak n ess and d i f f e r e n t i a l m ovem ents on
p a l e o t e c t o n i c e le m e n ts d u rin g th e P a le o z o ic .
77
CONCLUSION
\
The Big Snowy Group d is c o n fo rm a b ly o v e r l i e s th e M issio n Canyon
F o rm atio n in th e B r id g e r R ange.
The c o n ta c t i s c h a r a c t e r i z e d by a
change from c l e a n , o o l i t i c , M issio n Canyon lim e b io —w ack esto n e t o .
muddy, s i l t y , la m in a te d d o lo s to n e .
The ro c k s a r e n o t g r a d a t i o n a l and
do n o t i n t e r f i n g e r a c r o s s th e c o n ta c t i n d i c a t i n g t h a t a s i n g l e ,
e x te n s iv e s u r f a c e o f d is c o n f o r m ity d e v e lo p e d .
Sando (1 9 7 6 )
re c o g n iz e s t h a t th e r e a r e t h r e e , and p o s s ib ly f o u r , f a u n a l zones
m is s in g a c r o s s th e c o n t a c t.
The low er member o f th e K ibbey F o rm a tio n r e p r e s e n t s a sabkha
d e p o s ite d a t th e le a d in g edge o f th e Big Snowy s e a as i t tr a n s g r e s s e d
n o rth e a s tw a r d i n t o th e B rid g e r R ange.
A lg a l la m in a e , d e s s i c a t i o n
f e a t u r e s , and f e n e s t r a l f a b r i c a r e common in th e low er K ibbey
F o rm atio n d o lo s to n e .
L u cia (1972) a t t r i b u t e s such s t r u c t u r e s
p r im a r ily to i n t e r m i t t e n t l y flo o d e d s u p r a t i d a l and h ig h i n t e r t i d a l
m a rg in a l m a rin e z o n e s .
D o lo sto n e b r e c c ia s in th e low er K ibbey
F o rm atio n a r e i n t e r p r e t e d as e v a p o r i te s o l u t i o n z o n e s .
T h is
i n d i c a t e s t h a t a h o t, a r i d c lim a te p r e v a i le d a t t h i s tim e .
I n t e r t i d a l s i l i c i c l a s t i c c h a n n e ls ( c r e e k s ) m ig ra te d a c r o s s th e sabkha
in th e c e n t r a l ra n g e a r e a .
The low er K ibbey c h a n n e ls a r e s i m i l a r to
m odern c h a n n e ls on A ndros I s la n d a s d e s c r ib e d by S h in n , L lo y d , and
G insburg (1 9 6 9 ).
B oth f i n e up from s a n d s to n e o r c o n g lo m e ra te on a
s h a rp c o n ta c t c u t i n t o u n d e r ly in g s e d im e n ts i n t o s i l t s t o n e , and
78
m u d sto n e .
T id a l c h a n n e ls a r e r e s t r i c t e d to th e low l y i n g , i n t e r t i d a l
a r e a s on modern s a b k h a s .
. The u p p er member o f th e K ibbey F o rm atio n i s a p r im a r ily
r e g r e s s i v e o f f s h o r e to s h o r e f a c e d e p o s i t .
M udstones a t th e b a se w ere
d e p o s ite d by s u s p e n s io n in a low er s h o r e f a c e s e t t i n g .
In te rla y e re d
and g ra d e d s i l t s t o n e and s a n d s to n e w ith in th e m udstone w ere d e p o s ite d
by i n t e r m i t t e n t storm c u r r e n t s , p o s s ib ly com bined w ith t i d a l
c u rre n ts .
I n c r e a s in g g r a in s i z e and d e c r e a s in g mud c o n te n t u p s e c tio n
in th e s h o r e f a c e d e p o s it r e f l e c t h ig h e r en erg y c o n d itio n s a s s o c ia t e d
w ith th e s tr a n d l ir ie as th e r e g r e s s i o n c o n tin u e d .
H igh a n g le
t a n g e n t i a l and tro u g h c r o s s beds n e a r th e to p o f th e u p p e r member i s
s i m i l a r to m id d le and u p p e r s h o r e f a c e c r o s s b ed d in g r e p o r t e d in
modern se d im e n ts (C a m p b e ll,1971 ; Howard, 1971; R ein ech and S in g h ,
1 9 7 3 ).
L agoonal and t i d a l f l a t s i l t s t o n e , m u d sto n e, and d o lo s to n e a t
th e to p o f th e K ibbey F o rm atio n i n d i c a t e t h a t r e g r e s s io n c o n tin u e d
u n t i l i n t e r t i d a l , and p o s s ib ly s u p r a t i d a l , c o n d itio n s r e tu r n e d to th e
B rid g e t Range a r e a .
S e v e ra l h y p o th e se s may be p ro p o sed to e x p la in th e u p p e r K ibbey
r e g r e s s i o n a l e p is o d e .
A p e r io d o f r e l a t i v e se a le v e l lo w e rin g ,
c a u s e d e i t h e r by t e c t o n i c u p l i f t o r r e g io n a l s e a l e v e l f a l l , co u ld
c a u se a r e g r e s s i o n o f th e s h o r e f a c e f a c i e s .
A ls o , in c r e a s e d sedim ent
su p p ly to th e a r e a co u ld p ro d u ce o utw ard b u ild in g o r r e g r e s s i o n of
th e s h o r e f a c e , p a r t i c u l a r l y d u rin g a p e r io d o f r e l a t i v e s e a le v e l
s ta b ility .
R e g io n a l a n a l y s i s i s n eed ed to d e te rm in e w hich mechanism
i s p r im a r ily r e s p o n s i b l e .
The Big Snowy s e a r e tu r n e d a g a in b u t l e f t s c a n t r e c o r d in th e
79
K ibbey F o rm a tio n .
T r a n s g r e s s iv e s h o r e f a c e d e p o s it s su ch a s upward
c o a rs e n in g s a n d s to n e s a r e n o t re c o g n iz e d in th e u p p e r member.
H owever, t h i n l i t h o c l a s t c o n g lo m e ra te s a r e p r e s e r v e d a t s e v e r a l
s e c t i o n s on th e u p p er and lo w er c o n ta c t o f th e u p p er member.
These
c o n g lo m e ra te s r e p r e s e n t m inor d is c o n f o r m i ti e s g e n e r a te d d u rin g
t r a n s g r e s s i o n by s h o r e f a c e e r o s io n .
s i m i l a r s i t u a t i o n in m odern d e p o s i t s .
C l i f t o n (1973) r e p o r te d a
Where th e c o n g lo m e ra te i s
m is s in g in th e B rid g e t. R ange, a m inor t r a n s g r e s s i v e seq u en ce may be
p re s e n t.
The Lombard f a c i e s o v e r ly in g th e U pper member o f th e K ibbey
F o rm atio n was d e p o s ite d in a s h e l f lag o o n (a s d e f in e d by W ilso n ,
1975) .
The Lombard f a c i e s i s composed p r im a r ily o f d a rk c o lo re d lim e
m udstone and s h a le d e p o s ite d by s u s p e n s io n .
C i r c u l a t i o n in th e
Lombard la g o o n was r e s t r i c t e d by a to p o g ra p h ic b a r r i e r on th e s h e l f
m a rg in n e a r th e extrem e so u th w e st p a r t o f th e S how crest tro u g h (R ose,
1 9 7 6 ) , o r by th e g eom etry o f th e lag o o n i t s e l f as p ro p o sed f o r modern
e p i c o n t i n e n t a l s e a s (Logan and C e b u ls k i, 1 9 7 0 ).
I n t e r m i t t e n t storm
c u r r e n t s , p o s s ib ly com bined w ith t i d a l c u r r e n t s , b ro u g h t sm all
am ounts o f c r in o i d and s h e l l d e b r is in t o th e B r id g e t Range a r e a and
d e p o s ite d them as p a c k s to n e and w a c k e s to n e .
S edim ents a t th e b a se o f th e Lombard f a c i e s w ere b io t u r b a te d in
r e l a t i v e l y n o rm a lly o x y g en ated w a te r .
The lo s s o f e v id e n c e o f
b e n th ic fa u n a u p s e c tio n s u g g e s ts t h a t th e en v iro n m en t became
r e s t r i c t e d and somewhat a n o x ic , a t l e a s t n e a r th e b o tto m o f th e
la g o o n .
P e la g ic f o r a m i n if e r a w ack esto n e a t th e to p o f th e Lombard
f a c i e s , ho w ev er, i n d i c a t e s t h a t s u r f a c e w a te r s w ere o x y g en ated enough
80
to s u p p o rt s u r f a c e fa u n a d u rin g l a t e Lombard tim e .
T hus, th e u p p er
Lombard f a c i e s was d e p o s ite d u n d e r a s t r a t i f i e d w a te r column in much
th e same way as r e p o r te d f o r o th e r P a le o z o ic la g o o n a l d e p o s it s
( P o t t e r , M aynard, and P r y o r , 1 9 8 2 ).
A r e g i o n a l u n c o n fo rm ity i s d e v e lo p e d a t th e to p o f th e Lombard
f a c i e s ( Sando, 1976; Maughan, 1 9 8 4 ).
d u r a c r u s t m arks th e c o n t a c t .
In th e B rid g e r R ange, a
A sh a rp l i t h o l o g i c change a t m ost
e x p o su re s i s e v id e n c e o f change in d e p o s i t i o n a l en v iro n m e n ts a c ro s s
th e u n c o n fo rm ity .
The c e n t r a l B r id g e t Range s u b s id e d r e l a t i v e to th e Wyoming s h e l f
and Lombard a r c h d u rin g B ig Snowy d e p o s i t i o n .
E n v iro n m en ts o f th e
low er and u p p er members o f th e K ibbey F o rm atio n and th e Lombard
f a c i e s i n d i c a t e t h a t d e e p e r w a te r c o n d itio n s p e r s i s t e d in th e c e n t r a l
ra n g e a r e a due to t h i s s u b s id e n c e .
A ls o , s i g n i f i c a n t l y t h i c k e r
s t r a t i g r a p h i c s e c ti o n s a r e p re s e r v e d h e r e .
The e x c e p tio n to t h i s
g e n e r a l p a t t e r n betw een Ross Peak and B ig h o rn Lake was cau sed by a
l o c a l p o s i t i v e e lem en t w ith in th e c e n t r a l p a r t o f th e r a n g e .
The n e g a tiv e t e c t o n i c te n d en cy o f th e c e n t r a l B r id g e r Range was
a l s o i n f l u e n t i a l d u rin g u p l i f t and e r o s io n o f th e Big Snowy Group
p r i o r to th e Amsden t r a n s g r e s s i o n .
G r e a te s t e r o s io n and t h e r e f o r e
r e l a t i v e u p l i f t i s i n d i c a te d by rem oval o f th e e n t i r e Lombard f a c i e s ,
and in some c a s e s , th in n in g o f th e u p p e r member o f th e K ibbey
F o rm a tio n .
th in n e s t.
T h is o c c u rre d w here th e e n t i r e K ibbey F o rm a tio n i s
Where th e Lombard i s t h i c k e s t , th e K ibbey i s a l s o t h i c k .
The end r e s u l t i s t h a t e le m e n ts w hich w ere p o s i t i v e d u rin g d e p o s itio n
rem ain ed p o s i t i v e d u rin g pre-A m sden e r o s i o n , and t h i n d e p o s it s o f th e
81
Big Snowy Group on th e s e ele m e n ts w ere th in n e d m o re.
In v e rs e ly ,
t h i c k e r d e p o s its on n e g a tiv e ele m e n ts w ere p re s e r v e d from e r o s io n and
rem ain ed t h i c k .
In t h i s way b o th d e p o s i t i o n a l th in n in g and
p o s t - d e p o s i t i o n a l e r o s io n a r e r e s p o n s ib le f o r th ic k n e s s tr e n d s o f th e
Big Snowy Group in th e B r id g e r R ange.
The c e n t r a l B r id g e r Range i s an a n c ie n t zone o f c r u s t a l
w e ak n ess.
D uring th e P r o te r o z o i c , th e D illo n b lo c k s o u th o f th e
c e n t r a l B r id g e t Range was u p l i f t e d a lo n g a f a u l t th ro u g h Ross P ass
fo rm in g th e s o u th e r n m a rg in o f th e B e lt b a s in (McManni s , 1 9 6 3 ).
S im ila r ly d u rin g th e P a le o z o ic , th e Wyoming s h e l f s o u th o f th e
c e n t r a l B r id g e t Range was s t a b l e , o r n e u t r a l , r e l a t i v e to th e
u n s ta b l e tro u g h to th e n o r th ( P e te r s o n , 1 9 8 1 ).
These t e c t o n i c
c o n t r o l s r e s u l t e d in t h i c k s t r a t i g r a p h i c s e c ti o n s o f P r o te r o z o ic and
P a le o z o ic ro c k s n o r th o f th e c e n t r a l ra n g e a r e a , and t h i n o r m iss in g
s e c ti o n s to th e s o u th .
D uring l a t e M is s is s i p p ia n , th e c e n t r a l
B rid g e t Range a c te d as an u n s ta b l e tro u g h b etw een th e Wyoming s h e lf
and Lombard a r c h .
Big Snowy ro c k s a r e t h i n o r m is s in g on th e Wyoming
s h e l f and Lombard a r c h , and a r e t h i c k e r in th e c e n t r a l ra n g e a r e a .
These r e l a t i o n s h i p s p ro v id e i n s i g h t i n t o th e n a tu r e o f t e c t o n i c
in f lu e n c e t h a t th e zone o f c r u s t a l w eakness in th e B r id g e t Range had
on s e d im e n ta tio n o f P r o te r o z o ic and P a le o z o ic r o c k s .
Big Snowy Group
l i t h o f a c i e s and s t r a t i g r a p h y p ro v id e p a r t i c u l a r l y good e v id e n c e of
th i s in flu e n c e .
U n d o u b ted ly , f u t u r e s t u d i e s h e re w i l l p r o v id e more
e v id e n c e o f th e in f lu e n c e t h i s a n c ie n t zone o f w eakness had on
s e d im e n ta tio n o f o th e r r o c k s .
Such s t u d i e s m ight f u r t h e r e l u c i d a t e
th e d i s t r i b u t i o n o f p a l e o t e c t o n i c movements in tim e and s p a c e .
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S h in n , E .A ., R.M. L loyd and R .N . G in s b u rg , 1969, Anatomy o f a modern
c a rb o n a te t i d a l f l a t , A ndros I s l a n d , Bahamas: J o u r n a l o f S ed im en tary
P e tr o lo g y , v . 3 9 , p . 1 2 0 2 -1 2 2 8 .
S l o s s , L .L ., and R.H. H am blin, 1942, S tr a t ig r a p h y and in s o l u b l e
r e s id u e s o f M adison Group ( M is s i s s i p p ia n ) o f M ontana: A m erican
A s s o c ia tio n o f P e tro le u m G e o lo g is ts B u l l e t i n , v . 2 6 , n o . 3 , p .
3 0 5 -3 3 5 .
S w if t, D . J . P . , 1975, T id a l sand r id g e s and sh o a l r e t r e a t m a s s i f s :
M arine G eology, v . 1 8 , p . 105 -1 3 4 .
W alton, P .T ., 1946, E l l i s , Amsden and B ig Snowy G roup, J u d i t h B a s in ,
M ontana: A m erican A s s o c ia tio n o f P e tro le u m G e o lo g is ts B u l l e t i n , v .
3 0 , n o . 8 , p . 1 2 9 4 -1 3 0 5 .
86
Weed, W .H., 1896, Y e llo w sto n e N a tio n a l P a r k , s e d im e n ta ry r o c k s :
U n ite d S ta t e s G e o lo g ic a l Survey A t l a s , F o lio 3 0 , p . 4 - 5 .
1900, G eology o f th e L i t t l e B e lt M o u n tain s, M ontana: U n ite d
S ta t e s G e o lo g ic a l Survey A nnual R ep o rt 2 0 , 1 8 9 8 -9 9 , p t „ 3 , p .
2 5 7 -4 6 1 . *
W illia m s , L .A ., 1983, D e p o s itio n o f th e B ear G ulch L im e sto n e : a
C a rb o n ife ro u s p l a t t e n k a l k from c e n t r a l M ontana: S e d im e n to lo g y , v . 3 0 ,
p . 8 4 3 -8 6 0 .
W ilso n , J . L . , 1975, C arb o n ate f a c i e s in g e o lo g ic h i s t o r y : New Y ork,
S p rin g e r - V e r l a g , 471 p .
<
87
APPENDIX
D e s c r ip tio n s o f m easu red s e c ti o n s
C onodonts i n t e r p r e t e d by B ruce WardI aw,
U n ite d S ta t e s G e o lo g ic a l Survey
ROCKY CANYON SECTION
(H igh on w a ll o f rocky canyon n o r th o f i n t e r s t a t e , .
U n it t h i c k n e s s
S ec. 1 9 , T. 2 S . , R. 6 E .)
in fe e t
Amsden F o rm a tio n ( p a r t ) :
4 8 ' .............. C overed-brow n s o i l .
22'
8'
......... S i l t s t o n e - p u r p l e - r e d , sandy a t b a s e , c o v e re d in m id d le .
......... S a n d s to n e -p in k and m aroon, m in o r s i l t s t o n e .
16'
......... S i l t s t o n e -b ro w n -re d , p a r t l y c o v e re d .
10'
......... D o lo s to n e -Ia v e n d e r m o ttle d cream , t h i n to t h i c k bed d ed ,
m inor re d s i l t s t o n e .
12'
......... DoI o s to n e - w h it e , d e n s e , m a s s iv e , w e a th e rs to rounded
k n o l l s , ( t e c t o n i c ? ) b r e c c i a , c ru d e la m in a tio n s .
4'
......... C o v ered .
Upper Member K ibbey F o rm a tio n :
......... S a n d s to n e - g e n e r a lIy as b elo w , w ith s i l t s t o n e .
......... S a n d s to n e -w h ite and la v e n d e r , f i n e g r a in e d , m o d e ra te ly
s o r t e d , p o o rly p r e s e r v e d , t a n g e n t i a l tro u g h c r o s s b ed d ed , 15
d e g re e maximum d ip .
......... S a n d s to n e - b r ic k re d and p a le g r e e n , m in o r w h ite and p in k ,
f i n e to medium g r a in e d , m o d e ra te and p oor s o r t i n g , p a r t l y
c o v e re d .
10'
C o n g lo m e ra te -b ric k re d s a n d s to n e m a tr ix , f i n e to medium
88
g r a in e d , w ith d o lo s to n e and s a n d s to n e l i t h o c l a s t s up to 1 / 4
in c h s u p p o rte d by m a tr ix , s u b -ro u n d e d , p a r t l y c o v e re d .
4 ' ......... «- S i l t s t o n e - b r i c k r e d , low er re d m u d sto n e.
Lower Member K ibbey F o rm a tio n :
6 ' ..............D o lo s to n e - la v e n d e r m o ttle d g r e e n , a s below w ith a t h i n
re d m u d sto n e .
1 6 '.............. D o lo s to n e -p u rp le -m a ro o n , m in o r g r e e n , d e n s e , m a s s iv e ,
wavey la m in a tio n s , b a s a l y e llo w and muddy.
8 ' .............. D o lo sto n e b r e c c i a - p i n k , o r a n g e , y e llo w , g r a y , p u r p le and
maroon a t to p , c l a s t s up to 3 f e e t .
8 0 ' . . T o ta l Big Snowy Group
M issio n Canyon F o rm a tio n ( p a r t ) :
L im esto n e and D o lo s to n e - I ig h t g r a y to t a n , v e ry c o a r s le y
c r y s ta l lin e , te c h to n ic a lly b re c c ia te d .
BRIDGER CANYON SECTION
( 3 /4 m ile n o r th o f ro a d th ro u g h canyon.
S ec. 2 7 , T . I S ., R. 6 E .)
I'
Amsden F o rm atio n ( p a r t ) :
..............D o lo s to n e - y e llo w - g r e e n , f r a c t u r e d .
7 ' .............. C o v e re d -re d d is h s o i l , m in o r re d and g re e n m udstone
c ro p p in g o u t .
Lombard F a c i e s :
8 ' ..............L im e s to n e -d a rk g r a y , d e n s e , f i n e l y c r y s t a l l i n e , medium
b e d d in g , b la c k m udstone in te r b e d d e d , c a l c i t e f i l l e d f r a c t u r e s .
Upper Member K ibbey F o rm a tio n :
8 ' ................ S a n d s to n e -w h ite , m a ro o n -p u rp le , f i n e g r a in e d g e n e r a lly
m a ssiv e w ith t h i n muddy i n t e r b e d s , becomes s i l t s t o n e a t to p .
Lower Member K ibbey F o rm a tio n :
9 ' ................ S i l t s t o n e and DoI o s to n e -m aro o n m o ttle d g re e n and w h ite .
3 ' ................ S i l t s t o n e - b r i c k r e d .
89
12" ..............S i l t s to n e -m aro o n m o ttle d y e llo w -c re a m , b r e c c i a t e d , m inor
f i n e g r a in e d s a n d s to n e , b a s a l p a r t f i l l i n g K a r s t - s i n k in
M issio n Canyon F o rm a tio n .
4 8 " . . T o ta l Big Snowy F o rm atio n
M issio n Canyon F o rm a tio n ( p a r t ) :
L im estone-m edium b ro w n -g ra y , f i n e l y c r y s t a l l i n e , few
c o r a l s . Lower K ibbey s e d im e n ts f i l l i n g K a r s t - s i n k .
BRIDGER PEAK SECTION
( J u s t n o r th w e a s t o f S a d d le P eak , NW 1 /4 S ec. 3 6 , T. I N ., R 6 E .)
(M easurem ents and some d e s c r i p t i o n s from McMannis, 1 9 5 1 .)
Amsden F o rm a tio n ( p a r t ) :
1 8 " .............. D o lo s to n e - p u r p le , m o ttle d p a l e g r e e n , m a s s iv e .
7 " . ............ S i l t s t o n e - b r i c k r e d , m o t tle d , w ith in te r b e d d e d re d
m u d sto n e , p u r p le d o lo s to n e a t to p , p a r t i a l l y c o v e re d .
Upper Member K ibbey F o rm a tio n :
2 3 " .............. S a n d s to n e -re d a t to p and b o tto m , w h ite in m id d le , f i n e
g r a in e d , medium g r a i n in m id d le , f a i n t c r o s s b e d d in g .
1 0 " .............. S i l t s t o n e - b r i c k o ra n g e , p u r p l e , muddy.
Lower Member K ibbey F o rm a tio n :
3 5 " ..............D o lo s to n e - v a r io u s p a s t e l c o l o r s , p re d o m in a n tly maroon
m o ttle d g r e e n , s i l t y , la m in a te d i n p a r t , few b r e c c i a z o n e s.
14".......... D o l o s t o n e - p u r p l e an d c re a m m o ttle d , m a s s iv e , la m in a te d ,
muddy, few b r e c c i a s .
6 " . . ..........S i l t s t o n e - p u r p l e and r e d m o t tle d , b r e c c i a t e d .
1 4 " . . . . . . .D o lo s to n e -c re a m and p u r p le m o t tle d , b r e c c i a t e d .
1 0 2 " .. T o ta l Big Snowy Group
M is s io n Canyon F o rm a tio n ( p a r t ) :
L im estone-m edium g ra y -b ro w n , m a s s iv e , few f o s s i l s .
90
MAYNARD CREEK SECTION
( B rid g e r Bowl s k i a r e a , SW 1 /4 S ec. 2 4 , T. I N ., R 6 E .)
Amsden F o rm a tio n ( p a r t ) :
I ' ..............C la y - re d .
4 ' .............. D o lo s to n e - b u f f - p in k , g r a n u l a r , s l i g h t Iy sa n d y .
8 ' .............. DoI o s to n e - r e d and m aroon m o t tle d , p o s s ib ly b i o t u r b a t e d .
4 ' .............. Muds t o n e / c l a y s to n e -r e d - b ro w n .
1 2 ' .............. C o v e re d -p ro b a b le re d m udstone and s i l t s t o n e .
6 ' ...............D o le s to n e -v e ry l i g h t g r a y - la v e n d e r , f i n e l y c r y s t a l l i n e ,
sm ooth concave w e a th e re d s u r f a c e s , m a s s iv e .
2 ' ...............Q u a r t z - c a l c i t e d u r a c r u s t- y e l lo w , b a s a l
m u d s to n e - c la y s to n e .
re d
Lombard F a c i e s :
6 f .............. L im estone-m edium to l i g h t g r a y , medium
f o s s i l i f e r o u s , to p i s p in k m o t tle d .
c ry s ta llin e ,
4 1......... . L im e s to n e -a s a b o v e, w e ll d ev e lo p e d medium b e d d in g , few
b io -w a c k e s to n e b e d s , c h e r t y .
1 8 '................ L im e s to n e - o liv e d o lo m itic as ab o v e, s o l i t a r y c o r a l s a
to p , C h e s te r ia n CONODONTS from to p .
4 ' .............. D o lo s to n e - p u r p le and p in k - r e d , y e llo w w e a th e r in g .
1 2 '.............. L im e s to n e -a s ab o v e, w ith s ilty -m u d d y p a r t i n g s .
1 8 ' .............. L im e s to n e -d a rk g ra y —b la c k , b o tto m 2 /3 i s muddy, s i l t y ,
t h i n bedded to f i s s i l e , s o l i t a r y c o r a l s a t b a s e , C h e s te ria n
CONODONTS from to p .
8 ’ .............. L im e s to n e -a s above w ith m u d sto n e -g re e n and g r a y ,
C h e s te ria n CONODONTS from to p .
2 ’ ................. S a n d s to n e -g re e n and g r a y .
Upper Member K ibbey F o rm a tio n :
91
......... D o lo s to n e - I a v e n d e r , s i l t y , muddy.
......... S a n d s to n e -p in k and g r e e n , v e ry f i n e , f i n e , and medium
g r a in e d , m o d e ra te to poor s o r t i n g , t a b u l a r o u tc r o p , m inor
m u d sto n e.
Lower Member K ibbey F o rm a tio n :
.......... D o lo s to n e - p a le o ran g e to p in k , b o tto m 2 /3 b r e c c i a t e d ,
b a s a l g re e n and p u r p le m u d sto n e.
..........D o lo s to n e - o r a n g e - r e d , w ith s a n d s to n e , s i l t s t o n e and
m u d sto n e .
......... D o lo s to n e - p in k -r e d and p a le g r e e n - y e llo w , m o ttle d maroon
s i l t y , m in o r b r e c c i a .
T o ta l Big Snowy Group
M issio n Canyon F o rm a tio n :
Lim estone-m edium g r a y , f i n e l y c r y s t a l l i n e , m a s s iv e .
ROSS PEAK SECTION
(NE s lo p e o f p e a k , SW 1 /4 S ec. 2 , t . I N ., R. 6 E . )
Amsden F o rm a tio n ( p a r t ) :
I .............. D o lo s to n e - I ig h t g r a y , f i n e l y c r y s t a l l i n e .
5 0 ' .............. C o v e re d -re d s o i l and d e b r i s .
Lombard F a c i e s :
3 2 '.............. L im e s to n e -c re a m -p in k , o l i v e , and brow n, medium and t h i n
bedded f o s s i l i f e r o u s , one s o l i t a r y c o r a l c o l l e c t e d from to p ,
p o s s i b le sponge from to p , brown c h e r t n o d u le s , l e n s e s , and
fra g m e n ts , l a t e C h e s te ria n CONODONTS 18 f e e t from to p .
1 2 '.............. Lim estone-m edium b ro w n -re d , medium c r y s t a l l i n e , t h i n to
p la te y b e d d in g , b io -w a c k e sto n e and p a c k s to n e b ed s a b u n d a n t,
m inor o ran g e lim e s to n e .
1 6 '.............. L im e s to n e -b la c k , t h i n to p l a te y b ed d ed , f o s s i l i f e r o u s ,
a b u n d an t b la c k m udstone i n t e r b e d s , C h e s te ria n CONODONTS from
base.
16'
......... Lim estone-m edium g r a y - b la c k , f i n e l y c r y s t a l l i n e , v e ry
f o s s i l i f e r o u s , m a s s iv e .
92
6 ’ .............. S h a le - b la c k , s i l t y ,
lim y .
1 4 '.............. C overed-brow n s o i l , g r a s s y .
6 ' ...............Lim estone-m edium b ro w n -g ra y , a s below , p o s s i b l e
i n t r a c l a s t c o n g lo m e ra te .
2 0 ' .............. C o v e re d -d a rk brown s o i l , g r a s s y .
^ .............. L im esto n e—u p p er medium brown—g r a y , low er la v e n d e r , f i n e l y
c r y s t a l l i n e , sa n d y , 1 / 2 in c h h o r i z o n t a l worm t u b e s .
1 2 '............... C o v e re d -d a rk brown s o i l , g r a s s y .
6 ' ................Lim estone-m edium g ra y -b ro w n , medium and c o a r s le y
c r y s t a l l i n e , f o s s i l i f e r o u s , m id d le 2 f e e t i s b la c k m i c r i t i c ,
muddy lim e s to n e , C h e s te r ia n CONODONTS from b a s e and to p .
1 4 '................C o v e re d -d a rk brown s o i l , g r a s s y .
Upper Member K ibbey F o rm a tio n :
4 ' ................ C o n g lo m erate-m aro o n , su b ro u n d ed s a n d s to n e and d o lo s to n e
c l a s t s in s i l t s t o n e m a tr ix .
2 ' ............... M u d sto n e -re d .
6 ' ................ S a n d s to n e -d a rk red -m a ro o n , v e ry f in e g r a in e d , d a rk re d
m udstone c h ip s in m id d le .
1 4 ' . . . . . . . .S a n d s to n e - w h ite , v e ry f i n e g r a in e d , w e ll s o r t e d , w e ll
in d u r a t e d , m a s s iv e .
1 0 '................S a n d s to n e -a s ab o v e, f i n e g r a in e d .
2 ' ................M u d sto n e -b ric k r e d , la v e n d e r .
2 6 '................ S a n d s to n e -p a le y e llo w -o r a n g e , f i n e to medium g r a in e d ,
m o d e ra te ly w e ll s o r t e d , 10 in c h deep tro u g h c r o s s b e d s .
8 ' ................ S a n d s to n e - b r ic k r e d , v e ry f i n e g r a in e d , c a l c a r e o u s .
Lower Memeber K ibbey F o rm a tio n :
3 0 ' . . . . . . . . S i l t s t o n e —b r i c k r e d , m o ttle d y e llo w , f i n e wavey
la m in a tio n s , d o l o m i t i c , in te r b e d d e d w ith s a n d s to n e b r i c k r e d ,
v e ry f i n e g r a in e d , d o lo m iti c , and d o lo s to n e , m aro o n , la v e n d e r .
1 6 '.................D o lo s to n e —v a r io u s p a s t e l c o l o r s p re d o m in a n tly maroon
93
s i l t y , muddy, t h i n wavey la m in a tio n s , w ith in te r b e d d e d
s i l t s t o n e as a b o v e .
1 4 '.............. S ilts to n e - p r e d o m in a n tly b r i c k re d and g ra y r e d , m o ttle d
y e llo w , b lo c k y f r a c t u r e , few v e ry f in e s a n d s to n e s as ab o v e .
8 ' .............. D o lo s to n e -a s d o lo s to n e a b o v e .
1 9 '.............. D o lo s to n e -y e llo w -g ra y m o ttle d la v e n d e r , t h i n b ed d ed ,
t h r e e f i n i n g upw ard seq u en ce s o f p in k -m aro o n s a n d s to n e ,
s i l t s t o n e , m u d sto n e , s h a rp b a s a l c o n t a c t .
I ' .............. L i t h o c l a s t C o n g lo m erate-m aro o n , d is c o n tin u o u s , sa n d sto n e
and s i l t s t o n e c l a s t s .
3 3 6 ' . . T o t a l Big Snowy Group
M issio n Canyon F o rm atio n ( p a r t ) :
Lim estone-m edium g r a y , m a s s iv e , c o n ta c t w ith K ibbey has 3
f e e t o f r e l i e f , l i t h o c l a s t co n g lo m e ra te f i l l s low a r e a s .
STATION 191 SECTION
(One m ile n o r th o f Ross P eak , SW 1 /4 S ec. 3 5 , T. 2 N ., R. 6
E .)
Amsden F o rm a tio n ( p a r t ) :
10 ' .............. C o v e re d -o ra n g e -p in k ,
4 ' .............. L im e sto n e -v e ry l i g h t
s o il.
g r a y , b io - w a c k e s to n e .
1 4 '.............. D o lo s to n e - p u r p le and
g re e n m o ttle d .
Lim estone-m edium g r a y , d e n s e , b io - w a c k e s to n e , and
p a c k s to n e .
9 ' .............. P re d o m in a n tly co v ered -b ro w n s o i l w ith re d s i l t s t o n e and
s a n d s to n e c ro p p in g o u t a t to p , re d s o i l w ith r e d - o r a n g e s i l t y
m udstone a t b a s e .
Upper Member K ibbey F o rm a tio n :
3 4 '.............. S a n d s to n e -y e llo w a t b o tto m , w h ite in m id d le , r e d a t to p ,
f in e and medium g r a in e d , m o d e ra te ly w e ll s o r t e d , p la n a r
la m in a tio n s , p o o rly p r e s e r v e d lo w -a n g le c r o s s b e d s .
I ' .............. C o n g lo m e ra te -y e llo w , su b ro u n d ed lim e s to n e c l a s t s in muddy
s a n d s to n e .
6'
D o lo s to n e - r e d , san d y .
94
4 ' .............S i l t s t o n e - r e d .
4 ' ............. D o lo s to n e - r e d , s i l t y .
^ .............. S i l t s t o n e - r e d , b a s a l re d m u d sto n e .
^ ..............D o lo s to n e - p in k ,g r e e n and p u r p le m o ttle d , sm ooth concave
w e a th e re d s u r f a c e s .
Lower Member K ibbey F o rm a tio n :
12 .............. C o n g lo m e ra te -p u rp le and p a le g re e n
d o lo s to n e w ith
d o lo s to n e and s a n d s to n e c l a s t s , r e d , p u r p le , and p a le g re e n ,
su b -ro u n d e d .
7 1 To t a l Big Snowy Group
M issio n Canyon F o rm atio n ( p a r t ) :
® ..............L im esto n e B r e c c ia - p in k ,
I in c h c l a s t s .
SOUTHEAST SACAJAWEA SECTION
(One m ile s o u th o f F a ir y L ake,
NE 1 /4 S ec. 2 7 , T. 2 N ., R 6 E .)
Amsden fo r m a tio n ( p a r t ) :
2 2 ' ..............M u d sto n e-b ro w n -re d , t h i n l y bedded w ith 3 lim e s to n e b e d s,
y e llo w -g r a y , r e d , f i n e l y c r y s t a l l i n e , m a s s iv e .
1 8 '.............. L im e s to n e - p a le , r e d - p u r p l e , m o ttle d g r a y , f i n e l y
c r y s t a l l i n e , v e ry f o s s i l i f e r o u s .
1 4 ’ ...............C o v ere d -b ro w n -re d , sa n d y , m udstone
d e b ris .
6 ' .............. L im est o n e -w h it e - p a l e g r a y - r e d , medium to f i n e g r a in e d ,
m a s s iv e , f o s s i l i f e r o u s , e a r l y Morrow CONODONTS from b a s e .
Upper Member K ibbey F o rm a tio n : .
2 4 ' ............... S a n d sto n e -b ro w n -re d and p in k , f i n e g r a in e d , muddy, t h i n
b ed d ed .
1 4 '............... S a n d s to n e -a s s a n d s to n e b elo w .
4 *................M u d sto n e-b ro w n -re d , sa n d y .
............ S a n d s to n e -p a le g r e e n , w h ite and p in k m o t tle d , s p o tte d
o ran g e in p a r t , medium g r a in e d a t to p , f i n e g r a in e d a t b a s e ,
p a r t i a l c o v e r to w ard s b o tto m .
........... S i l t s to n e - p a le g re e n and p a le r e d , d o l o m i t i c , sa n d y .
■..........S a n d s to n e - b r ic k r e d , f i n e g r a in e d .
..........S i l t s to n e - b r i c k r e d .
..........M u d sto n e -b ric k r e d .
. . . . . S a n d s t o n e - r e d o ran g e m o ttle d p a le o r a n g e , d o lo m iti c , f i n e
g r a in e d , medium to t h i c k i r r e g u l a r b e d s .
.......... S ilt s to n e - r e d - b r o w n and b r i c k r e d , g r a d a t i o n a l from mud
below to s a n d s to n e a b o v e .
......... M u d sto n e -b ric k r e d .
.......... B r e c c ia l C o n g lo m e ra te -b ric k r e d .
Lower Member K ibbey F o rm a tio n :
.......... S an d sto n e and S i l t s t o n e - p u r p l e - r e d , m o ttle d g re e n in
p a r t , a t l e a s t 4 upward f i n i n g l e n s e s .
.......... M u d sto n e -re d .
..........D o lo s to n e - b r ic k r e d , s a n d y , s i l t y , w ith m in o r sa n d s to n e
and s i l t s t o n e b e d s .
......... DoI o s to n e —ta n , m inor p in k and p u r p l e , p re d o m in a n tly
co v ere d w ith re d s o i l , wavey la m in a tio n s .
• T o ta l Big Snowy Group
M issio n Canyon F o rm atio n ( p a r t ) :
Lim estone-m edium g ra y -b ro w n , m a s s iv e .
EAST SACAJAWEA SECTION
(M easurem ents and some d e s c r i p t i o n s
from M cMannis, 1 9 5 1 .)
(On r id g e j u s t SW o f F a ir y L ake,
NE 1 /4 S ec. 2 7 , T. 2 N ., R 6 E .)
S i l t s t o n e - b r i c k re d and p u r p le .
96
U t ...
Lim est O1I e - pUr p l e j m o ttle d g r a y , f o s s i l i f e r o u s , few
in te r b e d s o f b r i c k re d s i l t y m u d sto n e.
. . . . . S i l t s t o n e - b r i c k re d and p u r p le , few m udstone and
s a n d s to n e i n t e r b e d s , m o stly c o v e r e d »
......... L im e s to n e - p u r p le , m o ttle d g r a y , m a s s iv e , f o s s i l i f e r o u s .
Upper Member K ibbey F o rm a tio n :
SSt . . . . S a n d s to n e -w h ite -c re a m , f i n e g r a in e d a t b a s e and to p ,
medium g r a in e d in m id d le , p o o rly p r e s e r v e d c r o s s b e d s ,
m te r b e d d e d s i l t s t o n e «
.......... M u d sto n e -p u rp le , p re d o m in a n tly c o v e re d .
......... S i l t s t o n e - r e d , p re d o m in a n tly c o v e re d .
Lower Member K ibbey F o rm a tio n :
2 0 ' . . . . DoI o s t o n e - p i n k - g r a y , m o ttle d g r e e n , s i l t y , b r e c c i a a t
base.
D o lo s to n e - v a n o u s p a s t e l c o l o r s , p re d o m in a n tly y e llo w and
, s i l t y , wavwey la m in a tio n s , b r e c c ia zones
1 8 2 '. .T o ta l Big Snowy Group
M issio n Canyon F o rm atio n ( p a r t ) :
Lim estone-m edium g r a y , m a s s iv e , f o s s i l i f e r o u s .
BIGHORN LAKE SECTION
(On r id g e j u s t s o u th o f B ig h o rn L ak e,
SW 1 /4 S ec. 1 5 , T. 2 N ., R. 6 E .)
Amsden F o rm a tio n ( p a r t ) :
. . . . . S i l t s t o n e - r e d , p u r p le , d o lo m iti c , w ith in te r b e d d e d
lim e s to n e a s b elo w , b r i c k r e d s i l t y m u d sto n e, and s i l t y ,
p u r p le , d o lo s to n e .
..........-L im e s to n e -p u rp le , s i l t y , m a s s iv e , f o s s i l i f e r o u s .
......... C o v e re d -re d -p in k s o i l w ith p u r p le lim e s to n e d e b r i s .
Lombard F a c i e s :
20'
■■7 ; Lr ! ^ n e " J lg h t 8 r a y ’ c o a r s le y c r y s t a l l i n e , medium and
t h i c k b ed d ed , c h e r t y , f o s s i l i f e r o u s , few s o l i t a r y c o r a l s a t
97
to p .
3 0 ' .............. Lim estone-m edium g r a y , l i g h t g ra y a t b a s e , t h i n and
medium bedded a s ab o v e.
1 0 ' .............. Lim estone-m edium and d a rk g r a y , s i l t y , f o s s i l i f e r o u s ,
beds o f b io - p a c k s to n e , l a t e C h e s te r ia n CONODONTS from b a s e .
4 ' .............. C o v e re d -p in k s o i l .
2 ' .............. L im e s to n e - p u r p le ,p in k m o t tle d , and y e l lo w - t a n , d o lo m itic ,
v e ry f i n e c r y s t a l l i n e .
Upper Member K ibbey F o rm a tio n :
6 ' .............. S i l t s t o n e - p u r p l e , 2 f o o t m udstone a t b a s e .
1 6 '................ S a n d s to n e -p a le o r a n g e - r e d and p in k , d o lo m iti c , few t h i n
beds o f d o lo s to n e , f i n e g r a in e d , w e ll s o r t e d , medium b ed d ed .
2 ' ...............M u d sto n e -re d .
3 .............. S a n d s to n e -w h ite -v e ry p a le g r e e n , c l e a n , m a s s iv e .
8 ' .............. S a n d s to n e -a s below g r a d in g up i n t o r i p p l e d la m in a tio n s ,
p a r t i n g s 2 in c h e s a p a r t .
1 0 '.............. S a n d s to n e -w h ite , re d c h e r t g r a in s v i s i b l e , f i n e g r a in e d ,
c a l c a r e o u s , I in c h h ig h r i p p l e s on 5 in c h h ig h tro u g h c ro s s
beds.
6 .............. M u d s to n e -p u rp le , cream m o t tle d , d o lo m itic s i l t s t o n e a t
to p .
^ .............. S a n d s to n e - p in k -y e llo w , medium g r a in e d , m o d e ra te s o r t i n g ,
c le a n , 4 in c h b ed s w ith p la n a r la m in a tio n s .
8 ' ................S i l t s t o n e - p u r p l e , cream m o t tle d , d o lo m iti c , s i l t y
d o lo s to n e in p a r t , p u r p le m udstone a t b a s e .
8 *.............. S a n d s to n e - p u r p le , g r a y , and t a n , medium g r a in e d ,
m o d e ra te ly s o r t e d , 3 f o o t m a ss iv e le n s a t to p w ith tro u g h
c ro ss b ed s.
4 ' .............. M u d sto n e-d ark r e d , m in o r p u r p le .
3 ................ S a n d s to n e -p u rp le m o ttle d g r e e n , v e ry f i n e g r a in e d ,
m a s s iv e , d o lo m iti c .
Lower Member K ibbey F o rm a tio n :
2^ .............. S an d sto n e —s i l t s t o n e and m u d s to n e -re d and b r i c k r e d .
98
upw ard f i n i n g l e n s e s , l i t h o c l a s t c o n g lo m e ra te a t b a s e on sh a rp
lo w er c o n t a c t , few p a le p a s t e l d o lo s to n e b e d s .
1 0 '.......... DoI o s t o n e - u p p e r p u r p l e , lo w e r c re a m -b u ff m o ttle d re d and
p in k , sm ooth, con cav e w e a th e re d s u r f a c e s , t h i n i r r e g u l a r
la m in a tio n s a t b a s e .
1 0 ' . . ..........C o v ered -p in k -b ro w n s o i l .
1 9 8 ' . . T o ta l Big Showy G roup.
M issio n Canyon F o rm a tio n ( p a r t ) :
4 ' . . . . . . .L im e s t o n e - li g h t g r a y , m o ttle d y e llo w , medium to f i n e l y
c r y s t a l l i n e , somewhat f o s s i l i f e r o u s .
8 ' . . . . . . .C o v ered -b ro w n s o i l .
4 0 ' .............. L im estone-m edium g r a y , b a s a l y e llo w - ta n to g r a y .
g e n e r a l l y a s a b o v e, 12 f e e t b r e c c i a in m id d le , lo w er p in k
s i l t y lim e s to n e .
NORTHWEST BIGHORN LAKE SECTION
(On r i d g e j u s t NW o f B ig h o rn Lake,
NW 1 /4 S ec. 10, T. 2 N ., R. 6 E . )
Amsden F o rm a tio n ( p a r t ) :
1 4 '......... Sa n d s t o n e - l i g h t g r a y - w h i t e , v e ry f i n e g r a in e d , t h i n
b e d d ed , c a l c a r e o u s , s i l t y a t b a s e .
2 8 '.......... L i m e s t o n e - d a r k g r a y , la v e n d e r , n o d u la r w e a th e r in g , w ith
in te r b e d d e d s i l t s t o n e , p u r p le , p in k , muddy, p a r t l y c o v e re d .
C o v e re d -o ra n g e -re d s o i l .
8 ' .............. L im esto n e B r e c c ia - g r a y .
1 2 ' .............. C o v e re d -o ra n g e -re d s o i l .
3 0 ' .............. L im e s to n e -g ra y , la v e n d e r , d o lo m itic .
C o v e re d -p in k s o i l a t to p , r e d s o i l a t b a s e .
Lombard F a c i e s :
3 6 ' ............L im e s to n e - d a r k g r a y , m in o r p in k - y e llo w , f o s s i l i f e r o u s ,
s o l i t a r y c o r a l s in m id d le , c h e r t y .
1 6 ' .............. L im e s to n e -a s a b o v e, v e ry f o s s i l i f e r o u s
99
Upper Member K ibbey F o rm a tio n :
14'
10'
4'
10'
S i l t s t o n e —a s belo w , w ith in te r b e d d e d s a n d s to n e and
d o lo s to n e .
S a n d s to n e -p u rp le and g re e n m o ttle d , v e ry f i n e g r a in e d ,
medium b e d d e d , s i l t y .
......... M u d sto n e -o ra n g e , g ra d in g up i n t o s a n d s to n e , r e d .
.......... S an d sto n e —p in k a t b a s e , w h ite a t to p , medium and f in e
g r a in e d t h i n bedded d o lo m iti c .
NOTErFold o r f a u l t in s e c t i o n , m easurem ents below a r e
q u e s ti o n a b le .
26'
......... S a n d s to n e - d o lo m itie , m in o r s i l t s t o n e , f a i n t c r o s s beds
Lower Member K ibbey F o rm a tio n :
50'
. . — S i l ts to n e - m a r o o n - r e d and o r a n g e , d o lo m iti c , w ith
in te r b e d d e d d o lo s to n e and s a n d s to n e .
38'
.......... C o v e re d -b ric k re d s i l t s t o n e d e b r i s .
T o ta l Big Snowy Group
M issio n Canyon F o rm atio n ( p a r t ) :
.........."Lim estone B r e c c ia —m aroon, s i l t y .
......... C o v ered .
L im e s to n e -v e ry d a rk g r a y , m a s s iv e .
1 4 '.............. C o v ered .
^
.............. D o lo s to n e - o liv e , g r a y , g r a n u l a r .
3 0 ' ----- . . . C o v e r e d - p a r t i a l e x p o su re s o f s i l t y g r a y - la v e n d e r
lim e s to n e .
SOUTH ANGLER SECTION
(On r id g e j u s t s o u th o f A n g ler L ake,
S ec. 1 5 , T. 2 N ., R. 6 E . )
Amsden F o rm a tio n ( p a r t ) :
.......... C overed-m aroon s i l t s t o n e d e b r i s , g r a s s y .
100
.............. C o v e re d -d a rk brown s o i l , r e d in p a r t , g r a s s y .
Lombard F a c i e s :
30 . . . . . . . L im e s to n e -d a rk and medium g r a y , b ro w n -o ran g e in p a r t ,
f i n e to medium c r y s t a l l i n e , t h i c k and medium b e d d e d , s p a r s e
f o s s i l s , s o l i t a r y c o r a l s in to p 9 f e e t .
2 2 ' .............. L im e s to n e -d a rk g r a y , g ray -b ro w n a t to p , f i n e l y
c r y s t a l l i n e d e n s e , m a s s iv e , f o s s i l i f e r o u s , w ack esto n e and
p a c k s to n e i n t e r b e d s , c h e r t y .
1 2 ' .............L im estone-m edium and d a rk g r a y , y e llo w -g ra y w e a th e rin g ,
m a ss iv e w ith few s i l t y , muddy b e d s , f o s s i l i f e r o u s , b a s a l
. b io -w a c k e sto n e and p a c k s to n e b e d .
8 ' .............. M u d sto n e-b lac k and g r a y , re d a t b a s e , s i l t y , p a r t l y
c o v e re d .
Upper Member K ibbey F o rm a tio n :
4 4 ' .............. D o lp s to n e -u p p e r la v e n d e r , m id d le p in k m o ttle d p a le g r e e n ,
low er b r i c k r e d , s i l t y a t b a s e , t h i n l y la m in a te d in p a r t .
4 * ...............S i l t s t o n e - r e d - p u r p l e , muddy.
2 4 ' ...............D o lo s to n e -v a r io u s p a s t e l c o l o r s , p re d o m in a n a tIy
o ra n g e - y e llo w , b r i c k re d a t b a s e , s i l t y a t b a s e , t h i n
la m in a tio n s , i r r e g u l a r lumpy b e d d in g .
1 4 '.............. S i l t s t o n e - b r i c k r e d , d o lo m iti c , w ith s i l t y d o lo s to n e .
4 6 ' . . . . . . . S a n d s to n e - r e d , m in o r o r a n g e , y e llo w , p in k , f i n e g r a in e d ,
in te r b e d d e d b r i c k r e d s i l t s t o n e .
1 0 '.............. S i l t s t o n e - b r i c k re d and p u r p le , sa n d y , w ith s i l t y
s a n d s to n e .
4 0 ' .............. S a n d s to n e - b r ic k r e d , f i n e and v e ry f i n e g r a in e d , s i l t y a t
base.
Lower Member K ibbey F o rm a tio n :
1 0 ' .............. M u d sto n e -b ric k r e d , s i l t y , w ith s i l t s t o n e , m o s tly
c o v e re d .
10 ' ..........„ . S a n d s to n e -y e llo w , p in k , m in o r r e d - o r a n g e , f i n e and medium
g r a in e d , m o d e ra te s o r t i n g , d o lo m itic muddy s i l t s t o n e in
m id d le .
101
3 0 ' .............. S i l t s t o n e - r e d , yelow -brow n a t b a s e , d o l o m i t i c , few
b r e c c ia z o n e s , 5 sandy u n i t s .
1 0 '.............. S i l t s to n e -y e llo w -b ro w n , a s a b o v e .
3 6 ' .............. DoI o s to n e -m a ro o n , p in k , t a n , m inor p u r p le m o ttle d g r e e n ,
la m in a te d , s i l t y , muddy, few t h i n s i l t s t o n e b e d s .
4 ' ............. S i l t s t o n e —m aroon, d o lo m itic , 6 f e e t o f r e l i e f on M issio n
Canyon c o n t a c t , l i t h o c l a s t c o n g lo m e ra te in low a r e a s .
3 5 8 ' . . T o t a l B ig Snowy Group
M issio n Canyon F o rm a tio n ( p a r t ) :
Lim estone-m edium g ra y -b ro w n , m a ss iv e and t h i c k b edded,
b r e c c i a t e d in p a r t .
NORTH ANGLER SECTION
(On r id g e j u s t n o r th o f A n g ler L ak e,
SW 1 /4 S ec. 1 0 , T. 2 N ., R. 6 E .)
Amsden F o rm a tio n ( p a r t ) :
2 6 '.............. S i l t s t o n e - p u r p l e , m aroon, b r i c k r e d , v e ry f i n e sa n d s to n e
in p a r t , p a r t i a l l y c o v e re d .
1 0 '.............. L im e s to n e -p a le b ro w n -g ra y , t h i n lumpy b e d d in g ,
f o s s i lif e r o u s , c h e rty .
1 8 '.............. Lim estone-m edium g r a y - la v e n d e r , f o s s i l i f e r o u s , w ith
in te r b e d d e d p u r p le - r e d s i l t y m u d sto n e , p a r t l y c o v e re d .
6 ' .............. L im e s to n e -y e llo w -g ra y , sa n d y .
3 ' .............. C a l c i t e D u r a c r u s t.
Lombard F a c i e s :
4 ' .............. L im e s to n e -a s lim e s to n e b elo w .
6 ' .............. D o lo s to n e - p a le p u r p le - g r a y , lim e s to n e in p a r t , p a r t l y
c o v e re d .
2 4 '.............. L im e s to n e -p a le g r a y , brow n, la v e n d e r , w e ll d ev elo p ed t h i n
102
and medium b e d d in g , f o s s i l i f e r o u s , c h e r t y .
1 6 '.............. L im e s to n e -a s a b o v e , s i l t y and th i c k bedded a t b a s e .
1 0 '............ L im e s to n e - la v e n d e r , f i n e l y c r y s t a l l i n e , medium and th i c k
b e d d in g , c h e r t y , t h i n s i l t y s h a le i n t e r b e d s .
6 ' ............. L im e s to n e -g ra y , y e llo w w e a th e r in g , s i l t y , v e ry
f o s s i l i f e r o u s , t h i n b ed d ed , l a t e C h e s te ria n CONODONTS from
m id d le .
3 ' ............. L im e s to n e -g ra y ,
t h i c k b ed d ed .
4 ' ..............Lim estone-m edium b ro w n -g ra y , i r r e g u l a r and p la te y
b e d d in g , f o s s i l i f e r o u s .
3 ' ..............L im e s to n e -g ra y ,
t h i c k b ed d ed .
8 ' ..............C overed-brow n s o i l .
Upper Member K ibbey F o rm a tio n :
8 T..............D o lo s to n e - l i g h t y e llo w and p a le m aroon, l a t e r a l l y
d is c o n tin u o u s , wavey la m in a tio n s , s i l t y , s a n d y .
6 ' ..............C o v ered - p in k - o r a n g e s o i l .
3 0 ' ..............S i l t s t o n e - b r i c k r e d , p u r p l e , p in k , o r a n g e , w ith
in te r b e d d e d m udstone and s a n d s to n e .
2 0 ' ..............S a n d s to n e -y e llo w -o ra n g e , p in k , f i n e g r a in e d , t h i n to
medium b e d d ed , I 1 /2 in c h h ig h c r o s s - b e d s , 15 d e g re e maximum
d ip .
9 ' .............. S a n d s to n e -lo w e r r e d , u p p e r w h ite , le n s 150 f e e t w id e,
medium g r a in e d , 25 d e g re e s s o u th d ip p in g c r o s s s t r a t a a t b a s e ,
p la n a r la m in a te d a t to p , le n s e n c lo s e d by s i m i l a r f i n e g r a in e d
s a n d s to n e .
2 8 '.............. S a n d s to n e - b r ic k r e d , m a s s iv e , f i n e g r a in e d , tr o u g h
c r o s s - b e d s w ith r i p p l e c r o s s s t r a t a .
1 4 1. . . . . . . S i l t s t o n e - b r i c k r e d , muddy a t b a s e , sandy a t to p , .
d o lo m itic .
Lower Member K ibbey F o rm a tio n :
103
7 0 '.............. D o lo s to n e - I a v e n d e r and m aroon, s i l t y , muddy, b r e c c ia
z o n e s , la m in a te d z o n e s , in te r b e d d e d s i l t s t o n e .
1 0 '.............. D o lo s to n e - g r a y , y e llo w w e a th e r in g , la m in a te d , b a s a l 4
in c h e s b u rro w e d .
2 7 9 ' . . T o ta l Big Snowy Group
M issio n Canyon F o rm a tio n ( p a r t ) :
Lim estone-m edium g ra y -b ro w n , w e a th e rs g r a y , c o a r s e ly
c r y s t a l l i n e , f o s s i l f r a g m e n ts .
HORSE MOUNTAIN SECTION
(SN 1 /4 S e c . 18 and NW 1 /4 S ec. 1 9 , T. 3 N ., R. 6 E .)
Amsden F o rm a tio n ( p a r t ) :
5 0 ' .............. DoI o s t o n e - p i n k - r e d , v a r i a b l e b e d d in g .
3 0 ' .............. L im e s to n e -p in k and p u r p le , g r a n u la r a t to p , d en se a t
b a s e , s a n d y , s i l t y , e a r l y Morrow CONODONTS from to p .
4 ' .............. L im e s to n e - la v e n d e r , m o ttle d p a le g e r e e n , f o s s l i f e r o u s ,
e a r l y Morrow CONODONTS from b a s e .
Upper Member K ibbey F o rm a tio n :
3 0 '.............. Sands to n e - p u r p I e w ith i n t e n s e g re e n la m in a tio n s , f i n e and
medium g r a in e d , t h i c k b ed d ed , r e d m udstone in te r b e d d e d ,
c ro s s -b e d d e d a t b a s e .
1 2 '.............. S a n d s to n e - r e d - p u r p le , y e llo w ns w h ite a t to p , f i n e and
v e ry f i n e g r a in e d , t h i n b e d d ed , f la g g y a t b a s e .
2 4 '............... C o v e re d -re d s o i l .
2 2 ' ............... C overed-brow n s o i l .
Lower Member K ibbey F o rm a tio n :
4 ' ................. .D o lo s to n e - b r ic k r e d , s i l t y .
2 0 '............... C o v e re d -d a rk brown s o i l .
4 ' ............... M u d sto n e -re d , b r e c c i a t e d .
1 4 '............... B r e c c ia - y e llo w , re d p u r p l e , e q u a l am ounts o f d o lo s to n e ,
lim e s to n e , and s a n d s to n e c l a s t s up to 18 in c h e s i n d ia m e te r .
104
^ .............. D o lo s to n e - r e d , w ith m in o r re d s a n d s to n e .
13 2 ' . .T o ta l B ig Snowy Group
M issio n Canyon F o rm a tio n ( p a r t ) :
6 ' .............. L im e sto n e - l i g h t g r a y , f i n e l y c r y s t a l l i n e , medium b ed d ed .
4 ' ..............D o lo s to n e -b ro w n -g ra y , l a t e r a l l y d is c o n tin u o u s .
3 .............. L im e s to n e -re d , p u r p le , b r e c c i a a t to p .
Limes t o n e -g ra y -b ro w n , w e a th e rs g r a y , m a s s iv e , some
fo s s ils .
MIDDLE FORK SIXTEENMILE CREEK SECTION
(On s lo p e j u s t n o r th o f ro a d th ro u g h can y o n ,
S ec. 1 1 , T. 4 N ., R. 6 E . )
Amsden F o rm a tio n ( p a r t ) :
8 ' .............. C overed-brow n s o i l .
1 4 '.............. S a n d s to n e - w h ite - b u f f , v e ry f i n e g r a in e d , d o lo m iti c ,
s m a ll, b la c k c h e r t s t r i n g e r .
Lombard F a c i e s :
^ .............. L im e s to n e - p in k - o r a n g e , medium c r y s t a l l i n e , f o s s i l i f e r o u s ,
b io - p a c k s to n e , and g r a i n s t o n e , l a t e C h e s te ria n CONODONTS.
3 ' .............. L im e s to n e - o liv e , d a rk g ra y w e a th e r in g , b ra c h io p o d
g r a in s t o n e a t to p , c r i n o i d g r a in s t o n e a t b a s e .
2 ' .............. M u d sto n e -re d , y e llo w in p a r t , w h ite s p o tte d c a l c i t e
(re p la c in g e v a p o r ite ? ) .
2 ' .............. Lim estone-m edium g r a y - o l i v e , b ra c h io p o d wa ek es to n e j l a t e
C h e s te ria n CONODONTS.
4 ' .............. M u d sto n e -re d , w h ite s p o tte d in p a r t as a b o v e .
2 ' ..............L im e s to n e -v e ry sa n d y , o i l s t a i n in v u g s.
Upper Member K ibbey F o rm a tio n :
8 ' .............. S an d sto n e—w h ite w ith o ra n g e s p o t s , f i n e and v e r y f i n e
g r a in e d , d o lo m iti c .
2 ' .............. M u d sto n e -re d .
5 ' . . . . . . . S a n d s to n e -a s s a n d s to n e a b o v e , p a r t l y c o v e re d .
105
4 ' ............ M u d sto n e -re d .
5 ' ............. S a n d sto n e-m aro o n , f i n e g r a in e d .
5 .............. S i l t s t o n e —p in k and m aroon, p re d o m in a n tly c o v e re d .
4 ’ ..............S a n d s to n e -o ra n g e m o ttle d p in k , d o l o m i t i c .
1 2 '............M u d sto n e -re d , m in o r g r e e n .
6
S a n d s to n e - r e d , p in k , f i n e g r a in e d , re d mud c h i p s , c u r r e n t
rip p le s .
3 ’ ............. S i l t s t o n e - r e d , muddy a t b a s e , sandy a t to p .
9 ............M u d sto n e -re d , s i l t y a t to p , s h a rp b ase on s a n d s to n e
belo w .
8 ' ............. S a n d s to n e -a s s a n d s to n e a b o v e .
5 ’ ............. S i l t s t o n e - r e d , m aroon, m o ttle d g r e e n .
8 ' ............. M u d sto n e -g re e n , r e d , p re d o m in a n tly c o v e re d .
1 2 '............M udstone-m aroon.
6 ' ............M u d sto n e -g re e n .
Lower Member K ibbey F o rm a tio n :
............ C o v ered - m udstone d e b r is - o r a n g e - y e llo w , p in k , m a rro n ,
g re e n .
.............. M udstone in te r b e d d e d w ith s i l t s t o n e —la v e n d e r , m in o r
g re e n .
^ 2 '............D o lo sto n e -b ro w n , w h ite w e a th e r in g , b r e c c i a t e d in p a r t .
1 6 '..............M u d s to n e -ta n -p in k , b r e c c ia in p a r t ,
s i l t s t o n e p re d o m in a n a tIy c o v e re d .
gig;
m in o r p in k -b ro w n
T o ta l Big Snowy Group
M issio n Canyon F o rm atio n ( p a r t ) :
............ L im e s to n e -p a le y e llo w , d a rk g ray -b ro w n a t to p , medium
b e d d ed , t h i n l y bedded and s i l t y a t b a s e .
Lim estone-m edium g ra y -b ro w n , m a s s iv e .
AfJ78
<5 9 * 4
Plate I
Lombard Arch
(north)
Lithofocies Key =
Lombard Facies
L3
Lz
L1
Lime Mudstone Lithofacies
Ume Wackestone- Packstone Lithofacies
Shale Lithofacies
Upper Member Kibbey Formation
UKg Mudstone - Dolostone Lilhofacies
UK| Sandstone Lithofocies
Lower Member Kibbey Formation
LKg Laminated Dolostone Lithofacies
LK Sandstone Channel Lithofocies
Contacts =
Formation
Lithofacies
Big Snowy - Amsden
IOO Feet
Unconformity
1
50 Meters
2 Kilometer
c e p - a.
ILIfSSSZLSiTf
Wyoming S he lf
( so uth )
MONTANA STATE UNIVERSITY LIBRARIES
762 1001 4 1 5 8 7
I