The transition from the Judith River Formation to the Bearpaw Shale (Campanian), north-central
Montana
by Roger Elmer Braun
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Earth Sciences
Montana State University
© Copyright by Roger Elmer Braun (1983)
Abstract:
The upper 15 m of the Judith River Formation on and adjacent to the Fort Belknap Indian Reservation,
north-central Montana is composed mostly of overbank mudrock, siltstone, fine-grained sandstone, and
coal, with some cross-stratified channel sandstone in the lower part. The lower 15 m of the overlying
Bearpaw Shale is a transgressive deposit composed primarily of concretionary silty shale with some
clayey, silty sandstone zones and one bentonite bed. The source area for both formations was primarily
in western Montana and Idaho, with the Elkhorn Mountains volcanics a major source of debris.
The contact between the Judith River and Bearpaw formations is abrupt and lacks a transgressive
sandstone facies. The transgression of the Bearpaw sea across the study area is considered to have been
a nearly isochronous event because of the nature of the transition, small east-west differences in
thickness between marker horizons, and similar elevations of the contact from east to west across
undeformed parts of the study area.
The Bearpaw transgression was caused mainly by tectonic thickening in the western Cordillera, which
created subsidence primarily in the western and central portions of the Western Interior basin. The
transgression was a nearly isochronous event that took place approximately 72 m.y. ago according to
radiometric age dates on bentonite beds. Discrepancies between these radiometric dates and faunal
zonation that implies a diachronous transgression can be explained by paleogeography and salinity
stratification of the Bearpaw sea before and at the beginning of the transgression. The marine nektonic
organisms used in the faunal zonation studies are thought to have been excluded from the initial
transgressive pulse because of low salinity water in the surface layer of the sea. The sea probably
invaded subsided areas through inlets that formed around topographic highs, which are major Judith
River Formation deltaic complexes in central Montana and southern Alberta and Saskatchewan.
Nektonic fauna probably did not occupy the newly subsided basin until well after the entire region was
inundated, thus giving the impression of a diachronous transgression throughout the region. THE TRANSITION FROM.THE JUDITH RIVER FORMATION
TO THE BEARPAW SHALE (CAMPANIAN),
NORTH-CENTRAL MONTANA
by
Roger Elmer Braun
A th e s is subm itted in p a r t i a l f u l f i l l m e n t
o f th e req u irem en ts f o r th e degree
Of
M aster o f Science
in
E arth Sciences
MONTANA STATE UNIVERSITY
Bozeman, Montana
March 1983
M AIN
LIB.
H372
B 73 L 7
6 o p .3
ii
APPROVAL
o f a th e s is subm itted by
Roger Elmer Braun
T h is th e s is has been read by each member o f the th e s is committee
and has been found to be s a t is f a c t o r y re g a rd in g c o n te n t, E n g lis h usage,
fo rm a t, c i t a t i o n s , b ib lio g r a p h ic s t y l e , and c o n s is te n c y , and is ready
f o r submission to th e C o lle g e o f G raduate S tu d ie s .
Date
Approved f o r th e M ajo r Department
Date
Approved f o r th e C o lle g e o f Graduate S tu d ie s
7Date
it
-?3
Graduate Dean
iii
STATEMENT OF PERMISSION TO USE
In p re s e n tin g t h i s th e s is in p a r t i a l
f u l f i l l m e n t o f th e requirem ents
f o r a m a s te r's degree a t Montana S ta te U n iv e r s it y , I agree t h a t the
L ib r a r y s h a ll make i t a v a ila b le to borrow ers under ru le s o f th e L ib r a r y .
B r i e f q u o ta tio n s from t h i s th e s is a re a llo w a b le w ith o u t s p e c ia l p e r­
m is s io n , p ro vid ed t h a t a c c u ra te acknowledgment o f source is made.
Perm ission f o r e x te n s iv e q u o ta tio n from o r re p ro d u c tio n o f th is
th e s is may be g ra n te d by my m ajor p ro fe s s o r, o r in h is /h e r absence,
by th e D ir e c t o r o f L ib r a r ie s when, in th e o p in io n o f e i t h e r , th e pro­
posed use o f th e m a te ria l
use o f th e m a te ria l
is f o r s c h o la r ly purposes.
in t h i s th e s is f o r f in a n c ia l gain s h a ll n o t be
a llo w e d w ith o u t my w r it t e n p e rm is s io n .
S ig n a tu re
Date
Any copying o r
V
ACKNOWLEDGMENTS
G rea t a p p re c ia tio n is extended to D r. Donald L. Smith (Committee
C h a irm an ), D r. Stephan G. C u s te r, and D r. Robert A. Chadwick o f th e
Reading Com m ittee, who p ro vid ed many s u g g e s tio n s , much g u id a n c e ^ and
s e v e ra l c r i t i c a l
review s o f th e t h e s is .
A p p re c ia tio n is a ls o extended to D r. J . P. Wehrenberg f o r sug­
g e s tin g th e p r o je c t to th e a u th o r, and to th e F o rt Belknap In d ia n
R e s e rv a tio n T r ib a l
Council f o r funding and c o o p e ra tio n d u rin g f i e l d
w ork.
S p ec ia l thanks to my w i f e , M ary, f o r her u n d e rs ta n d in g , p a tie n c e ,
and su p p o rt d u rin g th e p re p a ra tio n and c om pletion o f th e t h e s is .
Vl
TABLE OF CONTENTS
Page
1.
LIST OF FIGURES.......................................................................................................
2.
ABSTRACT......................................................................................................................
x
3.
INTRODUCTION.............................................................................................................
I
L o c a tio n .................................................................................................................
Purpose...........................................................................
P ro ced u re...........................................................................
General G eology............................................................................................
P revious In v e s t ig a t io n s ...............................................................................
V iii
.
I
2
6
4.
PALEOGEOGRAPHY AND TECTONIC SETTING...........................................
5.
STRATIGRAPHY.............................................................................................................
14
Regional S t r a t i g r a p h y . ..................................................
S tr a tig r a p h y o f th e J u d ith R iver/B earp aw T r a n s it io n ..............
14
16
LITHOLOGY OF THE JUDITH RIVER/BEARPAW TRANSITION..........................
20
In t r o d u c t io n ........................................................................................................
J u d ith R iv e r Mudrock.............................
J u d ith R iv e r Sandstone Beds..................
J u d ith R iv e r Coal and L i g n i t e ..................................
Bearpaw S i l t y S h a le ................................................................
Bearpaw C la y e y , S i l t y Sandstone............................................................
Bearpaw B e n to n ite ...................................................................................
20
20
21
27
28
30
DESCRIPTIONS AND INTERPRETATIONS OF SPECIFIC OUTCROPS..............
31
Three M ile C re ek ................................................................... ........ , ................
Peoples C re ek ............................................................ ...................... — ...........
Cow C re e k ..............................................................................................................
31
34
36
DEPOSITIONAL MODELS............................................................................................
40
Development o f a M odel.......................................................................
Environments o f D e p o s itio n During th e T ra n s g re s s io n ..............
40
44
6.
7.
8.
7
30.
' vi i
TABLE OF CONTENTS— Continued
Page
9.
BEARPAW TRANSGRESSION............................................................ ...................... ..
50
In t e r p r e t a t io n s in th e L i t e r a t u r e .............................................. ..
Rate o f T ra n s g re s s io n ..........................................................................
Causes o f th e Bearpaw T ra n s g re s s io n .............. ...........................
Evidence f o r a Rapid T ran sg re s sio n in th e Study A re a ..........
50
. 50
51
56
1 0.
CONCLUSIONS............................ ............................................. .................................
59
1 1.
REFERENCES CITED.................................................................................................
61
v iii
LIST OF FIGURES
■
F ig u re
Page
1.
G e n e ra liz e d g e o lo g ic map o f th e study a r e a ................................ ..
3
2.
S t r a t ig r a p h ic c o r r e la t io n c h a rt o f Upper Cretaceous rocks
in Montana anda d ja c e n t a r e a ......................................................................
5
3.
N orth American p a leogepgraphic map during, th e C re ta c e o u s ..
8
4.
Schematic re c o n s tru c tio n showing m ig ra tio n o f th e Rocky
Mountain fo re d e ep from th e J u ra s s ic to th e T e r t i a r y ..............
9
Schem atic e a s t-w e s t cross s e c tio n from n orthw est Montana
to n o rth w est Minnesota showing th e geometry o f th e West­
ern I n t e r i o r b asin and g eneral s t r a t ig r a p h ic r e l a t i o n ­
s h ip s ........................................................................................................................
11
P aleogeographic map showing s h o re lin e and p o s itio n s o f 5
d e l t a i c complexes j u s t p r io r to th e Bearpaw tra n s g re s s io n .
12
Diagram m atic e a s t-w e s t cross s e c tio n through n o rth e rn
Montana showing th e g eneral r e la tio n s h ip s o f th e Montana
Group.......................................................................................................
15
Composite s t r a t ig r a p h ic s e c tio n o f th e upper 15 m o f the
J u d ith R iv e r Form ation and th e lo w er 15 m o f th e Bearpaw
Shale in th e study a r e a ..................................................................................
17
Photograph o f la r g e -s c a le omikron c r o s s - s t r a t i f i c a t i o n
t h a t occurs in th e t h i c k e r , lo w er sandstone beds in th e
upper 15 m o f th e J u d ith R iv e r Form ation in th e study
a r e a ...........................................................................................................................
22
C u rre n t rose diagram showing azim uths o f fo re s e ts o f 74
c r o s s - s t r a t i f i e d sandstone beds in th e upper p a r t o f th e
J u d ith R iv e r Form ation in th e study a r e a .......................................
23
Map o f th e study a re a showing lo c a tio n s o f v a rio u s o u t­
crops r e f e r r e d to in th e t e x t ............................................
25
Photograph o f c lim b in g r ip p le s o f r i p p l e - d r i f t c ro s s ­
la m in a tio n from near th e top o f th e J u d ith R iv e r Formar
t io n in th e study a r e a ..........................L..............................................
26
5.
6.
7.
8.
9.
10.
1 1.
1 2.
IX
LIST OF FIGURES-C o n tin u e d
F ig u re
1 3.
Page
Photograph o f in s it u s i l ic if i.e d t r e e stump in l i g n i t e
a t l o c a l i t y "A" in th e upper p a r t o f th e J u d ith R iv e r
Form ation in th e study a r e a .............................................................. ....
29
L ith o lo g ic log and d e s c r ip tio n o f th e Three M ile Creek
e x p o s u r e . . . . ........................................................................................................
32
L ith o lo g ic log and d e s c r ip tio n o f th e Peoples Creek
e xp o s u re .....................................................................
35
L ith o lo g ic lo g and d e s c r ip tio n o f th e Cow Creek
e xp o s u re ................................................................
37
1 7.
Id e a liz e d f in in g upward sequence o f a p o in t bar. d e p o s it . . .
42
1 8.
Diagram m atic o b liq u e view showing a l l u v i a l , d e l t a i c ,
and m arine environm ents a p p ro x im a te ly 7 2 .5 m .y. B .P .................
45
Diagram m atic o b liq u e view showing a l l u v i a l , d e l t a i c ,
and m arine environm ents a p p ro x im a te ly 72 m .y. B .P ......................
46
Diagram m atic o b liq u e view showing a l l u v i a l , d e l t a i c ,
and m arine environm ents a p p ro x im a te ly 7 1 .5 m .y. B .P ..............
47
Diagram m atic o b liq u e view showing a l l u v i a l , d e l t a i c ,
and m arine environm ents a p p ro x im a te ly 70 m .y . B . P . - . ..........
48
P aleogeographic map o f Montana and a d ja c e n t area showing
in f e r r e d p o s itio n o f s h o r e lin e , m ajor d e lt a ic com plexes,
and d ir e c t io n o f sedim ent t r a n s p o r t ...................................................
53
Map and cross s e c tio n showing m ajor J u d ith R iv e r d e l t a i c
complexes and c ir c u l a t i o n o f w a te r d u rin g i n i t i a l B e a rpaw tra n s g re s s io n near study a r e a ........................................................
55
14.
1 5.
1 6.
19.
20.
2 1.
22.
23.
X
ABSTRACT
The upper 15 m o f th e J u d ith R iv e r Form ation on and a d ja c e n t to
th e F o rt Belknap In d ia n R e s e rv a tio n , n o r th -c e n tr a l Montana is composed
m o s tly o f overbank mudrock, s i l t s t o n e , f in e -g r a in e d sandston e, and c o a l,
w ith some c r o s s - s t r a t i f i e d channel sandstone in th e lo w er p a r t .
The
lo w er 15 m o f th e o v e rly in g Bearpaw S hale is a tra n s g re s s iv e d e p o s it com
posed p r im a r ily o f c o n c re tio n a ry s i l t y s h a le w ith some c la y e y , s i l t y
sandstone zones and one b e n to n ite bed.
The source area f o r both
fo rm a tio n s was p r im a r ily in w estern Montana and Id a h o , w ith th e Elkhorn
Mountains v o lc a n ic s a m ajor source o f d e b r is .
The c o n ta c t between th e J u d ith R iv e r and Bearpaw fo rm a tio n s is
a b ru p t and la c k s a tra n s g re s s iv e sandstone f a c ie s .
The tra n s g re s s io n
o f th e Bearpaw sea across th e study a rea is considered to have been a
n e a r ly isochronous e v e n t because o f th e n a tu re o f th e t r a n s i t i o n , small
e a s t-w e s t d iffe r e n c e s in th ic k n e s s between m arker h o riz o n s , and s im ila r
e le v a tio n s o f th e c o n ta c t from e a s t to w est across undeformed p a rts o f
th e study a re a .
The Bearpaw tra n s g re s s io n was caused m a in ly by t e c to n ic th ic k e n in g
in th e w estern C o r d i l l e r a , which c re a te d subsidence p r im a r ily in th e
w estern and c e n tr a l p o rtio n s o f th e W estern I n t e r i o r b a s in . The tr a n s ­
g re s s io n was a n e a r ly isochronous e ve n t t h a t took p la ce a p p ro x im a te ly
72 m .y. ago a ccording to ra d io m e tric age dates on b e n to n ite beds.
D is ­
c rep a n c ies between th ese ra d io m e tric dates and fa u n a l z o n a tio n th a t
im p lie s a diachronous tra n s g re s s io n can be e x p la in e d by paleogeography
and s a l i n i t y s t r a t i f i c a t i o n o f th e Bearpaw sea b e fo re and a t th e begin­
ning o f th e tra n s g re s s io n .
The m arine n e k to n ic organisms used in the
fa u n a l zo n a tio n s tu d ie s a re thoug ht to have been excluded from th e
i n i t i a l tra n s g re s s iv e pu lse because o f low s a l i n i t y w a te r in th e s u r­
fa c e la y e r o f th e sea. The sea p ro b a b ly invaded subsided areas through
i n l e t s t h a t formed around to p o g ra p h ic h ig h s , which a re m ajo r J u d ith
R iv e r Form ation d e l t a i c complexes in c e n tr a l Montana and southern
A lb e r ta and Saskatchewan.
N ektonic fauna p ro b ab ly d id n o t occupy the
newly subsided b asin u n t i l w e ll a f t e r th e e n t i r e re g io n was in u n d a te d ,
thus g iv in g th e im pression o f a diachronous tra n s g re s s io n throughout
th e re g io n .
I
INTRODUCTION
L o c atio n
The study a rea is in n o r th -c e n tr a l Montana and in c lu d e s a l l
of
th e F o rt Belknap In d ia n R e s e rv a tio n as w e ll as an a d ja c e n t a rea to the
w es t.
It
is bounded on th e n o rth by th e M ilk R iv e r , on th e e a s t and
south by th e r e s e r v a tio n boundary, and on th e w est by 109° w est lo n g i­
tu d e .
The a re a l i e s j u s t n o rth o f th e L i t t l e
Rocky Mountains and
a p p ro x im a te ly 30 km e a s t o f th e Bearpaw Mountains in B la in e and P h i ll i p s
c o u n tie s .
Purpose
T h is study focuses on th e s tr a tig r a p h y o f th e upper 15 m o f the
J u d ith R iv e r Form ation and th e lo w er 15 m o f th e Bearpaw S hale and,
s p e c i f i c a l l y , on th e r a t e o f tra n s g re s s io n o f th e Bearpaw sea.
Toward
t h is end, an u nderstand ing o f re g io n a l s t r a t ig r a p h ic r e la t io n s h ip s ,
d e p o s itio n a l e n viro n m e n ts , provenence, and d ir e c tio n o f sedim ent tr a n s ­
p o rt is n ecessary.
Procedure
A fie ld
study was conducted on and a d ja c e n t to th e F o rt Belknap
In d ia n R e s e rv a tio n d u rin g th e summer o f 1981.
F ie ld work was i n i t i a t e d
by f i r s t fin d in g exposures through th e use o f a e r ia l
1\
m inute to p o g ra p h ic maps.
photographs and
Because o f th e f l a t - l y i n g
a t t i t u d e o f th e
2
s t r a t a and th e p ro p e n s ity o f th e J u d ith R iv e r and Bearpaw fo rm a tio n s
to erode and slump, most exposures were found o n ly near a c t iv e , modern
d ra in a g e system s.
F ie ld work c o n s is te d o f measuring s t r a t ig r a p h ic s e c tio n s a t a
t o t a l o f 64 l o c a l i t i e s w ith s te e l tape and Jacob's s t a f f .
in fo rm a tio n recorded a t each l o c a l i t y
in c lu d e s :
(I)
S p e c ific
th ic k n e s s and
geom etry o f beds; (2 ) a zim u th , d ip , and th ic k n e s s o f c r o s s - s t r a t i f i e d
s e ts ;
(3 ) d e s c r ip tio n o f c r o s s - s t r a t i f i c a t i o n ;
(4 ) n o ta tio n o f o th e r
p rim ary sedim entary fe a tu r e s such as graded bedding, f o s s i l s , and
ic h n o fo s s ils ;
in d u r a tio n ;
ta l
(5 ) d ia g e n e tic fe a tu re s such as c o n c re tio n s and degree o f .
(6 ) d e s c r ip tio n o f lit h o l o g y ; and (7 ) v e r t ic a l
v a r ia t io n o f l i t h o l o g i e s .
and h o rizo n ­
Samples were c o lle c te d a t e v e ry change
o f sed im en tary s tr u c tu r e o r lit h o l o g y .
L a b o ra to ry analyses in c lu d e d t h in s e c tio n and b in o c u la r m icro ­
scope exa m in atio n o f s e le c te d samples from v a rio u s l o c a l i t i e s .
S tan­
dard t h in s e c tio n s and rin g -m o u n t t h in s e c tio n s o f p o o rly c o n s o lid a te d
sedim ents were examined under the p e tro g ra p h ic m ircoscope.
P rec ise
d e te rm in a tio n o f m in era lo g y was not an o b je c t iv e o f t h is s tu d y .
However,
an alyses confirm ed more d e ta ile d p e tro g ra p h ic s tu d ie s o f Rubey (1 9 3 0 ),
McLean (1 9 7 1 ), and S c h u ltz and o th e rs
(1 9 8 0 ).
:
General Geology
Bedrock in th e study a rea is p re d o m in an tly o f C retaceous age,
e xc e p t f o r s e v e ra l T e r t i a r y in s tru s io n s in th e southern and w estern
p o rtio n s
( F ig .
I).
The la r g e r in tr u s io n s have t i l t e d
Precam brian rocks around t h e i r p e rim e te rs .
P a le o zo ic and
3
O
109
LEGEND
Pre-Judith River Formation
Intrusive rocks
(-Tertiary)
Bearpaw Shale (Cretaceous)
P
K
F ig u re I .
I Judith River
I Formation (Cretaceous)
f H
l Mesozoic rocks
5555
E=E=Bi Paleozoic
rocks undifferentiated
Precambrian metamorphic rocks
!"''-'.'-,I
G e n e ra lize d g e o lo g ic map o f th e study area (m o d ifie d from
Al v e rs o n , 1965; o th e r data from Ross and o th e r s , 1 9 5 8 ).
4
Upper Cretaceous sedim entary rocks o f th e re g io n a re p a r t o f the
Montana Group ( F ig . 2 ) .
focus o f t h is stu d y:
paw S h a le .
Two u n its near th e to p o f t h is group are the
th e J u d ith R iv e r Form ation and th e o v e rly in g B ear-
The J u d ith R iv e r Form ation is p r im a r ily a fr e s h w a te r , re g re s -
i
s i Ve d e p o s it o f sandstone, s i l t s t o n e , and mudrock .
The Bearpaw is a
tra n s g re s s iv e m arine d e p o s it composed p r im a r ily o f d a rk -c o lo re d s h a le .
Much o f th e study area is covered by a veneer o f a llu v iu m o r
g l a c i o f l u v i a l m a te ria l up to 30 m th ic k
G la c io f lu v ia l
(Hauptmann and Todd, 1 9 5 3 ).
d e p o s its a re u s u a lly a s s o c ia te d w ith uplands and
a llu v iu m w ith modern stream drain ag es as w e ll w ith the. p r e g la c ia l
M is so u ri R iv e r.
The dominant s tr u c tu r a l fe a tu re s in th e area were formed in response
to s tre s s e s t h a t formed th e L i t t l e
d ip s te e p ly o f f th e L i t t l e
Rocky and Bearpaw m ountains.
S tra ta ,
Rocky Mountains as w e ll as o f f a s s o c ia te d
o u tly in g in t r u s iv e bodies ( K n e c h te l, 1 9 5 9 ).
In the n o rth and n o rth ­
w est p a rts o f th e study a r e a , however, sedim entary s t r a t a a re n e a rly
f l a t - l y i n g , u s u a lly d ip p in g le s s than one degree ( A lv e rs o n , 1 9 6 5 ).
■
Two lo w -a n g le t h r u s t - f a u l t system s, th e tre n d s o f which are
m u tu a lly p e rp e n d ic u la r, a re d e p ic te d on maps by Erdmann and Koskinen
(1 9 5 3 ) and A lverson (1 9 6 5 ).
The t h r u s t f a u l t system in th e northw est
p a r t o f th e study a rea has a N40°W tr e n d ; th e system in th e southw estern
c o rn e r tre n d s N30°E.
These are b e lie v e d to be g r a v it y s lid e f a u lt s
The term mudrock, as used h e re , r e fe r s to a n o n f is s ile m ix tu re o f c la y ,
s i l t , and sand p a r t ic le s o f i n d e f i n i t e p ro p o rtio n s .
NW
STAGE
N -C
CRAZY MTS
SE
BIGHORN
MONTANA
BASIN
MONTANA
BASIN
SE
MONTANA
ALBERTA
St Mary
River
Battle Fm
White
Hills
O
t—
Cam p­
D
O
UJ
CC
o
Q
anian
Bearpaw
Bearpaw
Sh
Sh
Ct
Two
Medicine
U l
Q.
CL
CO
C
D
*5
CQ
Virgelle
CD
0.
CO
Santonian
F ig u re 2 .
Ss
Telegraph
Creek
a
D
O
-J
Pakowki
Milk River
CO
C
CO
C
O
Meeteetse
I
Claggett
Eagle
Pierre
Mesa
1
Dlaggett
Sh
O
Verde
CO
C
CO
O
S
5
Sh
O
O
Fm
.5
CO
C
CO
a
D
River
V)
o>
>
Sh
Judith
C
O
-J
O
5
Foremost
paw
E
Judith
River
Fm
>
a
Fm
Oldman
Ss
Ss
LL
O
O
O
oc
Q
D
O
Bearpaw
Sh
Hills
B ear-
O
<
Lennep
X
Ss
Fm
UJ
Fox
C O
L> S
-
Eastend
Ss
ZD
x
X
Mud Fm
Horsethief
CO
Lennep
Fox
S
Fm
Sh
Eagle
Fm
Virgelle
Telegraph
Creek
Virgelle
Telegraph
Creek
V
\
- A
x "
Cody
Sh
S tr a tig r a p h ic c o r r e la t io n c h a rt o f Upper Cretaceous rocks in Montana and
a d ja c e n t areas (fro m B a ls t e r , 1 9 8 0 ).
Shaded area is in te r v a l s tu d ie d .
6
a s s o c ia te d w ith th e Bearpaw Mountains (R eeves, 1 9 4 6 ).
Numerous normal
f a u l t s a re a ss o c ia te d , w ith domal fe a tu re s near the L i t t l e
(Al v ers o n , 1 9 6 5 ).
■ .
Rocky Mountains
.
Previous In v e s tig a tio n s
S tr a tig r a p h y o f Upper Cretaceous rocks has been th e s u b je c t o f
numerous d e t a ile d s tu d ie s through out th e Western I n t e r i o r .
The s tu d ie s
m entioned here a re those considered most p e r tin e n t to t h is study and a re
o n ly a f r a c t io n o f th e t o t a l
p u b lish e d m a t e r ia l.
E a rly d e s c rip tio n s and mapping o f th e J u d ith R iv e r and Bearpaw f o r ­
m ations were done by Meek and Hayden (1 8 5 6 ), H atcher and S tanton (1 9 0 3 ),
Pepperberg (1 9 0 8 , 1 9 1 0 ), and Bowen (1 9 1 2 , 1 9 1 5 ).
The s tr a tig r a p h y and
b io s tr a t ig r a p h y o f Upper. Cretaceous rocks in Montana and a d ja c e n t areas
were s tu d ie d by Cobban (T 9 5 5 ), Reeside (1 9 5 7 ), G i l l and Cobban (1 9 6 6 ,
1 9 7 3 ), and Tschudy (1 9 7 3 ).
G eologic maps were produced on and a d ja c e n t
th e study a rea by K n e c h te l.( 1 9 5 9 ) , Hearn and o th e rs
o th e rs (1 9 6 4 ), and A lverson (1 9 6 5 ).
(1 9 6 4 ), Schmidt and
S tu d ie s th a t focused on th e tr a n s ­
g re s s io n o f th e Bearpaw sea a re those o f McLean (1 9 7 1 ), G i l l
and Cobban
(1 9 7 3 ), and Lorenz (1 9 8 1 ). .
Rocks o f t h i s a rea d ep o s ited d u rin g th e Campanian Stage o f th e Upper
C retaceous have been s tu d ie d from many vantage p o in ts and a t numerous
s c a le s .
It
is g e n e r a lly agreed th a t th e J u d ith R iv e r Form ation was
d e p o s ited in nonmarine environm ents by streams th a t flow ed from the
C o r d ille r a n h ig h la n d eastw ard tow ard th e Cretaceous e p e ir ic sea.
The
Bearpaw Shale is con sid ered to be o f m arine o r i g i n , but th e r a te o f .
7
tra n s g re s s io n o f th e Bearpaw sea is v a r io u s ly in te r p r e t e d by s ev e ral
au th o rs w ith in th e r e g io n .■
PALEOGEOGRAPHY AND TECTONIC SETTING
The Cretaceous Western I n t e r i o r b asin was an a s y m m e tric a l, e lo n g a te
s tr u c tu r a l trough ly in g e a s t o f th e C o r d ille r a n g e a n t ic lin e
1 9 7 7 ).
(Kauffm an,
The e p e ir ic sea t h a t occupied t h i s basin was up to 1 ,6 0 0 km w id e ,
more than 5 ,0 0 0 km lo n g , and connected th e p re s e n t-d a y G u lf o f Mexico
w ith th e p r o to - A r c t ic Ocean (F ig . 3 ) .
The development o f th e Western I n t e r i o r basin began in th e Late
J u ra s s ic as a r e s u lt o f th e a c c re tio n o f e x o tic t e r r a i n onto th e North
American c ra to n d u rin g subduction o f th e P a c if ic p la te
( P r i c e , 1 9 7 3 ).
W ith th e development o f th e r e s u ltin g orogen and subsequent c ru s ta l
s h o rte n in g , th e mass o f th e e a s tw a rd -d is p la c e d s u p ra c ru s ta l rocks i n i ­
t i a t e d subsidence along th e w estern m argin o f th e basin ( P r ic e , 1 9 7 3 ).
T h is te c to n ic th ic k e n in g on top o f "an o ld , c o o l, t h i c k , lith o s p h e r ic
p la te "
(C a ld w e ll, 1 98 2 , p. 296) and r e s u lt a n t subsidence probably
a ffe c te d a la r g e p a r t o f th e e ve n tu a l b asin a re a .
Is o s t a t i c adju stm en t o f th e lith o s p h e r e due to te c to n ic th ic k e n in g
was supplemented by t h a t from sedim ent lo a d in g e a s t o f th e f o ld and
th r u s t b e l t .
These t h i c k , e a s tw a rd -b u ild in g prisms o f sedim ent gave
r i s e to th e " m ig ra tin g foredeep" concept o f B a lly and o th e rs
(1966)
( F ig . 4 ) .
O rogenic pulses in th e C o r d ille r a n h ig h la n d to th e w est provided
source areas f o r most o f th e sedim ent s u p p lie d to th e b a s in , w ith very
little
d e t r i t u s d e riv e d from the North American c ra to n to th e e a s t ( G i l l
8
LAND AREA
F ig u re 3 .
North American paleogeographic map showing g e n era l maximum
d is t r i b u t i o n o f Cretaceous e p e ir ic sea.
S ta te o f Montana
is o u t lin e d , study area shown in b la c k .
Dashed l i n e i n d i ­
c ates approxim ate p o s itio n o f s h o re lin e j u s t p r i o r to
Bearpaw tra n s g re s s io n (m o d ifie d from G i l l and Cobban,
1 9 7 3 ).
9
THE
F ig u re 4 .
MI GRATI NG
Schematic re c o n s tru c tio n showing eastward m ig ra tio n o f the
Rocky Mountain foredeep from th e J u ra s s ic to th e T e r t i a r y .
Note te c to n ic th ic k e n in g by successive t h r u s t f a u l t i n g and
i s o s t a t i c adjustm ent o f Precam brian basement rocks (from
B a lly and o th e r s , 1966, p. 3 6 6 ).
10
and Cobban, 1 9 7 3 ).
The geometry o f th e basin evolved in to an asymmetric
p r o f i l e , w ith much t h i c k e r , coarser, d e p o s its along th e more r a p id ly sub­
s id in g w estern m argin ( F ig . 5 ) .
Most o f th e sedim ent from th e C o r d ille r a n h ig h lan d was d e liv e re d
to th e basin by streams t h a t formed a t le a s t f i v e m ajor d e l t a i c com­
plexes in th e n o rth e rn p a r t o f th e b asin d u rin g th e Campanian Stage
( F ig . 6 ) .
Weimer (1 9 7 0 ) showed d e l t a i c complexes lo c a te d in n o rth e rn
C o lo ra d o , c e n tr a l Wyoming, and c e n tra l Montana, and W illia m s and S te lk
(1 9 7 5 ) found evidence f o r two o th e r s , one in .n o r th e r n A lb e r ta and a
second in southern A lb e rta and Saskatchewan.
Numerous m inor d e lt a ic
systems fe d by s m a lle r streams p ro b a b ly e x is te d between th e m ajor com­
p le xe s (McLean, 1 9 7 1 ).
Provenance o f th e J u d ith R iv e r Form ation and p a r t i a l
tim e -
s t r a t ig r a p h ic e q u iv a le n ts in w estern Montana was s tu d ie d by McLean
(1 9 7 1 ), McMannis (1 9 6 5 ), Roberts (1 9 6 3 ), V ie le and H a rris
Mudge and Sheppard (1 9 6 8 ).
(1 9 6 5 ), and
These au th o rs agree t h a t th e m a jo r ity o f
c la s ts in th e s e fo rm a tio n s were d e riv e d from v o lc a n ic sources.
The
p rim ary v o lc a n ic source was th e E lkhorn M ountains, a lthough th e Deer
Creek v o lc a n ic c e n te rs p ro b a b ly s u p p lie d some v o lc a n ic d e t r it u s in
southwest Montana (P a rso n s , 1 9 4 2 ).
V ie le and H a rris
(1 9 6 5 ) s tu d ie d
v o l c a n i c l a s t i c - r i c h Upper Cretaceous rocks in n orthw est Montana and
s p ec u la te d t h a t th e source o f th e la r g e v o lc a n ic fragm ents th e re was a
n o rth e rn e x te n s io n o f th e E lkhorn M ountains.
They p o s tu la te d th a t the
E lkhorn Mountains v o lc a n ic s extended n o rth o f t h e i r p re s e n t d i s t r i b u ­
t io n and have s in ce been covered by t h r u s t sheets emplaced du rin g th e
P aleocene.
O ther im p o rta n t source rocks f o r th e J u d ith R iv e r Formation
STUDY
F ig u re 5 .
Schematic e a s t-w e s t cross s e c tio n from northw est Montana to northw est Minnesota
showing th e geometry o f th e Western I n t e r i o r basin and general s tr a t ig r a p h ic
r e la tio n s h ip s (d a ta from S c h u ltz and o th e r s , 1980; G i l l and Cobban, 1965; Hansen,
1958; and McGooky and o th e r s , 1 9 7 2 ).
12
Study
area
F ig u re 6 .
P aleogeographic map showing s h o re lin e and p o s itio n s o f f i v e
d e l t a i c complexes j u s t p r io r to the Bearpaw tra n s g re s s io n .
N orth ern two from data o f W illia m s and S te lk (1 9 7 5 ), southern
th re e from data o f Weimer (1 9 7 0 ).
13
are th e B e lt Supergroup, Archean metamorphic ro c k s , and P a le o zo ic and
Mesozoic sedim entary ro c k s , a l l
o f which were u p l i f t e d d u rin g the
S e v ie r and e a r ly p a r t o f th e Laramide orogeny (McLean, 1 9 7 1 ).
14
STRATIGRAPHY
Regional S tr a tig r a p h y
The Montana Group was named by E ld rid g e (1 8 8 9 ) f o r Upper Cretaceous
sedim entary rocks which a fe b e st exposed in th e s ta te o f Montana.
With
more re c e n t r e v is io n s by Cobban and Reeside (1 9 5 2 ), th e fo rm a tio n s o f
t h is group a r e , in ascending o rd e r:
Bearpaw, and Fox H i l l s
( F ig . 2 ) .
E a g le , C la g g e tt, J u d ith R iv e r,
E q u iv a le n t r o c k - s t r a t ig r a p h ic u n its
a ls o in c lu d e d in th e Montana Group a re th e P ie r r e S h a le , Telegraph Creek
F o rm atio n , V i r g e l l e Sandstone, Parkman Sandstone, Two M edicine Forma­
t i o n , Lennep Sandstone, and H o rs e th ie f Sandstone (F ig . 2 ) .
These u n its
g e n e r a lly c o n s is t o f e a s tw a rd -th in n in g wedges o f re g r e s s iv e , nonmarine
and m arg in al m arine d e p o s its t h a t enclose w e s tw a rd -th in n in g wedges o f
tr a n s g r e s s iv e , m arine s t r a t a .
F ig u re 7 is a diagram m atic e a s t-w e s t
.
cross s e c tio n showing th e g eneral r e la tio n s h ip s o f th e Montana Group.
The s t r a t ig r a p h ic u n its o f in t e r e s t in t h is study a re th e J u d ith
R iv e r Form ation and th e o v e rly in g Bearpaw S h a le .
The J u d ith R iv e r
Form ation is an e a s tw a rd -th in n in g , p r im a r ily fre s h w a te r d e p o s it th a t
accum ulated d u rin g th e m iddle Campanian.
The Bearpaw S hale is a
com plim entary westward th in n in g fo rm a tio n d ep o sited in m arine e n v i­
ronments d u rin g th e l a t e r Campanian and th e e a r l i e s t p a r t o f the
M a e s tric h tia n
(F ig s . 2 and 7 ) .
In w estern Montana, th e t im e - s t r a t ig r a p h ic e q u iv a le n t o f the
J u d ith R iv e r Form ation is th e upper p a r t o f th e Two M edicine Form ation.
15
Bearpaw
Glacier
National Park
Bowdoin
Dome
Horsalhief Sandstone
Bearpaw Shale
West flank
Williston Basin
Fox Hills Ss
STUDY AREA
Fox Hlls Ss
Bearpaw Shale
Pierre
Shale
VirgeII^r S
F ig u re 7 .
Telegraph Creek Formation
Diagram m atic e a s t-w e s t cross s e c tio n through n o rth e rn Montana
showing th e g eneral r e la tio n s h ip s o f the Montana Group (modi­
f i e d from Rice and S h u rr, 1 9 8 0 ).
On th e west s id e o f th e Sweetgrass A rch, a p p ro x im a te ly 250 km west o f
th e study a r e a , the Two M edicine Form ation is g r e a te r than 600 m th ic k
and is alm ost e n t i r e l y nonmarine sandstone, s i l t s t o n e , and s h ale (Cobban,
1 9 5 5 ).
The l i t h o l o g i c a l l y s im ila r t im e - s t r a t ig r a p h ic e q u iv a le n t o f the
J u d ith R iv e r Form ation in southern A lb e rta and Saskatchewan is c a lle d
th e B e lly R iv e r Group t h a t is d iv id e d in to th e Oldman and Foremost
fo rm a tio n s
(McLean, 1 9 7 1 ).
In Wyoming, th e J u d ith R iv e r e q u iv a le n t is
th e Mesa Verde Form ation which has been re p o rte d as being unconformably
o v e r la in by th e M eeteetse S h a le , th e Bearpaw Shale e q u iv a le n t ( G i l l and
Cobban, 1 9 6 6 ).
16
S tr a tig r a p h y o f th e J u d ith R iver/B earp aw T r a n s itio n
T h is study focuses oh th e s tr a tig r a p h y o f th e upper 15 m o f the
J u d ith R iv e r Form ation and th e t r a n s it io n
Bearpaw S h a le .
A composite l i t h o l o g i c sequence f o r t h i s
shown in F ig u re 8 .
a ll
in to th e lo w er p a r t o f the
This diagram i l l u s t r a t e s
s ig n if ic a n t h o rizo n s and lit h o l o g i e s
in t e r v a l
is
th e r e l a t i v e p o s itio n s o f
in th e upper p a r t o f th e J u d ith
R iv e r and lo w er p a r t o f th e Bearpaw w ith in th e study a re a .
No s in g le
'
ou tcro p s tu d ie d in c lu d e d a l l h o rizo n s o r l i t h o l o g i e s shown in th e f ig u r e .
The upper p a r t o f th e J u d ith R iv e r is a l i g h t - c o l o r e d , heterogenous m ix­
tu r e o f sandstone, s i l t s t o n e , and mudrock w ith a few t h in coal beds near
th e to p .
Although th e l i t h o lo g y o f a c e r t a in bed may rem ain co n stan t
f o r s e v e ra l hundred m e te rs , l a t e r a l
and v e r t ic a l
v a r ia t io n e f f e c t i v e l y
e lim in a te s use o f most beds as re g io n a l m arker h o riz o n s .
The geometry o f in d iv id u a l beds w ith in th e upper p a r t o f th e J u d ith
R iv e r Form ation in th e study a rea is g e n e r a lly l e n t i c u l a r , although some
coal and mudrock beds appear t a b u la r across s h o rt exposures o f is o la te d
o u tc ro p s .
Mudrock and sandstone lith o so m es a re commonly in te rto n g u e d
a t both la r g e s c a le (beds up to 3 m t h ic k ) and small s c a le (beds les s
than 5 cm t h i c k ) .
C ontacts between f i n e and o v e rly in g coarse l i t h o ­
somes a re u s u a lly s h a rp , and a g ra d a tio n a l t r a n s it io n u s u a lly occurs
between coarse and o v e rly in g mudrock f a c ie s .
The t r a n s it io n from th e J u d ith R iv e r in to th e Bearpaw is d i s t i n c t
in a l l
exposures but is most a b ru p t in th e e a s te rn p o rtio n o f th e study
a re a .
The c o n ta c t is c h a ra c te riz e d by an upward decrease in g ra in s iz e ,
an in c re a s e in f i s s i l i t y , and d a rk e r c o lo r .
Because th e Bearpaw Shale
is a r e l a t i v e l y n o n re s is ta n t u n it c o n ta in in g a s ig n if ic a n t amount o f
17
Vertical
Lower
scale
Bearpaw
Shale
[
I M assive
__I sandstone
I Carbonaceous
shale
I- . • I Sand and
I- * I gravel
Silty shale
□
C ro s s -s tra tifie d
sandstone
I . y , ; | Gypaum
Upper
L -y e. I Bivalves
U a a !G astropods
Concretions
Judith River
Formation
F ig u re 8 .
Composite s t r a t ig r a p h ic s e c tio n o f th e upper 15 m o f the
J u d ith R iv e r Form ation and th e lo w er 15 m o f th e Bearpaw
Shale in th e study a re a .
18
expandable c la y , i t forms a subdued topography t h a t supports l i t t l e
vege­
t a t i o n , which f u r t h e r d e lin e a te s th e c o n ta c t.
The lo w er p a r t o f th e Bearpaw is composed o f d a rk -c o lo re d s .ilty
s h ale t h a t w eathers l i g h t gray and c o n ta in s a t le a s t one y e llo w -w h ite
to w h ite b e n to n ite bed.
S an id in e and b i o t i t e
from b e n to n ite beds in
th e Bearpaw S hale have been dated by potassium -argon tech n iq u es
( F o lin sb e e and o th e r s , 1 96 1 , 1965) and th ese ages have been a p p lie d to
ammonite and fo r a m in ife r a n zo n a tio n f o r g r e a t e r r e s o lu tio n o f tim e r e l a ­
tio n s h ip s in th e Western I n t e r i o r ( G i l l
C a ld w e ll, 1 9 7 5 ).
and Cobban, 1973; North and
However, th e lo w er p a r t o f th e Bearpaw S hale lacks
th e b io s t r a t ig r a p h ic c o n tro l used thro u g h o u t the r e s t o f th e fo rm a tio n
(Hearn and o th e r s , 1965; C a ld w e ll, 1 9 6 8 ).
T h is la c k o f f o s s il s may have
been caused by u n fa v o ra b le c o n d itio n s f o r fa u n a l o c cup ation and w i l l
addressed in a l a t e r s e c tio n .
be
In th e study a r e a , th e lo w e r p a r t o f the
Bearpaw S hale c o n ta in s numerous c o n c re tio n s but none were found th a t
c o n ta in ed ammonites as d e s c rib e d from h ig h e r p a rts o f th e fo rm a tio n
elsew h ere ( G i l l
and Cobban, 1973; S c h u ltz and o th e r s , 1 9 8 0 ).
Exposures o f th e Bearpaw Shale e x h i b i t a popcorn-w eathered s u rfa c e
t h a t compacts e a s ily under w eig h t and, when w e tte d , produces a very
s tic k y o r gumbo s u rfa c e .
T h is type o f w ea th e rin g is due to expansion
and c o n tr a c tio n by s m e c tite c la y s t h a t l o c a l l y comprise a la r g e propor­
t io n o f th e s h ale (S c h u ltz and o th e rs , 1 9 8 0 ).
Although th e Bearpaw Shale was re p o rte d to c o n ta in numerous bento­
n i t e beds ( K n e c h te l, 1959; G i l l and Cobban, 1973; C a ld w e ll, 1968; S c h u ltz
and o th e r s , 1 9 8 0 ), th e lo w er 15 m in th e study area c o n ta in s o n ly one
d is c r e t e , r e l a t i v e l y continuous bed.
This b e n to n ite bed occurs
19
a p p ro x im a te ly 13 to 1 5 m above th e J u d ith R iver/B earpaw c o n ta c t in the
n o rth e rn p a r t o f th e study area and has a maximum th ic k n e s s o f 0 .6 5 m.
A p p a re n tly t h i s b e n to n ite bed was d e p o s ite d in l o c a l l y a g ita t e d w a te r,
s in ce th e th ic k n e s s v a r ie s c o n s id e ra b ly w ith in in few m eters l a t e r a l l y
and th e upper s u rfa c e is i r r e g u l a r and appears scoured.
C u rren ts may
have s h if t e d th e v o lc a n ic ash from one a rea to a n o th e r where tr a n q u il
c o n d itio n s a llo w e d a b n o rm ally th ic k a cc u m u la tio n s.
O th er b e n to n ite beds occur in th e study area a t a le v e l approx­
im a te ly 30 to 35 m above th e J u d ith R iver/B earp aw c o n ta c t.
These beds
appear le s s d is ru p te d than th e lo w er b e n to n ite bed and may no t have been
s u b je c te d to c u rre n ts d u rin g o r s h o r tly a f t e r d e p o s itio n .
Sandstone tongues were re p o rte d w ith in th e Bearpaw Shale by Knechtel
(1 9 5 9 ) and C a ld w ell
i
(1 9 6 8 ).
C a ld w ell d e sc rib e d these sandstone tongues
as w e ll- s o r t e d , f in e - g r a in e d , and l o c a l l y c r o s s - s t r a t i f i e d .
No compar­
a b le sandstone lit h o l o g i e s occur in th e lo w er p a r t o f th e Bearpaw in
th e study a r e a , a lthough zones o f c la y e y and s i l t y sandstone do occur
a t v a rio u s h o rizo n s
(F ig . 8 ) .
The maximum th ic k n e s s o f th ese zones in
th e study area is 0 .5 m and th e y u s u a lly pinch out w ith in 50 m l a t e r a l l y .
M in e ra lo g y o f g ra in s w ith in th e sandstone zones suggests th e y are r e la t e d
to th e J u d ith R iv e r F o rm ation.
These may be o ffs h o re o r d e lt a f r o n t
bars t h a t accum ulated in response to in c re a s e d sedim ent in p u t from the
a llu v ia l
system.
20
LITHOLOGY OF THE JUDITH RIVER/BEARPAW TRANSITION
,In tro d u c tio n
The .upper 15 m o f th e J u d ith R iv e r Form ation is an heterogenous
m ix tu re o f l i g h t - c o lo r e d mudrock and sandstone w ith some t h in l i g n i t e
beds in th e upper p a r t .
J u d ith R iv e r sandstone is t e x t u r a l l y and m in-
e r a l o g i c a l l y immature and is c l a s s i f ie d as fe ld s p a th ic l i t h a r e n i t e
1974 c l a s s i f i c a t i o n t r i a n g l e ) .
(F o lk ,
Mudrbck c o n s titu te s a m ajo r p a r t o f th e
J u d ith R iv e r Form ation and is g e n e r a lly p o o rly s o rte d and h ig h ly bento­
n itic .
M in e ra lo g y o f th e s i l t - s i z e
g ra in s in th e mudrock is s im ila r to
th e m in era lo g y o f J u d ith R iv e r sandstone.
The lo w er 15 m o f th e Bearpaw Shale is composed p r im a r ily o f d a rk c o lo re d s i l t y
s h ale t h a t c o n ta in s numerous s i d e r i t e c o n c re tio n s .
n i t e and gypsum beds and s e v e ra l l i g h t - c o lo r e d c la y e y and s i l t y
Bento­
sand­
stone zones a ls o occur in th e lo w er p a r t o f th e fo rm a tio n .
J u d ith R iv e r Mudrock
In th e upper 15 m o f th e J u d ith R iv e r Form ation in th e study a re a ,
mudrock is th e predom inant rock ty p e w ith some in t e r f in g e r e d sandstone
and t h in coal beds.
The mudrock is u s u a lly l i g h t brown to g ra y , m assive,
p o o rly s o r te d , and c o n c re tio n a ry .
McLean (1 9 7 1 ) determ ined t h a t the mud­
rock was composed o f m ontm oriI I o n i t e ,
illite ,
k a o l i n i t e , and c h l o r i t e ,
and s i l t - s i z e d g ra in s o f q u a r tz , fe ld s p a r , c l i n o p t i l o l i t e , d o lo m ite ,
cal c i t e , b i o t i t e , and p y r i t e .
Both septan* an and n o n s e p ta ria n
21
c o n c re tio n s occur w ith in th e mudrock.
Most c o n c re tio n s a re composed o f
s i d e r it e a lthough some c o n ta in abundant c la s ts s im ila r to those in th e
host ro c k .
The cementing agent in c l a s t i c - r i c h c o n c re tio n s is u s u a lly
s i d e r i t e , c a l c i t e , o r sometimes a w e ll-in d u r a te d s ilic e o u s , c la y m a tr ix .
No f o s s il s were found in any c o n c re tio n s in th e mudrock.
The mudrock
is g e n e r a lly p o o rly in d u ra te d and on w eathered exposures o fte n co n tain s
gypsum and j a r o s i t e .
J u d ith R iv e r Sandstone Beds
Sandstone beds w ith in th e upper 15 m o f th e J u d ith R iv e r Formation
a re o f two ty p e s :
those t h a t a re g r e a t e r .th a n 3 m t h i c k , t h a t a re mod­
e r a t e l y s o rte d , and d is p la y la r g e -s c a le c r o s s - s t r a t i f i c a t i o n , and those
t h a t a re le s s than 3 m t h i c k , are w e ll- s o r t e d , and d is p la y s m a ll-s c a le
c r o s s - s t r a t i f i c a t i o n o r r i p p l e - d r i f t la m in a e .
The t h i c k , la r g e -s c a le
c r o s s - s t r a t i f i e d sandstone beds g e n e r a lly occur low er in th e sequence,
and th e t h i n , s m a ll-s c a le c r o s s - s t r a t i f i e d u n its u s u a lly occur h ig h e r
in th e J u d ith R iv e r Form ation near th e c o n ta c t, although th e y sometimes .
are found j u s t above th e t h ic k e r sandstone beds' ( F ig . 8 ) .
Both types o f .
sandstones a re composed m a in ly o f p la g io c la s e , rock fra g m e n ts , q u a r tz ,
and potassium f e ld s p a r , w ith c a l c i t e and g la u c o n ite as m inor c o n s t it -
. .
u e n ts .
The th ic k -b e d d e d sandstone fa c ie s is g e n e r a lly p o o rly to m oderately
s o rte d and up to 6 m t h i c k .
Sedim entary s tru c tu re s w ith in th e t h ic k e r ,
lo w er sandstone beds a re la r g e -s c a le omikron c r o s s - s t r a t i f i c a t i o n
(F ig . 9 ) .
•
Most c r o s s - s t r a t i f i e d sets a re c h a ra c te riz e d by ta n g e n tia l
bases and e ro s io n a l upper s u rfa c e s .
The th ic k n e s s o f in d iv id u a l
22
F ig u re 9 .
Photograph o f la r g e -s c a le omikron c r o s s - s t r a t i f i c a t i o n th a t
occurs in th e t h i c k e r , lo w er sandstone beds in th e upper
15 m o f th e J u d ith R iv e r Form ation in th e study a re a .
23
c r o s s -s tr a tifie d
s ets ranges up to 40 cm.
S e v e n ty -fo u r azim uths o f
c r o s s - s t r a t i f i e d sandstone beds in the upper 15 m o f th e J u d ith R iv er
Form ation were measured through out th e study area and a re p lo tte d on
a rose diagram shown in F ig u re 10.
N
Percent
of total
F ig u re 10.
C u rre n t rose diagram showing azim uths o f fo re s e ts o f 74
c r o s s - s t r a t i f i e d sandstone beds in th e upper p a r t o f the
J u d ith R iv e r Form ation in th e study a re a .
Compass head­
ings were taken in th e d ir e c t io n toward which fo r e s e t beds
d ip p ed .
I
24
Cosets o f th e lo w e r , t h i c k sandstone beds d i s p la y a fin in g -u p w a rd
g r a in s iz e sequence and upward decreased th ic k n e s s o f c r o s s - s t r a t i f i e d
sets.
At l o c a l i t y
"A" ( F i g .
11) the f l u v i a l
system produced c y c l i c
d e p o s its as f o l l o w s .
TOP
D
0 - 2 m
Mudrock w ith t h i n lenses o f very f i n e - g r a i n e d sand­
stone c o n ta in in g c lim b in g r i p p l e s :
i n t e r p r e t e d as
le v e e o r overbank d e p o s it s .
C
O - I m
Coal and coal fragm ent mudstone:
f l o o d p l a i n and swamp d e p o s its .
B
I
Clayey sandstone and mudrock:
i n t e r p r e t e d as l e v e e ,
crevasse s p la y , and chute d e p o s its .
BOTTOM A
- 3 m
3 - 6 m
i n t e r p r e t e d as
Omikron c r o s s - s t r a t i f i e d sandstone w ith upward
decrease in g r a in s i z e , sometimes capped w ith r ip ^
p ie d r i f t c r o s s - l a m in a t i o n :
i n t e r p r e t e d as m ig r a t ­
ing f l u v i a l p o in t bar d e p o s its .
The c y c le s a re o f t e n m u l t i s t o r i e d w ith a “stacked" sequence t h a t fo llo w s
a (n)A - B -
(n)A - B - C - yD p a t t e r n where (n ) could be a number from
I to 3 , g iv in g s ev e ral r e p e t i t i o n s o f omikron c r o s s - s t r a t i f i e d sand­
s to n e s , each w ith an e r o s io n a l upper s u r fa c e .
to t h a t de sc rib e d by A l i e n
This p a t t e r n is s i m i l a r
(1965b) f o r fin in g -u p w a r d c y c le s o f meander­
ing stream s.
!
T h i n , w e ll
s o rte d sandstone beds t h a t c o n ta in s m a l l - s c a l e c ross -
s t r a t i f i c a t i o n o f t e n occur w ith a t h i c k e r sequence o f mudrock, 9 to
10 m below the J u d ith R iver/Bearpaw c o n ta c t ( F i g . 8 ) .
These t h i n
sandstone beds accumulated to a maximum thic k n e s s o f I m and o fte n
d i s p la y c lim b in g r i p p l e s o r r i p p l e - d r i f t c ro s s -la m in a e ( F i g . 1 2 ) .
At some exposures th e t h i n sandstone beds c o n ta in burrows and small
coal fra g m e n ts .
S m a ll-s c a le c r o s s - s t r a t i f i c a t i o n a ls o occurs as a
25
I
O
°0)
O
study area boundary
O
T32N
-f.
'T i
"hree Mile Creek
r * _1
\
i
O
O
i °
I
I
i
Peoples Creek-* *
T31N
T30N
I
T29N
________
e
i
I
•
R22E
T27N
_
_
L
J
i
•
•i
Cow Cree
k•
L
R21E
T28N
r
..J
R23E
O
R24E
km
I
I
T26N
____ A___
V
R25E
T25N
J
R26E
8
/
N
• selected localities with coal seam exposed
O exposures with Judith River-Bearpaw transition exposed
•
Figure 11.
selected stratigraphic section locations
Map o f study area showing lo c a tio n s o f v a rio u s outcrops
r e f e r r e d to in the t e x t .
26
Figure 12.
Photograph o f c lim b in g r i p p l e s o r r i p p l e - d r i f t c ro ss ­
la m in a tio n from near th e top o f the J u d ith R iv e r Forma­
t i o n in the study a r e a .
27
"cap" on top o f t h i c k , l a r g e - s c a l e c r o s s - s t r a t i f i e d sandstone beds,
u s u a l l y 12 to 13 m below th e J u d ith R iver/B earpaw c o n ta c t (Fig.. 8 ) .
Climbing r i p p l e s o r r i p p l e d r i f t c r o s s -la m in a tio n is i n d i c a t i v e o f
r a p id s e d im e n ta tio n and i s produced by m ig r a tin g s m a l l - s c a l e c u r r e n t
r i p p l e s w ith both th e stoss and l e e s id e o f th e r i p p l e preserved
( A l l e n , 19,65b).
O ther i s o l a t e d beds o f c r o s s - s t r a t i f i e d , m oderately w e l l - s o r t e d
sandstone have f o r e s e t s t h a t d ip as much as 3 2 ° .
These sandstone beds
pinch ou t l a t e r a l l y and grade i n t o la m in a te d mudrock and l i g h t - c o l o r e d
s i lts to n e -.
In th e study a r e a , these beds occur j u s t below th e c o n ta c t
I
w ith th e o v e r l y i n g Bearpaw.Shale and u s u a l l y above th e coal seam
(F ig . 8 ) .
No r e c u r r i n g sequences occur in these s te e p ly dipping beds.
J u d ith R iv e r Coal and L i g n i t e
/
Near th e top o f the J u d ith R iv e r Form ation, thin .s ea m s o f l i g n i t e
and subbituminous coal occur w i t h i n mudrock beds ( F i g . 8 ) .
The m axi-
mum l i g n i t e o r coal th ic k n e s s in th e study area is 1 .0 6 m, but a seam
2 . 5 m t h i c k was de sc rib e d by Pepperberg (1910) w i t h i n th e upper p a rt.
io f th e J u d ith R iv e r Formation about 25 km west o f the study a r e a .
In .
those l o c a l i t i e s where coal could be s tu d ie d in outcrop ( F i g . 1 1 ) ,
th e d e p o s its were g e n e r a l l y l e n t i c u l a r ,
a few hundred m e te rs ;
i n v a r i a b i l y p in c h in g out w i t h i n
Weathered exposures o f coal o f t e n c o n ta in the
m inera l j a r o s i t e i d e n t i f i e d by J . P. Wehrenberg using X -r a y d i f f r a c t i o n
techniques a t th e U n i v e r s i t y o f Montana (personal communication,
1 0 /1 6 /8 1 ).
J a ro s ite
(KFe 3 (OH) 6 (SO4 ) 2 ) a p p a r e n tly forms as a w eather­
ing product o f p y r i t e and probably would, not occur in unweathered coal
.
28
(Warshaw, 1 9 5 6 ).
The coal conta in s abundant fragments o f s i l i c i f i e d
a n d /o r carbonized wood.
At the Peoples Creek exposure ( F i g . 11) an
in s i t u stump o f s i l i c i f i e d wood w ith r o o ts i n t a c t appears to have
grown a t an angle o b liq u e t o th e coal seam ( F i g . 1 3 ) .
Bearpaw S i l t y Shale
The low er 15 m o f th e Bearpaw Shale in the study area i s composed
p r i m a r i l y o f d a r k - c o l o r e d , popcorn- o r f r o t h y - w e a t h e r i ng s i l t y s h a le .
Most d e f i n i t i o n s o f shale emphasize th e f i s s i l e
g ra in e d d e t r i t a l
s i l t y s h a le .
rock and t h i s
is t r u e o f a l l
c h a r a c te r o f the f i n e ­
rocks r e f e r r e d to here as
The percentage o f s i l t - s i z e g ra in s in t h i s u n i t v a r ie s
from p la c e to p la c e but averages 15 to 20 p e rce n t as e s tim a te d from
b in o c u la r microscope a n a ly s e s .
S c h u ltz and o th e rs (1 9 80 ) analyzed th e low er Bearpaw Shale a t a.
l o c a t i o n a p p ro x im a te ly 55 km south o f th e study area and found t h a t the
s ilt-s iz e
p a r t i c l e s were composed o f m ostly q u a r t z , p l a g i o c l a s e , b i o t i t e
potassium f e l d s p a r , cal c i t e , d o lo m ite , a n d ' c r i s t o b a l i t e .
Clay m in e ra ls
comprise 75 to 80 p e rc e n t o f the f r o t h y - o r popcorn-weathered Bearpaw
S h ale .
M in e r a ls in c lu d e m ix e d -la y e r c l a y s ,
k a o lin ite
(S c h u ltz and o t h e r s , 1 9 8 0 ).
illite ,
c h l o r i t e * and
M ix e d - la y e r c la y in f r o t h y - o r
popcorn-weathered shale is composed o f a high p r o p o r tio n o f m ontm orill o n i t e and i l l i t e
w ith sub o rd in ate b e i d e l l i t e
(S c h u ltz and o t h e r s , 1980)
The popcorn- o r f r o t h y - w e a t h e r i ng is u s u a l l y e x p la in e d as th e r e s u l t o f
s w e llin g and s h r in k in g o f c la y s r i c h in th e exchangeable c a tio n s Na.
and Ca+ .
29
Figure 13.
Photograph o f in s i t u s i l i c i f i e d t r e e stump in
l i g n i t e a t l o c a l i t y "A" ( F i g . 11) in th e upper p a r t
o f the J u d ith R iv e r Formation in the study a r e a .
Diam eter o f stump is a p p ro xim a te ly 20 cm.
30
Bearpaw C la y e y , S i l t y Sandstone
S everal t h i n , p o o r ly s o rte d and p o o rly in d u ra te d zones o f sandstone
occur in th e low er p a r t o f th e Bearpaw Shale in th e study a r e a . . M in e r­
a lo g y o f th e s i l t and sand f r a c t i o n o f these n o n r e s is ta n t beds is s im i­
l a r to t h a t o f th e upper p a r t o f th e J u d ith R iv e r Formation except t h a t
q u a r tz is the m ajor c o n s t i t u e n t fo llo w e d in abundance by f e l d s p a r and
rock fr a g m e n ts .
Clay m in era lo g y w i t h i n th e sandy tongues was not
d e te rm ine d.
Bearpaw B e n to n ite
The s in g le d i s c r e t e b e n to n ite bed in th e low er 15 m o f th e Bearpaw
Shale in th e study area is w h ite to y e l l o w i s h - w h i t e and has a maximum
th ic k n e s s o f 0 .6 5 m.
b e n to n ite .
Compositional analyses were not performed on t h i s
However, S c h u ltz and o th e rs (1 9 80 ) used X -r a y d i f f r a c t i o n
analyses and determ ined t h a t a t y p i c a l
Bearpaw b e n to n ite i s composed o f
80 to 100 p e rc e n t c la y m in e ra ls o f which 100 p e rcent is b e i d e l l i t e m o n t m o r illo n it e m i x e d - l a y e r .
tite
They found q u a r t z , p l a g i o c l a s q , and b i o -
to comprise up to 20 p e rce n t o f th e t y p i c a l b e n to n ite ;
31
DESCRIPTIONS AND INTERPRETATION OF SPECIFIC OUTCROPS
Three w id e ly s epa ra te d lo c a tio n s
( F i g . 11) were s e le c te d f o r
d e t a i l e d l i t h o l o g i c study and i n t e r p r e t i v e d is c u s s io n .
Al I th r e e
in c lu d e good exposures o f th e J u d ith R iv e r fo r m a tio n ; t r a n s i t i o n a l
beds, and th e o v e r ly in g Bearpaw S h a le .
Inclu d e d w ith th e discussion
o f each l o c a l i t y i s a f i g u r e showing the s t r a t i g r a p h i c s e c tio n and a
l i t h o l o g i c d e s c r i p t i o n o f the J u d ith River/B earpaw t r a n s i t i o n a t t h a t
lo c a tio n .
Three M i l e Creek Exposure
This outcrop is lo c a te d in th e northw est p a r t o f th e study area
near th e c e n t e r o f Sec. 1 1 , T31N, R22E along th e i n t e r f l u e v e s o f Three
M i l e Creek and the M i l k R iv e r .
of th is lo c a lity
A s t r a t i g r a p h i c s e c tio n and d e s c r ip t io n
is given in Figure 14.
The basal u n i t o f t h i s exposure i s a l i g h t - c o l o r e d , s m a ll-s c a le
c r o s s - s t r a t i f i e d sandstope t h a t may re p r e s e n t th e upper p a r t o f a p o in t
bar d e p o s it or an accum ulation w i t h i n a crevasse s p la y o r l e v e e .
The
o v e r ly in g f i n e - g r a i n e d sandstone, s ilt s to n e ,;m u d r o c k and c o a l/b e d s are
i n t e r p r e t e d as overbank d e p o s its t h a t r e s u l t e d from l a t e r a l
stream m ig ra ­
t i o n , which brought th e f l o o d p l a i n environment over th e form er channel.
Above th e coal seam a re l i g h t - c o l o r e d s i l t s t o n e , mudrock, and carbo­
naceous shale beds w ith a t o t a l
th ic k n e s s o f 2 .5 m.
These beds may be.,
a c o n tin u a tio n , o f overbank d e p o s its o r , a l t e r n a t i v e l y , th e r e s u l t o f
32
White to yellowish-white bentonite (45 cm) with
overlying dark gray to brown s ilty shale.
Dark gray to brown s ilty shale, locally concretion­
ary and gypsiferous (2.5 m).
Bearpaw
Buff to gray, poorly sorted, clayey, s ilty
sandstone (.05 m).
I
Dark gray to brown s ilty shale, locally con­
cretionary and gypsiferous (4.5 m).
th River Formation
Dark gray to gray gypsiferous s ilty claystone
and mudrock (I m).
Medium gray to brown, massive, gypsiferous,
sandy shale with jarosite (I m).
Buff to gray, fin e- to medium-grained, friable
gypsiferous clayey sandstone with jarosite
and hematite (1.5 m).
Dark brown carbonaceous shale with hematite (0.5 m).
Buff to gray laminated mudrock and siltstone (0.5 m).
Black subbituminous coal with gypsum, ja ro site ,
and petrified wood (I m).
Gray, clay-rich, massive mudrock with jarosite (0.5 m).
Dark brown carbonaceous shale (10 cm).
Light gray to buff, fine-grained massive, hematitic
sandstone with small-scale cross-stratified
sandstone at base (base covered).
LEGEND
I Massive
I sandstone
Silty shale
Carbonaceous
shale
Mudrock
X-sratified
sandstone
Siltstone
Figure 14.
: . « I Sand and
I gravel
’
Coal
<Y,' I Gypsum
i Bivalves
A A A !Gastropods
Bentonite
Concretions
Lithologic log and description of the Three Mile Creek
exposure. Vertical scale in meters.
33
b r a c k is h -w a t e r e s t u a r y , l a g o o n a l , o r d e l t a p l a i n s e d im e n ta tio n .
Lack
o f b r a c k is h -w a t e r f o s s i l s fa v o r s the form e r e x p la n a tio n but i s not con­
c lu s iv e .
O v e rly in g th e f l o o d p l a i n d e p o s its is a 4 m t h i c k , g e n e r a l l y
d a r k e r c o lo re d sequence o f c la y e y sandstone, sandy s h a le , and mudrock
beds.
These a re i n t e r p r e t e d as crevasse s p la y , l e v e e , and d e l t a p l a in
d e p o s its t h a t accumulated in fr e s h o r b r a c k is h -w a t e r environm ents.
Simi
I a r d e p o s its were de sc ribe d by Coleman and P r i o r (1 9 82 ) from th e M is s is ­
s ip p i d e l t a .
No f o s s i l s were found a t t h i s s t r a t i g r a p h i c h o rizo n a t th e
Three M ile Creek outcrop t h a t might c o n firm t h i s environment o f deposi­
tio n .
S harply o v e r ly in g th e l a s t mudrock bed i s dark grayish-brow n shale
t h a t is i n t e r p r e t e d as th e i n i t i a l
d e p o s it o f th e Bearpaw tr a n s g r e s s io n .
A p proxim ately 3 m above the f i r s t appearance o f d a r k - c o lo r e d s h a le , i s a
t h i n zone o f p o o r ly s o r t e d , m assive, c la y e y sandstone t h a t may be the
r e s u l t o f storm a c t i v i t y o r increased sediment in p u t from th e f l u v i a l
system.
Currents were a p p a r e n tly strong enough to b rin g in sand g r a in s ,
but o f i n s u f f i c i e n t d u r a tio n or magnitude to produce a. w e l l - s o r t e d
d e p o s it .
The th ic k n e s s and l a t e r a l
e x t e n t o f t h i s c o a r s e -g r a in e d f a c ie s
v a r i e s s i g n i f i c a n t l y w i t h i n 50 m.
A pproxim ately 8 m above the low es t m arine d e p o s it , a 30 to 45 cm
t h i c k b e n to n ite bed o c c u r s .
This a s h f a l l
d e p o s it has v a r i a b l e t h i c k ­
ness and an i r r e g u l a r , a p p a r e n tly scoured upper s u rfa ce t h a t suggests
some e ro s io n occurred b e fo re b u r i a l and, t h e r e f o r e , d e p o s itio n w i t h i n
wave base.
34
Peoples Creek Exposure
This outcrop i s lo c a te d in th e e a s t - c e n t r a l
in the NEi o f Sec. 2 1 , T29N, R25E.
p a r t o f th e study area
A s t r a t i g r a p h i c s e c tio n and d e s c r ip ­
t i o n o f t h i s l o c a l i t y is given in Fig u re 15.
The sequence o f l i t h o l o g i e s a t t h i s outcrop is s i m i l a r to t h a t a t
Three M i l e Creek w ith a basal f i n e - g r a i n e d , s m a ll- s c a le and r i p p l e - d r i f t
c r o s s -la m in a te d sandstone bed.
This type o f c r o s s - s t r a t i f i c a t i o n can
occur in two environments o f a f l u v i a l
fin a l
system ( A l l e n , 1 9 6 5 a ) :
as the
d e p o s it in p o in t bar sequences, o r on levees and crevasse s p la y s .
Above th e basal sandstone, a gradual upward decrease in g r a in s iz e
occurs.
M o d e ra te ly s o r t e d , f i n e - g r a i n e d sandstone is in te rb ed d ed w ith
mudrock t h a t grades upward i n t o a t h i c k bed o f sandy mudrock c o n ta in in g
lenses o f carbonaceous shale and a i m
t h i c k coal seam.
A t h i n bed con­
s i s t i n g o f l i g h t - c o l o r e d , f i n e - g r a i n e d , massive s i l t y sandstone con­
t a i n i n g abundant coal fragments o v e r l i e s th e coal-mudrock sequence.
The e n t i r e low er 7 m o f th e exposure is i n t e r p r e t e d as an accumulation
t h a t occurred in overbank environm ents, beginning w ith le v e e deposits
t h a t were succeeded by f i n e - g r a i n e d f l o o d p l a i n and swamp d e p o s it s .
v e rtic a l
The
j u x t a p o s i t i o n o f l i t h o l o g i e s may be e x p la in e d by s h i f t i n g
environments as a stream channel m ig r a te d , as p o s tu la te d by A lle n
(1965b) from s i m i l a r r e c e n t d e p o s its .
O v e r ly in g these overbank l i t h o l o g i e s
i s a b i o t u r b a t e d , dark gray
shale t h a t c o n ta in s numerous " f l o a t i n g " f i n e - g r a i n e d sand p a r t i c l e s
near the base o f th e u n i t .
This bed i s p robably o f m arine o r i g i n and
may have been de pos ited on a flo o d e d d e l t a p l a i n o r in. a lagoon c re a te d
35
'
Glaciofluvial deposits.
Dark gray, coarsely laminated, gypsiferous s ilty
shale with some hematite staining (I m).
Bearpaw Shal
Medium gray, laminated, gysiferous mudrock with
brackish-water gastropods ( Euspira subcrassum) (0.5 m).
Dark gray to brown, coarsely laminated, gypsiferous,
concretionary shale with some hematitic patches
(3 m).
I
White selenite gypsum in dark gray s ilty shale (15 cm).
Dark gray to brown, coarsely laminated, gypsiferous,
concretionary s ilty shale with some hematite
staining ( I .5 m).
irmation
Dark gray, bioturbated sandy shale and mudrock (I m).
Light gray to buff, fine-grained, massive sandstone
with coal fragments and jarosite (I m).
Buff to gray, massive mudrock and s iltstone (20 cm).
Black subbituminous coal with gypsum and hematite
o
U-
( I .2 m).
£
>
,
AC
£
"O
3
’
Buff to lig h t gray, wavy bedded sandy mudrock with
thin carbonaceous shale lenses (1.5 m).
Light gray, wavy bedded to massive, moderately
sorted, fine-grained sandstone and interbedded
mudrock (I m).
e Buff, fine-grained, rip p le -d rift laminated and
I
small-scale cross-stratified sandstone (base
1
covered).
LEGEND
Figure 15.
Silty shale
Massive
sandstone
Mudrock
Carbonaceous
shale
Cross-stratified
sandstone
Sand and
gravel
Bentonite
Siltstone
Coal
Concretions
« \V
< »' I
Gypsum
Bivalves
Gastropods
Lithologic log and description of the Peoples Creek
exposure. Vertical scale in meters.
36
du rin g a tr a n s g r e s s iv e pulse o f th e Bearpaw sea.
The b io tu r b a te d
sandy shale grades upward i n t o a dark g r a y , c o a rs e ly la m in a te d ,
g y p s ife ro u s shale i n t e r p r e t e d as th e i n i t i a l
d e p o s it o f the advancing
Bearpaw sea.
The j u x t a p o s i t i o n o f s t r a t a a t t h i s h o rizo n suggests t h a t some
f l u c t u a t i o n o f the d e p o s it io n a l
environment occurred a t t h i s l o c a t i o n .
A 0 . 5 m t h i c k mudrock c o n ta in in g Euspira subcraSsum, a b r a c k is h -w a te r
gastropod (W. A. Cobban, personal communication, 2 / 3 / 8 2 ) , occurs approx­
im a t e ly 5 m above th e J u d ith River/B earpaw c o n ta c t w i t h i n the Bearpaw
S h a le .
F lu c t u a t io n o f marine and b r a c k is h -w a t e r environments may have
been caused by s l i g h t re g re s s io n s o f th e sea o r l o c a l l y decreased t i d a l
range.
O v e rly in g t h i s b r a c k is h -w a t e r d e p o s it i s dark g ra y is h -b ro w n ,
c o n c r e t io n a r y , g y p s if e r o u s , s i l t y shale t y p i c a l o f th e Bearpaw Forma­
tio n .
Cow Creek Exposure
This outcrop is lo c a te d in the southwest p a r t o f th e study area in
Sec. 3 3 , T26N, R21E, a p p ro x im a te ly 0 . 5 km e a s t o f Cow Creek ( F ig s . 11
and 1 6 ) .
The lo w e s t s t r a t a exposed are an a l t e r n a t i n g sequence o f carbon­
aceous shale and b u f f to l i g h t g r a y , v e ry f i n e - g r a i n e d , h e m a t it e s ta in e d sandstone, which has s m a l l - s c a l e c r o s s - s t r a t i f i c a t i o n a t the
base.
The i n f e r r e d d e p o s it io n a l environm ent is a le v e e fo llo w e d by
overbank swamps t h a t were o c c a s io n a lly covered by crevasse splay s e d i­
m entation s i m i l a r to r e c e n t de pos its de sc ribe d by C o llin s o n (1 9 7 8 ) .
37
Glaciofluvial deposits.
ro
«
5
IO
Dark brown to gray, gypsiferous, concretionary,
fis s ile , s ilty shale (5 m).
<o
O
m
Light gray, fria b le , fine-grained s ilty sandstone
interbedded with dark brown carbonaceous shale (I m).
Light gray, well indurated, fine-grained sandstone
and mudrock with Euspira subcrassum (0.5 m).
c
o
Z
Light gray to buff, concretionary, fine-grained
sandstone with mudballs and large and small-scale
cross-stratification near the middle (2.5 m).
E
o
LL
S
>
cc
£
Light gray to buff, poorly indurated, poorly sorted,
very fin e- to medium-grained sandstone and mud­
stone (2.5 m).
Buff, massive, fine-grained sandstone (I m).
3-
Light gray, massive mudrock containing Ostrea
subtrigonalis (0.5 m).
Dark brown carbonaceous shale and thin lig n ite
interbedded with buff to gray, very fin e - to
fine-grained, well indurated, hematitic sandstone
with small-scale cross-stratification at base (2 m).
O-
LEGEND
Silty shale
Massive
sandstone
Mudrock
Carbonaceous
shale
""I Cross-stratified | : « » I Sand and
I" »
" I gravel
L ____ sandstone
Siltstone
Figure 16.
Coal
<k , I Gypsum
r— r r = ] Bivalves
A A A !Gastropods
Bentonite
Concretions
Lithologic log and description of the Cow Creek exposure.
V ertical scale in meters.
38
The n ext l i t h o l o g y in th e sequence is a l i g h t g r a y , massive mudrock
c o n ta in in g numerous b r a c k is h -w a t e r o y s t e r s , in c lu d in g O strea subtrig o n a l i s .
This 0 . 5 m t h i c k b r a c k is h -w a t e r f a c ie s occurs between
t h i c k e r sequences o f fr e s h w a te r a l l u v i a l
d e p o s it s .
The environment
t h a t produced th e b r a c k is h -w a t e r d e p o s it may have formed by lo c a l
subsidence and t i d a l
fl o o d i n g o r in an e s tu a r y by a b r i e f advance o f .
the Bearpaw sea.
The o y s t e r - b e a r i n g h o rizo n is o v e r l a i n by a m assive, n o n r e s is ta n t
f i n e - g r a i n e d sandstone, which grades i n t o a p o b rly s o r t e d , massive to
p o o r ly bedded, f i n e - g r a i n e d sandstone and mudrock.
stone and mudrock sequence i s a w e ll
O v e rly in g the sand­
i n d u r a t e d , f i n e - to medium-grained,
c r o s s - s t r a t i f i e d sandstone bed up to 3 m t h i c k . . C r o s s - s t r a t a dip as
much as 22° and c o n ta in calc are o u s c o n c re tio n s and m udballs.
Cross­
s t r a t a a re in s e ts up t o 60 cm t h i c k w ith e ro s io n a l upper surfaces
and t a n g e n t i a l
1 9 6 3 ).
bases and a re c l a s s i f i e d as omikron c r o s s - s t r a t a
(A lle n ,
G rain s iz e decreases upward in c o s e ts , w ith some c lim b in g r i p ­
p les in th e upper p a r t o f th e f a c i e s .
From th e top o f the b r a c k is h -w a t e r o y s t e r horizon upward, the
environments o f d e p o s itio n appear to have been dominated by a l l u v i a l
processes as i n d i c a t e d by prim ary sedim entary s tr u c tu r e s and f a c ie s
re la tio n s h ip s .
The t h i n , massive sandstone and o v e r ly in g mudrock is
s i m i l a r t o those d e sc rib e d by C o llin s o n
f l o o d p l a i n d e p o s it s .
(1 9 78 ) as crevasse splay and
The c r o s s - s t r a t i f i e d sandstone is i n t e r p r e t e d as
a p o in t bar d e p o s it t h a t accumulated on top o f the f l o o d p l a i n f a c ie s
as a channel m ig ra te d l a t e r a l l y .
I
39
O v e rly in g the c r o s s - s t r a t i f i e d sandstone is a t h i n , l i g h t g ra y ,
g y p s if e r o u s , c o n c r e t i o n a r y , g a s t r o p o d -b e a rin g , f i n e - g r a i n e d sandstone
w ith inte rbed ded mudstone.
The gastropods were i d e n t i f i e d by W. A.
Cobban (personal communication, 2 / 3 / 8 2 ) as Euspira subcrassum (Meek
and Hayden) t h a t he presumed to be s h allo w w a te r forms from t h e i r
a s s o c ia tio n w ith o th e r proven marine fa u n a .
This f a c i e s probably
was de pos ited in an environment t h a t r e s u l t e d from a n o th e r b r i e f advance
o f th e sea o r by l a t e r a l
s h i f t o f fa c ie s .
O v e rly in g t h i s gastropod bed is a succession o f a l t e r n a t i n g s i l t y
sandstone and dark brown carbonaceous shale beds, th e l a t t e r p r e ­
dom inating in th e upper p a r t .
These a re i n t e r p r e t e d as c o a s ta l or
d e l t a p l a i n swamp d e p o s its t h a t formed as th e Bearpaw sea advanced,
c r e a t i n g a h ig h e r g round-w ater t a b l e .
The carbonaceous s h a le grades
upward i n t o dark g r a y , g y p s if e r o u s , s i l t y s h a le , th e f i r s t Bearpaw
lith o lo g y .
40
DEPOSITIONAL MODELS
Development o f a Model
Sedimentary rocks a t th e J u d ith R iv e r Form ation/Bearpaw Shale
t r a n s i t i o n a re so v a r ie d t h a t no s in g le environm ental model s a t i s f a c ­
t o r i l y e x p la in s them.
The technique used here has been t o d e sc rib e the
c h a r a c t e r i s t i c s o f th e d e p o s its and then a p p ly d i f f e r e n t models as
they f i t ,
r e s u l t i n g in a composite m odel.
A model can be devised using modern analogues by e x e r c is in g some
re s tra in t.
Several authors have observed t h a t modern f l u v i a l
do not p a r a l l e l
ments.
c o n d itio n s t h a t a f f e c t e d most a n c ie n t f l u v i a l
systems ■
e n v ir o n ­
D iff e r e n c e s between modern and a n c ie n t c o n d itio n s p ointed out
by Frien d (1 978) in c lu d e d i f f e r e n t c li m a t e s , v e g e t a t i o n , and orogenic
c o n d it io n s .
(1 978) a l l
Leeder ( 1 9 7 8 ) , Frien d ( 1 9 7 8 ) , and McLean and J e r z y k ie w ic z
agreed t h a t th e th ic k n e s s and l a t e r a l
e xten t o f flo o d p la in
d e p o s its o f a n c ie n t r i v e r systems is f a r g r e a t e r than those o f modern
analogues.
This i s e s p e c i a l l y t r u e when subsidence a llo w s fr e q u e n t
f lo o d events t h a t pro v id e th e accum ulation and b u r i a l o f f l o o d p l a i n
sedim ents, which may have been the case during J u d ith R iv e r tim e .
A number o f f a c t o r s suggest t h a t streams which d e p o s ited the
J u d ith R iv e r Formation were r e l a t i v e l y s t a b le and not prone to l a t e r a l
m i g r a t io n .
G rain s i z e , c om position, and sedimentary packaging i n d ic a te s
t h a t th e upper 15 m o f th e J u d ith R iv e r Formation in th e study area is
composed o f 50 to 60 p e rc e n t overbank d e p o s it s .
Gant (1 9 8 2 ) pointed
41
out t h a t when stream banks are composed o f f i n e - g r a i n e d , cohesive s e d i­
ments w ith t h i c k v e g e t a t i v e grow th, th e r e is a high r e s is t a n c e to e r o ­
sion and channel m i g r a t io n .
Overbank accum ulation o f p e a t , a f i b r o u s ,
cohesive m a t e r i a l a ls o r e s i s t s e ro s io n and th e development o f a new
channel across i t by r i v e r a v u ls io n
( R y e r, 1 9 8 1 ).
Channel m ig ra tio n
would a ls o be i n h i b i t e d by c la y plugs formed in abandoned meanders.
Another f a c t o r to be considered i s t h a t J u d ith R iv e r streams may have
been c o n fin e d to broad, in c is e d v a l l e y s produced i n i t i a l l y by low ering
o f base l e v e l during r e g re s s io n o f the C la g g e t t sea and p o s s ib ly sus­
ta in e d by s poradic u p l i f t s
in the w est.
A l l o f these f a c t o r s probably
c o n tr ib u te d to l a t e r a l l y s t a b le streams t h a t confined d e l t a i c complexes
'
to a r e a l l y l i m i t e d c e n te rs o f d e p o s itio n f o r r e l a t i v e l y long periods o f
tim e .
D e te rm in a tio n o f a l l u v i a l
environments i s o ft e n f a c i l i t a t e d by
r e c o g n it io n o f p o i n t bar d e p o s its t h a t d i s p la y c h a r a c t e r i s t i c sequen­
tia l
p ro p e rtie s
1 9 7 8 ).
( A l l e n , 1965a; Reineck and Singh, 1973; and Le ede r,
This id e a l p o in t bar model is d is tin g u is h e d by a d i s t i n c t
v e rtic a l
p a t t e r n t h a t d is p la y s an upward decrease in g r a in s iz e and an
upward decrease in th ic k n e s s o f sedim entary u n i t s , which a llo w s the
d e p o s it to be d iv id e d i n t o upper and low er p a r t s .
Figure 17 is a diagram showing a complete sequence i n a p o in t
bar d e p o s it as d e sc rib e d by Reineck and Singh ( 1 9 7 3 ).
id e a l
The base o f an
sequence i s c h a r a c t e r iz e d by a scoured s u rfa ce fo llo w e d by chan­
nel la g d e p o s its and l a r g e - s c a l e , r e l a t i v e l y c o a r s e -g r a in e d , c ro ss s tra tifie d
sandstone.
T r a n s i t i o n to th e upper p o in t bar i s o ft e n
marked by a zone o f h o r i z o n t a l
la m in a tio n .
The upper p o r tio n o f an
42
id e a l
p o in t bar d is p la y s s m a l l - s c a l e c r o s s - s t r a t i f i c a t i o n and r i p p l e
d r i f t c r o s s - l a m in a t i o n , which are sometimes capped by a mud l a y e r .
NEW SEQUENCE
MUD LAYER
SMALL-SCALE CROSS-STRATIFICATION
RIPPLE-DRIFT CROSS-LAMINATION
SMALL-SCALE CROSS-STRATIFICATION
RIPPLE-DRIFT CROSS-LAMINATION
SMALL-SCALE CROSS-STRATIFICATION
HORIZONTAL LAMINATION
LARGE-SCALE CROSS-STRATIFICATION
CHANNEL LAG DEPOSIT
Figure 17.
I d e a l i z e d f i n i n g upward sequence o f a p o in t bar d e p o s it.
G rain s iz e and sedim entary u n i t th ic kn es s decrease upward
(from Reineck and Singh, 1973, p. 2 4 0 ) .
E n t i r e id e a l
p o in t bar sequences seldom occur.
The upper o r f i n e ­
g ra in ed de pos its are o f t e n missing due to e i t h e r e ro sio n during reoccupa­
t i o n o f an o ld channel by a stream o r by simple n o n d e p o s itio n .
However,
most p o in t bar sequences d is p la y s u f f i c i e n t d ia g n o s tic f e a t u r e s f o r
c o n fid e n t i n t e r p r e t a t i o n o f the d e p o s it .
In th e study a r e a , the th ic kn es s o f p o in t bar sequences in the
upper 15 m o f the J u d ith R iv e r Formation i n d ic a te s d e p o s itio n by
43
r e l a t i v e l y s m a l l , w e ll o rg a n ized f l u v i a l
d e lta ic p la in s .
Much l a r g e r a l l u v i a l
systems on a l l u v i a l
and
and d e l t a i c systems e x is t e d 100
to 200 km to th e north and the south (Weimer, 1960; W illia m s and S t e l k ,
1 9 7 5 ).
The marine d e p o s it io n a l environment o f the Bearpaw Shale has been
e s t a b lis h e d through p a le o n t o lo g ic e v id e n c e .
However, oceanographic
c o n d itio n s in t h i s m arine environment a re not as e a s i l y documented.
Ryer and Kauffman (1980) suggested m esotidal
ranges e x i s t e d in the Cre­
taceous e p e i r i c sea o f th e Western I n t e r i o r .
As evidence th e y c i t e d
l i n e a r sand bars and t h i c k fo r e s h o r e sequences..
a ls o proposed th e e x is te n c e o f t i d a l
o f r i v e r systems, and t i d a l
fla ts .
Ryer and Kauffman
i n l e t s , t i d a l I y in flu e n c e d reaches
However, l i n e a r sand bars and t h i c k
fo re s h o re sequences do not occur in the study a r e a , which may r e f l e c t a
s m a lle r t i d a l
range b e fo re the Bearpaw tr a n s g r e s s io n o r sim ply a la c k
o f d e t r i t u s f o r such accum ulations.
a c t i v i t y was suggested by Caldw ell
The p o s s i b i l i t y o f d a i l y wave
(1 9 68 ) from evidence he found in
th e low er Bearpaw Shale in southern Saskatchewan.
This evidence
in c lu d e s thanatocoenosis., p rim ary sedim entary s tr u c tu r e s w i t h i n sand­
stone tongues, and scoured b e n to n ite beds, a l l
w i t h i n wave base.
suggesting d e p o s itio n
However, no shoreface sequences o r l a t e r a l l y con­
tin u o u s d e l t a f r o n t sandstones, which Ryer (1981) considered as e v i ­
dence o f s i g n i f i c a n t wave a c t i v i t y , occur in the study a r e a .
Lacking d e t a i l e d l i t h o l o g i c o r p a le o n t o lo g ic e v id e n c e , i t
is d i f f i ­
c u l t to p r e c i s e l y c h a r a c t e r iz e the d e p o s it io n a l environment o f the low er
Bearpaw Shale in th e study a r e a .
However, the presence o f b r a c k i s h -
w a te r fauna and a scoured b e n to n ite bed a t some lo c a t io n s in th e study
44
a r e a , and s t r a t i g r a p h i c r e l a t i o n s h i p s t h a t i n d i c a t e a r a p id tr a n s g r e s ­
s io n , a l l
suggest s h a llo w marine and b r a c k is h -w a t e r environments e x is te d
du rin g d e p o s itio n o f the low er Bearpaw S h ale .
Environments o f D e p o s itio n During the Transgression
As subsidence occurred in the w estern p a r t o f the b a s i n , s e d i m e n t a ­
t i o n o f the J u d ith R iv e r Formation in th e study area probably decreased
and sea l e v e l
ro s e , s h i f t i n g d e p o s it io n a l environments to th e w est.
Figures 18 through 21 a re a s e r ie s o f block diagrams showing the
a l l u v i a l , d e l t a i c , and marine systems b e lie v e d r e s p o n s ib le f o r deposi­
t i o n o f the upper J u d ith R iv e r Formation and th e low er Bearpaw Shale.
The diagrams show a tim e - t r a n s g r e s s iv e sequence w ith the. encroachment
o f th e Bearpaw sea over th e top o f th e J u d ith R iv e r Formation and the
i n f e r r e d environments produced during the tr a n s g r e s s io n .
The lo c a tio n s
o f th e s t r a t i g r a p h i c s e c tio n s shown in Figures 1 4, 1 5, and 16 are i n d i ­
cated i n Figures 18 through 21.
Fig u re 18 shows th e e n v iro n m e n ts .a t a p p ro xim a te ly 7 2 .5 m .y . B . P . ,
b e fo re the m ajor tr a n s g r e s s iv e pulse o f th e Bearpaw sea.
development o f J u d ith R iv e r a l l u v i a l
approaching the f i n a l
s ta g e .
At t h i s tim e ,
and d e l t a i c f a c ie s was probably
With the i n i t i a t i o n o f in c re a se d sub­
sidence in th e western p a r t o f th e Western I n t e r i o r basin due to
t e c t o n i c t h i c k e n i n g , p eat bogs were p robably common in overbank and
d e l t a i c swamps.
The la c k o f w e ll developed p o in t bar d e p o s its a t
t h i s stage suggests decreased stream load and t h e r e f o r e decreased
stream g r a d i e n t s .
45
le v ee
c r e v a s s e splay
p e a t bog
flo od pla in
a p proxim ate
sc ale
LEGEND
Judith River Formation
M u d r o c k w it h some s il ts to n e . shale, and
Coal and lignite;
c o a l; o v e r b a n k and de lt a plain de posits
o v e r b a n k d e p o s it s
S a n d -r ic h mud rock;
o v e r bank d e p o s its
Bar lin ger sa n d s to n e and siltstone;
delta front and delta margin depo sits
L a r g e - s c a l e c r o a s - s t r a t H l e d sa nds to n e;
point bar and ch annel de po sits
R i p p l e - d r i f t and s m a l l - s c a l e
c r o s s -s tra tifie d sandstone; crevasse
sp lay and upper point bar de posits
Figure 18.
Bearpaw Shale
.....- j
-------
Silty shale with minor be nto nite ;
Marin e de po sits
Diagrammatic o b liq u e view showing a l l u v i a l , d e l t a i c , and
marine environments a p p ro x im a te ly 7 2 .5 m .y. B.P.
Numbers
i n d i c a t e lo c a tio n s o f s t r a t i g r a p h i c s ec tio n s shown in
Figures 1 4, 1 5, and 16.
Number I r e f e r s to Three M ile
Creek s e c t io n ; 2 r e f e r s to Peoples Creek s e c t i o n ; and
3 r e f e r s to Cow Creek s e c t i o n .
46
LEGEND
Judith River Formation
M u drock with some s i l ts to n e , sh ale, and
C oa l and lignite;
co al; o v e r b a n k and d e lt a plain de po sits
o v e r b a n k d e p o sit s
S a n d -r ic h mud rock;
o v e r bank d e p o s its
Bar finger s a nds to n e and siltstone;
delta fron t and delta margin depo sits
L a r g e - s c a l e c r o s s - s t r a t i f i e d sa ndsto n e;
point bar and channel de po sits
R i p p l e - d r i f t and s m a l l - s c a l e
c ro s s -s tra tifie d sandstone; crevasse
sp lay and upper point bar depo sits
Figure 19.
Bearpaw Shale
S il ty sh ale with minor b e n to n it e ;
Ma ri n e de po sits
Diagrammatic o b liq u e view showing a l l u v i a l , d e l t a i c , and
marine environments a p p ro x im a te ly 72 m.y. B.P.
Numbers
i n d i c a t e lo c a t io n s o f s t r a t i g r a p h i c s ec tio n s shown in
Figures 1 4, 15, and 16.
Number I r e f e r s to Three M ile
Creek s e c t io n ; 2 r e f e r s to Peoples Creek s e c t io n ; and 3
r e f e r s to Cow Creek s e c t io n .
47
LEGEND
Judith River Formation
M u d r o c k w it h some s il ts to n e , shale, and
C oa l and lignite;
co al; o v e r b a n k and d e lt a plain depo sits
o v e r b a n k d e po sit s
S a n d - r ic h m udrock;
o v e r bank d e p o s its
Bar finge r s a nds to n e and sil tsto ne ;
de lta front and delta margin de posits
L a r g e - s c a l e c r o s s - s t r a t i f i e d sa ndsto n e;
point bar and ch annel d e po sit s
R i p p l e - d r i f t and s m a l l - s c a l e
c r o s s -s tra tifie d sandstone; crevasse
sp lay and upper point bar de posits
Figure 20.
Bearpaw Shale
S il ty shale with minor b e n to n it e ;
M a rin e de po sits
Diagrammatic o b liq u e view showing a l l u v i a l , d e l t a i c , and
marine environments a p p ro x im a te ly 7 1 .5 m .y. B.P.
Numbers
i n d i c a t e l o c a t io n s o f s t r a t i g r a p h i c s ec tio n s shown in
Figures 1 4, 1 5, and 16.
Number I r e f e r s to Three M ile
Creek s e c t io n ; 2 r e f e r s to Peoples Creek s e c t i o n ; and
3 r e f e r s to Cow Creek s e c t io n .
48
LEGEND
Judith River Formation
—
-Jrr=L
M u d r o c k with some s il ts to n e , shale, and
Coal and lignite;
c o a l; o v e r b a n k and de lt a plain depo sits
o v e r b a n k d e p o s its
S a n d -r ic h m ud rock;
o v e r ba nk d e p o s its
Bar finger sa n d s to n e and siltstone;
delta fron t and de lt a margin depo sits
L a r g e - s c a l e c r o s s - s t r a t i f i e d sa ndsto n e;
point bar and ch annel d e po sit s
R i p p l e - d r i f t and s m a l l - s c a l e
c ro s s -s tra tifie d sandstone; crevasse
sp lay and upper point bar depo sits
Figure 2 1.
Bearpaw Shale
Silty shale with minor be nto nit e;
Ma ri n e depo sits
Diagrammatic o b liq u e view showing a l l u v i a l , d e l t a i c , and
marine environments a p p ro x im a te ly 71 m.y. B.P.
Numbers
i n d i c a t e lo c a tio n s o f s t r a t i g r a p h i c s ec tio n s shown in
Figures 1 4, 1 5, and 16.
Number I r e f e r s to Three M ile
Creek s e c t io n ; 2 r e f e r s to Peoples Creek s e c t io n ; and 3
r e f e r s to Cow Creek s e c t io n .
49
In Fig u re 19 th e i n i t i a l
t r a n s g r e s s iv e pulse o f th e Bearpaw sea
is shown w ith a t te n d a n t westward m ig r a t io n o f a l l u v i a l
systems.
and d e l t a i c
A f u r t h e r decrease in J u d ith R iv e r sedim e n ta tio n is im p lie d ,
probably caused reduced sediment supply due to reduced stream g r a d ie n t s .
D e l t a i c accum ulations were probably n e g l i g i b l e , la c k in g s u f f i c i e n t time
and d e t r i t u s f o r development.
C o n tin u a tio n o f the tr a n s g r e s s io n o f th e Bearpaw sea is shown in
Fig u re 20 w ith th e a l l u v i a l
environment S h i f t e d f a r t h e r w e s t.
In the
study area a t t h i s s ta g e , stream discharge, and g r a d i e n t were probably
reduced to th e p o in t where sediment accum ulation was i n s i g n i f i c a n t .
Marine and b r a c k is h -w a t e r c o n d itio n s dominated a t t h i s tim e except f o r
proximal c o n tin e n ta l environments in western Montana.
Figure 21 shows probable environments a t a p p ro xim a te ly 71 m.y.
ago w ith in u n d a tio n by the Bearpaw sea across the e n t i r e study a re a .
The r e s u l t i n g marine s i l t y
shale and occasional c la y e y sandstone and
b e n to n ite beds accumulated on top o f th e fr e s h and b r a c k is h -w a te r
J u d ith R iv e r Formation.
50
BEARPAW
t r a n s g r e s s io n
I n t e r p r e t a t i o n s in th e L i t e r a t u r e
Rate o f Transgression
The r a t e o f tra n s g r e s s io n o f th e Bearpaw sea in th e n o rth e rn United
S ta te s and southern Canada has long been debated w ith s ev e ral authors
s upporting a r a p id tra n s g r e s s io n using s t r a t i g r a p h i c and r a d io m e tr ic
e v id e n c e , w h ile o th e rs found faunal evidence t h a t suggested a d i a ­
chronous e v e n t.
Russell
(1 9 39 ) f i r s t suggested t h a t the Bearpaw tr a n s g r e s s io n was
a n e a r l y isochronous e vent from s t r a t i g r a p h i c work in southern A lb e r ta .,
McLean (1 9 71 ) a ls o considered th e tr a n s g r e s s io n to have been r a p id over
much o f A lb e r t a and western Montana because he found no tr a n s g r e s s iv e
sandstones a t th e J u d ith River/B earpaw c o n t a c t .
In n o rth e rn Montana, •
Lorenz (1981) showed th e t r a n s g r e s s iv e Bearpaw s h o r e lin e m igrated
r a p i d l y , a t a r a t e o f a p p ro x im a te ly 450 km/m.y.
F olinsbee and others
(1 9 6 1 , 1965) r a d i o m e t r i c a l l y dated b e n to n ite beds a t th e base o f the
Bearpaw Shale from l o c a t i o n s in s o u th -c e n tr a l A lb e r ta in th e Cypruss
H ills ,
in southern Saskatchewan along th e Saskatchewan R i v e r , and along
th e M issou ri R iv e r in n o r t h - c e n t r a l Montana.
These potassium-argon
dates were ob ta in e d from s a n id in e and b i o t i t e and i n d i c a t e d a ra p id
tra n s g r e s s io n o f th e Bearpaw sea.
Folin sb e e and o th e rs (1 9 6 5 ) dated t h i s
isochronous tr a n s g r e s s io n a t 72 - 73 m .y. ago w ith a maximum d e v ia t io n
o f ±5 p e r c e n t.
51
C on v e rse ly, Caldw ell
(1 9 6 8 , 1978, 1982) used ammonite z o n a tio n to
show t h a t the tra n s g r e s s io n in southern A l b e r t a and Saskatchewan was
markedly diachronous.
G ill
and Cobban (1 9 73 ) provided s i m i l a r evidence
in c e n t r a l and southern Montana and a d ja c e n t areas where th e y used
ammonite zo n a tio n to c a l c u l a t e t h a t the t r a n s g r e s s iv e s h o r e lin e had
m ig rated about H O km/m.y.
Both s tu d ie s found c o n s id e ra b le age d i f ­
fe re n c e s i n d i c a t e d by fau n a l change in th e low est f o s s i l s found in the
Bearpaw Shale from e a s t to w est.
In some areas o f th e Western I n t e r i o r b a s in , t r a n s g r e s s iv e move­
ment o f th e Bearpaw s h o r e lin e has been re p o rte d as minimal to n o n e x is ta n t.
G ill
and Cobban (1 9 66 ) re p o rte d a m ajor unconform ity a t the base
o f th e Teapot Sandstone Member o f th e Mesa Verde Formation in several
basins o f Wyoming.
G ill
and Cobban placed a date o f 72 m .y. B.P. on the
u nconform ity and suggested i t r e s u l t e d from e ro sio n f o l l o w i n g a broad
re g io n a l u p l i f t .
This occurred a t about th e same tim e as th e r e l a t i v e l y
r a p id tra n s g r e s s io n o f th e Bearpaw sea in n o rth e rn Montana, which was
r a d i o m e t r i c a l l y dated a t about 72 m .y.
(F o lin s b e e and o t h e r s , 1 9 6 5 ).
Causes o f th e Bearpaw Transgression
I n t e r p r e t a t i o n s o f th e cause o f th e Bearpaw tra n s g r e s s io n are
d iv id e d between te c to n o e u s ta s is and l o c a l t e c t o n i c c o n t r o l .
Hancock
and Kauffman (1 9 79 ) proposed a e u s t a t i c c o n tr o l on sea l e v e l f l u c ­
t u a t i o n in th e Western I n t e r i o r as w e ll as in northw est Europe, N i g e r i a ,
and I n d i a .
C o n v e rse ly, G i l l
and Cobban (1 9 6 6 , 1973) and J e l e t s k y (1 9 7 1 ,
1977) considered t e c t o n i c mechanisms l a r g e l y r e s p o n s ib le f o r s h i f t s o f
th e s tra n d l i n e .
B a l ly and oth e rs ( 1 9 6 6 ) , McLean and J e r z y k ie w ic z
( 1 9 7 8 ) , and Lorenz (1 9 8 1 ) r e l a t e d t e c t o n i c th ic k e n in g along th e western
.
52 ■
C o r d i l l e r a to basin subsidence, which th e y p o s tu la te d i n i t i a t e d the
Bearpaw tr a n s g r e s s io n .
Considering th e v a rio u s i n t e r p r e t a t i o n s re g a rd ­
ing th e tra n s g r e s s io n o f th e Bearpaw sea and mechanisms t h a t caused i t ,
no s in g le e x p la n a tio n is l i k e l y to be c o r r e c t f o r th e e n t i r e Western
I n t e r i o r b a s in .
Tecto n o e u stas is was p robably a m ajor cause f o r th e i n i t i a l
forma­
t i o n o f th e Cretaceous e p e i r i c sea because s i m i l a r seas e x i s t e d a t the
same tim e on o th e r c o n tin e n ts
(Hancock and Kauffman, 1 9 7 9 ) .
However,
tr a n s g re s s io n s and re g re s s io n s a p p a r e n tly occurred too r a p i d l y to have
been caused by changes in mid-ocean r id g e spreading r a t e s .
Pitman
(1 978) considered th e maximum r a t e o f sea l e v e l r i s e due t o changes in
mid-ocean r id g e spreading would be about 10 m/m.y.
O ther causes o f
eustasy suggested by Donovan and Jones (1 9 79 ) such as g l a c i a t i o n or
d e s ic c a te d basins a re not known to have occurred during th e Cretaceous.
Fig u re 22 i s a paleogeographic map o f Montana and a d ja c e n t areas
d u ring d e p o s it io n o f upper J u d ith R iv e r rocks (a p p ro x im a te ly 72 m.y.
ago) showing th e i n f e r r e d s h o r e lin e and m ajor d e l t a i c c e n t e r s .
Lorenz
(1 9 81 ) suggested a mechanism o f s t r a n d l i n e movement t h a t invokes r e g io n a l
t h r u s t events t h a t m o d ifie d the l a r g e r e e n t r a n t d e l t a s , which appears to
account f o r w id e ly d i f f e r e n t r a te s o f tr a n s g r e s s io n r e p o r te d f o r the
Bearpaw sea in d i f f e r e n t regio ns ( F i g . 2 2 ) .
The r a t e o f tra n s g r e s s io n
c r e a te d would have v a r ie d from pla ce t o p la c e depending on th e r e l a t i v e
p o s i t i o n w ith re s p e c t to th e t h r u s t sheets (and t h e r e f o r e subsidence)
and d e l t a i c c e n t e r s .
According to t h i s m odel, the tr a n s g r e s s io n was
n e a r l y isochronous in r a p i d l y subsiding areas and slow er o v er l a r g e ,
r e c e n t l y abandoned d e l t a s .
Lorenz (1 9 8 1 ) p o s tu la te d t h a t th e Bearpaw
.
53
Study
250
Figure 22.
km
Paleogeographic map o f Montana and a d ja c e n t area showing
i n f e r r e d p o s i t i o n o f s h o r e l i n e , m ajor d e l t a i c complexes,
and d i r e c t i o n o f sediment t r a n s p o r t (d a ta from Weimer,
1970; W illia m s and S t e l k, 1975; W illia m s and Burke, 1964;
W ils o n , 1 9 7 0 ).
54
tra n s g r e s s io n was slow est in t h a t area occupied by the m ajor J u d ith R iv e r
Formation d e l t a i c complexes in c e n t r a l Montana and southern A lb e r ta and
Saskatchewan.
In a d d i t i o n , the r a d io m e tr ic age date o f 72 m .y. B.P.
o b ta in e d by Hoffman and o th e rs (1976) f o r the beginning o f th e l a s t
episode o f t h r u s t i n g in northw estern Montana c o r r e l a t e s v ery w ell w ith
th e 72-73 m .y. B.P. date given by Folinsbee and o thers
(1 9 65 ) f o r the
isochronous advance o f th e Bearpaw sea.
L o r e n z 1 mechanism has several
p o s i t i v e p o in ts .
However, h is t r a n s ­
g r e s s iv e r a te s c o n t r a d i c t those im p lie d by faunal z o n a tio n re p o rte d by
Caldw ell
(1 9 6 9 , 1978) and G i l l
and Cobban ( 1 9 7 3 ) .
This in c o n g r u it y may
be e x p la in e d by examining p o s s ib le d i f f e r e n c e s in paleotopography.
The
r a p id tra n s g r e s s io n occurred over th e l o w - r e l i e f , n e ar-se a le v e l coastal
p l a i n d e p o s its o f the J u d ith R iv e r Formation and not as r a p i d l y over
th e m ajor d e l t a i c complexes to the north and south.
S m a ll, shallow
i n l e t s between the major d e l t a i c c e n te rs may have allow ed b ra c k is h w ater to invade the subsiding western p a r t o f the basin around and
a d ja c e n t to th e t o p o g r a p h i c a l l y high d e l t a i c complexes ( F i g . 2 3 ) .
Kauffman (1 9 7 7 , F ig . 1 1, p. 15) p o s tu la te d th e r e was a s a l i n i t y s t r a t i ­
fic a tio n
in th e Cretaceous sea w ith a s l i g h t l y b r a c k is h , lo w -d e n s ity
s u rfa c e l a y e r produced by fre s h w a te r i n f l o w o f r i v e r systems d r a in in g
a d ja c e n t land a r e a s .
Beneath the b r a c k is h -w a t e r s u rfa c e l a y e r , Kauffman
suggested the e x is te n c e o f a warm, n o r m a l - s a l i n i t y marine bottom l a y e r .
Because n e k to n ic ammonites a p p a r e n tly p r e f e r r e d normal m arine surface
w a te r th e y may not have in h a b it e d th e s u rfa c e b r a c k is h -w a t e r l a y e r .
Thus, the b r a c k is h -w a t e r o f newly subsided areas would have lacked the
55
DELTA
B R A C K IS H 4 V
.W A T E R ///!
DELTA
RAPID
TRANSG RESSIO N 1
_ SLO W
I
TR A N S G R ESS IO N
B R A C K IS H ^
RAPID
TR A N S G R ESS IO N
W ATER
DELTA
A
A'
FOLD AND
TH R U S T BELT
C O A S TA L PLAIN
AND ’ NEW" DELTA
Two Medicine
Formation
/
B R A C K IS H W ATER
O L D ' DELTA
LEVEL
JUDITH RIVER FM.
BEARPAW SHALE
OLDER P H A N E R O ZO IC ROCKS
PRECAMBRIAN ROCKS
Figure 23.
Map ; 1 cross s e c tio n showing major J u d ith R iv e r d e l t a i c
compi
„ t h a t probably prevented f u l l c i r c u l a t i o n o f
normal marine s a l i n i t y w a te r but allowed s u rfa c e b r a c k is h w ater to e n t e r subsiding basin to west.
Hachure area in
map view is b r a c k is h - w a t e r , dark black l i n e is s h o r e lin e .
56
r ic h fauna o f th e r e s t o f th e b a s in , accounting f o r th e e a s t-w e s t age
d iffe r e n c e s o f ammonites and th e a tte n d e n t diachronous in t e r p r e t a t io n s .
Evidence f o r a Rapid T ran sg ressio n in th e Study Area
In th e study a r e a , s e v e ra l lin e s o f evidence suggest a n e a rly
isochronous tra n s g re s s io n o f th e Bearpaw sea.
This evidence in c lu d e s
a la c k o f sandy tra n s g re s s iv e d e p o s its a t th e c o n ta c t, sm all e a s t-w e s t
d iffe r e n c e s in th ic k n e s s between m arker h o riz o n s , and a p p a re n tly s im ila r
e le v a tio n s o f th e c o n ta c t from e a s t to w est in undeformed a re a s .
Although none o f
th e s e c o n s id e ra tio n s i s , by i t s e l f , , c o n c lu s iv e , when
co n sid ered to g e th e r w ith o th e r s tu d ie s in th e re g io n , th e im p lic a tio n s
a re c o n v in c in g .
The most s t r i k i n g l i t h o l o g i c fe a tu r e suggesting a n e a r ly iso ch ro n ­
ous tra n s g re s s io n is th e la c k o f a sandy tra n s g re s s iv e s h o re lin e fa c ie s
a t th e c o n ta c t.
R ussell
(1 9 3 9 ), McLean (1 9 7 1 ) and Lorenz (1 9 8 1 ), among
o th e r s , in f e r r e d t h a t th e absence o f n e a rs h o re , s a n d -ric h d e p o s its is
evidence t h a t th e Bearpaw tra n s g re s s io n was so ra p id t h a t such d ep o sits
n ever developed.
S w ift. (1 9 6 8 ) and Ryer (1 9 7 7 ) s tu d ie d more diachronous tra n s g re s ­
s iv e sequences t h a t lac k ed nearshore sand f a c ie s .
They e x p la in e d th is
absence as r e s u lt in g from sh oreface e ro s io n and sedim ent d is p e rs a l
d u rin g th e tra n s g re s s io n .
The r e s u ltin g d is co n fo .rm ity between nonmarine
an d .m arin e d e p o s its is termed a ra v in e m e n t.
S w ift (1 9 6 8 ) l i s t e d f i v e c r i t e r i a
I)
f o r re c o g n iz in g a ravinem ent:
a sharp c o n ta c t, 2 ) no nearshore sandstones, 3) a s l i g h t a n g u la r
u n c o n fo rm ity , 4 ) few in t e r f in g e r in g lithosom es o f m arine and nonmarine
57
sed im en ts, and 5 ) th e presence o f a basal la g g ra ve l
m arine u n i t .
in th e o v e rly in g
Only th e f i r s t and fo u rth o f th e f i v e fe a tu re s S w ift
d e sc rib e d were found a t th e c o n ta c t between th e J u d ith R iv e r and B earpaw fo rm a tio n s .
M oreover, no evidence f o r an unco n fo rm ity a t th is
h o rizo n in t h i s p a r t o f th e Western I n t e r i o r basin has been re p o rte d
in th e l i t e r a t u r e .
A nother p o s s ib le e x p la n a tio n f o r a la c k o f nearshore sands a t the
c o n ta c t may be t h a t i n s u f f i c i e n t d e t r it u s was d e liv e r e d to th e .m a rin e
en viro n m en t.
I f te c to n ic th ic k e n in g to th e w est produced contemporaneous
su b sid en ce, stream g ra d ie n ts in th e d i s t a l
p a r t o f th e basin may have
decreased so t h a t th e y were no lo n g e r a b le to c a rry s a n d -s iz e g r a in s ,
w ith most coarse d e t r it u s d e p o s ited a d ja c e n t to th e mountain f r o n t .
The second l i n e o f evidence suggesting a ra p id tra n s g re s s io n in
th e study a rea is th e sm all d iffe r e n c e in th ic k n e s s between c e r t a in
h o rizo n s from e a s t to w e s t.
B e n to n ite beds w ith in th e Bearpaw Shale
can serve as is o c h ro n ic h o riz o n s .
U n fo r tu n a te ly , subsu rface c o n tro l
is i n s u f f i c i e n t and those b e n to n ite , beds found in is o la te d outcrops
could not be c o r r e la te d w ith a high degree o f c o n fid e n c e .
However,
coal beds can be used as m arker h o rizo n s under some c ircu m stan ces.
Although coal seams near th e top o f th e J u d ith R iv e r a re no t l a t e r a l l y
continuous as d is c r e te beds, th e y do serve as evidence t h a t s im ila r
environm ents e x is te d a t about the same tim e across th e study a re a .
From th e e a r l i e r d is c u s s io n on s p e c if ic s t r a t ig r a p h ic s e c tio n s , i t can .
be noted t h a t coal seams o f th e w id e ly sep arated s e c tio n s a l l
occur
w ith in 6 to 9 m below th e f i r s t occurrence o f Bearpaw .Shale (F ig s . 1 4 1 6 ).
58
The e le v a tio n o f th e c o n ta c t between th e J u d ith R iv e r and Bearpaw
fo rm a tio n s is s im ila r in s e v e ra l lo c a tio n s w ith in the study a re a .
Because a la r g e p a r t o f th e study area appears to have escaped te c to n ic
d is tu rb a n c e w ith th e beds s t i l l
f l a t - l y i n g , comparison o f c o n ta c t e le v a ­
tio n s in both th e e a s t and w est may g iv e a v e ry gross f i r s t a p p ro x i­
m ation o f tra n s g re s s iv e synchrony.
In th e case o f s t r a t ig r a p h ic s e c tio n s
from Three M ile Creek ( F ig . 14) and Peoples Creek (F ig . 1 5 ) , th e J u d ith
R iver/B earp aw c o n ta c t occurs a t an e le v a tio n o f a p p ro x im a te ly 780 m
a t both lo c a tio n s , which a re sep arated by about 34 km ( F ig . 1 1 ) .
If .
c o n ta c t e le v a tio n s a re v a l i d in d ic a to r s o f th e r a te o f tra n s g re s s io n in
th e study a r e a , i t would appear t h a t th e tra n s g re s s io n o f th e Bearpaw
sea o ccu rred a t about th e same tim e a t both lo c a tio n s .
In summary, th e r a te o f th e Bearpaw tra n s g re s s io n a p p a re n tly
v a r ie d g e o g ra p h ic a lly in response to paleo to p o g rap h y.
Those areas
t h a t were th e s i t e o f m ajor d e lta s were to p o g ra p h ic a lly high and not
inundated as r a p id ly as a d ja c e n t c o a s ta l p la in s .
The study area was
occupied by a c o a s ta l p la in and r e l a t i v e l y small f l u v i a l
systems d u r­
ing J u d ith R iv e r d e p o s itio n and was th e s i t e o f ra p id tra n s g re s s io n by
th e Bearpaw sea.
59
CONCLUSIONS
(1 )
In th e study a r e a , th e upper 15 m o f th e J u d ith R iv e r Forma­
t io n was d e p o s ited p r im a r ily by s m a ll, meandering streams t h a t produced
a lo w -r e lie f flu v ia l
and d e l t a i c p l a i n .
The lo w er 15 m o f th e Bearpaw
Shale was d e p o s ited in a s h a llo w , m arine and b ra c k is h -w a te r environm ent.
(2 ) The tra n s g re s s io n o f th e Bearpaw sea was p ro b ab ly ra p id in
th e study area because th e a rea was r e l a t i v e l y to p o g ra p h ic a lly low .
Areas to th e n o rth and south were s it e s o f m ajor d e l t a i c complexes
d u rin g J u d ith R iv e r d e p o s itio n , were to p o g ra p h ic a lly h ig h , and were
inundated by th e Bearpaw sea more s lo w ly .
(3 ) Faunal zo n a tio n t h a t suggests a diachronous tra n s g re s s io n o f .
th e Bearpaw sea is pro b ab ly m is le a d in g .
The n e k to n ic fauna used in
t h i s zo n a tio n p ro b a b ly d id not in h a b it th e s u rfa c e b ra c k is h -w a te r la y e r
t h a t was th e f i r s t w a te r to flo o d th e su b sid in g b a s in .
T h is fauna may
not have occupied th e w estern p a r t o f th e basin u n t i l w e ll a f t e r the
in itia l
tra n s g re s s io n .
(4 ) The Bearpaw tra n s g re s s io n was p ro b a b ly caused by subsidence in
response to te c to n ic th ic k e n in g durin g th e S e v ie r and Laramide o ro g e n ie s .
The basin subsided most in th e west w ith added subsidence due to sediment
lo a d in g .
(5 ) A d d itio n a l d e t a ile d study o f th e re g io n a l s tr a tig r a p h y is
re q u ire d b e fo re any f ir m conclusions can be made concerning th e r a te o f
th e Bearpaw tra n s g re s s io n .
A c a re fu l
study o f th e b io s tr a tig r a p h y o f
60
th e low er Bearpaw Shale should be made, p r e c is e ly documenting horizons
where f o s s ils a re found.
Local s t r a t ig r a p h ic s tu d ie s o f th e J u d ith
R iver/B earp aw t r a n s it io n should be done on and a d ja c e n t to th e m ajor
d e l t a i c complexes to d eterm ine th e im pact these fe a tu re s had on the
tra n s g re s s io n .
61
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