Abstract A regional study, based on detailed descriptions of 17 outcrops across east-central Idaho and southwestern Montana, provides a dip-oriented cross section in which to better understand the distribution of Upper Mississippian (Chesterian) stratigraphy on the distal margin of the Antler foreland basin. Chesterian strata constitute an eastward-thinning wedge of mixed carbonate and siliciclastic rocks that formed on a west-facing ramp. Foreland-basin tectonism subdivided the ramp into three distinct depositional settings: the western, central, and eastern ramp. The western ramp records nearly continuous Chesterian deposition, whereas the central and eastern ramps have significant unconformities. Mud-rich subtidal carbonate predominates on the western ramp, but this interfingered during the late Chesterian with tidally influenced siliciclastics. The central ramp contains an intraramp basin with deep subtidal siliciclastics and carbonate that formed adjacent to shallower-water facies to the west and east. The eastern ramp has mostly peritidal carbonate and shallow marine to fluvial siliciclastics, but a transgression from the north during the late Chesterian inundated this portion of the ramp with open marine carbonate. New conodont biostratigraphic constraints indicate that these Chesterian strata are a second-order megasequence (10-12 My duration) composed of more than seven third-order depositional sequences (S0-S7), each having a duration of 1-5 My. The sequences are grouped into three composite sequences (I, II, and III) that define long-term changes in accommodation controlled by syndepositional tectonism. Composite sequence I was deposited during a period of tectonic loading that partitioned the ramp via subsidence loading and extension. Composite sequence II records a period of tectonic stabilization and deposition during the most extensive eustatic flooding, whereas composite sequence III is dominated by a localized subsidence event in the Big Snowy Trough. Higher-frequency (fourth- and fifth- order) parasequences are common throughout the study interval, but they are only locally correlative. A change from thick-bedded carbonate- dominated parasequences in the early and middle Chesterian to thinner-bedded mixed carbonate and siliciclastic parasequences in the late Chesterian likely reflects the onset of moderate- to high-amplitude, high-frequency eustatic fluctuations caused by the initiation of Gondwanan glaciation.

Abstract Mesoproterozoic strata from east-central Idaho and the Belt Supergroup of southwest Montana (eight new samples) contain several age groupings of detrital zircon grains: from old to young: a) Laurentian grains older than 1.85 Ga, b) a flood of 1655 to 1790 Ma Paleoproterozoic grains; b) non-North American zircon populations (ca. 1510 to 1625 Ma) with no known source on Laurentia, and c) syn- Belt grains, with groupings at 1480 Ma (syn-lower Prichard) and 1450 Ma (upper Piegan Group). The 1450 Ma = grain age population overlaps a 1454 ± 9 Ma fallout tuff in Glacier National Park. Strata of the Yellowjacket, Hoodoo, Apple Creek, and western Gunsight formations of Idaho all contain the 1450 Ma population, sparse non-North American grains, and dominant Paleoproterozoic populations at 1670 to 1790 Ma. This detrital-zircon signature is comparable to that of the Wallace Formation of the Belt Supergroup. The Swauger Quartzite in Idaho and the eastern Gunsight Formation at the ca. 900 Ma Beaverhead impact site in southwest Montana contain 1710 and 1780 Ma zircon populations identical to those of the Missoula Group of the Belt Supergroup. The E member of the lower Belt Prichard Formation from Plains, Montana, contains a population of syndepositional zircons at 1479 ± 19 Ma. Like the Revett Formation of the Ravalli Group, the E member contains Mesoproterozoic non-North American detrital-zircon populations as well as Paleoproterozoic grains at 1750 to 1790 Ma. The thick east-central Idaho Mesoproterozoic section was deposited after 1450 Ma, with deposition of the 10 km thickness from the lower Yellowjacket Formation through the Gunsight Formation spanning only 10 to 20 My. Given this very high rate of deposition, previous correlations of the Apple Creek Formation with the Piegan Group are permissible if problematic. However, previous correlations of the eastern Gunsight and Swauger formations with the Missoula Group are supported. Their detrital-zircon grain populations are identical.

Abstract The sediment-hosted gold–bearing cobalt–copper Blackbird deposits of east–central Idaho, U.S.A., are unique deposits hosted in enigmatic and poorly known Mesoproterozoic strata. New regional mapping studies elucidate the geographic distribution of units of the very thick Mesoproterozoic succession and the geometry of major structures. Blackbird deposits are hosted predominantly by the banded siltite unit of the Mesoproterozoic Apple Creek Formation; a few mineralized zones are in the underlying coarse siltite unit of the Apple Creek and the overlying basal Gunsight Formation. The depositional mechanism for hosting strata was predominantly turbidity– current flow. Blackbird deposits were long thought to be hosted in the Yellowjacket Formation, which was mapped as the dominant formation throughout central Idaho but, as originally defined, formed in a relatively shallow–water marine environment. Our conclusions about the host strata, in conjunction with previous interpretations that Blackbird deposits formed in a rift setting, resolves previous contradictions between models for environments of sediment deposition and origin of metals accumulation. Along the west side of the Blackbird district, the Apple Creek and Yellowjacket formations are separated by the northwest–trending Late Cretaceous Iron Lake fault. The Yellowjacket Formation is confined to the Iron Lake thrust plate west of the district and does not contain sediment–hosted deposits. In contrast, the Blackbird deposits lie in the complexly deformed upper part of the underlying Poison Creek plate, specifically at the northeast–trending hinge zone of the Blackbird Mountain oblique ramp. Near the ramp, both hanging–wall and footwall rocks are strongly deformed, resulting in major overturned folds in the hanging wall and imbricate thrust plates in the footwall. The upper imbricate in the Poison Creek plate contains chloritoid–garnet schists that were long recognized as being above the garnet isograd and having a different mineralization style. The intermediate imbricate forms the lower to middle greenschist Blackbird subplate containing bedding–parallel and structurally remobilized cobalt–copper deposits. The structurally lowest imbricate forms the Haynes– Stellite subplate and includes the youngest rocks (lower Gunsight Formation) and mineralized tourmaline breccias. Subsequent to deformation related to the Iron Lake thrust fault, several normal faults reactivated the Iron Lake thrust fault, the Blackbird Mountain ramp, and the buried Great Falls tectonic zone in the basement. This complex deformation resulted in juxtaposition of metamorphic facies and stratigraphic units from different structural levels as well as from different parts of the stratigraphic section. Present exposure of strata of the Apple Creek sedimentary basin, in which mineralized strata formed, is controlled by northwest– trending thrust faults and normal faults. Our study results in limiting the permissive stratigraphy that hosts the deposits in a meaningful way for future identification of related deposits. It also shows that the previous outline of an Idaho cobalt belt did not represent geometry of the rift basin per se, but instead is the expression of structural control of exposure of the hosting Apple Creek Formation, primarily those strata that lie in the Poison Creek plate.

Abstract An enigmatic quartzite breccia previously mapped within a late Neoproterozoic–Early Paleozoic unit exposed in Leaton Gulch near Challis, Idaho, U.S.A., may be related to the Beaverhead meteorite impact. The fragments in the breccia are highly deformed, but no unequivocal features of shock metamorphic deformation have been observed. The brecciated rocks are entirely in the late Mesoproterozoic Swauger Formation. The Swauger overlies crumpled rocks of the younger Mesoproterozoic Lawson Creek Formation, across a planar and nearly flat surface interpreted to be a thrust fault. Neither the planar surface nor the breccia is crumpled. The breccia is part of a thrust slice carried into the Leaton Gulch area from farther west during the Late Cretaceous thrust faulting characteristic of this region. The erosion– smoothed upper surface of the breccia is overlain by the Wilbert Formation of Neoproterozoic–Early Cambrian age. The Wilbert, in turn, is overlain by the Middle Ordovician Kinnikinic Quartzite. The age of the breccia therefore must be post–Swauger and pre–Wilbert, and Neoproterozoic. The breccia could be debris that fell from one of the outermost ring faults defining the impact crater. Such rocks would have been too far away from the point of impact to experience pressures sufficient to cause any shock deformation.

Abstract The Doublespring Pass road site is an excellent location at which to examine the surface faulting and ground breakage that accompanied the 1983 Borah Peak earthquake. The site can be reached by traveling 23 mi (37 km) northwest from Mackay, Idaho, or 30 mi (48 km) southeast from Challis, Idaho (Fig. 1), on U.S. 93 to the Doublespring Pass road turnoff. The Doublespring Pass road heads northeast from the highway. The intersectionof the Doublespring Pass road with the highway is identified by a sign indicating the direction to the towns of May and Patterson. The turnoff is also identified at the intersection by a historical marker commemorating William E. Borah, after whom Borah Peak was named. The Doublespring Pass road crosses thefault scarps 2.5 mi (4 km) northeast of the intersection of U.S. 93 (Fig. 2). Doublespring Pass road is a wide, well-maintained gravel road that can be safely traveled by passenger car and bus to the site (although the road is impassable at times in the winter and early spring when snow covers the area). The site lies within Challis National Forest and is open to the public.

Abstract The Lemhi Range, Idaho, is one of three northwest-trending basin-ranges along the northeastern margin of the eastern Snake River Plain (Fig. 1). The southern end of the range is about 55 mi (88 km) northwest of Idaho Falls. The small town of Howe is the closest location for food, water, gasoline, and a trailer park. The nearest motels and service stations are in Arco, about 22 mi(35 km) to the southwest. The area is covered by six 1:24,000 topographic quadrangles. They are listed in the caption of Figure 2A and will be essential for detailed planning of access. Access to the outcrops is provided by graded dirt roads from Howe to the alluvial apron where rougher roads (traversable by pickups) lead to the seven major canyons in which the stratigraphic sections are exposed (Fig. 2A). From west to east, these canyons are South Creek Canyon, Black Canyon, Middle Canyon, East Canyon, Box Canyon (informal name), an unnamed canyon with a jeep road that crosses the range, and, on the east side of the range, Kyle Canyon.