Evolution of the Cordilleran Foreland Basin System in Northwestern Montana, U.S.A

Home , Bearpaw Formation, Blackleaf Formation, Ellis Group, Eucommiidites, Kootenai Formation, Lewis and Clark Range

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 episode of marine inundation and black shale 1989; Fermor and Moffat, 1992; Stockmal et al., deposition (Marias River Shale) occurred be- 1992; Beaumont et al., 1993; Plint et al., 1993; New lithostratigraphic and chronostrati- tween the Cenomanian and mid-Santonian, Ross et al., 2005; Miall et al., 2008; Yang and graphic, geochronologic, and sedimentary and was followed by a regressive succession Miall, 2009). This bimodal focus was mainly petrologic data illuminate the history of represented by the Upper Santonian–mid- driven by either the presence of anomalously development of the North American Cor- Campanian Telegraph Creek, Virgelle, and good surface exposures, as in the case of the dilleran foreland basin system and adjacent Two Medicine Formations. Provenance data western interior United States, or by hydro- thrust belt from Middle Jurassic through do not resolve the timing of individual thrust carbon exploration and a large subsurface data- Eocene time in northwestern Montana. The displacements during Cenomanian–early base, as in Canada (Miall et al., 2008). The oldest deposits in the foreland basin system Campanian time. The Upper Campanian ~300-km-long segment of the foreland basin consist of relatively thin, regionally tabu- Bearpaw Formation represents the last major lying within and east of the Cordilleran belt in lar deposits of the marine Ellis Group and marine inundation in the foreland basin . By northwestern Montana remains comparatively fl uvial-estuarine Morrison Formation, which latest Campanian time, a major epi sode of poorly understood in terms of its stratigraphy, accumulated during Bajocian to Kimmerid- slip on the Lewis thrust system had com- basin evolution, and relationship with the kine- gian time. U-Pb ages of detrital zircons and menced, as recorded in the foreland by the matics of the developing fold-and-thrust belt. sandstone modal petrographic data indicate Willow Creek and St. Mary River Forma- In this paper, we present new stratigraphic that by ca. 170 Ma, miogeoclinal strata were tions in the proximal foredeep depozone. The and provenance data that, coupled with previ- being deformed and eroded in hinterland re- fi nal stage in the evolution of the Cor dilleran ous work, establish a coherent model for the gions. Sandstones of the Swift and Morrison fold-and-thrust belt and foreland basin evolution of the foreland basin in the context Formations contain detrital zircons derived system is recorded by the Paleocene–early of an integrated orogenic system in this region from the Intermontane belt. The Jurassic Eocene Fort Union and Wasatch Formations, from Jurassic to Eocene time. Besides fi lling deposits probably accumulated in the distal, which were preserved in the distal foreland a regional gap, the results of this paper should back-bulge depozone of an early foreland region. Regional extensional faulting along help to establish links and comparisons between basin, as suggested by the slow rates of tec- the fold-and-thrust belt began during the what is known from previous work in Canada tonic subsidence and tabular geometry. A middle Eocene. The results presented here and in better known parts of the Cordilleran regional unconformity separates the Jurassic enable the establishment of links between foreland basin system in the United States. strata from late Barremian(?) foredeep de- previous geological work in Canada and the posits. This unconformity possibly resulted better known parts of the Cor dilleran fore- REGIONAL SETTING as a combined effect of forebulge migration, land basin in the United States. decreased dynamic subsidence, and eustatic The foreland basin of northwestern Montana sea-level fall. The late Barremian(?)–early INTRODUCTION occupies a central location along the ~3000 km Albian Kootenai Formation is the fi rst unit length of the Cordilleran fold-and-thrust belt that consistently thickens westward, as Foreland basin systems are the stratigraphic and foreland basin system (Fig. 1). The North would be expected in a foredeep depozone. recorders of processes occurring in contiguous American Cordillera developed from mid- The subsidence curve at this time begins to orogenic belts. The history of terrane accretions, Mesozoic to Eocene times (e.g., Burchfi el et al., show the convex-upward pattern character- fold-and-thrust belt development, magmatism, 1992; Coney and Evenchick, 1994; DeCelles, istic of foredeeps. By Albian time, the fold- and major exhumation events is registered in 2004; Dickinson, 2004), and was subsequently and-thrust belt had propagated to the east the stratigraphy of these basins. Previous efforts modifi ed by gravitational collapse and later, by and incorporated Proterozoic rocks of the to link the foreland basin fi ll with the tectonic mid- to late Cenozoic Basin and Range exten- Belt Supergroup, as indicated by sandstone development of the North American Cordillera sion (Constenius, 1996). The initial evolution compositions, detrital zircon ages in the have been concentrated in two large regions: of the Cordillera was marked by subduction of Blackleaf Formation, and by crosscutting Utah, Colorado, and Wyoming in the United oceanic plates of the Panthalassa Ocean, and ac- relationships in thrust sheets involving Belt States (e.g., Royse et al., 1975; Jordan, 1981; cretion of parautochthonous terranes, fringing Supergroup rocks in the thrust belt. A major Lamerson, 1982; Heller et al., 1986; DeCelles, arcs, and exotic terranes (e.g., Coney et al., 1980; 1994; Currie, 2002), and Alberta and British Monger et al., 1982; Coney and Evenchick , †E-mail: [email protected] Columbia in Canada (e.g., Cant and Stockmal, 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 ﬁ gures; 2 tables; Data Repository item 2011002.

141°W Alaska 120°W 115°W (Cache N Creek) Banff U.S.A. (Stikinia) metamorphic Accreted core-complex N terranes Insular extension Frontal and superterrane Foreland b 50°N magmatic (Quesnell+Kootenay) f arc old-thrust belt Fold-thrust CANADA belt Intermontane asin USA

subduction

complex terranes

Canada J-K subduction complex Libby U.S.A. 49°N Forearc and L&C line Spokane

Laramide province system Foreland basin Columbia River basalts Idaho Helena batholith 46°N 100 km Mexico 30°N B 500 km A 110°W

116°W 110°W Sr = 0.706 Hall Lake Alberta rch St. Mary a WigwamFernie s N syncline Snowshoe

Purcell CANADA 49°N Moyie Sweetgras A’ USA Anticlinorium Whitefis Kevin Williston Basin

Pinkham Browning Sunburst Hope f h LEDSH Montana Bearpaw Mtns. Libby dome ault Li Sawtooth bby Range Choteau Lewi Flathead South s and Clark Lake A foreland bas arch Spokane line Augusta Highwood Mtns.

in Alta Sask

L MT o Helena mbar Idaho 0 100 km d-

batholith Eldorado Crazy Mtns. WY 46°N Butte ID C Bozeman

KEY FIGURES 1B AND 1C: Cenozoic extensional basin fill Paleogene forearc and

t Cenozoic volcanic rocks

t subduction complex s

e Cretaceous subduction i Cretaceous intrusive rocks

complex - a Mesozoic to lower Cenozoic

e foreland basin deposits Insular superterrane o

l Paleozoic and Mesozoic

a t Stikinia deformed sedimentary rocks

h Middle and Late Proterozoic

f Cache Creek p

a Belt and Windermere

A Kootenay and r

g Late Proterozoic Hamill Group

Intermontan terranes Quesnell

t and Paleozoic sedimentary rocks

e Late Proterozoic e

S Windermere Supergroup u

g Middle Proterozoic i

F Belt Supergroup Early Proterozoic pre- Belt metamorphic rocks

Figure 1. (A) Simpliﬁ ed 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 STRATIGRAPHY AND Island” complex (Suttner et al., 1981) and the mostly a coherent system, and was developing a SEDIMENTOLOGY Sweetgrass Arch and related structures, and fold-and-thrust belt and a foreland basin system thicken across the Williston Basin to the east (DeCelles, 2004). Approximately 2.5–3 km of Jurassic–Lower (Carlson, 1968). At the latitude of northwestern Montana, Paleocene strata currently lie in front of and The Sawtooth Formation is 15–50 m thick, southwestern Alberta, and southern British Co- within the frontal part of the fold-and-thrust belt and is composed of cross-bedded and ripple- lumbia (~46–51°N), the Cordilleran orogenic in northwestern Montana (Figs. 2 and 3). Most laminated sandstones and laminated mudstones, belt is composed of: (1) a fold-and-thrust belt, of the Lower Paleogene succession has been deposited in nearshore marine environments characterized by closely spaced thrust faults in- erosionally removed from the proximal fore- during a regional transgressive event. The Rier- volving Paleozoic and Mesozoic sedimentary land basin, but remnants are preserved to the don Formation contains mostly gray mudstone, rocks in its frontal part, and a system of mega- east in isolated localities. Subsurface regional varies in thickness from ~25 to ~70 m (Cobban, thrusts carrying Proterozoic and Paleozoic strata basement uplift has exhumed part of the fore- 1945), and indicates offshore deposition. The in the hinterland; (2) a zone of metamorphic and land basin fi ll along the Sweetgrass Arch and Swift Formation is characterized by glauconitic plutonic rocks located west of the generally un- related structures. To the east, in proximity to cross-bedded and rippled sandstones. Its lower metamorphosed fold-and-thrust belt, known as the North and South Dakota borders, subsidence part consists of shale with subordinate biotur- the Omineca belt in Canada; (3) remnants of the associated with the Williston Basin combined bated sandstone. Thickness ranges between 20 magmatic arc, mainly in the form of gran itoid plu- with distal fl exural subsidence due to orogenic and 40 m. The Swift Formation marks a high- tons; and (4) a series of accreted terranes and ac- loading has created additional accommoda- stand/regressive episode, with progradation of cretionary complexes that make up the rest of the tion. The following sections address the major shoreface deposits over distal facies of the Rier- Cordilleran system to the west. The easternmost sedimentologic and stratigraphic aspects of the don Formation. Rippled fi ne-grained sandstones group of these, including the Kootenay, Quesnell, Middle Jurassic–Lower Paleocene basin fi ll of and mudstones in the lower part of the Swift pos- Cache Creek, and Stikinia terranes, is generally northwestern Montana, and provide context sibly represent tidal-fl at deposits (Porter, 1989). included in the composite Intermontane super- for discussion of basin evolution relative to the Reported ages for the Ellis Group range from terrane (Fig. 1), which was accreted to North Cordilleran orogenic belt. An extended version Bajocian-Bathonian to late Oxfordian–Kimmer- America by the Middle to Late Jurassic (Monger of the stratigraphic and sedimentologic descrip- digian time, and the detailed stratigraphy and et al., 1982; Gabrielse et al., 1991; Murphy et al., tions is available in the GSA Data Repository regional extent of internal unconformities within 1995; Dickinson, 2004; Colpron et al., 2007; Table DR1.1 the unit have been the subject of numerous in- Dorsey and LaMaskin, 2007; Ricketts , 2008). vestigations (e.g., Cobban, 1945; Carlson, 1968; The eastern region of this superterrane is part of Middle to Upper Jurassic Mudge, 1972; Imlay, 1980; Porter, 1989; Parcell the morphotectonic Omineca belt. The western- and Williams, 2005). Recent stratigraphic analy- most element in the Cordilleran collage at these In northwestern Montana, an unconformity sis suggests that these uncon formi ties are gener- latitudes, the Insular Superterrane, had a long representing ~150 m.y. separates the youngest ally local, and resulted in part from tectonically and complex accretion history that commenced preserved miogeoclinal rocks of the Mississip- active basement structures (Parcell and Williams, during the Middle Jurassic (Monger et al., 1982; pian Madison Group from the oldest strata, of 2005). Our new detrital zircon and palynology Colpron et al., 2007; Ricketts, 2008). Middle to Late Jurassic age, that can be linked results yield additional constraints for the age of The general stratigraphy of northwest to Cordilleran orogenic evolution (Fuentes the Ellis Group. A sample collected 0.5 m above Montana can be divided into four major tec- et al., 2009). These Jurassic deposits are referred the pre–Middle Jurassic unconformity yielded tonostratigraphic packages developed over to as the Ellis Group and the Morrison Forma- one zircon grain with an age of 171.1 ± 2.5 Ma North American cratonic basement: (1) a thick tion. The tectonic setting of these and equivalent (see Isotopic Results and Provenance Interpreta- (≥15 km) Proterozoic succession of clastic, deposits farther south in the U.S. Cordillera re- tions sections and Table DR2 [see footnote 1]), carbonate, and igneous rocks of the Belt Super- mains controversial, and no consensus exists for which constrains the maximum depositional group (Harrison, 1972; Harrison et al., 1974), possible tectonothermal, dynamic, or fl exural age of the Sawtooth Formation as early Bajo- possibly deposited in an intracontinental rift mechanisms of subsidence (for a discussion, see cian (±error). The youngest age obtained from (Cressman, 1989; Price and Sears, 2000) or a DeCelles, 2004). the detrital zircons of a Swift Formation sample backarc extensional or strike-slip basin (Ross The Middle–Upper Jurassic Ellis Group is was 157.1 ± 6 Ma (mid-Oxfordian ± error). Paly- and Villeneuve, 2003); (2) a succession of composed of ~80–200 m of marine strata that nology from three Morrison Formation samples carbonate and relatively minor clastic rocks, are divided into the Sawtooth, Rierdon, and discussed later herein constrain the minimum deposited as a miogeoclinal wedge during late Swift Formations (Figs. 2 and 4). These units possible depositional age of the Swift as Ox- Precambrian rifting and subsequent Paleo- are characterized by an irregular distribution, fordian. These data, together with the previ- zoic passive-margin development (McMannis, abrupt lateral facies changes, and local internal ously reported paleontological ages, indicate a 1965; Bond et al., 1985; Poole et al., 1992); unconformities (McMannis, 1965; Peterson, Bajocian–mid-Oxfordian age for the Ellis Group (3) a Middle Jurassic to Lower Cenozoic clastic 1981; Parcell and Williams, 2005). At a regional in northwestern Montana. wedge, mainly deposited in a foreland basin set- scale, the Middle Jurassic deposits thin mark- At a regional scale, the Ellis Group correlates ting (McMannis, 1965; Bally et al., 1966; Peter- edly across a region of paleohighs of the “Belt with the upper part of the Fernie Formation of son, 1981); and (4) middle Eocene to Holocene southwestern Canada (Poulton et al., 1994), and clastic rocks deposited in discrete extensional 1GSA Data Repository item 2011002, Table the Gypsum Spring and Sundance Formations of basins in the western part of the fold-and-thrust DR1 (extended stratigraphy and sedimentology); northern Wyoming (Parcell and Williams, 2005). Table DR2 (U-Pb ages of detrital zircons); and Table belt (McMannis, 1965; Constenius, 1982, DR3 (point-counting results), is available at http:// Marine environments were replaced by 1996). The focus of this paper is on the Middle www.geosociety.org/pubs/ft2010.htm or by request fl uvial and lacustrine environments during Late Jurassic–Lower Cenozoic stratigraphic interval. to [email protected]. Jurassic deposition of the Morrison Formation.

Geological Society of America Bulletin, March/April 2011 509 Fuentes et al.

AGE LITHOLOGYGP. FORMATION DOMINANT APPROX. DEPOSITIONAL THICKNESS

) (m)

E O ENVIRONMENTS

( E Y

G . Wasatch Fluvial, ash fall >250 (central MT)

O C T

E 60 O

L ? E S

A Fort Union (P. Hills) Fluvial, lacustrine (coals) 300 (central MT)

P D Willow Creek M St. Mary River Fluvial >800 Bearpaw/Horsethief Offshore/shoreface 75–100 C Fluvial, 550–600 80 E Two Medicine lacustrine (coals) T Virgelle Shoreface 40–60

Montana A Telegraph Creek Shoreface, estuarine 90–170

L S C Marias T o River Offshore marine 350–450

C r Shale

o 100 l Fluvial (upper part)

S o Shoreface 200–500

U C Blackleaf A Offshore (lower part)

E Fluvial (mainly braided), C Y 100–>300 L Kootenai lacustrine (clastic and 120 A A

R carbonatic). Paleosols.

R B

C KEY H Transitional Coarse sand/ Coal/continental silt/claystone conglomerate organic shale Marine Continental 140 V Limestone sandstone sandstone B Marine Continental Tuff/ silt/claystone siltstone bentonite T

T Fluvial, shallow lacustrine, K 60–80

A Morrison

C estuarine. Paleosols (top)

I 160 L O S Swift Shoreface

E C l Rierdon Offshore marine 80–200

E A B D Sawtooth Nearshore marine

D B

U A

J 180 T

Figure 2. Simpliﬁ ed Jurassic–early Eocene stratigraphy of northwestern Montana.

510 Geological Society of America Bulletin, March/April 2011 Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.

′ A Sweetgrass arch A records the ﬁ rst major Cretaceous marine trans- E (Kevin Sunburst dome-

P Wasatch Fm. gression along the Western Interior Basin dur- Willow Creek Fm. South arch) Fort Union Fm. Hell Creek Fm. ing the late Albian. Lower Cretaceous strata in St. Mary River Fm. Claggett/Judith Bearpaw Shale northwestern Montana are the ﬁ rst unequivo- Horsethief River Fms. Sandstone Two Medicine Fm. cally synorogenic foredeep deposits related to

LC Virgelle Marias River Shale Sandstone Cordilleran tectonics (Suttner, 1969; DeCelles, Telegraph 1986; Schwartz and DeCelles, 1988). Creek Fm. Throughout western Montana, the base of the Blackleaf Fm.

C Kootenai Formation is conspicuously deﬁ ned E Kootenai Fm. an Precambrian basement by a several-meter-thick, coarse-grained to J Ellis Gr./ ppi

Morrison Fm. e conglomeratic, trough cross-bedded sandstone

- Mississinian i Siltstone, sandstone, conglomerate

r vo 1 km n an a o characterized by abundant chert grains (Fig. 4). De Shale, siltstone, sandstone

m N Cambri Sandstone These deposits are overlain by a succession of

r Shale, siltstone, sandstone variegated siltstone, cross-bedded and rippled 0 a

0 50 km M Shale, siltstone sandstone, limestone beds, and reworked tuffs. Depositional environments of the Kootenai Figure 3. West-east simplifi ed stratigraphic cross section of the northwestern Montana fore- Formation were dominantly fl uvial, with ex- land basin (modifi ed after Peterson, 1981). Lithologies correspond to the dominant type. Only tensive mud-dominated overbank environments major unconformities are depicted. Location of section is shown in Figure 1C. J—Middle- containing calcic paleosols. Relatively long- Late Jurassic, EC—Early Cretaceous, LC—Late Cretaceous, P—Paleocene, E—Eocene. lived lacustrine systems allowed deposition of limestones. Infl ux of siliceous volcanic ash, reworked by fl uvial systems, contributed sig- In northwestern Montana, this unit consists of fl uvial deposits. This zone of multiple red and nifi cantly to the net sedimentation. Paleocurrent ~60–80 m of fi ne-grained clastic strata (Fig. 4). gray paleosol horizons is mostly developed over data from fl uvial channel deposits show consis- Whether or not the basal contact of the Mor- silty overbank facies, contains carbonate nod- tently eastward transport (Fig. 4). rison represents an unconformity has been de- ules, multicolor mottles, and calcareous and lo- The age of the Kootenai Formation and cor- bated (Suttner, 1969; Pipiringos and O’Sullivan, cally iron-oxide and hydroxide cements. relative deposits is poorly constrained (Cobban , 1978; Peterson, 1981; Porter, 1989; Gillespie The Morrison Formation has been sparsely 1955; DeCelles, 1986; Heller and Paola, 1989; and Heller, 1995). This issue was clarifi ed by dated in northwestern Montana. Palynological Gillespie and Heller, 1995). Sparse pollen and Demko et al. (2004), who indicated that the analyses from our measured section yielded fossil ages have yielded mostly Aptian to Al- base of the formation is marked by the J-5 un- Oxfordian to Kimmeridgian ages (Table 1; bian ages in mudstones above the basal coarse conformity in the southern part of the Morrison Fig. 4). The youngest detrital zircons from a beds to the south. No direct dates had been depositional basin, but this contact becomes sandstone bed in the middle part of the unit obtained for the lower coarse section prior to conformable from northern Utah and Colo- yielded middle to late Oxfordian ages. These this work. Four new palynology samples and rado northward. The Morrison’s upper limit is new data constrain the age of the Morrison as new detrital zircon data constrain its age to be marked by a regional unconformity separating it latest Oxfordian to Kimmeridgian in the region, late Barremian(?)–early Albian. In particular, from the Kootenai Formation or equivalent units which is in general agreement with previous a detrital zircon sample from the basal con- (Mudge, 1972; DeCelles, 1986, 2004; Dolson work in Utah, Wyoming, and Colorado (Litwin glomeratic sandstone yielded two grains with and Piombino, 1994; Currie, 1998). et al., 1998; Kowallis et al., 1998; Turner and Hauterivian ages (131.6 ± 4.5 and 133.5 ± In northwestern Montana, the basal Mor- Peterson, 2004). 1.8 Ma), providing a maximum possible age rison Formation is characterized by shale and In southwestern Canada, Upper Jurassic rocks for this interval. These data confl ict with recent fi ne-grained sandstone arranged in heterolithic considered equivalent to the Morrison Forma- suggestions by Roca and Nadon (2007) for sedimentary structures. This interval is followed tion include most of the Passage Beds at the top continuous deposition between the Morrison by ~65 m of siltstone, with subordinate cross- of the Fernie Formation, and the Morrisey and Formation and the overlying conglomeratic bedded sandstone and beds of limestone and lower part of the Mist Mountain Formations, beds, and the proposition that the K-1 uncon- marl. The top ~30 m of the Morrison Formation included in the Kootenay Group (Poulton et al., formity is higher in the section. A detrital zir- consists of gray, green, and purple mud rocks 1994; Gillespie and Heller, 1995; Turner and con sample from the upper part of the Kootenai with pervasive evidence for pedogenesis. Peterson, 2004). contained a population of euhedral crystals that The lower part of the Morrison probably yielded early Albian ages, refl ecting syndepo- represents a tide-dominated marginal marine Lower Cretaceous sitional volcanism. Thus, the unconformity at environment. Estuarine conditions are inferred the base of the Kootenai Formation can be at- from the heterolithic sedimentary structures and A regional unconformity, representing more tributed to the time interval between the late from new palynological assemblages from three than 20 m.y., cuts into the Morrison Formation Tithonian and late Aptian. samples that indicate estuarine or deltaic envi- and, locally, Ellis Group deposits, and separates The Blackleaf Formation is divided into four ronments (Table 1). Most of the unit in northern the Jurassic from the Lower Cretaceous succes- members: the Flood Shale, Taft Hill, Vaughn, Montana, however, represents low-energy fl u- sion. The oldest Cretaceous sedimentary rocks and Bootlegger Members (Mudge, 1972; vial and local shallow lacustrine environments in Montana consist of ~50–400 m of conglom- Dyman et al., 1996). The Bootlegger Member (Peterson, 1981). The upper Morrison paleosol erate, sandstone, and siltstone of the Kootenai thins toward the west, where it is replaced by complex is incompletely preserved owing to ex- Formation. This unit is overlain by the Black- Vaughn Member facies. Thicknesses range from tensive erosional truncation beneath Cretaceous leaf Formation of the Colorado Group, which ~200 m in the east to ~500 m in the west.

Geological Society of America Bulletin, March/April 2011 511 Fuentes et al.

r M Marias River Shale

R T

940

900

980 960

990

920

950

970

930 910

1020

1050

1030 1000 1010

1040

Marias River Shale vc

890

860

820

840

830

800 780

810

900

870

880

850

790

770

760 750

g Marias River Shale a

740

710

650

690 680

670 630

600

660

610

640

620

750

720

730

700

g Blackleaf Fm. (Vaughn Mb.) 1 Blackleaf Fm. (Taft Hill Mb.)

=10

590

570

530

520 490

500

600

580

550

510 480

470

560

540 460

450

)

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Blackleaf Fm. (Flood Shale Mb.) vc

(

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S ). Composite log of measured sections (in meters) of Jurassic–early Eocene strata. Paleocurrent sections (in meters) of Jurassic–early Eocene strata. Paleocurrent ). Composite log of measured

Fm. T

c DeCelles et al. (1983). cation according to method I from

440

410

380 310

390 340 330 360 350

450

420

430

400

370

320

300

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i Kootenai Fm.

) a

M Fm. T

=28

a p

L (

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c on this and following three pages on this and following

290

260

240

220 180

210 170

200

300

270

280

250

160

230 190

150

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n Ellis Gr.

Morrison Fm. d

r Ellis Gr.

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1M5

o G Swift Fm. Rierdon Fm. Sawtooth Fm.

l o

1 TS

50 80

140

150

120

130

100

512 Geological Society of America Bulletin, March/April 2011 Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A. N

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8 B l R d

8 0 o p i ( ( ( ( ( ( ( B ( ( r i u 1 b U t r 1

3 4 T H S u t ( o P ( n e c F R vc i D F B R c B R C R M 9° o 2° 2° 1 v B e a i C e 0 V S P W B P S S T S D G 1 1 C B c Ry 1 1 1 R 1 aughn Mbr 19 1 1 Mbrs.): aughn 3 1 S 1 2: 1 H 1 1 ( 1 B S 1 P m 1 S T S f T

Covered Covered interval not well constrained well not interval s c 1820 1940 1830 1850 1840 1860 1870 1880 1890 1910 1900 1920 1930 1950 1810 1800 2

4 Bearpaw Shale B Two Medicine Fm. . P g 1 m F vc e c o n i w m c i T M f d b p T e 1 o vf T M S s T c 1670 1790 1680 1690 1700 1720 1710 1730 1740 1750 1760 1780 1770 1800 1660 1650 ). 1 M g E

Two Medicine Fm. * vc c m f b b vf continued s c 1520 1530 1540 1560 1580 1570 1640 1630 1500 1550 1590 1510 1600 1620 1650 1610 Figure 4 ( Figure 1TM3 =10

Two Medicine Fm. n g =13 vc n c m f b b vf s c 1470 1400 1440 1490 1480 1370 1390 1460 1350 1360 1380 1420 1430 1450 1500 1410 4 8 8 5 1 * 2 0 =28 R Two Medicine Fm. R 4 n S S 2 2 2 g * R =16 =10 S n n vc 2 * c m f b vf s c 1250 1220 1260 1300 1200 1230 1240 1270 1320 1350 1210 1290 1280 1310 1330 1340 . e e l h l m n p e F o a t g r r k s i

Two Medicine Fm. g Telegraph Creek Fm. e d =7 7 R 6 V e n 8 g p l e 7 n Virgelle Ss. S r p R 2 R e o a =29 3 S o n 0 S T C T vc S S 2 1 2 T T R c S 2 m f vf s c 0 0 0 0 0 0 0 0 0 0 1 6 9 3 4 8 2 5 7 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1080 1 1 1 1070 1 1200 1060 1050 1090

Geological Society of America Bulletin, March/April 2011 513 Fuentes et al. . m F h c t a U 1 g s F 2

Wasatch Fm. W a C 1 vc W H 1 W c S p T m o b b b b b b b b b b b b b b T b

1W TS vf s c 590 570 490 520 530 500 600 580 470 480 510 550 560 450 460 540 . m t

Wasatch Fm. r Fort Union Fm. F g o n F o vc i p n c o U T m f b b b b vf s c 440 410 310 380 330 340 360 350 390 450 420 430 400 320 370 300

Fort Union Fm. g 3 5 U U F vc F =19 =8 1 1 n n c m f b b vf s c 290 260 170 180 200 210 220 240 300 270 280 250 160 150 190 230 ). . m l l F e Hell Creek Fm. (Upper part) Fort Union Fm. k H g e p e r vc o C T c continued m f ? vf s c Figure 4 ( Figure 0 0 40 20 30 90 10 1 60 70 50 80 140 1 150 120 130 100 s g r o l e V m d c t i w F e r r o t s r l k A t c a d e . e o d e 7 b n l v e - n i n m r a e 1 a m s R C 3 n d s e F 3 o y n m g i e o w 2 r t k a F c d o ~ p l r e a i l l r Willow Creek Fm. vc n i l r u e s e r i M o c e o i v c W l t s i . C s l w t f p e e e R i f o m S r w d o R w y c o s r l f g p s l y n s i w a n . . o e b i o e l o i t l s vf T d M W t t n b d e g c . f u k s g n t e o i w e c c o l t i n S i s a a t h n c c e n f r t i t r R t r e d o o l u i t e e s t u c c a l p m g n c t e e c r p o l n e o o e r n p T e D f s o L i U 310 380 350 330 340 360 320 370 300 g vc

St. Mary River Fm. c m b b b b b b b b b f vf b s c b b b b b 290 260 170 180 220 210 200 240 270 300 280 250 160 150 190 230 g vc

St. Mary River Fm. c m b b b f vf b b b b b b b b b b b b b b b b b s c 0 0 40 20 30 90 10 1 60 70 50 80 140 1 120 150 130 100

514 Geological Society of America Bulletin, March/April 2011 Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.

LEGEND marine equivalents in central Montana include the Eagle and Judith River Formations, which Sandstone Pebbles thin eastward into the fully marine Claggett and Bearpaw Shales. The 40Ar/39Ar dating of biotite Marine clay/siltstone Intraclasts and plagioclase from bentonites located near Transitional clay/siltstone Calcareous nodules/concret. the base and top of the Two Medicine Forma- Terrestrial mud/siltstone Shell material tion constrains its age between ca. 80 and 74 Ma Limestone Plant material/organic matter (Rogers et al., 1993; Rogers, 1994). Deposition of the Two Medicine Formation Marl/sandy limestone Burrows/bioturbation continued until maximum transgression of the Tuff/tuffaceous silt/sandstone Roots Bearpaw Sea (Gill and Cobban, 1973). The Poorly exposed mud/siltstone interval Dessication cracks Campanian-Maastrichtian Bearpaw Shale and the Horsethief Sandstone represent the fi nal Covered interval Fish material widespread marine units in northwestern Mon- Vertebrate bones Trough cross-bedding tana. In the study area, the total thickness of the Ripples Climbing ripples Water-escape structures/load casts Bearpaw and Horsethief is ~100 m (Fig. 4). The Planar cross-bedding Seismites Bearpaw Shale consists of offshore black shale, Planar bedding/lamination b Bentonite layer which grades upward into fossiliferous, trough cross-stratifi ed sandstone. The Horsethief Sand- Hummocky cross-bedding Coal stone consists of a succession of cross-stratifi ed Flaser/wavy/lenticular bedding Palynology sample with age shallow marine sandstone. Strong pedogenesis Point counting sample The youngest foreland basin deposits pre- Gypsum/anhydrite Detrital zircon sample served in regions adjacent to the thrust belt are Paleocurrent direction the St. Mary River and Willow Creek Forma- c s vf f m c vc g n=20 Gravel/pebble tions, which are broadly dated by vertebrate with number of measurements Sand Silt and invertebrate fossils as Maastrichtian–early TS 1V Top of measured section with Clay section name Paleocene (Russel, 1950, 1968; Tozier, 1956; Catuneanu and Sweet, 1999). The thickness of Figure 4 (continued). these two units is diffi cult to estimate owing to the discontinuity of outcrops and lack of marker intervals to establish correlations. Well data in- The Flood Shale Member is composed of Marias River Shale correlates with the Frontier dicate that the St. Mary River Formation is on ~50–60 m of marine shale capped by rippled Formation of southwestern Montana (Dyman the order of 300 m thick (Fig. 4), similar to the and trough cross-bedded sandstone (Fig. 4). et al., 1996), and with the Blackstone, Cardium, value estimated by Stebinger (1916). The com- The Taft Hill Member is a stack of upward- and Wapiabi Formations of southern Alberta bined thickness of the two units probably is more coarsening shale and cross-bedded and rippled (Yang and Miall, 2009). The Marias River Shale than 800 m (Mudge et al., 1982). The St. Mary sandstone. The Vaughn Member consists of is dominated by dark shale, with an increase in River Formation consists of mudstone and beds nonmarine mudstone, cross-bedded sandstone, sandstone content toward the top (Fig. 4). Thin of cross-stratifi ed fl uvial sandstone. Cuttings and and conglomeratic channel fi lls that represent bentonite beds occur in the unit. electric logs from the Rainbow Resources 1–7 fl uvial and alluvial plain deposits. The Santonian is characterized by a regres- Art V Dresen well located near the international In Montana, the Blackleaf Formation has sive sedimentation pattern, which continues into border show a dominance of mudstone with fre- been dated as late Albian–early Cenomanian the Campanian. The Telegraph Creek Forma- quent bentonitic beds. The Willow Creek For- (Cobban and Kennedy, 1989). The mean age tion consists of ~90–170 m of mudstone and mation is only locally exposed, and it consists of of the eight youngest detrital zircons from a siltstone with sandstone intercalations. Ripples, variegated mudstone with thin beds of sandstone. sample of the Vaughan Member is ca. 97 Ma, trough cross-stratifi cation, hummocky stratifi ca- A sample of fl uvial channel sandstone from which is consistent with the previously reported tion, and burrows are abundant (Fig. 4). Facies the St. Mary River Formation yielded abundant paleontological ages. and abundant body and trace fossils indicate detrital zircons with ages up to mid-Maastrich- nearshore marine and estuarine conditions. The tian. The mean age from the ten youngest grains Upper Cretaceous Virgelle Sandstone is 40–60 m thick and is com- is ca. 68.5 Ma, providing an additional maxi- posed almost entirely of trough cross-stratifi ed mum age constraint for deposition of this unit. A second widespread marine transgression af- sandstone that was deposited in a nearshore en- fected the foreland basin during the early Late vironment. The Telegraph Creek Formation is Paleocene–Eocene Cretaceous (Porter et al., 1982; Stott, 1984). In late Santonian in age (Cobban, 1955; Cobban northwestern Montana, late Cenomanian to early et al., 2005), and the Virgelle Sandstone is early The youngest part of the foreland basin has Santonian black shales reach a thickness of more Campanian (Cobban, 1955). been erosionally removed along proximal areas. than 350 m (Schmidt, 1978; Yang and Miall , Nonmarine deposition resumed during the Lower Danian to middle Paleocene deposits are 2009). These deposits are referred to as the Campanian. The Two Medicine Formation preserved in the Porcupine Hills Formation in Marias River Shale. This unit and the Blackleaf in Montana consists of ~600 m of fl uvial and southern Canada, and in the Fort Union Forma- Formation are formally included in the Colorado minor lacustrine deposits with volcanic material tion in central Montana (Douglas, 1950; Mack Group (Cobban et al., 1959; Mudge, 1972). The and coal beds (Fig. 4). Nonmarine and nearshore 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 Kootenai Formation 1GR20 1GR22 1GR85 1GR189 1FG50 1FG69 1SFSR67 Spores and pollen Aequitriradites spinulosus R Apiculatisporis sp. A R Araucariacites australis A R R R R Callialasporites dampieri A R R *R *R C. triangularis RCRR C. trilobatus R R *R Cerebropollenites mesozoicus RCRR Cicatricosisporites australiensis R Cicatricosisporites australis R C. hallei R C. purbeckensis? R C. venustus *R Classopollis classoides A A R A Concavissimisporites punctatus R F R Concavissimisporites southeyensis R Concavissimisporites variverrucatus R *R Coronatispora sp. *R Corrugatisporites anagrammensis *R Cyathidites australis R Deltoidospora spp. A A R A R R Eucommiidites troedsonii R Exesipollenites tumulus ARRR A Foraminisporis dailyi R R Gleicheniidites apilobatus R Gleicheniidites senonicus R Klukisporites pseudoreticulatus R Lycopodiacidites baculatus R Lycopodiacidites irregularis RR Lycopodiacidites tortus R L. triangularis R Lycopodiumsporites austroclavatidites F R Lycopodiumsporites pseudoannotinus *C Klukisporites pseudoreticulatus R Mathesisporites tumulosus *R *R Maumia irregularis *R Microreticulatisporites sp. R Osmundacites wellmanni RR Parvisaccites radiatus RR Perinopollenites halonatus R R R P. sp. R Perotrilites sp. R Platysaccus sp. A R R Podocarpites sp. R Reticulatisporites sp. R Rubinella sp. R R Schizosporis parvus R S. reticulates R Taxodiaceae A R R R A Trilobosporites hannonicus *R Trilobosporites humilis *R T. minor R T. perverulentus *R *R T. sphaerulentus *R Triporeletes reticulatus *R Undifferentiated Bissacates A A A A R F A Verrucosisporites staplinii *R V. sp. R Marine organisms/microplankton Aldorﬁ a 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 signiﬁ cant 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 Potential Source Terrane Compositions northeastern Oregon (Dickinson, 2004; Dorsey Montana, the youngest rocks that can be linked and LaMaskin , 2007). The implication of this is to deformation in the thrust belt are in the Lower From west to east, potential sources of sedi- that the Pacifi c Northwest of the United States Eocene Wasatch Formation on the fl anks of the ments (Fig. 5) include the assemblage that and adjacent Canada would have produced sedi- Bearpaw Mountains and in the Missouri Breaks constitutes the Intermontane superterrane, the ments of similar compositions. diatremes, ~200 km east of the thrust belt front region of the Omineca belt, a series of hinterland To the east, the Omineca belt consists of (Reeves, 1946; Hearn, 1976). thrust sheets carried by the Moyie, Libby, and metamorphic and plutonic rocks (Monger et al., The Fort Union Formation in the Bearpaw other thrust systems involving Belt Supergroup 1982; Gabrielse et al., 1991). Omineca belt Mountains consists of ~300 m of sandstone, rocks, the major thrust sheets carried by the rocks were polydeformed and metamorphosed siltstone, shale, and coal representing fl uvial Lewis-Eldorado-Steinbach-Hoadley (LEDSH) during the Jurassic, and some structural levels and lacustrine deposits (Fig. 4). The Wasatch thrust system, a series of closely spaced thrust were further deformed and overprinted as re- Formation consists of variegated siltstone, sheets in the Sawtooth Range and foothills, cently as the early Eocene (Parrish et al., 1988; bentonitic mudstone, cross-stratifi ed fi ne- to and North American basement. Possible along- Evenchick et al., 2007). The Omineca belt con- coarse-grained sandstone, and lenticular beds strike transport could have provided sediments tains exposures of the North American craton of cobble-boulder conglomerate. The Wasatch from distal sources. and abundant mid-Cretaceous plutons. The Formation is overlain by rocks of the Bearpaw The Intermontane belt at the latitude of north- eroded upper stratigraphic levels presumably Mountains volcanic fi eld (Hearn, 1976). In western Montana and southern Canada is com- contained distal facies of the Belt Supergroup measured sections, the Wasatch Formation is posed of the Stikinia, Cache Creek, Quesnell, and the miogeocline. ~250 m thick, but its original thickness is un- and Kootenay terranes (Figs. 1 and 5). Stikinia A zone of hinterland thrust systems involv- known. Eroded proximal equivalents of the Fort consists of Upper Paleozoic to Lower Jurassic ing Belt Supergroup rocks extends east of the Union and Wasatch Formations could have been volcanic and marine rocks. The Cache Creek Omineca belt. Major thrusts include the Moyie, much thicker. In Alberta, coal moisture, vit rinite terrane is interpreted to be the accretionary Libby, Pinkham, and a number of other thrusts refl ectance, and thermokinematic modeling in- complex of the Quesnellia arc, and contains (Harrison et al., 1980, 1986; Harrison et al., dicates that 2–4 km of synorogenic Cenozoic Mississippian to Middle to Late(?) Triassic 1992; Fillipone and Yin, 1994) for which de- strata have been erosionally removed (Beau- radiolarian chert, argillite, basalt, ultramafi c tailed geometry is poorly known owing to post- mont, 1981; Hardebol et al., 2009). rocks, limestone, and local blueschist (Coney orogenic extensional faulting and a widespread The Fort Union Formation has been assigned and Evenchick, 1994). Stikinia was probably Quaternary cover. These thrust systems termi- to the Paleocene (Rice, 1976). Recent work based not a signifi cant sediment source for the western nate to the south against the strike-slip system on plant content and magnetostratigraphy (Hart- Montana foreland basin because it was appar- of the Lewis and Clark line (Fig. 1), but thrust man, 2002; Lund et al., 2002) placed the Fort ently fl exed under the load of the Cache Creek systems involving Belt Supergroup rocks also Union Formation of the Williston Basin across the accretionary complex and remained close to or exist to the south, at the latitude of the Helena Maastrichtian-Danian boundary or the very early below sea level during the late Mesozoic and salient. Phanerozoic miogeocline and foreland Danian, up to the late Paleocene. The Wasatch early Cenozoic (Coney and Evenchick, 1994). basin strata are mostly eroded from these thrust Formation has been assigned an early Eocene The Cache Creek terrane shed radiolarian chert sheets. Although sediments produced during age (ca. 57–54 Ma) based on fl ora and vertebrate clasts westward, but apparently not eastward, early stages of slip on these thrusts were prob- fossils (Brown and Pecora, 1949; Marvin et al., implying the existence of a drainage divide ably recycled early foreland basin deposits 1980). Volcanic rocks overlying the Wasatch have east of it, at least in southern Canada (Mack and Paleozoic strata, diagnostic petrographic been dated as late early to early middle Eocene and Jerzykiewicz, 1989). The Quesnell terrane elements associated with deeper exhumation (ca. 54–50 Ma) (Marvin et al., 1980; Wing and is composed of late Paleozoic to early Meso- should be grains of argillite, polycrystalline Greenwood, 1993). The Wasatch Formation was zoic submarine mafi c and ultramafi c volcanic quartz, micas, and chlorite derived from Belt the last unit deposited in a foreland basin setting, rocks, volcaniclastic and carbonate rocks, and Supergroup strata. A key to differentiating this and the early to middle Eocene igneous rocks that chert, sitting on Devonian to Permian rocks of from provenance areas to the west in the Inter- cover and intrude this unit herald the beginning of arc affi nity. Although traditionally interpreted montane belt would be the ages of detrital zir- crustal extension and magmatism in the northern as a freestanding intraoceanic arc, recent work cons: Intermontane terrane zircons should be Cordillera (Constenius, 1996). (Erdmer et al., 2002; Unterschutz et al., 2002) dominated by Phanerozoic ages, whereas Belt demonstrates an autochthonous origin for Supergroup–derived zircons should have Lau- PROVENANCE the Quesnell terrane. The Kootenay terrane, the rentian ages >1.85 Ga and 1655–1790 Ma, easternmost component of the Intermontane non-Laurentian ages in the 1510–1625 range, Forty sandstone samples for modal petro- belt, contains Lower to Middle Paleozoic off- and synsedimentary (Belt) ages ranging from graphic analysis and 10 samples for U-Pb ages shelf facies of the miogeoclinal wedge (Colpron 1440 to 1480 Ma (Ross and Villeneuve, 2003; of detrital zircons were selected to characterize and Price, 1995; Colpron et al., 2007). Its lower Link et al., 2007). Although no detrital zircon the provenance of Middle Jurassic–early Eo- part is composed of quartzo-feldspathic schist studies for miogeoclinal rocks from the north- cene sedimentary rocks. These analyses were and gneiss, quartzite, and carbonate, and its western United States are available, Gehrels and carried out to assess major tectonic variations upper section is characterized by meta-volcanic Ross (1998) reported >1.75 Ga and “Grenville” in the development of the foreland basin with rocks, orthogneiss, dark graphitic pelites, lime- 1.1–1.4 Ga ages of zircons from miogeoclinal time, and to determine the unroofi ng history of stone, argillite, and chert (Coney and Evenchick, strata in British Columbia and Alberta. the fold-and-thrust belt. Additionally, U-Pb ages 1994; Colpron and Price, 1995). The frontal part of the northwestern Montana in some cases help constrain maximum ages of To the south, these terranes are covered by fold-and-thrust belt is dominated by a mega- the sedimentary units from which the zircons the Columbia River basalts, but correlative thrust sheet composed of several kilometers of were collected, as discussed already. terranes are present in the Blue Mountains of Belt Supergroup and minor Paleozoic rocks in

Geological Society of America Bulletin, March/April 2011 517 Fuentes et al.

K the hanging wall of the LEDSH thrust system.

O The Sawtooth Range and the foothills struc-

: tures involve Cambrian to early Paleocene strata

N A (Mudge, 1980, 1982). The development of these

o n foothills structures passively deformed rocks

e of the overlying Lewis thrust sheet, as clearly

a seen along the Lewis thrust salient in the U.S.-

- ature ature

t n Canada border region (e.g., Gordy et al., 1977;

r e

b Fermor and Moffat, 1992).

a a Finally, a potential source of sediments is

C s

O North American basement. Sedimentary rocks

p y

O with provenance from this source should con-

t tain abundant quartz and feldspar, and zircons

w a with ages older than 1.8 Ga (Gehrels and Ross,

E 1998; Ross and Villeneuve, 2003).

o .

u i Petrographic and Isotopic Provenance Data

r d

t tems:

h S Methods

e g

r r

H sys

A l

m L About 450 grains per slide were point counted

S using the Gazzi-Dickinson method, as explained

S by Ingersoll et al. (1984), in order to facilitate

f a comparison of the results with the provenance

d s

l s models of Dickinson and Suczek (1979) and

t s

c s Dickinson et al. (1983). Only medium-grained

a i

n sandstones were point counted. Half of each

r x

. e i

n g thin section was stained for both calcium and

l potassium feldspars. Grain types are listed in

O A

M A p

, Table 2. The stratigraphic position of individ-

z ual samples is shown in the measured sections

i (Fig. 4). Individual point-counting results are

f z tabulated in Table DR3 (see footnote 1). Paleo-

e current data were obtained to provide additional

f paleogeographic and provenance information.

d ,

i a Most of the data are from the limbs of medium-

t i

p to large-scale trough cross-strata (method I of

r o

chert. l K L p o S DeCelles et al., 1983). Additional paleocurrent

c information was provided by imbricated con-

a s glomerate clasts, and, in a few cases, by orienta-

w tions of trough axes.

v f Zircon U-Pb geochronology was conducted

r by laser ablation–multicollector–inductively

c coupled plasma–mass spectrometry (LA-MC-

l i ICP-MS) at the University of Arizona Laser-

n n

n r

e s Chron Center (analytical procedures in Gehrels

s f

i et al., 2008). Approximately 100 grains per sam-

M U l

S ple were dated, with the exception of a sample

a ) from the Two Medicine Formation, from which

(

R e only 19 grains were recovered. Analyses that

m t yielded isotopic data with acceptable discor-

H d dance, precision, and in-run fractionation are

t shown in Table DR2 (see footnote 1). In total,

h : 873 zircon ages are reported here. The ages are

n Figure 5. Cartoon illustrating potential major source areas for the foreland basin system of northwest Montana. Terrane nomencl Terrane basin system of northwest Montana. the foreland for areas source 5. Cartoon illustrating potential major Figure corresponds to southern Canada. Autochthonous source terranes over North American basement represent predeformed state. LEDSH— predeformed American basement represent North terranes over Autochthonous source to southern Canada. corresponds Lewis-Eldorado-Steinbach-Hoadley thrust system. Not to scale.

c e plotted on relative probability diagrams. Age

l peaks on these diagrams are considered robust

a if deﬁ ned by three or more analyses.

C R

T Petrographic Results

I Sandstones in the Jurassic Ellis Group and Morrison Formation have mature quartzo-lithic

518 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 1992) to southern Montana (Suttner et al., 1981; Quartzose grains Dyman et al., 1988). Qm Monocrystalline quartz Two sandstone samples from the Marias Qp Polycrystalline quartz Qpt Foliated polycrystalline quartz River Shale produced modes of Qm/F/Lt = Qss Quartz in sandstone/quartzite lithic grain 14/81/5 and Qt/F/L = 17/81/2, with feldspar S Siltstone Qt Total quartzose grains (=Qm + Qp + Qpt + Qss + S + C=Cb) fractions dominated by plagioclase. The high proportion of plagioclase refl ects active vol- Feldspar grains K Potassium feldspar canism at the time, signaled throughout the P Plagioclase feldspar unit by numerous bentonite beds (Figs. 4, 5, F Total feldspar grains (=K + P) and 7). These samples also contain anoma- Lithic grains lously large amounts of detrital biotite, of Metamorphic probable volcanic origin. Lph Phyllite Lsm Schist Telegraph Creek and Virgelle sandstone Lss Serpentinite schist samples have very similar petrographic com- M Marble positions, with average modes of Qm/F/Lt = Lm Metamorphic lithic grains (=Lph + Lsm + Lss + M + Qpt) 42/32/26 and Qt/F/L = 50/32/18. These sand- Volcanic stones contain signifi cant amounts of chert, Vvl Lathwork volcanic grains Lvx Microlitic volcanic grains phyllite, schist, different volcanic grain types, Lvf Felsic volcanic grains limestone, shale, and micas. Feldspar grains are Lvv Vitric volcanic grains Lvm Mafi c volcanic grains dominated by plagioclase, but traces of potas- Lv Total volcanic lithic grains (=Lvl + Lvx + Lvf + Lvv + Lvm) sium feldspar are also present. Samples of the Sedimentary Two Medicine Formation and Horsethief Sand- D Dolostone stone show compositions similar to those from Lc Limestone the Telegraph Creek–Virgelle, with average Lsh Shale/mudstone C Chert Qm/F/Lt = 49/25/26 and Qt/F/L = 59/24/17. Cb Black chert Relatively important constituents include shale, Ls t Total sedimentary lithic grains (=D + Lc + Lsh + C + Cb + Qss + S) limestone, micas, and low-grade metamorphic Lt Total lithic grains (=Ls + Lv + Lm) L Unstable lithic fragments (Lt-(C + Cb + Qss + S + Qpt)) lithic fragments, and traces of chlorite and am- Note: Accessory minerals: Epidote/zoisite, chlorite, garnet, amphibole, muscovite, biotite, tourmaline, pyroxene, phibole. A relatively high content of potassium kyanite, cordierite, sillimanite, zircon, kaolinite, glauconite, apatite. feldspar was found in a sample of the Horse- thief Sandstone. Saint Mary River and Willow Creek Forma- compositions, with average Qm/F/Lt = 85/1/14 Qt/F/L = 53/16/31. These samples contain much tion sandstones have average modes of Qm/F/ and average Qt/F/L = 94/1/5 (Figs. 6 and 7; higher proportions of plagioclase, volcanic lithic Lt = 35/33/32 and Qt/F/L = 44/32/24. Samples Table DR3 [see footnote 1]). The quartz frac- grains, and cherty mudstone fragments than from the lower St. Mary River Formation con- tion is dominated by unstrained monocrystalline sandstones lower in the section (Fig. 8). Other tain less quartz and more feldspar (Fig. 7). These quartz (Fig. 8). Lithic grains mostly consist of minor components include limestone, chlorite, samples also contain greater amounts of phyllite sedimentary chert, with subordinate amounts muscovite, and zircon. This major change in and schist grains (Figs. 6 and 7). Clasts of lime- of shale and limestone. Low-grade metasedi- composition was also identifi ed in correlative stone, shale, and mudstone are relatively im- mentary and volcanic grains are scarce. A few rocks in southern Canada (Ross et al., 2005). portant, and detrital chlorite is abundant in two grains of muscovite and glauconite are present Samples from the Blackleaf Formation have samples of the basal St. Mary River Formation. in samples from the Swift Formation. The Mor- average Qm/F/Lt = 38/23/39 and Qt/F/L = Sandstones from the Fort Union Formation, rison Formation contains traces of amphibole, 50/22/28, with a wide range of compositions. the equivalent Porcupine Hills Formation, and tourmaline, and zircon. The lithic fraction consists of chert, phyllite, and the Wasatch Formation show similar modes Samples from the lower, braided stream- schist, a variety of volcanic grains, and shale. with average Qm/F/Lt = 63/10/27 and Qt/F/L = dominated part of the Kootenai Formation have Muscovite, biotite, chlorite, and limestone 75/10/15. These samples contain a few to abun- average Qt/F/L = 95/0/5 and average Qm/F/Lt clasts are additional minor components. The dant potassium feldspar grains. Phyllite, schist, = 61/0/39. The discrepancy in Qm versus Qt key petrographic features of Blackleaf samples, limestone, and fi ne-grained sedimentary lithic results from abundant sedimentary chert grains however, are the relatively abundant low-grade fragments are abundant. Traces of chlorite, am- that dominate the lithic fraction (Fig. 8). Other metamorphic lithic fragments (Figs. 6, 7, and 8; phibole, tourmaline, pyroxene, and zircon are constituents of the lithic fraction are shale, mud- Table DR3 [see footnote 1]), different varieties present as well. The uppermost sample from stone, and phyllite. Traces of volcanic lithic of polycrystalline quartz, and a higher propor- our section, collected from the Wasatch Forma- grains, plagioclase, limestone, and muscovite tion of detrital muscovite and biotite. The in- tion, is dominated by plagioclase grains, with are present. The main difference between lower crease in feldspar and volcanic lithic fragments abundant volcanic lithic fragments and biotite Kootenai and Jurassic sandstone compositions toward the top of the Kootenai Formation, and the grains. Additionally, 68 clasts of a pebble con- is the abundance of chert in the former. appearance of notable quantities of slate and glomerate bed in the Wasatch Formation were A marked change in the detrital composition phyllite, plus an increased proportion of detrital counted in the fi eld. Clasts of Belt Supergroup occurs in the upper part of the Kootenai Forma- chlorite and micas by the time of deposition of quartzite and fi ne-grained mafi c igneous rocks tion. Samples of this interval are distinguished the Blackleaf Formation, seem to be of regional dominate (~28% of the total for each type). by average modes of Qm/F/Lt = 22/17/61 and extent, from Canada (Potocki and Hutcheon, Shale, mudstone, and low-grade metamorphic

Geological Society of America Bulletin, March/April 2011 519 Fuentes et al.

Qm Qt 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 CB RO syndepositional age of ca. 171 Ma (Table DR2 CB [see footnote 1]). Sample 1GR14 from the Swift Formation has RO an age spectrum similar to that of sample Eb, with most zircon grains showing ages older than MA MA 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. F 50% Lm Lt F L These Jurassic zircons were probably derived from the magmatic arc to the west. Addition- ally, 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 Lv Ls Qm Qt 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. Addi- tionally, both samples contain several Carbonif- erous–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 50% Lm F Lt F L 131–177 Ma for sample 1SFSR1. These sam- KEY: Fort Union/Wasatch Formations ples contained a few grains with late Permian St. Mary River/Willow Creek Formations (1GR100) and Early Triassic (1SFSR1) ages, T.Creek/Virgelle/T. Medicine/Horsethief Formations a few Late Proterozoic (1GR100) and Eocam- Marias River Shale Blackleaf Formation brian (1SFSR1) grains, and a dominant popula- Upper section Kootenai Formation Lower section tion of zircons with ages older than 1 Ga. Lv Ls Ellis/Morrison Formations Sample 1FG70 from the middle part of the Kootenai Formation provided detrital zircons Figure 6. Ternary diagrams illustrating modal framework-grain compositions of indi vidual with mostly young ages, ranging between 104 sandstone samples (above) and means with standard deviations (below). Provenance fi elds and 238 Ma. Additionally, two grains yielded are after Dickinson and Suczek (1979). RO—recycled orogen; CB—continental block; ages close to the Proterozoic-Cambrian bound- MA—magmatic arc. Framework components are explained in Table 2. Data are available ary (540 and 558 Ma), and four grains provided in Table DR3 (see text footnote 1). Wasatch Formation mean does not include uppermost Early Proterozoic or older ages. sample. Zircons from a Blackleaf Formation sample (1SR80) produced a dominant cluster of ages in the Early Cretaceous to mid-Cenomanian, from pebbles constitute 17% of the total, followed 40% Late Cretaceous–Paleocene fi ne-grained 132 to 96.6 Ma. This sample also produced ages closely by limestone clasts (16%). The remain- and porphyritic volcanic rocks, and 1% chert in the ranges 150–176 Ma and 953–2926 Ma. ing ~10% consists of similar proportions of fi ne- and conglomerate. Only 19 zircons were retrieved from the grain felsic igneous, granite, and schist clasts. sample 2SR240 of the Two Medicine Forma- Estimates of clast composition in Wasatch con- Isotopic Results tion. Most grains cluster in the 85–113 Ma glomerates by Hearn et al. (1964) provided val- Ages of detrital zircons of individual samples range, predating the known depositional age of ues of 50%–80% argillite and quartzite of Belt are plotted on relative probability diagrams in this unit by a few million years. The remaining Supergroup, 1%–5% Paleo zoic rocks, 20%– Figure 9. Sample Eb was collected from the six grains have Precambrian ages that extend

520 Geological Society of America Bulletin, March/April 2011 Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.

AGE FORMATION Qm FLtLsLv Lm

)

O (Qm+F+Lt) (Qm+F+Lt) (Qm+F+Lt) (Ls+Lv+Lm) (Ls+Lv+Lm) (Ls+Lv+Lm)

( N Y

G . Wasatch

O C T

E 60 O

L E S

P A Fort Union (P. Hills)

P D Willow Creek M St. Mary River Bearpaw/Horsethief C 80 E Two Medicine T Virgelle

A Telegraph Creek

L S C Marias T River C Shale 100

U A Blackleaf

L Kootenai 120 A A

R B C H 140 V B T

T I K

A Morrison

160 S O Swift

A E C

i Rierdon

B l

E Sawtooth

D B

J 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 A

Figure 7. Plot showing the stratigraphic distribution of ratios of main grain types. Deﬁ nitions of grain types are given in Table 2.

from 1518 to 3186 Ma. Six zircons have ages that proves a western, orogenic provenance is found in coeval deposits. This suggests the ex- that fall in the typical age range of Belt Super- found in the detrital zircon ages. Samples of the istence of a drainage divide along the Omineca group zircons. Ellis and Morrison Formations contain abun- belt that prevented Intermontane belt detritus Most grains from sample 1BB44, collected dant zircons with late Paleozoic and Triassic from reaching the Canadian sector of the retro- from a fl uvial channel of the St. Mary River ages derived from accreted elements in the east- arc foreland basin (Mack and Jerzykiewicz, Formation, have ages from 66 to 98 Ma. The ern part of the Intermontane belt, mid-Paleozoic 1989; Ross et al., 2005). youngest cluster of grains includes Maastrich- ages with possible origin in Kootenay arc rocks, Grains with Late Proterozoic–Cambrian ages tian ages (Fig. 9), providing a maximum deposi- and a mix of different miogeoclinal source units were probably recycled from Jurassic San Rafael tional age for this poorly dated unit. Only seven (Gehrels and Ross, 1998), refl ecting an early Group eolianites in the Colorado Plateau area out of a total of 95 grains yielded Precambrian fold-and-thrust belt involving Paleozoic strata (Dickinson and Gehrels, 2008) and transported ages, in the range 1307–2528 Ma. in the hinterland. Volcanogenic zircons of syn- by axial fl uvial systems that fl owed northward depositional age were retrieved in every Jurassic toward marine shorelines located near the in- Provenance Interpretations sample. The presence of zircons derived from ternational border (Demko et al., 2004; Turner the eastern part of the Intermontane belt indi- and Peterson, 2004). This view agrees with the Jurassic sandstones of the Ellis Group and cates that terranes in the Intermontane belt were work of Suttner et al. (1981), who inferred that Morrison Formation plot in the recycled oro- structurally elevated and supplying sediments some Morrison Formation detritus was derived genic fi elds (Fig. 6), and their modal compo- to a basin located hundreds of kilometers to the from a southern source, possibly in Colorado. sitions suggest deposition in a foreland basin east (Fuentes et al., 2009). The presence of de- Most signifi cantly, thousands of paleocurrent setting (Dickinson and Suczek, 1979; Suttner trital zircons likely derived from the Quesnell measurements from Morrison sandstones indi- et al., 1981). Lithic fragments suggest deriva- terrane in the Morrison Formation contrasts cate generally north-northeastward paleofl ow tion from deformed miogeoclinal strata and with results from southern Canada, where no during the Late Jurassic (Suttner et al., 1981; vol canic sources. However, the key information evidence of Intermontane belt debris has been DeCelles and Burden, 1992; Currie, 1998).

Geological Society of America Bulletin, March/April 2011 521 Fuentes et al.

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 Koo- tenai 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 Forma- tion, 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 Intermontane Belt Miogeocline SW Miogeocline volcanism upper part/ U.S.A (NA basement, Kootenay Grenville, terrane Appalachian)

1BB44 (St. Mary River Fm.) - n = 95

t 2SR240 (Two Medicine Fm.) - n = 19

g 1SR80 (Blackleaf Fm.) - n = 95

1FG70 (Kootenai Fm.) - n = 99

1SFSR1 (Kootenai Fm.) - n = 97

1GR100 (Kootenai Fm.) - n = 95 1GRZ (Morrison Fm.) - n = 87

)

m 70

u 1GRX (Morrison Fm.) - n = 91

p 50

a . 1GR14 (Ellis Gr.-Swift Fm.) - n = 98

/ 30

m Eb (Ellis Gr.-Sawtooth Fm.) - n = 97

M ( 10

0 100 200 300 400 500 550 550 1000 1500 2000 2500 3000 3500 A Age (Ma)

116 178 Morrison Formation - 1GRZ Kootenai Formation - 1FG70 Figure 9. (A) Age probability 114 174 plots of U-Pb ages of detrital zir- 112

170 ) 110

)

a cons. Shaded areas indicate main a

( 108 origins of zircons (from Coney ( 166

g 106

162 A and Evenchick, 1994; Roback and A 104 Walker, 1995; Gehrels and Ross, 158 1998; Ross and Villeneuve, 2003; 102 154 100 Ross et al., 2005; Link et al., 2007). TuffZirc Age = 162.47 +9.84 -5.97 Ma TuffZirc Age = 109.85 +0.70 -1.25 Ma 98 Horizontal bars indicate deposi- 150 (96.9% conf, from coherent group of 6) (93% conf, from coherent group of 6) tional age of samples. Note change Blackleaf Formation - 1SR80 St. Mary River Formation - 1BB44 in horizontal scale in plots at 105 72 550 Ma. The Coast Mountain 103 70 batholith magma ﬂ ux curve from 101

)

a Gehrels et al. (2009) is shown in the a 99 68

(

lower left corner. (B) TuffZirc ages e e 97 66

A of youngest detrital zircon popula- A 95 tion of selected samples. Box heights 64 93 are 1σ. NA—North America. 62 91 TuffZirc Age = 97.39 +1.49 -0.46 Ma TuffZirc Age = 68.47 +0.77 -0.68 Ma B 89 60 (93% conf, from coherent group of 8) (97.9% conf, from coherent group of 10)

Geological Society of America Bulletin, March/April 2011 523 Fuentes et al.

Late Early Proterozoic (ca. 1650–1775 Ma) zir- placement on thrusts involving Belt Supergroup River Formation suggests exhumation of Belt cons in these samples could have been derived strata is well constrained near the Canada-U.S. Supergroup rocks along the LEDSH system, from Neoproterozoic and early Paleozoic mio- border, where 94–96 Ma plutons crosscut the St. and passive transport of rocks in the Omineca geoclinal rocks of Alberta and British Columbia Mary and Hall Lake thrusts (Price and Sears, belt to shallower levels. The cosmopolitan (Gehrels and Ross, 1998), or from Yavapai- 2000). In northwestern Montana, the Moyie provenance of the St. Mary River and Willow Mazatzal basement rocks in southwest United fault deformed the margin of the 71 Ma Dry Creek Formations could be related to either can- States (Dickinson and Gehrels, 2008). Creek stock (Fillipone and Yin, 1994), but ear- nibalization of older foreland basin deposits or The basal coarse-grained sandstones of the lier episodes of slip along segments of this fault an integrated drainage system. Detrital zircons Kootenai Formation are also composition- cannot be ruled out (Harrison et al., 1986; Price from the St. Mary River Formation indicate pre ally mature, but they contain a much higher and Sears, 2000). Additionally, movement on and syndepositional volcanic sources, and input proportion of chert. This abundance of chert hinterland thrusts at the latitude of the Helena from Paleozoic and Belt Supergroup strata in in basal Kootenai and correlative samples of salient, and transpressive slip along faults in deformed thrust sheets. Montana and southern Canada has long been the Lewis and Clark line may have contributed Most samples of the Fort Union, Porcupine interpreted to refl ect derivation from Permian– Belt Supergroup–derived sediments. Blackleaf Hills, and Wasatch Formations show a rela- Pennsylvanian rocks to the west (Rapson, 1965; Formation sandstones also contain lithic grains tive increase in the proportion of quartz and a Suttner et al., 1981; DeCelles, 1986; Schwartz of siltstone and very fi ne-grained sandstone, trend from magmatic arc and mixed provenance and DeCelles, 1988). The high ratio of sedimen- which may have been cannibalized from older to quartzose recycled orogenic provenance tary lithic to total lithic fragments (Figs. 6 and 7) foreland basin deposits that were incorporated (Fig. 6). Fort Union deposits are considerably and the large proportion of detrital zircons ages into the eastward-propagating fold-and-thrust sandier than underlying units, even at distal typical of miogeoclinal strata indicate a prov- belt. Rocks of the Intermontane terrane and locations (Fig. 4). The increase in the sandstone/ enance dominated by thrust-imbricated Paleo- Omineca belt continued to provide sediment as shale ratio may be related to deeper exhumation zoic rocks. Uncertainty exists regarding the they were carried passively(?) eastward above of Belt Supergroup rocks in the LEDSH thrust location of this early fold-and-thrust belt, but a structurally lower thrust faults. The youngest sheets. This effect can also explain increased possibility is that the locus of deformation was population of zircon grains, which extends up to quartz content and low-grade metamorphic in the region occupied today by the Omineca the mid-Cenomanian (Fig. 9), refl ects syn depo- clasts derived from Belt Supergroup quartz- belt or along the western fl ank of the Purcell sitional volcanism and agrees with published ite and argillite. The increase in quartz content anticlinorium, prior to the involvement of Belt stratigraphic ages (Cobban and Kennedy, 1989; could also be explained by a longer transport Supergroup rocks in the thrust belt. Middle Dyman et al., 1996). distance, insofar as these units were sampled Juras sic to Lower Cretaceous zircons originated The late Cenomanian–mid-Santonian black at distal positions in the basin. The uppermost in the magmatic arc to the west. shales provided little information in terms of Wasatch Formation sample has a strong vol- The upper part of the Kootenai Formation detrital provenance, but active volcanism in the canic imprint, as seen in frequent bentonite beds contains signifi cant amounts of plagioclase and magmatic arc to the west may be inferred from in outcrops. Conglomerate clast counts from volcanic lithic grains, and seems to mark the the abundant tuffs and plagioclase-rich sand- this unit support a source terrane dominated by beginning of a change in provenance. The high stone beds. Belt Supergroup and volcanic rocks. infl ux of volcanic material is also registered in The Upper Cretaceous Telegraph Creek, In general, the detrital zircon probability the detrital zircons, indicating dominant sources Virgelle, Two Medicine, and Bearpaw-Horse- plot (Fig. 9) shows a large proportion of grains in the magmatic arc and the Intermontane belt. thief Formations have similar provenance from derived from the magmatic arc, particularly in This change is also recorded along the Alberta thrust sheets involving Belt Supergroup and rocks younger than the lower Kootenai Forma- foreland basin with an abrupt appearance of miogeoclinal rocks, plus cannibalized older tion. Magmatic fl ux curves calculated along the juvenile material documented by petrography, foreland basin deposits. Possible major thrust Coast Mountain batholith in southern Canada zircon geochronology, and isotope and trace- systems active at the time of deposition of these (Gehrels et al., 2009) exhibit a marked fl are-up element geochemistry (Potocki and Hutcheon, units include the Moyie, Snowshoe, Libby, during the 120–78 Ma interval (Fig. 9), coeval 1992; Ross et al., 2005). Pinkham, and other associated thrusts. High- with deposition of the Upper Kootenai Forma- Albian-Cenomanian samples of the Black- lands produced by slip along faults in the Lewis tion through Lower Two Medicine Formation. leaf Formation show an abrupt increase in and Clark system, thrust faults in the western The correlation of the Jurassic arc fl are-up is low-grade metamorphic lithic fragments (Figs. part of the Helena salient, and the Lombard not as striking, but still visible. The magmatic 6, 7, and 8), possibly refl ecting displacement thrust (Lageson et al., 2001) also may have con- lull in the Coast Mountains batholith during the on thrust systems involving Belt Supergroup tributed sediment. Detrital zircons of the Two 78–55 Ma period is not refl ected in detrital zir- metasedimentary rocks in the hinterland and Medicine Formation were derived from Creta- cons of the St. Mary River Formation, which are progressive exhumation of the Omineca belt. ceous volcanic centers, with contributions from still dominated by young grains. However, these Depositional age zircons from the Blackleaf miogeoclinal and Belt Supergroup strata. grains had probable origin in volcanism associ- Formation indicate coeval arc volcanism. Zir- Displacement along the LEDSH thrust sys- ated to the Idaho and Boulder batholiths, active cons with ages in the range 1639–1789 Ma were tem, the dominant structure in the frontal part from Santonian to early Eocene, and Campanian likely derived from rocks in the miogeocline or of the thrust belt, has been bracketed between time, respectively (e.g., Lageson et al., 2001). Belt Supergroup (Link et al., 2007). Pebble and ca. 74 and 59 Ma (Hoffman et al., 1976; Sears, cobble conglomerates in the Vaughn Member 2001; Osadetz et al., 2004). Major slip on the SUBSIDENCE HISTORY contain clasts of quartz, chert, quartzite, silici- LEDSH system commenced during deposi- fi ed carbonate, and igneous rocks that Mudge tion of the Bearpaw-Horsethief Formation. An Middle Jurassic–Danian strata in northwest- (1972) interpreted as being derived from the increase in low-grade metamorphic grains in ern Montana were decompacted and back- Belt Supergroup. Pre–mid-Cenomanian dis- sandstones of the lower part of the St. Mary stripped according to the methods described

524 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. indicate a continental condensed section that Formations. However, the total thickness of (1990) using stratigraphic sections measured is transitional into the interval 151–127 Ma, these units is difﬁ cult to estimate in northwest- during this study. Short-term eustatic variations marking a regional unconformity between Mor- ern Montana because of incomplete sections and were not taken into account during the back- rison and Kootenai strata. In places, this uncon- a paucity of stratigraphic markers (Mudge et al., stripping because of uncertainties in the existing formity represents even longer periods of time, 1982). This interval thickens to ~2 km toward eustatic models (Allen and Allen, 2005). The and basal Kootenai deposits accumulated in southern Alberta. Additional strata equivalent to resulting decompacted and tectonic subsidence paleovalleys incising Ellis Group strata. From the Fort Union and Wasatch Formations were curves (Fig. 10) update previous work (Cross, ca. 127 Ma onward, subsidence rates increased probably deposited in the frontal thrust belt and 1986; Gillespie and Heller, 1995) by incorporat- rapidly, a trend that is typical of classic fore- proximal foredeep, but subsequent erosion has ing new stratigraphic and geochronologic data. deep deposits (Angevine et al., 1990; DeCelles removed all traces of these deposits. The thick- The onset of regional subsidence is recorded and Giles, 1996). ness of exhumed and removed Cenozoic syn- at ca. 171 Ma. Subsidence rates were relatively Estimated subsidence for the youngest inter- orogenic strata in Alberta has been estimated in slow in this part of the basin during deposition val is conservative. A thickness of 800 m was the order of 2–4 km (Beaumont, 1981; Hardebol of the Ellis Group and Morrison Formation. used for the 73–65 Ma interval, during deposi- et al., 2009). Paleo sols in the uppermost Morrison Formation tion of the St. Mary River and Willow Creek The initial episode of slow subsidence, from ca. 171 to ca. 151 Ma, and the ensuing latest Jurassic–Neocomian period of erosion or non- Age (Ma) deposition were also depicted in subsidence 170 150 130 110 90 70 curves published by Cross (1986) and Gillespie and Heller (1995). These authors, however, F considered the onset of deposition in a fore-

0.5 - land basin setting to coincide with the marked

C )

W increase in subsidence rate at ca. 110–100 Ma. m

k BF Our data, however, indicate westerly prov- ( 1

n enance starting as early as Middle Jurassic. An

i t

MF c

T alternative view is that the sigmoidal shape of a p 1.5 the subsidence curve can be interpreted as the m VTF

o result of progressive stacking of foreland basin c

S e

R depozones in front of a migrating thrust-belt D 2 M load (DeCelles and Giles, 1996). The period BF 171–151 Ma, during deposition of the Ellis KF Group and Morrison Formation, may represent 2.5 MF EG initial, moderate subsidence in distal regions of the foreland basin system, possibly inboard of the ﬂ exural forebulge. The paleosols at the 170 150 130 110 90 70 top of the Morrison Formation and the unconformity below the Kootenai Formation would represent condensed deposition and erosion 0.5 asso ciated with passage of the forebulge. Fi- nally, post–127 Ma deposits represent the fore-

Tect )

m deep depozone. The principal argument for a onic k 1

( later Cretaceous onset of foreland basin devel- e

c opment in this region is based on the assump-

n e

d 1.5 tion that foreland basin sediments are conﬁ ned

i Total s

b to a relatively thick moat of ﬂ exural subsidence u

S directly adjacent to the thrust-belt load (Gil- 2 lespie and Heller, 1995). Ellis and Morrison

F Formation deposits do not thicken westward as

2.5 S expected for foredeep deposits. However, sedi-

C T ment derived from the thrust belt could have

W EG MF KF BF MRS V been deposited in distal regions on top of the Figure 10. Decompacted thicknesses, and total and tectonic sub- forebulge as well as in the back-bulge depozone sidence curves for Bajocian–Maastrichtian deposits of northwest- (e.g., Horton and DeCelles, 1997; Yu and Chou, ern Montana. Paleocene and younger deposits are not included due 2001; Roddaz et al., 2005; Chase et al., 2009). to postdepositional erosional removal and unknown original thick- The biggest potential problem with our in- ness. EG—Ellis Group, MF—Morrison Formation, KF—Kootenai terpretation is the absence of a Middle to Late Formation, BF—Blackleaf Formation, MRS—Marias River Shale, Jurassic foredeep. However, palinspastic resto- VTF—Telegraph Creek and Virgelle Formations, TMF—Two Medi- ration of the major thrusts east of the Kootenai cine Formation, BF—Bearpaw Shale and Horsethief Formation, arc at the latitude of northwestern Montana WC-SMR F—St. Mary River and Willow Creek Formations. (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 The basin geometry and paleogeography of thrust system in Montana. In southern Canada, Late Jurassic foredeep and forebulge deposits. Jurassic deposits in Montana were complicated Jurassic foredeep deposits are preserved along A similar situation has been described along the by the presence of preexisting basement highs major synclines. A good example is the Fernie Utah-Idaho segment of the Late Jurassic Cor- of the Sweetgrass and South Arches and the “basin,” where a thick section of Jurassic– dillera (Royse, 1993; DeCelles, 2004). “Belt Island” complex (Cobban, 1945; Carlson, Cretaceous strata is exposed in a major synform 1968; Suttner, 1969; Suttner et al., 1981; Peter- in the hanging wall of the Lewis thrust. BASIN EVOLUTION AND son, 1981; Parcell and Williams, 2005) (Fig. 11). A possible explanation for the paleosols in TECTONIC IMPLICATIONS A similar situation was shown by Demko et al. the Morrison Formation and the major uncon- (2004) for the earliest stages of deposition of the formity that separates the Jurassic from the An ~120 m.y. coherent story of Cordilleran Morrison Formation in the central part of the U.S. Lower Cretaceous strata is decreased accom- foreland basin evolution spans from Middle Cordilleran foreland basin, where highs of the modation associated with eastward migration Jurassic to Eocene time in western Montana Ancestral Rockies and Front Range partitioned of the fl exural forebulge. Forebulges migrate (Figs. 11 and 12). The North American conti- the back-bulge depozone. A further complica- at rates equal to the sum of thrust belt propa- nent started converging with offshore subduction in Montana was subsidence associated with gation plus shortening (DeCelles and DeCelles, tion zones at ca. 185 Ma (e.g., Monger and the Williston Basin, which interfered with distal 2001), and deposition along them is character- Price, 2002; Evenchick et al., 2007). The Inter- back-bulge subsidence. Additional subsidence ized by condensed sections when suffi cient montane terranes accreted to North America during the Middle and Late Jurassic could have sediment is available to bury the forebulge, or during the Middle Jurassic (e.g., Monger et al., been driven by dynamic coupling with the upper net erosion occurs in underfi lled settings. The 1982; Coney and Evenchick, 1994; Murphy mantle above low-angle subducting oceanic temporal magnitude of this stratigraphic hiatus et al., 1995; Dickinson, 2004; Colpron et al., lithosphere. This mechanism has been proposed in northwestern Montana is ~24 m.y., a typical 2007; Dorsey and LaMaskin, 2007; Ricketts, to explain accumulations of Jurassic strata be- value in many foreland basins worldwide. This 2008), and the Insular superterrane began to ac- yond the wavelength of the fl exural foredeep in unconformity has been recognized through- crete soon after (e.g., Dickinson, 2004; Colpron the central part of the U.S. Cordilleran foreland out the U.S. Cordilleran foreland basin from et al., 2007; Ricketts, 2008) (Fig. 12). Regional basin (Currie, 1998), and again during the Late southwestern Montana to southern Utah and subsidence in the foreland commenced during Cretaceous (Cross, 1986; Mitrovica et al., 1989; is partly attributed to migration of the fl exural the Bajocian (ca. 170 Ma), when marine depos- Liu and Nummedal, 2004). wave (DeCelles, 2004). Currie (1998) noted its of the Ellis Group began to accumulate in The coeval Jurassic foredeep depozone that that the unconformity extends far eastward of the distal retroarc region. Although provenance should have existed to the west was removed by the most eastward point of forebulge migration, information from the Sawtooth Formation is exhumation and erosion in northwestern Mon- however, and suggested that it resulted from the not conclusive, it suggests derivation from de- tana, once the major structures involving Belt cessation of dynamic subsidence, enhanced by formed strata of the distal miogeoclinal wedge Supergroup rocks in the hinterland became ac- migration of the forebulge. Other authors have and the Kootenay terrane. The Omineca belt in tive. Virtually no Mesozoic strata are preserved suggested that this unconformity indicates a re- southern Canada contains compressive struc- in the fold-and-thrust belt west of the LEDSH laxation of orogenic stresses (e.g., Poulton et al., tures 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 Bajo- Figure 11. Simplifi ed maps showing a summarized evolution of the foreland basin system in cian time also has been inferred for southern northwest Montana, based on a number of sources discussed in the text and data from this Canada (Stronach, 1984). Detrital zircon ages paper. (A) Late Jurassic, showing the potential confi guration during deposition of the Mor- from Oxfordian–Kimmeridgian sandstones of rison Formation. The initial foreland basin was established earlier during the Bajocian, the Swift and Morrison Formations provide with deposition of the Ellis Group. Area enclosed by dashed line in southeast corner of map unequivocal proof that sediments were derived shows approximate position of the Belt Island during the Middle Jurassic (from Parcell from the eastern part of the Intermontane belt. and Williams, 2005). The position of the forebulge is conjectural. Structure of the Kootenay The slow subsidence rates (Fig. 10) and the re- and Quesnell terranes is not interpreted. (B) Aptian–early Albian, during deposition of gionally tabular geometry of these units suggest the Kootenai Formation. (C) Albian–Santonian, during deposition of the Colorado Group. deposition in a back-bulge depozone. Map represents dominant environments during this time; coastal positions varied during Ward and Sears (2007) also proposed that a this time. (D) Campanian–early Eocene, during deposition of the Two Medicine–Wasatch foreland basin was active in this region as early interval. Map does not display the late Campanian marine deposits of the Bearpaw Shale as Bajocian time. However, they attributed the and Horsethief Sandstone. Major thrusts show possible location before pre-Cenozoic ex- basal Jurassic unconformity to forebulge devel- tension. LEDSH thrust system shows its potential pre-erosion position. Sawtooth Range opment, which implies that overlying Middle is represented by a single thrust in the map, but includes a complexly deformed imbri- and Upper Jurassic strata would have accumu- cated system. Forebulge represents its location during the mid-Campanian, although it lated in a foredeep depozone. This interpretation actively migrated during this time (Catuneanu, 2004). In all fi gures, the locations of the is inconsistent with the subsidence history and early Eocene thrust belt front and intraforeland highs are shown as a spatial reference. tabular geometry of these units, and it provides Location of thrusts in dotted lines is conjectural; dashed lines indicate incipient move- no explanation for the >20-m.y.-duration dis- ment. Only prevalent sedimentary environments are shown. Thrusts key: SM—St. Mary, conformity at the top of the Morrison Formation. HL—Hall Lake, M—Moyie, HC—Hawley Creek, CA—Cabin, S—Snowshoe, L—Libby, This unconformity stretches into southern Utah, P—Pinkham, WW—Wigwam-Whitefi sh, LEDSH—Lewis-Eldorado-Steinbach-Hoadley, indicating that it had a regional geo dynamic, Lo-E—Lombard-Eldorado, SR—Sawtooth Range imbricate. Lewis and Clark strike-slip rather than local, origin. system is simplifi ed as a single line.

526 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 CANADA 49°N ? Williston Basin Conjectural Sweetgrass arch U.S.A. frontal part Libby of early Forebulge Broken Kevin Sunburst Deformed fold-thrust back bulge belt Spokane dome South arch miogeocline Foredeep ?? Additional Helena dynamic subsidence? Kootenay WA MT and Quesnell 46°N A 100 km ID Late Oxfordian-Kimmeridgian

116°W 110°W

Sweetgrass CANADA 49°N arch U.S.A. Williston Basin Libby Forebulge Deformed SM Kevin (controlled Sunburst Back bulge HL by preexisting dome structures) South miogeoclineSpokane arch M Foredeep Helena Kootenay Forebulge and Quesnell HC 46°N B 100 km Aptian-Early Albian

116°W 110°W CANADA Forebulge 49°N U.S.A. Williston Basin Libby SM Foredeep Back bulge HL M S Spokane L

Helena Hinterland Lewis not shown and Clark Line Albian-Santonian Late Albian-early Cenomanian 46°N 100 km CA continental deposits of the Vaughn C Member not displayed on map.

116°W 110°W Forebulge KEY S CANADA 49°N Fluvial and alluvial WW U.S.A. M LEDSH deposits Foredeep Estuarine deposits H SR L P Lacustrine deposits ??

Coastal deposits Lewis and Clark Line 100 km Marine deposits

Helena Lo-E Campanian-Early Eocene 46°N Sediment dispersal Marine deposits of the Bearpaw direction D Idaho-Boulder batholiths Sea are not displayed in map.

Geological Society of America Bulletin, March/April 2011 527

Fuentes et al.

90 110 150 130 70 170 not (Ma) Age

calculated Paleogene subsidence km 1

f e 0.5 TECTONIC SUBSIDENCE asin system asin system

16 r e n C a i L k l

d n a s i w e L g n o l a

t n e m e o m n i v a M X, P 7,13, 15,17 ?

S.R./ Foothills

. S . T

H S D E

L ? X, P T, 7,8,9, 10,11, 12,13, 14,17

. S . T W W P L ? ?

? t s o u r h t e w o n h S s X 6 references therein

1) Evenchick et al., 2007 and 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 1994 Yin, 7) et al., 1976 Hoffman 8) Schmidt, 1978 9) Whipple et al., 1987 10) Sears, 2001 van der Pluijm et al., 2001, 2006 11) 12) Osadetz et al., 2004 13) Harlan et al., 2005 14) Hardebol et al., 2009 15) Schmidt, 1972 et al., 1990 16) Wallace 17) This work

t s u r h t e i y o

M X 5 P 4,17 X 6 ? ?

s s t r e h M t a k . a t y r L u S a / l l H X, P 2,3,4, 17

AGES OF THRUSTING

e c n a v d a e g l e r o F u b

l r e i t d n n e i a n h i l c n o e g o i m d e m r o f e

D P 17 ?

e v s t e a n n m o i r t o f p e d e i t l u l M

s t l e b a c e / n e i n m a t O n o m r e t n I

e t n l e b o t n n I r a e t m

e t r r e a r p r u s e n u s n I a l TERRANE ACCRETION sh-Wigwam thrusts system; LEDSH T.S.—Lewis-Eldorado-Steinbach-Hoadley thrust system; S.R.— T.S.—Lewis-Eldorado-Steinbach-Hoadley thrusts system; LEDSH sh-Wigwam Virgelle Swift Marias River Shale Rierdon Telegraph Creek Telegraph Kootenai Morrison Wasatch Blackleaf Sawtooth Bearpaw/Horsethief

Willow Creek FORMATION Fort Union (P. Hills) Fort Union (P.

St. Mary River Two Medicine Two o d a r o l o C E s i l l Montana GP. ? LITHOLOGY ssion tracks). ssion Figure 12. Stratigraphic chart, major deformation events in the fold-and-thrust belt, and tectonic subsidence of the foreland b deformation events in the fold-and-thrust belt, and tectonic subsidence of foreland 12. Stratigraphic chart, major Figure at the latitude of northern Montana. Sources of data for timing of thrust movements are numbered and given in diagram. Timing o Timing and given in diagram. numbered timing of thrust movements are of data for at the latitude of northern Montana. Sources terrane accretion and major episodes of metamorphism and exhumation in hinterland regions is discussed in the text. SLPWW T.S.— is discussed in the text. SLPWW episodes of metamorphism and exhumation in hinterland regions and major terrane accretion Snowshoe-Libby-Pinkham-Whiteﬁ ﬁ Sawtooth Range. Time constraints of thrust systems: X—crosscutting relationships; P—provenance data; T—thermochronology (apatit T—thermochronology data; P—provenance relationships; constraints of thrust systems: X—crosscutting Time Sawtooth Range.

T T

B S B B B S Y H A A V K A C D C C C O M

) E ( E L D D I M E T A L E T A L Y L R A E

C O . E L A P

. O E

S U O E C A T E R C C I S S A R U J

A P E E L E G N AGE O 60 80 140 100 160 180 120

528 Geological Society of America Bulletin, March/April 2011 Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.

1994) and/or isostatic rebound (McMechan and shale basin was not confi ned to a narrow fl ex- be explained by the following: (1) the bulk of Thompson, 1993). These last two explanations, ural foredeep (e.g., McMechan and Thompson, material that was initially eroded in the hang- however, are inconsistent with the abundant evi- 1993), and, in any case, no evidence from the ing wall of the Lewis thrust was fi ne-grained dence for ongoing crustal shortening and thick- Cordilleran hinterland exists for a long-duration or unconsolidated foreland basin deposits, not ening in the Cordilleran hinterland (DeCelles, episode of “orogenic quiescence” (Evenchick prone to generating gravel; (2) coarse alluvial 2004). An episode of global sea-level fall peak- et al., 2007). The major Cenomanian–Turonian fans were restricted to the most proximal areas ing during the Valanginian may have contrib- transgression (McDonough and Cross, 1991), in front of the Lewis thrust, where they were uted to development of this unconformity. In coupled with continuing creation of accom- vulnerable to erosion during continuing dis- our view, a combination of forebulge migration, modation by fl exural subsidence, seems to have placement on the Lewis and underlying thrusts eustatic sea-level fall, and possibly decreased controlled the shale deposition event. Additional in the Sawtooth Range duplex; and (3) coarse- dynamic subsidence provides a reasonable ex- dynamic subsidence may have increased basin grained sediment accumulation in the foreland planation for the regional and temporal extent of accommodation on a regional level (Liu and basin was highly localized along the front of the the basal Cretaceous unconformity. Nummedal, 2004). Cordilleran thrust belt and was partly controlled The model of early foreland basin develop- The late Santonian–early Campanian interval by local across-strike structural discontinuities ment presented here is similar to models pro- was marked by a strongly regressive system, (Lawton et al., 1994). posed further south in the United States, in with rapid, relatively coarse clastic sedimenta- The fi nal, Paleocene–early Ypresian record of which the Morrison Formation is interpreted tion (Figs. 4 and 10). Thrust systems possibly thrust belt shortening and synorogenic sedimen- as back-bulge deposits, and the unconformity active during this period include the Moyie, tation in the foreland is preserved in erosional at its top is attributed to the migration of the Snowshoe, Libby, Pinkham, Whitefi sh, and remnants of the Fort Union and Wasatch Forma- forebulge with a component of dynamic uplift Wigwam. The geometry and shortening on tions in the distal foreland. Regional extensional owing to changes in the angle of the subduct- these structures are poorly known owing to faulting in the fold-and-thrust belt commenced ing plate (e.g., Currie, 1998; DeCelles, 2004). postorogenic normal faulting and a widespread during middle Eocene time (Constenius, 1996) In Montana, however, deposition in a distal cover of Cenozoic deposits. Concurrently, dur- and coincided with low-angle detachment fault- foreland setting seems to have started during ing the Cenomanian–Campanian interval, the ing, growth of metamorphic core complexes, the Middle Jurassic, as discussed already. The Lewis and Clark strike-slip system was active, and regional-scale magmatism. Coarse-grained, aerially extensive distribution of Jurassic dis- and foreland basin deposits north and south proximal foredeep deposits were removed by tal foreland basin strata in the western interior of it show differences in thickness and facies uplift and erosion during postglacial isostatic United States terminates near the international (Wallace et al., 1990). rebound, erosion-driven isostatic rebound of border, and in Canada, these deposits are re- The marine shales of the Bearpaw Forma- the thrust belt (Sears, 2001), or exhumation stricted to a relatively narrow strip running tion represent the last marine inundation of the in response to generalized extension since the approximately parallel to the Alberta–British foreland basin. The Bearpaw Sea was confi ned middle Eocene. Columbia border (e.g., Carlson, 1968; Miall mainly to Montana and the Canadian part of the et al., 2008, and references therein). basin (Miall et al., 2008). Bearpaw-Horsethief CONCLUSIONS The late Barremian(?)–early Aptian Kootenai deposition was roughly coincident with the on- Formation is the fi rst unit in the foreland that set of major slip along the LEDSH thrust system (1) Regional subsidence in the northwestern consistently thickens westward. Moreover, the at ca. 75 Ma (Sears, 2001; Osadetz et al., 2004). Montana foreland commenced during the Bajo- subsidence curve begins to exhibit a convex- By early Campanian time, eastward propa- cian (ca. 170 Ma), when marine deposits of the upward pattern characteristic of foredeeps at gation of the thrust belt had driven migration Ellis Group began to accumulate in the distal this time. By Albian time, the fold-and-thrust of the forebulge, and the foredeep-forebulge retro arc region. The onset of subsidence and belt had propagated to the east and was involv- hinge line was located ~150 km east of the cur- lithic-rich clastic sediment accumulation indicate ing rocks of the Belt Supergroup, as indicated rent thrust front (Catuneanu et al., 2000; Miall development of the Cordilleran orogenic belt to by sandstone provenance data, detrital zircons et al., 2008). By late Campanian, and coeval the west. Detrital zircon ages from Oxfordian– in the Blackleaf Formation with ages typical with the onset of major slip along the Lewis Kimmeridgian sandstones of the Swift and Mor- of Belt Supergroup strata, and by crosscutting thrust, the forebulge had migrated 200–250 km rison Formations provide unequivocal proof that relationships in thrust sheets in the hinterland. farther eastward and was located in eastern sediments were derived from the eastern part of Coeval with deformation in the hinterland of Montana. However, the position of the fore- the Intermontane belt. The slow subsidence rates the Purcell anticlinorium, an episode of long- bulge in this region is diffi cult to map during and regionally tabular geometry of these units lasting marine transgression was being recorded much of the Cretaceous owing to the magnitude suggest deposition in a back-bulge depozone. in ~400 m of offshore marine deposits of the of dynamic loading (Miall et al., 2008). (2) Jurassic deposits are capped by a zone Marias River Shale. This episode of sedimenta- High rates of sedimentation in proximity to of paleosols in the upper Morrison Formation, tion was widespread along the Western Interior the advancing thrust front are evident in the and/or are truncated by a regional unconformity Basin (Miall et al., 2008), and contradictory St. Mary River and Willow Creek Formations that probably represents most of Tithonian– opinions exist regarding its cause. Some authors (Fig. 10). Paradoxically, these units lack coarse- Neocomian time. This unconformity possibly have suggested that these shales refl ect a period grained, conglomeratic facies. The absence resulted from the combined effects of eastward of orogenic quiescence (Price and Mountjoy, of coarse-grained facies within 20 km of the forebulge migration, eustatic sea-level fall, 1970; Porter et al., 1982; Tankard, 1986). In present erosional trace of the Lewis thrust lead and decreased dynamic subsidence owing to particular, Tankard (1986) suggested that shale McMannis (1965) to conclude that the main changes in the angle of the subducting plate. deposition was due to postloading viscous phase of shortening on the Lewis thrust post- (3) The preserved Barremian(?)–early Eocene lithospheric relaxation, which would produce dated deposition of the Willow Creek Forma- succession was deposited in a foredeep depo- a narrower, but deeper, basin. However, the tion. Alternatively, the lack of coarse facies may zone and contains a record of the development

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Prichard Formation (Middle Proterozoic) and the Early system commenced its episode of major slip, Burchfi el, B.C., Cowan, D.S., and Davis, G.A., 1992, Tec- Development of the Belt Basin, Washington, Idaho, which is recorded by rapid sediment accumula- tonic overview of the Cordilleran orogen in the western and Montana: U.S. Geological Survey Professional U.S., in Burchfi el, B.C., Lipman, P.W., and Zoback, Paper 1490, 80 p. tion rates in the foreland basin. As a result of M.L., eds., The Cordilleran Orogen: Conterminous Cross, T.A., 1986, Tectonic controls of foreland basin sub- the structural elevation gain of this megathrust U.S.: Boulder, Colorado, Geological Society of Amer- sidence and Laramide style deformation, western ica, The Geology of North America, v. G-3, p. 407–480. United States, in Allen, P.A., and Homewood, P., eds., sheet, earlier foreland basin deposits were can- Cant, D.J., and Stockmal, G.S., 1989, The Alberta foreland Foreland basins: International Association of Sedimen- nibalized and redeposited to the east. The strati- basin—Relationship between stratigraphy and Cor- tologists Special Publication 8, p. 15–39. graphic record of terminal thrust belt shortening dilleran terrane-accretion events: Canadian Journal of Currie, B.S., 1998, Jurassic–Cretaceous Evolution of the Earth Sciences, v. 26, no. 10, p. 1964–1975. Central Cordilleran Foreland Basin System [Ph.D. during the Paleocene–early Ypresian time is Carlson, C.E., 1968, Triassic–Jurassic of Alberta, Saskatche- thesis ]: Tucson, University of Arizona, 239 p. preserved in distal erosional remnants of the wan, Manitoba, Montana, and North Dakota: The Currie, B.S., 2002, Structural confi guration of the Early Cre- Fort Union and Wasatch Formations. Equiva- American Association of Petroleum Geologists Bul- taceous Cordilleran foreland-basin system and Sevier letin, v. 52, p. 1969–1983. thrust belt, Utah and Colorado: The Journal of Geol- lent, more proximal deposits were erosionally Catuneanu, O. Sweet, A.R., and Miall, A.D., 2000, Re- ogy, v. 110, p. 697–718, doi: 10.1086/342626. removed, and no vestige of a wedge-top depo- ciprocal stratigraphy of the Campanian-Paleocene DeCelles, P.G., 1986, Sedimentation in a tectonically Western Interior of North America: Sedimentary partitioned, nonmarine foreland basin—The Lower zone exists today in northwestern Montana. Geology, v. 134, no. 3–4, p. 235–255, doi: 10.1016/ Cretaceous Kootenai Formation, southwestern Mon- S0037-0738(00)00045-2. tana: Geological Society of America Bulletin, v. 97, ACKNOWLEDGMENTS Catuneanu, O., 2004, Retroarc foreland systems—Evo- no. 8, p. 911–931, doi: 10.1130/0016-7606(1986)97 lution through time: Journal of African Earth <911:SIATPN>2.0.CO;2. Funding for this work was provided by Exxon- Sciences, v. 38, no. 3, p. 225–242, doi: 10.1016/ DeCelles, P.G., 1994, Late Cretaceous–Paleocene syn- Mobil, with additional grants from ChevronTexaco, j.jafrearsci.2004.01.004. orogenic sedimentation and kinematic history of the The American Association of Petroleum Geologists, Catuneanu, O., and Sweet, A.R., 1999, Maastrichtian–Paleo- Sevier thrust belt, northeast Utah and southwest Wyo- and the Geological Society of America. A Fulbright cene foreland-basin stratigraphies, western Canada: A ming: Geological Society of America Bulletin, v. 106, scholarship was granted to Fuentes. Charles Park, reciprocal sequence architecture: Canadian Journal of no. 1, p. 32–56, doi: 10.1130/0016-7606(1994)106 Earth Sciences, v. 36, no. 5, p. 685–703, doi: 10.1139/ <0032:LCPSSA>2.3.CO;2. David Gingrich, Ylenia Almar, Erin Brenneman, and cjes-36-5-685. DeCelles, P.G., 2004, Late Jurassic to Eocene evolution of Nicole Russell assisted during fi eld work. Gerald Chase, C.G., Sussman, A.J., and Coblentz, D.D., 2009, the Cordilleran thrust belt and foreland basin system, Waanders analyzed the palynology samples. Bill Dick- Curved Andes: Geoid, forebulge, and fl exure: Litho- western USA: American Journal of Science, v. 304, inson, Ted Doughty, and Jerry Kendall provided valu- sphere, v. 1, no. 6, p. 358–363, doi: 10.1130/L67.1. no. 2, p. 105–168, doi: 10.2475/ajs.304.2.105. able comments. 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