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.

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.

Abstract Study of the distribution of the age-populations of detrital zircons in the Snake River system suggest that specific stream systems can be identified based on the detrital-zircon age-population signature (“barcode”) of ancient and Holocene sand deposits. Detrital zircon studies can be used on regional and local scales to determine changes in drainage patterns using both surface and subsurface data. Regional study of drainage patterns using detrital zircons found in Neogene strata of Idaho and southwest Montana suggest northeastward late Miocene to Holocene migration of the Continental Divide from the western side of the Pioneer Mountains to the current position in southwest Montana. Specifically, mixed populations of recycled Proterozoic detrital zircons that define the Wood River drainage are not found in the western Snake River Plain until after 7 Ma. Late Miocene eastward drainage from the central Snake River Plain to southwest Montana is suggested by 9–12 Ma detrital zircons found in fluvial strata less than 6 million years old, of the Sixmile Creek Formation Basalt eruptions of the Eastern Snake River Plain during the Pliocene and Pleistocene also caused drainage diversion. Detrital zircons in Pliocene sands from coreholes at Wendell and Mountain Home Air Force Base contain Big Lost River zircon provenance, suggesting that during the Pliocene, the Big Lost River flowed west along the central Snake River Plain. Late Pliocene and early Pleistocene basaltic volcanoes and rhyolite dome eruptions resulted in volcanic highlands, the Axial Volcanic Zone of the eastern Snake River Plain and the northwest-trending Arco Volcanic Rift Zone (which includes Craters of the Moon volcanic center). The development of these volcanic highlands disrupted the ancestral drainage of the Pliocene Big Lost River system, confining it to the Big Lost Trough, a volcanically dammed basin of internal drainage on the Idaho National Laboratory. After the Big Lost Trough was cut off from the main Snake River, basalt eruptions, local subsidence, and climate controlled the courses of the rivers that flowed into it. Detrital-zircon populations in core samples reveal the provenance of specific sand beds from the Big or Little Lost River systems.

Abstract Detailed mapping along the east face of Oxford Ridge in the southern Bannock Range, southeast Idaho determines the stratigraphic placement and lateral extent of strata in the Scout Mountain Member of the Neoproterozoic Pocatello Formation. The lower “transitional unit” overlies the Bannock Volcanic Member and consists of 70 m of massive diamictite with argillitic and vesicular basaltic clasts up to cobble size intercalated with thin metabasalt and hyaloclastite units. Overlying the transitional unit is a 150–190-m-thick, massive, brown-green to purple sandy diamictite with dominantly quartzose cobble clasts. Interbedded with this middle unit is a 60-m-thick epiclastic volcanic interval informally named the Oxford Mountain tuffite. An upper sandstone unit up to 100 m thick lies above the diamictite at the head of Fivemile Creek in the southern portion of the map area. The volcanic interval contains plagioclase-phyric volcanic lithic sandstone, porphyritic volcanic lithic fragments and rounded cobbles in tuffaceous diamictite and a reworked stratified lapilli-tuff. It is interstratified with quartzose and volcanogenic diamictite and can be traced along 5.5 km of strike. On Oxford Mountain, laser ablation–inductively coupled plasma mass spectrometry U-Pb zircon ages presented here and additional sensitive high-resolution ion microprobe ages constrain the underlying Bannock Volcanic Member to be 717–686 Ma and require that the overlying Scout Mountain Member is younger than 685 Ma.

Abstract This volume is the fourth decadal compendium of research on the Belt Supergroup. It is an outgrowth of Belt Symposium IV, held in Salmon, Idaho, in July, 2003, in conjunction with the Tobacco Root Geological Society annual field conference. A full abstract and field–trip volume for that meeting is Lageson and Christner (2003) . In keeping with previous Belt Symposia, the scope of the meeting and subsequent papers was broad and included Neoproterozoic strata of the western U.S. and Siberia. In the preface to the first Belt Symposium Savage (1973) acknowledges A.C. Peale as "Founder" of Belt Geology and C.P. Ross as "Father" of Belt Geology. These men, and C.D. Walcott, who was first to sub–divide the Belt, were strong–willed field geologists who spent decades mapping this thick pile of thrust– faulted quartzite, siltite, and argillite. Because of the geographic extent and great thickness of the Belt Supergroup, years of work have been required before one's conclusions are "bona fide", and only a few have been able to pay their dues. A core of these geologists composes the non–profit Belt Association, founded in 1984 by officers Jack Harrison, Greg McKelvey, Jon Thorson, and Jim Whipple, and board members Dick Berg, Ian Lange, Chet Wallace, and Don Winston. Subsequent board members John Balla, Earl Bennett, Lisa Hardy, Nancy Joseph, David Kidder, David Lidke, and Brian White kept the Belt Association active through the 1980s and 1990s. Present board members Larry Appelgate, Art Bookstrom, Jim Browne, Reed Lewis, Paul

Abstract High–precision U–Pb TIMS, SHRIMP, and LA–ICPMS dating of magmatic and detrital zircons from the core of the Clearwater complex, northern Idaho, U.S.A., provide new ages and new tectonic interpretations for potential Precambrian basement rocks in this part of the Cordillera. The Boehls Butte anorthosite, which is exposed in lens–like masses within the core of the Clearwater complex, crystallized at 1787 ± 2 Ma. Amphibolites, which are intercalated with the anorthosite, crystallized during distinctly different magmatic episodes around 1587 Ma, 1467 Ma, and 1453 Ma. These dates better define the age of Precambrian basement in this region and document a new exposure of 1580 Ma igneous rocks along the western edge of the North American craton. Surrounding the anorthosite are metasedimentary rocks (Boehls Butte Formation) that have been interpreted as predating the anorthosite and the Mesoproterozoic BeltPurcell Supergroup. Detrital zircons from these metasedimentary rocks yield age populations that are predominantly Paleoproterozoic with some Archean grains. The youngest concordant 207 Pb/ 206 Pb ages are between 1597 and 1761 Ma, well after crystallization of the anorthosite. On this basis, we conclude that most of the rocks once assigned to the Boehls Butte Formation are better correlated with the lower part of the Belt–Purcell Supergroup. The only part of the Boehls Butte Formation that remains potential basement is the Al–Mg– rich schist that borders the masses of anorthosite. We propose that the anorthosite and bordering Al–Mg schists are displaced tectonic slivers that were juxtaposed against the metasedimentary rocks by shear zones that predate peak metamorphism. This zone of shear may be related to the basal decollement for the Rocky Mountain fold–and–thrust belt.