Sedimentology of the early cretaceous lower sandstone member of the Thermopolis Shale,
southwestern Montana
by Alan Dennis Stine
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Earth Sciences
Montana State University
© Copyright by Alan Dennis Stine (1986)
Abstract:
The Early Cretaceous (Albian) lower sandstone member of the Thermopolis Shale in the Bozeman,
Montana area has previously been interpreted as a transgressive marine shoreface sequence. Detailed
observation and interpretation of lithologies and sedimentary structures in the lower sandstone member
document deposition in a setting that more closely resembles modern shallow marine shelves rather
than the marine shoreface. The proposed shelf environment is characterized by simultaneous deposition
of sand and mud, with their distribution controlled by sediment availability, bathymetry, and
distribution of marine currents and storm paths. First order geomorphic features on this shelf consist of
sand ridges which exhibit a hierarchy of bedforms. Megaripples and ripples are the second and third
order bedforms, respectively. The sand ridges, as well as isolated sand bedforms, migrated upon a
substrate comprised of mud and complexly interbedded sand and mud (heterolithic deposits).
Heterolithic intervals are interpreted as storm deposits while the mud is thought to be deposited in low
energy environments where storm influence is minimal. The hydraulic regime controlling deposition on
the shelf was characterized by genetically related uni-directional and oscillatory currents, that share an
origin in storm events.
Documented transgressive processes recorded in the strata include ravinement, or shoreface erosion, of
marginal marine deposits between the lower sandstone member and underlying non-marine Kootenai
Formation. Additionally, rapid transgression subsequent to lower sandstone member deposition
resulted in the sharp contact separating the lower sandstone member from the overlying offshore
marine, middle shale member of the Thermopolis Shale. This transgression is also recorded by
deposition of a thin, discontinuous, argillaceous micrite near the base of the middle shale member. SEDIMENTOLOGY OF THE EARLY CRETACEOUS LOWER SANDSTONE
MEMBER OF THE THERMOPOLIS SHALE
SOUTHWESTERN, MONTANA
by
Alan D ennis S t in e
A t h e s i s su bm itted in p a r t ia l f u lf il lm e n t
o f th e req u irem en ts fo r th e degree
of
Master o f S c ie n c e
in
Earth S c ie n c e s
MONTANA STATE' UNIVERSITY
Bozeman, Montana
January 1986
A /379'
St 6 b
Cap.dU
ii
APPROVAL
o f a t h e s i s su bm itted by
Alan D ennis S tin e
T his t h e s i s has been read by each member o f th e t h e s i s com m ittee
and has been found to be s a t is f a c t o r y regard in g c o n te n t, E n g lish
u sa g e , form at, 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 t e n c y , and i s
ready fo r su b m issio n to th e C o lle g e o f Graduate S tu d ie s .
Date
a ir p e r so n , Graduate Committee
Approved fo r th e Major Department
Date
Head, Major Department
Date
Graduate Dean
iii
STATEMENT OF PERMISSION TO USE
In p r e s e n tin g t h i s t h e s i s in p a r t ia l f u lf il lm e n t o f th e
req u irem en ts fo r a m a ste r ’ s d eg ree a t Montana S ta te U n iv e r s it y , I
a g ree th a t th e L ib rary s h a ll make i t a v a ila b le to borrow ers under
r u le s o f th e L ib ra ry .
B r ie f q u o ta tio n s from t h i s t h e s i s are a llo w a b le
w ith ou t s p e c ia l p e r m issio n , provided th a t a ccu ra te acknowledgement o f
sou rce i s made.
P erm issio n fo r e x t e n s iv e q u o ta tio n from or rep ro d u ctio n o f t h i s
t h e s i s may be gran ted by my major p r o fe s s o r , or in h i s a b sen ce, by th e
D ir e c to 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 proposed use
o f th e m a te r ia l i s f o r s c h o la r ly p u rp o ses.
Any cop yin g or u se o f the
m a te r ia l in t h i s t h e s i s f o r f in a n c ia l g a in s h a l l not be allow ed
w ith o u t my w r it t e n p erm issio n .
S ig n a tu re
Date
I /Co /
iv
ACKNOWLEDGEMENTS
Forem ost, th e author w ish es to acknowledge th e l a t e Dr. Donald L.
Smith fo r s u g g e s tin g th e t h e s i s t o p ic and p ro v id in g gu idance through
th e i n i t i a l s t a g e s o f t h i s p r o j e c t .
The author i s in d eb ted to Dr.
James G. Schm itt fo r h i s encouragement and many d is c u s s io n s which
c o n tr ib u te d to a l l p a r ts o f t h i s work.
The comments and c o n tr ib u tio n s
o f Dr. Stephan J . C uster and Dr. W illiam W. Locke I I I , who a s s is t e d by
rev iew in g f i n a l d r a f t s o f th e t h e s i s , a re g r e a t ly a p p r e c ia te d .
The
au th or thanks Dr. Robert K. Schw artz, o f A llegheny C o lle g e , fo r h is
s u g g e s tio n s made through correspondence and w h ile in th e f i e l d . Thanks
a r e a ls o due Susan K. Vuke, o f th e Montana Bureau o f Mines and
G eology, fo r her comments on t h i s stu d y .
V
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS...............................................................................................................•
LIST OF FIGURES.........................................................
LIST OF TABLES............................................................................................
iv
vi
v ii
ABSTRACT.................................................................................................................................... v i i i
INTRODUCTION............................................................................................................. s ..........
I
P u rp ose.......................................................................................
Study A rea...........................................................
P rev io u s S tr a tig r a p h ic I n v e s t i g a t io n s ........................................................
M ethods.............................................................................................................................
1
3
6
9
FACIES DESCRIPTIONS.......................................................................................................
11
Sandstone F a c ie s ...........................
Sandstone F a c ie s I n t e r p r e t a t io n .....................................................................
H e t e r o lit h ic F a c ie s ..................
H e t e r o lit h ic F a c ie s I n t e r p r e t a t io n .......................................................
Mud F a c ie s .....................
Mud F a c ie s I n t e r p r e t a t io n ...................................................................................
12
16
20
23
25
25
FACIES RELATIONS.........................................................................................
26
V e r tic a l F a c ie s R e la t io n s ...................................................................................
L a te r a l F a c ie s R e la t io n s .....................................................................................
F a c ie s R e la tio n s I n t e r p r e t a t io n .....................................................................
26
29
29
PETROGRAPHY AND PROVENANCE..................................................................................... .•
39
P etro g ra p h ic D e s c r ip t io n ............................................................ ■.......................
Provenance I n t e r p r e t a t io n .................................'.................................................
39
42
SUMMARY............................................................................................
48
REFERENCES CITED....... ...............................................................................................
51
' APPENDIX...................................
55
vi
LIST OF FIGURES
F igu re
Page
1.
Map o f E arly C retaceou s seaw ay..............................................................
2
2.
Study a rea and s e c t io n l o c a t i o n s ..........................................................
4
3.
P a l in s p a s t ic a l ly r e s to r e d s e c t io n l o c a t i o n s .................................
5
4.
S tr a tig r a p h ic c o r r e la t io n c h a r t . ..................................................
7
5.
Photograph
o f p lan ar c r o s s b e d s ...........................................
6.
Photograph
o f r ip p le c r o s s - la m in a t io n .............................................. 13
7.
Photograph
o f r ip p le marks.......................................................
14
8.
Photograph
o f san dston e body g e o m e t r y ..........................................
15
9.
Photograph
o f sand-dom inated s u b - f a c i e s ...........................
21
10.
Photograph
o f f la s e r b ed d in g...................................................
22
11.
Rose diagram s o f cro ssb ed d in g a n d .r ip p le c r e s t s ....................... 34
12.
E arly C retaceou s d e p o s it io n a l p r o v in c e s .......................................... 36
13.
Diagram o f t e c t o n ic p ro v en a n ces............................................................ 44
14.
S tr a tig r a p h ic column, S lu ic e b o x o u tcro p .......................................... 55
15.
S tr a tig r a p h ic colum ns, B ra ck ett Creek andF a iry C reek............ 56
16.
S tr a tig r a p h ic colum ns, Nixon Gulch and Cottonwood Gulch__ 57
17.
S tr a tig r a p h ic colum ns, Paddys Run and S tr ic k la n d C r e e k .... 58
18.
S tr a tig r a p h ic colum ns, Park Branch and W in eglass Mountain. 59
19.
S tr a tig r a p h ic colum ns, Johnny Gulch and J e f f e r s o n C anyon.. 60
20 .
S tr a tig r a p h ic colum ns, Rocky Canyon and Goose Pond................ 6 1
21.
S tr a tig r a p h ic column, Middle Fork S ix t e e n m ile ............................ 62
13
vii
LIST OF TABLES
Table
Page
1.
Markov Chain o f F a c ie s T r a n s it io n s ....................... .................... ..........
2.
Data From P o in t Counts-................................................................................... 40
28
viii
ABSTRACT
The E arly C retaceou s (A lb ian ) low er san d ston e member o f th e
Therm opolis S h ale in th e Bozeman, Montana a rea has p r e v io u s ly been
in te r p r e te d as a t r a n s g r e s s iv e marine sh o refa ce seq u en ce. D e ta ile d
o b se r v a tio n and in t e r p r e t a t io n o f l i t h o l o g i e s and sedim entary
s tr u c tu r e s in th e low er sa n d sto n e member document d e p o s it io n in a
s e t t i n g th a t more c l o s e l y resem b les modern sh a llo w marine s h e lv e s
r a th e r than th e m arine s h o r e fa c e . The proposed s h e l f environm ent i s
c h a r a c te r iz e d by sim u lta n eo u s d e p o s itio n o f sand and mud, w ith t h e ir
d is t r ib u t io n c o n tr o lle d by sedim ent a v a i l a b i l i t y , bathym etry, and
d is t r ib u t io n o f marine c u r r e n ts and storm p a th s. F i r s t order
geomorphic f e a t u r e s on t h i s s h e l f c o n s is t o f sand r id g e s which e x h ib it
a h ier a rch y o f bedform s. M egaripples and r ip p le s a re th e second and
th ir d ord er b e d f o r m s ,r e s p e c t iv e ly . The sand r id g e s , a s w e ll as
i s o l a t e d sand bedform s, m igrated upon a s u b s tr a te com prised o f mud and
com p lexly in terb ed d ed sand and mud ( h e t e r p li t h i c d e p o s i t s ) .
H e t e r o lit h ic i n t e r v a l s a r e in te r p r e te d a s storm d e p o s it s w h ile th e mud
i s thought to be d e p o s ite d in low energy environm ents where storm
in flu e n c e i s m inim al. The h y d ra u lic regim e c o n t r o llin g d e p o s itio n on
th e s h e l f was c h a r a c te r iz e d by g e n e t i c a l l y r e la te d u n i- d ir e c t io n a l and
o s c i l l a t o r y c u r r e n ts , th a t sh are an o r ig in in storm e v e n t s .
Documented t r a n s g r e s s iv e p r o c e s s e s recorded in th e s t r a t a in clu d e
ravinem ent, or s h o r e fa c e e r o s io n , o f m arginal marine d e p o s it s between
th e low er sa n d sto n e member and u n d e rly in g non-marine K ooten ai
Form ation. A d d it io n a lly , rap id t r a n s g r e s s io n subsequent t o low er
san d ston e member d e p o s it io n r e s u lte d in th e sharp c o n ta c t sep a ra tin g
th e low er san d ston e member from th e o v e r ly in g o ffs h o r e m arine, m iddle
s h a le member o f th e Therm opolis S h a le . T h is t r a n s g r e s s io n i s a ls o
recorded by d e p o s itio n o f a t h in , d is c o n tin u o u s , a r g illa c e o u s m ie r ite
near th e base o f th e m id d le .s h a le member.
I
INTRODUCTION
Purpose
P rev io u s s t r a t ig r a p h ic in v e s t ig a t i o n s o f E arly C retaceou s rocks
in th e Bozeman, Montana a rea in t e r p r e t th e low er san dston e member o f
th e Therm opolis S h ale as a n earsh ore marine u n it d e p o s ite d during
i n i t i a l t r a n s g r e s s io n o f th e C retaceous seaway ( F ig . I) in t o southw est
Montana (R o b erts, 1972; Schw artz, 1972, 1982; James, 1977; Vuke,
1 9 8 2 ).
However, th e s e e a r l i e r i n v e s t ig a t i o n s do n o t a d d ress in d e t a il
th e sed im en to lo g y and h y d r a u lic regim e o f th e d e p o s it io n a l environm ent
o f th e low er san d ston e member.
B ecause t r a n s g r e s s iv e m arginal marine
d e p o s it s are r a r e ly p reserv ed in th e rock record (R yer, 1 9 7 7 ), t h is
in v e s t ig a t io n was undertaken to examine th e sed im en tology o f what has
been in te r p r e te d as an a n c ie n t t r a n s g r e s s iv e sa n d sto n e.
The s p e c i f i c g o a ls o f t h i s in v e s t ig a t i o n a re to : I) t e s t the
v a l i d i t y o f e a r l i e r t r a n s g r e s s iv e marine in t e r p r e t a t io n s o f th e low er
san d ston e member; 2) d e fin e a more s p e c i f i c d e p o s it io n a l environm ent
fo r th e low er san d ston e member; 3) d eterm ine ty p es and mechanisms o f
sedim ent tr a n s p o r t in th e low er sa n d sto n e member; 4) e v a lu a te p o s s ib le
provenances o f th e low er san dston e member; and 5) c o n tr ib u te to
knowledge o f E arly C retaceou s paleogeograp hy in so u th w est Montana.
T his stu d y i s based on d e t a ile d com parison o f s e d im e n to lo g ic a l
a t t r ib u t e s o f th e low er san dston e member w ith c h a r a c t e r is t ic s o f
modern sh a llo w marine san d s.
From W illiam s and S te lc k ( 1 9 7 5 )
F igure I
Map showing e x te n t o f e a r ly
America.
la te
Albian
seas
in North
3
Study Area
Exposures o f th e low er san d ston e member occur in th e north end o f
th e G a lla tin Range, B rid ger Range, S ix te e n m ile a rea , H orseshoe H ills '
and J e ffe r s o n Canyon in G a lla t in , Park, J e ffe r s o n and Broadwater
C o u n ties, sou th w est Montana (F ig . 2 ) .
Here, th e low er san dston e
member crop s out a lo n g s t r u c t u r e s produced by Laramide and S e v ie r
s t y l e d eform ation and subsequent C enozoic B asin and Range e x te n s io n .
The d isp la cem en t o f s t r a t a a s s o c ia te d w ith th e s e o r o g e n ie s has a
p o t e n t ia l fo r d is r u p tin g f a c i e s r e l a t io n s w ith in th e low er san dston e
member.
R e sto r a tio n o f s e c t io n lo c a t i o n s to t h e ir approxim ate
d e p o s it io n a l p o s it io n s (F ig . 3 ) was accom plished u sin g e s tim a te s o f
d isp la cem en t from Schmidt and o th e r s (1 9 8 1 ), Woodward (1 9 8 1 ), and D.
Lageson, (p e r so n a l com m unication, 1 9 8 5 ).
Accuracy o f th e e s tim a te s o f
t e c t o n ic d isp la cem en t i s u n c e r ta in due t o lim it e d knowledge o f
su b su rface s t r u c t u r e s .
The stu d y area was d iv id e d in t o th r e e
s t r u c t u r a lly d i s t i n c t g eo g ra p h ic a l a r e a s in order to a s s e s s th e amount
o f d isp la c e m e n t.
S tr a ta in th e Elkhorn Mountains and J e ffe r s o n Canyon
have been d is p la c e d 3 0 -6 0 k ilo m e te r s eastw ard by movement a lo n g the
Lombard-Eldorado th r u s t zone (Schm idt and o th e r s , 1981; Woodward,
1981).
The B rid ger Range, H orseshoe H i l l s , and S ix te e n m ile area are
p art o f th e f r o n t a l f o ld and th r u st b e lt where d isp la cem en t o f s t r a t a
i s t o th e e a s t and on th e order o f 5 k ilo m e te r s (Woodward, 1981;
Lageson, D ., p erso n a l com m unication, 1 9 8 5 ).
S tr a ta in th e northern
G a lla tin range have been d is p la c e d 1-2 k ilo m e te r s to th e sou th w est by
Laramide s t y l e basement u p l i f t (L ageson, D ., p erso n a l communication,
1985).
4
R4E
ELKHORN
T5N
MOUNTAINS
I
njg
H ORSESHOE
HILLS
Three Forks
Bozeman
Study Area
Miles
X
GALLATIN
RANGE
Kilometers
F igu re 2.
Map o f stu d y a r e a . L in es mark th e ou tcrop tr a c e o f the
low er sa n d sto n e member o f th e Therm opolis S h a le .
L e tte r s n o te th e lo c a t io n s o f measured s e c t io n s .
JG-Johnny Gulch
J C -J e ffe r so n Canyon
CG-Cottonwood Gulch
NG-Nixon Gulch
PR-Paddys Run
MF-Middle Fork S ix te e n m ile
FC-Fairy Creek
BC -Brackett Creek
S B -S lu iceb ox
RC-Rocky Canyon
GP-Goose Pond
S C -S trick la n d Creek
WM-Wineglass Mountain
PB-Park Branch
5
N
|
<w— P R
o t— M F
•#— NO
W -C G
o 4 —• F C
#
o f — BC
Three Forks
I— ------
<*-S
B
LEGEND
#
R estored
Present
location
location
Direction and m agnitude
of r e s to r a t io n
I-"* 4
F igu re 3•
P o t e n t i a l e r r o r In r e s t o r a t i o n
Bozeman
9....? 1P
Miles
#
Livingston
RC,
GP jf
Li-L0
W M
s c '/
PB
Kilom eters
Map showing p a lin s p a s t i c a ll y r e sto r e d lo c a t io n s o f
measured s e c t io n s o f the low er sandstone member,
Therm opolis S h a le .
W ithin each s t r u c t u r a l a rea , s t r a t a have undergone s im ila r amounts
o f d isp la c e m e n t.
W hile th e g en era l tren d o f th e p a le o s h o r e lin e may be
somewhat a f f e c t e d , major j u x t a p o s itio n o f d e p o s itio n a l environm ents
w ith in each s t r u c t u r a l area i s not thought to have occu rred .
The amounts o f d i f f e r e n t i a l movement vary between th e s tr u c tu r a l
areas.
D i f f e r e n t i a l movement between s t r a t a in th e f r o n t a l fo ld and
th r u st b e lt and s t r a t a in th e northern G a lla tin Range i s estim a ted to
be o n ly 6-7 k ilo m e te r s .
The d e p o s it io n a l environm ent o f th e lower
san d ston e member in th e s e two s t r u c t u r a l regim es i s thought to be
s im ila r , as w i l l be d is c u s s e d l a t e r , and th e d isp la cem en t i s not
thought to ju x ta p o se d e p o s it io n a l en vironm en ts.
On th e o th er hand,
t e c t o n ic movement in th e Elkhorn Mountains and a t J e f f e r s o n Canyon
6
produced ap p roxim ately 25-6.0 k ilo m e te r s o f t e le s c o p in g o f s t r a t a
r e l a t i v e to th e f r o n t a l f o ld and th r u st a rea .
S tr a ta o f th e two
r e g io n s are much c lo s e r to g e th e r than when th ey were d e p o s ite d .
D e sp ite th e eastw ard com p ression , th e Elkhorn M ountains and J e ffe r s o n
Canyon exp osu res are s t i l l lo c a te d more than 15. k ilo m e te r s w est o f th e
r e s t o f th e stu d y a r e a .
Thus, a lth o u g h th e a b s o lu te p o s it io n i s n o t
th e same, th e r e l a t i v e e a s t - w e s t p o s it io n s remain s im ila r .
P rev io u s S tr a tig r a p h ic I n v e s t ig a t io n s
P rev io u s s t r a t ig r a p h ic in v e s t ig a t i o n s o f th e low er san dston e
member d e s c r ib e r e g io n a l c o r r e la t io n s and g iv e g e n e r a l environm ental ■
in t e r p r e t a t io n s (R o b erts, 1965 and 1972; Schw artz, 1972 and 1 982).
A
thorough rev iew o f th e n om en clatu re! h is t o r y o f th e low er sandstone
member i s g iv e n 1by R oberts (1972, p .C l6 ) .
T his term in o lo g y fo r th e
Lower C retaceou s s tr a tig r a p h y in th e L iv in g sto n , Montana a rea i s used
h ere.
R egion al rock s t r a t ig r a p h ic e q u iv a le n ts (F ig . 4) o f th e low er
san dston e member o f th e Therm opolis S h ale in c lu d e the b a sa l sandstone
o f th e Flood Member o f th e B la c k le a f Form ation in n orth w est Montana
(R o b erts, 1965); I n te r v a l A o f th e B la c k le a f Formation in extreme
sou th w est Montana (Schw artz, 1972); th e F i r s t Cat Creek Sand in
c e n tr a l Montana (R ich a rd s, 1957); th e wRusty Beds" and G reybull
Sandstone Member o f th e Therm opolis S h ale in c e n tr a l Wyoming (R ob erts,
1965); and th e F a ll R iver Form ation o f th e Black H i l l s in n o rth ea st
Wyoming and w estern North Dakota (R o b erts, 1 9 6 5 ).
Between c e n tr a l
Montana and th e Black H i l l s t h i s in t e r v a l changes to a s i l t s t o n e or
sh a le (R o b erts, 1 9 7 2 ).
MONTANA
Aption [
Albion
HBlockIeof Form ation
LOWER
CRETACEOUS
N orthw estern
B o u ld e r t o
C u t Bank
^Bootlegger
Lbr
Vaughn mbr
Taft
Hill
member
Flood
member
T l
Int D
Int C
Int B
Int A
rXmm
K ootena i
F o r m a t ion
Figure 4.
Southw estern
Drummond
t o Dillon
K ootenai
Form ation
WYOMING
Livingston
Area
S o u th C e n t r a l
Columbus
to Hardin
Mowry S h a le M ow ry Shale
Central
Powd er River
Basin
Mowry
M u d d y SS
member
U p p e r SS
member
M iddle
shale
member
L o w e r SS
member
T
L IT T T
K ootenai
Form ation
Shale
N ortheastern
B la ck
Hills
M o w r y Shale
N ew castle
Sandstone
S k u ll
Creek
Shale
R usty beds
T
Fall River
T i i IT i
C loverly
F o r m a t io n
Cloverly
Form ation
Lakota
Formation
Stratigraphic correlation chart of Early Cretaceous rocks in Montana and Wyoming.
Modified from Roberts (1965 and 1972) and Schwartz (1972).
8
The low er sa n d sto n e member o f th e Therm opolis S h ale has been
d escrib ed a s a 15 to 30 m eter th ic k sequence o f in terb ed d ed very f in e
to medium grain ed q u a r tz a r e n ite , s i l t s t o n e and sh a le (R o b erts, 1965
and 1972; Schw artz, 19 7 2 ). The sa n d sto n es a re th in to th ic k bedded and
c o n ta in a v a r ie t y o f sedim entary s t r u c t u r e s , in c lu d in g cro ssb ed d in g ,
r ip p le c r o s s -la m in a tio n , and symmetric and asymmetric r ip p le form s.
Burrows and t r a i l s w ith in th e low er sa n d sto n e member, in c lu d in g
Ophiomorpha. a re a sc r ib e d to marine bottom d w e llin g organ ism s.
S h a les
are dark gray and appear s im ila r to s h a le in th e m iddle s h a le member
o f th e Therm opolis S h a le .
R oberts (1972) and Schwartz (1972) both
in t e r p r e t th e low er sa n d sto n e member a s a n earsh ore marine u n i t .
d e p o s ite d d u rin g i n i t i a l tr a n s g r e s s io n o f th e C retaceou s seaway in to
sou th w est Montana.
Throughout c e n t r a l and w estern Montana th e low er san d ston e member
i s u n d e r la in by th e Lower C retaceou s (A ptian) K ootenai Formation
( S u ttn e r , 1969; R ob erts, 1 9 7 2 ).
R oberts (1972) d e s c r ib e s th e upper 75
m eters o f th e K ooten ai Form ation as a
p o o rly exposed sequence o f
v a r ie g a te d red , p u rp le, g reen and gray c a lc a r e o u s s i l t s t o n e ,
c la y s t o n e , m udstone, lim e sto n e and t u f f th a t c o n ta in s d isc o n tin u o u s
beds o f c a lc a r e o u s sa n d sto n e.
T his sequence was d e p o s ite d in stream
ch a n n els and upon a s s o c ia te d flo o d p la in s o f an e x t e n s iv e lowland
w ith in th e embryonic w estern in t e r io r fo rela n d b a s in .
At th e top o f
th e in t e r v a l i s a b io m ic r ite c o n ta in in g u n id e n t if ia b le le a ch ed f o s s i l
molds and l o c a l l y f o s s i l i f e r o u s lim e sto n e c o n ta in in g sm a ll fr e s h w ater
g a str o p o d s, p elecy p o d s and o s tr a c o d e s .
The lim e sto n e was d ep o sited in
a fr e s h to p o s s ib ly b ra ck ish w ater la k e or s e r i e s o f la k e s ex ten d in g
9
from n orth w est Wyoming in t o Canada and a s f a r e a s t as Harlowtown in
c e n tr a l Montana (S u ttn e r , 1969; R ob erts,
1972; Holm and o th e r s ,
1977).
The n atu re o f th e c o n ta c t between th e K ootenai Form ation and the
o v e r ly in g T herm opolis
S h ale v a r ie s throughout Montana.
R oberts
(1972) s t a t e s th a t th e low er san d ston e member commonly f i l l s
top ograp h ic d e p r e s s io n s on an u n d e rly in g e r o s io n a l s u r fa c e in the
L iv in g sto n , Montana a r e a .
The B la c k le a f-K o o te n a i c o n ta c t i s a ls o
d iscon form a b le on th e S w e etg ra ss. Arch in n orth w est Montana (Cobban and
o th e r s , 1 9 7 6 ).
Schwartz (1972) observed th e c o n ta c t to be g ra d a tio n a l
in w esternm ost Montana and noted th a t i t becomes more unconform able
ea stw a rd .
For th e p u rposes o f t h i s stu d y , th e c o n ta c t i s reco g n ized
by th e p resen ce o f n o n -o a lca reo u s q u a r tz a r e n ite or g r a y .s h a le o f .t h e
low er san d sto n e member, o v e r ly in g v a r ie g a te d , c a lc a r e o u s mudstone or
b io m ic r ite o f th e K ooten ai Form ation.
The m iddle sh a le member o f th e T herm opolis Shale l i e s in sharp
c o n ta c t w ith th e u n d e rly in g low er san d ston e member.
member i s composed o f dark gray s h a le , c o n ta in s minor
The m iddle sh a le
t h in sandstone
le n s e s and was d e p o s ite d in an o ffs h o r e marine environm ent s u b je c t to
red u cin g c o n d itio n s (R o b erts, 1972; Vuke, 1 982).
Methods
S tr a tig r a p h ic and l i t h o f a c i e s d a ta were gath ered from outcrop
e x p o su res.
S ev era l o f th e exp o su res were c it e d in th e li t e r a t u r e
(R ob erts, 1972; Schw artz, 1972; James, 1 9 7 7 ), most were lo c a te d by
r e c o n n a issa n c e .
Fourteen s e c t io n s (F ig . 2) were measured in d e t a il
u sin g a Jacobs s t a f f or ta p e .
Many o th er more p o o rly exposed s e c t io n s
10
were b r i e f l y d e s c r ib e d .
L ith o lo g y and p h y s ic a l and b io g e n ic
sedim entary s t r u c tu r e s were d escrib ed in d e t a i l .
u sin g th e c l a s s i f i c a t i o n scheme o f Folk (1 9 8 0 ).
S an dston es are named
Mudrocks are
c l a s s i f i e d a cco rd in g to th e method o f Lundegard and Samuels (1 9 8 0 ).
Samples were c o l le c t e d o f a l l l i t h o l o g i e s a t each measured s e c t io n .
T h irty th in s e c t io n s were prepared, o f which f i f t e e n were s ta in e d to
d eterm ine p otassium f e ld s p a r c o n t e n t . In ord er to a s s e s s p o s s ib le
provenance o f th e low er san d ston e member, a l l sam ples were stu d ie d
w ith a b in o c u la r p etro g ra p h ic m icroscope and te n th in s e c t io n s were
p o in t counted to determ ine r e l a t i v e abundances o f d e t r i t a l g r a in s .
FACIES DESCRIPTIONS
Roberta (1972) and Schwartz (1972) have both d esc r ib e d gen era l
c h a r a c t e r is t ic s o f th e low er san d ston e member and have a ssig n e d i t to
a n earsh ore marine s e t t i n g .
T his stu d y expands upon t h e i r work by
p ro v id in g a more d e t a ile d in t e r p r e t a t io n o f th e sed im en to lo g y and
h y d ra u lic regim e o f th e d e p o s it io n a l environm ent o f th e low er
san d ston e member.
In ord er to f a c i l i t a t e in t e r p r e t a t io n o f
d e p o s it io n a l p r o c e s s e s , s e v e r a l f a c i e s have been d is c r im in a te d in th e
low er san d ston e member.
A primary way in which d if f e r e n t d e p o s itio n a l
p r o c e s s e s may be ex p ressed i s in th e typ e o f sedim ent d e p o s ite d by
each p r o c e s s .
I n i t i a l d is c r im in a tio n o f f a c i e s ty p e s i s th e r e fo r e
based upon l i t h o l o g i e s and l i t h o l o g i c a s s o c ia t io n s p r e s e n t.
Sandstone
and s h a le a re th e prim ary l i t h o l o g i e s in th e low er sa n d sto n e member
and f a c i e s a re su b d ivid ed on th e r e l a t i v e abundance o f ea ch .
This
method o f f a c i e s d is c r im in a tio n i s commonly used in s i l i c i c l a s t i c
f a c i e s a n a ly s is (R eading, 1 9 7 8 ).
F a c ie s ty p e s d e fin e d f o r th e low er
san d ston e member a r e : Sandstone (>90% sa n d ), H e t e r o lit h ic (50-90% sand
and 10-50% mud) and Mud (>90% mud).
These f a c ie s r e p r e se n t a near
continuum o f p h y s ic a l en ergy c o n d itio n s w ith th e e x c e p tio n o f the
absence o f d e p o s it s w ith 10-50% sand and 50-90% mud.
These
l i t h o l o g i e s may a c t u a lly be p resen t in th e low er san d ston e member but
were n ot observed b ecause o f t h e ir low r e s is t a n c e to w ea th erin g
coupled w ith th e g e n e r a lly in com p lete exp o su res o f th e lo w er sandstone
member. As w i l l be shown l a t e r , th e r e i s no c o n s is t e n t , r e g io n a l
12
s ta c k in g o rd er to th e s e f a c i e s in th e low er san dston e member and th e
ord er in which th ey are d is c u s s e d d oes n ot imply any v e r t i c a l order to
th e low er sa n d sto n e member.
.Sandstone F a c ie s
The san d ston e f a c i e s com p rises 64% o f th e low er san d ston e member
and c o n s i s t s o f >90% sand.
Sandstones a re composed of. very f in e to
medium g r a in e d , c h e r t-b e a r in g ,, s ilic a -c e m e n te d q u a r tz a r e n ite .
M u d sto n e-c la sts ( t o p eb b le s i z e ) a re a m inor, y e t p e r s is t e n t ,
component o f t h i s f a c i e s and occur a s i s o la t e d c l a s t s or d isc o n tin u o u s
la g s w ith in sa n d sto n e seq u en ces.
S h ale in te r b e d s com prise <10% o f th e
san d ston e f a c i e s and a re u s u a lly p r e se n t as very t h in (<5 m illim e te r s
t h ic k ) , d is c o n tin u o u s le n s e s or s l i g h t l y th ic k e r (a p p ro x im a tely I
c e n tim e te r t h ic k ) , p e r s is t e n t b ed s.
Sandstones are medium to th ic k bedded (t o 2 m eters) and co n ta in
d ecim eter s c a l e , h ig h a n g le (1 5 -3 0 d eg ree) ta b u la r to wedge planar
(F ig . 5) and trough c r o ssb e d s; h o r iz o n ta l to low a n g le p la n a r
s t r a t i f i c a t i o n ; hummocky c r o s s - s t r a t i f i c a t i o n ; and r ip p le c r o s s ­
la m in a tio n .
S e v e r a l ty p e s o f r ip p le c r o s s -la m in a tio n a re p resen t in
th e san d ston e f a c i e s .
F o r e se t ty p e c r o s s -la m in a tio n s w ith a ta b u la r
or trough geom etry are p r e se n t but th e most abundant c r o s s -la m in a tio n
typ e i s c h a r a c te r iz e d by an in t e r n a l s tr u c tu r e o f s tr o n g ly t a n g e n t ia l
c r o s s r la m in a tio n s which may form o f f s h o o t s th a t ascend on to a d ja cen t
r ip p le s (F ig . 6 ) .
These c r o s s -la m in a tio n s have a v a r ia b le a n g le o f
rep ose and th e low er bounding su r fa c e o f r ip p le s e t s i s sco o p in g and
u a d u la to r y .
Even, low a n g le and u n d u latory la m in a tio n s a re fr e q u e n tly
a s s o c ia te d w ith th e s e c r o s s - la m in a t io n s .
Symmetric and asym m etric,
13
F igu re 5.
Planar cro ssb ed s in th e san dston e f a c i e s . L in es on Jacob
s t a f f r e p r e se n t 6 in ch i n t e r v a l s . From W in eglass Mountain
ex p o su re.
F igure 6 .
Wave r ip p le c r o s s -la m in a tio n in crossbedded san dston e
f a c i e s . From W ineglass Mountain ex p o su re.
14
lin g o id (F ig . 7) r ip p le forms are common in th e san d ston e f a c ie s w ith
symmetric forms th e more common t y p e .
F igure 7 .
Asymmetric lin g o id r ip p le marks in th e sa n d sto n e f a c i e s .
From W in eglass Mountain ex p o su re.
A s s o c ia tio n s o f sedim entary s t r u c t u r e s w ith in th e sandstone
f a c i e s are h ig h ly v a r ia b le .
Many sa n d sto n e in t e r v a ls a re composed
e n t i r e l y o f one ty p e o f sedim entary s t r u c t u r e . These in t e r v a l s may
c o n s is t o f e i t h e r s in g le bed or m u ltip le bed seq u en ces.
In a d d itio n ,
some san dston e in t e r v a l s c o n s is t o f in terb ed d ed sedim entary
s t r u c t u r e s , e s p e c i a l l y d ecim eter s c a le c r o s s b e d s, p lan ar la m in a tio n s
and c r o s s -la m in a t io n s . No ordered s p a t i a l arrangem ent o f th e se
s tr u c tu r e s was o b serv ed .
T his may in p art be due to th e o f t e n poor
exposure o f sedim entary s t r u c t u r e s .
R e la tiv e abundance o f th ese
15
s tr u c tu r e s i s h ig h ly v a r ia b le .
In g e n e r a l, ta b u la r to w edge-planar
cro ssb ed s are most common, w h ile trough c r o s s b e d s, r ip p le c r o s s ­
la m in a tio n , and h o r iz o n ta l to low a n g le s t r a t i f i c a t i o n a re l e s s
abundant. Hummocky c r o s s - s t r a t i f i c a t i o n i s p resen t o n ly in th e Elkhorn
M ountains.
The geom etry o f th e se san d ston e in t e r v a ls i s m ostly
obscured because san d ston e in t e r v a ls u s u a lly p e r s is t beyond the li m it s
o f most e x p o su res.
Where observed (W in eglass Mountain e x p o su r e ), th e
geom etry i s markedly p lan ocon vex, w ith a f l a t base and convex-up,
furrow ed, upper-bounding su r fa c e (F ig . 8 ) .
The in t e r n a l
s t r a t i f i c a t i o n o f th e san d ston e b o d ie s i s form d isc o r d a n t and i s
com prised o f c r o s s b e d s, c r o s s - la m in a t io n s , and p lan ar la m in a tio n s.
16
B io tu r b a tio n in th e san d ston e f a c i e s ta k es two form s. In
g e n e r a l, tr a c e f o s s i l s a re r a r e , but where observed a re u s u a lly
c u r v ilin e a r , w rin k led , knobby c y lin d e r s , 1 -1 .5 c e n tim e te r s in diam eter
and s e v e r a l c e n tim e te r s lo n g th a t emerge o b liq u e ly from bedding
p la n e s .
These are probably a form o f Ophiomorpha.
A c o n c e n tr a tio n o f
la r g e r Ophiomorpha. 5 c e n tim e te r s in d iam eter and 30 c e n tim e te r s lo n g
i s p r e se n t a t th e top o f a crossbedded san dston e in t e r v a l a t th e
B ra ck ett Creek s e c t io n .
H o r iz o n ta l, c u r v ilin e a r , n on-branching to
s l i g h t l y branching burrows, 1-2 m illim e t e r s in d iam eter and s e v e r a l
c e n tim e te r s lo n g , occur in convex e p i - and h y p o - r e lie f a t many
s a n d s to n e /s h a le c o n t a c t s .
These u n id e n t if ie d t r a i l s a re common
fe a tu r e s throughout th e low er san dston e member.
Sandstone i n t e r v a l s may a ls o be th o ro u g h ly b io tu rb a ted w ith
in t e n s e ly d is t u r b e d . la m in a tio n s or a m o ttled te x tu r e o f w h ite sand
b le b s in a gray san d ston e m a trix .
V e r t ic a l burrows, s e v e r a l
m illim e t e r s in d ia m eter, a re p resen t in s e v e r a l b io tu rb a ted beds
■
though in g e n e r a l, in d iv id u a l burrows cannot be d is t in g u is h e d in th e se
b ed s.
Where b io tu rb a ted san d ston e beds o v e r lie and a r e in sharp
co n ta c t w ith sh a le in t e r v a l s th e san d ston e bed fr e q u e n tly c o n ta in s
numerous U-shaped burrows on i t s low er bounding's u r fa c e .
Sandstone F a c ie s I n te r p r e ta tio n
The sa n d sto n e f a c i e s i s in te r p r e te d to have been d e p o s ite d under
a v a r ie t y o f flo w c o n d it io n s .
Tabular to w edge-planar d ecim eter s c a le
cro ssb ed s (F ig . 5) in th e san dston e f a c i e s were produced by m igratin g
s t r a ig h t c r e s te d m eg a rip p les. ( R eineck and S in gh , 1980, p . 3 8 ) .
D ecim eter s c a le trough cro ssb ed s a re a t t r ib u t e d to m ig ra tin g dunes or
17
lu n a te m egarip p les (R eading, 1978, p. 233; Reineck and S in g h , 1980, p.
3 9 ).
M egaripples are low er flo w regim e bedform s, produced under u n i­
d ir e c t io n a l flo w c o n d it io n s , which have d im en sion s o f 60 c en tim e ters
to 30 m eters in le n g th and 6 c e n tim e te r s to 1 .5 m eters in h eig h t
( R eineck and S in gh , 1980, p . 4 1 ). Lower flo w regim e bedform
developm ent i s m o stly dependent on flo w v e l o c i t y and g r a in s i z e , w ith
a r e l a t i v e l y minor in f lu e n c e by w ater depth in w e ll sorted , quartz
sands l i k e th o se o f th e low er sa n d sto n e member.
For f i n e to medium
san d s, v e l o c i t i e s o f 4 0 -7 0 cm /sec a re n e c e ssa r y fo r s t r a ig h t - c r e s t e d
m egaripple form ation w h ile lu n a te m egarip p les re q u ir e s l i g h t l y h ig h er
flo w v e l o c i t i e s , m o stly more than 70 cm /sec in w ater d ep th s o f more
than 2 m eters ( R eineck and S in gh , 1980, p. 4 3 ) .
R ip p le c r o s s -la m in a tio n in th e san d ston e f a c i e s i s d ia g n o s tic o f
both o s c i l l a t o r y and u n i- d ir e c t io n a l c u r r e n ts . W ave-generated r ip p le
c r o s s -la m in a tio n s (F ig . 6) were rec o g n ized u sin g c r i t e r i a developed by
Boersma (1970; c it e d in Reading, 1978, p . 2 5 0 ).
These c r i t e r i a
in c lu d e c r o s s -la m in a tio n s th a t are c h a r a c t e r i s t i c a l l y form d isco rd a n t
w ith in t e r n a l s tr u c tu r e th a t may: I) c o n ta in bundled u p b u ild in g where
bu nd les a r e th in and o p p o s it e ly d ip p in g , 2) c o n ta in s t r o n g ly
ta n g e n t ia l c r o s s la m in a tio n s which in extrem e c a s e s form o f f s h o o t s
th a t sweep up and ascend on to a d ja cen t r i p p l e s , 3) have a v a r ia b le
a n g le o f r e p o se , an d /or 4) have
c r o s s -la m in a tio n s t r u c tu r e which
d i f f e r s in sep a ra te c r o s s s e c t io n s through a s in g le r i p p le .
In
a d d itio n , th e low er bounding su r fa c e o f r ip p le s e t s i s sco o p in g and
u n d u latory and th e s t r u c t u r e s are fr e q u e n tly a s s o c ia te d w ith even
18
la m in a tio n o f th e s t r a i g h t , hummocky or low a n g le ty p e .
Symmetrical
and l e s s freq u en t asym m etrical r ip p le marks (F ig . 7) a re a s s o c ia te d
w ith th e r ip p le c r o s s - la m in a t io n s .• T h is v a r ie t y o f r ip p le s tr u c tu r e
in d ic a t e s th a t bedform developm ent d u rin g d e p o s itio n o f th e low er
san d ston e member was in flu e n c e d by both u n i- d ir e c t io n a l and
o s c i l l a t o r y flo w s .
Some th in seq u en ces o f r ip p le c r o s s -la m in a tio n , in terb ed d ed w ith
m egaripple c r o s s b e d s, e x h ib it sharp b a sa l c o n ta c ts and interw oven
upper c o n ta c ts w ith th e r ip p le f o r e s e t s d ip p in g in a d ir e c t io n
o p p o s ite to th e o v e r ly in g m egaripple f o r e s e t s .
These sm a ll r ip p le s
were probably formed by backflow c u r r e n ts in th e flo w se p a r a tio n zone
t o th e l e e o f m ig ra tin g m egarip p les ( R eineck and S in gh , 1980, p. 2 8 ).
H o r izo n ta l to lo w -a n g le p lan ar s t r a t i f i c a t i o n may be a ttr ib u te d
to e i t h e r u n i- d ir e c t io n a l or o s c i l l a t o r y flo w .
A sso c ia te d sym m etrical
r ip p le forms s u g g e st th a t th e s e s t r u c tu r e s were probably produced
under o s c i l l a t o r y flo w c o n d it io n s . However, i f h o r iz o n ta l
s t r a t i f i c a t i o n was produced under u n i- d ir e c t io n a l flow c o n d itio n s ,
flo w v e l o c i t i e s o f >100 cm /sec (Harms and o th e r s ,
1982, p . 2 -1 4 ) must
have been common d u rin g d e p o s itio n o f th e low er san d ston e member.
Hummocky c r o s s - s t r a t i f i c a t i o n , which was observed o n ly in the
Elkhorn M ountains, i s a t tr ib u te d to storm wave a c tio n w ith su rges o f
g r e a te r d isp la cem en t and v e l o c i t y than th o se req u ired to form wave
r ip p le s (Harms and o th e r s , 1982, p. 7 - 9 ) .
Hummocky c r o s s ­
s t r a t i f i c a t i o n i s t y p ic a l o f a wave-dom inated sh o r e fa c e s e t t i n g
( R eineck and S in gh , 1980, p. 398; Harms and o th e r s , 1982, p. 7 - 9 ) .
H ighly b io tu rb a ted sa n d sto n es in d ic a t e p e r io d s o f slow
19
d e p o s it io n .
The low sed im en ta tio n r a t e s may be due to l o c a l f a c t o r s
which cau se sed im en t-b ea rin g c u r r e n ts to circum vent th e area or low
r a te s o f su b sid en ce th a t in h ib it accu m u lation o f sed im en t.
M u ltip le-b ed sa n d sto n e in t e r v a ls ( e . g . a t W in eglass Mountain
exp osu re) are c h a r a c te r iz e d by th e p resen ce o f in terb ed d ed cro ssb ed s
and c r o s s -la m in a tio n s th a t are fo rm -d isco rd a n t to th e san dston e
b o d ie s ’ planoconvex geom etry.
T his r e f l e c t s a h iera rch y o f bedforms
in which th e la r g e , f i r s t order f e a t u r e s a re th e sand b o d ie s
th e m s e lv e s .
The in t e r n a l s tr u c tu r e o f th e s e san dston e b o d ie s i s form
d is c o r d a n t, a s th e s t r u c t u r e s were produced by th e sm a lle r m egaripp les
and r ip p le s which r e p r e se n t th e second and th ir d ord er bedform s,
r e s p e c t i v e ly .
W hile most sedim entary s tr u c tu r e s in th e san dston e f a c i e s are
/
a t tr ib u te d t o bedload tr a n s p o r t, th e r e i s a ls o ev id en ce fo r c e s s a t io n
o f flo w .
The p resen ce o f sh a le in te r b e d s w ith in some san d ston e
in t e r v a l s documents q u ie t p e r io d s , as do h o r iz o n ta l f o s s i l t r a i l s
produced by d e p o s it fe e d e r s t y p ic a l o f low energy c o n d it io n s (Howard,
1 978).
D ir e c t ev id e n c e fo r marine in flu e n c e in th e sa n d sto n e f a c ie s i s
la c k in g due to th e a b sen ce o f body f o s s i l s in th e low er san dstone
member.
The occu rren ce o f Ophiomorpha i s u s u a lly i n d ic a t i v e o f a high
energy m arine environm ent, though i t has a ls o been found in t u r b id it e s
and f l u v i a l d e p o s it s (Hoyt and Weimer , 1965; Frey and o t h e r s , 1978).
In th e absence o f body f o s s i l s , Oohiomoroha i s th e b e s t fa u n a l
ev id en ce fo r marine in f lu e n c e in th e low er san dston e member.
20
Heterolithic Facies
. The h e t e r o l i t h i c f a c i e s com p rises 14% o f th e low er san dston e
member and c o n s i s t s o f 50-90% very f in e to f i n e g r a in e d , c h e r t and
m u d sto n e -c la st b ea rin g q u a r tz a r e n ite in terb ed d ed w ith gray s h a le .
■
F in e -g r a in e d carbonaceous d e b r is i s a common, y e t minor component o f
t h i s f a c i e s and i s u s u a lly co n cen tra ted on la m in a tio n or bedding
su r fa c e s.
Trace f o s s i l s vary in abundance from n o n - e x is t e n t to
abundant on bedding s u r f a c e s .
They in c lu d e sm all h o r iz o n ta l t r a i l s
and l e s s freq u en t U-shaped burrow s.
Where U-shaped burrows are
p r e s e n t, th ey a re abundant and u s u a lly merge in t o v e r t i c a l burrows in
th e o v e r ly in g san d ston e bed.
S m all, h o r iz o n ta l T h a la s s in o id e s burrows
a re p r e se n t a lo n g s a n d s to n e -s h a le c o n ta c ts a t s e v e r a l s e c t io n s .
The
h e t e r o l i t h i c f a c i e s i s fu r th e r su b d iv id ed in t o th e -sand-dominated and
mixed s u b - f a c ie s on th e b a s is o f r e l a t i v e abundances o f sand and mud.
The sand-dom inated s u b - f a c ie s c o n s is t o f 75-90% sand w ith th e
rem ainder b ein g mud.
T h is s u b - f a c ie s i s c h a r a c te r iz e d by ta b u la r to
s l i g h t l y l e n t i c u l a r , p e r s is t e n t sa n d sto n e b ed s, 5-25 c e n tim e te r s
th ic k , sep a ra ted by s h a le in t e r v a ls app roxim ately 5 m illim e t e r s th ic k
(F ig . 9 ) .
Sandstone beds are t y p i c a l l y in sharp c o n ta c t w ith the
a d ja cen t s h a le and upper bedding s u r fa c e s are fr e q u e n tly sym m etrical
an d . s l i g h t l y asym m etrical rip p ie-m a rk ed .
H o r iz o n ta l^ p a r a ll e l
la m in a tio n s a re p resen t in th e low er 1 /3 o f san dstone b ed s.
The
m iddle and upper p o r tio n s o f each bed u s u a lly appear m a ssiv e, though
some e x h ib it d isc o n tin u o u s r ip p le c r o s s -la m in a tio n .
S m a ll, h o r iz o n ta l
t r a i l s a re common on upper r ip p le d bedding su r fa c e s as a re minor
amounts (<1%) o f f in e - g r a in e d carbonaceous d e b r is .
T h is s u b - f a c ie s
21
F igu re 9.
Sandstone beds sep arated by th in sh a le in t e r v a l s in sanddominated s u b - f a c i e s . From outcrop a lon g South Fork
S ix te e n m ile Creek.
u s u a lly forms r e p e t i t i v e seq u en ces o f a lt e r n a t in g san d ston e and sh a le
in bed s e t s up to 3 m eters t h ic k .
The mixed s u b - f a c ie s c o n ta in s 50-75$ sand and 25-50$ mud. This
s u b - f a c ie s i s u s u a lly com prised o f r ip p le c r o ss-la m in a te d sandstone
w ith a s s o c ia t e d sym m etrical and asym m etrical r ip p le forms (F ig . 1 0 ).
A gain, r ip p le c r o s s -la m in a tio n s w ith t a n g e n t ia l c r o s s -la m in a tio n s and
scoop in g to u n du latory low er bounding s u r fa c e s are more abundant than
f o r e s e t typ e c r o s s -la m in a tio n s c h a r a c te r iz e d by a ta b u la r or trough
geom etry.
H o r iz o n ta l, p lan ar to wavy lam inated and s t r u c t u r e le s s
th in ly -b ed d ed sa n d sto n es a re a s s o c ia te d w ith th e r ip p le c r o s s lam inated sa n d sto n e.
S h ale i s p resen t as d ra p es, th in l e n s e s , or
22
F igu re 10.
F la s e r bedded san dston e and s h a le in th e mixed s u b - f a c ie s .
From W in eglass Mountain s e c t io n .
p e r s is t e n t beds g e n e r a lly <1 c e n tim e te r t h ic k . This v a r ie t y o f
com plexly in terb ed d ed san dston e and s h a le i s o fte n r e fe r r e d to as
f l a s e r bedding (R eading, 1978, p. 233; Reineck and S in gh , 1980, p.
1 1 3 ).
Fining-upw ard seq u en ces th a t a re l o c a l l y p resen t w ith in t h is
s u b - f a c ie s are c h a r a c te r iz e d by a b a sa l bed, 5-30 c e n tim e te r s th ic k ,
which u s u a lly appears s t r u c t u r e le s s but may co n ta in h o r iz o n t a l, planar
to wavy la m in a tio n s .
The b a sa l bed i s o v e r la in by s e v e r a l cen tim e ters
o f t h in ly le n tic u la r -b e d d e d , plan ar t o wavy lam inated and c r o s s lam inated san dston e w ith minor in terb ed d ed s h a le .
in c r e a s e s upward in th e s e seq u en ces.
S h ale co n ten t
23
Heterolithic Facies Interpretation
D e p o sitio n o f com p lexly in terb ed d ed san d ston e and s h a le req u ir es
high energy e v e n ts , in which sand i s in tro d u ced , a lt e r n a t in g w ith
q u ie t p e r io d s , d u rin g which c la y i s d e p o s ite d from su sp e n sio n . In
sh a llo w marine en vironm en ts, th e s e req u irem en ts may be met by t id a l
a c tio n (A lle n , 1982, p . 511) or e p is o d ic storm e v e n ts (G oldrin g and
B r id g e s, 1973; De Raaf and o th e r s , 1 9 7 7 ).
G oldring and B rid g es (1973) d e s c r ib e a s e r i e s o f t h in , sh e e t
sa n d sto n e s, which th ey r e f e r to as s u b l i t t o r a l sh e e t sa n d sto n e s, th a t
a r e common t o a n c ie n t sandy s h e l f seq u en ces.
T y p ic a lly th e main part
o f th e san d sto n e bed i s f i n e to very f i n e , w e ll s o r te d , p a r a lle l
lam inated sa n d sto n e.
W ave-generated r ip p le c r o s s -la m in a tio n s may be
p r e se n t near th e to p o f san dston e beds and upper bounding su r fa c e s may
be m od ified by o rg a n ic burrow ing.
B ecause o n ly th e to p s o f sandstone
beds are burrowed, i t i s thought th a t th e s e san dston e beds are s in g le
ev en t d e p o s it s .
G oldring and B rid g es (1973.) su g g est th a t storm waves,
p o s s ib ly in com bination w ith ebb t i d a l c u r r e n ts or storm s u r g e s , were
im portant p r o c e s s e s d u rin g d e p o s itio n o f s u b l i t t o r a l s h e e t sa n d sto n es.
Shallow w ater t u r b id it y c u r r e n ts , r ip c u r r e n ts and tsunam is may have
a ls o played a r o le in t h e ir d e p o s it io n .
G oldring and B rid g es (1973)
n o te th a t s u b l i t t o r a l s h e e t sa n d sto n es occur e it h e r as amalgamations
o f in d iv id u a l san dston e beds sep a ra ted by th in sh a le i n t e r v a l s or as
amalgamated beds w ith l i t t l e or no in terb ed d ed s h a le .
They su g g est
th a t amalgamated beds th a t la c k in terb ed d ed sh a le a re a s s o c ia te d w ith
s h o r e lin e s w h ile s h e e t sa n d sto n es sep a ra ted by sh a le in te r b e d s are
c h a r a c t e r is t ic o f a sh a llo w marine open s h e l f .
In. th e low er sandstone
24
member, d e p o s it s o f th e sand dominated s u b - f a c ie s are s im ila r to
G oldring and B r id g e s' (1973) sh a llo w m arine open s h e l f , sh e e t
sa n d sto n e s.
The sand-dom inated s u b - f a c ie s i s th e r e fo r e in te r p r e te d as
a storm - and w ave-gen erated sequence d e p o s ite d in a sh a llo w marine
open s h e l f s e t t i n g .
F la s e r bedded san d ston e and s h a le , c h a r a c t e r is t ic o f th e mixed
s u b - f a c i e s , i s g e n e r a lly co n sid ered t o be ev id en ce o f a t i d a l l y
in flu e n c e d d e p o s it io n a l s e t t i n g (A lle n , 1982, p. 511) .
However, De
Raaf and o th e r s (1977) have dem onstrated th a t f la s e r bedding can a ls o
be produced in a wave dom inated environm ent p r a c t ic a lly d evoid o f u n i­
d ir e c t io n a l c u r r e n ts .
T h eir model i s based on in t e r p r e t a t io n o f
sedim entary s t r u c tu r e s in f l a s e r bedding in th e Lower C arb oniferous
near Cork, I r e la n d .
They su g g e st th a t th e p resen ce o f w ave-generated
c r o s s -la m in a tio n and near absence o f u n i- d ir e c t io n a l c u r r e n t generated
c r o s s -la m in a tio n in f l a s e r bedding i s d ia g n o s t ic o f a wave dominated
environm ent.
R ipple c r o s s -la m in a tio n s tr u c tu r e in th e mixed s u b - f a c ie s o f th e
low er san d ston e member i s in te r p r e te d a s b ein g dom inantly wave­
gen erated (w ave-gen erated c r o s s -la m in a tio n s tr u c tu r e i s d is c u s se d in
san d ston e f a c i e s in t e r p r e t a t io n s e c t i o n ) .
U n i- d ir e c t io n a l cu rren t
gen erated c r o s s -la m in a tio n i s p r e s e n t, but not n e a r ly as abundant.
Furtherm ore, sym m etric r ip p le forms a re much more common than
asym m etric r ip p le forms in th e mixed s u b - f a c ie s .
T his ev id en ce
in d ic a t e s th a t th e mixed s u b - f a c ie s was d e p o site d in a wave dominated
r a th e r than t i d a l l y in flu e n c e d s e t t i n g .
25
Mud Facies
The mud f a c i e s com prises 22% o f th e low er san d ston e member and i s
c h a r a c te r iz e d by l i g h t to medium gray c la y and mud s h a le w ith <10%
in terb ed d ed v ery f i n e g ra in ed san dston e and s i l t s t o n e .
The sandstone
u s u a lly occu rs a s sym m etrical and r a r e r asym m etrical r ip p le d le n s e s <1
c e n tim e te r th ic k and s e v e r a l c e n tim e te r s lo n g .
These le n s e s co n ta in
h o r iz o n ta l to wavy la m in a tio n s and r ip p le c r o s s - la m in a t io n s , or may be
s t r u c t u r e le s s .
Rare, more p e r s i s t e n t , s t r u c t u r e le s s san d ston e and
s i l t s t o n e beds up to s e v e r a l c e n tim e te r s th ic k a re a ls o p r e s e n t.
S m all, h o r iz o n ta l t r a i l s in convex e p i - and h y p o - r e lie f a re f a i r l y
abundant a t some s a n d s to n e /s h a le c o n t a c t s . Minor amounts o f f in e
carbonaceous d e b r is a r e p resen t on many o f th e la m in a tio n s u r fa c e s .
Mud F a c ie s I n te r p r e t a t io n
The p resen ce o f both s h a le and h o r iz o n ta l f o s s i l t r a i l s in d ic a t e s
a q u ie t d e p o s it io n a l s e t t i n g th a t p e r i o d i c a ll y ex p erien ced minor low
en ergy e v e n ts in which sand was in tro d u ced arid worked in t o r ip p le
form s.
The u n i- d ir e c t io n a l c u r r e n ts req u ired fo r sand tra n sp o r t onto
th e mud s u b s tr a te were probably storm induced a s sy m m etrical, wave- .
gen erated r ip p le forms dom inate.
The gray s h a le in th e low er
san d ston e member i s alm ost id e n t ic a l to marine, s h a le in th e.m id d le
s h a le member and d oes n ot resem ble th e v a r ie g a te d , c a lc a r e o u s mudstone
p resen t in th e non-m arine K ootenai Form ation.
T h is su p p o rts
in t e r p r e t a t io n o f a marine environm ent f o r th e low er san d ston e member.
26 .
FACIES RELATIONS
O b servation and in t e r p r e t a t io n o f f a c i e s r e l a t io n s in th e low er
san d ston e member i s hampered by g e n e r a lly s p o tty and in com p lete
exp osu res.
Com plete, w e ll exposed s e c t io n s a re p resen t a t o n ly two
lo c a t io n s ( Rocky Canyon and Paddys Run) and even th e s e a r e n ot
l a t e r a l l y e x t e n s iv e .
D e s p ite th e poor ex p o su re, th e f a c i e s r e la t io n s
ob served p rovid e v a lu a b le in fo rm a tio n in d e c ip h e r in g th e d e p o s itio n a l
h is t o r y o f t h i s u n i t .
V e r t ic a l F a c ie s R e la tio n s
V e r tic a l f a c i e s r e l a t io n s w ith in th e low er san dston e member are
very s im ila r throughout, th e stu d y area w ith th e e x c e p tio n o f exp osures
in' th e Elkhorn Mountains and J e ffe r s o n Canyon.
The f o llo w in g
d e s c r ip t io n co v er s a l l ex p o su res e x ce p t th e s e anomalous s e c t io n s .
A ll f a c i e s a r e thought t o be p r e se n t a t each o u tcro p , though
som etim es l e s s r e s i s t a n t h e t e r o l i t h i c , b io tu rb a ted sa n d sto n e, and
s h a le i n t e r v a l s a re covered a t th e p oorer ex p o su res.
V e r t ic a l f a c ie s
changes w ith in th e low er san dston e member t y p i c a l l y occu r on a
d ecim eter to m eter s c a le ( s e e s t r a t ig r a p h ic columns o f measured
s e c t io n s in A ppendix). No apparent s ta c k in g order was ob served fo r th e
low er san d ston e member a s a w hole, a s each f a c ie s can o ccu r a t any
v e r t i c a l p o s it io n w ith in th e s e c t io n .
In spite of the absence of an overall, vertical stacking order to
the lower sandstone member, potential exists for smaller scale facies
27
tr e n d s .
In ord er to d eterm ine th e p resen ce o f th e s e tr e n d s , Markov
ch ain a n a ly s is was a p p lie d to f a c i e s t r a n s it io n s w ith in th e low er
san d ston e member (T able I ) .
In t h i s method, observed t r a n s it io n
p r o b a b ili t ie s a re determ ined and random t r a n s it io n p r o b a b ili t ie s are
c a l c u la t e d .
Random t r a n s it io n p r o b a b ili t ie s depend o n ly on th e
a b s o lu te abundance o f th e v a r io u s f a c i e s and can be c a lc u la t e d u sin g
th e eq u a tio n r . . = n ./(N -n . ) where r . . i s th e random p r o b a b ilit y o f
1J
J
1
1J
t r a n s it io n from f a c i e s i to f a c i e s j j n^ and n ^ are th e number o f
o ccu rren ces o f f a c i e s i and j r e s p e c t i v e ly , and N i s th e t o t a l number
o f occu rren ces o f a l l f a c i e s (W alker, 1979, p . 3 ) .
The d if f e r e n c e
between th e num erical v a lu e s o f observed t r a n s it io n p r o b a b ili t ie s and
random t r a n s it io n p r o b a b ili t ie s i s used to determ ine whether
t r a n s it io n s a re more common than random.
For f a c ie s t r a n s it io n s which
occu r more fr e q u e n tly than random, th e observed minus random
t r a n s it io n p r o b a b ili t ie s are g r e a te r than z e r o .
Those t r a n s it io n s
which have a p o s i t i v e num erical v a lu e fo r observed minus random
t r a n s it io n p r o b a b ili t ie s in th e low er sa n d sto n e member a re th o se
t r a n s it io n s from th e h e t e r o l i t h i c to th e san dston e f a c i e s and from th e
mud t o th e san d ston e f a c i e s (T able I ) .
These tren d s in d ic a t e the
occu rren ce o f coarsening-upw ard c y c le s th a t term in a te in developm ent
o f san d ston e in t e r v a l s .
V e r tic a l f a c i e s r e l a t io n s a t ex p o su res in J e f f e r s o n Canyon d i f f e r
from th a t p a tte r n .
Here crossb ed d ed , p lan ar lam inated
and c r o s s -
lam inated sa n d sto n es are ra re and in s te a d th e sequence i s dominated" by
b io tu r bated sa n d sto n e.
A d d itio n a lly , th e low er sa n d sto n e member i s
th in n e r than a v era g e, b ein g o n ly 10-15 m eters t h ic k .
28
TO
S
'
H
M
S
—
.43
.57
H
.83
—
.17
M
.95
.05
S
———
.4 2
.5 8
H
.60
——
.40
M
.67
.33
———
S
———
.01
Observed T r a n s itio n
P r o b a b ilit ie s
FROM
C a lcu la ted Random
T r a n sitio n P r o b a b ilit ie s
- .0 1 .
Observed Minus Random
I
———
CO
OJ
CO
CXJ
H '
I
CXJ
CO
Table I .
ro
CO
T r a n s itio n P r o b a b ilit ie s
M
Markov ch a in a n a ly s is o f f a c i e s t r a n s it io n w ith in th e low er
san d ston e member. S=Sandstone f a c i e s , H = H e te ro lith ic
f a c i e s , M=Mud f a c i e s .
Exposures in th e Elkhorn Mountains d i f f e r in th a t a 5 m eter th ic k
in t e r v a l o f p lan ar lam inated san d ston e caps th e low er san d ston e
member. T his i s th e o n ly occu rren ce o f a san dston e body in th e lower
san d ston e member in which p lan ar la m in a tio n s are th e dominant
sed im entary s t r u c t u r e .
Sym m etrical r ip p le forms and hummocky c r o s s ­
s t r a t i f i c a t i o n a re in t e r c a la t e d w ith th e plan ar lam in ated sa n d sto n e.
The low er 12 m eters o f s e c t io n s in th e Elkhorn M ountains a re comprised
o f in terb ed d ed s h a le , and crossbedded and r ip p le c r o ss-la m in a te d
san d ston e t y p ic a l o f o th e r exp osu res o f th e low er sa n d sto n e member.
29
Lateral Facies Relations
L a te r a l f a c i e s changes a re r a r e ly observed on ou tcrop s c a le .
Most th ic k (>3 m eter) sa n d sto n e b o d ies can be tra ced f o r a t l e a s t
s e v e r a l hundred m eters, and a t s e v e r a l l o c a l e s , such a s th e c e n tr a l
B rid ger Range ( s e e s t r a t ig r a p h ic columns fo r B ra ck ett Creek and F airy
Creek in Appendix) and th e n o r th e a ste r n G a lla tin Range, some sandstone
in t e r v a l s are b e lie v e d to c o r r e la t e a c r o s s s e v e r a l o u tcro p s fo r
d is t a n c e s o f a t l e a s t 5 k ilo m e te r s . T his c o r r e la t io n i s based on
v is u a l c o r r e la t io n o f
s t r a t ig r a p h ic p r o f i l e s and w alk in g out
san d ston e i n t e r v a l s betw een th e measured o u tcro p s where p o s s i b l e .
The
san d ston e i n t e r v a l s may, and probably do, p e r s i s t beyond th e
c o r r e la te d s e c t io n s , but th e la c k o f exp osu re p rev en ts fu r th e r
c o r r e la t io n . The san d ston e body a t W in eg la ss Mountain, which i s
c h a r a c te r iz e d by a p lan oconvex geom etry, i s t y p ic a l o f th e s e th ic k ,
p e r s is t e n t san d ston e in t e r v a l s .
H e t e r o lit h ic in t e r v a l s which bound
th e s e c o r r e la te d sa n d sto n es probably have an e q u iv a le n t l a t e r a l
e x t e n t . Thinner f a c i e s in t e r v a l s o f t e n do n ot c o r r e la t e from one
ou tcrop to th e n ex t and th e p r e c is e e x te n t o f th e s e th in n e r seq uences
cannot be a s c e r t a in e d .
In g e n e r a l, th ic k san dston e seq u en ces are
s h e e t - l i k e , w ith a p lan oconvex geom etry observed a t th e ed ge o f
san d ston e b o d ies as su g g este d by o u tcro p s a t W in eglass Mountain (F ig .
8 ).
These sa n d sto n es o c c a s io n a lly have a l a t e r a l e x te n t o f a t l e a s t 5
k ilo m e te r s w h ile th in n e r seq u en ces e x h ib it more l a t e r a l v a r i a b i l i t y .
F a c ie s R e la tio n s I n te r p r e t a t io n
The sharp c o n ta c t a t th e base o f th e low er sa n d sto n e member i s
probably a r e s u l t o f s h o r e fa c e e r o s io n which occurred d u rin g
30
t r a n s g r e s s io n o f th e s e a a c r o s s non-m arine c o a s t a l p la in d e p o s it s o f
th e K ooten ai Form ation.
T h is typ e o f d isc o n fo r m ity has been termed a
ravinem ent ( S w if t , 1968) where ob served on th e U .S. A t la n t ic c o a s t .
M arginal marine d e p o s it s such as t i d a l i n l e t f i l l s , dune sa n d s, beach
sa n d s, and sh o r e fa c e sands are n ot p r e se n t and were presum ably removed
by s h o r e fa c e e r o s io n due t o wave a c t io n .
The sharp c o n ta c t a t th e to p o f th e low er san d ston e member marks
a change from a sand-dom inated sh a llo w marine environm ent in th e low er
san d ston e member to a mud-dominated o ffs h o r e marine environm ent
(R o b erts, 1972; Schw artz, 1 9 7 2 ).
Schwartz (1972) s t a t e s t h a t .t h i s
r e s u l t s from fu r th e r westward t r a n s g r e s s io n o f th e s e a .
The t h in ,
d is c o n tin u o u s , m ic r it e n ear th e base o f th e m iddle s h a le member
su p p orts t h i s in t e r p r e t a t io n as lim e s to n e d e p o s itio n i s a common
resp o n se t o a rap id tr a n s g r e s s io n , d u rin g which c l a s t i c sedim ent
su p p ly to th e o ffs h o r e environm ent i s g r e a t ly reduced (R yer, 1977).
The d e p o s it io n a l environm ent f o r th e low er sa n d sto n e member was
probably a sh a llo w marine s e t t i n g s im ila r to modern m arine s h e lv e s in
d e p o s itio n a l p r o c e ss and s t y l e .
Mud and sand were d e p o s ite d
sim u lta n e o u s ly , w ith t h e ir d is t r ib u t io n c o n tr o lle d by sedim ent
a v a ila b ilit y ,.b a t h y m e t r y , and d is t r ib u t io n o f marine c u r r e n ts and ■
storm p a th s .
Sandstone was d e p o site d by m ig ra tio n o f in d iv id u a l
bedform s, in c lu d in g r i p p le s and m eg a rip p les, and by m ig ra tio n o f
bedforms a s s o c ia t e d w ith sand b o d ies which may have had a la t e r a l
e x te n t o f a t l e a s t
5
san d ston e i n t e r v a l s .
k ilo m e te r s a s in d ic a te d by c o r r e la t io n o f
However, th e e x te n t o f san d ston e b o d ies in th e
low er san d sto n e member may n o t m erely r e p r e se n t th e d im en sion s o f th e
31
sand b o d ies as th ey appeared du rin g d e p o s it io n .
The observed
d im en sion s may in p art be due to l a t e r a l a c c r e t io n o f th e sand body as
i t m igrated on th e bottom .
These sand b o d ies c o n ta in c r o ssb e d s,
p lan ar la m in a tio n s and c r o s s -la m in a tio n s th a t are form d isc o r d a n t to
sand body geom etry..
F ie ld ( 1980) d e s c r ib e s a sand r id g e topography which i s a dominant
fe a tu r e o f th e e n t ir e U .S . A t la n t ic in n er c o n tin e n ta l s h e l f .
These
sand r id g e s t y p i c a l l y e x h ib it 6 -9 m eters o f r e l i e f , and vary from 9-15
k ilo m e te r s in le n g th and 1-3 k ilo m e te r s in w id th .
S w ift and F ie ld
(1981) n o te asym m etric and symmetric r ip p le forms from th e s e rid g e
c r e s t s and sym m etric r i p p l e s , m egarip p les and sandwaves in r id g e
tr o u g h s.
Much o f th e r id g e su r fa c e has been b u lld o zed smooth by
e c h in o id s (sand d o l l a r s ) .
F ie ld and o th e r s (1981) d e s c r ib e s im ila r
sand r id g e s in th e N orthern B ering S ea.
They n o te a h ier a rch y o f
th r e e s i z e s o f superim posed bedforms in c lu d in g la r g e sand waves,
(h e ig h t 2 m eters, sp a cin g 200 m e te r s ), sm a lle r sand waves (h e ig h t 1
m eter, sp a c in g 200 m eters) and o s c i l l a t o r y and u n i- d ir e c t io n a l cu rren t
gen erated r i p p le s .
Sand r id g e s in both th e North A t la n t ic and
N orthern B erin g Sea e x h ib it documented h i s t o r i c a l m ig ra tio n (F ie ld ,
1980; F ie ld and o t h e r s , 1 9 8 1 ).
F ie ld and o th e r s (1981) s p e c u la te th a t
sand r id g e s in th e a n c ie n t rock record would c o n s is t o f le n s e s o f
sand, 10-15 m eters t h ic k , en cased in a sandy s h a le .
Sedim entary
s tr u c tu r e s in th e sand b o d ie s should c o n s is t o f r ip p le c r o s s ­
la m in a tio n s and ta b u la r - and w edge-shaped c r o s s b e d s.
Bedding p lan es
should show ev id en ce o f lin g o id and p o s s ib le o s c i l l a t o r y r i p p le s .
T h eir d e s c r ip t io n i s rem arkably s im ila r to th e sedim entary s tr u c tu r e s
32
and geom etry o f sand b o d ie s observed in th e low er sa n d sto n e member.
Thus, th e m u ltip le bed san d ston e b o d ies in th e low er sa n d sto n e member
are in te r p r e te d as sand r id g e s .
M igration or c o n s tr u c tio n a l u p b u ild in g o f sand r id g e s and
in d iv id u a l sand bedforms
upon a s s o c ia t e d muds and storm d e p o s its
r e s u lte d in th e c o a rsen in g -u p seq u en ces su g g ested by Markov chain
a n a ly s is .
The ab sen ce o f an o v e r a ll v e r t i c a l tren d or s ta c k in g o f
f a c i e s in th e low er sa n d sto n e member in d ic a t e s th a t th e r e was no
d ir e c t io n a l change in d e p o s it io n a l environm ent, such a s a change in
se a l e v e l , d u rin g d e p o s it io n o f th e lo w er san dston e member.
S im ila r sand r id g e s e x i s t where s u f f i c i e n t u n i- d ir e c t io n a l
c u r r e n ts are gen erated by storm (S w ift and o th e r s , 1971; S ta h l and
o t h e r s , 1974; S t u b b le f ie ld and o t h e r s , 1975; Hunt and o t h e r s , 1977;
S w ift and o th e r s , 1979; F ie ld , 1980; F ie ld and o th e r s , 1981; S w ift and
F ie ld , 1981) or t i d a l p r o c e s s e s ( S w if t , 1975; S w ift, 1 9 7 6 ).
The
p resen ce o f in terb ed d ed storm seq u en ces and w ave-gen erated r ip p le
s t r u c t u r e s w ith sand r id g e d e p o s it s in th e low er san d ston e member
s u g g e s ts th a t sand r id g e developm ent and m aintenance was due to storm
and wave-dom inated p r o c e s s e s .
A d d itio n a l support fo r in t e r p r e t a t io n
o f th e h y d r a u lic regim e can be found in r e la te d n earsh ore d e p o s it s and
p a le o d is p e r s a l p a t t e r n s .
Because th ey are fa r t h e r w e s t,, s e c t io n s o f th e low er san dstone
member in th e Elkhorn Mountains must have been d e p o site d n ea rer the
in fe r r e d p a le o s h o r e lin e du rin g low er sa n d sto n e member d e p o s itio n
(Vuke, 1 984 ).
These s e c t io n s a re capped by a sequence o f h o r iz o n ta l
to low a n g le , p lan ar la m in a ted , and sym m etrical r ip p le marked
33
sa n d sto n e.
Hummocky c r o s s - s t r a t i f i e d san d ston e i s p r e se n t near th e
b ase o f t h i s capping seq u en ce.
T his v a r ie t y o f sedim entary s tr u c tu r e s
i s t y p ic a l o f wave dominated upper sh o r e fa c e seq u en ces ( Reineck and
S in gh , 1 980 ).
Thus th e p resen ce o f wave g en era te d , r a th e r than t i d a l ,
n earsh ore d e p o s it s su p p o rts a model in which storm s dom inated th e
h y d ra u lic regim e in t h i s p a rt o f th e seaway.
P a le o d is p e r s a l p a tte r n s in th e low er san dston e member were
ob tain ed from d ecim eter s c a le p lan ar c r o s s b e d s.
The lim it e d sample
p o p u la tio n (n = l4 ) i s due to th e g e n e r a l la c k o f th r e e -d im e n sio n a l
ex p o su res o f sed im en tary s t r u c t u r e s .
Q u a lit a t iv e o b s e r v a tio n o f tw o-
d im en sion al exp o su res o f cro ssb ed s s u g g e s ts th a t th e observed
p a le o d is p e r s a l p a tte r n s a re probably s im ila r to tr e n d s th a t would be
ob tain ed from a la r g e r p o p u la tio n .
P a le o d is p e r s a l d ir e c t io n s range
from 329 t o 75 d eg ree s azim uth w ith no modes to th e so u th (F ig . 1 1 ).
A bimodal d is p e r s a l p a tte r n would be ex p ected from t i d a l d e p o s it s
though a unimodal p a tte r n d oes n ot p reclu d e th e p o s s i b i l i t y o f t i d a l
in flu e n c e (R eading, 1978, p . 2 3 7 ).
However, tren d s o f sym m etrical
r ip p le c r e s t s a re s tr o n g ly e a s t - w e s t (F ig . 11) and markedly
p erp en d icu la r t o dominant p la n a r cro ssb ed d ip s .
T his s u g g e s ts a
g e n e t ic r e la t io n s h ip between th e u n i- d ir e c t io n a l c u r r e n ts which formed
th e m egarip p les and th e o s c i l l a t o r y flo w s which produced th e
'
sym m etrical r i p p le s . Such a r e la t io n s h ip has a ls o been n oted in l a t e
Pre-Cambrian s h e l f d e p o s it s in Norway.
T here, a model was developed
in which u n i- d ir e c t io n a l c u r r e n ts were g e n e t ic a lly r e la t e d to wave
a c t i v i t y ( L e v e ll, 1 9 8 0 ).
Such a model may a ls o ap p ly t o low er
san d ston e member d e p o s it io n where th e h y d ra u lic regim e was probably
34
Tr end of P l a n a r C r o s s b e d s
S = 56° X =341°
SEM = 4°
S c a l e in p e r c e n t
Tr e n d of S y mme t r i c a l Ripple C r e s t s
SEM=2
S=35
Scal e in percent
F igu re 1 1 .
Rose diagram s showing both th e trend o f p la n a r cro ssb ed s
and th e tren d o f sym m etrical r ip p le c r e s t s in the low er
san dston e member o f th e Therm opolis S h a le . X r e p r e se n ts
th e v e c to r mean, S th e standard d e v ia tio n and S.E.M. the
standard erro r o f th e mean. Data from exp o su res a t Fairy
Creek, Rocky Canyon, S tr ic k la n d Creek, Nixon Gulch,
Cottonwood Gulch, Paddys Run, S lu ic e b o x , and W in eglass
M ountain.
35
c h a r a c te r iz e d by g e n e t i c a l l y r e la t e d u n i- d ir e c t io n a l and o s c i l l a t o r y
flo w c o n d itio n s which shared an o r ig in in e p is o d ic storm e v e n ts .
The
.e p is o d i c it y o f th e s e e v e n ts i s r e f l e c t e d in th e p resen ce o f sh a le
in terb ed d ed w ith th e sa n d sto n e.
I n te r p r e t a t io n o f f a c i e s r e l a t io n s f o r th e low er san d ston e member
in th e Bozeman a rea a ls o c a r r ie s r e g io n a l se d im e n to lo g ic im p lic a t io n s .
A r e g io n a l d e p o s it io n a l model fo r th e low er san d ston e member, I n te r v a l
A o f th e B la c k le a f Form ation (Schw artz, 1 9 7 2 ), in sou th w est Montana
has been p resen ted by Schwartz (1972 and 1 9 8 2 ).
In t h i s m odel, two
d i s t i n c t E arly C retaceou s d e p o s it io n a l p ro v in ce s in sou th w est Montana
have been d e fin e d (F ig . 1 2 ).
They a re: I) a w estern p ro v in c e , lo c a te d
w est and sou th w est o f th e B oulder b a t h o lit h ; and 2) an e a s te r n
p ro v in ce c o n s is t in g o f th e a rea e a s t and so u th e a st o f th e Boulder
b a t h o lit h .
The stu d y a r e a fo r t h i s in v e s t ig a t io n i s lo c a te d in the
e a s te r n p ro v in c e .
Schwartz (1972 and 1982) n o te s marked d if f e r e n c e s in d e p o s itio n a l
environm ent fo r th e low er san dston e member between th e e a s te r n and
w estern p r o v in c e s .
In th e w estern p ro v in c e , th e low er san dston e
member i s com prised o f in terb ed d ed very f i n e to f in e g ra in ed
l i t h a r e n i t e , s i l t s t o n e and s h a le .
I t i s in te r p r e te d a s a mixed
d i s t a l - f l u v i a l and la c u s t r in e c l a s t i c sequence d e p o s ite d in a fr e s h to b ra ck ish -w a te r s e t t i n g .
In c o n t r a s t , th e low er san d ston e member in
th e e a s te r n p ro v in ce c o n s i s t s o f f in e g ra in ed q u a r tz a r e n ite and
su b o rd in a te s u b - l i t h a r e n i t e in terb ed d ed w ith minor amounts o f
s i l t s t o n e and s h a le .
Schwartz (1972 and 1982) and Vuke (1982)
in te r p r e t th e low er sa n d sto n e member in th e e a s te r n p ro v in ce as b ein g
LaJ
CTv
Figure 12.
Isopach map of the lower sandstone member of the Thermopolis Shale in southwest Montana.
All sections are restored to their approximate depositional locations. Also depicted
are locations of western province (non-marine) and eastern province (marine). Data
outside of study area from Schwartz (1982) and Vuke (1982).
37
d e p o s ite d in a t r a n s g r e s s iv e marine s h o r e fa c e s e t t i n g .
O b serv a tio n s and in t e r p r e t a t io n s p resen ted in t h i s paper in d ic a t e
th a t th e low er sa n d sto n e member in th e Bozeman, Montana area was
d e p o s ite d in a sh a llo w marine s h e l f r a th e r than th e ex p ected
tr a n s g r e s s iv e sh o r e fa c e s e t t i n g .
T his f a c t does not n e c e s s a r ily
in v a lid a t e th e model o f Schwartz (1 9 8 2 ).
Because o f s h o r e fa c e e r o sio n
d u rin g tr a n s g r e s s io n , m arginal and n earsh ore marine i n t e r v a l s such as
la g o o n a l and s h o r e fa c e d e p o s it s were reworked and l e f t no d e p o s itio n a l
rec o rd .
D e p o sitio n o f th e low er sa n d sto n e member in th e Bozeman area
i s th e r e fo r e a t t r ib u t e d t o an in f lu x o f sedim ent in t o a sh a llo w marine
s e t t i n g where d e p o s it io n was c o n tr o lle d by sh a llo w m arine p r o c e s s e s ,
r a th e r than t r a n s g r e s s iv e rew orking o f sedim ent in a n earsh ore
environment..
S in ce e q u iv a le n t s h o r e lin e d e p o s it s were n ot
r e c o g n iz e d , i t i s d i f f i c u l t to a s c e r t a in a c tu a l s h o r e lin e f lu c t u a t io n s
and p o s s ib le in flu e n c e o f t r a n s g r e s s iv e p r o c e s s e s d u rin g low er
san d ston e member d e p o s it io n .
Because d e s c r ip t io n s o f th e low er
san d ston e member in th e r e s t o f th e e a s te r n p rovin ce (S ch w artz, 1972;
Vuke, 1982) c l o s e l y resem b le th e low er san dston e member in th e Bozeman
a rea , i t i s l i k e l y th a t much o f th e low er san dston e member in th e
rem ainder o f th e e a s te r n p rovin ce was a ls o d e p o site d in a sh a llo w
m arine ra th e r than t r a n s g r e s s iv e sh o r e fa c e s e t t i n g .
Schwartz (1982) a ls o n o te s th e e x is t e n c e o f E arly C retaceous
fo r e la n d s tr u c tu r e s in sou th w est Montana th a t in flu e n c e d d e p o s itio n a l
p a tte r n s .
The p resen ce o f an a c t iv e p o s i t i v e elem ent in
southw esternm ost Montana i s in d ic a te d by an isop ach map o f th e low er
san d ston e member ( F ig . 1 2 ).
T his p o s i t i v e elem ent i s c o in c id e n t w ith
th e n o rth w esternmost e x p r e s s io n o f Laramide, basement in v o lv e d u p l i f t s
in Montana.
Though th e r e i s no ev id en ce th a t t h i s elem en t co n trib u ted
sedim ent d u rin g low er san d ston e member d e p o s it io n , i t d oes appear to
have been an a rea o f slo w er su b sid en ce a s r e f l e c t e d by th e th in n in g o f
th e low er sa n d sto n e member.
Exposures o f th e low er sa n d sto n e member
in J e ffe r s o n Canyon occur a lon g th e n orth ern edge o f t h i s tr e n d .
The
low er su b sid en ce r a t e s th e r e may have been in stru m en ta l in producing
th e anomalous s e c t io n o f b io tu r bated sa n d sto n e, which i s u s u a lly
in d ic a t i v e o f slow d e p o s it io n .
39
PETROGRAPHY AND PROVENANCE
The provenance fo r th e low er sa n d sto n e member in th e Bozeman area
has n ot been w e ll d e fin e d .
Schwartz (1982) s u g g e s ts th a t sands in th e
low er sa n d sto n e member o f th e e a s te r n p ro v in ce were d e r iv e d dom inantly
from th e d ir e c t io n o f t r a n s g r e s s io n .
I t i s u n c le a r w hether t h is means
th a t sedim ent was d eriv ed by t r a n s g r e s s iv e rew orking o f u n d erly in g
sedim ent or i f th e southward t r a n s g r e s s in g sea was a c t i v e l y
tr a n s p o r tin g sedim ent from th e n o rth .
The fo llo w in g d is c u s s io n
rev iew s th e petrography and p a le o d is p e r s a l p a tte r n s o f th e low er
san d ston e member in ord er to a s s e s s p o s s ib le provenance fo r th e low er
san d ston e member.
P etro g ra p h ic D e s c r ip tio n
P etrog ra p h ic a n a ly s is o f th e low er san d ston e member in c lu d e s
p o in t c o u n ts, to d eterm ine r e l a t i v e abundances o f d e t r i t a l m ineral
g r a in s , o f 10 sam ples c o l le c t e d from v a r io u s ex p o su res in th e study
area (Table 2 ) .
Cursory exam ination o f 20 more sam ples in d ic a te d no
d if f e r e n c e s th a t w arranted fu r th e r d e t a ile d exa m in a tio n .
Coarser
grain ed sam ples were ch osen fo r p etro g ra p h ic a n a ly s is in ord er to
p rovid e c o n s is te n c y in sam pling and becau se th e s e sam ples g e n e r a lly
c o n ta in a g r e a te r v a r ie t y o f framework g r a in s and so sh ould more
a c c u r a te ly r e f l e c t co m p o sitio n o f th e so u rce a r e a .
Most sam ples
average f i n e sand s i z e though th ey range from very f i n e t o medium
sand.
Mean
Mid
Chert Feldspar Clasts
Mid
Matrix
2
-
I
-
2
-
4
I
-
5
I
-
81
3
4
-
9
3
I
fine
86
2
I
-
8
2
-
Nixon
Gulch
fine
72
5
2
tr
6
12
N. Fork
Brackett
fine
72
6
I
-
4
Paddys
Run
fine
90
4
2
-
Jefferson
Canyon
very
fine
54
I
3
Rocky
Canyon
fine
86
5
N. Fork
16-mile
medium
71
8
Sample
location
Sand
Size
Mmo
Quartz
Poly
Quartz
Wineglass
Mauntain
fine
76
2
Strickland
Creek
fine
70
Johnny
Creek
very
fine
Cottomood
Gulch
Table'2.
.
FeO Biotite Muscovite Zircon Tourmaline Porosity
I
tr
-
15
-
tr
-
-
. 18
-
tr
tr
-
tr
tr
2
-
I
5
I
-
tr
2
tr
tr
-
38
2
tr
I
I
7
tr
12
-
-
-
-
-
-
-
-
-
.10
-
tr
tr
-
I
tr
tr
-
tr
tr
-
tr
-
tr
-
tr
-
—
I
tr
' -
Data set from point counts of thin sections of the lower sandstone member.
41
The f a b r ic o f th e san d ston e i s c h a r a c te r iz e d by lo n g g ra in
c o n t a c t s , squashed mud c l a s t s and g e n e r a lly low p o r o s it y .
Quartz
overgrow ths a re abundant in a l l but one sam ple, which was c la y
cem ented.
Quartz i s th e most common cement ty p e , though c la y , which
was probably d eriv ed from squashed det r i b a l mud c l a s t s , a c t s as a
patch y m atrix or cem ent.
P o r o s ity i s h ig h ly v a r ia b le , ran gin g from 0 -
18 p ercen t and i s b e s t d evelop ed in th ic k e r san d ston e b o d ies in th e
n o r th e a s te r n G a lla tin Range. Q u a lit a t iv e o b se r v a tio n o f th in s e c t io n s
in d ic a t e s th a t sa n d sto n es are m o stly w e ll so r te d and d e t r i t a l g r a in s
a r e subrounded in d ic a t in g a h ig h d eg ree o f t e x t u r a l m a tu rity
( P e ttijo h n and o t h e r s , 1972, p. 5 8 8 ) ..
Quartz ty p e s p r e se n t in c lu d e m o n o c r y sta llin e and p b ly c r y s t a llin e
q u a r tz . A u th ig en ic q u artz i s common as q u artz overgrow th s, but due to
th e d i f f i c u l t y in d is t in g u is h in g i t from
d e t r i t a l g r a in s , i t i s
in clu d ed in th e m o n o c r y sta llin e qu artz c a te g o r y .
For r e l a t i v e
abundances o f d e t r i t a l g r a in s , se e T a b le .2.
M u d sto n e-c la sts a re th e second most common c l a s t - t y p e o b serv ed .
They t y p i c a l l y c o n ta in s i l t - s i z e d q u artz g r a in s .
c l a s t s may range up t o p eb b le s i z e .
In d iv id u a l mudstone-r
B ecause o f th e c o m p o sitio n a l and
t e x t u r a l m a tu rity o f t h i s sa n d sto n e, mudstone c l a s t s a re thought to be
in tr a fo r m a tio n a l and have no d ir e c t b ea rin g on provenance
d eterm in a tio n fo r th e low er san d ston e member.
Chert i s p r e se n t in a l l
sam p les, but i s a minor component and ex ce ed s 5% in o n ly one sam ple.
M uscovite and b i o t i t e a re p r e se n t in tr a c e amounts in most sam ples.
Mica p la t e s a re u s u a lly bent around com petent g r a in s .
M icro clin e and
potassium f e ld s p a r s a re p r e se n t in tr a c e amounts in s e v e r a l o f th e
42
sam p les.
Z ircon i s a ls o p r e se n t in tr a c e amounts as i s tou rm alin e,
which i s alw ays w e ll rounded.
Clay m in era ls are u s u a lly co n fin ed to
mud c l a s t s , though in one sample th e c la y m atrix amounted t o 37? o f
th e sam ple.
T h is c la y d id n ot appear to be w e ll c r y s t a l l i z e d and was
probably in tro d u ced in t o th e san d ston e bed by burrow ing.
Provenance I n te r p r e t a t io n
Provenance o f th e low er san d ston e member i s d i f f i c u l t to
a s c e r t a in due to th e co m p o sitio n a l m a tu rity o f th e sa n d sto n e.
P o s s ib le so u rces in c lu d e exposed land a rea s t o th e w est o f th e Western
I n t e r io r C retaceou s Seaway as w e ll as t r a n s g r e s s iv e rew orking o f .t h e
u n d e rly in g K ootenai Form ation.
S in ce th e K ootenai Form ation had a
so u rce in th e S e v ie r o ro g en ic b e lt (R o b erts, 1972; S u ttn er and o th e r s ,
1 9 8 1 ), th e o r ig in a l sou rce o f sedim ent fo r th e low er sa n d sto n e member
should be w est o f th e W estern I n t e r io r C retaceou s Seaway.
The
fo llo w in g d is c u s s io n e v a lu a te s t h i s h y p o th e s is .
MacKenzie and P o o le (1962) d e lin e a t e two p e t r o lo g ic s u i t e s o f
d e t r i t a l framework g r a in s in Dakota Group sa n d sto n es o f th e Western
I n t e r io r , in c lu d in g Wyoming, South Dakota, Nebraska, Colorado and
K ansas.
The low er sa n d sto n e member o f th e Therm opolis S h ale i s
l i t h o s t r a t i g r a p h i c e q u iv a le n t to Dakota Group sa n d sto n es th ey stu d ied
in Wyoming.
They found an "eastern" s u i t e which was d eriv ed from th e
C o n tin en ta l Block P rovin ce e a s t o f th e C retaceou s seaway and a
"western" s u i t e which was d eriv ed from th e S e v ie r o ro g en ic b e lt w est
o f th e seaway.
abundance.
Both s u i t e s a re c h a r a c te r iz e d by very h ig h quartz
In a d d itio n , th e " eastern " s u i t e c o n ta in s , on th e a v era g e,
2% fe ld s p a r and o n ly t r a c e amounts o f c h e r t .
The heavy m in eral
43
f r a c t io n i s dominated by z ir c o n , to u rm a lin e, r u t i l e and c h lo r it o id .
The " w estern” s u i t e c o n t a in s , on th e a v e r a g e , about 15% ch e r t and on ly
tr a c e amounts o f f e ld s p a r .
The heavy m in eral f r a c t io n i s comprised
m o stly o f z ir c o n and to u rm a lin e.
Tourmaline g r a in s found in the
"western" s u i t e a re g e n e r a lly more rounded than to u rm a lin es in th e
" eastern " s u i t e .
W ithin th e low er sa n d sto n e member, qu artz g r a in s a re th e
p r in c ip le component and c h e r t ranks second in abundance o f d e t r i t a l
g r a in s .
sam p les.
F eld sp a r i s p r e se n t in o n ly tr a c e amounts in a few o f th e
Z ircon and tou rm alin e a re th e o n ly heavy m in e r a ls observed
and th e tou rm a lin e i s w e ll rounded.
In com parison w i t h .MacKenzie and
P o o le ’s (1962) stu d y , th e co m p o sitio n and th e r e fo r e provenance fo r the
low er san d sto n e member most c l o s e l y resem b les th a t o f th e "western"
s u i t e whose so u rce was th e S e v ie r orog e n ic b e lt t o th e w e s t.
Support
fo r t h i s in t e r p r e t a t io n can be found in a p lo t o f mean framework modes
o f th e low er san d ston e member and t h e ir r e la t io n s h ip t o d if f e r e n t
t e c t o n ic provenances (F ig . 13) a s determ ined by D ick in so n and Suczek
(1 9 7 9 ).
Though most sam ples p lo t between d efin ed p roven an ces, some
occu r in th e r e g io n occu p ied by th e r e c y c le d orogen provenance w h ile
none occur in th e c o n tin e n ta l b lock provenance.
T h is in d ic a t e s th a t
th e o r ig in a l sou rce fo r th e low er sa n d sto n e member was l i k e l y th e
S e v ie r e r o g e n ic b e lt r a th e r than th e e a s te r n c o n tin e n ta l b lock
provenance.
The S e v ie r o ro g en ic b e lt may have p rovided sedim ent to th e low er
san d ston e member by two d if f e r e n t m echanism s.
F i r s t , b ecau se the
sou rce fo r th e u n d e rly in g K ootenai Form ation was th e S e v ie r orogen ic
Qm
Continental
R ecycled
Orogen
Block
Provenance ,
\
Provenance
Magm atic
<
Arc
Provenance
F igu re 13.
Ternary diagram showing mean framework modes o f th e low er
san d ston e member and th e r e la t io n s h ip to d if f e r e n t
t e c t o n ic provenances as determ ined by D ick in so n and Suczek
(1 9 7 9 ): Qm i s m o n o c r y sta llin e qu artz g r a in s , F i s t o t a l
fe ld s p a r g r a in s , Lt i s t o t a l l i t h i o g r a in s , in c lu d in g
p o ly c r y s t a l lin e qu artz g r a in s .
45
b e lt (R o b erts, 1972; S u ttn e r and o th e r s , 1 9 8 1 ), t r a n s g r e s s iv e
rew orking o f th e K ooten ai Form ation may have su p p lied sedim ent to th e
low er san d sto n e member.
A lt e r n a t iv e ly , f l u v i a l p r o c e s s e s may have
tra n sp o r ted sedim ent from th e S e v ie r e r o g e n ic b e lt to th e c o a s t .
Once
t h e r e , th e sedim ent may have been r e d is t r ib u t e d by c o a s t a l p r o c e sse s
and in co rp o ra ted in t o th e low er sa n d sto n e member.
The K ootenai Form ation v a r ie s from a c h e r t-b e a r in g q u a r tz a r e n ite
to a c h e r t - r ic h l i t h a r e n i t e w ith upper K ooten ai sa n d sto n es becoming
p r o g r e s s iv e ly more q u artz r ic h upwards ( S u ttn er and o t h e r s , 1981).
Reworking o f th e s e sands in a s h o r e fa c e and sh a llo w m arine s e t t in g
cou ld c o n c e iv a b ly r e s u lt in c h e r t-b e a r in g q u a r tz a r e n ite s s im ila r to
th o se in th e low er san d ston e member.
K ooten ai sa n d sto n es in th e
Bozeman a rea occur ap p roxim ately 30 m eters below th e c o n ta c t w ith th e
low er san d sto n e member and a re sep a ra ted from th e low er san dston e
member by an in t e r v a l o f v a r ie g a t e d , c a lc a r e o u s mudstone and
b io m ic r it e .
No K ootenai sa n d sto n es were observed betw een th e low er
san d ston e member and th e mudstone and lim e s to n e a t th e to p o f th e
K ootenai Form ation.
I t i s p o s s ib le th a t another in t e r v a l o f sandstone
was p r e se n t a t th e to p o f. th e K ootenai Form ations but was t o t a l l y
reworked by th e t r a n s g r e s s in g s e a .
However, no ev id e n c e was observed
which supported t h i s h y p o th e sis and no m ention o f an oth er sandstone
in t e r v a l above th e lim e sto n e a t th e top o f th e K ootenai Form ation was
found in th e l i t e r a t u r e ( S u ttn e r , 1969; R ob erts, 1972; James, 1977;
S u ttn er and o t h e r s , 1 9 8 1 ).
The S e v ie r e r o g e n ic b e lt may a ls o have su p p lied sedim ent to th e
low er san dston e member in a more d ir e c t, manner.
Sediment may have
46
been f l u v i a l l y tra n sp o r ted to th e c o a s t and d is t r ib u t e d by c o a s t a l
p r o c e s s e s to th e s h e l f where i t was in co rp o ra ted in t o th e low er
san d ston e member.
Evidence fo r t h i s h y p o th e sis can be found in
f l u v i a l e q u iv a le n ts o f th e low er sa n d sto n e member.
D i s t a l - f l u v i a l and la c u s t r in e rock s t r a t ig r a p h ic e q u iv a le n ts o f
th e low er san d ston e member in th e w estern p rovin ce in c lu d e very f in e
to f i n e g r a in e d , l i t h a r e n i t e s and s u b - lit h a r e n it e s o f I n te r v a l A o f
th e B la c k le a f Form ation (Schw artz, 1972, 1 9 8 2 ).
I n te r v a l A san d ston es
are composed dom inantly o f q u artz and l i t h i c fragm ents o f " d e t r it a l
carb on ate c l a s t s w ith minor p resen ce o f c h e r t , q u a r t z it e ,
q u a r tz a r e n ite and q u artz wacke fragm ents" ( Schw artz, 1 9 7 2 ).
Winn and
o th e r s (1984) have shown th a t s i g n i f i c a n t c o m p o sitio n a l d if f e r e n c e s
can e x i s t betw een f l u v i a l and marine sa n d sto n es d eriv ed from th e same
so u r c e .
They found th a t marine sands o f th e Late C retaceou s F ro n tier
Form ation in sou th w estern Wyoming, co n ta in ed 18% more m o n o c r y sta llin e
q u a rtz,
8%
l e s s ch e r t and p o ly c r y s t a l lin e qu artz and 9% l e s s oth er
rock fragm en ts than r e la t e d f l u v i a l d e p o s it s .
Causes o f t h i s are: I)
tra p p in g o f th e c o a r s e r , more rock fragm ent r ic h sedim ent in f l u v i a l
environm ents and 2) d e s t r u c t io n o f s o f t e r rock fragm ents by wave
a c tio n in e it h e r a beach or o ffs h o r e s e t t i n g .
Thus fu r th e r reworking
o f I n te r v a l A sands In a high energy n ea rsh o re s e t t i n g co u ld r e s u lt in
ch e r t b ea rin g q u a r tz a r e n ite s s im ila r to th o se observed in th e low er
san d ston e member.
Though low er san d ston e member e q u iv a le n ts in th e w estern p rovince
may have su p p lie d some sedim ent to th e low er san d ston e member in the
e a s te r n p ro v in c e , Schwartz (1982) s u g g e s ts th a t p o s i t i v e elem en ts in
47
sou th w estern Montana may have a cted as b a r r ie r s to p a leodis p e r s a l from
th e w estern to th e e a s te r n p r o v in c e .
Thus, th e r e may have been o th er
lo c a l e s in th e S e v ie r e r o g e n ic b e lt th a t c o n tr ib u te d sedim ent to th e
low er san d sto n e member in th e e a s te r n p r o v in c e .
F igure 12
d em on strates a r e l a t i v e l y th ic k e r re g io n o f th e low er sa n d sto n e member
th a t ex ten d s from th e Bozeman area sou th s o u th e a s t in t o Y ellow ston e
N a tio n a l Park.
P a leo cu rren t d a ta in th e Bozeman area (F ig . 11 and 12)
s u g g e s ts th a t sand d e r iv a tio n may have been from th e s o u th - s o u t h e a s t .
These p a le o c u r r e n t d a ta are not d e f i n i t i v e s in c e th ey r e ly upon such a
sm all sample p o p u la tio n ( n = l4 ) .
A la r g e r sample p o p u la tio n was not
ob ta in ed due to th e la c k o f w e ll ex p o sed , th r e e -d im e n sio n a l exp osu res
o f sed im en tary s t r u c t u r e s n e c e ssa r y fo r p a leo c u r ren t measurement.
48
SUMMARY
I n i t i a l t r a n s g r e s s io n o f th e C retaceou s seaway in t o southw est
Montana was accompanied by sh o r e fa c e e r o s io n which produced th e sharp
c o n ta c t or ravinem ent th a t se p a r a te s non-m arine d e p o s it s o f th e
K ootenai Form ation from th e sh a llo w marine low er san d ston e member o f
th e Therm opolis S h a le .
Subsequent to tr a n s g r e s s io n , d e p o s it io n o f th e
low er san d ston e member occurred in a sh a llo w marine en v iro n m en t. o f low
r e lie f.
Sedim ent was probably su p p lied to t h i s s e t t i n g by two
p rocesses.
F i r s t , rew orking o f upper K ooten ai Form ation sands in th e
sh o r e fa c e environm ent and u ltim a te str a n d in g o f th e d e t r i t u s on th e
s h e l f by tr a n s g r e s s io n may have p rovid ed some o f th e sedim ent in the
low er san d sto n e member.
Second, sedim ent may have been in trod u ced by
f l u v i a l system s to th e c o a s t from th e S e v ie r ero g e n ic b e lt lo c a te d
w est o f th e seaway.
C o a sta l storm p r o c e s s e s th en tra n sp o r ted sedim ent
o ffs h o r e to th e s h e l f where i t was in co rp o ra ted in t o th e low er
san d ston e member.
F i r s t o rd er geomorphic fe a tu r e s on t h i s s h e l f are la r g e s c a le sand
r id g e s , w ith d im en sion s o f up to 5 k ilo m e te r s .
The sand r id g e s are
c h a r a c te r iz e d by an in h e r e n t h eira rch y o f bedform s.
M egaripples and
r ip p le s are second and th ir d order f e a t u r e s r e s p e c t i v e ly .
The
in te r n a l s tr u c tu r e o f sand r id g e s i s form d isco rd a n t and c o n s is t s o f a
p o tp o u r ri o f in terb ed d ed d ecim eter s c a le c r o ssb e d s, r ip p le c r o s s ­
la m in a tio n s and h o r iz o n ta l to low a n g le p lan ar la m in a tio n s .
D ecim eter
s c a le p lan ar cro ssb ed s a re th e most abundant s tr u c tu r e in sand r id g e
49
d e p o s it s .
Developm ent and m aintenance o f sand r id g e s i s a ttr ib u te d to
storm -induced u n i- d ir e c t io n a l c u r r e n ts w ith a dominant northwestward
(341 +.4 d e g r e e s) flo w d ir e c t io n ..
Some san d ston e in t e r v a ls occur as
is o la t e d beds bounded b y -o th e r f a c i e s i n t e r v a l s .
The crossb ed d ed ,
c r o ss-la m in a te d and p la n a r-la m in a ted sed im entary s t r u c t u r e s in the
san d ston e f a c i e s th u s r e p r e se n t both sand r id g e s and is o la t e d sand
bedform s.
Storm d e p o s it s , b io t u r bated san d ston e and sh a le occur in areas
n ot occu p ied by sand r id g e s o r .i s o l a t e d sand bedform s.
The storm
d e p o s it s are r e p re se n ted by in t e r v a l s o f th e h e t e r o l i t h i c f a c i e s . .
B io tu rb a ted sa n d sto n es r e p r e se n t d e p o s itio n in a r e a s o f low
se d im en ta tio n r a t e s , a llo w in g th e fauna to t o t a l l y d is r u p t in te r n a l
str u c tu r e s.
Low se d im e n ta tio n r a t e s may be accounted f o r by r e g io n a l
low su b sid en ce r a t e s , a s in th e J e f f e r s o n Canyon a r e a , or lo c a l
f a c t o r s such as absen ce o f sedim ent la d en c u r r e n ts or tem porary
b a r r ie r s which in h ib it e d sedim ent a ccu m u lation .
Mud was l o c a l l y
d e p o s ite d in low energy or s h e lte r e d environm ents where storm
in flu e n c e was m inim al.
M igration or rep ea ted developm ent and abandonment o f sand r id g e s
upon r e la t e d d e p o s it s r e s u lt e d in a com plex v e r t i c a l seq u en ce o f
sed im en ts c h a r a c te r iz e d by in terb ed d ed sa n d sto n e, h e t e r o l i t h i c and mud
f a c i e s i n t e r v a l s . These l o c a l tem poral changes may r e f l e c t changes in
sedim ent su p p ly , storm p a th s, d is t r ib u t io n o f marine c u r r e n ts , sea
l e v e l , or a com bination o f th e s e f a c t o r s .
Because o f r e in te r p r e t a t io n
o f th e low er sa n d sto n e member as a sh a llo w marine r a th e r than
s h o r e fa c e seq u en ce, i t i s n ot p o s s ib le to a s c e r t a in w hether th e low er
50
san d ston e member i s an a c tu a l t r a n s g r e s s iv e d e p o s it .
T his would
r e q u ir e r e c o g n itio n o f e q u iv a le n t s h o r e lin e d e p o s it s and
in t e r p r e t a t io n o f s h o r e lin e f lu c t u a t io n s during low er sa n d sto n e member
d e p o s it io n .
Subsequent to low er san d ston e member d e p o s it io n , tr a n s g r e s s io n
reduced sedim ent su pp ly t o th e.m a rin e environm ent.
T h is produced the
sharp c o n ta c t s e p a r a tin g th e low er sa n d sto n e member from th e o ffsh o r e
m arine, m iddle s h a le member o f th e Therm opolis S h a le .
The low c l a s t i c
se d im en ta tio n r a te a ls o allow ed carb on ate d e p o s itio n t o l o c a l l y
p r e v a il, r e s u lt i n g in th e t h in , d is c o n tin u o u s a r g illa c e o u s m ic r ite bed
near th e base o f th e m iddle sh a le member.
Though t h i s stu d y d e s c r ib e s th e h y d ra u lic regim e and d e p o s itio n a l
c h a r a c t e r i s t i c s o f th e low er san d ston e member in th e Bozeman a rea ,
l i t t l e i s known o f th e r e g io n a l s e d im e n to lo g ic p a tte r n s o f t h i s s h e lf
environm ent.
Future work in th e Crazy Mountains B asin and alon g th e
n orth fla n k o f th e B ea rto o th u p l i f t would h e lp in d e f in in g th e l a t e r a l
lo n g sh o re a s w e ll a s o ffs h o r e e x te n t o f th e p a leo -sa n d r id g e
top ograp h y.
Such work should prove u s e f u l in d eterm in in g mechanisms
o f sedim ent tr a n sp o r t a s w e ll a s p r e d ic t iv e models fo r a n c ie n t sand
r id g e f i e l d s .
51
REFERENCES CITED
A lle n , J . R . L . , 1982, Sedim entary s t r u c t u r e s , t h e ir c h a r a c te r and
p h y s ic a l b a s is , volume I: Amsterdam, th e N eth erla n d s, E ls e v ie r
S c i e n t i f i c P u b lish in g Company, 593 p.
Boersma, J . R . , 1970, D is tin g u is h in g f e a t u r e s o f w a v e -r ip p le c r o s s ­
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55
APPENDIX
Sluicebox
NE 1/4 Sec. 24 TIN R6E
LEGEND
I
I
In terO etid ed
I- - I
I -
4
s e n d s t o n e / s n e te
S ite te
U m e e to n e
| r v
j
S e d w w m e rv S tr u c tu r e s
M en e r c r o e e o e d e
M
T ro u g h c r o e e o e d e
W
H e m o m e t s tr a n h c e n o n
LJ
R toow c r o s e - is m m e n o n e
S y m m e tn c e t n o g te m e re s
B
T r e c e fo e ed e
M
t e
T h o rougnty o to tu rtte te d
i
Foeede
| G #
E
3
I
O s e tr o e o d e
Ol
O e tr e c o d e
grain size
Figure 14.
Location and stratigraphic column of Sluicebox outcrop.
56
Brackett Creek
NE 1/4 Sec. 35 T2N R6E
Fairy Creek
NW 1/4 Sec. 24 T2N R6E
1
J ilf
Lower sandstone member,
Thermopoila Shale
IS -
Kootenai Formation
Figure 15.
Locations and stratigraphic columns of Brackett Creek and
Fairy Creek outcrops.
57
Nixon Gulch
NE 1/4 Sec. I T2N R3E
I
Cottonwood Gulch
SW 1/4 Sec. 13 T2N R2E
I
UJ1
M J
Lower sandstone member,
Thermopolla Shale
B
;7
I'
Kootenai Formation
Figure 16.
Locations and stratigraphic columns
Cottonwood Gulch outcrops.
of
Nixon Gulch and
58
Paddys Run
NW 1/4 Sec. 32 T5N R4E
Strickland Creek
SE 1/4 Sec. 19 T3S R9E
.V**
■»s
30J
i-l
Middle ahele member,
Thermooolla Shale
s ir
Lower aandatone member,
Thermopolla Shale
IOjI
X
X
Kootenai Formation
xN
X
Figure 17.
I
Locations and stratigraphic columns of Paddys Run and
Strickland Creek outcrops.
59
Park Branch
NE 1/4 Sec. 28 T3S R9E
Figure 18.
Wineglass Mountain
SE 1/4 Sec. 15 T3S R9E
Locations and stratigraphic columns of Park Branch and
Wineglass Mountain outcrops.
60
Johnny Gulch
NE 1/4 Sec. 22 T5N RlW
Jefferson Canyon
Sec. 25 TlS R2W
-K
Lower sandstone member,
Thermooolls Shale
:
Kootenai Formation
Figure 19.
Locations and stratigraphic columns of Johnny Gulch and
Jefferson Canyon sections.
61
Rocky Canyon
SW 1/4 Sec. 20 T2S R7E
Goose Pond
NE 1/4 Sec. 3 T3S R7E
Middle shale member.
Thermooolis Shale
Lower sandstone member,
Thermopolla Shale
Kootenai Formation
Figure 20.
Locations and stratigraphic columns of Rocky Canyon and
Goose Pond sections.
62
Middle Fork Sixteenmile
NW 1/4 Sec. 11 T4N R6E
Figure 21. Location and stratigraphic column of Middle Fork
Sixteenmile outcrop