Regional Structure and Kinematic History of the Cordilleran Fold-Thrust Belt in Northwestern Montana, USA

Home , Blackleaf Formation, Fold and thrust belt, Sawtooth Range (Idaho), Swan Range, Thrust tectonics

Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana, USA

Facundo Fuentes*, Peter G. DeCelles, and Kurt N. Constenius Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

ABSTRACT data from the foreland basin and U-Pb ages 1999). This focused work left a large region in from crosscutting intrusive rocks establish northwestern Montana relatively undocumented The Cordilleran thrust belt of northwest- a preliminary kinematic model for this seg- in terms of structure, kinematic history, and its ern Montana (United States) has received ment of the Cordilleran thrust belt. The relationship with the greater Cordilleran thrust much less attention than its counterparts emerging pattern is a relatively simple fore- belt (DeCelles, 2004). This region contains sig- in the western interior of USA and Canada. landward progression of thrusting events. nifi cant structural features that have no apparent The structure of the thrust belt in this region Most shortening in the Lewis thrust system, counterparts in other parts of the Cordilleran is well preserved and has not been strongly Sawtooth Range, and foothills occurred thrust belt. Of particular interest is the Lewis overprinted by Cenozoic extension, provid- roughly between mid-Campanian and Early thrust system, an immense thrust system that has ing an opportunity to reconstruct its geom- Eocene time (ca. 75–52 Ma), yielding a short- tectonically transported a slab of rock >7 km in etry and to relate it to the foreland basin ening rate of ~5.9 mm/yr. This pattern differs thickness more than 100 km toward the foreland system. The thrust belt in this region consists from the pattern of shortening in the better region. In contrast to the other major Precam- of a frontal part of highly deformed Paleo- known Sevier thrust belt to the south, where brian rock–carrying thrust faults in the Cordi- zoic, Mesozoic, and Paleocene sedimentary regional far-traveled Proterozoic quartzite- llera, such as the Paris, Willard, Sheep Rock, and rocks, and a western region dominated by bearing thrust sheets were mainly active Canyon Range thrusts, which formed in the inte- a >15-km-thick succession of Proterozoic during Early Cretaceous time. From Middle rior part of the thrust belt in Idaho and Utah dur- Belt Supergroup strata underlain by faults Eocene to Early Miocene time, this sector of ing the Early Cretaceous, the Lewis is a frontal of the Lewis thrust system. The frontal part the Cordillera collapsed, generating a num- thrust system that sustained motions as late as can be subdivided into the foothills and the ber of extensional depocenters. Late Paleocene–Early Eocene. However, sur- Sawtooth Range. At the surface, the foot- prisingly little was previously known regarding hills show deformed Mesozoic and Paleo- INTRODUCTION the evolution of this thrust system as recorded cene rocks; at depth, refl ection seismic data in the foreland basin stratigraphy and unraveled indicate numerous thrust faults carrying Classic studies of the Cordilleran orogenic through detailed cross-section balancing. Paleozoic strata. The Sawtooth Range, south belt in the USA and Canada were among the During the 1960s and 1970s, the frontal part from the Lewis thrust salient, is defi ned by fi rst to demonstrate modern concepts of thrust of the northwestern Montana thrust belt was steeply dipping imbricate thrusts that detach belt structure, kinematics, and foreland basin mapped by M.R. Mudge and collaborators at the basal Cambrian stratigraphic level. development (e.g., Bally et al., 1966; Armstrong, (see Mudge et al., 1982; Mudge and Earhart, The Sawtooth Range plunges northward 1968; Dahlstrom, 1970; Price and Mountjoy, 1983) under the auspices of the U.S. Geologi- beneath the Lewis thrust salient and diverges 1970; Price, 1973, 1981; Gordy et al., 1977; cal Survey (USGS), resulting in a set of 1:24,000 into a pair of independent thrust systems that Royse et al., 1975; Burchfi el and Davis, 1972, quadrangle maps and large-scale compilations. form the Flathead and Waterton duplexes in 1975; Jordan, 1981; Lamerson, 1982; Wiltschko Subsequently, 1° × 2° quadrangle maps of the Canada. The relatively minor internal defor- and Dorr, 1983). However, these and subsequent northernmost part of this segment of the thrust mation in the western part of the thrust belt studies were concentrated in two large regions: belt and the hinterland regions were compiled resulted from the great rheological strength the Sevier thrust belt of southern Nevada, Utah, by Harrison et al. (1986, 1992, 1998), based on of the Belt Supergroup rocks and initial and Wyoming, USA (e.g., Burchfiel and previous work. The region was mostly mapped high taper of the preorogenic stratigraphic Davis, 1972; Royse et al., 1975; Lamerson, just prior to the widespread acceptance of mod- wedge. A new ~145-km-long balanced cross 1982; Wiltschko and Dorr, 1983; Coogan, 1992; ern ideas about thrust belt systematics and tim- section indicates ~135 km of shortening, a Lawton, 1994; DeCelles, 1994; Mitra, 1997; ing (Bally et al., 1966; Dahlstrom, 1970; Boyer value similar to that in the southern part of Constenius et al., 2003; DeCelles and Coogan, and Elliott, 1982) and the relationships between the Canadian thrust belt. Previous work and 2006; Schelling et al., 2007; Yonkee and Weil, thrust belts and foreland basin systems (Jordan, new conventional and isotopic provenance 2010), and the frontal Canadian thrust belt in 1981; Beaumont, 1981). As a result, the early southern Alberta and British Columbia, Canada structural cross sections are not balanced (e.g., *Corresponding author, present address: Tecpetrol, (e.g., Bally et al., 1966; Gordy et al., 1977; Price, Harrison et al., 1980; Mudge, 1982; Mudge Della Paolera 299, P. 21, Buenos Aires, Argentina, 1981, 1994, 2000; Fermor and Moffat, 1992; et al., 1982; Mudge and Earhart, 1983) and vir- C1001ADA; [email protected]. McMechan and Thompson, 1993; Fermor, tually no attempt has been made to integrate the

Geosphere; October 2012; v. 8; no. 5; p. 1104–1128; doi:10.1130/GES00773.1; 15 ﬁ gures; 2 tables; 3 supplemental ﬁ gures; 1 supplemental table. Received 15 December 2011 ♦ Revision received 29 May 2012 ♦ Accepted 30 May 2012 ♦ Published online 18 September 2012

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structural geometry and kinematics with the sub- arc region; in northwestern Montana (Fig. 2), considerable thickness of Paleocene–Early sidence history and provenance of the adjacent it involved rocks of three major tectonostrati- Eocene proximal deposits was erosionally foreland basin. Subsequent published structural graphic packages on top of autochthonous removed during the Cenozoic (Hardebol et al., cross sections have been very local (e.g., Mitra, North American cratonic basement (Fig. 3). The 2009). Jurassic strata are characteristically thin 1986; Holl and Anastasio, 1992), of very low Belt Supergroup comprises a thick (>15 km) and tabular, and possibly refl ect deposition in reso lution in the frontal part of the thrust belt Proterozoic succession of clastic, carbonate, and a distal, backbulge depozone (Fuentes et al., (e.g., Fritts and Klipping, 1987; Sears, 2001, igneous rocks (Harrison, 1972; Harrison et al., 2009). These deposits are bounded above by a 2007), or unbalanced (e.g., Harrison et al., 1992). 1974), possibly deposited in an intracontinental regional-scale unconformity that may be partly Our primary objectives are to document the rift (Cressman, 1989; Price and Sears, 2000), or related to the migration of the fl exural fore- regional-scale structure and kinematic history a backarc extensional or strike-slip basin (Ross bulge in response to shortening and propaga- of the northwestern Montana thrust belt, and to and Villeneuve, 2003). On top of these deposits, tion of the thrust belt into the foreland (Fig. 4). link it with the evolution of the foreland basin. Paleozoic strata consist of shelfal carbonate and The bulk of the preserved foreland basin fi ll An almost complete record of sedimentation in relatively minor clastic rocks, deposited after was deposited in a foredeep depozone. Strata the foreland basin system, local igneous rocks the Precambrian rifting event (McMannis, 1965; of the wedge-top depozone are not preserved and crosscutting relationships, and excellent Bond et al., 1985; Poole et al., 1992). Jurassic– along the thrust belt front, but potentially cor- subsurface data provide control on the major Paleocene mainly clastic sedimentary rocks relative distal deposits represented by the Fort kinematic events in the thrust belt. We fi nd that recycled from rocks of the growing Cordilleran Union and Wasatch Formations are preserved major structural elements in northwestern Mon- orogenic belt were deposited in a foreland basin in isolated localities on the Great Plains. Start- tana were controlled by preexisting stratigraphic to the east; these strata eventually became incor- ing in Eocene time, the hinterland region was architecture and lithology, and that the kine- porated into thrust sheets in the frontal part of overprinted by extensional collapse and subse- matic history of this region is signifi cantly dif- the thrust belt. quently by Basin and Range extension (Con- ferent from that farther south in the Sevier belt. Western regions of the retroarc thrust belt stenius, 1996). are dominated by rocks of the Belt Super- REGIONAL SETTING group (Figs. 1 and 2). The leading edge of the THRUST BELT STRUCTURE structures involving Belt rocks is defi ned by The Cordilleran orogenic belt of western the Lewis thrust, and the related Eldorado and The classic southern Canadian tectonomor- North America formed during Jurassic–Eocene Steinbach thrusts to the south (Fig. 2). The phic zones of the thrust belt, characterized by time in response to convergence between Pacifi c frontal part of the thrust belt is characterized distinct topography, stratigraphy, and structural domain plates and the North American plate by strongly deformed Paleozoic and Meso- style, continue southward into Montana with (Monger et al., 1982; Burchfi el et al., 1992; zoic strata of the Sawtooth Range and foothills some minor differences. The Foothills compose Saleeby, 1992; Allmendinger, 1992; Coney structures. The Sawtooth Range is composed of the frontal part of the thrust belt and mainly and Evenchick, 1994; DeCelles, 2004; Dickin- thrust sheets detached at Cambrian to Missis- consist of strongly deformed, imbricated thrust son, 2004). In northwestern USA and Canada, sippian levels. The foothills region is character- slices of Cretaceous–lower Cenozoic strata that subduction was accompanied by accretion of ized by deformed Mesozoic strata, with blind detach from Paleozoic deposits at depth, in a fringing arcs and parautochthonous and exotic structures involving Paleozoic rocks at depth. manner similar to the Alberta Foothills (Ross, terranes (Coney et al., 1980; Monger et al., The foothills, Sawtooth Range, and the frontal 1959; Mudge, 1982). The conspicuous frontal 1982; Coney and Evenchick, 1994; Dickinson, structures involving Belt Supergroup strata are syncline along the triangle zone in Canada, i.e., 2004; Evenchick et al., 2007; Colpron et al., commonly referred to as the Montana Disturbed the Alberta syncline (Price, 1981; Jones, 1982), 2007; Ricketts, 2008). The terrane structure and Belt (Mudge, 1982). is less well represented in Montana. evolution are better understood in southwestern At the latitude of Helena, the thrust belt forms The Sawtooth Range is partially equivalent Canada than in the USA Pacific Northwest. a pronounced salient that has been interpreted to the Front Ranges of southern Canada (Price, These terranes disappear southward beneath as the result of the inversion of an eastward 1981). It is composed of closely spaced imbri- the Columbia River Basalt Group (Fig. 1), prolongation of the Belt basin, perhaps a failed cated panels of Paleozoic and Mesozoic rocks but correlative terranes are present in the Blue rift (Harrison et al., 1974; Winston, 1986; Price (McMannis, 1965; Mudge, 1982). The regional Mountains of northeastern Oregon (Dorsey and Sears, 2000; Sears and Hendrix, 2004). A detachment of the Sawtooth Range structures and Lamaskin, 2007). The current view is that complex array of strike-slip faults, known as the cuts downsection progressively to the west, the Pacifi c Northwest of the USA and adjacent Lewis and Clark line, bounds the Helena salient eventually reaching the Cambrian level (Mudge Canada share a similar early accretion history to the north, and extends westward into the and Earhart, 1983; Boyer, 1992; Harrison et al., (Dickinson, 2004). In Canada, the easternmost hinterland (Figs. 1 and 2). The Lewis and Clark 1998). Whereas the Sawtooth Range involves part of these terranes is grouped into a com- line has a long history of recurrent movement exclusively Phanerozoic rocks, in Canada fron- posite unit, the Intermontane superterrane, which since Precambrian time (Wallace et al., 1990). tal thrusts involving Belt rocks are included was accreted to the North American plate by McClelland and Oldow (2004) interpreted the within the Front Ranges. the Middle to Late Jurassic (Dickinson, 2004; Lewis and Clark line as an oblique ramp con- The dominant structure in northwestern Mon- Colpron et al., 2007). The Insular superterrane, necting basement-detached hinterland struc- tana is the Lewis thrust and the related Eldorado the westernmost element in the Cordilleran col- tures of the Shuswap complex in southern Brit- and Steinbach thrusts (Fig. 2). The hanging wall lage at these latitudes, had a long and complex ish Columbia and Washington with the frontal of this thrust system consists of several kilometers accretion history that started during the Middle Laramide uplifts in the USA. of Proterozoic and Lower Paleozoic rocks, which Jurassic (Colpron et al., 2007; Ricketts, 2008). The Middle Jurassic to Early Eocene fore- are in thrust contact with complexly deformed Under this regime of subduction and colli- land basin system (Fig. 4) preserves a thick- footwall Paleozoic and Mesozoic strata of the sion, a fold-thrust belt developed in the retro- ness of strata of as much as 2.5–3 km, but a Sawtooth Range (Mudge, 1982; Sears, 2001).

Geosphere, October 2012 1105

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141°W Alaska 120°W 115°W

c (Cache Creek)

r a N

U.S.A. Banff

t a metamorphic (Stikinia)

m core-complex N g

a Insular extension Frontal fold-thrust belt

m Superterrane 50°N

d n

a (Quesnell+Kootenay) Foreland basin

CANADA t

l USA e b terranes subduction Intermontane Libby

complex

J-K Terranes Canada subduction complex

thrust U.S.A. Forearc and 49°N L&C line Accreted Spokane Fold-

Laramide system province Foreland basin Columbia River basalts Idaho Helena batholith 46°N B 100 Km Mexico 30°N

500 km A 110°W

116°W 110°W

Sr=0.706 Hall Lake Alberta syncline Fernie St. Mary N

Priest River Complex Purcell Sweetgrass arch CANADA Purcell Moyie 49°N

Snowshoe USA Anticlinorium Kevin Williston basin

Trench Sunburst Pinkham Browning Bearpaw Mtns. dome Montana foreland basin Libby Sawtooth Newport Libby LTS fault Hope fault Range Choteau Flathead South Lewis and Cl Lake Spokane Augusta arch ark line Highwood Mtns. Alta Sask

Helena MT Lombard- Idaho

0 100km El

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

Figure 1. (A) Simpliﬁ ed tectonic map of the North America Cordi lleran KEY FIGURES 1B AND 1C: Cenozoic extensional basin fill Paleogene forearc and

orogenic system. Box shows location of map of Figure 1B. (B) Terrane s

t Cenozoic volcanic rocks subduction complex

map of northwest USA and southwest Canada (terranes based on Cretaceous subduction i Cretaceous intrusive rocks

complex s

a e Mesozoic to lower Cenozoic

l Dickin son, 2004; Colpron et al., 2007). J-K—Jurassic–Cretaceous; u

r e foreland basin deposits Insular Superterrane h

L&C—Lewis and Clark line. (C) Tectonic-index map of the fold-thrust n f Paleozoic and Mesozoic

l n Stikinia deformed sedimentary rocks

belt and foreland basin of northwestern Montana (MT) and adjacent i

t h Middle and Late Proterozoic

f Cache Creek p

areas (ID—Idaho; WY—Wyoming). Only names of major thrusts and a Belt and Windermere

c Kootenay and

rranes

termontane g Late Proterozoic Hamill Group

thrust systems are indicated. LTS—Lewis thrust system (Lewis-Eldo- C

te In Quesnell

t and Paleozoic sedimentary rocks

e Late Proterozoic e

rado-Steinbach thrusts). Thrusts in the Sawtooth Range and foothills r

b Windermere Supergroup u

g regions of Montana and Canada are schematic and not ornamented. Middle Proterozoic i

F Belt Supergroup Compiled from Alpha (1955), Mudge et al. (1982), Harrison et al. (1986, Early Proterozoic pre- 1992, 1998), Wheeler and McFeely (1991), Constenius (1996), Doughty Belt metamorphic rocks and Price (2000), Lageson et al. (2001) and Vuke et al. (2007). Figures modiﬁ ed from Fuentes et al. (2011).

1106 Geosphere, October 2012

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Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana

47°N 47°N

48°N 48°N

2°W 2°W

. u

h 11

er Cz er elds

w t

pper K- pper

′

)

Chotea 20 km 20

tooth Steinbac t

t Thrus

Lake

Greenfi

brian Belt Sgr. Belt brian

ation front ation

Figure 15-A Figure

Augusta

Mary River Fm. River Mary

retaceous rocks retaceous

illow Creek Fm. Creek illow

ajor thrust faul thrust ajor

ormal faul ormal eform

C Thrust

St. St.

Range and foothills and Range

Thrust fault (Saw fault Thrust

Major anticlin Major

(undiferentiated)

Cretaceous rocks Cretaceous

Cretaceous igneous rocks igneous Cretaceous

Paleozoic rocks Paleozoic

Paleozoic to Lower to Paleozoic

Upper Cretaceous rocks Cretaceous Upper

Cenozoic igneous rocks igneous Cenozoic

Precam

Major synclin Major

of fold-thrust bel fold-thrust of

Cenozoic halfgrabens) Cenozoic

Quaternary (covering Quaternary

slip fault (L&C Line) (L&C fault slip

Strike-slip or oblique or Strike-slip

servoi

Syncli

Augusta

Re Bynum

KEY Hoadley

n .A.

N Range

Reservoir

Gibso

U.S ANADA

C Sawtooth Figure 10-A Figure

10A. Box in southeast corner indicates location 10A. Box in southeast corner W W

ms are displayed. Box in the southeast quadrant ms are

′

3°

1 113°W Browning Thrust e t in t orado Divide Syncl aul L&C—Lewis and Clark. so indicated. Sgr—supergroup; Eld sThrus F Lewi 6 Continental Fork South Thrust s Figure Lewi t e Range Swan Faul quadrangle maps (Mudge et al., 1982; Harrison 1986, 1992, 1998) ° Swan y alient r lle

Va × 2 Swan ° Akamina Synclin ion Miss Lewis Thrust S ld ange e R t Hungry Horse Reservoi

Lak S

4°W 4°W McDona Faul 114°W 11 t Mission Faul y Flathead lle

Basin d

Flathead Va Flathea

Kishenehn Lake

Kalispell Whitefish h enc

Tr ne

Mountain s

lley Rocky System Gibb #1 Arco-Marathon

Va 5°W 15°W 15°W

Thrust 11 1 Tobacco Purcell anticlinorium

Pinkham Li Clark and Lewis Lake Koocanusa System

Libby Thrust Hope Fault System Libby

Thrust ion Snowshoe

ed geologic map of northwest Montana compiled from U.S. Geological Survey 1 ed geologic map of northwest Montana compiled from

stock

Creek

Dry

River stock River Vermil

Sylvanitee R

Anticlin Moyie Thrust System

47°N 47°N

48° 49° and the geologic map of Montana (Vuke et al., 2007). Only major faults of the Moyie, Horseshoe, Libby, and Pinkham thrust syste faults of the Moyie, Horseshoe, Libby, et al., 2007). Only major and the geologic map of Montana (Vuke 6. Box in the northern part of Sawtooth Range show location map Figure of the map indicates location Figure of Figure 15A. Red lines indicate seismic lines discussed in the text. Locations of cross sections discussed in the text are al sections discussed in the text are 15A. Red lines indicate seismic discussed in the text. Locations of cross of Figure Figure 2. Simpliﬁ Figure

Geosphere, October 2012 1107

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Upper section is eroded )

Figure 3. Simplified stratigraphy of the e

LITHOSTRATIGRAPHICLITHOSTRATIGRAPHIC m

C (

e I G thrust belt frontal part in northwestern UNITUNIT G

S I

O ONIC

S T Montana. Lithostratigraphic units after e

E T

a C

Willow Creek Fm. N Mudge (1972). Main detachment horizons E

SETTIN S

TECT T

(lower part) C

are indicated by arrows. Note change in I H

scale for Belt Supergroup. Logs: continen- T tal; bivalves: shallow marine; ammonites: St. Mary River Fm.

deep marine deposits. m

Horsethief Sandstone e t

. Bearpaw Shale s

r y

G s

e n

p i

a s

t U Two Medicine Fm.

In this paper, we refer to the frontal thrusts car- a

n b

rying Belt Supergroup rocks as the Lewis thrust o M d

system; when referring to individual thrusts n

Virgelle Sandstone a

(Lewis, Eldorado, and Steinbach thrusts), we l e

Telegraph Creek Fm. r

use their names as they were mapped by USGS o

f f

geologists (e.g., Mudge and Earhart, 1980). s o

2–3 (preserved)

To the west, Belt Supergroup rocks are r p e Marias River Shale

G deformed by broad open folds and widely e

a e

e d

spaced thrusts, in contrast to the foothills and the r a e

r Sawtooth Range. This region contains a series r

o o

o of thrust systems with poorly known geometry F because of postorogenic extensional faulting C Blackleaf Fm.

and Cenozoic sedimentary cover. Thrust sys- e

tems of this region include the Pinkham, Libby, L Snowshoe, and Moyie (Fig. 2). Mid-Cenozoic extensional collapse of the orogenic belt and Kootenai Fm.

) subsequent Basin and Range deformation pro- r

p ?

duced a number of half-grabens in the interior s Morrison Fm. g

b Swift Fm. z region of the thrust belt (Fig. 2). l

p i Rierdon Fm.

J Figure 5 shows a simpliﬁ ed crustal-scale cross e

M B Sawtooth Fm. d

( .

n section at the latitude of the Lewis thrust salient, r

i south of the international border (Constenius, G Castle Reef

p n

i Dolomite 1996). The internal structure of the thrust belt o

s s

s i

s Allan Mountain

in Paleozoic and Mesozoic rocks in the frontal d

s a part of the section is not displayed because of i Limestone

M M

scale limitations. As shown in this ﬁ gure and )

n Three Forks Fm.

mentioned above, the dominant feature of the s

i thrust belt is the thrust sheet of Belt Supergroup - Jefferson Fm.

d rocks in the Lewis thrust system hanging wall. z

1–2

D Maywood Fm.

The giant Purcell anticlinorium possibly repre- l

( sents a fault-bend fold developed over a major Devils Glen Dolomite a

P ramp separating autochthonous from alloch- Switchback Shale

)

r Steamboat Limestone

thonous Belt Supergroup rocks. Western thrust p a Pentagon Shale

- systems have relatively minor slip, and were b Pagoda Limestone

m Dearborn Limestone severely disrupted by Cenozoic extension. The d

d i Damnation Limestone

C thrust belt basal detachment was reactivated M ( Gordon Shale during Cenozoic extension, and listric normal Flathead Sandstone faults fed slip down to it (Constenius, 1996). Change in scale Regional detachment faulting and metamorphic Garnet Range Fm.

d f . Mc Namara Fm.

i e

core complex development to the west produced r

r n f

i i

l l

g Bonner Quartzite l

u s

p r

basement upwarping in the hinterland (Fillipone i a

o Mount Shields Fm.

m t

s e

o i

s b

i Shepard Fm. n

and Yin, 1994; Doughty and Price, 2000; John- p d

c u

n Snowslip Fm.

n a

son, 2006). i S

a t

Piegan. Gr. Helena Fm. d

r -

t n

l k s Empire Fm.

5–>15 s

e e

a c

r Spokane Fm. a

Balanced Cross Section v

B a

a b

Greyson Fm. r

R t

r o

Prichard Fm. n

C o We constructed a new regional balanced cross (“Lower Belt”) I section to illustrate the geometry of the thrust North American crystalline basement belt in northwestern Montana. The section is

1108 Geosphere, October 2012

AGE LITHOLOGYGR FORMATION DOMINANT APPROX. DEPOSITIONAL THICKNESS

C (m)

) ENVIRONMENTS

O N Y

( E

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 Offshore marine 350-450 d River

C r

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 carbonate). 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 4. Simpliﬁ ed Jurassic–Early Eocene (EOC.) stratigraphy of northwestern Montana (MT) (modiﬁ ed from Fuentes et al., 2011). P. Hills—Porcupine Hills.

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R R′ SW NE Lightning Purcell anticlinorium Rocky Kishenehn km Hope Creek Moyie Dry Creek Arco-Marathon Pinkham Mountain South basin Flathead Lewis fault stock thrust stock #1 Gibbs thrust Trench Fork basin fault thrust 0

Paleozoic & Mesozoic Hefty rocks 10 Reactivated thrust Lewis Autochthonous 20 Belt Supergroup thrust Archean crystalline basement 30 50 km Moho 40

KEY Middle and upper Belt Supergroup Cenozoic Lower Belt Cretaceous Supergroup intrusive rocks Basement Paleozoic and Mesozoic rocks

Figure 5. Simplifi ed regional geologic cross section R–R′ of northwest Montana based on results of seismic refl ection and refraction profi les, Bouguer gravity data, well control, and balanced cross sections (Bally et al., 1966; Harris, 1985; Fritts and Klipping, 1987; Constenius, 1981, 1988; Boberg et al., 1989; Yoos et al., 1991; Harrison et al., 1992; Sears and Buckley, 1993; Van der Velden and Cook, 1994) (based on Constenius, 1996). The Dry Creek stock is projected southward to the cross section to show its relation with the Moyie thrust.

located in the south-central part of the Saw- perpendicular to the regional strike direction seismic data in frontal and western areas of the tooth Range, and extends from the undeformed and approximately parallel to the direction of thrust belt indicate that the base of the Cambrian foreland east of the Augusta syncline westward shortening of the major thrust faults (Fig. 6). section dips ~3.3° beneath the foothills and to the Mission Range, where structures of the The cross section is line-length balanced. Sawtooth Range, and ~4° beneath the Lewis thrust belt interfere with structures related to Industry refl ection seismic profi les and oil and thrust sheet. A gradual westward increase in the Lewis and Clark strike-slip system (Figs. gas wells were used to interpret the foothills sub- basement dip was noted both in Canada (Bally 2 and 6). The location of the cross section was surface structure. Most of these seismic profi les et al., 1966; Dahlstrom, 1970; Fermor, 1999) chosen in part because no regional, detailed, were shot during the early 1980s; we reprocessed and the USA (Boyer, 1995). balanced cross sections exist for the area. Exist- a 10 km section of a key dipline (Fig. 6). Forma- Seismic refl ection profi les from the foothills ing sections are local (e.g., Mitra, 1986; Holl tion tops from the Montana Power Co. State 1 region show a stacking of thrust sheets con- and Anastasio, 1992), unbalanced (e.g., Mudge well (Section 15, Township 21N, Range 7W, taining mechanically strong Paleozoic strata et al., 1982; Mudge and Earhart, 1983), or have Lewis and Clark County) (Fig. 6) were used to (Fig. 8; Supplemental Figure 11). Based on cali- poor resolution in the structures east of the tie the seismic data via a synthetic seismogram. bration with synthetic seismograms from nearby Lewis thrust system (e.g., Fritts and Klipping, Correlation information for the deepest strati- wells, the basal detachment for these structures 1987; Sears, 2001, 2007). Previous work por- graphic interval was obtained using a synthetic is inferred near the base of the Devonian section. traying the frontal part of the thrust belt lacked seismogram from the Oxy Larson 1 (Section 1, An additional indicator comes from published subsurface control (e.g., Sears et al., 2005). Township 22N, Range 5W, Teton County), a well USGS geological maps (Mudge, 1965, 1968) Moreover, published USGS geological maps to the east that reached basement. A synthetic showing that the Home thrust is located directly of the Sawtooth Range and the foothills in this seismogram from the Husky Gulf Mercier 1 well below Devonian Jefferson Formation strata. region are superb (Mudge, 1965, 1966a, 1966b, to the north (Section 32, Township 32N, Range Upper detachment levels for these structures are 1968), and we have obtained high-quality indus- 11W, Glacier County) provided additional con- controlled by fi ne-grained clastic deposits of the try seismic lines and well data from this area. trol in the identifi cation of stratigraphic horizons Cretaceous Kootenai Formation, Blackleaf For- from the seismic data. Two contiguous seismic mation, and the Marias River Shale. The west- Methods profi les across the Lewis thrust sheet in the Swan ern fl ank of the frontal synclinorium, referred to The cross section is based on published valley (Fig. 2) facilitated geometric reconstruc- as the Augusta syncline, seems to be the result USGS geological maps and our own mapping. tion of the hinterland part of the section and in of the emplacement of thrust sheets that, at shal- A number of 1:24,000 scale quadrangle maps the determination of depth to undeformed base- low levels, juxtapose strata of the Marias River and a subsequent compilation (Mudge, 1965, ment. The reasoning, justifi cation, and uncertain- Shale on top of the lower Two Medicine For- 1966a, 1966b, 1968, 1972) provide data from ties behind different aspects of the cross section mation. Long fl at-on-fl at relationships between the frontal syncline up to the leading edge of are discussed in the following and explained in the Lewis thrust system. Maps at 1:125,000 and Figure 7. 1Supplemental Figure 1. Color version of seis- 1:250,000 scales (Mudge et al., 1982; Mudge mic section displayed in Figure 8. Vertical scale is and Earhart, 1983) were used in the mostly Results two-way traveltime. Location is in Figure 6 (SL 1-a). If you are viewing the PDF of this paper or read- undeformed plains, and along most of the Lewis Figure 7 shows an ~145-km-long balanced ing it offl ine, please visit http://dx.doi.org/10.1130 thrust sheet, where the structural wavelengths cross section of the northwest Montana thrust /GES00773.S1 or the full-text article on www are large. The cross section is oriented ~N80°E, belt. Petroleum wells drilled in the foothills and .gsapubs.org to view Supplemental Figure 1.

1110 Geosphere, October 2012

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Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana

ambria Prec N n ’ 5 4 ° W 7 ° 47°45’N 4 W 2

° the Marias River Shale and the Two Medicine 1 2 112°W 1 1 5 112°W 1

e r

e k e

v i k

a a Formation are evident in geological maps to the R

River

n L

o s Lake

t d e

eton lld e T T e t ffiei t n

u e re north and northwest of the Augusta syncline B

G t s e N Mount Shields Fm Snowslip Fm Garnet Range Fm Shepard Fm Spokane Fm Helena Fm i Empire Fm McNamara Fm Basement Greyson Fm Belt Supergroup (undivided) Bonner Quartzite Lower Belt Sgr rocks Precambrian igneous rocks r 5 (Mudge and Earhart, 1983; Fig. 6). Closely Priest Butte Lake P 0 40 4

’ imbricated Cretaceous rocks are restricted to 3

S’ S

ceous Creta urass. J shallow levels, as suggested by geological maps n o s r a y L x - u Oxy O 1-Larson 1

a and seismic data. e t o h 5 Choteau C The classic structures of the Sawtooth Range 0 10 1 4 0

20 2 comprise a set of closely spaced, hinterland dip- PLAINS 0 30 3

5 ping thrust sheets involving Paleozoic to Lower Quaternary Cambrian (undivided) Cenozoic (undivided) Quaternary and Neogene(?) Two Medicine Fm Two Creek Telegraph Fms and Virgelle Marias River Shale Blackleaf Fm Devonian (undivided, includes upper part of M-D Forks Fm) Three Ellis Gr and Morrison Fm St. Mary River Fm Mississippian (undivided) Cretaceous igneous rocks Kootenai Fm Horsethief and Bearpaw Fms 5 a r t i

s Cretaceous sedimentary rocks (Mudge, 1982). o u v r g e e u v s r e t e Augusta A S e g r

i a

e The steep dip angles in these thrust sheets are a 4 e s R o Y o

- Gose Steve G 1 1-Yeager v k r e e e s r

e result of back rotation during the emplacement C

r w n e o u l

6 v i l k i of successively younger and structurally lower

h R 3 Willow Creek Reservoir W s i n L . SLS 3

Pishkun Reservoir P u o a C

SunS River n r a e t e t n

w E thrust sheets to the east. The frontal structures a o as added N o t LI Montana M P Power Co. S State YNC A S USTAST SYNC 5 AUGUAUG detach at the base of the Devonian section, simi- 5 5 b - 0 10 1 0 1 0 50 5 50 5 lar to the buried thrust sheets beneath the foot- L SLS 1-b

5 S 5 15 1 55 5 0 30 3

35 3 5

l 15 1 55 5

n hills. We infer that the basal detachment cuts

u k

h 2

55 5 s i

P Pishkun Canal Pishkun L

Well - 5 FOOTHILL Re-processed seismic line (Figure 7) Town Train rail Train 0 Unpublished seismic lines used in cross-section construction Road 0

25 2 downsection westward, because thrust sheets in 40 4 0 60 6 SLS 2 1 40 4 0 0 5 40 4 30 3 65 6 L 0 0 30 3 0 60 6 60 6 SLS 1-a 0 0 5 40 4 n 20 2 85 8 o 0 0 0 s 5 80 8 20 2 50 5 r 85 8 0 a 0 y 60 6 40 4 0 0 L x 0 the western part of the Sawtooth Range contain - 20 2 60 6 50 5 0 Oxy O 1 1-Larson 30 3 5 45 4 5 55 5 5 35 3 0 0 5 60 6 30 3 55 5 5 0 0 45 4 0 40 4 40 4 5 5 40 4 15 1 35 3 5 25 2 0 20 2 0

5 Cambrian strata (Figs. 6 and 7). Upper-level 30 3 35 3 0 5 0 30 3 5 35 3 50 5 25 2 0 0 20 2 20 2 5 5 5 35 3 0 45 4 5 35 3 50 5 35 3 5 15 1 5 0 5 35 3 40 4 45 4 5 5 55 5 45 4 detachments are localized in the Ellis Group and 0 40 4 5 Anticline 0 Overturned syncline Thrust fault Overturned anticline Syncline Normal fault 65 6 60 6 5 r 5 0 0 45 4 55 5 i 50 5 70 7 0 0 60 6 60 6 o 0 70 7 0 60 6 v 5 0 0 35 3 0 r 70 7 0 50 5 0 30 3

60 6 Kootenai and Blackleaf Formations. 50 5 0 5 e 40 4 45 4 5 5 65 6 5 s 55 5 65 6 0 0 5 70 7 5 KEY 30 3 55 5 e 0 65 6 0 60 6 5 30 3 75 7 0 70 7 R 0 5

60 6 55 5 0 5 60 6

n To the west of the Sawtooth Range, thrust 45 4 0 5 50 5 55 5 5 o 0 35 3 0 70 7 60 6 s 5 0

0 WTOOTH RANGE WTOOTH 0 T 65 6 60 6 70 7 0 60 6 5 5 70 7 0 b 55 5 45 4 S 0 50 5 i 50 5 5 U 65 6

5 SA 45 4 0 5 Gibson Reservoir G R 60 6 5 65 6 5

55 5 slices containing Paleozoic rocks are buried by 5 65 6 H 0 55 5 0 0 50 5 50 5 70 7 TTHRUS 0 70 7

0 O 5 50 5 55 5

5 D 0 45 4 0 70 7 5 40 4 AADO 0 0 55 5 0 50 5 50 5 40 4

0 R 0 50 5 5 5 70 7 imbricated, highly deformed Mesozoic strata, 35 3 55 5 ORO 0 0 0 D 50 5 50 5 60 6 L

5 E 45 4 0 0 70 7 0 0 40 4 and farther westward, by Belt Supergroup rocks 60 6 70 7 0 60 6 0 40 4 5 25 2 0 40 4 5 35 3 0

30 3 in the hanging wall of the Lewis thrust system. 5 5 25 2 35 3

0 T 70 7 5 0 45 4 40 4 S 5 0 25 2 60 6 0 5 0 U 0 0 40 4 45 4 40 4 40 4 60 6 R 5

5 T 45 4 0 0 35 3 H 50 5 30 3 5 5 STS Our interpretation of the subsurface geometry 15 1 35 3 0

0 THRUST 5 0 5 40 4 40 4 25 2 30 3 U 25 2 5

45 4 Y 0 R 70 7 W

0 E

H ° 40 4 L 0 W THRUT 3 20 2

° D 1

3 S I ADLEYA 113°W 1 shows imbricated thrust faults repeating the 1 O 113°W 1 W

5 E H 25 2 LEWISL 5 0 10 1 5

55 5 Paleozoic section; this is required to avoid an 5 45 4 0 30 3 excessive amount of shortening in the imbri- 0 10 1 5 0 15 1 30 3 20 km 5 0 25 2 30 3 cated Cretaceous rocks, the regional elevation 5 75 7 5 35 3 0 5 20 2 35 3 0 10 1 NE

0 I 10 1 CCLINEL of which is ~3 km beneath the topographic

YN L

S 5 E 75 7 0 D 10 1 I

V 15 AL I L DIVIDED SYN 5 5

A 45 4 15 1 T 5 surface. Moreover, the imbricate geometry ﬁ lls EN 65 6 TIINENTALN ONNT COC 5 55 5 0 50 5 space beneath the frontal part of the Lewis thrust 5 35 3 0 60 6

5 system. The western extent of thrust sheets con- 55 5 0 40 4 0 0 40 4 30 3 taining Paleozoic rocks is based on the inter- 0 30 3 510

5 r 25 2 e Riv ead pretation of unpublished proprietary seismic

THRUST SYSTEM HANGINGW SYSTEM THRUST latheadth River k F

Y or F 0 th Fork Fla 20 2 SoutSou reﬂ ection proﬁ les shot across the northern part 5 0 35 3 5

HOADLE 15 1 of the Continental Divide syncline and the 0 0 20 2 30 3 0 30 3 0 50 5 0 10 1

5 South Fork of the Flathead River Basin (Figs. 45 4 5 5 15 1 15 1 5

15 1 STEINBACH- 0 0 50 5 20 2 2 and 7, points 21, 28). Although the seismic 5 5

25 2

25 2 DO- 5 25 2

5 line that shows this relationship is located more 35 3

0 LDORA 40 4 5 25 2 0 0 40 4 30 3 than 30 km to the north, the continuity of sur- 5 35 3 0 50 5 0 30 3 0 0 30 3 30 3 5

25 2 LEWIS-E 5 25 2 face structures along strike makes this projec- GE RAN 5 N 45 4 SWANSWA RANGE tion permissible. The importance of the western indicates position of cross section of Figure 7. Also shown is location of 7. section of Figure indicates position of cross ′

0 extent of Paleozoic strata is that it provides an 40 4 0 30 3 indirect but signiﬁ cant constraint for the possible location of the easternmost footwall ramp in Belt Supergroup strata. A conservative view 5 15 1 r is that this ramp may be located just westward of iverve n R

aan R 5 SwSw 25 2 the footwall cutoff in the Paleozoic section, once 0 20 2 5 5 25 2 25 2 thrust sheets of the Sawtooth Range and buried 5 15 1 equivalents are restored to an undeformed posi- 0 0 10 1 30 3 0 5 0 30 3 0 15 1 30 3 0 20 2 20 2 0 10 1 0 20 2 5 15 1 5 0 15 1 20 2 0 10 1 at www.gsapubs.org.

0 tion. The location of the frontal footwall cutoffs 10 1 5 15 1 ANGE ION R in Belt rocks, plus the unknown extent of eroded MISSIONMISS RANGE 0 10 1 strata in thrust sheets of the Lewis thrust system, W S ° W ° 4 4 1 are the major sources of uncertainty in estimat- 114°W 1 1 N ’ 114°W 1 0 3 ° 7 47°30’N 4 along the Sawtooth Range from several U.S. Geological Survey 1:24,000 quadrangle maps (Mudge, 1965, 1966a, 1966b, along the Sawtooth Range from own mapping. S-S 1968), and our Figure 6. Geologic map along the balanced cross-section region (based on Mudge et al., 1982; and Earhart, 1983). Detail w region 6. Geologic map along the balanced cross-section Figure seismic line of Figure 8, additional seismic data used to construct the balanced cross section, 8, additional seismic data used to construct the balanced cross line of Figure seismic 14. Location of map is shown in and location of U-Pb sample KR2 Figure 6, please visit of Figure view the full-sized PDF To 2. Figure the full- or http://dx.doi.org/10.1130/GES00773.S2 text article ing shortening in this region.

Geosphere, October 2012 1111

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1104/3342493/1104.pdf by guest on 28 September 2021 Fuentes et al. –3 2 –1 –2 3 km S.L. 1 Stacks of thrust slices of Mesozoic strata

20 km occur on both sides of the Sawtooth Range. –3 2 –1 –2 3 km S.L. 1 ′

S These thrusts propagated upward through the Oxy 1-Larson Paleozoic section from the basal detachment, 20 km 15

Augusta syncline were deﬂ ected eastward toward the foreland State 1 (projected) Montana Power Co. along detachments located low in the Meso- 15 zoic section, and were subsequently offset by 10 underlying younger thrust faults. Because of its 10 Gose Steve

Yeager 1 (projected) Yeager small scale and relative insigniﬁ cance for cal- 2 5 1 culating total shortening, the frontal imbricate 5 3

ttp://dx.doi.org east of the Diversion thrust is not shown in the

t Projected seismic data 0

s u

r the South st from restored section (Fig. 7). The detailed geometry th

Augusta syncline e

t iv

State 1 (projected) D 0

Montana Power Co. u

Seismic coverage of Figure 7 in the similar imbricated system to the west of r

4 e th e

m o

5 H the Sawtooth Range is not strongly constrained, mainly because of the poor map resolution in 6 7

Sawtooth Range this part of the thrust belt. 11 8

t Projected seismic data

h The frontal structure of the Lewis thrust t

sion sion

s 9 Dive

u Seismic coverage of Figure 7 r

m system along the trace of the cross section is

o H 10 14 the Eldorado thrust. The traces of the Lewis Sawtooth Range 15

14 and Eldorado thrusts diverge from a branch

16 point located south of the Lewis thrust salient (Figs. 2 and 9). The Eldorado thrust follows a 17 18 Shallow, small-scale imbricate Shallow, not restored.

19 detachment in the Proterozoic Empire-Spokane Formations interval, as suggested by the long Zoom-in of frontal part cross-section hanging-wall ﬂ at along these units shown in the thrust thrust Eldorado Eldorado Mudge and Earhart (1983) map. These two units 20

23 are mapped as a single package at this position Lewis thrust Lewis thrust Hoadley thrust in the thrust belt because of their reduced thick-

24 ness. In map view, the Lewis thrust also exhibits thrust Hoadley a relatively long hanging-wall ﬂ at in the Mount Shields Formation (Figs. 6 and 9). The Hoadley 22 Continental Divide syncline thrust has a relatively small amount of displacement in the cross section, consistent with the 21 location of a tip point on the map a few kilome- river valley South Fork Flathead Continental Divide syncline ters north of the cross section (Figs. 6 and 9). The Continental Divide syncline resulted partly from the juxtaposition of a hanging-wall ramp with a footwall ﬂ at beneath the western limb of the syncline (Fig. 7). Further tighten- river valley

South Fork Flathead ing of the syncline took place by back rotation

Projected seismic data of the eastern limb of the fold during internal not differentiated. Belt Supergroup units

Swan Range imbrication and additional passive tilting dur-

28 ing emplacement of structurally lower thrust 25

26 sheets in Paleozoic strata. The dip of bedding in the back limb is potentially accentuated by restored. Hinterland part of thrust belt not 2 3 1 –1 –2 –3 –4 –5 –6 –7 –8

S.L. ﬂ exural slip, as suggested by the apparently Swan River Valley half-graben Swan River Valley high hanging-wall cutoff angle in lower Belt e ters f s 27 on. ted

the Supergroup units. 26 The western part of the cross section shows

Autochthonous Belt Supergroup extensional faults produced by orogenic collapse and subsequent Basin and Range tectonics (Constenius, 1996). The location of the extensional fault just west of the Continental Divide S Fork of the Flathead River Valley, is not shown in the restored section. To view the full-sized PDF of Figure 7, please visit h of Figure view the full-sized PDF To section. is not shown in the restored Valley, Fork of the Flathead River the full-text article at www.gsapubs.org. or /10.1130/GES00773.S3 Figure 7. Balanced cross section. See Figure 6 for location and key for lithologic units. Western part of the cross section, we part of the cross Western lithologic units. location and key for 6 for section. See Figure 7. Balanced cross Figure syncline controls the position of the South Fork of the Flathead River. This fault probably accom- 2 3 1 –4 –5 –6 –3 –8 –9 –1 –7 –2 S.L. –11 –12 –14 –10 –13 modated the collapse of a structural culmina- tion in a major fault-bend fold, which acquired additional structural elevation above superposed ramps along deeper thrust faults. The estimated transferred from structures of the Sawtooth Range. 1: Basement dip, estimated from well and seismic data, is ~3.3 in the frontal part of thrust belt. Increases to ~4 beneath 10: sheet geometry and thickness projected from interpreted seismic line (Figure 8). Thrust 24: Hoadley thrust requires minimum displacement along the trace of cross-section because it shows a tip point few kilome KEY POINTS IN CROSS-SECTION: Lewis thrust sheet, based on interpretation of depth to the basal Cambrian section from seismic lines in hinterland positions. 2: Thickness of strata in the eastern part section derived from 3 wells. 3: Shallow deformation in Medicine Formation is transferred from a deeper detachment in Marias River Shale as evidenced Two 4: Minor anticline controled by deeper structure detached in Marias River Shale (deduced from map Mudge and Earhart, 1983). 5: Minor anticline involving Formations with detachment in Marias River Shale from interpretation o Creek and Virgelle Telegraph unpublished seismic line. 8: Basal detachment in thrust sheets involving the Paleozoic section interpreted near base of Devonian.Thicknesses Fm strata in it evidence for the position of basal detachment comes from surface data: Home thrust sheet contains Jefferson 9: Thrust sheet geometry and thickness projected from interpreted seismic line (Figure 8). Note minor backthrust simplified 11: section of Marias River Shale interpreted as a result combination wedging and backthrusting. Other Thickened geometric solutions are possible. 12: Relative small imbricates in Cretaceous strata not displayed retrodeformed model. Partial shortening of these structures 13: sheet interpreted and projected from seismic line (Figure 8). Thrust 14: Basal detachment in frontal structures of the Sawtooth Range remains base Devonian section. Base Home thrust Fm deposits in its base. Regional geologic maps (e.g. Mudge and Earhart, 1983) show that thrusts th sheet contains Jefferson 15: Presence of (thin) Cambrian deposits in the hanging wall indicates a deepening basal detachment into upper 16: Outcropping Cambrian section indicates a deepening of the basal detachment into strata. 17: Geometry of highly deformed imbricated zone Cretaceous strata and igneous sill is simplified in the balanced cross secti Widespread vegetation cover and intrusion of sill complex make map relations uncertain. 18: Stacking of thrust sheets repeating the Paleozoic section is required to avoid an excessive amount shortening in imbrica Cretaceous rocks, whose regional is ~3 Km beneath the topographic surface. 19: syncline with Marias River Shale in its core is preserved (untruncated) to the north where it exposes rocks of Truncated Medicine Fm. Two 20: Stacking of thrust sheets is required to alleviate room problems beneath the Eldorado thrust. 23: Flat on Mount Shields Fm deduced from regional extent of this unit Lewis thrust hangingwall. north of the trace section. 25: Geometry and thickness of Cenozoic half-graben basin fill interpreted from seismic line located ~30 km to the north, projected and adapted to trace of section (see location in Figure 2). 27: Presence of, and wedging out geometry of autochtonous Belt Supergroup strata interpreted from seismic line. 28: Limit of basal Cambrian reflections (Flathead/Mowry equivalents) traceable from seismic sections. 6: Anticline geometry from seismic line (Figure 8) and geologic map. 7: Minor deformation in Paleozoic section simplified from seismic data interpretation. Paleozoic strata and position of basal detachment deduced from seismic data calibrated with synthetic seismograms. Additional base (see point 14). seismic data interpretation. frontal structures of the Sawtooth Range cut down-section into Devonian deposits at low angle. deposits. 21: Extent to the west of thrust sheet carrying Paleozoic strata is inferred from seismic data. thrust along a regional extent on geologic maps (Mudge and Earhart, 1983). 26: Geometry is based on interpretation of Proterozoic sills and Belt strata from seismic line. from seismic data and the geologic map (Mudge Earhart, 1983). 22: Flat in strata of the Empire/Spokane Fms along Eldorado thrust is deduced from presence this unit on top normal slip along this fault is ~6 km.

1112 Geosphere, October 2012

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Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana

Two-way Two-way time (sec) time 2 0 2 0 1 Shale Lower Cambrian Virgelle Fm. Virgelle Cut Bank Ss. Blackleaf Fm. Bakken Fm. Kootenai Fm. Intra-Cambrian Top Paleozoic Top Marias River E raveltime. Loca- m K Figure 8. Uninterpreted and interpreted seismic section from the foothills, south of the Sun River. Vertical scale is two-way t Vertical the foothills, south of Sun River. seismic section from and interpreted 8. Uninterpreted Figure tion is in Figure 6 (SL 1-a). A color version of this seismic section is available in Supplemental Figure 1 (see footnote 1). version of this seismic section is available in Supplemental Figure color A 1-a). 6 (SL tion is in Figure 1 W

Geosphere, October 2012 1113

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GLACIER NATIONAL Key points: PARK N 1- Lewis-Eldorado thrusts branch-point.

′

0 2- Lewis thrust cuts up-section from the Lewis thrust

3 salient at the latitude of Glacier National Park to the

1 1 LEWIS THRUST south. 48°15′N 3- Lewis thrust displays long hangingwall flat in Mount Shields Fm; cuts down-section into Shepard Fm to the south. 1 4- South tip-point of Lewis thrust in Mississippian rocks. 2 5- In vicinity of line of cross-section Lewis thrust footwall changes from a flat in Devonian, to a ramp in Cambrian-Mcnamara and Bonner Fms, to a flat in ELDORADO THRUST Mount Shields Fm.

6- Eldorado thrust displays long hangingwall flat or very low angle ramp in condensed deposits of the Sheppard -Spokane interval, possibly with a dominant glide horizon in the Empire-Spokane interval (undifferentiated at the mapping scale).

7- Eldorado-Steinbach thrusts branch-point. 6 3 8- Hoadley thust tip point.

CONTINENTAL DIVIDE SYNCLINE 9- Hoadley thrust displays hangingwall flat in Mount Shields Fm; cuts downsection to the south. LEWIS THRUST

′

1 47°45′N

8 Figure 9. Map of the Lewis thrust system south of the Lewis thrust salient, showing key map relations of individual thrusts. SOUTH FORK FAULT Local trace of Branch and tip points, and location of main glide horizons and cross-section ramps for the Lewis, Eldorado, Steinbach, and Hoadley thrusts 5 are indicated. Locations of faults are in Figures 1 and 2. Note that several lithostratigraphic units were combined in single mapping units to simplify key relations and due to scale limita- HOADLEY THRUST tions. Hanging-wall rocks along the Lewis thrust salient are not displayed. Map and thrust nomenclature are simpliﬁ ed from Mudge and Earhart (1983).

EXPLANATION 9 LEWIS THRUST Jurassic-Cretaceous rocks (undifferentiated) 7 STEINBACH T Mississippian-Devonian rocks

Cambrian rocks ELDORADO THRHRUST Garnet Range-McNamara-Bonner Formations Mount Shield Formation 4 UST Shepard to Greyson Formations

1114 Geosphere, October 2012

The geometry of the major half-graben that required to fi ll space as discussed herein. The known ages, provenance of dated synorogenic controls the Swan River basin is partially based extent of the westernmost thrust sheet of Paleo- sedimentary units in the foreland basin system, on projection of seismic data collected north zoic rocks beneath the Lewis thrust system adds and previously published apatite fi ssion-track of the plane of the cross section (Fig. 2). Thick a few kilometers of shortening and is inferred ages (Fig. 11). mafi c sills in Belt Supergroup strata generate from relatively ambiguous seismic data. The strong seismic refl ections that help to illustrate largest sources of uncertainty in the shorten- Early Hinterland Deformation (Jurassic) the structure. Depth estimates of extensional ing estimate are the westward extent of these growth strata in the hanging wall of this struc- buried thrust sheets, the location of the footwall By Jurassic time, shortening in hinterland ture indicate ~2.3 km of Cenozoic basin fi ll. Slip ramp in Belt Supergroup strata, and the length regions of the eastern part of the Intermontane along the fault in the cross section is ~12 km. In of eroded strata in the leading edge of the Lewis and Omineca belts was driven by the colli- addition, seismic data show stratifi ed refl ections thrust system. The position of the basal detach- sion of the Intermontane terrane with North beneath the basal Cambrian event that converge ment is well constrained by subsurface data America (Evenchick et al., 2007, and references markedly toward the east. Our preferred inter- and cannot be brought to shallower levels to therein). In Canada, accretion of the Intermon- pretation is that these refl ections correspond to decrease shortening. tane terrane between 187 and 173 Ma involved autochthonous Belt Supergroup rocks tapering The location of the footwall cutoffs in Belt tectonic burial of the outboard part of the Cordi- toward the east. Supergroup strata derived from the cross-sec- lleran miogeocline to depths of 20–25 km, and Total shortening along the cross section is tion restoration coincide with the inferred posi- was accompanied by regional metamorphism ~135 km, or ~70% if measured from the axis of tions of the footwall megaramp of Price and and the emplacement of synkinematic granitic the Augusta syncline to the western fl ank of the Sears (2000) and Sears (2001, 2007) at this lati- plutons. Subsequent southwest-verging defor- Continental Divide syncline. The Lewis thrust tude in the thrust belt; shortening may decrease mation of the outboard part of the miogeocline system accommodates ~50 km of shortening southward from this position, as suggested by involved ~10 km of rapid exhumation between along this cross section. these authors. 173 and 168 Ma (Archibald et al., 1983; Col- In southern Canada near the international bor- The transition from the deeply exhumed Saw- pron et al., 1996, 1998; Evenchick et al., 2007). der, van der Velden and Cook (1994) calculated tooth Range, in the south, to the less exhumed Evidence of hinterland deformation at the lati- ~115 km of total displacement for the Lewis Lewis thrust salient with its thick Belt Super- tude of northwestern Montana is deduced from thrust sheet by matching the Lewis thrust hang- group section to the north (Figs. 2, 5, and 10) modal petrographic data and U-Pb detrital zircon ing wall and interpreted footwall cutoffs on a results from two factors: (1) regional basement ages from Jurassic sandstones (Fuentes et al., deep seismic refl ection profi le. Their estimate of elevation increases from north to south, as indi- 2009, 2011). Ellis Group and Morrison Forma- shortening included the cumulative shortening cated by seismic profi les, well data, and balanced tion sandstones contain lithic fragments that of the Lewis thrust and all the other thrusts that cross sections; and (2) the Flathead and Waterton suggest derivation from deformed miogeoclinal affect the Paleozoic strata, and did not include duplexes beneath the Lewis thrust salient appar- strata with some infl uence from volcanic sources additional shortening in western structures. ently merge southward into, or are replaced by, (Fig. 12; Table 1). The key information that Revision of the seismic profi les used in their a single large exhumed duplex in the Sawtooth proves a western, orogenic provenance is found study suggests that the location of the footwall Range. Consequently, structural elevations of in detrital zircon ages. Samples of the Swift and ramp could be ~10 km farther west. Bally (1984) Paleozoic strata in thrust sheets in the Sawtooth Morrison Formations contain abundant zircons calculated a minimum of 165 km of shortening Range reach elevations more than 2 km above sea with late Paleozoic and Triassic ages derived from a balanced cross section extending from level. In contrast, the top of the Paleozoic section from accreted elements in the eastern part of the the frontal part of the thrust belt in Canada to between the Flathead and Waterton duplexes, Intermontane belt, mid-Paleozoic ages with pos- the Moyie thrust along the Idaho-Montana beneath the Akamina syncline, is ~3.5–3.6 km sible origin in Kootenay arc rocks, and a variety border. The cross section in Constenius (1996) below sea level (Gordy et al., 1977; Bally, 1984). of different miogeoclinal source units (Fig. 13; indicates shortening of ~135 km, including the A possible third factor is that the Lewis thrust Supplemental Table2). This interpretation is con- Lewis thrust system and all the structures to sheet salient is bounded on its northwest and sistent with the Late Jurassic onset of foreland the east (Fig. 5). Based on these numbers, it southeast sides by lateral ramps, across which basin subsidence recorded by ~1000 m of Koo- appears that shortening remains relatively con- the Lewis thrust climbs upsection along its hang- tenay Group deposits that crop out near Fernie, stant from the southern part of the Canadian ing wall from a detachment located deep in the British Columbia (Ross et al., 2005). thrust belt to the line of our cross section. Prichard Formation (or lateral equivalents) into The main uncertainties on our shortening much higher stratigraphic levels. The Lewis Western Thrust Systems (Cretaceous) estimate are as follows. The geometry in the thrust sheet is thicker and stronger within the frontal part of the section is well constrained salient than in the fl anking regions. North of Basal coarse-grained sandstone and con- by seismic data; minor variations in the inter- Marias Pass the Lewis thrust sheet includes a glomerate of the Lower Cretaceous Kootenai pretation are possible but could not signifi cantly thick succession of the Prichard and Appekunny Formation and equivalent strata in southwest- affect the shortening amount. The structure in Formations that is absent along the hanging wall ern Montana, Wyoming, and southern Canada the Sawtooth Range does not permit many geo- of the Lewis thrust system south of Marias Pass. contain a high proportion of chert, which has metric solutions, and the solutions chosen in the cross section represent minimum shortening. KINEMATIC HISTORY 2Supplemental Table. Excel fi le of U-Pb (zircon) The detailed geometry of the partially buried geochronologic analyses by laser-ablation multi- stack of thrust slices of Paleozoic strata between A preliminary kinematic history of the Cor- collector–inductively coupled plasma–mass spec- the Sawtooth Range and the Lewis thrust system dilleran thrust belt at the latitude of northwestern trometry. If you are viewing the PDF of this paper is conjectural owing to the lack of subsurface Montana can be deduced from crosscutting and or reading it offl ine, please visit http://dx.doi.org /10.1130/GES00773.S4 or the full-text article on data. These repetitions add shortening, but are overlapping relationships between rock units of www.gsapubs.org to view the Supplemental Table.

Geosphere, October 2012 1115

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′ 113°15 W 113°W Alb

erta syncline ′ N N C

i o L

n e e CANADA

c w

v a is t U.S.A. e t e C h

r D Kish ru

C s d t

n d enehn

i A a Foothills

p kamina syncline

s basin

M 48°15′N C

K Sawtooth A 10 km Range C

Lewis C Kishenen basin Akamina syncline C′ thrust KEY

SL Paleozoic Cenozoic Proterozoic Upper Belt Supergroup Cretaceous Jurassic to Basement Waterton Lower Cretaceous Flathead duplex 10 Km B duplex

Figure 10. (A) Simpliﬁ ed geologic map of the northern end of the Sawtooth Range showing the divergence of the single imbricate fan into two plunging individual systems to the north. Map is based on Mudge and Earhart (1983). (B) Simpliﬁ ed cross section showing the Water- ton and Flathead duplexes and their relation with the Akamina syncline in proximity of the international border. Based on Gordy et al. (1977). (C) Relative location map showing trace of cross section and detailed map (A).

long been interpreted to refl ect derivation from crosscutting plutons indicate pre–mid-Creta- et al., 1991). Early Cretaceous deformation mid- to upper Paleozoic rocks that were uplifted ceous movement (Wanless et al., 1968; Foo, involving Belt Supergroup rocks in hinterland and eroded in the growing thrust belt to the west 1979; Archibald et al., 1984; Höy and van der regions seems to have been a regional process, (Rapson, 1965; Suttner et al., 1981; DeCelles, Heyden, 1988, Price and Sears, 2000). Recent as indicated by an abrupt increase in the propor- 1986; Schwartz and DeCelles, 1988). The high work in southern British Columbia provides tion of low-grade metamorphic lithic fragments ratio of sedimentary lithic to total lithic frag- additional information about the timing of and detrital zircons in the Blackleaf Formation ments (Fig. 12), together with the high pro- early contractional deformation in this region: with U-Pb ages typical of Belt Supergroup strata portion of detrital zircons with ages typical of 40Ar/39Ar dating of stocks intruding and ther- (Figs. 12 and 13; Supplemental Table [see foot- miogeoclinal strata, indicates a provenance mally overprinting the Lussier River fault and note 2]) (Fuentes et al., 2011). dominated by thrust-imbricated Paleozoic deformed Cambrian–Ordovician shales and car- The Moyie thrust system extends from rocks. Uncertainty exists regarding the location bonate rocks yielded ages of ca. 108 Ma (Larson British Columbia into northwestern Montana of this early fold-thrust belt; the locus of defor- et al., 2006). All these geochronometric results (Figs. 1 and 2). Shortening estimates range mation may have resided in the region occupied in southern Canada fi x the minimum age of dis- from ~8 km in southeastern British Columbia today by the Omineca belt, or along the western placement on the St. Mary–Lussier fault system (McMechan and Price, 1982) to ~4 km in the fl ank of the Purcell anticlinorium. between mid-Albian and late Cenomanian time. central Cabinet Mountains of Montana (Filli- The oldest known structures in the unmeta- In northern Idaho the Early and Middle Eocene pone and Yin, 1994). Slip along this thrust has morphosed western part of the thrust belt are extensional exhumation of the Priest River meta- been bracketed between ca. 71 and 69 Ma (Filli- the Hall Lake and St. Mary thrusts, and the morphic core complex from depths of 35–40 km pone and Yin, 1994), based on 40Ar/39Ar ages of Lussier River fault, in the vicinity of the United removed the thrust belt, locally exposing its mylonitic rocks from the west side of the Dry States–Canada border. Palinspastic map recon- basal detachment and the underlying Archean Creek stock (Fig. 2) that were interpreted to be structions by Price and Sears (2000) indicate basement (Doughty et al., 1998; Doughty and deformed by the Moyie thrust. This interpreta- that part of the Hall Lake and St. Mary thrusts Price, 1999). Although the preextensional struc- tion, however, can only limit the timing of the were located mainly south of the international tural confi guration of this region is uncertain, youngest slip along this thrust. Approximately border prior to the movement of thrust systems thrusts involving Belt Supergroup and younger 50 km to the south, a ca. 114 Ma 40Ar/39Ar horn- to the east. The timing of initial slip along these rocks west of the Priest River complex have been blende age from the Vermillion River stock, structures is unknown, but mid-Cenomanian identifi ed (Rhodes and Hyndman, 1988; Stoffel which was interpreted to crosscut the Moyie

1116 Geosphere, October 2012

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Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana

70 km /Ma per km per /Ma km 3

I G 50

T L 30

T O 10

B ing

90 110 70 Ma 70 150 130 170 not system at

constrained

calculated Paleogene subsidence 1km 0.5 TECTONIC SUBSIDENCE

16b E

e Clark Lin Clark

is and is 16 Lew along

ement Main mov Main X, P 7,13, 15,17 ?

Foothills S.R./

T.S.

Lewis ?

X, P T, 7,8,9, 10,11, 12,13, 14,17 PWT S. T. LPWW ? ? T.S.—Lewis-Eldorado- thrust system; Lewis sh-Wigwam

? Snowshoe thrust Snowshoe 6 X Price and Sears, 2000 Archibald et al., 1983, 1984 references therein

2) 6) Fillipone and 1994 Yin, 8) Schmidt, 1978 10) Sears, 2001 12) Osadetz et al., 2004 13) Harlan et al., 2005 et al., 1990 16) Wallace 17) work This 5) Harrison et al., 1986 3) Höy and van der Heyden,1988 4) 9) Whipple et al., 1987 van der Pluijm et al., 2001, 2006 11) 14) Hardebol et al., 2009 15) 1972 Schmidt, 1) Evenchick et al., 2007 and 7) et al., 1976 Hoffman 16b) Sears and Hendrix, 2004 Moyie thrust Moyie X 5 P 4,17 6 X ? ?

ssion tracks). Subsidence curve from Fuentes et al. (2009, 2011). Tim- Fuentes et al. (2009, 2011). ssion tracks). Subsidence curve from Hall Lake/St. Mary thrusts Mary Lake/St. Hall X, P 2,3,4, 17

AGES OF THRUSTING

Forebulge advance Forebulge et al. (2009). Gehrels ux is from

Deformed miogeocline in hinterland in miogeocline Deformed P 17 ?

tion events tion ltiple deforma ltiple Mu

ts Intermontane/Omineca bel Intermontane/Omineca

W belt Intermontane

Start ca. 187 Ma Insular Superterrane Insular ACCRETION TERRANE Virgelle River Swift Marias Rierdon Kootenai Telegraph Creek Telegraph Morrison Blackleaf Wasatch Sawtooth Bearpaw/Horsethief

Willow Creek FORMATION

St. Mary River Fort Union (P. Hills) Fort Union (P. Two Medicine Two Colorado Ellis Montana GP. ? the latitude of northern Montana. Sources of data for timing of thrust movement numbered and given in diagram. Timing of terran Timing and given in diagram. timing of thrust movement numbered of data for the latitude of northern Montana. Sources in southern Canada. Coast Mountains batholith magmatic fl Figure 11. Stratigraphic chart, major deformation events in the fold-thrust belt, and tectonic subsidence of the foreland basin deformation events in the fold-thrust belt, and tectonic subsidence of foreland Stratigraphic chart, major 11. Figure T.S.—Snowshoe-Libby-Pinkham-Whitefi discussed in the text. SLPWW accretion constraints of thrust systems: X—crosscutt Time Hills. Hills—Porcupine Steinbach-Hoadley thrust system; S.R.—Sawtooth Range; P. (apatite fi T—thermochronology data; P—provenance relationships; of the Intermontane and Omineca belts at latitude Montana is unknown, but it ing of deformation in hinterland regions LITHOLOGY

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

(E) MIDDLE LATE LATE EARLY

PALEOC.

EO.

CRETACEOUS JURASSIC AGE PALEOGENE 60 80 140 160 100 180 120

Geosphere, October 2012 1117

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Qm Qt

F 50% Lm Lt F L KEY: Fort Union/Wasatch Formations - n=4 St. Mary River/Willow Creek Formations - n=5 T.Creek/Virgelle/T. Medicine/Horsethief Formations - n=9 Marias River Shale - n=2 Blackleaf Formation - n=7 Upper section - n=2 A Kootenai Formation Lower section - n=3 Lv Ls Ellis/Morrison Formations - n=7 B AGE FORMATION Qm FLtLsLv Lm

)

E (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 100

A Blackleaf

120 L A Kootenai

E B CRETACEOUS H 140 V B T

T I K

A Morrison

160 S O Swift

A E C

i Rierdon

R 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 12. (A) Ternary diagrams illustrating mean modal framework-grain compositions of foreland basin sandstones with standard deviations. Qm—monocrystalline quartz; Qt—total quartz; Lt—total lithics; F—feldspar; Lv—volcanic lithics; Ls—sedimentary lithics. (B) Plot showing the stratigraphic distribution of ratios of main grain types. For explanation of petrographic parameters, see Table 1. Both ﬁ gures after Fuentes et al. (2011).

1118 Geosphere, October 2012

TABLE 1. PETROGRAPHIC PARAMETERS ment of the Lewis thrust ca. 72 and ca. 52 Ma. Quartzose grains Forward thermokinematic modeling by Harde- Qm Monocrystalline quartz bol et al. (2009) is in agreement with onset of Qp Polycrystalline quartz Qpt Foliated polycrystalline quartz deformation ca. 75 Ma. None of these data Qss Quartz in sandstone or quartzite lithic grain exclude the possibility of earlier displacement. S Siltstone Qt Total quartzose grains (Qm + Qp + Qpt + Qss + S + C + Cb) The apatite and zircon cooling ages could be a result of passive uplift during duplex devel- Feldspar grains K Potassium feldspar opment beneath the Lewis thrust. Dates from P Plagioclase feldspar fault gouge at isolated localities may represent F Total feldspar grains (K + P) discrete episodes of movement, especially Lithic grains considering that van der Pluijm et al. (2006) Metamorphic showed two different age clusters separated by Lph Phyllite Lsm Schist ~20 m.y. The illite ages could also be related to Lss Serpentine schist diagenetic processes in fault zones, rather than M Marble Lm Metamorphic lithic grains (Lph + Lsm + Lss + M + Qpt) kinematic events. Price (2007) and Pevear et al. (2007) presented an interesting discus- Volcanic Lvl Lathwork volcanic grains sion about the validity of the van der Pluijm Lvx Microlitic volcanic grains et al. (2006) ages. Lvf Felsic volcanic grains Sears (2001) proposed that the Lewis thrust Lvv Vitric volcanic grains Lvm Mafi c volcanic grains in Montana did not move until 74 Ma, based on Lv Total volcanic lithic grains (Lvl + Lvx + Lvf + Lvv + Lvm) two lines of argument: (1) correlative ca. 76 Ma Sedimentary sills intruded undeformed Cretaceous rocks in D Dolostone both footwall and hanging-wall positions prior Lc Limestone Lsh Shale or mudstone to thrusting; and (2) 74 Ma ash-fall deposits C Chert covered footwall and hanging-wall rocks of the Cb Black chert Lewis thrust sheet. Andesite sills in the hang- Ls t Total sedimentary lithic grains (D + Lc + Lsh + C + Cb + Qss + S) ing wall yielded a biotite 40Ar/39Ar age of 75.9 ± Lt Total lithic grains (Ls + Lv + Lm) 1.2 Ma (Sears et al., 1997, 2000). However, L Unstable lithic fragments [Lt (C + Cb + Qss + S + Qpt)] these sills are restricted to the Garrison depres- Note: Accessory minerals: epidote, zoisite, chlorite, garnet, amphibole, muscovite, biotite, sion, located southwest of the Lewis thrust tourmaline, pyroxene, kyanite, cordierite, sillimanite, zircon, kaolinite, glauconite, and apatite. sheet, in the domain of the Lewis and Clark line and Helena salient (Sears et al., 2000). The assumed correlative sills in the Lewis thrust sys- thrust (Harrison et al., 1986), suggests that this pone and Yin (1994) suggested that the Moyie tem footwall intruded concordant to bedding in fault was active earlier than Late Cretaceous thrust postdates the Snowshoe thrust system. the Kootenai and Blackleaf Formations; these time. Possible explanations for this discrep- Uncertainty also exists for the timing of slip sills are now exposed in strongly deformed ancy include: (1) the lack of clear crosscutting along the Libby and Pinkham thrust systems, thrust sheets to the west of the Sawtooth Range relations led to erroneous interpretations; and as well as other thrusts to the east, mapped as (Fig. 6). A sample of one sill yielded a biotite (2) the Vermillion River stock cuts a splay of the the Whitefi sh and Wigwam thrusts by Harrison 40Ar/39Ar age of 58.8 ± 1.5 Ma, with a disturbed Moyie system different from the one that cuts et al. (1992). Provenance data do not provide gas release spectrum, and a sericite 40Ar/39Ar the Dry Creek stock. If this were the case, it constraints for the timing of displacement on age of ca. 60 Ma (Sears et al., 2000; Sears, would be possible that movement on this thrust these faults because of the similar hanging-wall 2001); these ages were interpreted as cooling system occurred during pre–late Aptian time, stratigraphy of the thrust sheets involved. All of ages (Sears, 2001). New U-Pb geochronology with selective reactivation of splays during these thrusts sheets detach in and expose Belt of zircons from a sill sample collected west the late Campanian–Maastrichtian. This view Supergroup strata, and prior to exhumation and of Gibson Reservoir (Fig. 6) was conducted aligns with the suggestion of Early Cretaceous erosion contained miogeocline rocks and possi- by laser ablation–multicollector–inductively or older slip along the Moyie thrust (Price and bly early foreland basin deposits. coupled plasma mass spectrometry (LA-MC- Sears, 2000) based on structural similarities ICPMS) at the Arizona LaserChron Center. among the Moyie, St. Mary, and Hall Lake Lewis Thrust System The analytical data are reported in Table 2. A faults, and the recent age data from Larson et al. (Late Cretaceous–Early Paleogene) weighted mean age of 82.8 ± 0.8 Ma for this (2006) discussed herein. sill sample (Fig. 14) shows that not all of the The Snowshoe thrust, mapped as a forethrust Kinematic history information is better for sills in the Cretaceous rocks cut by the Lewis by Harrison et al. (1992), was reinterpreted as an the eastern part of the thrust belt (Fig. 11). Apa- thrust can be correlated confi dently. The 74 Ma east-dipping backthrust system by Fillipone and tite and zircon fi ssion track dating in the Lewis ash could have been deposited on top of mov- Yin (1994). The timing of slip on this thrust sys- thrust salient in southern Canada suggest rapid ing thrust sheets. Although the proposed onset tem is unknown. Based on local juxtaposition of hanging-wall cooling ca. 75 Ma (Osadetz et al., of slip along the Lewis thrust by Sears (2001) west-dipping cleavage in the Moyie thrust sheet 2004; Feinstein et al., 2007). The 40Ar/39Ar dat- is coincident with timing deduced from work in over east-dipping cleavage related to structures ing of clay-bearing gouge (illite) led van der Canada, we suggest that earlier episodes of slip of the Snowshoe thrust system, and truncation Pluijm et al. (2006) to suggest pulses of con- along this structure cannot be ruled out. The of west-vergent folds by the Moyie thrust, Filli- tractional deformation for the Canadian seg- new U-Pb age provides a ca. 83 Ma limit for

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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

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

l 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

1GRX (Morrison Fm.) - n=91

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

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

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

Figure 13. Age probability plots of U-Pb ages of detrital zircons of selected samples from foreland basin deposits. Shaded areas indicate main origins of zircons (from Coney and Evenchick, 1994; Roback and Walker, 1995; Gehrels and Ross, 1998; Ross and Villeneuve, 2003; Ross et al., 2005; Link et al., 2007). Yellow bars indicate depositional age of samples. Note change in horizontal scale in plots at 550 Ma. Modiﬁ ed from Fuentes et al. (2011). Data are in Supplemental Table (see footnote 2).

the maximum age of deformation of Cretaceous ment along this thrust system (Sears, 2001). termination of slip on the Lewis thrust system rocks east of the Lewis thrust system. New U-Pb ages were obtained from 20 zircon has not been precisely dated because the thrust Key constraints on the timing of displace- crystals from a sample of this intrusive at the sheets have been deeply eroded along their lead- ments on the Lewis thrust system and the fron- University of Arizona LaserChron Center. A ing edges, removing any Late Cretaceous–Early tal structures are provided by sills and dikes weighted mean age of 52.6 ± 0.4 Ma limits the Eocene strata that may have overlain or been associated with the Adel Mountain Volcanics, youngest possible age of slip along this fault truncated by these faults. The youngest unit in the southern part of this segment of the thrust to the Early Eocene (Fig. 15). To the east, a truncated by the Lewis thrust system is the Cam- belt. In the Rogers Pass area (Fig. 15), a mon- number of monzonite porphyry sills intruded panian Two Medicine Formation. An increase in zonite intrusion postdates motion along the into Cretaceous rocks are folded and cut by low-grade metamorphic grains in sandstones of Steinbach thrust (Whipple et al., 1987; Mudge minor thrusts, indicating that sill emplacement the lower part of the late Campanian–Maastrich- et al., 1982; Harlan et al., 2005). Although the predated or was synchronous with shorten- tian St. Mary River Formation (Fig. 12) possibly intrusion has a complex geometry, it clearly ing (Schmidt, 1972). The K/Ar and 40Ar/39Ar reﬂ ects exhumation of Belt Supergroup rocks cuts across the trace of the fault (Fig. 15). A ages of these sills (Harlan et al., 2005) suggest above this thrust system. A similar increase in K-Ar age (biotite) of 59.6 ± 1.6 Ma (Schmidt, thrusting younger than 55.5 Ma. low-grade metamorphic detritus at equivalent 1978; Whipple et al., 1987) has been used to The new U-Pb ages bracket the youngest stratigraphic levels was noted by Mack and establish the minimum possible age of move- movement on the Steinbach thrust; however, the Jerzykie wicz (1989) in southern Canada.

1120 Geosphere, October 2012

TABLE 2. U-Pb GEOCHRONOLOGIC ANALYSES Apparent ages Isotope ratios (Ma) 206Pb 206Pb* ± 207Pb* ± 206Pb* ± error 206Pb* ± 207Pb* ± 206Pb* ± Analysis 204Pb 207Pb* (%) 235U* (%) 238U (%) correction 238U* (Ma) 235U (Ma) 207Pb* (Ma) 2RP-1 8364 16.6804 13.6 0.0689 14.3 0.0083 4.3 0.30 53.5 2.3 67.6 9.4 601.8 296.2 2RP-2 14980 17.4316 10.3 0.0624 11.2 0.0079 4.4 0.39 50.6 2.2 61.4 6.7 505.7 226.6 2RP-3 7588 23.2611 22.1 0.0487 22.2 0.0082 1.8 0.08 52.7 1.0 48.3 10.5 –168.2 556.9 2RP-5 16788 25.8982 85.2 0.0434 85.2 0.0082 1.3 0.02 52.4 0.7 43.2 36.0 –442.9 2851.5 2RP-6 113360 24.7918 15.3 0.0475 15.4 0.0085 1.3 0.08 54.8 0.7 47.1 7.1 –329.4 395.8 2RP-7 16560 24.5352 28.5 0.0455 28.6 0.0081 1.8 0.06 52.0 0.9 45.2 12.6 –302.7 742.5 2RP-8 20760 18.1797 24.1 0.0621 24.2 0.0082 2.2 0.09 52.5 1.2 61.1 14.4 412.5 545.8 2RP-9 6404 18.2371 45.1 0.0625 45.1 0.0083 2.2 0.05 53.1 1.1 61.6 27.0 405.4 1060.5 2RP-10 14448 24.7605 20.8 0.0474 20.8 0.0085 1.6 0.08 54.6 0.9 47.0 9.6 –326.1 537.9 2RP-11 15944 20.1302 28.4 0.0617 28.4 0.0090 1.7 0.06 57.8 1.0 60.8 16.8 179.9 673.6 2RP-12 5336 12.9089 55.4 0.0907 55.5 0.0085 2.9 0.05 54.5 1.6 88.1 46.9 1133.2 1202.2 2RP-13 13512 22.5989 7.7 0.0509 8.3 0.0083 3.0 0.36 53.6 1.6 50.4 4.1 –96.7 190.1 2RP-14 9644 18.4367 25.7 0.0636 26.0 0.0085 4.0 0.15 54.6 2.2 62.6 15.8 381.0 585.7 2RP-15 14536 20.4294 28.1 0.0536 28.1 0.0079 1.5 0.05 51.0 0.8 53.0 14.5 145.4 670.1 2RP-16 85040 22.0611 16.6 0.0505 16.7 0.0081 1.4 0.08 51.9 0.7 50.0 8.2 –37.9 406.4 2RP-17 27588 22.6283 11.1 0.0499 11.1 0.0082 1.0 0.09 52.6 0.5 49.5 5.4 –99.9 272.4 2RP-18 33244 19.8337 9.8 0.0547 10.2 0.0079 2.8 0.27 50.6 1.4 54.1 5.4 214.4 227.2 2RP-19 10376 19.7461 14.8 0.0565 14.9 0.0081 2.3 0.15 52.0 1.2 55.8 8.1 224.6 342.8 2RP-20 25084 23.4852 11.5 0.0458 11.7 0.0078 2.0 0.17 50.0 1.0 45.4 5.2 –192.1 287.9 2RP-21 2252 13.7260 23.8 0.0851 23.9 0.0085 2.7 0.11 54.4 1.5 83.0 19.1 1009.9 489.0 KR2-1C 5986 18.6375 23.0 0.0891 23.8 0.0120 5.8 0.24 77.2 4.4 86.7 19.7 356.6 526.5 KR2-2C 9985 18.9472 15.2 0.0902 15.7 0.0124 3.9 0.25 79.4 3.1 87.7 13.2 319.3 347.5 KR2-3C 14415 21.0558 1.2 0.0817 2.8 0.0125 2.5 0.89 79.9 2.0 79.7 2.1 74.1 29.7 KR2-4C 18273 21.1273 1.7 0.0842 3.1 0.0129 2.6 0.84 82.6 2.1 82.1 2.4 66.0 39.6 KR2-5R 17771 20.6464 3.3 0.0860 3.8 0.0129 1.9 0.49 82.5 1.5 83.7 3.1 120.6 78.3 KR2-6R 21552 20.9000 2.2 0.0831 2.8 0.0126 1.8 0.64 80.7 1.4 81.0 2.2 91.7 51.0 KR2-8 17434 21.2500 7.5 0.0861 8.1 0.0133 3.0 0.37 85.0 2.5 83.9 6.5 52.2 179.5 KR2-9 19930 21.8703 3.2 0.0798 4.3 0.0127 2.9 0.68 81.1 2.4 78.0 3.3 –16.9 77.3 KR2-10 15117 16.6684 27.1 0.1081 27.1 0.0131 2.2 0.08 83.7 1.8 104.2 26.9 603.3 595.6 KR2-11 30097 21.0068 3.6 0.0870 4.1 0.0133 1.9 0.47 84.9 1.6 84.7 3.3 79.6 85.9 KR2-12 20500 20.8042 3.8 0.0862 4.8 0.0130 2.9 0.60 83.3 2.4 83.9 3.9 102.6 91.0 KR2-13 18877 21.0911 5.0 0.0862 5.2 0.0132 1.3 0.25 84.5 1.1 84.0 4.2 70.1 118.9 KR2-14 13490 21.6132 5.0 0.0758 10.0 0.0119 8.6 0.87 76.1 6.5 74.2 7.1 11.6 120.3 KR2-15 38379 20.8751 3.3 0.0857 3.9 0.0130 1.9 0.50 83.1 1.6 83.5 3.1 94.5 79.2 KR2-16 23194 20.4416 5.5 0.0873 6.0 0.0129 2.3 0.39 82.9 1.9 85.0 4.9 144.0 129.5 KR2-18 21508 20.1581 4.2 0.0875 4.4 0.0128 1.5 0.34 81.9 1.2 85.2 3.6 176.6 97.0 KR2-19 16619 20.2485 2.4 0.0891 3.6 0.0131 2.6 0.74 83.8 2.2 86.6 3.0 166.2 56.6 KR2-20 17869 22.4863 4.7 0.0785 5.6 0.0128 3.1 0.56 82.0 2.6 76.8 4.2 –84.5 114.0 KR2-21 19908 20.8994 5.5 0.0811 6.8 0.0123 4.0 0.59 78.8 3.2 79.2 5.2 91.8 129.7 KR2-22 2613 17.8995 13.8 0.0993 14.1 0.0129 2.6 0.18 82.5 2.1 96.1 12.9 447.1 308.7 KR2-23 13805 18.8866 20.5 0.0959 20.6 0.0131 2.1 0.10 84.1 1.8 92.9 18.3 326.5 469.3 KR2-24 27896 20.9723 2.7 0.0871 3.6 0.0133 2.3 0.66 84.9 2.0 84.8 2.9 83.5 63.6 Note: All uncertainties are reported at the 1σ level, and include only measurement errors. Systematic errors would increase age uncertainties by 1%–2%. U concentrations and U/Th are calibrated relative to our Sri Lanka zircon standard, and are accurate to ~20%. Common Pb correction is from 204Pb, with composition interpreted from Stacey and Kramers (1975) and uncertainties of 1.0 for 206Pb/204Pb, 0.3 for 207Pb/204Pb, and 2.0 for 208Pb/204Pb. U/Pb and 206Pb/207Pb fractionation is calibrated relative to fragments of a large Sri Lanka zircon of 564 ± 4 Ma (2σ). U decay constants and composition as follows: 238U = 9.8485 x 10 – 10, 235U = 1.55125 x 10 – 10, 238U/235U = 137.88.

Sawtooth Range and Foothills from stacking of thrust sheets (Hoffman et al., the 100–175 °C indicated by the low-grade (Late Cretaceous?–Early Paleogene) 1976). As Lageson (1987) pointed out, this age metamorphic mineralization. Recent thermo- range does not necessarily indicate the begin- kinematic work in southern Canada indicates Thrust displacement along the structures of ning and end of thrusting; instead it represents that a thickness of 3–5 km of synorogenic strata the Sawtooth Range postdates, at least in part, the time span during which maximum tempera- was removed from proximal positions (Harde- displacement on the Lewis thrust. Map relations tures were reached, which may correspond to bol et al., 2009), calling into question the inter- and cross sections along the Lewis thrust salient thrust emplacement. Moreover, the K-Ar ages pretation of burial metamorphism driven solely show that the Akamina syncline resulted from for bentonites provide crude constraints, but by thrust sheet burial. In any case, the timing passive folding during the emplacement of the detailed interpretations are problematic as rela- proposed by Hoffman et al. (1976) agrees with Waterton and Flathead duplexes (Bally et al., tive contributions from tectonic and sedi mentary the general timing of structure formation in the 1966; Gordy et al., 1977; Fermor and Moffat, burial, potential effects of ﬂ uids, and potential frontal part of the thrust belt. 1992; Mudge and Earhart, 1983). These two problems from incomplete resetting and mix- The youngest sedimentary unit truncated by duplexes are the northern equivalent of the Saw- ing of small detrital components are difﬁ cult to thrusts in the foothills region is the Willow Creek tooth Range (Fig. 10). evaluate. Hoffman et al. (1976) and Hoffman Formation (Mudge and Earhart, 1983), which Samples from Cretaceous bentonite beds in and Hower (1979) assumed that the maximum is broadly dated by vertebrate and in verte brate thrust sheets of the Sawtooth Range and foot- thickness of strata above the stratigraphic inter- fossils as Maastrichtian–Early Paleocene (Rus- hills yielded K/Ar ages (illite-smectite) of val sampled (Blackleaf Formation to Two Medi- sell, 1950, 1968; Tozer, 1956; Catuneanu and between 72 and 56 Ma that were interpreted as cine Formation) was 350–1350 m; this requires Sweet, 1999). Along the triangle zone of the the results of low-grade burial metamorphism a considerable tectonic thickening to reach Alberta syncline, rocks of the early Paleocene

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12 Sample Kr2 (n=22) 80

75 Age (Ma)

10 probability Relative 70

8 65 500 1000 1500 2000 2500 3000 3500 4000 U conc (ppm)

6 90 85

4 80 Number of grains 75 Age (Ma) 2 70

A 65 0 0.4 0.6 0.8 1.0 1.2 1.4 U/Th 60 70 80 90 100 90 Age (Ma) 85

Final age = 82.8 ± 1.3 Ma (1.6%) 80

Weighted Mean Age = 82.8 ± 0.8 Ma (1.0%) Age (Ma)

Systematic Error = 1.3 % 70 MSWD = 0.9 65 –600 –400 –200 0 200 400 600 800 B Concordance (%)

Figure 14. (A) U-Pb relative probability plot of 22 analyzed zircon crystals. MSWD—mean square of weighted deviates. (B) Ages are from the Age Pick program of G. Gehrels (University of Arizona). Blue error bars are at 1σ. Uncertainties for ﬁ nal age and weighted mean are at 2σ; conc.—concentration. Location of sample is in Figure 6.

Porcupine Hills Formation are tilted as a result in the sandstone/shale ratio in this unit may be Late Cretaceous–Paleocene fi ne-grained and of wedging of blind thrust sheets. These are related to deeper exhumation of Belt Super- porphyritic volcanic rocks, 1%–5% Paleozoic the youngest preserved deposits that were hori- group rocks in the hanging wall of the Lewis rocks, and 1% chert and conglomerate. zontally shortened in the region. The youngest thrust system, which also could have increased Synorogenic sedimentation in the foreland synorogenic units in the foreland are the Paleo- the abundance of quartz and low-grade meta- continued until the Early Eocene. Extensional cene–Lower Eocene Fort Union and Wasatch morphic clasts as byproducts of Belt quartz- faulting along the fold-thrust belt initiated soon Formations, preserved on the fl anks of the Bear- ite and argillite erosion. The increased quartz after, during Middle Eocene time (Constenius, paw Mountains and in the Missouri Breaks dia- content may also result from greater transport 1996). The development of extensional basins in tremes (Hearn, 1968, 1976; Hearn et al., 1964). distance, insofar as these units were sampled at the thrust belt overlapped in time with low-angle Coeval deposits along the front of the thrust belt distal positions in the basin, or enhanced chemi- detachment faulting and development of meta- possibly accumulated in a wedge-top depozone, cal weathering. The uppermost Wasatch Forma- morphic core complexes in hinterland regions and were subsequently eroded. Erosion of the tion contains abundant volcanogenic sediment, (Doughty and Price, 2000; Johnson, 2006). proximal foreland basin was a regional process. as seen in multiple bentonite beds. Conglomer- Nurkowski (1984) estimated that 900–1900 m ate clast counts from this unit indicate deriva- DISCUSSION of stratigraphic over burden was eroded from tion from Belt Supergroup and volcanic rocks. above near-surface coals in the Alberta Plains. Pebble conglomerates in the Wasatch Forma- Controls on Thrust Belt Regional Geometry As mentioned here, more recent thermal and tion are dominated by clasts of Belt Supergroup forward kinematic modeling in southern Canada quartzite (~28%) and fi ne-grained mafi c igne- The geometry, structure, and stratigraphic suggests ~3 km of postorogenic erosion in the ous rocks (~28%). Shale, mudstone, and low- architecture of the Belt-Purcell basin controlled proximal foredeep and 3–5 km of exhumation in grade metamorphic pebbles constitute 17% of the architecture of the Cordilleran thrust belt the foothills (Hardebol et al., 2009). the total, followed closely by limestone clasts of north-central Montana and southern Canada Sandstone samples of the Fort Union, Por- (16%). The remaining ~10% consists of similar (Price and Sears, 2000). North of the Lewis cupine Hills, and Wasatch Formations show an proportions of fi ne-grained felsic igneous, gran- and Clark line, the Belt-Purcell deposits form a increase in the proportion of quartz relative to ite, and schist clasts. Estimates of clast composi- thick, rheologically strong prism of quartz-rich lower stratigraphic units (Fig. 12). Fort Union tion in Wasatch conglomerates by Hearn et al. metasedimentary rocks and mafi c sills (Sears, deposits are considerably sandier than underly- (1964) provided values of 50%–80% argillite 2007) with relatively minor internal shortening ing units, even at distal locations. The increase and quartzite of Belt Supergroup, 20%–40% (Figs. 5 and 7). As suggested for other major

1122 Geosphere, October 2012

112°20′

KEY 60 45 Paleogene monzonite intrusion N 32 Cretaceous volcanic member of Two Medicine Formation 55 Cretaceous Blackleaf Formation Yb Mississippian Lodgepole Limestone Proterozoic Belt Supergroup Age (Ma) 50

Ktm 45 0 200 400 600 800 1000 1200 1400 1600 1800 U conc (ppm) Pi

′ 60 0

Steinbach thrust °1

Kb 47 55

Age (Ma) 50 Sample 2RP 45 29 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 U/Th 500 m A 60

10 55

9 Sample 2RP (n=20) probability Relative

8 Age (Ma) 50 7 6 45 –150 –100 –50 0 50 5 Concordance (%) 4 Final age = 52.6 ± 1.9 Ma (3.6%) 3 Weighted Mean Age = 52.6 ± 0.4 Ma (0.8%) 2 C Systematic Error = 3.5 %

Number of grains 1 MSWD = 2.0 0 42 44 46 48 50 52 54 56 58 60 62 64 66 B Age (Ma)

Figure 15. (A) Simpliﬁ ed map showing structural relations between the Steinbach thrust and the monzonite intrusion (after Harlan et al., 2005; based on R.G. Schmidt, personal commun.) in the Roger Pass area. In the northwest corner of the map, the intrusion cuts across the trace of the fault as a high-angle intrusion. In the hanging wall of the Steinbach thrust, deposits of the Belt Supergroup show thermal altera- tion. A complete discussion of the structural relation between the intrusion and the thrust was provided in Harlan et al. (2005). Also shown is location of sample 2RP. Approximate map location is in Figure 2. (B) U-Pb relative probability plot of 20 analyzed zircon crystals. (C) Ages are from the Age Pick program of G. Gehrels (University of Arizona). Blue error bars are at 1σ (light blue sample excluded). Uncertainties for ﬁ nal age and weighted mean are at 2σ. Previous K-Ar reported age was 59.6 ± 1.6 Ma (Schmidt, 1978; Whipple et al., 1987); conc.—concentration; MSWD—mean square of weighted deviates.

thrust sheets in the Cordilleran thrust belt (Mitra, hence, trailing parts of thrust belts attain critical to slope and basin along the preorogenic rifted 1997), the relative lack of internal shortening taper with relatively minor internal deformation, basin margin. Flexure beneath the Paleozoic could be a result of this rheological strength. provided the rocks involved are rheologically miogeocline and early foredeep depozone would Sedimentary basin taper, or the predeformation strong (DeCelles and Mitra, 1995; Mitra, 1997; have further increased basement dip toward the basement dip, may be another key control in Yonkee, 2005). In northwestern Montana, this hinterland. Calculations of the depth to basement the structural style (Boyer, 1995). A hinterland may be particularly true; deposits of the Belt from seismic proﬁ les and wells along our cross increase in basement dip is characteristic of most Supergroup thicken markedly toward the west, section support an increase of ~0.7° in the dip of basins deformed by thrust belts (Boyer, 1995); probably the result of the transition from shelf the basal décollement (β) over a relatively short

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distance, from ~3.3° beneath the frontal part of Sevier thrust belt, these major thrust sheets were Clark line and western structures of the Helena the thrust belt to ~4° beneath the Lewis thrust actively emplaced during Early Cretaceous (and salient region. system. In reality, the basement dip along any possibly latest Jurassic) time (e.g., Yonkee, Deposition of the thick offshore facies of the regional section varies with time once an oro- 1992; DeCelles, 2004). In contrast, the Lewis Cenomanian–Santonian Marias River Shale, genic system is established. This owes to the thrust system in northwestern Montana was and similar deposits (such as the lower part of complex interaction of sedimentary and tectonic mainly active during Campanian–Paleocene the Mancos Shale) are conspicuous along the loads, the propagation toward the foreland of the time. Whereas older thrust faults in northwest- Western Interior basin, and refl ect high eustatic fold-thrust belt and the fl exural wave of the fore- ern Montana and adjacent British Columbia sea-level conditions (Miall et al., 2008). land basin, and possible dynamic subsidence. involved Proterozoic rocks, it was not until the The late Santonian–early Campanian interval Thus, a westward increase in β was probably a Lewis thrust system became active that Protero- was marked by a strongly regressive system, major control in the low magnitude of internal zoic rocks were transported >100 km eastward. with rapid, relatively-coarse grained, clastic shortening west of the leading edge of the Lewis Another key difference between this part sedimentation. Thrust systems possibly active thrust system. of the Cordillera and other more intensively during this period include the Moyie, Snow- Toward the foreland, the decrease in origi- studied regions is the history of arc magma- shoe, Libby, Pinkham, Whitefi sh, and Wigwam. nal dip of the basement-cover interface and tism; in particular, the Cretaceous magmatic Concurrently, the Lewis and Clark strike-slip the relatively thin prism of lithologically and arc was situated directly west and southwest system was active, and foreland basin deposits rheologically variable Paleozoic and Mesozoic of the northwestern Montana thrust belt. High- north and south of it show differences in thick- strata available to construct the thrust belt likely volume magmatism in the region took place ness and facies (Wallace et al., 1990; Sears and required the formation of the tightly imbricated mainly shortly after the emplacement of the Hendrix, 2004). stack of thrust sheets in the Sawtooth Range in Lewis thrust system (Gaschnig et al., 2010), Major slip on the Lewis thrust system com- order to maintain critical taper. In other words, and may have been fueled by melt-fertile lower menced during deposition of the late Cam panian high initial taper and great internal strength crust and lithosphere that was underthrust west- Bearpaw-Horsethief Formation. An increase in characterized the Lewis thrust system, whereas ward while the Lewis thrust system was active low-grade metamorphic grains in sandstones of lower initial taper and generally lower strength in the upper crust. the lower part of the latest Campanian–Maas- but greater rheological heterogeneity (i.e., layer- trichtian St. Mary River Formation suggests ing) characterized the Sawtooth Range system. Relations with the Foreland Basin System exhumation of Belt rocks along the Lewis sys- The value of taper necessary for continued tem. Detrital zircons from the St. Mary River thrust belt forward propagation was neverthe- The foreland basin system records major tec- Formation indicate predepositional and syn- less attained, as the thrust belt remained active tonic events in the retroarc region. Jurassic strata deposi tional volcanic sources, as well as input until Eocene time. of the Ellis Group and Morrison Formation in from Paleozoic and Belt Supergroup strata. Both Inherited structures of the Proterozoic rifted Montana were deposited in the distal part of an the St. Mary River and Willow Creek Formations margin also controlled the structure of the thrust early foreland basin (possibly in a slowly sub- indicate high sedimentation rates proximal to the belt hinterland (Harrison et al., 1974; Winston, siding back-bulge depozone) during collision of advancing thrust front. The paradoxical absence 1986). The Snowshoe and Pinkham thrusts have the Intermontane terrane (Fuentes et al., 2009, of coarse-grained conglomerate in these forma- been interpreted as inverted Proterozoic normal 2011) (Fig. 11). The regional unconformity at tions may be explained by: (1) most material ini- faults (Sears, 2007). A question that arises with the top of the Morrison Formation originated in tially eroded from the hanging wall of the Lewis these interpretations, however, is the reason response to cratonward migration of a fl exural thrust system was fi ne-grained or unconsolidated for the selective reactivation of only the upper forebulge, eustatic sea-level fall, and possible foreland basin deposits, not prone to generating segments of these structures, without base- decreased dynamic subsidence. The Lower Cre- gravel; (2) coarse-grained alluvial fans were ment involvement. Several thrust segments with taceous Kootanai Formation is the oldest unit of restricted to the most proximal areas in front of anomalous orientations seem to represent inver- clearly synorogenic sedimentary strata that con- the Lewis thrust system, where they were vul- sion of synextensional transform faults (Price sistently thickens to the west, as expected for nerable to erosion during continuing displace- and Sears, 2000; Sears, 2007); a good example foredeep deposits. Provenance information indi- ment on the Lewis and underlying thrusts in the is the northeast-striking segment of the Moyie cates that sediment was derived from deformed Sawtooth Range duplex; and (3) coarse-grained thrust in Canada, aligned with the crustal dis- miogeoclinal strata, volcanic sources in the sediment accumulation in the foreland basin was continuity of the Vulcan suture zone (Price and magmatic arc, and the Intermontane belt. At that highly localized along the front of the Cordi- Sears, 2000). time, the tectonic subsidence curve shows the lleran thrust belt and was partly controlled by A major difference between this sector of the onset of a convex-upward pattern, typical of a local across-strike structural discontinuities Cordilleran thrust belt and areas of the Sevier foredeep depozone. All these elements suggest (Lawton et al., 1994). belt farther south is in the timing of slip on the the propagation of the thrust belt toward the east The fi nal record of synorogenic sedimenta- regional-scale, far-traveled Proterozoic quartz- and lateral migration of the fl exural wave. tion in the foreland extended from Paleocene ite-bearing thrust sheets. In northern Utah and The Albian–lower Cenomanian Blackleaf until the Early Eocene, during the young- Idaho, the Paris-Willard-Meade thrust system Formation shows an abrupt increase in the pro- est movement of the Lewis thrust system and carries a thick succession of Proterozoic quartz- portion of low-grade metamorphic lithic frag- deformation along the Sawtooth Range, and ite and overlying Paleozoic shelfal strata, with ments, suggesting involvement of Belt Super- is recorded in erosional remnants of the Fort more than 50 km of slip (Yonkee et al., 1997). group rocks in thrust sheets in the Cordilleran Union and Wasatch Formations in the distal In central and southern Utah, the Canyon Range hinterland. Candidate thrusts include the Hall foreland. Any coarse-grained, more proximal and Sheeprock thrusts and their lateral equiva- Lake, St. Mary, and Moyie thrust system. Alter- deposits were removed by uplift and erosion lents carry thick Proterozoic quartzite sections native or additional sediment sources include during post glacial isostatic rebound, erosion- more than 100 km to the east. Throughout the the transpressive structures in the Lewis and driven isostatic rebound of the thrust belt (Sears,

1124 Geosphere, October 2012

2001), or exhumation in response to generalized of shortening substantially decreases southward tana thrust belts, in McClay, K.R., ed., Thrust tectonics: extension since the Middle Eocene (Harde- in Montana, as suggested by Price and Sears London, England, Chapman & Hall, p. 377–390. Boyer, S.E., 1995, Sedimentary basin taper as a factor bol et al., 2009). Regional extensional faulting (2000) and Sears (2001, 2007). controlling the geometry and advance of thrust belts: along the thrust belt commenced during Middle American Journal of Science, v. 295, p. 1220–1254, ACKNOWLEDGMENTS doi:10.2475/ajs.295.10.1220. Eocene time (Constenius, 1996) and brought Boyer, S.E., and Elliott, D., 1982, Thrust systems: American to a close the preceding ~110 m.y. episode of Funding for this work was provided by the Association of Petroleum Geologists Bulletin, v. 66, growth in the Cordilleran orogenic belt. ExxonMobil COSA (Convergent Orogenic Systems p. 1196–1230. Analysis) project at the University of Arizona, with Burchfi el, B.C., and Davis, G.A., 1972, Structural framework and evolution of the southern part of the Cordi lleran CONCLUSIONS additional grants from ChevronTexaco, the Ameri- orogen, western United States: American Journal of can Association of Petroleum Geologists, and the Science, v. 272, p. 97–118, doi:10.2475/ajs.272.2.97. Geological Society of America. A Fulbright schol- Burchfi el, B.C., and Davis, G.A., 1975, Nature and controls The Cordilleran retroarc at the latitude of arship was granted to Fuentes. Charles Park, David of Cordilleran orogenesis, western United States— northwestern Montana started its evolution Gingrich, Ylenia Almar, Erin Brenneman, and Nicole Extension of an earlier synthesis: American Journal of during the Middle Jurassic. Although the early Russell assisted during fi eld work. We thank Steve Science, v. 275A, p. 363–396. Boyer, Paul Kapp, George Gehrels, Jerry Kendall, Burchfi el, B.C., Cowan, D.S., and Davis, G.A., 1992, Tec- structures of the thrust belt have been removed Steve Lingrey, Mike McGroder, and Lynn Peyton for tonic overview of the Cordilleran orogen in the west- by erosion, the foreland basin system records ern United States, in Burchfi el, B.C., et al., eds., The discussions and insights into Cordilleran thrust belt Cordilleran orogen: Conterminous U.S.: Boulder, the initial stages of deformation in regions that tectonics. LithoTect software used to construct the Colorado, Geological Society of America, Geology of are now located in the orogenic hinterland. balanced cross section was provided by GeoLOGIC North America, v. G-3, p. 407–480. The westernmost preserved structures, includ- Systems. We thank Excel Geophysical Services for Catuneanu, O., and Sweet, A.R., 1999, Maastrichtian-Paleo- reprocessing some of the seismic data. B. 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