ABSTRACT The Windermere Supergroup in southern British Columbia and its correlatives (such as the Pocatello Formation and lower Brigham Group in southeastern Idaho) along the western North American Cordilleran margin are an archetype of Neoproterozoic to early Paleozoic tectonic, sedimentary, and climatic processes. The central Idaho portion of the margin remains relatively understudied when compared to regions to the south in southeastern Idaho or to the north in northeastern Washington. This is in part a legacy of early workers, who identified the absence of Neoproterozoic and Cambrian strata in east-central Idaho across the Lemhi arch. However, Neoproterozoic and Cambrian rocks are indeed present west of the Lemhi arch within the central Idaho section of the Cordillera. Here, we summarize recent advances in our understanding of these strata within central Idaho and correlate the Pocatello Formation and Brigham Group rocks from northern Utah/southeastern Idaho through central Idaho to northeastern Washington. We also provide new constraints that link Cambrian strata from central Idaho across the Lemhi arch to southwestern Montana. Collectively, this emerging tectono-stratigraphic framework suggests extensive, some likely diachronous, stratigraphic boundaries and magmatic events relating to (1) widespread rifting ca. 720–680 Ma; (2) early and late Cryogenian (Sturtian and Marinoan) glacial sedimentation; (3) base-level drawdown and formation of incised valleys, previously correlated to the Marinoan glacial interval, but which now appear to be younger (ca. 600 Ma) and perhaps related to tectonic activity; (4) onset of the Sauk I transgression 560–530(?) Ma; (5) the ca. 515 Ma Sauk II lowstand, perhaps related to final rifting in southern Laurentia; and (6) the Sauk III lowstand coeval with exhumation of 500–490 Ma Beaverhead plutons within the Lemhi arch. Magmatism occurred ca. 680 Ma, 660 Ma, 600 Ma, and 500 Ma, providing age ties. These observations suggest that Neoproterozoic and lower Paleozoic strata in the central Idaho sector of the North American Cordillera record similar processes and sedimentation as strata elsewhere along the margin.

This Special Paper focuses on the evolution of the crust of the hinterland of the orogen during the orogenic cycle, and describes the evolution of the crust and basins at metamorphic core complexes. The volume includes a regional study of the Sevier-Laramide orogens in the Wyoming province, a regional seismic study, strain analysis of Sevier and Laramide deformation, and detrital zircon provenance from the Pacific Coast to the foreland between the Jurassic and the Eocene.

ABSTRACT Strata preserved within the Sevier foreland basin of North America contain a suite of lithologic variations influenced by hinterland tectonic processes. Using U-Pb detrital zircon geochronology, we compared provenance signals of Upper Jurassic and Lower Cretaceous strata from a west-to-east, foredeep-to-forebulge-to-backbulge depozone transect across the state of Wyoming and evaluated major tectonic mechanisms operating during the early evolution of the Sevier orogeny. Our data included new and compiled U-Pb detrital zircon ages ( n = 6013) from 50 localities that were integrated into a revised chronostratigraphic framework and subsidence history for the basin. At the onset of the Sevier orogeny, we found evidence for uplift and erosion of early Mesozoic and late Paleozoic strata within the nascent Sevier fold-and-thrust belt. This event occurred prior to the Aptian Stage of the Early Cretaceous and is recorded by the coordinated progradation of coarse-grained fluvial systems across the overfilled foreland basin. Continued emplacement of thrust loads in the hinterland generated accommodation in excess of sediment supply, a condition likely exacerbated by a relative reduction of siliciclastic sediment supply due to greater unroofing of Paleozoic carbonates during the Aptian and Albian Stages of the Early Cretaceous. This led to an underfilled condition characterized by widespread calcareous lacustrine deposition across much of the foredeep depozone and condensed stratigraphic intervals in the forebulge and backbulge depozones. During the late Albian–earliest Cenomanian, fluvial systems sourced in the Appalachians invaded the foreland basin from the east, followed by the rapid incursion of the Western Interior Seaway, driven by accelerated thrust emplacement, flexural subsidence, and potentially the onset of dynamic subsidence.

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.

Proterozoic mafic magmatic rocks exposed along the western side of North America, or western Laurentia, from Kimberley, British Columbia, through to northwestern Wyoming have been previously divided into two large igneous provinces: the ca. 1460 Ma Moyie-Purcell and the ca. 780 Ma Gunbarrel large igneous provinces. New geochemical analysis from this study demonstrates that there are additional intraplate mafic magmatic rocks present. Distinguishable by variable normalized rare earth element patterns combined with differing slopes on a binary Ti versus V plot, there are 17 identifiable geochemical signatures in the 307 whole-rock and trace-element analyses from this study. Only seven of these signatures can be linked to the ca. 1460 Ma Moyie-Purcell large igneous province, and one signature to the 780 Ma Gunbarrel large igneous province. This study has identified two groups of intrusions with distinct geochemical signatures previously linked with the ca. 1460 Ma Moyie-Purcell large igneous province but now recognized to be separate events, a single unique geochemical signature with a U-Pb age correlative with the Moyie-Purcell large igneous province and seven other heretofore unidentified signatures interpreted to belong to additional undated events.

Abstract Prepared for the 2016 GSA Rocky Mountain Section Meeting, this well-illustrated volume offers guides to the lavas of the Columbia River basalts, megaflood landscapes of the Channeled Scablands, Mesozoic accreted terranes, metamorphic Precambrian Belt and pre-Belt rocks, and other features of this tectonically active region.

Abstract Ancient and modern types of sedimentary placer deposits formed in both alluvial and coastal environments have been signficant sources of the rare earth elements (REEs). The REE-bearing minerals in placer-type deposits are primarily monazite [(Ce,La,Nd,Th)PO4] and sometimes xenotime (YPO4), which are high-density (heavy) minerals that accumulate with the suite of heavy minerals. Monazite has been extracted from many heavy mineral placers as a coproduct of the economic recovery of associated industrial minerals, such as titanium oxide minerals (ilmenite, rutile), zircon, sillimanite, garnet, staurolite, and others. Xenotime has been produced from some alluvial deposits as a coproduct of tin (cassiterite) placer mining. Placers are mineral deposits formed by the mechanical concentration of minerals from weathered debris. Placers can be classified as eluvial, alluvial, eolian, beach, and fossil (paleo) deposit types. Monazite-bearing placer-type deposits can occur in residual weathering zones, beaches, rivers and streams, dunes, and offshore areas. The detrital mixture of sand, silt, clays, and heavy (dense) minerals deposited in placers are derived primarily from the erosion of crystalline rocks, mainly igneous rocks and moderate- to high-grade metamorphic rocks (amphibolite facies and higher). In fluvial settings, slope is an important factor for the concentration of heavy minerals from detritus. In coastal settings, the actions of waves, currents, tides, and wind are forces that concentrate and sort mineral particles based on size and density. Placer deposits containing monazite are known on all continents. In the past, by-product monazite has been recovered from placers in Australia, Brazil, India, Malaysia, Thailand, China, New Zealand, Sri Lanka, Indonesia, Zaire, Korea, and the United States. More recently, monazite has been recovered from coastal and alluvial placers in India, Malaysia, Sri Lanka, Thailand, and Brazil. In particular, along the southwestern and southeastern coasts of India, beach deposits rich in heavy minerals have experienced renewed exploration and development, partly to recover monazite for its REEs as well as its Th, to be used as a nuclear fuel source. Exploration designed to locate heavy mineral placers in coastal environments should identify bedrock terranes containing abundant high-grade metamorphic rocks or igneous rocks and identify ancient or modern coastal plains sourced by streams and rivers that drain these terranes. Trace elements associated with heavy mineral placers, useful as pathfinder elements, primarily include Ti, Hf, the REEs, Th, and U. Radiometric methods of geophysical exploration are useful in discovering and delineating deposits of heavy mineral sands. Several minerals in these deposits can produce a radiometric anomaly, but especially monazite, due to its high thorium content. Some beach districts in India and Brazil have been demonstrated as areas of high background radiation with potential dose exposure to humans and others, primarily due to the Th and U in detrital grains of monazite and zircon. Monazite- or xenotime-bearing placers offer several advantages as sources of REEs. Ancient and modern deposits of heavy mineral sands that formed in coastal settings can be voluminous with individual deposits as much as about 1 km wide and more than 5 km long. Grains of monazite or xenotime in placer deposits are mingled with other heavy minerals of industrial value. Monazite and xenotime are durable and often the heaviest minerals within the sand-silt deposit, which makes them relatively easy to mechanically separate. Thus, the REE ore minerals, monazite or xenotime, can be recovered from heavy mineral placers as a low-cost coproduct along with the economic production of the associated industrial minerals.