The Leaton Gulch Breccia in East-Central Idaho, U.S.A., and its Relation to the Beaverhead Meteorite Impact
Published:January 01, 2007
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R.B. Hargraves, E.T. Ruppel, J.W. Gephart, 2007. "The Leaton Gulch Breccia in East-Central Idaho, U.S.A., and its Relation to the Beaverhead Meteorite Impact", Proterozoic Geology of Western North America and Siberia, Paul K. Link, Reed S. Lewis
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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.
The Beaverhead meteorite impact was first recognized in 1990 (Hargraves et al., 1990), when shatter cones were found in Prot– erozoic rocks in the Beaverhead Mountains, near Medicine Lodge Creek in southwest Montana (Fig. 1). Because the Proterozoic rocks were known to be allochthonous, the search for the impact area, and for other pieces of fragmented, allochthonous rocks from the impact area, was extended to the west. Ultimately, it led to the region surrounding Challis, Idaho, where McCafferty (1995) identified the probable impact on the basis of gravity and magnetic data, and to Leaton Gulch, an area east of Challis where an enigmatic breccia was known to occur.
This breccia, almost alone in this region, might have been formed by the meteorite impact.
Enigmatic breccias and associated other rocks in Leaton Gulch, in the northern part of the Lost River Range, had earlier been mapped in reconnaissance and described by McIntyre and Hobbs (1987). Carr and Link (1999) and Merrill (2003) later studied the area and concluded that the breccia probably was impact–related but still enigmatic.
Because of uncertainties posed by these earlier studies, we concluded that additional study and more detailed mapping was needed. We started that study in 1998 and continued it intermittently until 2002. In all, we spent several weeks mapping the small Leaton Gulch area (Fig. 2) and resolving the principal strati– graphic problems. We concluded that the stratigraphic units, which are the fundamental key to understanding the geology of the Leaton Gulch area, are much as described by McIntyre and Hobbs (1987). The interpretation of structure in the area has been confused by stratigraphic uncertainties and isolated outcrops, but is relatively simple. A plate of fractured and previously brecciated Swauger Formation quartzite has been thrust over younger rocks and broken by a few steep faults. Although there is some coincidence in the time of brecciation and the probable time of the Beaverhead meteorite impact, and although we believe that the Leaton Gulch breccia probably is a thrust–faulted piece of the western crater rim, there is no conclusive evidence that the two are related.
Summary of Earlier Investigations
The shatter cones in the Medicine Lodge Creek area of southwest Montana have been the subject of a number of investigations since their discovery in 1990 (Hargraves et al., 1990). In subsequent years, much of the area was mapped to learn the distribution of the shatter cones and the distribution of pseudotachylites and bleached quartzitic rocks of the Swauger Formation in and near the area of shatter cones (Ruppel, 1994; Kellogg et al., 1999; Kellogg et al., 2003). As a result of the mapping, and of further study of the distribution and attitudes of the shatter cones, a more complete description of the Medicine Lodge exposures was published by Hargraves et al. (1994). At the same time, Fiske et al. (1994) described pseudotachylites in crystalline metamorphic rocks at the east margin of the area of shatter cones but noted that the pseudotachylites and the shatter cones never occur together. Their attempts at dating the pseudotachylites were not successful, but Kellogg et al. (1999) and Kellogg et al. (2003), on the basis of later studies of biotite, muscovite, and zircons in these rocks, suggested that the impact occurred in the Early Neoproterozoic, between about 850 and 900 Ma.
A summary article on the impact was published by Fiske and Hargraves in 1998. It included brief statements on the breccias at Leaton Gulch, and on a rock tentatively identified as an impact– related silicic lithic tuff at the bottom of a deep drill hole west of Challis. The lithic tuff was examined by B.F. Leonard (written communication, 1990) and R.J. Klee (written communication, 1995) and did not exhibit any shock features or other characteristics that would suggest that it is impact related.
In the Leaton Gulch area, the breccias and other rocks mapped by McIntyre and Hobbs (1987) were described as an undivided unit that might include correlatives of the Wilbert and Summerhouse Formations, which are present farther south in the Lost River Range and in the adjacent Lemhi Range (Ross, 1947; Ruppel et al., 1975; McCandless, 1982; Janecke and Wilson, 1992). The discussion of the Leaton Gulch rocks and breccias was expanded slightly in a later report (Hobbs and Hayes, 1990) but without resolution of the stratigraphic confusion. The geology of the area was later reinterpreted by Carr and Link (1999) and Merrill (2003), who divided the rocks into two units and suggested that the breccia unit was related to the Beaverhead impact event, either as a basal conglomerate deposited in an outer ring crater or as a valley–fill deposit of locally derived debris.
Geophysical evidence interpreted by Bankey (1992) and McCafferty (1992, 1995) defined large, overlapping roughly circular magnetic and gravity anomalies in the Challis region and suggested that the anomalies reflect the disruption of crustal rocks at the site of the Beaverhead impact.
In summary, these reports describe an area in and near the Beaverhead Mountains in southwest Montana where shatter cones are abundant and pseudotachylites are locally present, in thrust–faulted rocks of the Mesoproterozoic Gunsight Formation of the Lemhi Group and in some crystalline metamorphic rocks. Shatter cones or other evidence of the Beaverhead impact have not been found in any younger rocks or in any other place in either southwest Montana or central Idaho, but geophysical evidence suggests that the impact was in what is now central Idaho, near the community of Challis, and near the enigmatic breccia in Leaton Gulch. Shallower parts of the impacted area are exposed at the surface in the thrust–faulted rocks of the Beaverhead Mountains of southwestern Montana. The widespread distribution of shatter cones there suggests that the impact crater may have been as much as 50 to 60 miles (80 to 96 km) in diameter and is one of the largest ever identified in the United States. The crater has been so dismembered by thrust faulting and later detachment and related extensional faulting, however, that only the remnants in the Medicine Lodge basin are known with any certainty (Hargraves et al., 1990; Hargraves et al., 1994; Fiske et al., 1994; Ruppel and O’Neill, 2003; Janecke and Blankenau, 2003).
The Leaton Gulch area reflects the complexity of the geology of east–central Idaho. All of the pre–Tertiary sedimentary rocks of this region have been carried eastward on Late Cretaceous thrust faults (Ruppel, 1978; Ruppel and Lopez, 1984; Janecke and Wilson, 1992). Later, steeply dipping northwest– and northeast– trending faults, all of them probably controlled at least partly by ancient basement faults, have broken the region into rhombohe– dral blocks. Eocene low–angle detachment faults and extensional collapse have defined the present mountain ranges. Movement on the basement–controlled faults and on younger north–trending faults continues today in this earthquake–prone region. Some more recent reports, for example Tysdal (1999, 2000, 2002) and Winston et al. (1999), have suggested other interpretations of the sequence of Proterozoic rocks and of the regional structural framework than those in earlier reports. The significance of the Chief Joseph metamorphic core complex and of the related detachment faulting that pervades this region (Ruppel and O’Neill, 2003; Desmarais, 1983), are not recognized in these more recent reports, however, and the conclusions reached in them are seriously, perhaps fatally, flawed. In this report on the Leaton Gulch breccia, any more extensive discussion of these regional strati– graphic and structural issues, or of continuing studies by others on these issues, is not appropriate. We retain the earlier strati– graphic framework and much of the earlier structural framework.
The sedimentary rocks in the Leaton Gulch area were deeply eroded after thrusting, and the resulting irregular erosion surface was flooded by the tuffs and lavas of the Challis Volcanics, of Eocene age. The pre–volcanic surface is now being exhumed, at least partly, and the pre–volcanic rocks crop out as islands in a sea of tuffs, lavas, and landslides (Fig. 2). The sequence of sedimentary rocks was described by MacIntyre and Hobbs (1987) and Hobbs and Hays (1990) as a “sequence of strata comprising predominant quartzite with subordinate interbeds of dolomite, siltstone, and argillite”, which also includes coarse conglomerate or intraformational breccias. In their limited study, they could not correlate this sequence of strata with other formations in the region, and they mapped it as a single, undivided unit. They suggested, however, that it included rocks that resembled the Mesoproterozoic Swauger and Lawson Creek Formations, the late Neoproterozoic (?) Wilbert Formation, the Lower Cambrian rocks above the Wilbert Formation included in the formation of Tyler Peak of McCandless (1982), and the Lower Ordovician Summerhouse Formation.
Carr and Link (1999) and Merrill (2003) divided the formation into an upper unit (OZlu), of conglomerate, sandstone, and siltstone, and a lower unit (OZll), of fine– to medium–grained sandstone that constitutes most of their formation of Leaton Gulch. They did not suggest correlation with any of the better– known formations of the region. They interpreted the principal and best–exposed breccia at the head of Leaton Gulch to be a basal conglomerate in their upper unit, and they concluded that it was a debris flow or talus derived from impact–related scarps on normal faults.
We conclude that McIntyre and Hobbs (1987) were correct in their identification of the formations present in Leaton Gulch. We have subdivided the sequence of sedimentary rocks into the Swauger, Lawson Creek, Wilbert, and Summerhouse formations, and the Kinnikinic Quartzite. The Lawson Creek Formation is overridden by a Late Cretaceous thrust plate of brecciated and intensely fractured Swauger Formation quartzite. The thrust plate, in turn, is overlain disconformably by the Wilbert Formation and the Kinnikinic Quartzite.
The Swauger Formation and the Kinnikinic Quartzite are the most easily recognized of these formations. The Swauger Formation, of Late Mesoproterozoic age, is almost 10,000 feet (3,050 m) thick in its type area in the Lemhi Range (Ross, 1947; Ruppel, 1975). Unlike other Proterozoic rocks in east–central Idaho, it consists of distinctive, purplish colored, medium– to coarsegrained, hematitic quartzite that contains little or no feldspar. It is mainly thick–bedded, and the beds commonly show pronounced cross–bedding (Ruppel, 1975). The Kinnikinic Quartzite (Hobbs et al., 1968) is vitreous, white to light gray massive quartzite of Middle Ordovician age. It ranges from almost 3,000 feet (915 m) thick in the Lost River Range to as little as 500 feet (152 m) thick in the southern part of the Lemhi Range (Ross, 1961; James and Oaks, 1977).
The other formations in the Leaton Gulch area are heterogeneous groups of rocks that are known to crop out only in relatively small areas. The Lawson Creek Formation (Hobbs, 1980) of late Mesoproterozoic age is known primarily in and near its type area in the east flank of the Lost River Range, east of Leaton Gulch. It is a sequence of reddish purple to maroon, predominantly thin– bedded impure quartzite, siltite, and argillite with abundant mud chips, ripple marks, and micaceous partings. Hobbs (1980) noted that the heterogeneous rocks in the formation indicate an unstable depositional environment. It is more than 5,000 feet (1525 m) thick in its type area. The Lawson Creek Formation gradationally overlies the Swauger Formation and is the uppermost of the Mesoproterozoic formations in east–central Idaho. Because of extensive Neoproterozoic deformation and erosion, the original top and regional extent of the Lawson Creek Formation are unknown (Hobbs, 1980).
Elsewhere in east–central Idaho, the Swauger Formation is overlain with angular unconformity by the upper Neoproterozoic and Lower Cambrian Wilbert Formation or younger formations. In some places, where the Swauger Formation has been completely removed by Neoproterozoic erosion, these younger rocks overlie the Gunsight Formation at the top of the Lemhi Group, also with angular unconformity (Ruppel et al., 1975; Ruppel and Lopez, 1988). The Wilbert Formation is known mainly in its type area in the south part of the Lemhi Range and in a few places in the Lost River Range and Beaverhead Mountains (Ross, 1947, 1961; Janecke and Wilson, 1992). It is about 400–500 feet (122–152 m) thick at its type area and thins to disappear about 20 miles (32 km) farther north in the Lemhi Range. It is mainly light–gray to brownish–gray to pale red quartzitic, poorly sorted, fine– to coarse–grained sandstone and conglomerate (Ruppel et al., 1975). It probably is mainly late Neoproterozoic in age, but McCandless (1982) and Derstler and McCandless (1981) reported fossils of middle Early Cambrian age from the upper part of the formation. McCandless (1982) separated these fossiliferous rocks, which are mainly impure sandstones and shale about 100 feet (30.5 m) thick, from the type Wilbert Formation as the formation of Tyler Peak.
The Lower Ordovician Summerhouse Formation overlies the Wilbert Formation with slight angular unconformity, or, north of the area of disappearance of the Wilbert, overlies the Swauger Formation with pronounced angular unconformity (Ruppel et al., 1975). It is exposed mainly in the central part of the Lemhi Range and in a few places in the Lost River Range. The formation ranges greatly in thickness from place to place, from about 150 to 200 feet (46 to 61 m) at its type section to as much as 1,000 feet (305 m), and it thins to disappear northward in the central part of the Lemhi Range. It differs widely in composition from one section to another, but in general it consists of poorly sorted brownish to reddish quartzite, Skolithos–bearing, glauconitic, calcareous sandstone, pink to white quartzite, glauconitic shale, and olive–gray limestone and dolomite. In addition to abundant Skolithos tubes and other trace fossils in some beds, the formation also has yielded a varied and distinctive fauna of Early Ordovician age (Ruppel et al., 1975). The formation is overlain with slight angular unconformity by the Kinnikinic Quartzite.
Description of Formations at Leaton Gulch
Swauger Formation (Upper Mesoproterozoic)–
The Swauger Formation crops out in a southwest–dipping thrust plate in the upper part of Leaton Gulch (Fig. 2). It is composed of distinctive light–pink to pale–red and purple, me– dium–to coarse–grained quartzite that is hematitic, in some places slightly feldspathic. The quartzite is in beds as much as 10 ft (3 m) thick, and commonly is prominently cross–laminated. Quartz grains typically are well–rounded and glassy, and some are amethyst. The formation is strongly fractured and in places brecciated, and the Leaton Gulch breccia is composed entirely of fragments of Swauger Quartzite. The total thickness of the faulted and eroded remnant of the formation in Leaton Gulch is about 200 to 250 feet (61 to 76 m). The lower unit OZll of Carr and Link (1999) includes both the Swauger Quartzite and the Lawson Creek Formation of this report.
The Leaton Gulch breccia is a unique rock, without a counterpart elsewhere in central Idaho. It includes both angular fragments, some of them as much as 10 to 13 feet (3 to 4 m) across, and rounded fragments from several centimeters to a few meters in diameter, in a silicified and in places mylonitic matrix of smaller fragments and quartz grains that themselves show abundant deformation laminae (Carr and Link, 1999; Merrill, 2003). Brecci– ated zones within the formation are as much as 15 feet (4.6 m) thick, and tens of feet long, grading almost imperceptibly into less intensely fractured quartzites. The top of the breccias, and of the Swauger Formation in Leaton Gulch, is a smooth surface, an angular unconformity that cuts across fragments and matrix alike, and clearly shows that the breccia was strongly cemented, silicified, and brittle before the erosion that beveled and channeled its upper surface. The brecciation and fracturing in the Swauger Formation clearly preceded deposition of the Wilbert Formation, and is Neoproterozoic in age.
Lawson Creek Formation (Upper Mesoproterozoic).—
The Lawson Creek Formation (Hobbs, 1980; McIntyre and Hobbs, 1987) is widely exposed in the Leaton Gulch area. It is a heterogeneous, interbedded sequence of reddish purple and maroon to medium brown, fine–grained, feldspathic and hema– titic quartzite, impure quartzitic sandstone, siltstone, and argillite. It is mostly in beds 0.5 to 3 ft (0.15 to 1 m) thick, with mudcracked argillite bedding partings. Although the formation normally gradationally overlies the Swauger Formation, in Leaton Gulch it is in locally tightly folded and contorted thin beds beneath the thrust–faulted Swauger Formation.
Wilbert Formation (Neoproterozoic and Lower Cambrian).—
In Leaton Gulch, the Wilbert Formation overlies the fractured and brecciated Swauger Formation with angular unconformity. It consists of a basal conglomeratic or pebbly sandstone 2 to 5 ft (0.6 to 5.2 m) thick, overlain by purplish gray to yellowish gray, fine–to medium–grained, poorly sorted quartzitic sandstone, and grayish–red to dark–purple, chippy siltstone, mudstone, and shale with interbedded yellowish gray and grayish orange quartz– itic sandstone. The formation is overlain with apparent conformity by the Middle Ordovician Kinnikinic Quartzite in the northeastern part of the Leaton Gulch area. West of the map area near Challis Hot Springs, rocks tentatively mapped as part of the Wilbert Formation include a few lenticular beds of conglomerate as much as 20 ft (6.1 m) thick, containing well–rounded pebbles and cobbles up to 0.5 ft (0.15 m) in diameter of white quartz and Swauger quartzite. Similar conglomerates are common in the formation in the type area in the southern Lemhi Range, and much of the formation is poorly sorted, gritty, or conglomeratic throughout its outcrop area (Ruppel, et al., 1975).
Summerhouse Formation (Lower Ordovician)–
The Summerhouse Formation is exposed in fault–bounded, isolated outcrops south of Leaton Gulch and is more widely exposed west of the map area, near the mouth of Leaton Gulch. The exposures south of Leaton Gulch are light gray to white, massive to thick–bedded, fine–to medium–grained quartzitic sandstone that contains Skolithus tubes and Planolites , in some beds, and have been included in the upper (OZlu) unit of Carr and Link (1999), Merrill (2003). Near Challis Hot Springs, the Summerhouse Formation is a heterogeneous sequence of thin beds of yellowish brown, light gray, and pale or grayish pink, fine– to medium–grained quartzitic sandstone and interbedded olive gray, platy argillite and shale, grayish red thinly laminated sandstone, and grayish brown, partly laminated, partly algal dolomite and dolomitic sandstone. Some dolomite, dolomitic sandstone, and sandy dolomite beds are as much as 3 ft (1 m) thick, in units as much as 100 ft (30 m) thick. Trace fossils are abundant throughout the formation. Some of these rocks resemble the lower Middle Ordovician Ella Dolomite and may be an eastern equivalent of that formation, thrust–faulted eastward into the Challis region (Hobbs and Hays, 1990).
Kinnikinic Quartzite (Middle Ordovician).—
The Kinnikinic Quartzite is exposed only in the northeast corner of the mapped area, where it overlies the Wilbert Forma–tion with apparent conformity. The formation is thick–bedded to massive, vitreous quartzite composed almost entirely of subrounded to subangular fine to medium grains of quartz cemented by silica. The Summerhouse Formation is not present beneath the Kinnikinic in this thrust–faulted outcrop.
Challis Volcanic Group (Eocene).—
The volcanic rocks that surround the outcrops of sedimentary rocks in Leaton Gulch are part of the Challis Volcanics and include widespread tuffs and lava flows. McIntyre and Hobbs (1987) included most of the tuff in the tuffs of Pennal Gulch, a unit of “massive or crudely bedded, pumice–rich coarse tuff and pumice lapilli tuff”, and interbedded volcanic sandstone and mudstone. Small areas of tuff in the upper part of Leaton Gulch, and other red, reddish purple, and yellowish brown welded ashflow tuffs, were included in the tuff of Challis Creek, which was deposited on an erosion surface cut on the Pennal Gulch tuff.
Lavas range from andesite to basalt and mostly are gray or greenish gray, reddish–brown–weathering, fine–grained, and blocky. Olivine and pyroxene phenocrysts are present in some beds. Other volcanic rocks in the lower part of Leaton Gulch, west of the map area, include rhyodacitic flow rocks and poorly sorted pyroclastic rocks.
Landslides (Pleistocene and Holocene).—
Landslides are nearly as widespread as Challis Volcanic rocks in the upper part of Leaton Gulch, and, like the volcanic rocks, they surround and partly conceal the older sedimentary rocks. The landslides are composed of large blocks and smaller fragments from adjacent sedimentary rocks and lava, and are moving down Leaton Gulch on the underlying tuffs. Outcrops of the Swauger Formation are a major source of many of the landslides, and large blocks of fractured and brecciated Swauger Quartzite have broken from the west–dipping thrust plate and moved down Leaton Gulch. All of the landslides have hummocky surfaces that suggest recent movement, but they appear to be stable under present conditions.
In Leaton Gulch, the Proterozoic and lower Paleozoic rocks crop out as islands surrounded by Challis Volcanics, and they have been so broken by faults that one outcrop commonly cannot be related to another. The faults that we recognize differ from those of earlier maps. The most critical fault, in terms of understanding the Leaton Gulch breccias, is the planar surface at the base of the Swauger Formation. The fractured and brecci– ated quartzite of the Swauger Formation overlies the folded, younger Lawson Creek Formation across a thin sheared zone and planar surface that dips 5° to 15° to the southwest. The sheared zone and planar surface is interpreted to be a Late Cretaceous thrust fault, which carried the earlier brecciated and cemented Swauger Formation (and unconformably overlying Wilbert Formation) over the crumpled Lawson Creek Formation. Neither the thrust zone nor the Swauger breccia are folded like the underlying rocks of the Lawson Creek Formation, although there are some folds within the thrust plate (Carr and Link, 1999; Merrill, 2003). The thin, sheared thrust zone is exposed in a single, isolated outcrop in the northwest corner of the mapped area (locality B, Fig. 2).
The folded and thrust–faulted rocks are broken by a series of northeast–striking faults and by a group of north–striking faults that are the youngest faults in the area. Northeast–striking faults in the upper part of Leaton Gulch are part of a group of similarly striking faults and fracture zones that extend across the north end of the Lost River Range. The dips on the faults are unknown, and the displacements on the faults are uncertain. The straight traces suggest that they are very steep normal faults. They are about parallel to the major fault in Pennal Gulch, which places Challis Volcanics against the Lawson Creek and Summerhouse Formations (MacIntyre and Hobbs, 1987). Leaton Gulch is controlled by one of these faults, although the fault is largely concealed by landslides. Younger north–trending, nearly vertical strike–slip faults break the northeast–striking faults, the volcanic rocks, and some of the surficial deposits. They are common in the lower reaches of Leaton Gulch, but they have not been recognized in the upper part of the gulch (Ruppel, 1964).
The limited exposures of the Lawson Creek Formation beneath the thrust plate of brecciated and fractured Swauger Formation suggest only that the formation is broadly folded and locally tightly crumpled. In general, the formation dips 30° to 40° to the southeast (McIntyre and Hobbs, 1987; Carr and Link, 1999; Merrill, 2003). In the area west of the map area near Challis Hot Springs, however, both the Lawson Creek Formation and the younger Summerhouse Formation are tightly folded into a series of northwest–trending anticlines and synclines that partly are overturned to the east.
Summary and Conclusions
The stratigraphic relations of the sedimentary rocks exposed in Leaton Gulch show that the Leaton Gulch breccia is confined to a thrust plate of Swauger quartzite. The breccia contains only angular to rounded blocks and fragments of the quartzite, set in a matrix of sand and coarser fragments also derived from the quartzite. It is overlain with angular unconformity by the upper Neoproterozoic (?) and Lower Cambrian Wilbert Formation. The surface of the unconformity bevels both fragments and matrix, and shows that the breccia was tightly cemented and brittle when it was beveled. In the broadest sense, the Leaton Gulch breccia is post–Swauger and pre–Wilbert in age, and Neoproterozoic.
The relation of the upper Mesoproterozoic Lawson Creek Formation to the Leaton Gulch breccia is known only from the outcrops in Leaton Gulch. Much of the formation was stripped from east–central Idaho as a result of Neoproterozoic deformation and erosion (Hobbs, 1980; Ruppel and Lopez, 1988), but the impure, feldspathic and heterogeneous rocks of the Lawson Creek Formation suggest unstable depositional conditions that were different from those indicated by the generally cleaner and more homogeneous older Mesoproterozoic formations (Hobbs, 1980). They probably reflect the beginning of deformation that ended the long period of conformable Mesoproterozoic sedimentation. The deformation and associated deep erosion of folded Mesoproterozoic rocks continued into the early or middle Neoproterozoic. The Leaton Gulch breccia, and later the poorly sorted rocks of the upper middle and upper Neoproterozoic Wilbert Formation and red mudrocks of the Lower Cambrian formation of Tyler Peak, were deposited across this deeply eroded terrain (McCandless 1982; Hobbs, 1980; Ruppel et al., 1975; Ruppel and Lopez, 1988).
Kellogg et al. (1999) and Kellogg et al. (2003) have reported 40Ar/39Ar and U/Pb data that suggest ages of about 850–900 Ma on biotite, muscovite, and zircon in metamorphic rocks that reportedly contain shatter cones in the Medicine Lodge Greek area of southwestern Montana. Although it is difficult to relate shatter cones in basement crystalline metamorphic rocks in southwest Montana to a meteorite impact nearly 100 miles (160 km) away, the coincidence of these ages with the geologic evidence (which suggests an Early or Middle Neoproterozoic age for the Leaton Gulch breccia) supports the conclusion that the breccia might be impact–related. We believe that it probably is, even though we have not found, and do not expect to find, any other more positive evidence. The breccia is unique, and there is no tectonic event likely to have formed so unusual a breccia that is known to have occurred in this region at this time, other than the Beaverhead meteorite impact. If it was formed by the meteorite impact, it may be landslide debris from the western crater rim or debris from one of the outermost ring faults defining the crater. The rocks do not contain unequivocal evidence of significant shock deformation that would indicate nearness to the central uplift.
Finally, considering the probable size of the central uplift and of the crater (Hargraves et al., 1994; Carr and Link, 1999), the impact might be expected to have had a major and lasting effect on the regional geologic history. It is possible that at least some of the uplift and erosion in the early and middle Neoproterozoic, attributed to the uplift of the Lemhi Arch (Sloss, 1954; Ruppel, 1986) and coincident in time with the inferred age of the impact, could have been caused by the impact instead (Hargraves et al., 1994; McCafferty, written communication, 1994; McCafferty, 1995)
We are indebted to many colleagues who have visited Leaton Gulch and who have shared their observations with us. The late S. Warren Hobbs, who with David McIntyre originally mapped the Leaton Gulch breccias and noted their unusual textures, visited the area with us in 1992, and we profited enormously from his insights. Donald P. Elston of the U.S. Geological Survey had visited the area earlier in the course of his paleomagnetic studies of Proterozoic sedimentary rocks in the Northern Rocky Mountains and commented on the wildly disoriented fragments in the breccias and on the paleomagnetic signatures of both the Swauger Formation and the Lawson Creek Formation. Anne E. McCafferty of the U.S. Geological Survey freely shared her information and conclusions on gravity and magnetic anomalies in the Challis, Idaho, area, and her conclusions on the likelihood of these representing the original impact site of the Beaverhead meteorite. Similarly, Karen Lund of the U.S. Geological Survey briefly visited the area and provided help on some of the stratigraphic problems. Paul Link and Jennifer Carr Merrill, of Idaho State University, in Pocatello, Idaho, have been continuing sources of information, based on the studies that led to their 1999 report (Carr and Link, 1999) and on Jennifer Carr Merrill’s continuing studies and petrographic work (Merrill, 2003). Technical reviews by Paul Link, Karen Lund, and an anonymous reviewer helped us strengthen the manuscript and improve its organization; we are indebted to them. Diana Boyack at Idaho State University Geosciences produced the figures.
The late David Roddy of the U.S. Geological Survey was enthusiastic about the extension of our work to Leaton Gulch and strongly supported our request for a research grant from the Barringer Crater Company. That generous grant enabled us to continue field work in both 1999 and 2000.
Finally, and sadly, we, the junior authors of this report, must note that Professor Robert B. Hargraves, the senior author, died during the preparation of the report. The study of the Beaverhead meteorite impact was almost entirely guided by him, from 1990, when he first identified the shatter cones in the Medicine Lodge area in southwest Montana, until only a few days before his death in March, 2003. We have completed the report in his absence, and we believe that it faithfully represents his views and conclusions on both the Beaverhead meteorite impact and on the possible relation of the breccias in Leaton Gulch to that impact. But we have missed his enthusiasm and wise counsel, and his broad knowledge that guided us along the way.
Figures & Tables
Proterozoic Geology of Western North America and Siberia
This volume is a compendium of research on the Belt Supergroup. It is an outgrowth of Belt Symposium IV, held in Salmon, Idaho, in July, 2003, in conjunction with the Tobacco Root Geological Society annual field conference. Because of the geographic extent and great thickness of the Belt Supergroup, years of work have been required before conclusions are “bona fide”. The Mesoproterozoic Belt Supergroup of western Montana and adjacent areas is geologically and economically important, but it has been frustratingly hard to understand. The previous Belt Symposium volumes offer an historical view of the progress of the science of geology in the western United States. The advent of U-Pb geochronology, especially using the ion microprobe (SHRIMP) and laser-ablation ICPMS, has injected geochronometric reality into long-standing arguments about Belt stratigraphy. Several papers in this volume utilize these new tools to provide constraints on age and correlation of Belt strata (Chamberlain et al., Lewis et al., Link et al., and Doherty et al.)