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 geology across north-central Idaho from the Snake River in the west across the Bitterroot Mountains to the east to near Missoula, Montana. The regional geology includes a much-modified Mesozoic accretionary boundary along the western side of Idaho across which allochthonous Permian to Cretaceous arc complexes of the Blue Mountains province to the west are juxtaposed against autochthonous Mesoproterozoic and Neoproterozoic North American metasedimentary assemblages intruded by Cretaceous and Paleogene plutons to the east. The accretionary boundary turns sharply near Orofino, Idaho, from north-trending in the south to west-trending, forming the Syringa embayment, then disappears westward under Miocene cover rocks of the Columbia River Basalt Group. The Coolwater culmination east of the Syringa embayment exposes allochthonous rocks well east of an ideal steep suture. North and east of it is the Bitterroot lobe of the Idaho batholith, which intruded Precambrian continental crust in the Cretaceous and Paleocene to form one of the classical North American Cordilleran batholiths. Eocene Challis plutons, products of the Tertiary western U.S. ignimbrite flare-up, intrude those batholith rocks. This guide describes the geology in three separate road logs: (1) The Wallowa terrane of the Blue Mountains province from White Bird, Idaho, west into Hells Canyon and faults that complicate the story; (2) the Mesozoic accretionary boundary from White Bird to the South Fork Clearwater River east of Grangeville and then north to Kooskia, Idaho; and (3) the bend in the accretionary boundary, the Coolwater culmination, and the Bitterroot lobe of the Idaho batholith along Highway 12 east from near Lewiston, Idaho, to Lolo, Montana.

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.

Sedimentary interbeds preserved between flows of the Columbia River Basalt Group provide a record of the depositional and erosional conditions that characterized the Columbia Plateau between eruptions of basalt. Examination of the sedimentary, stratigraphic, and petrologic character of the Sweetwater Creek interbed from within the Lewiston basin of southeastern Washington and north-central Idaho allows insight into the paleogeographic conditions that existed following eruption of the Priest Rapids Member of the Wanapum Basalt, ca. 14.5 Ma. The Sweetwater Creek interbed is composed of generally unconsolidated and inter-stratified beds of clay, silt, sand (with local thin gravel stringers), and volcanic ash-rich sediment. Three broadly defined sedimentary facies are identified on the basis of lithology and texture. The spatial distribution of these facies, abundance of clay- and silt-rich sediment, and internal sedimentary structures suggest that deposition of the interbed resulted primarily from fluvial and mixed fluvial-lacustrine sedimentation. Fluvial drainages that headed in the ancestral Clearwater Mountains entered the Lewiston basin on the east and exited to the northwest. Basin streams appear to have been primarily of the meandering, mixed-load type. Channel sands deposited by these streams were concentrated east and north of the basin center, and transported extrabasinal sediments are characterized by plutonic and metamorphic sand- and gravel-sized clasts. Fine-grained silt- and clay-rich flood-plain and associated lacustrine deposits extend across the basin, but are thickest near the basin center. The Umatilla basalt flow entered the Lewiston basin during deposition of the Sweetwater Creek interbed and locally invaded fine-grained lacustrine sediments. A later flow, the Wilbur Creek basalt, partially buried the interbed. Complete burial of the Sweetwater Creek interbed sediments followed eruption of the Asotin flow.

New structural, metamorphic, and geochronologic data from the Clearwater complex, north-central Idaho, define the origin and exhumation history of the complex. The complex is divisible into an external zone bound by normal faults and strike-slip faults of the Lewis and Clark Line, and an internal zone of Paleoproterozoic basement exposed in two shear zone–bounded culminations. U-Pb sensitive high-resolution ion microprobe (SHRIMP) dating of metamorphic zircon overgrowths from the external zone yield zircon growth at ca. 70–72 Ma and 80–82 Ma, during peak metamorphism and before tectonic exhumation of the external zone. U-Pb SHRIMP dating of metamorphic zircon rims from the internal zone record growth at ca. 64 and between 59 and 55 Ma. The older ages record pre-extension metamorphism. The younger rim ages were derived from fractured zircons in the Jug Rock shear zone, and they document the beginning of exhumation of the internal zone along deep-seated shear zones that transported the basement rocks to the west. The 40 Ar/ 39 Ar ages record quenching of the external zone starting ca. 54 Ma and the internal zone between 53 and 47 Ma by movement along the bounding faults and internal shear zones. After ca. 47 Ma, extension was accommodated via a west-dipping detachment that was active until after ca. 41 Ma. The Clearwater complex is interpreted as an Eocene metamorphic core complex that formed in an extensional relay zone between faults of the Lewis and Clark Line.

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.