ABSTRACT The Montana metasedimentary terrane (MMT) forms the NW margin of the Wyoming Province in present coordinates. The MMT preserves a multistage Paleoproterozoic tectonic history that clarifies the position of the Wyoming craton during assembly and breakup of the Precambrian Kenorland supercontinent and the subsequent assembly of Laurentia’s Precambrian basement. In SW Montana, burial, metamorphism, deformation, and partial melting attributed to orogeny were superimposed on Archean quartzofeldspathic orthogneisses and paragneisses at ca. 2.55 and ca. 2.45 Ga during the Tendoy and Beaverhead orogenies, respectively. Subsequent stability was disrupted at 2.06 Ga, when probable rift-related mafic dikes and sills intruded the older gneisses. The MMT was profoundly reworked by tectonism again as a consequence of the ca. 1.8–1.7 Ga Big Sky orogeny, during which juvenile metasupracrustal suites characteristic of an arc (the Little Belt arc) and back-arc basin collapsed against the Wyoming craton continental margin. The northern margin of the Wyoming craton occupied an upper-plate position south of a south-dipping subduction zone at that time. Lithostratigraphic correlations link the southeastern Wyoming and southern Superior cratons at ca. 2.45 Ga with the Wyoming craton joined to the Kenorland supercontinent in an inverted position relative to present coordinates. This places the MMT along an open supercontinental margin, in a position permissive of collision or accretion and orogeny during a time when other parts of Kenorland were experiencing mafic volcanism and incipient rifting. The ca. 2.45 Ga Beaverhead orogeny in the MMT was most likely the consequence of collision with one of the Rae family of cratons, which share a history of tectonism at this time. The Beaverhead collision enveloped the Wyoming craton in a larger continental landmass and led to the 2.45–2.06 Ga period of tectonic quiescence in the MMT. Breakup of Kenorland occurred ca. 2.2–2.0 Ga. In the MMT, this is expressed by the 2.06 Ga mafic dikes and sills that crosscut older gneisses. The Wyoming craton would have been an island continent within the Manikewan Ocean after rifting from Kenorland on one side and from the Rae family craton on the MMT side. Subduction beneath the MMT in the Wyoming craton started no later than 1.87 Ga and was active until 1.79 Ga. This opened a back-arc basin and created the Little Belt arc to the north of the craton, contributed to the demise of the Manikewan Ocean, and culminated in collision along the Big Sky orogen starting ca. 1.78 Ga. Collision across the Trans-Hudson orogen in Canada occurred during a slightly earlier period. Thus, docking of the Wyoming craton reflects the final stage in the closure of the Manikewan Ocean and the amalgamation of the Archean cratons of Laurentia.

New regional mapping documents that a thick quartzite sequence in the Lemhi subbasin of the Belt-Purcell basin lies near the top of the Mesoproterozoic stratigraphic column, and that two finer-grained units have been miscorrelated. This observation requires reassessment of the subbasin's stratigraphy, which we present here. Determination of the relationships between the stratigraphic units of the Lemhi Range and Salmon River and Beaverhead Mountains and better-known Belt Supergroup units to the north has been hampered by miscorrelation of this upper quartzite sequence with older strata, and by miscorrelation of the type Apple Creek Formation with a similar but stratigraphically lower unit. The base of the upper quartzite sequence includes the Swauger and Lawson Creek Formations, which are the highest units previously identified in the Lemhi subbasin. This sequence continues upward through quartzite units described here that underlie or comprise lateral equivalents of the type Apple Creek Formation in the Lemhi Range. The spatial distribution of these quartzite units extends the Lemhi subbasin farther east and north in Montana and northwest in Idaho. The complete stratigraphy reflects the stratigraphic separation of the two “Apple Creeks” and expands the type Apple Creek Formation to accommodate the quartzite units into the regional Mesoproterozoic stratigraphy. Our proposed correlation of the thick upper quartzite sequence with the Bonner Formation and higher units of the Missoula Group in the main part of the Belt basin requires that subsidence of the Lemhi subbasin was significantly faster than that of the main part of the Belt basin during deposition of the upper Missoula Group. Therefore, the two parts of the Belt basin were distinct tectonically, although they shared common sediment sources.

A recent 1:24,000 scale mapping project within the northern Beaverhead Mountains along the Idaho-Montana border has resulted in a reinterpretation of both the Mesoproterozoic stratigraphy and the regional structural framework. A 15-km-thick stratigraphic section of the Mesoproterozoic Lemhi subbasin was initially deformed by northeast-southwest shortening into giant northwest-striking, northeast-verging folds, probably during Cretaceous Sevier orogenesis. These initial folds were then dissected by a system of subparallel and anastomosing, oblique-slip reverse, thrust, and normal faults that generally strike northwest, but that exhibit east-west–oriented lineations, suggesting components of strike-slip displacement. Contractional faulting appears to have been followed by Eocene to Miocene extensional faulting, with many normal faults following the preexisting fabrics. Extension opened Tertiary basins along some of these faults, including the Salmon Basin along the southwestern side of the Beaverhead Range. Subparallel faults in the surrounding region appear to have a similar complex history, and all appear to be part of a major northwest-striking Cretaceous fold-and-thrust belt that was later dissected by Tertiary extension. Although the faults of the Beaverhead Mountains are significant and long-lived, they are not terrane-bounding structures separating the Belt and Lemhi sedimentary sequences. Instead, Lemhi strata extend across the range and northward to Missoula, where they grade into correlative Missoula Group strata.

We used laser ablation–inductively coupled plasma–mass spectrometry to determine the U-Pb ages for 1206 detrital zircons from 15 samples of the Lemhi subbasin, upper Belt Supergroup, in southwest Montana and east-central Idaho. We recognize two main detrital-zircon provenance groups. The first is found in the Swauger and overlying formations. It contains a unimodal 1740–1710 Ma zircon population that we infer was derived from the “Big White” arc, an accretionary magmatic arc to the south of the Belt Basin, with an estimated volume of 1.26 million km 3 —a huge feature on a global scale. The ɛ Hf(i) values for magmatic 1740–1710 Ma zircons from the Lawson Creek Formation are +8–0, suggesting that they were derived from more juvenile melts than most other Lemhi subbasin strata, which have values as evolved as −7 and may have been derived from an arc built on Proterozoic or Archean crust in the Mojave Province. Since paleocurrents in cross-bedded sandstones indicate northward flow, the proximate source terrane for this sand was to the south. The second provenance group is that of the Missoula Group (and Cambrian strata recycled from the Missoula Group), with significant numbers of 1780–1750 Ma grains and more than 15% Archean grains. This provenance group is thought to represent mixing of Yavapai Province, Mojave Province, and Archean Wyoming Province sources. Both of these provenance groups differ from the basal Belt Prichard Formation, and strata of the Trampas and Yankee Joe Basins of Arizona and New Mexico, which contain a major population of 1.61–1.50 Ga non–North American grains. The 12 youngest grains from the several Swauger Formation samples suggest the formation is younger than 1429 Ma. The three youngest grains from Apple Creek Formation diamictite suggest the rock is younger than 1390 Ma. This makes the Apple Creek diamictite the youngest part of Belt Supergroup strata south of the Canadian border. Though the Big White magmatic arc was produced before 1.7 Ga, the sediment may have been recycled several times before being deposited as locally feldspathic sandstone in the Lemhi subbasin depositional site 300 m.y. later. Because the detrital-zircon provenance does not change from Idaho east to Montana, our data do not support the existence of a major Great Divide megashear separating the Lemhi subbasin from the Belt Basin. In southwest Montana, unfossiliferous sandstones of Cambrian age contain the same detrital-zircon assemblages as the Swauger Formation and Missoula Group, suggesting reworking of a local Belt Supergroup source.

The Neihart Quartzite and LaHood Formation are the lowermost units exposed in the Helena embayment, which forms the eastern and southeastern margins of the Belt Basin. Ages of detrital zircons from the Neihart Quartzite (quartz arenite) and a range of lithologies in the LaHood Formation (conglomerates to arkoses to siltstones) show that these units do not share a common provenance. The dominant provenance is Paleoarchean for the LaHood Formation and Paleoproterozoic for the Neihart Quartzite. Provenance is further constrained by the geochemistry and U-Pb ages of zircons from cobbles from the classic LaHood conglomerate in Jefferson Canyon (Tobacco Root Mountains), ages of Paleoproterozoic crystalline basement in the Beaverhead-Tendoy Mountains (1.8–2.45 Ga), and elemental and Sm-Nd isotopic data for select samples of both sedimentary rocks and crystalline basement within the basin. These data show a pronounced lack of detritus from abundant, proximal Neoarchean (2.7–2.9 Ga) and Paleoproterozoic (1.9–2.5 Ga) crystalline basement exposed in Laramide uplifts and the soles of Sevier-style thrust faults within and near the basin. Analyses of detrital mineral assemblages in the Lower Belt Supergroup units clearly indicate that the finer-grained portions of the LaHood Formation were not locally derived, based on abundant white mica in sections overlying tonalite-trondhjemite-granodiorite (TTG) basement and lack of amphibole in units overlying hornblende tonalites. Significant fractionation also exists between sand- and cobble-size components in conglomerate of the LaHood Formation in terms of elemental abundances, isotopic compositions, and the U-Pb ages of zircons. Stratigraphically, the differences in the ages of the youngest zircons in all LaHood Formation samples and the Neihart Quartzite (1.71 Ga, Neihart; 1.78 Ga, LaHood) do not refute any proposed stratigraphic correlations. Nonetheless, age spectra of detrital zircons from the Neihart Quartzite, all LaHood lithologies, and previously published data for the Newland Formation show distinctions of provenance and an apparent lack of interaction among the sediment-supply systems of these three formations. This contrast suggests that distinct, likely fault-bounded, sedimentologically restricted subbasins characterized the initial stages of development of the eastern Belt Basin along the Perry line (southeastern margin of the Helena embayment), in the manner of a modern, but partially submerged, Basin and Range topography. The time of development of this topography is not clear, but it may have been related to the collapse phase of the Great Falls orogeny at ca. 1.7 Ga for the Helena embayment. The primary, north-south–trending Belt Basin also developed subsequent to the Great Falls orogeny along the western paleomargin of the newly amalgamated Wyoming–Medicine Hat–Hearne craton.

Abstract This field guide covers geology across north-central Idaho from the Snake River in the west across the Bitterroot Mountains to the east to near Missoula, Montana. The regional geology includes a much-modified Mesozoic accretionary boundary along the western side of Idaho across which allochthonous Permian to Cretaceous arc complexes of the Blue Mountains province to the west are juxtaposed against autochthonous Mesoproterozoic and Neoproterozoic North American metasedimentary assemblages intruded by Cretaceous and Paleogene plutons to the east. The accretionary boundary turns sharply near Orofino, Idaho, from north-trending in the south to west-trending, forming the Syringa embayment, then disappears westward under Miocene cover rocks of the Columbia River Basalt Group. The Coolwater culmination east of the Syringa embayment exposes allochthonous rocks well east of an ideal steep suture. North and east of it is the Bitterroot lobe of the Idaho batholith, which intruded Precambrian continental crust in the Cretaceous and Paleocene to form one of the classical North American Cordilleran batholiths. Eocene Challis plutons, products of the Tertiary western U.S. ignimbrite flare-up, intrude those batholith rocks. This guide describes the geology in three separate road logs: (1) The Wallowa terrane of the Blue Mountains province from White Bird, Idaho, west into Hells Canyon and faults that complicate the story; (2) the Mesozoic accretionary boundary from White Bird to the South Fork Clearwater River east of Grangeville and then north to Kooskia, Idaho; and (3) the bend in the accretionary boundary, the Coolwater culmination, and the Bitterroot lobe of the Idaho batholith along Highway 12 east from near Lewiston, Idaho, to Lolo, Montana.

The Priest River, Clearwater, Bitterroot, and Anaconda metamorphic core complexes of the northern Rocky Mountains were exhumed in Eocene time by crustal extension, which was linked via dextral displacement on the Lewis and Clark fault zone. Detailed geochronology and thermochronology (U-Pb, 40 Ar/ 39 Ar, and fission-track) from the Bitterroot complex indicates that extension started at 53 ± 1 Ma and continued until after 40 Ma. New U-Pb zircon and 40 Ar/ 39 Ar data from the Anaconda complex and published geochronology from the Priest River complex indicate a similar timing for the onset of major extension and exhumation. 40 Ar/ 39 Ar data from the Clearwater complex, which formed within a relay between strike-slip splays of the Lewis and Clark fault zone, are consistent with exhumation during the same time span. The Lewis and Clark fault zone separates ENE-directed extension in the Priest River complex from ESE-directed extension in the Bitterroot and Anaconda complexes. Large-scale extension was transferred eastward on the south side of this fault zone, where stretching lineations in core complex mylonites are oriented ∼104°–110° and coincide with the general trend of the transcurrent faults. Extension and exhumation of middle crustal rocks along the Lewis and Clark fault zone was concentrated in areas that also experienced voluminous Eocene midcrustal magmatism. Extension was probably initiated by a change in plate boundary conditions combined with the rapid influx of heat from the asthenosphere as a slab window opened beneath the western Cordillera, which led to collapse of the Cordilleran orogenic wedge and widespread early Eocene magmatism.