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.

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.

The late Eocene to early Miocene Renova Formation records initial post-Laramide sediment accumulation in the intermontane basin province of southwest Montana. Recent studies that postulate deposition of the Renova Formation were restricted to a broad, low-relief, tectonically quiescent basin on the eastern shoulder of an active rift zone vastly differ from traditional models in which the Renova Formation was deposited in individual intermontane basins separated by basin-bounding uplands. This study utilizes detrital zircon geochronology to resolve the paleogeography of the Renova Formation. Detrital zircon was selected as a detrital tracer that can be used to differentiate between multiple potential sources of similar mineralogy but with distinctly different U-Pb ages. Laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) U-Pb detrital zircon ages were determined for 11 sandstones from the Eocene-Oligocene Renova Formation exposed in the Sage Creek, Beaverhead, Frying Pan, Upper Jefferson, Melrose, and Divide basins. Detrital zircon ages, lithofacies, paleoflow, and petrography indicate that provenance of the Renova Formation includes Paleogene volcanics (Dillon volcanics and Lowland Creek volcanics), Late Cretaceous igneous intrusions (Boulder batholith, Pioneer batholith, McCartney Mountain pluton), Mesozoic strata (Blackleaf Formation, Beaverhead Group), Belt Supergroup strata, and Archean basement. The oldest deposits of the Renova are assigned Bridgerian to Uintan North American Land Mammal (NALM) ages and contain detrital zircons derived from volcanic, sedimentary, and metamorphic rocks constituting the “cover strata” to uplift-cored Late Cretaceous plutonic bodies. Regional unroofing trends are manifested by a decreased percentage of cover strata–sourced zircon and an increased percentage of pluton-sourced zircon as Renova deposits became younger. Zircon derived from Late Cretaceous plutonic bodies indicate that initial unroofing of the McCartney Mountain pluton, Pioneer batholith, and Boulder batholith occurred during Duchesnean time. Facies assemblages, including alluvial fan, trunk fluvial, and paludal-lacustrine lithofacies, are integrated with detrital zircon populations to reveal a complex Paleogene paleotopography in the study area. The “Renova basin” was dissected by paleo-uplands that shed detritus into individual intervening basins. Areas of paleo-relief include ancestral expressions of the Pioneer Range, McCartney Mountain, Boulder batholith–Highland Range, and Tobacco Root Range. First-order alluvial distributary systems fed sediment to two noncontiguous regional-trunk fluvial systems during the Chadronian. A “Western fluvial system” drained the area west of the Boulder batholith, and an “Eastern fluvial system” drained the area east of the Boulder batholith. Chadronian paleodrainages parallel the regional Sevier-Laramide structural grain and may exhibit possible inheritance from Late Cretaceous fluvial systems. Detrital zircons of the Renova Formation can be confidently attributed to local sources exposed in highlands that bound the Divide, Melrose, Beaverhead, Frying Pan, Upper Jefferson, and Sage Creek basins. The data presented in this study do not require an Idaho batholith provenance for the Renova Formation.

The greenhouse-icehouse change across the Eocene-Oligocene transition and associated Oi-1 glaciation event is the most profound climatic change in Earth’s recent geological history. Marine reconstructions of the Oi-1 glaciation using foraminiferal δ 18 O isotopic compositions suggest that much of the change was associated with Antarctic ice growth rather than climatic change. Nonetheless, some cooling is expected to have occurred on land in addition to drier conditions associated with water tied up in the polar ice caps, and some recent results based on stable isotope analyses of bones support this viewpoint. Nonmarine paleoclimatic conditions (mean annual temperature, mean annual precipitation) may be quantitatively reconstructed using paleosols preserved in continental successions to test this general model. Results from Oregon and Nebraska suggest moderate drying and cooling, not as a stepwise change at the time of the Oi-1 glaciation, but as part of a long-term aridification and cooling event associated in part with emplacement of the Cascade Range. In contrast, intermontane Montana’s paleoprecipitation and paleotemperatures fluctuated on short-term (i.e., Milankovitch) time scales but on balance were both essentially unchanged by the Oi-1 glaciation. Results from Europe (UK, Spain) suggest a different pattern characterized by stable (i.e., unchanging) paleotemperatures in both localities and increasingly wet conditions in the UK. Taken together, these results indicate that (1) strongly regionalized climatic change was associated with the Oi-1 glaciation, (2) physiographic position with respect to orographic features played a key role in determining those regional climatic responses to the global event, and (3) there was little or no cooling on land associated with the Oi-1 glaciation.