Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A. Facundo Fuentes†, Peter G. DeCelles, Kurt N. Constenius, and George E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA ABSTRACT episode of marine inundation and black shale 1989; Fermor and Moffat, 1992; Stockmal et al., deposition (Marias River Shale) occurred be- 1992; Beaumont et al., 1993; Plint et al., 1993; New lithostratigraphic and chronostrati- tween the Cenomanian and mid-Santonian, Ross et al., 2005; Miall et al., 2008; Yang and graphic, geochronologic, and sedimentary and was followed by a regressive succession Miall, 2009). This bimodal focus was mainly petrologic data illuminate the history of represented by the Upper Santonian–mid- driven by either the presence of anomalously development of the North American Cor- Campanian Telegraph Creek, Virgelle, and good surface exposures, as in the case of the dilleran foreland basin system and adjacent Two Medicine Formations. Provenance data western interior United States, or by hydro- thrust belt from Middle Jurassic through do not resolve the timing of individual thrust carbon exploration and a large subsurface data- Eocene time in northwestern Montana. The displacements during Cenomanian–early base, as in Canada (Miall et al., 2008). The oldest deposits in the foreland basin system Campanian time. The Upper Campanian ~300-km-long segment of the foreland basin consist of relatively thin, regionally tabu- Bearpaw Formation represents the last major lying within and east of the Cordilleran belt in lar deposits of the marine Ellis Group and marine inundation in the foreland basin . By northwestern Montana remains comparatively fl uvial-estuarine Morrison Formation, which latest Campanian time, a major epi sode of poorly understood in terms of its stratigraphy, accumulated during Bajocian to Kimmerid- slip on the Lewis thrust system had com- basin evolution, and relationship with the kine- gian time. U-Pb ages of detrital zircons and menced, as recorded in the foreland by the matics of the developing fold-and-thrust belt. sandstone modal petrographic data indicate Willow Creek and St. Mary River Forma- In this paper, we present new stratigraphic that by ca. 170 Ma, miogeoclinal strata were tions in the proximal foredeep depozone. The and provenance data that, coupled with previ- being deformed and eroded in hinterland re- fi nal stage in the evolution of the Cor dilleran ous work, establish a coherent model for the gions. Sandstones of the Swift and Morrison fold-and-thrust belt and foreland basin evolution of the foreland basin in the context Formations contain detrital zircons derived system is recorded by the Paleocene–early of an integrated orogenic system in this region from the Intermontane belt. The Jurassic Eocene Fort Union and Wasatch Formations, from Jurassic to Eocene time. Besides fi lling deposits probably accumulated in the distal, which were preserved in the distal foreland a regional gap, the results of this paper should back-bulge depozone of an early foreland region. Regional extensional faulting along help to establish links and comparisons between basin, as suggested by the slow rates of tec- the fold-and-thrust belt began during the what is known from previous work in Canada tonic subsidence and tabular geometry. A middle Eocene. The results presented here and in better known parts of the Cordilleran regional unconformity separates the Jurassic enable the establishment of links between foreland basin system in the United States. strata from late Barremian(?) foredeep de- previous geological work in Canada and the posits. This unconformity possibly resulted better known parts of the Cor dilleran fore- REGIONAL SETTING as a combined effect of forebulge migration, land basin in the United States. decreased dynamic subsidence, and eustatic The foreland basin of northwestern Montana sea-level fall. The late Barremian(?)–early INTRODUCTION occupies a central location along the ~3000 km Albian Kootenai Formation is the fi rst unit length of the Cordilleran fold-and-thrust belt that consistently thickens westward, as Foreland basin systems are the stratigraphic and foreland basin system (Fig. 1). The North would be expected in a foredeep depozone. recorders of processes occurring in contiguous American Cordillera developed from mid- The subsidence curve at this time begins to orogenic belts. The history of terrane accretions, Mesozoic to Eocene times (e.g., Burchfi el et al., show the convex-upward pattern character- fold-and-thrust belt development, magmatism, 1992; Coney and Evenchick, 1994; DeCelles, istic of foredeeps. By Albian time, the fold- and major exhumation events is registered in 2004; Dickinson, 2004), and was subsequently and-thrust belt had propagated to the east the stratigraphy of these basins. Previous efforts modifi ed by gravitational collapse and later, by and incorporated Proterozoic rocks of the to link the foreland basin fi ll with the tectonic mid- to late Cenozoic Basin and Range exten- Belt Supergroup, as indicated by sandstone development of the North American Cordillera sion (Constenius, 1996). The initial evolution compositions, detrital zircon ages in the have been concentrated in two large regions: of the Cordillera was marked by subduction of Blackleaf Formation, and by crosscutting Utah, Colorado, and Wyoming in the United oceanic plates of the Panthalassa Ocean, and ac- relationships in thrust sheets involving Belt States (e.g., Royse et al., 1975; Jordan, 1981; cretion of parautochthonous terranes, fringing Supergroup rocks in the thrust belt. A major Lamerson, 1982; Heller et al., 1986; DeCelles, arcs, and exotic terranes (e.g., Coney et al., 1980; 1994; Currie, 2002), and Alberta and British Monger et al., 1982; Coney and Evenchick , †E-mail: [email protected] Columbia in Canada (e.g., Cant and Stockmal, 1994; Dickinson, 2004; Colpron et al., 2007). GSA Bulletin; March/April 2011; v. 123; no. 3/4; p. 507–533; doi: 10.1130/B30204.1; 12 fi gures; 2 tables; Data Repository item 2011002. For permission to copy, contact [email protected] 507 © 2011 Geological Society of America Fuentes et al. 141°W Alaska 120°W 115°W (Cache N Creek) Banff U.S.A. (Stikinia) metamorphic Accreted core-complex N terranes Insular extension Frontal and superterrane Foreland b 50°N magmatic (Quesnell+Kootenay) f arc old-thrust belt Fold-thrust CANADA belt Intermontane asin USA subduction complex terranes Canada J-K subduction complex Libby U.S.A. 49°N Forearc and L&C line Spokane Laramide province system Foreland basin Columbia River basalts Idaho Helena batholith 46°N 100 km Mexico 30°N B 500 km A 110°W 116°W 110°W Sr = 0.706 Hall Lake Alberta rch St. Mary a WigwamFernie s N syncline Snowshoe Purcell CANADA 49°N Moyie Sweetgras A’ USA Anticlinorium Whitefis Kevin Williston Basin Pinkham Browning Sunburst Hope f h LEDSH Montana Bearpaw Mtns. Libby dome ault Li Sawtooth bby Range Choteau Lewi Flathead South s and Clark Lake A foreland bas arch Spokane line Augusta Highwood Mtns. in Alta Sask L MT o Helena mbar Idaho 0 100 km d- batholith Eldorado Crazy Mtns. WY 46°N Butte ID C Bozeman KEY FIGURES 1B AND 1C: Cenozoic extensional basin fill Paleogene forearc and t Cenozoic volcanic rocks s t subduction complex s n u n r e Cretaceous subduction i Cretaceous intrusive rocks h s t m complex - a Mesozoic to lower Cenozoic e d l b l e foreland basin deposits Insular superterrane o d f d n f e l Paleozoic and Mesozoic e o a t Stikinia deformed sedimentary rocks e e y r r h Middle and Late Proterozoic c o f Cache Creek p c a Belt and Windermere d A Kootenay and r n g Late Proterozoic Hamill Group i a t C Intermontan terranes Quesnell t and Paleozoic sedimentary rocks a 1 l r t e Late Proterozoic e r b S Windermere Supergroup u g Middle Proterozoic i F Belt Supergroup Early Proterozoic pre- Belt metamorphic rocks Figure 1. (A) Simplifi ed tectonic map of the North America Cordilleran orogenic system. Box shows location of map of B. (B) Terrane map of northwestern United States and southwestern Canada (terranes based on Dickinson, 2004; Colpron et al., 2007). (C) Tectonic index map of the fold-and-thrust belt and foreland basin of northwestern Montana and adjacent areas. Only names of major thrusts and thrust systems are indicated. Thrusts in the Sawtooth Range and foothills regions of Montana and Canada are schematic and not ornamented. Dashed line in the southeast quadrant of the map indicates Belt Island paleohigh during the Jurassic (from Parcell and Williams, 2005). A-A′ is line of section of Figure 3. Figure was compiled from Alpha (1955); Mudge et al. (1982); Harrison et al. (1986, 1988, 1992); Latham et al. (1988); Constenius (1996); Kleinkopf (1997); Lageson et al. (2001); and Parcell and Williams (2005). 508 Geological Society of America Bulletin, March/April 2011 Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A. By Late Jurassic time, this orogenic belt was STRATIGRAPHY AND Island” complex (Suttner et al., 1981) and the mostly a coherent system, and was developing a SEDIMENTOLOGY Sweetgrass Arch and related structures, and fold-and-thrust belt and a foreland basin system thicken across the Williston Basin to the east (DeCelles, 2004). Approximately 2.5–3 km of Jurassic–Lower (Carlson, 1968). At the latitude of northwestern Montana, Paleocene strata currently lie in front of and The Sawtooth Formation is 15–50 m thick, southwestern Alberta, and southern British Co- within the frontal part of the fold-and-thrust belt and is composed of cross-bedded and ripple- lumbia (~46–51°N), the Cordilleran orogenic in northwestern Montana (Figs. 2 and 3). Most laminated sandstones and laminated mudstones, belt is composed of: (1) a fold-and-thrust belt, of the Lower Paleogene succession has been deposited in nearshore marine environments characterized by closely spaced thrust faults in- erosionally removed from the proximal fore- during a regional transgressive event.