Abstract We analyzed detrital zircons in seven samples of metasedimentary rock from north–central Idaho, U.S.A., to test the previous assignment of these rocks to the Mesoproterozoic Belt–Purcell Supergroup. Correlating these rocks with known sedimentary units through field observations is difficult if not impossible due to the high metamorphic grade (amphibolite facies) and intensity of deformation. Zircon analysis by laser–ablation inductively coupled plasma mass spectrometry (LA–ICPMS) reveals that five of the seven samples contain multiple zircon populations between 1700 and 1400 Ma and a scatter of Paleoproterozoic and Archean ages, similar to results reported from the Belt Supergroup to the north and east. These results indicate that the likely protoliths of most high–grade metamorphic rocks northwest of the Idaho batholith were upper strata of the Belt Supergroup. In contrast, a quartzite at Bertha Hill north of Pierce lacks grains younger than 1600 Ma and thus is distinctly unlike the Ravalli Group of the Belt Supergroup, with which it was previously correlated. Possible correlatives that contain similarly old populations of zircons and feldspar–poor quartzite include the Neihart Formation (lowermost Belt Supergroup in Montana), Neoproterozoic quartzite (Syringa metamorphic sequence), and Cambrian quartzite. A sample from the North Fork of the Clearwater River yielded a large number of zircons with concordant Neoproterozoic ages, all of which had low Th/U ratios that suggest either a Neoproterozoic metamorphic event or the transport and deposition of zircons that were metamorphosed in the Neoproterozoic. SHRIMP (sensitive high–resolution ion microprobe) dating of a granite (now augen gneiss) that intruded sedimentary rocks west of Pierce, Idaho, yields an age of 1379 ± 12 Ma based on seven of fourteen analyses; this provides a lower age limit for sediment deposition of some rocks mapped as metamorphosed Belt Supergroup, and which had detrital zircon populations in the 1700 to 1400 Ma range. Additional analyses of three zircon rims yield an age range of 87–82 Ma, which is similar to the youngest ages from the North Fork sample. We interpret these ages to reflect the time of zircon overgrowth synchronous with the emplacement of the Cretaceous Idaho Batholith. None of the metasedimentary rocks dated can be older than Mesoproterozoic, and, with the exception of the Bertha Hill quartzite, none can be older than the Belt–Purcell Supergroup.

Abstract This field guide covers the Precambrian geology of the western portion of the Clearwater complex and surrounding area in north-central Idaho in the vicinity of Marble Creek within the St. Joe National Forest. The regional geology of the Marble Creek area includes Precambrian basement orthogneisses, possible basement metasupracrustal rocks, and overlying metamorphosed Belt Supergroup strata. These rocks are exhumed within the western portion of the Cretaceous-Eocene Clearwater metamorphic core complex. This guide focuses on the western part of the Clearwater complex in the vicinity of Marble Creek. Outcrops of Paleoproterozoic basement and overlying Mesoproterozoic metasedimentary units provide a better understanding of the Precambrian magmatic and metamorphic history of this region. The road log in this guide describes the regional geology in a south to north transect from Clarkia, Idaho, to the confluence of Marble Creek with the St. Joe River.

Abstract In the absence of land plants, broad pediments may have formed stable landforms that beveled Proterozoic continents. Braided streams would have transported a thin layer of clastic sediment across such Proterozoic epicontinental pediments. The Proterozoic pediment–braidplain system may be represented by extremely flat regional unconformities beneath locally preserved, supermature, braidplain sandstones. Continental rifting would have destabilized Proterozoic epicontinental pediments by funneling runoff along rift axes to create large rivers, which otherwise were not favored in the Proterozoic landscape. The sedimentological history and detrital–zircon provenance of the intracratonic Mesoproterozoic Belt–Purcell basin of western North America may be described in terms of destabilization of a late Paleoproterozoic to early Mesoproterozoic epicontinental pediment by a three–armed rift system with the Belt–Purcell basin at its center. A model using a Siberia–Laurentia–Australia paleocontinental reconstruction implies that the sedimentary veneer of the pediment washed down the western branch of the rift system to enter the Belt–Purcell basin at a point source on its western side. Capture of clastic sediment in delta fans on the western side of the basin permitted clean carbonate to precipitate on the northeast side. Reconfiguration of the basin by renewed rifting appears to have changed composition, grain size, and sedimentary provenance during deposition of the Missoula Group (upper Belt–Purcell Supergroup).

Abstract The Lewis thrust plate in Glacier National Park contains the displaced margin of the Mesoproterozoic Belt Basin, which was transported some 130 km (~80 mi) northeastward during the Late Cretaceous to late Paleocene Laramide orogeny. This two-day field trip examines the Lewis plate on an eastward transect along the Going-to-the-Sun Road, crossing the Continental Divide at Logan Pass. The trip begins in basinal facies of the Belt Supergroup, and crosses eastward into shelf facies. The shelf-to-slope transition coincides with the axis of the Akamina syncline along the Continental Divide. The trip visits classic exposures of the Lewis thrust along the Rocky Mountain front, where the lower Belt Supergroup overlies a duplex zone in the Late Cretaceous marine section.

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.