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.

A recent 1:24,000 scale mapping project within the northern Beaverhead Mountains along the Idaho-Montana border has resulted in a reinterpretation of both the Mesoproterozoic stratigraphy and the regional structural framework. A 15-km-thick stratigraphic section of the Mesoproterozoic Lemhi subbasin was initially deformed by northeast-southwest shortening into giant northwest-striking, northeast-verging folds, probably during Cretaceous Sevier orogenesis. These initial folds were then dissected by a system of subparallel and anastomosing, oblique-slip reverse, thrust, and normal faults that generally strike northwest, but that exhibit east-west–oriented lineations, suggesting components of strike-slip displacement. Contractional faulting appears to have been followed by Eocene to Miocene extensional faulting, with many normal faults following the preexisting fabrics. Extension opened Tertiary basins along some of these faults, including the Salmon Basin along the southwestern side of the Beaverhead Range. Subparallel faults in the surrounding region appear to have a similar complex history, and all appear to be part of a major northwest-striking Cretaceous fold-and-thrust belt that was later dissected by Tertiary extension. Although the faults of the Beaverhead Mountains are significant and long-lived, they are not terrane-bounding structures separating the Belt and Lemhi sedimentary sequences. Instead, Lemhi strata extend across the range and northward to Missoula, where they grade into correlative Missoula Group strata.

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.