ABSTRACT It is debated whether plate tectonics (horizontal tectonics) or single-lid tectonics (vertical tectonics) dominated the Mesoproterozoic Era. Either rifting of the Nuna/Columbia supercontinent or a localized vertical subsidence and tectonism mechanism within a single tectonic plate is likely recorded in Mesoproterozoic basins. This study summarizes detrital zircon samples from the Mesoproterozoic Belt and Purcell Supergroups and Lemhi subbasin of the western United States and Canada and tests competing rift and intracratonic basin models. Rift models take the observed detrital zircon trends to mean that a non-Laurentian (ca. 1.6–1.5 Ga) detrital zircon component becomes completely absent higher in the section, signifying rifting of the Nuna/Columbia supercontinent at ca. 1.4 Ga. Intracratonic models acknowledge this observed shift in provenance but interpret a long-lived intracratonic setting for the basin following an earlier failed rifting event. The fundamental question is whether the Belt basin represents a failed or successful rift. We used statistical comparison of 72 detrital zircon signatures, reported in the literature and presented in this study, to test the rift model. Samples are not evenly distributed across the basin or its stratigraphy. Non-Laurentian grains are spatially restricted to the northwest part of the basin but are present in all groups, suggesting that the apparent loss of the non-Laurentian population is an artifact of sampling bias. Like stratigraphic boundaries and facies changes, mixing trends are gradual, not sharp or sudden, signifying progressive reworking of Proterozoic zircons and transport from all sides. Archean zircons are localized near the edges of Archean blocks, signifying local down-dropping along cratonic margins. The rift model is therefore rejected in favor of the intracratonic model for the Belt basin on the basis of variable mixing between non-Laurentian and Laurentian sources in both pre–Missoula Group and Missoula Group strata. Far away from plate margins, sediment incrementally filled topographic depressions created by densified and thinned Proterozoic crustal blocks, resulting in vertical down-dropping along preexisting sutures with neighboring Archean blocks. More systematic detrital zircon studies are needed in order to accurately quantify provenance trends in space and time. Continued investigation of the Belt basin may reveal underappreciated or unrecognized vertical tectonic processes that may explain Mesoproterozoic rocks more accurately.

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.

The ca. 1.460 Ga Revett Formation is a gray and purple quartzite lithosome in northwestern Montana, and it interfingers eastward into red argillite of the Grinnell Formation in Glacier National Park. The Revett Formation was analyzed in northwestern Montana by identifying sedimentary structures in stratigraphic sections and by interpreting flow processes of the structures using the standard flow regime model (e.g., Simons et al., 1965). The sedimentary structures and thicknesses of the event beds were then organized into eight sediment types (lithofacies) that were grouped into three sediment complexes: the playa complex, the antidune complex, and the sheet sand complex. The arrangements of the sediment types and complexes within the stratigraphic framework of the lower informal Revett member indicated the configurations of the depositional environments in space, and the vertical configurations of the sediment types revealed the depositional history of the lower Revett member. The lower Revett member lithosome interfingers eastward into the red argillite of the Grinnell Formation lithosome, and has eight through-going descriptive, stacked, lithic units, called lithostromes. Lithostromes 2, 4, 6, and 8 (from the bottom up) are composed of the sheet sand complex and extend into playa complexes of the Grinnell Formation. They were deposited by sandy sheetfloods that flowed at grade and terminated as the water sank into the sand substrate. Between lithostromes 2, 4, 6, and 8 are lithostromes marked by playa lakes of the playa complex that spread from the east across western Montana during humid periods. They were overlain by sheetfloods of the antidune complex that built eastward over the playa complex as the playa lakes retreated with increasing aridity. The antidune complex was overlain by the sheet sand complex of a vast sand plain deposited by sheetfloods from the southwest that flowed at grade level across western Montana during arid periods. The sheetflood deposits of the Revett Formation were mostly deposited by the upper-flow regime element of the established fluvial facies model.

Crinkle cracks are sand-filled cracks up to 5 mm wide in plan view that pinch at their ends. In cross section, they are canted and crinkled. They cut mudstone beds that underlie hummocky cross-laminated sandstone lenses. They are here described from the Piegan Group, Proterozoic Belt Supergroup, but they are widespread in Proterozoic and Phanerozoic marine and lacustrine rocks. However, they represent a distinctive, descriptive style of mudcracks, not attributed to inferred syneresis processes, although they have been commonly attributed to syneresis. In plan view, crinkle cracks closely resemble cracks formed where oscillatory waves striking viscous mud banks are transformed into fluid solitary-like waves that open surface cracks on their trailing limbs and close the cracks on their leading limbs as they pass through the viscous mud. Crinkle cracks preserved in rocks are hypothetically attributed to oscillatory waves moving sand over viscous mud. The oscillatory waves are transformed into solitary-like waves as they pass down into the mud, forming the cracks. The surface sand falls down into the cracks, preserving them. With burial, the water escapes, and the viscous mud compacts, crinkling the sand-filled cracks.

The Chamberlain Formation, one of the lower members of the early Mesoproterozoic Belt Supergroup, has previously yielded low-diversity assemblages of microfossils but the reported fossils were of limited utility for inferring paleoenvironmental conditions. Here, we describe substantially more diverse microfossil assemblages from drill core of the Chamberlain Formation obtained from the Black Butte mine locality near White Sulphur Springs, Montana. The Chamberlain Formation biota contains abundant Valeria , Leiosphaeridia , Synsphaeridium , and Lineaforma , with lesser amounts of Satka , Symplassosphaeridium , and Coniunctiophycus. The assemblages partially overlap with, but are distinct from, microfossils recently reported from the Greyson Formation, another unit from the Helena embayment of the Belt Supergroup. Since the overlapping taxa exhibit similar states of preservation but dissimilar relative abundances, we interpret the assemblages as reflective of distinct paleoenvironmental conditions of the sampled sections of the Chamberlain and Greyson Formations. The Chamberlain Formation assemblages are most comparable to microfossil groupings reported from the Bylot Supergroup of Canada and the Roper Group of Australia from sediments from very shallow-water (supratidal to lower shoreface) marine environments. This comparison corroborates previous hypotheses on the basis of sedimentological data that the lower Chamberlain Formation sediments were formed in a lagoonal or mud-flat environment. By contrast, the Greyson Formation assemblages are most comparable to microfossil groupings associated with sediments from shallow-shelf marine environments. The fidelity of comparisons among the 1.2 Ga Bylot Supergroup, 1.49 Ga Roper Group, and 1.45 Ga Belt Supergroup assemblages indicates that the groups of microorganisms that produced these assemblages, and their associations with the paleoenvironments that they inhabited, may have been characteristic of the littoral marine biosphere throughout much of the Mesoproterozoic.

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.