Evolution of the Cordilleran foreland basin system
in northwestern Montana, U.S.A.
Facundo Fuentes†, Peter G. DeCelles, Kurt N. Constenius, and George E. Gehrels
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
ABSTRACT
New lithostratigraphic and chronostratigraphic, geochronologic, and sedimentary
petrologic data illuminate the history of
development of the North American Cordilleran foreland basin system and adjacent
thrust belt from Middle Jurassic through
Eocene time in northwestern Montana. The
oldest deposits in the foreland basin system
consist of relatively thin, regionally tabular deposits of the marine Ellis Group and
fluvial-estuarine Morrison Formation, which
accumulated during Bajocian to Kimmeridgian time. U-Pb ages of detrital zircons and
sandstone modal petrographic data indicate
that by ca. 170 Ma, miogeoclinal strata were
being deformed and eroded in hinterland regions. Sandstones of the Swift and Morrison
Formations contain detrital zircons derived
from the Intermontane belt. The Jurassic
deposits probably accumulated in the distal,
back-bulge depozone of an early foreland
basin, as suggested by the slow rates of tectonic subsidence and tabular geometry. A
regional unconformity separates the Jurassic
strata from late Barremian(?) foredeep deposits. This unconformity possibly resulted
as a combined effect of forebulge migration,
decreased dynamic subsidence, and eustatic
sea-level fall. The late Barremian(?)–early
Albian Kootenai Formation is the first unit
that consistently thickens westward, as
would be expected in a foredeep depozone.
The subsidence curve at this time begins to
show the convex-upward pattern characteristic of foredeeps. By Albian time, the foldand-thrust belt had propagated to the east
and incorporated Proterozoic rocks of the
Belt Supergroup, as indicated by sandstone
compositions, detrital zircon ages in the
Blackleaf Formation, and by crosscutting
relationships in thrust sheets involving Belt
Supergroup rocks in the thrust belt. A major
†
E-mail: ffuentes@email.arizona.edu
episode of marine inundation and black shale
deposition (Marias River Shale) occurred between the Cenomanian and mid-Santonian,
and was followed by a regressive succession
represented by the Upper Santonian–midCampanian Telegraph Creek, Virgelle, and
Two Medicine Formations. Provenance data
do not resolve the timing of individual thrust
displacements during Cenomanian–early
Campanian time. The Upper Campanian
Bearpaw Formation represents the last major
marine inundation in the foreland basin. By
latest Campanian time, a major episode of
slip on the Lewis thrust system had commenced, as recorded in the foreland by the
Willow Creek and St. Mary River Formations in the proximal foredeep depozone. The
final stage in the evolution of the Cordilleran
fold-and-thrust belt and foreland basin
system is recorded by the Paleocene–early
Eocene Fort Union and Wasatch Formations,
which were preserved in the distal foreland
region. Regional extensional faulting along
the fold-and-thrust belt began during the
middle Eocene. The results presented here
enable the establishment of links between
previous geological work in Canada and the
better known parts of the Cordilleran foreland basin in the United States.
INTRODUCTION
Foreland basin systems are the stratigraphic
recorders of processes occurring in contiguous
orogenic belts. The history of terrane accretions,
fold-and-thrust belt development, magmatism,
and major exhumation events is registered in
the stratigraphy of these basins. Previous efforts
to link the foreland basin fill with the tectonic
development of the North American Cordillera
have been concentrated in two large regions:
Utah, Colorado, and Wyoming in the United
States (e.g., Royse et al., 1975; Jordan, 1981;
Lamerson, 1982; Heller et al., 1986; DeCelles,
1994; Currie, 2002), and Alberta and British
Columbia in Canada (e.g., Cant and Stockmal,
1989; Fermor and Moffat, 1992; Stockmal et al.,
1992; Beaumont et al., 1993; Plint et al., 1993;
Ross et al., 2005; Miall et al., 2008; Yang and
Miall, 2009). This bimodal focus was mainly
driven by either the presence of anomalously
good surface exposures, as in the case of the
western interior United States, or by hydrocarbon exploration and a large subsurface database, as in Canada (Miall et al., 2008). The
~300-km-long segment of the foreland basin
lying within and east of the Cordilleran belt in
northwestern Montana remains comparatively
poorly understood in terms of its stratigraphy,
basin evolution, and relationship with the kinematics of the developing fold-and-thrust belt.
In this paper, we present new stratigraphic
and provenance data that, coupled with previous work, establish a coherent model for the
evolution of the foreland basin in the context
of an integrated orogenic system in this region
from Jurassic to Eocene time. Besides filling
a regional gap, the results of this paper should
help to establish links and comparisons between
what is known from previous work in Canada
and in better known parts of the Cordilleran
foreland basin system in the United States.
REGIONAL SETTING
The foreland basin of northwestern Montana
occupies a central location along the ~3000 km
length of the Cordilleran fold-and-thrust belt
and foreland basin system (Fig. 1). The North
American Cordillera developed from midMesozoic to Eocene times (e.g., Burchfiel et al.,
1992; Coney and Evenchick, 1994; DeCelles,
2004; Dickinson, 2004), and was subsequently
modified by gravitational collapse and later, by
mid- to late Cenozoic Basin and Range extension (Constenius, 1996). The initial evolution
of the Cordillera was marked by subduction of
oceanic plates of the Panthalassa Ocean, and accretion of parautochthonous terranes, fringing
arcs, and exotic terranes (e.g., Coney et al., 1980;
Monger et al., 1982; Coney and Evenchick,
1994; Dickinson, 2004; Colpron et al., 2007).
GSA Bulletin; March/April 2011; v. 123; no. 3/4; p. 507–533; doi: 10.1130/B30204.1; 12 figures; 2 tables; Data Repository item 2011002.
For permission to copy, contact editing@geosociety.org
© 2011 Geological Society of America
507
Fuentes et al.
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Stratigraphy of fold-thrust
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Paleogene forearc and
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Cretaceous subduction
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Intermontane
terranes
Accreted elements
KEY FIGURES 1B AND 1C:
Cenozoic extensional
basin fill
Cenozoic volcanic rocks
Cretaceous intrusive rocks
Mesozoic to lower Cenozoic
foreland basin deposits
Paleozoic and Mesozoic
deformed sedimentary rocks
Middle and Late Proterozoic
Belt and Windermere
Late Proterozoic Hamill Group
and Paleozoic sedimentary rocks
Late Proterozoic
Windermere Supergroup
Middle Proterozoic
Belt Supergroup
Early Proterozoic preBelt metamorphic rocks
Figure 1C
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Figure 1. (A) Simplified tectonic map of the North America Cordilleran orogenic system. Box shows location of map of B. (B) Terrane map
of northwestern United States and southwestern Canada (terranes based on Dickinson, 2004; Colpron et al., 2007). (C) Tectonic index
map of the fold-and-thrust belt and foreland basin of northwestern Montana and adjacent areas. Only names of major thrusts and thrust
systems are indicated. Thrusts in the Sawtooth Range and foothills regions of Montana and Canada are schematic and not ornamented.
Dashed line in the southeast quadrant of the map indicates Belt Island paleohigh during the Jurassic (from Parcell and Williams, 2005).
A-A′ is line of section of Figure 3. Figure was compiled from Alpha (1955); Mudge et al. (1982); Harrison et al. (1986, 1988, 1992); Latham
et al. (1988); Constenius (1996); Kleinkopf (1997); Lageson et al. (2001); and Parcell and Williams (2005).
508
Geological Society of America Bulletin, March/April 2011
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
By Late Jurassic time, this orogenic belt was
mostly a coherent system, and was developing a
fold-and-thrust belt and a foreland basin system
(DeCelles, 2004).
At the latitude of northwestern Montana,
southwestern Alberta, and southern British Columbia (~46–51°N), the Cordilleran orogenic
belt is composed of: (1) a fold-and-thrust belt,
characterized by closely spaced thrust faults involving Paleozoic and Mesozoic sedimentary
rocks in its frontal part, and a system of megathrusts carrying Proterozoic and Paleozoic strata
in the hinterland; (2) a zone of metamorphic and
plutonic rocks located west of the generally unmetamorphosed fold-and-thrust belt, known as
the Omineca belt in Canada; (3) remnants of the
magmatic arc, mainly in the form of granitoid plutons; and (4) a series of accreted terranes and accretionary complexes that make up the rest of the
Cordilleran system to the west. The easternmost
group of these, including the Kootenay, Quesnell,
Cache Creek, and Stikinia terranes, is generally
included in the composite Intermontane superterrane (Fig. 1), which was accreted to North
America by the Middle to Late Jurassic (Monger
et al., 1982; Gabrielse et al., 1991; Murphy et al.,
1995; Dickinson, 2004; Colpron et al., 2007;
Dorsey and LaMaskin, 2007; Ricketts, 2008).
The eastern region of this superterrane is part of
the morphotectonic Omineca belt. The westernmost element in the Cordilleran collage at these
latitudes, the Insular Superterrane, had a long
and complex accretion history that commenced
during the Middle Jurassic (Monger et al., 1982;
Colpron et al., 2007; Ricketts, 2008).
The general stratigraphy of northwest
Montana can be divided into four major tectonostratigraphic packages developed over
North American cratonic basement: (1) a thick
(≥15 km) Proterozoic succession of clastic,
carbonate, and igneous rocks of the Belt Supergroup (Harrison, 1972; Harrison et al., 1974),
possibly deposited in an intracontinental rift
(Cressman, 1989; Price and Sears, 2000) or a
backarc extensional or strike-slip basin (Ross
and Villeneuve, 2003); (2) a succession of
carbonate and relatively minor clastic rocks,
deposited as a miogeoclinal wedge during late
Precambrian rifting and subsequent Paleozoic passive-margin development (McMannis,
1965; Bond et al., 1985; Poole et al., 1992);
(3) a Middle Jurassic to Lower Cenozoic clastic
wedge, mainly deposited in a foreland basin setting (McMannis, 1965; Bally et al., 1966; Peterson, 1981); and (4) middle Eocene to Holocene
clastic rocks deposited in discrete extensional
basins in the western part of the fold-and-thrust
belt (McMannis, 1965; Constenius, 1982,
1996). The focus of this paper is on the Middle
Jurassic–Lower Cenozoic stratigraphic interval.
STRATIGRAPHY AND
SEDIMENTOLOGY
Approximately 2.5–3 km of Jurassic–Lower
Paleocene strata currently lie in front of and
within the frontal part of the fold-and-thrust belt
in northwestern Montana (Figs. 2 and 3). Most
of the Lower Paleogene succession has been
erosionally removed from the proximal foreland basin, but remnants are preserved to the
east in isolated localities. Subsurface regional
basement uplift has exhumed part of the foreland basin fill along the Sweetgrass Arch and
related structures. To the east, in proximity to
the North and South Dakota borders, subsidence
associated with the Williston Basin combined
with distal flexural subsidence due to orogenic
loading has created additional accommodation. The following sections address the major
sedimentologic and stratigraphic aspects of the
Middle Jurassic–Lower Paleocene basin fill of
northwestern Montana, and provide context
for discussion of basin evolution relative to the
Cordilleran orogenic belt. An extended version
of the stratigraphic and sedimentologic descriptions is available in the GSA Data Repository
Table DR1.1
Middle to Upper Jurassic
In northwestern Montana, an unconformity
representing ~150 m.y. separates the youngest
preserved miogeoclinal rocks of the Mississippian Madison Group from the oldest strata, of
Middle to Late Jurassic age, that can be linked
to Cordilleran orogenic evolution (Fuentes
et al., 2009). These Jurassic deposits are referred
to as the Ellis Group and the Morrison Formation. The tectonic setting of these and equivalent
deposits farther south in the U.S. Cordillera remains controversial, and no consensus exists for
possible tectonothermal, dynamic, or flexural
mechanisms of subsidence (for a discussion, see
DeCelles, 2004).
The Middle–Upper Jurassic Ellis Group is
composed of ~80–200 m of marine strata that
are divided into the Sawtooth, Rierdon, and
Swift Formations (Figs. 2 and 4). These units
are characterized by an irregular distribution,
abrupt lateral facies changes, and local internal
unconformities (McMannis, 1965; Peterson,
1981; Parcell and Williams, 2005). At a regional
scale, the Middle Jurassic deposits thin markedly across a region of paleohighs of the “Belt
1
GSA Data Repository item 2011002, Table
DR1 (extended stratigraphy and sedimentology);
Table DR2 (U-Pb ages of detrital zircons); and Table
DR3 (point-counting results), is available at http://
www.geosociety.org/pubs/ft2010.htm or by request
to editing@geosociety.org.
Island” complex (Suttner et al., 1981) and the
Sweetgrass Arch and related structures, and
thicken across the Williston Basin to the east
(Carlson, 1968).
The Sawtooth Formation is 15–50 m thick,
and is composed of cross-bedded and ripplelaminated sandstones and laminated mudstones,
deposited in nearshore marine environments
during a regional transgressive event. The Rierdon Formation contains mostly gray mudstone,
varies in thickness from ~25 to ~70 m (Cobban,
1945), and indicates offshore deposition. The
Swift Formation is characterized by glauconitic
cross-bedded and rippled sandstones. Its lower
part consists of shale with subordinate bioturbated sandstone. Thickness ranges between 20
and 40 m. The Swift Formation marks a highstand/regressive episode, with progradation of
shoreface deposits over distal facies of the Rierdon Formation. Rippled fine-grained sandstones
and mudstones in the lower part of the Swift possibly represent tidal-flat deposits (Porter, 1989).
Reported ages for the Ellis Group range from
Bajocian-Bathonian to late Oxfordian–Kimmerdigian time, and the detailed stratigraphy and
regional extent of internal unconformities within
the unit have been the subject of numerous investigations (e.g., Cobban, 1945; Carlson, 1968;
Mudge, 1972; Imlay, 1980; Porter, 1989; Parcell
and Williams, 2005). Recent stratigraphic analysis suggests that these unconformities are generally local, and resulted in part from tectonically
active basement structures (Parcell and Williams,
2005). Our new detrital zircon and palynology
results yield additional constraints for the age of
the Ellis Group. A sample collected 0.5 m above
the pre–Middle Jurassic unconformity yielded
one zircon grain with an age of 171.1 ± 2.5 Ma
(see Isotopic Results and Provenance Interpretations sections and Table DR2 [see footnote 1]),
which constrains the maximum depositional
age of the Sawtooth Formation as early Bajocian (±error). The youngest age obtained from
the detrital zircons of a Swift Formation sample
was 157.1 ± 6 Ma (mid-Oxfordian ± error). Palynology from three Morrison Formation samples
discussed later herein constrain the minimum
possible depositional age of the Swift as Oxfordian. These data, together with the previously reported paleontological ages, indicate a
Bajocian–mid-Oxfordian age for the Ellis Group
in northwestern Montana.
At a regional scale, the Ellis Group correlates
with the upper part of the Fernie Formation of
southwestern Canada (Poulton et al., 1994), and
the Gypsum Spring and Sundance Formations of
northern Wyoming (Parcell and Williams, 2005).
Marine environments were replaced by
fluvial and lacustrine environments during Late
Jurassic deposition of the Morrison Formation.
Geological Society of America Bulletin, March/April 2011
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Fuentes et al.
120
S
C
T
C
CRETACEOUS
100
C
A
A
Fluvial
>800
Bearpaw/Horsethief
Offshore/shoreface
75–100
Two Medicine
Fluvial,
lacustrine (coals)
550–600
Virgelle
Telegraph Creek
Shoreface
40–60
Shoreface, estuarine
90–170
Marias
River
Shale
Offshore marine
350–450
Blackleaf
Fluvial (upper part)
Shoreface
Offshore (lower part)
200–500
Kootenai
Fluvial (mainly braided),
lacustrine (clastic and
carbonatic). Paleosols.
100–>300
Transitional
silt/claystone
Marine
sandstone
V
Marine
silt/claystone
Coarse sand/
conglomerate
Continental
sandstone
Continental
siltstone
Coal/continental
organic shale
Limestone
Tuff/
bentonite
T
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T
Ellis
MIDDLE LATE
JURASSIC
Willow Creek
St. Mary River
KEY
B
Morrison
Swift
Rierdon
Sawtooth
Fluvial, shallow lacustrine,
estuarine. Paleosols (top)
60–80
Shoreface
Offshore marine
Nearshore marine
Figure 2. Simplified Jurassic–early Eocene stratigraphy of northwestern Montana.
510
>250 (central MT)
B
140
180
Fluvial, ash fall
Fort Union (P. Hills) Fluvial, lacustrine (coals) 300 (central MT)
H
160
DOMINANT
APPROX.
DEPOSITIONAL THICKNESS
(m)
ENVIRONMENTS
?
Montana
80
Wasatch
Colorado
EO.
(E)
PALEOC.
T
S
D
M
LATE
Y
EARLY
60
PALEOGENE
AGE LITHOLOGY GP. FORMATION
Geological Society of America Bulletin, March/April 2011
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Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
A
P E
St. Mary River Fm.
Horsethief
Sandstone
Virgelle Two Medicine Fm.
Sandstone
Telegraph
Creek Fm.
Wasatch Fm.
Bearpaw Shale
Fort Union Fm.
Hell Creek Fm.
Claggett/Judith
River Fms.
Marias River Shale
Blackleaf Fm.
Kootenai Fm.
Ellis Gr./
Morrison Fm.
ian
ipp
iss ian
s
s
Mi von n
De bria
m
Ca
0
Precambrian basement
1 km
0
50 km
NonMarine marine
EC
LC
Willow Creek Fm.
J
A′
Sweetgrass arch
(Kevin Sunburst domeSouth arch)
Siltstone, sandstone, conglomerate
Shale, siltstone, sandstone
Sandstone
Shale, siltstone, sandstone
Shale, siltstone
Figure 3. West-east simplified stratigraphic cross section of the northwestern Montana foreland basin (modified after Peterson, 1981). Lithologies correspond to the dominant type. Only
major unconformities are depicted. Location of section is shown in Figure 1C. J—MiddleLate Jurassic, EC—Early Cretaceous, LC—Late Cretaceous, P—Paleocene, E—Eocene.
In northwestern Montana, this unit consists of
~60–80 m of fine-grained clastic strata (Fig. 4).
Whether or not the basal contact of the Morrison represents an unconformity has been debated (Suttner, 1969; Pipiringos and O’Sullivan,
1978; Peterson, 1981; Porter, 1989; Gillespie
and Heller, 1995). This issue was clarified by
Demko et al. (2004), who indicated that the
base of the formation is marked by the J-5 unconformity in the southern part of the Morrison
depositional basin, but this contact becomes
conformable from northern Utah and Colorado northward. The Morrison’s upper limit is
marked by a regional unconformity separating it
from the Kootenai Formation or equivalent units
(Mudge, 1972; DeCelles, 1986, 2004; Dolson
and Piombino, 1994; Currie, 1998).
In northwestern Montana, the basal Morrison Formation is characterized by shale and
fine-grained sandstone arranged in heterolithic
sedimentary structures. This interval is followed
by ~65 m of siltstone, with subordinate crossbedded sandstone and beds of limestone and
marl. The top ~30 m of the Morrison Formation
consists of gray, green, and purple mud rocks
with pervasive evidence for pedogenesis.
The lower part of the Morrison probably
represents a tide-dominated marginal marine
environment. Estuarine conditions are inferred
from the heterolithic sedimentary structures and
from new palynological assemblages from three
samples that indicate estuarine or deltaic environments (Table 1). Most of the unit in northern
Montana, however, represents low-energy fluvial and local shallow lacustrine environments
(Peterson, 1981). The upper Morrison paleosol
complex is incompletely preserved owing to extensive erosional truncation beneath Cretaceous
fluvial deposits. This zone of multiple red and
gray paleosol horizons is mostly developed over
silty overbank facies, contains carbonate nodules, multicolor mottles, and calcareous and locally iron-oxide and hydroxide cements.
The Morrison Formation has been sparsely
dated in northwestern Montana. Palynological
analyses from our measured section yielded
Oxfordian to Kimmeridgian ages (Table 1;
Fig. 4). The youngest detrital zircons from a
sandstone bed in the middle part of the unit
yielded middle to late Oxfordian ages. These
new data constrain the age of the Morrison as
latest Oxfordian to Kimmeridgian in the region,
which is in general agreement with previous
work in Utah, Wyoming, and Colorado (Litwin
et al., 1998; Kowallis et al., 1998; Turner and
Peterson, 2004).
In southwestern Canada, Upper Jurassic rocks
considered equivalent to the Morrison Formation include most of the Passage Beds at the top
of the Fernie Formation, and the Morrisey and
lower part of the Mist Mountain Formations,
included in the Kootenay Group (Poulton et al.,
1994; Gillespie and Heller, 1995; Turner and
Peterson, 2004).
Lower Cretaceous
A regional unconformity, representing more
than 20 m.y., cuts into the Morrison Formation
and, locally, Ellis Group deposits, and separates
the Jurassic from the Lower Cretaceous succession. The oldest Cretaceous sedimentary rocks
in Montana consist of ~50–400 m of conglomerate, sandstone, and siltstone of the Kootenai
Formation. This unit is overlain by the Blackleaf Formation of the Colorado Group, which
records the first major Cretaceous marine transgression along the Western Interior Basin during the late Albian. Lower Cretaceous strata in
northwestern Montana are the first unequivocally synorogenic foredeep deposits related to
Cordilleran tectonics (Suttner, 1969; DeCelles,
1986; Schwartz and DeCelles, 1988).
Throughout western Montana, the base of the
Kootenai Formation is conspicuously defined
by a several-meter-thick, coarse-grained to
conglomeratic, trough cross-bedded sandstone
characterized by abundant chert grains (Fig. 4).
These deposits are overlain by a succession of
variegated siltstone, cross-bedded and rippled
sandstone, limestone beds, and reworked tuffs.
Depositional environments of the Kootenai
Formation were dominantly fluvial, with extensive mud-dominated overbank environments
containing calcic paleosols. Relatively longlived lacustrine systems allowed deposition of
limestones. Influx of siliceous volcanic ash,
reworked by fluvial systems, contributed significantly to the net sedimentation. Paleocurrent
data from fluvial channel deposits show consistently eastward transport (Fig. 4).
The age of the Kootenai Formation and correlative deposits is poorly constrained (Cobban,
1955; DeCelles, 1986; Heller and Paola, 1989;
Gillespie and Heller, 1995). Sparse pollen and
fossil ages have yielded mostly Aptian to Albian ages in mudstones above the basal coarse
beds to the south. No direct dates had been
obtained for the lower coarse section prior to
this work. Four new palynology samples and
new detrital zircon data constrain its age to be
late Barremian(?)–early Albian. In particular,
a detrital zircon sample from the basal conglomeratic sandstone yielded two grains with
Hauterivian ages (131.6 ± 4.5 and 133.5 ±
1.8 Ma), providing a maximum possible age
for this interval. These data conflict with recent
suggestions by Roca and Nadon (2007) for
continuous deposition between the Morrison
Formation and the overlying conglomeratic
beds, and the proposition that the K-1 unconformity is higher in the section. A detrital zircon sample from the upper part of the Kootenai
contained a population of euhedral crystals that
yielded early Albian ages, reflecting syndepositional volcanism. Thus, the unconformity at
the base of the Kootenai Formation can be attributed to the time interval between the late
Tithonian and late Aptian.
The Blackleaf Formation is divided into four
members: the Flood Shale, Taft Hill, Vaughn,
and Bootlegger Members (Mudge, 1972;
Dyman et al., 1996). The Bootlegger Member
thins toward the west, where it is replaced by
Vaughn Member facies. Thicknesses range from
~200 m in the east to ~500 m in the west.
Geological Society of America Bulletin, March/April 2011
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Geological Society of America Bulletin, March/April 2011
Sawtooth Fm.
* *Eb
150
160
c s vf f m c vc g
Oxfordian to
Kimmeridgian
300
310
320
n=17
*1FG83
c s vf f m c vc g
Late *
Jurassic
to Early
Albian
(Undiff.)
* *
1FG70
TS 1SFSR
450
460
470
480
490
500
510
520
530
1SR80
1SR6
1SR15
c s vf f m c vc g
1SR57
1SR91
n=10
TS 1SR
1SR118
600
610
620
630
640
650
660
670
680
690
700
710
720
730
740
750
b
c s vf f m c vc g
1V2
Top Blackleaf Fm.
T S 1V
b
b
750
760
770
780
790
800
810
820
830
840
850
860
870
880
890
900
b
b
c s vf f m c vc g
Marias River Shale
900
910
920
930
940
950
960
970
980
990
1000
1010
1020
1030
1040
1050
b
b
b
b
b
b
3SR5
Top Marias
River Shale
3SR3
TS 3SR
c s vf f m c vc g
Figure 4 (on this and following three pages). Composite log of measured sections (in meters) of Jurassic–early Eocene strata. Paleocurrent
data are from trough cross-stratification according to method I from DeCelles et al. (1983).
c s vf f m c vc g
Ellis Gr.
0
Rierdon Fm.
10
T S 1 GR
Top
Morrison
Fm.
1SFSR1
* *1GR100
330
180
170
340
190
n=20
350
Top
Kootenai
Fm.
540
550
560
570
580
590
600
Blackleaf Fm. (Taft Hill Mb.)
20
30
Swift Fm.
200
360
210
380
370
1SFSR54
n=28
220
230
390
400
410
420
430
TS 1PC
Marias River Shale
40
Top
Ellis Gr.
50
60
70
1DD1
1DD2
Aptian to
Early Albian
Kootenai Fm.
TS
1DD
240
Barremian to
Aptian*
Late Jurassic to
Early Albian
(Undiff.)*
440
450
Blackleaf Fm. (Flood Shale Mb.)
80
1GR17
1GR14
250
260
270
280
290
300
Blackleaf Fm. (Vaughn Mb.)
90
Early to Middle
Oxfordian
Oxfordian to
Kimmeridgian
1GRx
1GRz
1M5
1GR44
Morrison Fm.
100
110
120
130
140
150
Fuentes et al.
Marias River Shale
Geological Society of America Bulletin, March/April 2011
1050
c s vf f m c vc g
2SR76
Top
Telegraph
Creek Fm.
* 2SR240
*2SR184
1200
1210
1220
c s vf f m c vc g
n=28
1350
1360
1370
c s vf f m c vc g
b
*EM1
c s vf f m c vc g
Figure 4 (continued).
1500
1510
1520
1650
1660
1670
1680
1530
1230
1380
1240
1690
1400
1250
1540
1710
1560
1390
1720
1730
1570
1580
1740
1750
1760
1770
1780
1700
n=10
1TM3
1590
1600
1610
b
1790
1800
1550
1410
1260
n=10
1420
1270
b
n=13
1620
1630
1640
1650
Two Medicine Fm.
1060
1070
1080
1090
1100
1110
1120
Virgelle Ss.
2SR87
1430
1280
1450
b
Two Medicine Fm.
n=29
*
2SR258
n=16
1460
1440
b
1290
1300
1310
1470
1480
1490
1500
c s vf f m c vc g
b
TS 1TM
Top Two
Medicine Fm.
1PB42
Bearpaw Shale
1130
2SR103
n=7
Top Virgelle
Sandstone
1320
Two Medicine Fm.
1140
1150
Two Medicine Fm.
TS 2SR
1330
1340
1350
1800
1810
1820
1830
1840
1850
1860
1870
1880
1890
1900
1910
1920
1930
1940
1950
1BB25
n=37
1BB44
1BB61
Covered interval not well constrained
1PB66
Top Bearpaw Shale/
Horsethief Sandstone
c s vf f m c vc g
TS 1PB
1DD (Ellis Gr.): 112°42′ 29″W - 47°37′ 45″N
1GR (Morrison Fm.): 112°42′ 35″W - 47°37′ 3″N
1SFSR (Kootenai Fm.): 112°51′ 32″W - 47°37′ 47″N
1PC (Flood Shale Mbr.): 112°40′W - 47°37′ 29″N
1SR (Taft Hill and lower Vaughn Mbrs.):
112°40′ 17″W - 47°37′ 16″N
1V (upper Vaughn Mbr.): 112°41′ 8″W - 47°36′ 11″N
3SR (Marias River Shale): 112°38′ 39″W - 47°37′ 16″N
2SR (Telegraph Creek and Virgelle Fms.):
112°38′ 16″W - 47°37′ 39″N
1TM (Two Medicine Fm.): 112°15′ 47″W - 48°26′ 28″N
1PB (Bearpaw/Horsethief Fm.): 112°31′ 42″W - 47°49′ 49″N
1BB (lower St. Mary River Fm.): 112°35′ 22″W - 47°31′ 12″N
Rainbow Resources 1-7 ART V DRESEN well (St. Mary
River and lower Willow Creek Fm.): 113°5′46″W- 48°59′ 6 ″N
HCFU (Fort Union and lower Wasatch Fms.):
109°08′ 56″W - 48°06′ 07″N
1W (upper Wasatch Fm.): 109°54′ 01″W - 48°15′ 52″N
(Coordinates are for base of sections, except for well)
on
Seeccttiioonnlsocloactia
tion
TS 1BB
St. Mary River Fm.
1160
1170
1180
1190
1200
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
Horsethief Sandstone
Two Medicine Fm.
Telegraph Creek Fm.
513
514
b
Geological Society of America Bulletin, March/April 2011
c s vf f m c vc g
b
b
b
b
150
160
170
180
b
b
St. Mary River Fm.
c s vf f m c vc g
b
b
b
300
310
320
330
340
350
360
370
Log of St. Mary River Fm and lower
section of Willow Creek Fm based
on cutting description and electric logs
from Rainbow Resources 1-7 Art V
Dresen well.
Total thickness of Willow Creek Fm
penetrated by well is ~233 m.
380
Upper section of well does not
include cutting description and
electric logs.
c s vf f m c vc g
Top St. Mary River
Willow Creek Fm.
0
b
b
b
b
b
b
b
b
b
0
10
20
30
40
50
60
70
80
90
100
110
120
130
Top Hell
Creek Fm.
150
160
170
180
190
c s vf f m c vc g
n=19
1FU3
300
310
320
330
340
350
n=8
1FU5
200
370
380
390
400
360
b
410
420
210
220
230
240
250
260
b
430
440
450
Top Fort
Union Fm.
c s vf f m c vc g
b
b
b
b
Fort Union Fm.
Hell Creek Fm. (Upper part)
Figure 4 (continued).
c s vf f m c vc g
?
270
280
290
300
Fort Union Fm.
10
20
30
190
b
b
b
Fort Union Fm.
40
b
200
220
230
240
250
260
270
280
50
St. Mary River Fm.
210
b
b
b
140
150
Wasatch Fm.
b
b
b
b
b
b
b
b
b
290
300
450
460
470
480
490
500
510
520
530
540
550
560
570
580
590
600
TS 1W
TS HCFU
1W1
1W2
c s vf f m c vc g
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
Top Wasatch Fm.
Wasatch Fm.
60
70
80
90
100
110
120
130
140
150
Fuentes et al.
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
LEGEND
Sandstone
Pebbles
Marine clay/siltstone
Intraclasts
Transitional clay/siltstone
Calcareous nodules/concret.
Terrestrial mud/siltstone
Shell material
Limestone
Plant material/organic matter
Marl/sandy limestone
Burrows/bioturbation
Tuff/tuffaceous silt/sandstone
Roots
Poorly exposed mud/siltstone interval
Dessication cracks
Covered interval
Fish material
Trough cross-bedding
Vertebrate bones
Ripples
Water-escape structures/load casts
Climbing ripples
Seismites
Planar cross-bedding
Planar bedding/lamination
Bentonite layer
b
Coal
Hummocky cross-bedding
Flaser/wavy/lenticular bedding
Palynology sample with age
Strong pedogenesis
Point counting sample
Gypsum/anhydrite
Detrital zircon sample
Paleocurrent direction
n=20 with number of measurements
c s vf f m c vc g
TS 1V Top of measured section with
Gravel/pebble
Sand
Silt
Clay
section name
Figure 4 (continued ).
The Flood Shale Member is composed of
~50–60 m of marine shale capped by rippled
and trough cross-bedded sandstone (Fig. 4).
The Taft Hill Member is a stack of upwardcoarsening shale and cross-bedded and rippled
sandstone. The Vaughn Member consists of
nonmarine mudstone, cross-bedded sandstone,
and conglomeratic channel fills that represent
fluvial and alluvial plain deposits.
In Montana, the Blackleaf Formation has
been dated as late Albian–early Cenomanian
(Cobban and Kennedy, 1989). The mean age
of the eight youngest detrital zircons from a
sample of the Vaughan Member is ca. 97 Ma,
which is consistent with the previously reported
paleontological ages.
Upper Cretaceous
A second widespread marine transgression affected the foreland basin during the early Late
Cretaceous (Porter et al., 1982; Stott, 1984). In
northwestern Montana, late Cenomanian to early
Santonian black shales reach a thickness of more
than 350 m (Schmidt, 1978; Yang and Miall,
2009). These deposits are referred to as the
Marias River Shale. This unit and the Blackleaf
Formation are formally included in the Colorado
Group (Cobban et al., 1959; Mudge, 1972). The
Marias River Shale correlates with the Frontier
Formation of southwestern Montana (Dyman
et al., 1996), and with the Blackstone, Cardium,
and Wapiabi Formations of southern Alberta
(Yang and Miall, 2009). The Marias River Shale
is dominated by dark shale, with an increase in
sandstone content toward the top (Fig. 4). Thin
bentonite beds occur in the unit.
The Santonian is characterized by a regressive sedimentation pattern, which continues into
the Campanian. The Telegraph Creek Formation consists of ~90–170 m of mudstone and
siltstone with sandstone intercalations. Ripples,
trough cross-stratification, hummocky stratification, and burrows are abundant (Fig. 4). Facies
and abundant body and trace fossils indicate
nearshore marine and estuarine conditions. The
Virgelle Sandstone is 40–60 m thick and is composed almost entirely of trough cross-stratified
sandstone that was deposited in a nearshore environment. The Telegraph Creek Formation is
late Santonian in age (Cobban, 1955; Cobban
et al., 2005), and the Virgelle Sandstone is early
Campanian (Cobban, 1955).
Nonmarine deposition resumed during the
Campanian. The Two Medicine Formation
in Montana consists of ~600 m of fluvial and
minor lacustrine deposits with volcanic material
and coal beds (Fig. 4). Nonmarine and nearshore
marine equivalents in central Montana include
the Eagle and Judith River Formations, which
thin eastward into the fully marine Claggett and
Bearpaw Shales. The 40Ar/39Ar dating of biotite
and plagioclase from bentonites located near
the base and top of the Two Medicine Formation constrains its age between ca. 80 and 74 Ma
(Rogers et al., 1993; Rogers, 1994).
Deposition of the Two Medicine Formation
continued until maximum transgression of the
Bearpaw Sea (Gill and Cobban, 1973). The
Campanian-Maastrichtian Bearpaw Shale and
the Horsethief Sandstone represent the final
widespread marine units in northwestern Montana. In the study area, the total thickness of the
Bearpaw and Horsethief is ~100 m (Fig. 4). The
Bearpaw Shale consists of offshore black shale,
which grades upward into fossiliferous, trough
cross-stratified sandstone. The Horsethief Sandstone consists of a succession of cross-stratified
shallow marine sandstone.
The youngest foreland basin deposits preserved in regions adjacent to the thrust belt are
the St. Mary River and Willow Creek Formations, which are broadly dated by vertebrate
and invertebrate fossils as Maastrichtian–early
Paleocene (Russel, 1950, 1968; Tozier, 1956;
Catuneanu and Sweet, 1999). The thickness of
these two units is difficult to estimate owing to
the discontinuity of outcrops and lack of marker
intervals to establish correlations. Well data indicate that the St. Mary River Formation is on
the order of 300 m thick (Fig. 4), similar to the
value estimated by Stebinger (1916). The combined thickness of the two units probably is more
than 800 m (Mudge et al., 1982). The St. Mary
River Formation consists of mudstone and beds
of cross-stratified fluvial sandstone. Cuttings and
electric logs from the Rainbow Resources 1–7
Art V Dresen well located near the international
border show a dominance of mudstone with frequent bentonitic beds. The Willow Creek Formation is only locally exposed, and it consists of
variegated mudstone with thin beds of sandstone.
A sample of fluvial channel sandstone from
the St. Mary River Formation yielded abundant
detrital zircons with ages up to mid-Maastrichtian. The mean age from the ten youngest grains
is ca. 68.5 Ma, providing an additional maximum age constraint for deposition of this unit.
Paleocene–Eocene
The youngest part of the foreland basin has
been erosionally removed along proximal areas.
Lower Danian to middle Paleocene deposits are
preserved in the Porcupine Hills Formation in
southern Canada, and in the Fort Union Formation in central Montana (Douglas, 1950; Mack
and Jerzykiewicz, 1989; Fox, 1990; Catuneanu
Geological Society of America Bulletin, March/April 2011
515
Fuentes et al.
TABLE 1. PALYNOLOGY RESULTS OF MORRISON AND KOOTENAI FORMATIONS
Species
Samples
Morrison Formation
1GR20
1GR22
1GR85
Spores and pollen
Aequitriradites spinulosus
Apiculatisporis sp. A
Araucariacites australis
Callialasporites dampieri
C. triangularis
C. trilobatus
Cerebropollenites mesozoicus
Cicatricosisporites australiensis
Cicatricosisporites australis
C. hallei
C. purbeckensis?
C. venustus
Classopollis classoides
Concavissimisporites punctatus
Concavissimisporites southeyensis
Concavissimisporites variverrucatus
Coronatispora sp.
Corrugatisporites anagrammensis
Cyathidites australis
Deltoidospora spp.
Eucommiidites troedsonii
Exesipollenites tumulus
Foraminisporis dailyi
Gleicheniidites apilobatus
Gleicheniidites senonicus
Klukisporites pseudoreticulatus
Lycopodiacidites baculatus
Lycopodiacidites irregularis
Lycopodiacidites tortus
L. triangularis
Lycopodiumsporites austroclavatidites
Lycopodiumsporites pseudoannotinus
Klukisporites pseudoreticulatus
Mathesisporites tumulosus
Maumia irregularis
Microreticulatisporites sp.
Osmundacites wellmanni
Parvisaccites radiatus
Perinopollenites halonatus
P. sp.
Perotrilites sp.
Platysaccus sp. A
Podocarpites sp.
Reticulatisporites sp.
Rubinella sp.
Schizosporis parvus
S. reticulates
Taxodiaceae
Trilobosporites hannonicus
Trilobosporites humilis
T. minor
T. perverulentus
T. sphaerulentus
Triporeletes reticulatus
Undifferentiated Bissacates
Verrucosisporites staplinii
V. sp.
1GR189
Kootenai Formation
1FG50
1FG69
1SFSR67
R
R
A
A
R
R
R
R
R
C
R
C
R
R
R
R
*R
R
R
R
R
*R
*R
R
R
R
R
*R
A
R
R
A
R
R
*R
R
A
R
A
A
R
F
*R
*R
A
R
A
R
R
R
R
R
R
A
R
R
R
R
R
R
R
R
R
R
F
*C
R
*R
*R
R
*R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
A
R
R
R
A
*R
*R
R
*R
*R
*R
A
A
*R
R
A
A
R
F
*R
A
Marine organisms/microplankton
Aldorfia dictyota
*R
Cribroperidinium nuciformis
*R
Ctenidodinium sp.
R
Lithodinia sp.
R
Micrhystridium sp.
R
Rhynchodiniopsis cladophora
*R
*R
*R
Scriniodinium crystallinun
*R
*F
Stephanelytron redcliffense
*R
Note: R—rare; F—frequent; C—common; A—abundant. Taxa marked with an asterisk (*) are especially significant to the age assigned for the sample in which they
were found.
516
Geological Society of America Bulletin, March/April 2011
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
and Sweet, 1999; Lund et al., 2002). In northern
Montana, the youngest rocks that can be linked
to deformation in the thrust belt are in the Lower
Eocene Wasatch Formation on the flanks of the
Bearpaw Mountains and in the Missouri Breaks
diatremes, ~200 km east of the thrust belt front
(Reeves, 1946; Hearn, 1976).
The Fort Union Formation in the Bearpaw
Mountains consists of ~300 m of sandstone,
siltstone, shale, and coal representing fluvial
and lacustrine deposits (Fig. 4). The Wasatch
Formation consists of variegated siltstone,
bentonitic mudstone, cross-stratified fine- to
coarse-grained sandstone, and lenticular beds
of cobble-boulder conglomerate. The Wasatch
Formation is overlain by rocks of the Bearpaw
Mountains volcanic field (Hearn, 1976). In
measured sections, the Wasatch Formation is
~250 m thick, but its original thickness is unknown. Eroded proximal equivalents of the Fort
Union and Wasatch Formations could have been
much thicker. In Alberta, coal moisture, vitrinite
reflectance, and thermokinematic modeling indicates that 2–4 km of synorogenic Cenozoic
strata have been erosionally removed (Beaumont, 1981; Hardebol et al., 2009).
The Fort Union Formation has been assigned
to the Paleocene (Rice, 1976). Recent work based
on plant content and magnetostratigraphy (Hartman, 2002; Lund et al., 2002) placed the Fort
Union Formation of the Williston Basin across the
Maastrichtian-Danian boundary or the very early
Danian, up to the late Paleocene. The Wasatch
Formation has been assigned an early Eocene
age (ca. 57–54 Ma) based on flora and vertebrate
fossils (Brown and Pecora, 1949; Marvin et al.,
1980). Volcanic rocks overlying the Wasatch have
been dated as late early to early middle Eocene
(ca. 54–50 Ma) (Marvin et al., 1980; Wing and
Greenwood, 1993). The Wasatch Formation was
the last unit deposited in a foreland basin setting,
and the early to middle Eocene igneous rocks that
cover and intrude this unit herald the beginning of
crustal extension and magmatism in the northern
Cordillera (Constenius, 1996).
PROVENANCE
Forty sandstone samples for modal petrographic analysis and 10 samples for U-Pb ages
of detrital zircons were selected to characterize
the provenance of Middle Jurassic–early Eocene sedimentary rocks. These analyses were
carried out to assess major tectonic variations
in the development of the foreland basin with
time, and to determine the unroofing history of
the fold-and-thrust belt. Additionally, U-Pb ages
in some cases help constrain maximum ages of
the sedimentary units from which the zircons
were collected, as discussed already.
Potential Source Terrane Compositions
From west to east, potential sources of sediments (Fig. 5) include the assemblage that
constitutes the Intermontane superterrane, the
region of the Omineca belt, a series of hinterland
thrust sheets carried by the Moyie, Libby, and
other thrust systems involving Belt Supergroup
rocks, the major thrust sheets carried by the
Lewis-Eldorado-Steinbach-Hoadley (LEDSH)
thrust system, a series of closely spaced thrust
sheets in the Sawtooth Range and foothills,
and North American basement. Possible alongstrike transport could have provided sediments
from distal sources.
The Intermontane belt at the latitude of northwestern Montana and southern Canada is composed of the Stikinia, Cache Creek, Quesnell,
and Kootenay terranes (Figs. 1 and 5). Stikinia
consists of Upper Paleozoic to Lower Jurassic
volcanic and marine rocks. The Cache Creek
terrane is interpreted to be the accretionary
complex of the Quesnellia arc, and contains
Mississippian to Middle to Late(?) Triassic
radiolarian chert, argillite, basalt, ultramafic
rocks, limestone, and local blueschist (Coney
and Evenchick, 1994). Stikinia was probably
not a significant sediment source for the western
Montana foreland basin because it was apparently flexed under the load of the Cache Creek
accretionary complex and remained close to or
below sea level during the late Mesozoic and
early Cenozoic (Coney and Evenchick, 1994).
The Cache Creek terrane shed radiolarian chert
clasts westward, but apparently not eastward,
implying the existence of a drainage divide
east of it, at least in southern Canada (Mack
and Jerzykiewicz, 1989). The Quesnell terrane
is composed of late Paleozoic to early Mesozoic submarine mafic and ultramafic volcanic
rocks, volcaniclastic and carbonate rocks, and
chert, sitting on Devonian to Permian rocks of
arc affinity. Although traditionally interpreted
as a freestanding intraoceanic arc, recent work
(Erdmer et al., 2002; Unterschutz et al., 2002)
demonstrates an autochthonous origin for
the Quesnell terrane. The Kootenay terrane, the
easternmost component of the Intermontane
belt, contains Lower to Middle Paleozoic offshelf facies of the miogeoclinal wedge (Colpron
and Price, 1995; Colpron et al., 2007). Its lower
part is composed of quartzo-feldspathic schist
and gneiss, quartzite, and carbonate, and its
upper section is characterized by meta-volcanic
rocks, orthogneiss, dark graphitic pelites, limestone, argillite, and chert (Coney and Evenchick,
1994; Colpron and Price, 1995).
To the south, these terranes are covered by
the Columbia River basalts, but correlative
terranes are present in the Blue Mountains of
northeastern Oregon (Dickinson, 2004; Dorsey
and LaMaskin, 2007). The implication of this is
that the Pacific Northwest of the United States
and adjacent Canada would have produced sediments of similar compositions.
To the east, the Omineca belt consists of
metamorphic and plutonic rocks (Monger et al.,
1982; Gabrielse et al., 1991). Omineca belt
rocks were polydeformed and metamorphosed
during the Jurassic, and some structural levels
were further deformed and overprinted as recently as the early Eocene (Parrish et al., 1988;
Evenchick et al., 2007). The Omineca belt contains exposures of the North American craton
and abundant mid-Cretaceous plutons. The
eroded upper stratigraphic levels presumably
contained distal facies of the Belt Supergroup
and the miogeocline.
A zone of hinterland thrust systems involving Belt Supergroup rocks extends east of the
Omineca belt. Major thrusts include the Moyie,
Libby, Pinkham, and a number of other thrusts
(Harrison et al., 1980, 1986; Harrison et al.,
1992; Fillipone and Yin, 1994) for which detailed geometry is poorly known owing to postorogenic extensional faulting and a widespread
Quaternary cover. These thrust systems terminate to the south against the strike-slip system
of the Lewis and Clark line (Fig. 1), but thrust
systems involving Belt Supergroup rocks also
exist to the south, at the latitude of the Helena
salient. Phanerozoic miogeocline and foreland
basin strata are mostly eroded from these thrust
sheets. Although sediments produced during
early stages of slip on these thrusts were probably recycled early foreland basin deposits
and Paleozoic strata, diagnostic petrographic
elements associated with deeper exhumation
should be grains of argillite, polycrystalline
quartz, micas, and chlorite derived from Belt
Supergroup strata. A key to differentiating this
from provenance areas to the west in the Intermontane belt would be the ages of detrital zircons: Intermontane terrane zircons should be
dominated by Phanerozoic ages, whereas Belt
Supergroup–derived zircons should have Laurentian ages >1.85 Ga and 1655–1790 Ma,
non-Laurentian ages in the 1510–1625 range,
and synsedimentary (Belt) ages ranging from
1440 to 1480 Ma (Ross and Villeneuve, 2003;
Link et al., 2007). Although no detrital zircon
studies for miogeoclinal rocks from the northwestern United States are available, Gehrels and
Ross (1998) reported >1.75 Ga and “Grenville”
1.1–1.4 Ga ages of zircons from miogeoclinal
strata in British Columbia and Alberta.
The frontal part of the northwestern Montana
fold-and-thrust belt is dominated by a megathrust sheet composed of several kilometers of
Belt Supergroup and minor Paleozoic rocks in
Geological Society of America Bulletin, March/April 2011
517
Hinterland and LEDSH thrust
systems:
Precambrian Belt Supergroup,
with Phanerozoic stratigraphy
largely eroded.
Argillite, quartzite, low-grade
metamorphic rocks.
Cache Creek:
Mississippian to Middle-Late(?)
Triassic accretionary complex.
Radiolarian chert, argillite, basalt,
ultramafic rocks.
Quesnell:
Upper Paleozoic to lower
Mesozoic submarine volcanic
deposits sit on DevonianPermian rocks of arc affinity.
Mafic and ultramafic rocks,
limestone, chert.
Kootenay:
Low to mid-Paleozoic
possible off-shelf facies
of miogeocline.
Schist and gneiss, quartzite,
limestone, meta-volcanics,
graphitic pelite, argillite,
chert.
Omineca belt:
Metamorphic and plutonic
rocks. Exposures of North
American basement.
Abundant mid-Cretaceous
plutons. Original upper
stratigraphy mostly eroded.
STIKINIA
CACHE CREEK
INTERMONTANE TERRANES
QUESNELL
OMINECA BELT
KOOTENAY
J-K MAGMATIC ARC
HINTERLAND
THRUST
SYSTEMS
518
Figure 5. Cartoon illustrating potential major source areas for the foreland basin system of northwest Montana. Terrane nomenclature
corresponds to southern Canada. Autochthonous source terranes over North American basement represent predeformed state. LEDSH—
Lewis-Eldorado-Steinbach-Hoadley thrust system. Not to scale.
Sawtooth Range/Foothills:
Cambrian-Paleocene
sedimentary rocks.
LEDSH SAWTOOTH
THRUST
RANGE/
SYSTEM FOOTHILLS
N.A. BASEMENT
ALONG-STRIKE
TRANSPORT
Fuentes et al.
the hanging wall of the LEDSH thrust system.
The Sawtooth Range and the foothills structures involve Cambrian to early Paleocene strata
(Mudge, 1980, 1982). The development of these
foothills structures passively deformed rocks
of the overlying Lewis thrust sheet, as clearly
seen along the Lewis thrust salient in the U.S.Canada border region (e.g., Gordy et al., 1977;
Fermor and Moffat, 1992).
Finally, a potential source of sediments is
North American basement. Sedimentary rocks
with provenance from this source should contain abundant quartz and feldspar, and zircons
with ages older than 1.8 Ga (Gehrels and Ross,
1998; Ross and Villeneuve, 2003).
Petrographic and Isotopic Provenance Data
Methods
About 450 grains per slide were point counted
using the Gazzi-Dickinson method, as explained
by Ingersoll et al. (1984), in order to facilitate
comparison of the results with the provenance
models of Dickinson and Suczek (1979) and
Dickinson et al. (1983). Only medium-grained
sandstones were point counted. Half of each
thin section was stained for both calcium and
potassium feldspars. Grain types are listed in
Table 2. The stratigraphic position of individual samples is shown in the measured sections
(Fig. 4). Individual point-counting results are
tabulated in Table DR3 (see footnote 1). Paleocurrent data were obtained to provide additional
paleogeographic and provenance information.
Most of the data are from the limbs of mediumto large-scale trough cross-strata (method I of
DeCelles et al., 1983). Additional paleocurrent
information was provided by imbricated conglomerate clasts, and, in a few cases, by orientations of trough axes.
Zircon U-Pb geochronology was conducted
by laser ablation–multicollector–inductively
coupled plasma–mass spectrometry (LA-MCICP-MS) at the University of Arizona LaserChron Center (analytical procedures in Gehrels
et al., 2008). Approximately 100 grains per sample were dated, with the exception of a sample
from the Two Medicine Formation, from which
only 19 grains were recovered. Analyses that
yielded isotopic data with acceptable discordance, precision, and in-run fractionation are
shown in Table DR2 (see footnote 1). In total,
873 zircon ages are reported here. The ages are
plotted on relative probability diagrams. Age
peaks on these diagrams are considered robust
if defined by three or more analyses.
Petrographic Results
Sandstones in the Jurassic Ellis Group and
Morrison Formation have mature quartzo-lithic
Geological Society of America Bulletin, March/April 2011
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
TABLE 2. PETROGRAPHIC PARAMETERS
Quartzose grains
Qm
Qp
Qpt
Qss
S
Qt
Monocrystalline quartz
Polycrystalline quartz
Foliated polycrystalline quartz
Quartz in sandstone/quartzite lithic grain
Siltstone
Total quartzose grains (=Qm + Qp + Qpt + Qss + S + C=Cb)
Feldspar grains
K
P
F
Potassium feldspar
Plagioclase feldspar
Total feldspar grains (=K + P)
Lithic grains
Metamorphic
Lph
Lsm
Lss
M
Lm
Phyllite
Schist
Serpentinite schist
Marble
Metamorphic lithic grains (=Lph + Lsm + Lss + M + Qpt)
Volcanic
Vvl
Lvx
Lvf
Lvv
Lvm
Lv
Lathwork volcanic grains
Microlitic volcanic grains
Felsic volcanic grains
Vitric volcanic grains
Mafic volcanic grains
Total volcanic lithic grains (=Lvl + Lvx + Lvf + Lvv + Lvm)
Sedimentary
D
Dolostone
Lc
Limestone
Lsh
Shale/mudstone
C
Chert
Cb
Black chert
Ls t
Total sedimentary lithic grains (=D + Lc + Lsh + C + Cb + Qss + S)
Lt
Total lithic grains (=Ls + Lv + Lm)
L
Unstable lithic fragments (Lt-(C + Cb + Qss + S + Qpt))
Note: Accessory minerals: Epidote/zoisite, chlorite, garnet, amphibole, muscovite, biotite, tourmaline, pyroxene,
kyanite, cordierite, sillimanite, zircon, kaolinite, glauconite, apatite.
compositions, with average Qm/F/Lt = 85/1/14
and average Qt/F/L = 94/1/5 (Figs. 6 and 7;
Table DR3 [see footnote 1]). The quartz fraction is dominated by unstrained monocrystalline
quartz (Fig. 8). Lithic grains mostly consist of
sedimentary chert, with subordinate amounts
of shale and limestone. Low-grade metasedimentary and volcanic grains are scarce. A few
grains of muscovite and glauconite are present
in samples from the Swift Formation. The Morrison Formation contains traces of amphibole,
tourmaline, and zircon.
Samples from the lower, braided streamdominated part of the Kootenai Formation have
average Qt/F/L = 95/0/5 and average Qm/F/Lt
= 61/0/39. The discrepancy in Qm versus Qt
results from abundant sedimentary chert grains
that dominate the lithic fraction (Fig. 8). Other
constituents of the lithic fraction are shale, mudstone, and phyllite. Traces of volcanic lithic
grains, plagioclase, limestone, and muscovite
are present. The main difference between lower
Kootenai and Jurassic sandstone compositions
is the abundance of chert in the former.
A marked change in the detrital composition
occurs in the upper part of the Kootenai Formation. Samples of this interval are distinguished
by average modes of Qm/F/Lt = 22/17/61 and
Qt/F/L = 53/16/31. These samples contain much
higher proportions of plagioclase, volcanic lithic
grains, and cherty mudstone fragments than
sandstones lower in the section (Fig. 8). Other
minor components include limestone, chlorite,
muscovite, and zircon. This major change in
composition was also identified in correlative
rocks in southern Canada (Ross et al., 2005).
Samples from the Blackleaf Formation have
average Qm/F/Lt = 38/23/39 and Qt/F/L =
50/22/28, with a wide range of compositions.
The lithic fraction consists of chert, phyllite, and
schist, a variety of volcanic grains, and shale.
Muscovite, biotite, chlorite, and limestone
clasts are additional minor components. The
key petrographic features of Blackleaf samples,
however, are the relatively abundant low-grade
metamorphic lithic fragments (Figs. 6, 7, and 8;
Table DR3 [see footnote 1]), different varieties
of polycrystalline quartz, and a higher proportion of detrital muscovite and biotite. The increase in feldspar and volcanic lithic fragments
toward the top of the Kootenai Formation, and the
appearance of notable quantities of slate and
phyllite, plus an increased proportion of detrital
chlorite and micas by the time of deposition of
the Blackleaf Formation, seem to be of regional
extent, from Canada (Potocki and Hutcheon,
1992) to southern Montana (Suttner et al., 1981;
Dyman et al., 1988).
Two sandstone samples from the Marias
River Shale produced modes of Qm/F/Lt =
14/81/5 and Qt/F/L = 17/81/2, with feldspar
fractions dominated by plagioclase. The high
proportion of plagioclase reflects active volcanism at the time, signaled throughout the
unit by numerous bentonite beds (Figs. 4, 5,
and 7). These samples also contain anomalously large amounts of detrital biotite, of
probable volcanic origin.
Telegraph Creek and Virgelle sandstone
samples have very similar petrographic compositions, with average modes of Qm/F/Lt =
42/32/26 and Qt/F/L = 50/32/18. These sandstones contain significant amounts of chert,
phyllite, schist, different volcanic grain types,
limestone, shale, and micas. Feldspar grains are
dominated by plagioclase, but traces of potassium feldspar are also present. Samples of the
Two Medicine Formation and Horsethief Sandstone show compositions similar to those from
the Telegraph Creek–Virgelle, with average
Qm/F/Lt = 49/25/26 and Qt/F/L = 59/24/17.
Relatively important constituents include shale,
limestone, micas, and low-grade metamorphic
lithic fragments, and traces of chlorite and amphibole. A relatively high content of potassium
feldspar was found in a sample of the Horsethief Sandstone.
Saint Mary River and Willow Creek Formation sandstones have average modes of Qm/F/
Lt = 35/33/32 and Qt/F/L = 44/32/24. Samples
from the lower St. Mary River Formation contain less quartz and more feldspar (Fig. 7). These
samples also contain greater amounts of phyllite
and schist grains (Figs. 6 and 7). Clasts of limestone, shale, and mudstone are relatively important, and detrital chlorite is abundant in two
samples of the basal St. Mary River Formation.
Sandstones from the Fort Union Formation,
the equivalent Porcupine Hills Formation, and
the Wasatch Formation show similar modes
with average Qm/F/Lt = 63/10/27 and Qt/F/L =
75/10/15. These samples contain a few to abundant potassium feldspar grains. Phyllite, schist,
limestone, and fine-grained sedimentary lithic
fragments are abundant. Traces of chlorite, amphibole, tourmaline, pyroxene, and zircon are
present as well. The uppermost sample from
our section, collected from the Wasatch Formation, is dominated by plagioclase grains, with
abundant volcanic lithic fragments and biotite
grains. Additionally, 68 clasts of a pebble conglomerate bed in the Wasatch Formation were
counted in the field. Clasts of Belt Supergroup
quartzite and fine-grained mafic igneous rocks
dominate (~28% of the total for each type).
Shale, mudstone, and low-grade metamorphic
Geological Society of America Bulletin, March/April 2011
519
Fuentes et al.
Qm
Qt
CB
RO
CB
RO
MA
MA
F
Lv
F
50% Lm
Qm
50% Lm
Lt
Ls
Lt
L
F
Qt
L
F
KEY:
Fort Union/Wasatch Formations
St. Mary River/Willow Creek Formations
T.Creek/Virgelle/T. Medicine/Horsethief Formations
Lv
Ls
Marias River Shale
Blackleaf Formation
Upper section
Kootenai Formation
Lower section
Ellis/Morrison Formations
Figure 6. Ternary diagrams illustrating modal framework-grain compositions of individual
sandstone samples (above) and means with standard deviations (below). Provenance fields
are after Dickinson and Suczek (1979). RO—recycled orogen; CB—continental block;
MA—magmatic arc. Framework components are explained in Table 2. Data are available
in Table DR3 (see text footnote 1). Wasatch Formation mean does not include uppermost
sample.
pebbles constitute 17% of the total, followed
closely by limestone clasts (16%). The remaining ~10% consists of similar proportions of finegrain felsic igneous, granite, and schist clasts.
Estimates of clast composition in Wasatch conglomerates by Hearn et al. (1964) provided values of 50%–80% argillite and quartzite of Belt
Supergroup, 1%–5% Paleozoic rocks, 20%–
520
40% Late Cretaceous–Paleocene fine-grained
and porphyritic volcanic rocks, and 1% chert
and conglomerate.
Isotopic Results
Ages of detrital zircons of individual samples
are plotted on relative probability diagrams in
Figure 9. Sample Eb was collected from the
lower part of the Sawtooth Formation and produced age clusters with a typical miogeoclinal
signature (Gehrels and Ross, 1998). Most grains
yielded ages older than ca. 1 Ga; a few provided
ages in the range 419–467 Ma; two grains had
Late Proterozoic ages; and one zircon yielded a
syndepositional age of ca. 171 Ma (Table DR2
[see footnote 1]).
Sample 1GR14 from the Swift Formation has
an age spectrum similar to that of sample Eb,
with most zircon grains showing ages older than
1 Ga, but with the addition of a cluster of ages
in the range 234–329 Ma (Fig. 9). The youngest detrital zircons are in the 157–179 Ma range.
These Jurassic zircons were probably derived
from the magmatic arc to the west. Additionally, both Ellis Group samples show grains with
Late Proterozoic–early Paleozoic ages from
unknown sources in southwestern Canada and
northwestern United States.
Detrital zircons from two samples of the
Morrison Formation (1GRx, 1GRz) are dominated by Precambrian ages between ca. 1030
and 2900 Ma. A dozen grains in each sample
yielded ages in the range 400–480 Ma. Additionally, both samples contain several Carboniferous–Triassic grains. A few grains have Middle
Jurassic ages, with one grain in sample 1GRz
yielding a mid-Oxfordian age of 156.5 ± 3 Ma.
Finally, 20 grains from both samples have Late
Proterozoic to Early Cambrian ages.
Two samples—1GR100, 1SFSR1—from the
basal sandstones of the Kootenai Formation at
different locations produced abundant zircons
of Middle Jurassic to Lower Cretaceous age,
from 141 to 188 Ma for sample 1GR100, and
131–177 Ma for sample 1SFSR1. These samples contained a few grains with late Permian
(1GR100) and Early Triassic (1SFSR1) ages,
a few Late Proterozoic (1GR100) and Eocambrian (1SFSR1) grains, and a dominant population of zircons with ages older than 1 Ga.
Sample 1FG70 from the middle part of the
Kootenai Formation provided detrital zircons
with mostly young ages, ranging between 104
and 238 Ma. Additionally, two grains yielded
ages close to the Proterozoic-Cambrian boundary (540 and 558 Ma), and four grains provided
Early Proterozoic or older ages.
Zircons from a Blackleaf Formation sample
(1SR80) produced a dominant cluster of ages in
the Early Cretaceous to mid-Cenomanian, from
132 to 96.6 Ma. This sample also produced ages
in the ranges 150–176 Ma and 953–2926 Ma.
Only 19 zircons were retrieved from the
sample 2SR240 of the Two Medicine Formation. Most grains cluster in the 85–113 Ma
range, predating the known depositional age of
this unit by a few million years. The remaining
six grains have Precambrian ages that extend
Geological Society of America Bulletin, March/April 2011
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
EO.
(E)
PALEOC.
60
PALEOGENE
AGE
Qm
(Qm+F+Lt)
FORMATION
Y
F
(Qm+F+Lt)
Lt
(Qm+F+Lt)
Ls
(Ls+Lv+Lm)
Lv
(Ls+Lv+Lm)
Lm
(Ls+Lv+Lm)
Wasatch
T
S
D Fort Union (P. Hills)
M Willow Creek
St. Mary River
100
120
CRETACEOUS
80
EARLY
LATE
Bearpaw/Horsethief
C Two
Medicine
Virgelle
Telegraph Creek
S
C
T
C
Marias
River
Shale
A
Blackleaf
A
Kootenai
B
H
V
140
T
K
O
C
B
B
A
Ellis Gr.
MIDDLE LATE
160
JURASSIC
B
Morrison
Swift
Rierdon
Sawtooth
0
0.2 0.4 0.6 0.8
1 0
0.2 0.4 0.6 0.8
1 0
0.2 0.4 0.6 0.8
1 0
0.2 0.4 0.6 0.8
1 0
0.2 0.4 0.6 0.8
1 0
0.2 0.4 0.6 0.8
1
Figure 7. Plot showing the stratigraphic distribution of ratios of main grain types. Definitions of grain types are given in Table 2.
from 1518 to 3186 Ma. Six zircons have ages
that fall in the typical age range of Belt Supergroup zircons.
Most grains from sample 1BB44, collected
from a fluvial channel of the St. Mary River
Formation, have ages from 66 to 98 Ma. The
youngest cluster of grains includes Maastrichtian ages (Fig. 9), providing a maximum depositional age for this poorly dated unit. Only seven
out of a total of 95 grains yielded Precambrian
ages, in the range 1307–2528 Ma.
Provenance Interpretations
Jurassic sandstones of the Ellis Group and
Morrison Formation plot in the recycled orogenic fields (Fig. 6), and their modal compositions suggest deposition in a foreland basin
setting (Dickinson and Suczek, 1979; Suttner
et al., 1981). Lithic fragments suggest derivation from deformed miogeoclinal strata and
volcanic sources. However, the key information
that proves a western, orogenic provenance is
found in the detrital zircon ages. Samples of the
Ellis and Morrison Formations contain abundant zircons with late Paleozoic and Triassic
ages derived from accreted elements in the eastern part of the Intermontane belt, mid-Paleozoic
ages with possible origin in Kootenay arc rocks,
and a mix of different miogeoclinal source units
(Gehrels and Ross, 1998), reflecting an early
fold-and-thrust belt involving Paleozoic strata
in the hinterland. Volcanogenic zircons of syndepositional age were retrieved in every Jurassic
sample. The presence of zircons derived from
the eastern part of the Intermontane belt indicates that terranes in the Intermontane belt were
structurally elevated and supplying sediments
to a basin located hundreds of kilometers to the
east (Fuentes et al., 2009). The presence of detrital zircons likely derived from the Quesnell
terrane in the Morrison Formation contrasts
with results from southern Canada, where no
evidence of Intermontane belt debris has been
found in coeval deposits. This suggests the existence of a drainage divide along the Omineca
belt that prevented Intermontane belt detritus
from reaching the Canadian sector of the retroarc foreland basin (Mack and Jerzykiewicz,
1989; Ross et al., 2005).
Grains with Late Proterozoic–Cambrian ages
were probably recycled from Jurassic San Rafael
Group eolianites in the Colorado Plateau area
(Dickinson and Gehrels, 2008) and transported
by axial fluvial systems that flowed northward
toward marine shorelines located near the international border (Demko et al., 2004; Turner
and Peterson, 2004). This view agrees with the
work of Suttner et al. (1981), who inferred that
some Morrison Formation detritus was derived
from a southern source, possibly in Colorado.
Most significantly, thousands of paleocurrent
measurements from Morrison sandstones indicate generally north-northeastward paleoflow
during the Late Jurassic (Suttner et al., 1981;
DeCelles and Burden, 1992; Currie, 1998).
Geological Society of America Bulletin, March/April 2011
521
Fuentes et al.
A
B
C
D
E
Figure 8. Photomicrographs of selected sandstones representing
major changes in clast composition with time, all under crossed
polarizers. See Table 2 for description of petrographic parameters.
(A) Sandstone of the Swift Formation (Ellis Group) with clasts
dominated by monocrystalline quartz, with minor lithic fragments
(mainly chert), and plagioclase. (B) Basal sandstone of the Kootenai Formation, showing a marked increase in the proportion of
chert, likely derived from the upper section of the miogeocline.
(C) Sandstone from the upper part of the Kootenai Formation, with abundant plagioclase and volcanic lithic fragments.
(D) Blackleaf Formation sample, registering the introduction of
relatively abundant low-grade metamorphic lithic fragments into
the basin. (E) Sandstone of the Two Medicine Formation, with a
mixed composition, indicating a variety of source terranes.
522
Geological Society of America Bulletin, March/April 2011
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
J-K
volcanism
Miogeocline SW
upper part/ U.S.A
Kootenay
terrane
Intermontane Belt
Miogeocline
(NA basement,
Grenville,
Appalachian)
Age probability
1BB44 (St. Mary River Fm.) - n = 95
2SR240 (Two Medicine Fm.) - n = 19
1SR80 (Blackleaf Fm.) - n = 95
1FG70 (Kootenai Fm.) - n = 99
1SFSR1 (Kootenai Fm.) - n = 97
1GR100 (Kootenai Fm.) - n = 95
Magmatic flux
3
(km /m.y. per km)
1GRZ (Morrison Fm.) - n = 87
70
1GRX (Morrison Fm.) - n = 91
50
1GR14 (Ellis Gr.-Swift Fm.) - n = 98
30
Eb (Ellis Gr.-Sawtooth Fm.) - n = 97
10
200
400
300
500
550
550
1500
1000
2000
2500
3000
3500
Age (Ma)
116
Morrison Formation - 1GRZ
Kootenai Formation - 1FG70
114
174
112
170
110
166
162
108
106
104
158
102
154
150
105
100
TuffZirc Age = 162.47 +9.84 -5.97 Ma
(96.9% conf, from coherent group of 6)
98
Blackleaf Formation - 1SR80
72
103
TuffZirc Age = 109.85 +0.70 -1.25 Ma
(93% conf, from coherent group of 6)
St. Mary River Formation - 1BB44
70
101
Age (Ma)
Figure 9. (A) Age probability
plots of U-Pb ages of detrital zircons. Shaded areas indicate main
origins of zircons (from Coney
and Evenchick, 1994; Roback and
Walker, 1995; Gehrels and Ross,
1998; Ross and Villeneuve, 2003;
Ross et al., 2005; Link et al., 2007).
Horizontal bars indicate depositional age of samples. Note change
in horizontal scale in plots at
550 Ma. The Coast Mountain
batholith magma flux curve from
Gehrels et al. (2009) is shown in the
lower left corner. (B) TuffZirc ages
of youngest detrital zircon population of selected samples. Box heights
are 1σ. NA—North America.
Age (Ma)
178
Age (Ma)
A
100
Age (Ma)
0
99
97
95
68
66
64
93
62
91
B
89
TuffZirc Age = 97.39 +1.49 -0.46 Ma
(93% conf, from coherent group of 8)
60
Geological Society of America Bulletin, March/April 2011
TuffZirc Age = 68.47 +0.77 -0.68 Ma
(97.9% conf, from coherent group of 10)
523
Fuentes et al.
Late Early Proterozoic (ca. 1650–1775 Ma) zircons in these samples could have been derived
from Neoproterozoic and early Paleozoic miogeoclinal rocks of Alberta and British Columbia
(Gehrels and Ross, 1998), or from YavapaiMazatzal basement rocks in southwest United
States (Dickinson and Gehrels, 2008).
The basal coarse-grained sandstones of the
Kootenai Formation are also compositionally mature, but they contain a much higher
proportion of chert. This abundance of chert
in basal Kootenai and correlative samples of
Montana and southern Canada has long been
interpreted to reflect derivation from Permian–
Pennsylvanian rocks to the west (Rapson, 1965;
Suttner et al., 1981; DeCelles, 1986; Schwartz
and DeCelles, 1988). The high ratio of sedimentary lithic to total lithic fragments (Figs. 6 and 7)
and the large proportion of detrital zircons ages
typical of miogeoclinal strata indicate a provenance dominated by thrust-imbricated Paleozoic rocks. Uncertainty exists regarding the
location of this early fold-and-thrust belt, but a
possibility is that the locus of deformation was
in the region occupied today by the Omineca
belt or along the western flank of the Purcell
anticlinorium, prior to the involvement of Belt
Supergroup rocks in the thrust belt. Middle
Jurassic to Lower Cretaceous zircons originated
in the magmatic arc to the west.
The upper part of the Kootenai Formation
contains significant amounts of plagioclase and
volcanic lithic grains, and seems to mark the
beginning of a change in provenance. The high
influx of volcanic material is also registered in
the detrital zircons, indicating dominant sources
in the magmatic arc and the Intermontane belt.
This change is also recorded along the Alberta
foreland basin with an abrupt appearance of
juvenile material documented by petrography,
zircon geochronology, and isotope and traceelement geochemistry (Potocki and Hutcheon,
1992; Ross et al., 2005).
Albian-Cenomanian samples of the Blackleaf Formation show an abrupt increase in
low-grade metamorphic lithic fragments (Figs.
6, 7, and 8), possibly reflecting displacement
on thrust systems involving Belt Supergroup
metasedimentary rocks in the hinterland and
progressive exhumation of the Omineca belt.
Depositional age zircons from the Blackleaf
Formation indicate coeval arc volcanism. Zircons with ages in the range 1639–1789 Ma were
likely derived from rocks in the miogeocline or
Belt Supergroup (Link et al., 2007). Pebble and
cobble conglomerates in the Vaughn Member
contain clasts of quartz, chert, quartzite, silicified carbonate, and igneous rocks that Mudge
(1972) interpreted as being derived from the
Belt Supergroup. Pre–mid-Cenomanian dis-
524
placement on thrusts involving Belt Supergroup
strata is well constrained near the Canada-U.S.
border, where 94–96 Ma plutons crosscut the St.
Mary and Hall Lake thrusts (Price and Sears,
2000). In northwestern Montana, the Moyie
fault deformed the margin of the 71 Ma Dry
Creek stock (Fillipone and Yin, 1994), but earlier episodes of slip along segments of this fault
cannot be ruled out (Harrison et al., 1986; Price
and Sears, 2000). Additionally, movement on
hinterland thrusts at the latitude of the Helena
salient, and transpressive slip along faults in
the Lewis and Clark line may have contributed
Belt Supergroup–derived sediments. Blackleaf
Formation sandstones also contain lithic grains
of siltstone and very fine-grained sandstone,
which may have been cannibalized from older
foreland basin deposits that were incorporated
into the eastward-propagating fold-and-thrust
belt. Rocks of the Intermontane terrane and
Omineca belt continued to provide sediment as
they were carried passively(?) eastward above
structurally lower thrust faults. The youngest
population of zircon grains, which extends up to
the mid-Cenomanian (Fig. 9), reflects syndepositional volcanism and agrees with published
stratigraphic ages (Cobban and Kennedy, 1989;
Dyman et al., 1996).
The late Cenomanian–mid-Santonian black
shales provided little information in terms of
detrital provenance, but active volcanism in the
magmatic arc to the west may be inferred from
the abundant tuffs and plagioclase-rich sandstone beds.
The Upper Cretaceous Telegraph Creek,
Virgelle, Two Medicine, and Bearpaw-Horsethief Formations have similar provenance from
thrust sheets involving Belt Supergroup and
miogeoclinal rocks, plus cannibalized older
foreland basin deposits. Possible major thrust
systems active at the time of deposition of these
units include the Moyie, Snowshoe, Libby,
Pinkham, and other associated thrusts. Highlands produced by slip along faults in the Lewis
and Clark system, thrust faults in the western
part of the Helena salient, and the Lombard
thrust (Lageson et al., 2001) also may have contributed sediment. Detrital zircons of the Two
Medicine Formation were derived from Cretaceous volcanic centers, with contributions from
miogeoclinal and Belt Supergroup strata.
Displacement along the LEDSH thrust system, the dominant structure in the frontal part
of the thrust belt, has been bracketed between
ca. 74 and 59 Ma (Hoffman et al., 1976; Sears,
2001; Osadetz et al., 2004). Major slip on the
LEDSH system commenced during deposition of the Bearpaw-Horsethief Formation. An
increase in low-grade metamorphic grains in
sandstones of the lower part of the St. Mary
River Formation suggests exhumation of Belt
Supergroup rocks along the LEDSH system,
and passive transport of rocks in the Omineca
belt to shallower levels. The cosmopolitan
provenance of the St. Mary River and Willow
Creek Formations could be related to either cannibalization of older foreland basin deposits or
an integrated drainage system. Detrital zircons
from the St. Mary River Formation indicate pre
and syndepositional volcanic sources, and input
from Paleozoic and Belt Supergroup strata in
deformed thrust sheets.
Most samples of the Fort Union, Porcupine
Hills, and Wasatch Formations show a relative increase in the proportion of quartz and a
trend from magmatic arc and mixed provenance
to quartzose recycled orogenic provenance
(Fig. 6). Fort Union deposits are considerably
sandier than underlying units, even at distal
locations (Fig. 4). The increase in the sandstone/
shale ratio may be related to deeper exhumation
of Belt Supergroup rocks in the LEDSH thrust
sheets. This effect can also explain increased
quartz content and low-grade metamorphic
clasts derived from Belt Supergroup quartzite and argillite. The increase in quartz content
could also be explained by a longer transport
distance, insofar as these units were sampled
at distal positions in the basin. The uppermost
Wasatch Formation sample has a strong volcanic imprint, as seen in frequent bentonite beds
in outcrops. Conglomerate clast counts from
this unit support a source terrane dominated by
Belt Supergroup and volcanic rocks.
In general, the detrital zircon probability
plot (Fig. 9) shows a large proportion of grains
derived from the magmatic arc, particularly in
rocks younger than the lower Kootenai Formation. Magmatic flux curves calculated along the
Coast Mountain batholith in southern Canada
(Gehrels et al., 2009) exhibit a marked flare-up
during the 120–78 Ma interval (Fig. 9), coeval
with deposition of the Upper Kootenai Formation through Lower Two Medicine Formation.
The correlation of the Jurassic arc flare-up is
not as striking, but still visible. The magmatic
lull in the Coast Mountains batholith during the
78–55 Ma period is not reflected in detrital zircons of the St. Mary River Formation, which are
still dominated by young grains. However, these
grains had probable origin in volcanism associated to the Idaho and Boulder batholiths, active
from Santonian to early Eocene, and Campanian
time, respectively (e.g., Lageson et al., 2001).
SUBSIDENCE HISTORY
Middle Jurassic–Danian strata in northwestern Montana were decompacted and backstripped according to the methods described
Geological Society of America Bulletin, March/April 2011
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
in Allen and Allen (2005) and Angevine et al.
(1990) using stratigraphic sections measured
during this study. Short-term eustatic variations
were not taken into account during the backstripping because of uncertainties in the existing
eustatic models (Allen and Allen, 2005). The
resulting decompacted and tectonic subsidence
curves (Fig. 10) update previous work (Cross,
1986; Gillespie and Heller, 1995) by incorporating new stratigraphic and geochronologic data.
The onset of regional subsidence is recorded
at ca. 171 Ma. Subsidence rates were relatively
slow in this part of the basin during deposition
of the Ellis Group and Morrison Formation.
Paleosols in the uppermost Morrison Formation
indicate a continental condensed section that
is transitional into the interval 151–127 Ma,
marking a regional unconformity between Morrison and Kootenai strata. In places, this unconformity represents even longer periods of time,
and basal Kootenai deposits accumulated in
paleovalleys incising Ellis Group strata. From
ca. 127 Ma onward, subsidence rates increased
rapidly, a trend that is typical of classic foredeep deposits (Angevine et al., 1990; DeCelles
and Giles, 1996).
Estimated subsidence for the youngest interval is conservative. A thickness of 800 m was
used for the 73–65 Ma interval, during deposition of the St. Mary River and Willow Creek
Age (Ma)
170
130
110
70
90
WC-SMR F
150
BF
TMF
1
1.5
VTF
MRS
Decompaction (km)
0.5
2
BF
KF
MF
EG
2.5
170
150
130
110
70
90
c
Te
1
ic
ton
1.5
al
Tot
Subsidence (km)
0.5
EG
MF
KF
BF
MRS
TMF
BF
WC-SMR F
2.5
VTF
2
Figure 10. Decompacted thicknesses, and total and tectonic subsidence curves for Bajocian–Maastrichtian deposits of northwestern Montana. Paleocene and younger deposits are not included due
to postdepositional erosional removal and unknown original thickness. EG—Ellis Group, MF—Morrison Formation, KF—Kootenai
Formation, BF—Blackleaf Formation, MRS—Marias River Shale,
VTF—Telegraph Creek and Virgelle Formations, TMF—Two Medicine Formation, BF—Bearpaw Shale and Horsethief Formation,
WC-SMR F—St. Mary River and Willow Creek Formations.
Formations. However, the total thickness of
these units is difficult to estimate in northwestern Montana because of incomplete sections and
a paucity of stratigraphic markers (Mudge et al.,
1982). This interval thickens to ~2 km toward
southern Alberta. Additional strata equivalent to
the Fort Union and Wasatch Formations were
probably deposited in the frontal thrust belt and
proximal foredeep, but subsequent erosion has
removed all traces of these deposits. The thickness of exhumed and removed Cenozoic synorogenic strata in Alberta has been estimated in
the order of 2–4 km (Beaumont, 1981; Hardebol
et al., 2009).
The initial episode of slow subsidence, from
ca. 171 to ca. 151 Ma, and the ensuing latest
Jurassic–Neocomian period of erosion or nondeposition were also depicted in subsidence
curves published by Cross (1986) and Gillespie
and Heller (1995). These authors, however,
considered the onset of deposition in a foreland basin setting to coincide with the marked
increase in subsidence rate at ca. 110–100 Ma.
Our data, however, indicate westerly provenance starting as early as Middle Jurassic. An
alternative view is that the sigmoidal shape of
the subsidence curve can be interpreted as the
result of progressive stacking of foreland basin
depozones in front of a migrating thrust-belt
load (DeCelles and Giles, 1996). The period
171–151 Ma, during deposition of the Ellis
Group and Morrison Formation, may represent
initial, moderate subsidence in distal regions
of the foreland basin system, possibly inboard
of the flexural forebulge. The paleosols at the
top of the Morrison Formation and the unconformity below the Kootenai Formation would
represent condensed deposition and erosion
associated with passage of the forebulge. Finally, post–127 Ma deposits represent the foredeep depozone. The principal argument for a
later Cretaceous onset of foreland basin development in this region is based on the assumption that foreland basin sediments are confined
to a relatively thick moat of flexural subsidence
directly adjacent to the thrust-belt load (Gillespie and Heller, 1995). Ellis and Morrison
Formation deposits do not thicken westward as
expected for foredeep deposits. However, sediment derived from the thrust belt could have
been deposited in distal regions on top of the
forebulge as well as in the back-bulge depozone
(e.g., Horton and DeCelles, 1997; Yu and Chou,
2001; Roddaz et al., 2005; Chase et al., 2009).
The biggest potential problem with our interpretation is the absence of a Middle to Late
Jurassic foredeep. However, palinspastic restoration of the major thrusts east of the Kootenai
arc at the latitude of northwestern Montana
(Price and Sears, 2000) provides at least 400–
Geological Society of America Bulletin, March/April 2011
525
Fuentes et al.
450 km of space for accumulation of Middle to
Late Jurassic foredeep and forebulge deposits.
A similar situation has been described along the
Utah-Idaho segment of the Late Jurassic Cordillera (Royse, 1993; DeCelles, 2004).
BASIN EVOLUTION AND
TECTONIC IMPLICATIONS
An ~120 m.y. coherent story of Cordilleran
foreland basin evolution spans from Middle
Jurassic to Eocene time in western Montana
(Figs. 11 and 12). The North American continent started converging with offshore subduction zones at ca. 185 Ma (e.g., Monger and
Price, 2002; Evenchick et al., 2007). The Intermontane terranes accreted to North America
during the Middle Jurassic (e.g., Monger et al.,
1982; Coney and Evenchick, 1994; Murphy
et al., 1995; Dickinson, 2004; Colpron et al.,
2007; Dorsey and LaMaskin, 2007; Ricketts,
2008), and the Insular superterrane began to accrete soon after (e.g., Dickinson, 2004; Colpron
et al., 2007; Ricketts, 2008) (Fig. 12). Regional
subsidence in the foreland commenced during
the Bajocian (ca. 170 Ma), when marine deposits of the Ellis Group began to accumulate in
the distal retroarc region. Although provenance
information from the Sawtooth Formation is
not conclusive, it suggests derivation from deformed strata of the distal miogeoclinal wedge
and the Kootenay terrane. The Omineca belt in
southern Canada contains compressive structures that were already active by the beginning of the Middle Jurassic (Evenchick et al.,
2007, and references therein). Evidence for a
western source of sediments as early as Bajocian time also has been inferred for southern
Canada (Stronach, 1984). Detrital zircon ages
from Oxfordian–Kimmeridgian sandstones of
the Swift and Morrison Formations provide
unequivocal proof that sediments were derived
from the eastern part of the Intermontane belt.
The slow subsidence rates (Fig. 10) and the regionally tabular geometry of these units suggest
deposition in a back-bulge depozone.
Ward and Sears (2007) also proposed that a
foreland basin was active in this region as early
as Bajocian time. However, they attributed the
basal Jurassic unconformity to forebulge development, which implies that overlying Middle
and Upper Jurassic strata would have accumulated in a foredeep depozone. This interpretation
is inconsistent with the subsidence history and
tabular geometry of these units, and it provides
no explanation for the >20-m.y.-duration disconformity at the top of the Morrison Formation.
This unconformity stretches into southern Utah,
indicating that it had a regional geodynamic,
rather than local, origin.
526
The basin geometry and paleogeography of
Jurassic deposits in Montana were complicated
by the presence of preexisting basement highs
of the Sweetgrass and South Arches and the
“Belt Island” complex (Cobban, 1945; Carlson,
1968; Suttner, 1969; Suttner et al., 1981; Peterson, 1981; Parcell and Williams, 2005) (Fig. 11).
A similar situation was shown by Demko et al.
(2004) for the earliest stages of deposition of the
Morrison Formation in the central part of the U.S.
Cordilleran foreland basin, where highs of the
Ancestral Rockies and Front Range partitioned
the back-bulge depozone. A further complication in Montana was subsidence associated with
the Williston Basin, which interfered with distal
back-bulge subsidence. Additional subsidence
during the Middle and Late Jurassic could have
been driven by dynamic coupling with the upper
mantle above low-angle subducting oceanic
lithosphere. This mechanism has been proposed
to explain accumulations of Jurassic strata beyond the wavelength of the flexural foredeep in
the central part of the U.S. Cordilleran foreland
basin (Currie, 1998), and again during the Late
Cretaceous (Cross, 1986; Mitrovica et al., 1989;
Liu and Nummedal, 2004).
The coeval Jurassic foredeep depozone that
should have existed to the west was removed by
exhumation and erosion in northwestern Montana, once the major structures involving Belt
Supergroup rocks in the hinterland became active. Virtually no Mesozoic strata are preserved
in the fold-and-thrust belt west of the LEDSH
thrust system in Montana. In southern Canada,
Jurassic foredeep deposits are preserved along
major synclines. A good example is the Fernie
“basin,” where a thick section of Jurassic–
Cretaceous strata is exposed in a major synform
in the hanging wall of the Lewis thrust.
A possible explanation for the paleosols in
the Morrison Formation and the major unconformity that separates the Jurassic from the
Lower Cretaceous strata is decreased accommodation associated with eastward migration
of the flexural forebulge. Forebulges migrate
at rates equal to the sum of thrust belt propagation plus shortening (DeCelles and DeCelles,
2001), and deposition along them is characterized by condensed sections when sufficient
sediment is available to bury the forebulge, or
net erosion occurs in underfilled settings. The
temporal magnitude of this stratigraphic hiatus
in northwestern Montana is ~24 m.y., a typical
value in many foreland basins worldwide. This
unconformity has been recognized throughout the U.S. Cordilleran foreland basin from
southwestern Montana to southern Utah and
is partly attributed to migration of the flexural
wave (DeCelles, 2004). Currie (1998) noted
that the unconformity extends far eastward of
the most eastward point of forebulge migration,
however, and suggested that it resulted from the
cessation of dynamic subsidence, enhanced by
migration of the forebulge. Other authors have
suggested that this unconformity indicates a relaxation of orogenic stresses (e.g., Poulton et al.,
Figure 11. Simplified maps showing a summarized evolution of the foreland basin system in
northwest Montana, based on a number of sources discussed in the text and data from this
paper. (A) Late Jurassic, showing the potential configuration during deposition of the Morrison Formation. The initial foreland basin was established earlier during the Bajocian,
with deposition of the Ellis Group. Area enclosed by dashed line in southeast corner of map
shows approximate position of the Belt Island during the Middle Jurassic (from Parcell
and Williams, 2005). The position of the forebulge is conjectural. Structure of the Kootenay
and Quesnell terranes is not interpreted. (B) Aptian–early Albian, during deposition of
the Kootenai Formation. (C) Albian–Santonian, during deposition of the Colorado Group.
Map represents dominant environments during this time; coastal positions varied during
this time. (D) Campanian–early Eocene, during deposition of the Two Medicine–Wasatch
interval. Map does not display the late Campanian marine deposits of the Bearpaw Shale
and Horsethief Sandstone. Major thrusts show possible location before pre-Cenozoic extension. LEDSH thrust system shows its potential pre-erosion position. Sawtooth Range
is represented by a single thrust in the map, but includes a complexly deformed imbricated system. Forebulge represents its location during the mid-Campanian, although it
actively migrated during this time (Catuneanu, 2004). In all figures, the locations of the
early Eocene thrust belt front and intraforeland highs are shown as a spatial reference.
Location of thrusts in dotted lines is conjectural; dashed lines indicate incipient movement. Only prevalent sedimentary environments are shown. Thrusts key: SM—St. Mary,
HL—Hall Lake, M—Moyie, HC—Hawley Creek, CA—Cabin, S—Snowshoe, L—Libby,
P—Pinkham, WW—Wigwam-Whitefish, LEDSH—Lewis-Eldorado-Steinbach-Hoadley,
Lo-E—Lombard-Eldorado, SR—Sawtooth Range imbricate. Lewis and Clark strike-slip
system is simplified as a single line.
Geological Society of America Bulletin, March/April 2011
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
116°W
110°W
Absent Jurassic deposits
Libby
ed
ge
oc
lin
e
Kootenay
and Quesnell
WA
Additional
dynamic
subsidence?
Helena
MT
46°N
ID
100 km
A
South
arch
??
ep
de
re
Fo
io
Late Oxfordian-Kimmeridgian
116°W
110°W
CANADA
U.S.A.
nB
sto
in
as
lge
Fo
Helena
Forebulge
ep
de
re
ne
cli
eo
M
South
arch
bu
Spokane
iog
d m
me
HL
Kevin
Sunburst
dome
ck
(controlled
by preexisting
structures)
Ba
for
De
B
Forebulge
SM
49°N
illi
Libby
W
Sweetgrass
arch
Kootenay
and Quesnell
49°N
sin
Ba
lge
bu
re
Fo
orm
D ef
m
CANADA
U.S.A.
Sweetgrass arch
Kevin
Broken
back bulge Sunburst
dome
n
sto
illi
W
?
Conjectural
frontal part
of early
fold-thrust
belt
Spokane
46°N
HC
100 km
Aptian-Early Albian
116°W
Helena
d Clark L
Albian-Santonian
ine
Late Albian-early Cenomanian
continental deposits of the Vaughn
Member not displayed on map.
CA
100 km
C
116°W
S
lge
ep
de
re
P
SR
Fo
L
SH
H
Lewis an
Coastal deposits
100 km
Marine deposits
??
d Clark L
ine
Lo-E
Helena
D
49°N
ebu
M
Lacustrine deposits
Sediment dispersal
direction
CANADA
U.S.A.
WW
LED
Fluvial and alluvial
deposits
Estuarine deposits
46°N
110°W
For
KEY
sin
Ba
Lewis an
lge
Hinterland
not shown
bu
ep
L
n
sto
illi
W
e
de
Spokane
ck
ulg
re
S
49°N
Ba
reb
Fo
M
CANADA
U.S.A.
Fo
Libby
SM
HL
110°W
Idaho-Boulder batholiths
Geological Society of America Bulletin, March/April 2011
Campanian-Early Eocene
Marine deposits of the Bearpaw
Sea are not displayed in map.
46°N
527
Geological Society of America Bulletin, March/April 2011
180
160
140
120
100
80
60
O
C
B
B
A
K
T
B
V
H
B
A
A
C
S
C
T
C
T
S
D
M
Morrison
Swift
Rierdon
Sawtooth
Kootenai
Blackleaf
Marias
River
Shale
Virgelle
Telegraph Creek
Two Medicine
Bearpaw/Horsethief
Willow Creek
St. Mary River
Fort Union (P. Hills)
Multiple deformation events
1
P
17
?
Hall Lake/St. Mary thrusts
X, P
2,3,4,
17
Moyie thrust
X
5
Snowshoe thrust
?
X
6
?
?
LPWW T.S.
X, T, P
7,8,9,
10,11,
12,13,
14,17
?
?
X, P
7,13,
15,17
1) Evenchick et al., 2007 and
references therein
2) Archibald et al., 1983, 1984
3) Höy and van der Heyden,1988
4) Price and Sears, 2000
5) Harrison et al., 1986
6) Fillipone and Yin, 1994
7) Hoffman et al., 1976
8) Schmidt, 1978
9) Whipple et al., 1987
10) Sears, 2001
11) van der Pluijm et al., 2001, 2006
12) Osadetz et al., 2004
13) Harlan et al., 2005
14) Hardebol et al., 2009
15) Schmidt, 1972
16) Wallace et al., 1990
17) This work
P
4,17
?
X
6
?
LEDSH
T.S.
Intermontane/Omineca belts
Intermontane belt
Insular superterrane
?
16
E
Paleogene
subsidence
not
calculated
TECTONIC
SUBSIDENCE
Figure 12. Stratigraphic chart, major deformation events in the fold-and-thrust belt, and tectonic subsidence of the foreland basin system
at the latitude of northern Montana. Sources of data for timing of thrust movements are numbered and given in diagram. Timing of
terrane accretion and major episodes of metamorphism and exhumation in hinterland regions is discussed in the text. SLPWW T.S.—
Snowshoe-Libby-Pinkham-Whitefish-Wigwam thrusts system; LEDSH T.S.—Lewis-Eldorado-Steinbach-Hoadley thrust system; S.R.—
Sawtooth Range. Time constraints of thrust systems: X—crosscutting relationships; P—provenance data; T—thermochronology (apatite
fission tracks).
?
Wasatch
S.R./
Foothills
0.5
PALEOGENE
AGES OF THRUSTING
1 km
Y
W
Main movement
along Lewis and
Clark Line
PALEOC.
LATE
EARLY
MIDDLE LATE
ACCRETION
Age (Ma)
EO.
(E)
CRETACEOUS
JURASSIC
Montana
Colorado
Ellis
Deformed miogeocline in hinterland
Forebulge advance
70
90
110
130
150
528
170
AGE LITHOLOGY GP. FORMATION TERRANE
Fuentes et al.
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
1994) and/or isostatic rebound (McMechan and
Thompson, 1993). These last two explanations,
however, are inconsistent with the abundant evidence for ongoing crustal shortening and thickening in the Cordilleran hinterland (DeCelles,
2004). An episode of global sea-level fall peaking during the Valanginian may have contributed to development of this unconformity. In
our view, a combination of forebulge migration,
eustatic sea-level fall, and possibly decreased
dynamic subsidence provides a reasonable explanation for the regional and temporal extent of
the basal Cretaceous unconformity.
The model of early foreland basin development presented here is similar to models proposed further south in the United States, in
which the Morrison Formation is interpreted
as back-bulge deposits, and the unconformity
at its top is attributed to the migration of the
forebulge with a component of dynamic uplift
owing to changes in the angle of the subducting plate (e.g., Currie, 1998; DeCelles, 2004).
In Montana, however, deposition in a distal
foreland setting seems to have started during
the Middle Jurassic, as discussed already. The
aerially extensive distribution of Jurassic distal foreland basin strata in the western interior
United States terminates near the international
border, and in Canada, these deposits are restricted to a relatively narrow strip running
approximately parallel to the Alberta–British
Columbia border (e.g., Carlson, 1968; Miall
et al., 2008, and references therein).
The late Barremian(?)–early Aptian Kootenai
Formation is the first unit in the foreland that
consistently thickens westward. Moreover, the
subsidence curve begins to exhibit a convexupward pattern characteristic of foredeeps at
this time. By Albian time, the fold-and-thrust
belt had propagated to the east and was involving rocks of the Belt Supergroup, as indicated
by sandstone provenance data, detrital zircons
in the Blackleaf Formation with ages typical
of Belt Supergroup strata, and by crosscutting
relationships in thrust sheets in the hinterland.
Coeval with deformation in the hinterland of
the Purcell anticlinorium, an episode of longlasting marine transgression was being recorded
in ~400 m of offshore marine deposits of the
Marias River Shale. This episode of sedimentation was widespread along the Western Interior
Basin (Miall et al., 2008), and contradictory
opinions exist regarding its cause. Some authors
have suggested that these shales reflect a period
of orogenic quiescence (Price and Mountjoy,
1970; Porter et al., 1982; Tankard, 1986). In
particular, Tankard (1986) suggested that shale
deposition was due to postloading viscous
lithospheric relaxation, which would produce
a narrower, but deeper, basin. However, the
shale basin was not confined to a narrow flexural foredeep (e.g., McMechan and Thompson,
1993), and, in any case, no evidence from the
Cordilleran hinterland exists for a long-duration
episode of “orogenic quiescence” (Evenchick
et al., 2007). The major Cenomanian–Turonian
transgression (McDonough and Cross, 1991),
coupled with continuing creation of accommodation by flexural subsidence, seems to have
controlled the shale deposition event. Additional
dynamic subsidence may have increased basin
accommodation on a regional level (Liu and
Nummedal, 2004).
The late Santonian–early Campanian interval
was marked by a strongly regressive system,
with rapid, relatively coarse clastic sedimentation (Figs. 4 and 10). Thrust systems possibly
active during this period include the Moyie,
Snowshoe, Libby, Pinkham, Whitefish, and
Wigwam. The geometry and shortening on
these structures are poorly known owing to
postorogenic normal faulting and a widespread
cover of Cenozoic deposits. Concurrently, during the Cenomanian–Campanian interval, the
Lewis and Clark strike-slip system was active,
and foreland basin deposits north and south
of it show differences in thickness and facies
(Wallace et al., 1990).
The marine shales of the Bearpaw Formation represent the last marine inundation of the
foreland basin. The Bearpaw Sea was confined
mainly to Montana and the Canadian part of the
basin (Miall et al., 2008). Bearpaw-Horsethief
deposition was roughly coincident with the onset of major slip along the LEDSH thrust system
at ca. 75 Ma (Sears, 2001; Osadetz et al., 2004).
By early Campanian time, eastward propagation of the thrust belt had driven migration
of the forebulge, and the foredeep-forebulge
hinge line was located ~150 km east of the current thrust front (Catuneanu et al., 2000; Miall
et al., 2008). By late Campanian, and coeval
with the onset of major slip along the Lewis
thrust, the forebulge had migrated 200–250 km
farther eastward and was located in eastern
Montana. However, the position of the forebulge in this region is difficult to map during
much of the Cretaceous owing to the magnitude
of dynamic loading (Miall et al., 2008).
High rates of sedimentation in proximity to
the advancing thrust front are evident in the
St. Mary River and Willow Creek Formations
(Fig. 10). Paradoxically, these units lack coarsegrained, conglomeratic facies. The absence
of coarse-grained facies within 20 km of the
present erosional trace of the Lewis thrust lead
McMannis (1965) to conclude that the main
phase of shortening on the Lewis thrust postdated deposition of the Willow Creek Formation. Alternatively, the lack of coarse facies may
be explained by the following: (1) the bulk of
material that was initially eroded in the hanging wall of the Lewis thrust was fine-grained
or unconsolidated foreland basin deposits, not
prone to generating gravel; (2) coarse alluvial
fans were restricted to the most proximal areas
in front of the Lewis thrust, where they were
vulnerable to erosion during continuing displacement on the Lewis and underlying thrusts
in the Sawtooth Range duplex; and (3) coarsegrained sediment accumulation in the foreland
basin was highly localized along the front of the
Cordilleran thrust belt and was partly controlled
by local across-strike structural discontinuities
(Lawton et al., 1994).
The final, Paleocene–early Ypresian record of
thrust belt shortening and synorogenic sedimentation in the foreland is preserved in erosional
remnants of the Fort Union and Wasatch Formations in the distal foreland. Regional extensional
faulting in the fold-and-thrust belt commenced
during middle Eocene time (Constenius, 1996)
and coincided with low-angle detachment faulting, growth of metamorphic core complexes,
and regional-scale magmatism. Coarse-grained,
proximal foredeep deposits were removed by
uplift and erosion during postglacial isostatic
rebound, erosion-driven isostatic rebound of
the thrust belt (Sears, 2001), or exhumation
in response to generalized extension since the
middle Eocene.
CONCLUSIONS
(1) Regional subsidence in the northwestern
Montana foreland commenced during the Bajocian (ca. 170 Ma), when marine deposits of the
Ellis Group began to accumulate in the distal
retroarc region. The onset of subsidence and
lithic-rich clastic sediment accumulation indicate
development of the Cordilleran orogenic belt to
the west. Detrital zircon ages from Oxfordian–
Kimmeridgian sandstones of the Swift and Morrison Formations provide unequivocal proof that
sediments were derived from the eastern part of
the Intermontane belt. The slow subsidence rates
and regionally tabular geometry of these units
suggest deposition in a back-bulge depozone.
(2) Jurassic deposits are capped by a zone
of paleosols in the upper Morrison Formation,
and/or are truncated by a regional unconformity
that probably represents most of Tithonian–
Neocomian time. This unconformity possibly
resulted from the combined effects of eastward
forebulge migration, eustatic sea-level fall,
and decreased dynamic subsidence owing to
changes in the angle of the subducting plate.
(3) The preserved Barremian(?)–early Eocene
succession was deposited in a foredeep depozone and contains a record of the development
Geological Society of America Bulletin, March/April 2011
529
Fuentes et al.
and propagation of the retroarc fold-andthrust belt. The late Barremian(?)–early Aptian
Kootenai Formation is the first unit in the foreland that consistently thickens westward and
exhibits an upward-convex subsidence curve
characteristic of foredeep deposits. Provenance
data from the Kootenai indicate contributions
from imbricated miogeoclinal strata in the
hinterland and volcanic sources in the magmatic arc and Intermontane belt. By Albian
time, the fold-and-thrust belt had incorporated
Proterozoic rocks of the Belt Supergroup. The
mid-Cenomanian–early Campanian interval
was characterized by marine deposition and relatively slow forward propagation of the thrust
belt. During the Campanian, the LEDSH thrust
system commenced its episode of major slip,
which is recorded by rapid sediment accumulation rates in the foreland basin. As a result of
the structural elevation gain of this megathrust
sheet, earlier foreland basin deposits were cannibalized and redeposited to the east. The stratigraphic record of terminal thrust belt shortening
during the Paleocene–early Ypresian time is
preserved in distal erosional remnants of the
Fort Union and Wasatch Formations. Equivalent, more proximal deposits were erosionally
removed, and no vestige of a wedge-top depozone exists today in northwestern Montana.
ACKNOWLEDGMENTS
Funding for this work was provided by ExxonMobil, with additional grants from ChevronTexaco,
The American Association of Petroleum Geologists,
and the Geological Society of America. A Fulbright
scholarship was granted to Fuentes. Charles Park,
David Gingrich, Ylenia Almar, Erin Brenneman, and
Nicole Russell assisted during field work. Gerald
Waanders analyzed the palynology samples. Bill Dickinson, Ted Doughty, and Jerry Kendall provided valuable comments. Associate Editor John Wakabayashi,
and Andrew Miall and Brian Currie provided constructive reviews that helped us to improve the paper.
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