ABSTRACT A large portion of the Belt-Purcell Supergroup is well exposed in the vicinity of Glacier and Waterton Lakes national parks of northwestern Montana, USA, and southwestern Alberta, Canada. These strata were deposited in the northeastern part of the Mesoproterozoic Belt Basin. The dramatic rate of subsidence combined with dominantly fine-grained sediment influx produced thick units of broadly uniform lithology, which constitute the spectacular and unusually colorful mountain scenery of this region. Seemingly fairly simple at first glance, in detail these rocks exhibit a great deal of facies heterogeneity and a number of unusual attributes. This has resulted in contrasting and controversial interpretations of sedimentary features, depositional dynamics, sedimentary environments, and consequently the overall understanding of the entire basin. The Belt Basin reveals itself to be a unique setting in many respects, but ideas stemming from these rocks have implications for other strata, not just those of pre-Cambrian age, but for the entire Phanerozoic as well. The Belt Supergroup is therefore a particularly stimulating field-trip destination that challenges textbook interpretations.

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.

The Blackbird cobalt-copper (Co-Cu) district in the Salmon River Mountains of east-central Idaho occupies the central part of the Idaho cobalt belt—a northwest-elongate, 55-km-long belt of Co-Cu occurrences, hosted in grayish siliciclastic metasedimentary strata of the Lemhi subbasin (of the Mesoproterozoic Belt-Purcell Basin). The Blackbird district contains at least eight stratabound ore zones and many discordant lodes, mostly in the upper part of the banded siltite unit of the Apple Creek Formation of Yellow Lake, which generally consists of interbedded siltite and argillite. In the Blackbird mine area, argillite beds in six stratigraphic intervals are altered to biotitite containing over 75 vol% of greenish hydrothermal biotite, which is preferentially mineralized. Past production and currently estimated resources of the Blackbird district total ~17 Mt of ore, averaging 0.74% Co, 1.4% Cu, and 1.0 ppm Au (not including downdip projections of ore zones that are open downward). A compilation of relative-age relationships and isotopic age determinations indicates that most cobalt mineralization occurred in Mesoproterozoic time, whereas most copper mineralization occurred in Cretaceous time. Mesoproterozoic cobaltite mineralization accompanied and followed dynamothermal metamorphism and bimodal plutonism during the Middle Mesoproterozoic East Kootenay orogeny (ca. 1379–1325 Ma), and also accompanied Grenvilleage (Late Mesoproterozoic) thermal metamorphism (ca. 1200–1000 Ma). Stratabound cobaltite-biotite ore zones typically contain cobaltite 1 in a matrix of biotitite ± tourmaline ± minor xenotime (ca. 1370–1320 Ma) ± minor chalcopyrite ± sparse allanite ± sparse microscopic native gold in cobaltite. Such cobaltite-biotite lodes are locally folded into tight F 2 folds with axial-planar S 2 cleavage and schistosity. Discordant replacement-style lodes of cobaltite 2 -biotite ore ± xenotime 2 (ca. 1320–1270 Ma) commonly follow S 2 fractures and fabrics. Discordant quartz-biotite and quartz-tourmaline breccias, and veins contain cobaltite 3 ± xenotime 3 (ca. 1058–990 Ma). Mesoproterozoic cobaltite deposition was followed by: (1) within-plate plutonism (530–485 Ma) and emplacement of mafic dikes (which cut cobaltite lodes but are cut by quartz-Fe-Cu-sulfide veins); (2) garnet-grade metamorphism (ca. 151–93 Ma); (3) Fe-Cu-sulfide mineralization (ca. 110–92 Ma); and (4) minor quartz ± Au-Ag ± Bi mineralization (ca. 92–83 Ma). Cretaceous Fe-Cu-sulfide vein, breccia, and replacement-style deposits contain various combinations of chalcopyrite ± pyrrhotite ± pyrite ± cobaltian arsenopyrite (not cobaltite) ± arsenopyrite ± quartz ± siderite ± monazite (ca. 144–88 Ma but mostly 110–92 Ma) ± xenotime (104–93 Ma). Highly radiogenic Pb (in these sulfides) and Sr (in siderite) indicate that these elements resided in Mesoproterozoic source rocks until they were mobilized after ca. 100 Ma. Fe-Cu-sulfide veins, breccias, and replacement deposits appear relatively undeformed and generally lack metamorphic fabrics. Composite Co-Cu-Au ore contains early cobaltite-biotite lodes, cut by Fe-Cu-sulfide veins and breccias, or overprinted by Fe-Cu-sulfide replacement-style deposits, and locally cut by quartz veinlets ± Au-Ag ± Bi minerals.

The Mesoproterozoic Belt-Purcell Basin of the United States–Canadian Rocky Mountains formed in a complex intracontinental-rift system. The basin contained three main fault blocks: a northern half-graben, a central horst, and a southern graben. Each had distinct internal stratigraphy and mineralization that influenced Phanerozoic sedimentation; the northern half-graben and horst formed a platform with a condensed section, whereas the southern graben formed the subsiding Central Montana trough. They formed major crustal blocks that rotated clockwise during Cordilleran thrust displacement, with transpressional shear zones deforming their edges. The northern half-graben was deepest and filled with a structurally strong prism of quartz-rich sedimentary rocks and thick mafic sills that tapered toward the northeast from >15-km-thick near the basin-bounding fault. This strong, dense prism was driven into the foreland basin as a readymade, critically tapered tectonic wedge and was inverted into the Purcell anticlinorium. Erosion did not breech the Belt-Purcell Supergroup in this prism during thrusting. The southern graben was thinner, weaker, lacked mafic sills, and was engorged with sheets of granite during thrusting. It was internally deformed to achieve critical taper and shed thick deposits of syntectonic Belt-Purcell–clast conglomerate into the foreland basin. A palinspastic map of the basin combined with a detailed paleocontinental map that juxtaposes the northeastern corner of the Siberian craton against western North America indicates that the basin formed at the complicated junction of three continental-scale rift zones.

The Lewis thrust, which is >225 km long and has a maximum displacement of >80 km, is a major Foreland belt structural element in the southeastern Canadian Cordillera. We use low-temperature thermochronometry in the preserved Lewis thrust sheet stratigraphic succession to constrain variations in both paleogeothermal gradient and Lewis thrust sheet thickness immediately prior to motion on the Lewis thrust fault. Fission-track and vitrinite reflectance data combined with stratigraphic data suggest that maximum Phanerozoic burial and heating occurred in the Lewis thrust sheet during a short interval (<15 m.y.) in late Campanian time immediately prior to thrusting (ca. 75 Ma). The data suggest that the late predeformational Lewis thrust sheet paleogeothermal gradient was between ∼18 and 22.5 °C/km, which is higher than that inferred for subsequent syn- and postdeformational intervals by other studies. The inferred paleotemperatures and geothermal gradients indicate that the preserved Lewis thrust sheet stratigraphic succession was overlain by ∼4–5.5 km of additional Late Cretaceous strata that were subsequently removed by erosional denudation. We estimate that the Lewis thrust sheet was ∼12–13.5 km thick when thrusting commenced. Deposition of the Late Cretaceous succession was terminated by the onset of displacement on the Lewis thrust (ca. 75 ± 5 Ma) and was followed by intervals of erosional denudation that are constrained stratigraphically by both early Oligocene and current erosion surfaces on the Lewis thrust sheet.