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onset of deposition in northwestern , USA: Implications for along-strike synchroneity of Cordilleran orogenic activity

F. Fuentes*, P.G. DeCelles, G.E. Gehrels Department of Geosciences, University of , Tucson, Arizona 85721, USA

ABSTRACT Stratigraphic, provenance, and subsidence analyses suggest that by the Middle to Late Jurassic a foreland basin system was active in northwestern Montana (). U-Pb ages of detrital zircons and detrital modes of indicate provenance from accreted terranes and deformed miogeoclinal rocks to the west. Subsidence commenced ca. 170 Ma and followed a sigmoidal pattern characteristic of foreland basin systems. Thin Jurassic deposits of the and accumulated in a backbulge depozone. A regional and/or paleosol zone separates the Morrison from Early foredeep deposits of the Kootenai Formation. The model presented here is consistent with regional deformation events registered in hinterland regions, and challenges previous interpretations of a strongly diachronous onset of Cordilleran foreland basin deposition from northwestern Montana to southern .

INTRODUCTION Formations (Mudge, 1972). The Ellis Group correlates with the upper part One of the most controversial aspects of the Cordilleran thrust of the of southwest and southeast British Colum- belt and foreland basin system is also one of the most fundamental, i.e., bia (Poulton et al., 1994). The overlying Morrison Formation consists of when did this system initially develop? Estimates for the onset of fore- ~60–80 m of fi ne-grained estuarine to nonmarine strata, its upper part land basin accumulation in the western interior of the United States span usually overprinted by strong pedogenesis. Palynology of three Morrison ~75 m.y. from Early Jurassic to time (Heller et al., 1986; Bjerrum samples yielded –Kimmeridgian ages (GSA Data Repository and Dorsey, 1995; DeCelles and Currie, 1996; Allen et al., 2000, Dorsey Table DR11), suggesting temporal continuity between the Ellis Group and and LaMaskin, 2007), whereas most workers agree that a foreland basin the Morrison Formation. was established in southern Canada by the Middle to Late Jurassic (Cant A regional unconformity or zone of extreme stratigraphic conden- and Stockmal, 1989; Fermor and Moffat, 1993; McMechan and Thompson, sation separates the Morrison from the overlying Lower Cretaceous 1993; Poulton et al., 1994; Ross et al., 2005). Gillespie and Heller (1995) Kootenai Formation. In places this unconformity cuts into the lower part inferred that foreland basin development in the western United States was of the Ellis Group along incised paleovalleys (Dolson and Piombino, ~40 m.y. later than in Canada, giving rise to the notion of strongly diachro- 1994). The Kootenai consists of ~200–400 m of fl uvial and minor lacus- nous . In this paper we present stratigraphic, geochronologic, and trine deposits. This and correlative units have been sparsely dated as provenance data from northwestern Montana that support the hypothesis Barremian(?)–Aptian (DeCelles, 2004). New palynology and detrital that deposition took place in a retroarc foreland basin zircon data extend this range into the lower Albian (Tables DR1 system. We conclude that initial foreland basin development was nearly and DR2). The Lower Cretaceous, as well as the overlying 2500 m synchronous along the United States and southern Canadian portions of succession of Albian–early age, was deposited in the foredeep the Cordilleran thrust belt. depozone of the Cordilleran foreland basin system (Suttner et al., 1981; Gillespie and Heller, 1995; DeCelles, 2004). REGIONAL SETTING AND JURASSIC– DETRITAL ZIRCON U-Pb AGES Three major tectonic elements compose the Cordilleran orogenic A total of 476 U-Pb ages of detrital zircon grains from fi ve samples system at the latitude of northwestern Montana and southern Canada of Jurassic and Early Cretaceous sandstones was obtained by laser- (Fig. 1): a western region of accreted terranes (e.g., Colpron et al., 2007; ablation–multicollector inductively coupled plasma–mass spectrometry Giorgis et al., 2008); a fold-and-thrust belt involving and Meso- at the University of Arizona LaserChron Center (analytical procedures in zoic sedimentary rocks in its frontal part, and mainly strata Gehrels et al., 2008). Analyses that yielded isotopic data with acceptable in the hinterland; and a –early foreland basin system discordance, precision, and in-run fractionation are shown in Table DR2. that has been partially incorporated into the fold-and-thrust belt. In north- The ages are plotted on relative probability diagrams (Fig. 3); peaks on western Montana an unconformity representing ~150 m.y. separates the these diagrams are considered robust if defi ned by three or more analyses. youngest preorogenic strata of the from In addition to provenance information, the youngest cluster of ages for the oldest strata that possibly could be linked to Cordilleran orogenic evo- each sample gives a maximum depositional age. lution, the Middle Jurassic Ellis Group (Fig. 2). The Ellis Group consists of ~100–200 m of –Oxfordian 1GSA Data Repository item 2009091, Table DR1 (palynology table), Table DR2 (geochronologic analyses), and Table DR3 (detrital modes of sandstones), clastic and carbonate marine deposits of the Sawtooth, Rierdon, and Swift is available online at www.geosociety.org/pubs/ft2009.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, *E-mail: [email protected]. CO 80301, USA.

© 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, April April 2009; 2009 v. 37; no. 4; p. 379–382; doi: 10.1130/G25557A.1; 4 fi gures; Data Repository item 2009091. 379 120°W 115°W (Cache Banff 141°W metamorphic Alaska Frontal fold-and-thrust belt (Stikinia) Creek core-complex N N Insular extension Foreland U.S.A. 50°N Superterrane ) (Quesnell+Kootenai)

Belt b CANADA as i USA a n

terranes ~Franciscan

Intermontane llochton t subductio N Terranes Libby bel o Forear Canada rth L&C line

Ame Spokane

thrust U.S.A. Accreted - 49°N nc c and ri - e a c nd Fold basin a omplex m Laramid Columbia River basalts Helena province reland syste batholith o 46°N A F 500 Km B 100 Km

Cenozoic extensional Paleogene forearc and basin fill subduction complex Cretaceous subduction

Tertiary volcanic rocks ents

d-thrust

n complex Figure 1. A: General location map of Cordilleran orogenic system showing area Cretaceous intrusive rocks Insular Superterrane of B. B: Simplifi ed tectonic map of the fold-and-thrust belt and foreland basin Mesozoic to lower

f fold-an

e

lnd basi of study area with accreted terranes to the west (terranes based on Dickinson, foreland basin deposits ned elem Stikinia 2004; Colpron et al., 2007). Blue box indicates area of sample collection. Red Paleozoic and Mesozoic circle shows location of section used for subsidence curve in Figure 2. deformed sedimentary rocks Cache Creek

Accretio

nd forea

rranes termontan graphy o Middle and late Proterozoic Kootenai and

In Belt and Windermere te Quesnell

belt a Strati Early Proterozoic pre- Belt metamorphic rocks

Sample Eb from the Sawtooth Formation produced age clusters with Simplified Terrane Regional Tectonic a miogeoclinal signature (Gehrels and Ross, 1998). Most detrital zircons stratigraphy accretion deformation subsidence yielded ages older than ca. 1 Ga; a few grains provided ages in the range events 80 419–467 Ma, typical of upper Paleozoic– strata in the thrust belt; T two grains have Late Proterozoic ages; and one zircon yielded a syndepo- C 100 100 sitional age of ca. 171 Ma. The range of grains older than 1 Ga suggests A

hrusts

y, Hall Lake,

Foredeep deposits that the zircons of this sample were recycled from several intervals within up to Early Eocene the miogeocline, and perhaps refl ects an early fold-and-thrust belt involv- A ment

St. Mar 120 Moyie t 120 ing Paleozoic strata in the hinterland region. However, because zircons in EARLY B

KOOTENAI FM.

t

mation

emplace

f H orphism the miogeocline were mainly recycled from North American basement CRETACEOUS

V upli

rusting

superterrane and Grenville sources, a potential eastern contribution cannot be ruled out. 140 rocks B 140

Th

itic The single Jurassic grain supports a western source. al metam T MORRISON Major exhu

Insular

rass arch Samples 1GR14 from the Swift Formation and 1GRZ from the K FM. Gran

LATE O Region Morrison Formation provided zircons with a similar age spectrum. Most 160 SW. ? 160 C belt

R. Sweetg grains are older than 1 Ga, a few provided Late Proterozoic ages, and B S. sample 1GRZ produced an additional cluster in the range 403–457 Ma. B

ELLIS GR.

JURASSIC

montane

MIDDLE However, the key aspect of these samples is that they contain clusters of 180 A ages in the ranges 223–329 Ma and 156–180 Ma. The Mississippian– T Inter 0 0.5 1Km Triassic range is characteristic of igneous rocks of the eastern part of the Intermontane belt (Monger et al., 1982). The lack of other sources of zircons with those ages in the area indicates that terranes in the Inter- Figure 2. Chart showing local Middle Jurassic–Early Cretaceous stratigraphy, main tectonic events in northwestern Montana and montane belt were structurally elevated and supplying sediments to a southwestern Canada, and tectonic subsidence curve for northwest- basin located hundreds of kilometers to the east. Relatively abundant ern Montana foreland basin system. Time scale is from Palmer and Jurassic, arc-derived grains, further substantiate a western provenance Geissman (1999) and tectonic events are from sources discussed in as early as Oxfordian time. text. Location of subsidence curve is in Figure 1B; paleobathymetric Sample 1GR100 from the basal of the Kootenai Formation uncertainty is shown in gray. yielded abundant grains with a miogeoclinal signature, a few and Triassic zircons, and a relative increase in zircons derived from Middle Jurassic–Lower Cretaceous volcanic centers, a pattern that is amplifi ed in MODAL SANDSTONE PETROGRAPHY sample 1FG70 from the middle part of the unit. Sample 1FG70 provided Framework modes of 12 medium-grained sandstones from the Ellis abundant ages in the range 104–238 Ma, indicating predominant sources Group and Morrison and Kootenai Formations were determined by point- in the magmatic arc and Intermontane belt. counting ~450 grains per slide according to the Gazzi-Dickinson method. Most samples contained grains of Late Proterozoic ages, with Raw data and recalculated norms are available in Table DR3. On QmFLt unknown sources in the region. A possible scenario is transport along axial (monocrystalline quartz–feldspar–total lithic) and QtFL diagrams (Fig. 4), fl uvial systems from the south, in the Plateau region, where zir- the samples plot in the recycled orogen fi elds of Dickinson and Suczek cons of that age have been identifi ed (Dickinson and Gehrels, 2008). (1979), indicating provenance from a suture or fold-and-thrust belt. The

380 GEOLOGY, April 2009 Jurassic–Cretaceous Miogeocline SWSW volcanismn (upper part) U.S.AU.S.A

y Intermontane belt

t

i

l

i

b

a

b

o Miogeocline r 1FG70 (Kootenai Fm.) – n = 9999 p (NA basement,

e Grenville,

g

a Appalachian)

d

e

z 1GR100 (Kootenai Fm.) – n = 95

i

l

a 1GRZ (Morrison Fm.) – n = 87

m

r

o

N 1GR14 (Swift Fm.) – n = 98 Eb (Sawtooth Fm.) – n = 97 0 100 200 300 400 500 550 550 1000 1500 2000 2500 3000 Age (Ma) Age (Ma)

Figure 3. Age-probability plots of U-Pb ages of detrital zircons. Black bars indicate depositional age of samples. Shaded areas indicate main origins of zircons (from Coney and Evenchick, 1994; Roback and Walker, 1995; Gehrels and Ross, 1998; Ross and Villeneuve, 2003; Ross et al., 2005; Link et al., 2007). Note change in horizontal scale in plots. lithic fraction is dominated by , , , and phyllite. These Figure 4. Ternary dia- Qm Qt data strongly implicate sediment sources in deformed (meta)sedimentary grams showing modal- framework grain com- rocks of the Cordilleran thrust belt. positions of sandstones. Prove nance fields after RO SUBSIDENCE HISTORY Dickin son and Suczek RO CB Middle Jurassic– strata in northwestern Montana were (1979). RO—recycled CB decompacted and backstripped according to the methods described in orogen; CB—continental block; MA—magmatic arc. MA MA Angevine et al. (1990) and Allen and Allen (2005). The resulting tec- Raw data are in Table DR3 tonic subsidence curve (Fig. 2) has a sigmoidal shape, as expected from (see footnote 1). Qm— F F 50% Lm Lt L progressive stacking of foreland basin depozones in front of a migrating mono crystalline quartz; KEY orogenic load (DeCelles and Currie, 1996). The period 171–151 Ma, Qt—total quartz; F—feld- Kootenai spar; L—nonquartzose Formation during deposition of the Ellis Group and Morrison Formation, repre- lithic grains; Lm, Lv, Ls— Morrison sents initial, moderate subsidence in the region inboard of the fl exural Formation metamorphic, volcanic, Lv Ls Ellis Group forebulge. The multistory paleosols in the uppermost Morrison Forma- and sedimentary lithic tion indicate a continental condensed section, which is transitional into grains. the interval 151–127 Ma, marking a regional unconformity between Morrison and Kootenai strata. This unconformity has been recognized throughout the Cordilleran foreland basin, and attributed to the migration Timing of thrusting is poorly constrained in hinterland regions, but is of a fl exural forebulge (DeCelles, 2004). The Late Jurassic–Valanginian pre–middle Cretaceous in the United States–Canada border region (Price eustatic fall (Haq et al., 1987) may have contributed to erosion and low and Sears, 2000), and perhaps as old as late Middle Jurassic (McMechan and net sediment accumulation. From ca. 127 Ma onward, subsidence rates Thompson, 1993). increased rapidly, a trend that is typical of foredeep deposits. The principal argument for a later Cretaceous onset of fore- land basin development in this region is based on the assumption that INTERPRETATION AND CONCLUSIONS foreland basin sediment is confi ned to a relatively thick moat of fl exural Data presented here demonstrate that a foreland basin system became subsidence directly adjacent to the thrust belt load (Gillespie and Heller , active in northwest Montana as early as Bajocian time. Onset of subsidence 1995). Ellis Group and Morrison deposits do not thicken westward as and lithic-rich clastic sediment accumulation ca. 170 Ma, after a hiatus expected for foredeep deposits. However, sediment derived from the of ~150 m.y., indicates the development of a tectonically active orogenic thrust belt can be deposited on top of the fl exural forebulge as well as in belt to the west. Detrital zircon ages from Oxfordian–Kimmeridgian sand- the backbulge depozone (e.g., Horton and DeCelles, 1997; Yu and Chou, stones of the Swift and Morrison Formations provide unequivocal proof 2001; Roddaz et al., 2005). We suggest that relatively tabular and thin that sediment was derived from the eastern part of the Intermontane belt. Ellis and Morrison deposits in northwestern Montana were deposited in a Ellis and Morrison sandstones suggest derivation from deformed miogeo- backbulge depozone, and that the unconformity and/or condensation zone clinal strata to the west. that caps them was produced in part by migration of a forebulge through Timing of terrane accretion and deformation in the hinterland is the region. The model presented here is consistent with previous interpre- consistent with Middle to Late Jurassic foreland basin initiation in north- tations to the south that suggest the onset of foreland basin sedimentation western Montana (Fig. 2). The Intermontane terranes accreted to North during the Jurassic, and supports synchronous development of a foreland America during the Middle Jurassic (e.g., Coney and Evenchick, 1994; basin system from as far south as central (Bjerrum and Dorsey, 1995; Dickinson, 2004; Colpron et al., 2007; Dorsey and LaMaskin, 2007), DeCelles and Currie, 1996) to southern Canada. This is consistent with and the Insular superterrane soon after. Regional metamorphism in the coeval growth of the western Cordillera in response to rapid westward Omineca belt occurred between ca. 180 and 162 Ma (Archibald et al., movement of North America as heralded by Middle Jurassic seafl oor 1983; Fermor and Moffat, 1993), with rapid exhumation at 165–150 Ma. spreading in the North Atlantic (Coney and Evenchick, 1994).

GEOLOGY, April 2009 381 ACKNOWLEDGMENTS ogy, v. 23, p. 723–726, doi: 10.1130/0091–7613(1995)023<0723:BOFSIT> Funding was provided by ExxonMobil, ChevronTexaco, the Geological 2.3.CO;2. Society of America, and the American Association of Geologists. We Giorgis, S., McClelland, W., Fayon, A., Singer, B.S., and Tikoff, B., 2008, Timing thank Ylenia Almar, Erin Brenneman, and Nicole Russell for assistance during fi eld of deformation and exhumation in the western Idaho shear zone, McCall, work. Alan Carroll, Guy Plint, Paul Heller, and Tina Niemi provided constructive Idaho: Geological Society of America Bulletin, v. 120, p. 1119–1133: doi: reviews that helped us to improve the paper. 10.1130B26291.1. Haq, B.U., Hardenbol, J., and Vail, P.R., 1987, Chronology of fl uctuating sea REFERENCES CITED levels since the Triassic: Science, v. 235, p. 1156–1167, doi: 10.1126/ Allen, P.A., and Allen, J.R., 2005, Basin analysis: Principles and applications science.235.4793.1156. 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