Dikes and pods of pseudotachylite up to one meter thick have been found over an area >50 km 2 in the same area as shatter cones and other possible features of shock metamorphism in the Beaverhead and Tendoy Mountains in southwestern Montana, defining the allochthonous remains of the Beaverhead impact structure (see also Hargraves et al., Chapter 19, this volume). They are not associated with any tectonic feature in the area and have several features uncommon in pseudotachylites formed by tectonic processes (large size, vesicles, pseudotachylite clasts within pseudotachylite), but which have been documented in pseudotachylites from other impact structures. Rare single sets of planar deformation features (PDFs) are found in quartz grains in the pseudotachylites with crystallographic orientations similar to those found in shocked quartz from other impact structures. The major and trace element chemistry of the pseudotachylites is similar to their host rocks, but with some enrichments (Al, Mg, Fe, K, volatiles) and depletions (Si, Na) indicating low-grade metasomatic alteration. 40 Ar/ 39 Ar laser microprobe analyses of pseudotachylites from three localities show a wide spectrum of ages, from Precambrian to Tertiary. The distribution of ages suggests two isotopic signatures, one of Precambrian age (although younger than the age of the protoliths), and the other of Cretaceous age. 29 Si Magic angle spinning (MAS) nuclear magnetic resonance (NMR) studies of the pseudotachylite failed to detect the presence of high-pressure polymorphs of quartz. The evidence suggests that these pseudotachylites were formed by the same event that formed the shatter cones. Related work (see Hargraves et al., Chapter 19, this volume) suggests that the original crater was at least 75 km in diameter and was formed in the late Proterozoic to Cambrian, 20 to 150 km to the west of the present location.

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 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.

Upward-pointing shatter cones in sandstones of uncertain age (Middle Proterozoic? to Lower Cambrian?) and older crystalline basement rocks are exposed over an area of approximately 25 × 8 km in southwestern Montana. These shatter cones, together with pseudotachylites and breccias of various types (particularly in basement gneisses), are inferred to be products of a meteorite or cometary impact. However, Late Cretaceous contraction and Tertiary extension have contributed to the structural complexity of the area, and distinguishing unequivocally the shock brecciation from that due to younger tectonism is difficult. Stratigraphic constraints suggest the structure is Late Proterozoic or Cambrian in age. The shocked rocks are present in the Cabin thrust plate—one of many in the Late Cretaceous Cordilleran Thrust belt—and hence are allochthonous, having been transported tens of kilometers from the west. They are considered to represent only a piece from the central uplift of an original complex crater at least 75 km in diameter. It is speculated that some of the considerable uplift and erosion inferred to have taken place in Late Proterozoic to early Paleozoic time in east-central Idaho (The Lemhi arch) may be related to the postulated impact event. Furthermore, quasi-circular magnetic and regional gravity anomalies (50 to 75 km diameter) centered south-southeast of Challis, Idaho, may mark the concealed scar of the original impact structure.