Jurassic onset of foreland basin deposition in
northwestern Montana, USA: Implications for along-strike
synchroneity of Cordilleran orogenic activity
F. Fuentes*, P.G. DeCelles, G.E. Gehrels
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
ABSTRACT
Stratigraphic, provenance, and subsidence analyses suggest that by the Middle to Late
Jurassic a foreland basin system was active in northwestern Montana (United States). U-Pb
ages of detrital zircons and detrital modes of sandstones indicate provenance from accreted
terranes and deformed miogeoclinal rocks to the west. Subsidence commenced ca. 170 Ma and
followed a sigmoidal pattern characteristic of foreland basin systems. Thin Jurassic deposits
of the Ellis Group and Morrison Formation accumulated in a backbulge depozone. A regional
unconformity and/or paleosol zone separates the Morrison from Early Cretaceous foredeep
deposits of the Kootenai Formation. The model presented here is consistent with regional
deformation events registered in hinterland regions, and challenges previous interpretations
of a strongly diachronous onset of Cordilleran foreland basin deposition from northwestern
Montana to southern Canada.
INTRODUCTION
One of the most controversial aspects of the Cordilleran thrust
belt and foreland basin system is also one of the most fundamental, i.e.,
when did this system initially develop? Estimates for the onset of foreland basin accumulation in the western interior of the United States span
~75 m.y. from Early Jurassic to Aptian time (Heller et al., 1986; Bjerrum
and Dorsey, 1995; DeCelles and Currie, 1996; Allen et al., 2000, Dorsey
and LaMaskin, 2007), whereas most workers agree that a foreland basin
was established in southern Canada by the Middle to Late Jurassic (Cant
and Stockmal, 1989; Fermor and Moffat, 1993; McMechan and Thompson,
1993; Poulton et al., 1994; Ross et al., 2005). Gillespie and Heller (1995)
inferred that foreland basin development in the western United States was
~40 m.y. later than in Canada, giving rise to the notion of strongly diachronous orogeny. In this paper we present stratigraphic, geochronologic, and
provenance data from northwestern Montana that support the hypothesis
that Middle Jurassic deposition took place in a retroarc foreland basin
system. We conclude that initial foreland basin development was nearly
synchronous along the United States and southern Canadian portions of
the Cordilleran thrust belt.
REGIONAL SETTING AND JURASSIC–
EARLY CRETACEOUS STRATIGRAPHY
Three major tectonic elements compose the Cordilleran orogenic
system at the latitude of northwestern Montana and southern Canada
(Fig. 1): a western region of accreted terranes (e.g., Colpron et al., 2007;
Giorgis et al., 2008); a fold-and-thrust belt involving Paleozoic and Mesozoic sedimentary rocks in its frontal part, and mainly Proterozoic strata
in the hinterland; and a Mesozoic–early Paleogene foreland basin system
that has been partially incorporated into the fold-and-thrust belt. In northwestern Montana an unconformity representing ~150 m.y. separates the
youngest preorogenic strata of the Mississippian Madison Group from
the oldest strata that possibly could be linked to Cordilleran orogenic evolution, the Middle Jurassic Ellis Group (Fig. 2).
The Ellis Group consists of ~100–200 m of Bajocian–Oxfordian
clastic and carbonate marine deposits of the Sawtooth, Rierdon, and Swift
*E-mail: ffuentes@email.arizona.edu.
Formations (Mudge, 1972). The Ellis Group correlates with the upper part
of the Fernie Formation of southwest Alberta and southeast British Columbia (Poulton et al., 1994). The overlying Morrison Formation consists of
~60–80 m of fine-grained estuarine to nonmarine strata, its upper part
usually overprinted by strong pedogenesis. Palynology of three Morrison
samples yielded Oxfordian–Kimmeridgian ages (GSA Data Repository
Table DR11), suggesting temporal continuity between the Ellis Group and
the Morrison Formation.
A regional unconformity or zone of extreme stratigraphic condensation separates the Morrison from the overlying Lower Cretaceous
Kootenai Formation. In places this unconformity cuts into the lower part
of the Ellis Group along incised paleovalleys (Dolson and Piombino,
1994). The Kootenai consists of ~200–400 m of fluvial and minor lacustrine deposits. This and correlative units have been sparsely dated as
Barremian(?)–Aptian (DeCelles, 2004). New palynology and detrital
zircon data extend this age range into the lower Albian (Tables DR1
and DR2). The Lower Cretaceous, as well as the overlying 2500 m
succession of Albian–early Eocene age, was deposited in the foredeep
depozone of the Cordilleran foreland basin system (Suttner et al., 1981;
Gillespie and Heller, 1995; DeCelles, 2004).
DETRITAL ZIRCON U-Pb AGES
A total of 476 U-Pb ages of detrital zircon grains from five samples
of Jurassic and Early Cretaceous sandstones was obtained by laserablation–multicollector inductively coupled plasma–mass spectrometry
at the University of Arizona LaserChron Center (analytical procedures in
Gehrels et al., 2008). Analyses that yielded isotopic data with acceptable
discordance, precision, and in-run fractionation are shown in Table DR2.
The ages are plotted on relative probability diagrams (Fig. 3); peaks on
these diagrams are considered robust if defined by three or more analyses.
In addition to provenance information, the youngest cluster of ages for
each sample gives a maximum depositional age.
1
GSA Data Repository item 2009091, Table DR1 (palynology table), Table
DR2 (geochronologic analyses), and Table DR3 (detrital modes of sandstones),
is available online at www.geosociety.org/pubs/ft2009.htm, or on request from
editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder,
CO 80301, USA.
© 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY,
April
2009
Geology,
April
2009;
v. 37; no. 4; p. 379–382; doi: 10.1130/G25557A.1; 4 figures; Data Repository item 2009091.
379
115°W
Banff
120°W
L&C li
ne
Spokane
Idaho
batholith
Columbia River basalts
50°N
CANADA
USA
Helena
46°N
100 Km
Mesozoic to lower Tertiary
foreland basin deposits
Paleozoic and Mesozoic
deformed sedimentary rocks
Middle and late Proterozoic
Belt and Windermere
Early Proterozoic preBelt metamorphic rocks
180
T
K
O
C
B
B
A
T
MORRISON
FM.
SW.
R.
S.
ELLIS GR.
LATE
JURASSIC
160
B
Cache Creek
Kootenai and
Quesnell
Tectonic
subsidence
St. Mary, Hall Lake,
Moyie thrusts
80
100
120
Thrusting
Sweetgrass arch uplift
V
140
Stikinia
Regional
deformation
events
Major exhumation
B
H
Insular superterrane
A
Intermontane belt
Regional metamorphism
KOOTENAI FM.
Foredeep deposits
up to Early Eocene
A
EARLY
120
T
C
CRETACEOUS
100
Insular Superterrane
Intermontane
terranes
Cretaceous intrusive rocks
Terrane
accretion
Paleogene forearc and
subduction complex
Cretaceous subduction
complex
Accretioned elements
Tertiary volcanic rocks
Simplified
stratigraphy
MIDDLE
Sample Eb from the Sawtooth Formation produced age clusters with
a miogeoclinal signature (Gehrels and Ross, 1998). Most detrital zircons
yielded ages older than ca. 1 Ga; a few grains provided ages in the range
419–467 Ma, typical of upper Paleozoic–Triassic strata in the thrust belt;
two grains have Late Proterozoic ages; and one zircon yielded a syndepositional age of ca. 171 Ma. The range of grains older than 1 Ga suggests
that the zircons of this sample were recycled from several intervals within
the miogeocline, and perhaps reflects an early fold-and-thrust belt involving Paleozoic strata in the hinterland region. However, because zircons in
the miogeocline were mainly recycled from North American basement
and Grenville sources, a potential eastern contribution cannot be ruled out.
The single Jurassic grain supports a western source.
Samples 1GR14 from the Swift Formation and 1GRZ from the
Morrison Formation provided zircons with a similar age spectrum. Most
grains are older than 1 Ga, a few provided Late Proterozoic ages, and
sample 1GRZ produced an additional cluster in the range 403–457 Ma.
However, the key aspect of these samples is that they contain clusters of
ages in the ranges 223–329 Ma and 156–180 Ma. The Mississippian–
Triassic range is characteristic of igneous rocks of the eastern part of
the Intermontane belt (Monger et al., 1982). The lack of other sources
of zircons with those ages in the area indicates that terranes in the Intermontane belt were structurally elevated and supplying sediments to a
basin located hundreds of kilometers to the east. Relatively abundant
Jurassic, arc-derived grains, further substantiate a western provenance
as early as Oxfordian time.
Sample 1GR100 from the basal sandstone of the Kootenai Formation
yielded abundant grains with a miogeoclinal signature, a few Permian and
Triassic zircons, and a relative increase in zircons derived from Middle
Jurassic–Lower Cretaceous volcanic centers, a pattern that is amplified in
sample 1FG70 from the middle part of the unit. Sample 1FG70 provided
abundant ages in the range 104–238 Ma, indicating predominant sources
in the magmatic arc and Intermontane belt.
Most samples contained grains of Late Proterozoic ages, with
unknown sources in the region. A possible scenario is transport along axial
fluvial systems from the south, in the Colorado Plateau region, where zircons of that age have been identified (Dickinson and Gehrels, 2008).
Cenozoic extensional
basin fill
Stratigraphy of fold-and-thrust
belt and forealnd basin
Acc
La
rete
d
p r ra m a n
terranes
ov i
dthrust
in c d e
belt
e
For
ela
n
sys d bas
tem in
Libby
Figure 1. A: General location map of Cordilleran orogenic system showing area
of B. B: Simplified tectonic map of the fold-and-thrust belt and foreland basin
of study area with accreted terranes to the west (terranes based on Dickinson,
2004; Colpron et al., 2007). Blue box indicates area of sample collection. Red
circle shows location of section used for subsidence curve in Figure 2.
380
n
hto
oc
all
lt
Be
B
n
ca
cis
ran
~F
500 Km
x
nd
rc a omple
rea
Fo tion c
duc
sub
a
Americ
A
North
ld -
Fo
49°N
Intermontane
Terranes
in
as
db
lan
elt
re
tb
Fo
us
thr
dan
dfol
(Quesnell+Kootenai)
Can
a
U.S da
.A.
N
tal
on
Fr
Insular
Superterrane
k)
ree
U.
metamorphic
core-complex
extension
(Stikinia)
N
.
A
S.
Granitic rocks emplacement
1
Alaska
C
che
(Ca
°W
41
140
?
160
0
0.5
1 Km
Figure 2. Chart showing local Middle Jurassic–Early Cretaceous
stratigraphy, main tectonic events in northwestern Montana and
southwestern Canada, and tectonic subsidence curve for northwestern Montana foreland basin system. Time scale is from Palmer and
Geissman (1999) and tectonic events are from sources discussed in
text. Location of subsidence curve is in Figure 1B; paleobathymetric
uncertainty is shown in gray.
MODAL SANDSTONE PETROGRAPHY
Framework modes of 12 medium-grained sandstones from the Ellis
Group and Morrison and Kootenai Formations were determined by pointcounting ~450 grains per slide according to the Gazzi-Dickinson method.
Raw data and recalculated norms are available in Table DR3. On QmFLt
(monocrystalline quartz–feldspar–total lithic) and QtFL diagrams (Fig. 4),
the samples plot in the recycled orogen fields of Dickinson and Suczek
(1979), indicating provenance from a suture or fold-and-thrust belt. The
GEOLOGY, April 2009
Normalized age probability
Jurassic–Cretaceous
n
volcanism
Intermontane belt
Miogeocline SW
(upper part) U.S.A
Miogeocline
(NA basement,
Grenville,
Appalachian)
1FG70 (Kootenai Fm.) – n = 99
1GR100 (Kootenai Fm.) – n = 95
1GRZ (Morrison Fm.) – n = 87
1GR14 (Swift Fm.) – n = 98
Eb (Sawtooth Fm.) – n = 97
0
100
200
300
Age (Ma)
400
500
550
550
1000
1500
3000
2500
2000
Age (Ma)
Figure 3. Age-probability plots of U-Pb ages of detrital zircons. Black bars indicate depositional age of samples. 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). Note change in horizontal scale in plots.
lithic fraction is dominated by chert, limestone, shale, and phyllite. These
data strongly implicate sediment sources in deformed (meta)sedimentary
rocks of the Cordilleran thrust belt.
SUBSIDENCE HISTORY
Middle Jurassic–Danian strata in northwestern Montana were
decompacted and backstripped according to the methods described in
Angevine et al. (1990) and Allen and Allen (2005). The resulting tectonic subsidence curve (Fig. 2) has a sigmoidal shape, as expected from
progressive stacking of foreland basin depozones in front of a migrating
orogenic load (DeCelles and Currie, 1996). The period 171–151 Ma,
during deposition of the Ellis Group and Morrison Formation, represents initial, moderate subsidence in the region inboard of the flexural
forebulge. The multistory paleosols in the uppermost Morrison Formation indicate a continental condensed section, which is transitional into
the interval 151–127 Ma, marking a regional unconformity between
Morrison and Kootenai strata. This unconformity has been recognized
throughout the Cordilleran foreland basin, and attributed to the migration
of a flexural forebulge (DeCelles, 2004). The Late Jurassic–Valanginian
eustatic fall (Haq et al., 1987) may have contributed to erosion and low
net sediment accumulation. From ca. 127 Ma onward, subsidence rates
increased rapidly, a trend that is typical of foredeep deposits.
INTERPRETATION AND CONCLUSIONS
Data presented here demonstrate that a foreland basin system became
active in northwest Montana as early as Bajocian time. Onset of subsidence
and lithic-rich clastic sediment accumulation ca. 170 Ma, after a hiatus
of ~150 m.y., indicates the development of a tectonically active orogenic
belt to the west. Detrital zircon ages from Oxfordian–Kimmeridgian sandstones of the Swift and Morrison Formations provide unequivocal proof
that sediment was derived from the eastern part of the Intermontane belt.
Ellis and Morrison sandstones suggest derivation from deformed miogeoclinal strata to the west.
Timing of terrane accretion and deformation in the hinterland is
consistent with Middle to Late Jurassic foreland basin initiation in northwestern Montana (Fig. 2). The Intermontane terranes accreted to North
America during the Middle Jurassic (e.g., Coney and Evenchick, 1994;
Dickinson, 2004; Colpron et al., 2007; Dorsey and LaMaskin, 2007),
and the Insular superterrane soon after. Regional metamorphism in the
Omineca belt occurred between ca. 180 and 162 Ma (Archibald et al.,
1983; Fermor and Moffat, 1993), with rapid exhumation at 165–150 Ma.
GEOLOGY, April 2009
Figure 4. Ternary diagrams showing modalframework grain compositions of sandstones.
Provenance fields after
Dick in son and Suczek
( 1 9 7 9 ) . R O — re c y c l e d
orogen; CB—continental
block; MA—magmatic arc.
Raw data are in Table DR3
(see footnote 1). Qm—
monocrystalline quartz;
Qt—total quartz; F—feldspar; L—nonquartzose
lithic grains; Lm, Lv, Ls—
metamorphic, volcanic,
and sedimentary lithic
grains.
Qm
Qt
RO
RO
CB
CB
MA
F
Lv
F
50% Lm
MA
Lt
L
KEY
Kootenai
Formation
Morrison
Formation
Ellis Group
Ls
Timing of thrusting is poorly constrained in hinterland regions, but is
pre–middle Cretaceous in the United States–Canada border region (Price
and Sears, 2000), and perhaps as old as late Middle Jurassic (McMechan and
Thompson, 1993).
The principal argument for a later Cretaceous onset of foreland basin development in this region is based on the assumption that
foreland basin sediment is confined to a relatively thick moat of flexural
subsidence directly adjacent to the thrust belt load (Gillespie and Heller,
1995). Ellis Group and Morrison deposits do not thicken westward as
expected for foredeep deposits. However, sediment derived from the
thrust belt can be deposited on top of the flexural forebulge as well as in
the backbulge depozone (e.g., Horton and DeCelles, 1997; Yu and Chou,
2001; Roddaz et al., 2005). We suggest that relatively tabular and thin
Ellis and Morrison deposits in northwestern Montana were deposited in a
backbulge depozone, and that the unconformity and/or condensation zone
that caps them was produced in part by migration of a forebulge through
the region. The model presented here is consistent with previous interpretations to the south that suggest the onset of foreland basin sedimentation
during the Jurassic, and supports synchronous development of a foreland
basin system from as far south as central Utah (Bjerrum and Dorsey, 1995;
DeCelles and Currie, 1996) to southern Canada. This is consistent with
coeval growth of the western Cordillera in response to rapid westward
movement of North America as heralded by Middle Jurassic seafloor
spreading in the North Atlantic (Coney and Evenchick, 1994).
381
ACKNOWLEDGMENTS
Funding was provided by ExxonMobil, ChevronTexaco, the Geological
Society of America, and the American Association of Petroleum Geologists. We
thank Ylenia Almar, Erin Brenneman, and Nicole Russell for assistance during field
work. Alan Carroll, Guy Plint, Paul Heller, and Tina Niemi provided constructive
reviews that helped us to improve the paper.
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Manuscript received 2 October 2008
Revised manuscript received 25 November 2008
Manuscript accepted 2 December 2008
Printed in USA
GEOLOGY, April 2009