Missoula Group Research Articles

Recently Identified Mesoproterozoic Strata in South‐Central Idaho Document Late‐Stage Rifting of the Nuna Supercontinent in Western Laurentia

AbstractSedimentary basins are valuable archives of tectonic processes involved in continental rifting. The northern Rocky Mountains preserve the Belt Supergroup, one of the most complete records of Mesoproterozoic strata on Earth; however, debate remains about its tectonic origin. We investigated a recently identified package of Mesoproterozoic strata at Leaton Gulch near Challis, Idaho, using a combination of traditional and newer sedimentological tools. Results suggest that the Leaton Gulch stratigraphic section was deposited in a fluvial setting ca. 1,380–1,317 Ma, spanning the poorly documented interval between late Belt Supergroup deposition at ∼1,370 Ma and recently characterized Deer Trail Group strata that are less than 1,300 Ma. Detrital zircon age distributions from Leaton Gulch demonstrate a similar provenance signature to Missoula Group rocks of the upper Belt Supergroup; however, Leaton Gulch strata are up to ∼70 Ma younger than most prior age constraints on Belt Supergroup rocks. Regional metabentonites (interpreted as metamorphosed reworked tuffs) found within Leaton Gulch and Missoula Group strata show dominantly radiogenic εHf(t), with a range of −8 to +15, interpreted as a mix of primary mantle and remelted metasedimentary sources. Zircon trace element data of the metabentonite from Leaton Gulch suggest a 1,450–1,300 Ma geochemically consistent and moderate–high silica melt source. Collectively, the strata of Leaton Gulch record basin sedimentation during a critical window of Mesoproterozoic time. We speculate that sedimentation during late‐stage Belt Supergroup deposition thickened and stepped westward, abandoning the main Belt basin, culminating with breakup of the Nuna Supercontinent.

Mesoproterozoic basins (Yukon, Canada) in the evolution of supercontinent Columbia

Geochemical and geochronological data from the Pinguicula Group and unit PR1 of the lower Fifteenmile Group (Yukon, Canada) provide information on sediment provenance and timing of break-up of supercontinent Columbia and seaway development on Laurentia’s northwestern margin. The older unit PR1, in the Coal Creek inlier, has a near-unimodal detrital zircon population with an age of 1499 ± 3 Ma. The Pinguicula Group detrital zircon data, in the Wernecke and Hart River inliers, display a polymodal detrital zircon population with a maximum age of <1322 ± 23 Ma. Using detrital zircon signatures, Sm–Nd isotopic data, and C-isotopic signatures, lithostratigraphic correlations between the Pinguicula Group in the Wernecke and Hart River inliers are confirmed, whereas the Pinguicula Group and unit PR1 are no longer considered correlative. The zircon population in unit PR1 requires a proximal source, but sources of this age are not known in western Laurentia. Based on detrital zircon and Sm–Nd data, sediment in unit PR1 was derived from the Mt. Isa inlier in northeastern Australia sometime after 1460 Ma. Unit PR1 correlates with older Mesoproterozoic successions, including the Belt-Purcell, that were deposited during break-up of supercontinent Columbia, and contain sediment from Australia and the Mawson continent. Mesoproterozoic successions deposited after 1.45 Ga, including the Missoula Group, lack North American Magmatic Gap (NAMG)-aged zircon and instead record sediment provenance from southern Laurentia, as north Australia and the Mawson continent rifted from Laurentia’s western margin. The Pinguicula Group has few NAMG-aged grains that were probably recycled from older Mesoproterozoic basins.

Paleogeographic implications of non–North American sediment in the Mesoproterozoic upper Belt Supergroup and Lemhi Group, Idaho and Montana, USA

A non-North American provenance for the lower Belt Supergroup of North America has been used to support various pre-Rodinian paleogeographic reconstructions. Unlike the lower Belt Supergroup, most upper Belt Supergroup provenance studies have inferred Laurentian sediment sources. We test this hypothesis by analyzing U-Pb and Lu-Hf isotopes on detrital zircons, and whole-rock Nd isotopes from the Missoula (upper Belt Supergroup) and Lemhi Groups, and comparing to possible Laurentian sources. Detrital zircons from 11 sandstones analyzed show dominant ages between 1680 and 1820 Ma. These zircons are predominantly magmatic in paragenesis. Belt Supergroup–aged (1400–1470 Ma) and 2400–2700 Ma populations represent minor components. Lu-Hf isotopic analyses for 1675–1780 Ma Missoula Group and Lemhi Group detrital zircons range from eHf(i) +9 to –12 and +8 and –7, respectively. Belt Supergroup–aged grains from the Bonner Formation, Missoula Group, have eHf(i) values between +5 and –9, exceeding coeval ranges from the Mojave and Yavapai terranes [eHf(i) between +5 and 0]. Whole-rock Nd isotopes from Lemhi Group argillites yield a range in eNd(1400) between +1.1 and –5.9. Immature feldspathic sediment, nearly unimodal detrital zircon spectra, and dissimilar Belt Supergroup–aged zircon Hf signatures suggest that distal portions of the Yavapai and Mojave terranes intruded by A-type magmas were not the source for the Missoula and Lemhi Groups. Instead, a slightly modified Mesoproterozoic proto-SWEAT (southwestern United States and East Antarctica) model can best account for the sedimentologic and isotopic characteristics of the Missoula and Lemhi Groups. An alternative model with a source from southeastern Siberia and the Okhotsk Massif is less preferred.

Provenance of the Mesoproterozoic (1.45 Ga) Belt basin (western North America): Another piece in the pre-Rodinia paleogeographic puzzle

The Mesoproterozoic (1.47-1.40 Ga) Belt basin in western North America contains a thick succession of siliciclastic strata dominated by sediment derived from source areas that lay to the west and southwest of the present basin. The age and identity of this source area is important for reconstruction of the Mesoproterozoic history of western North America. Over 400 new SHRIMP U-Pb ages for detrital zircons, supplemented by Ar-Ar ages of detrital muscovite from sandstones within the Belt basin, document distinctive provenances of the basin fill; the predominant sediment influx came from a western craton, whereas subordinate input came from Laurentian sources to the east and south of the basin. During deposition of the lower two-thirds of the Belt basin, the western source was composed predominantly of Proterozoic rocks with 1920-1460 Ma ages; abundant grains have 1610-1500 Ma ages. These latter ages are unrepresentative of Laurentian sources, falling within the North American magmatic gap (1610-1490 Ma), but are widespread in central and eastern Australia and Baltica, the former being the most likely candidate for a former fluvial connection to western North America. The presence of detrital-zircon grains (1480-1440 Ma) with ages contemporary with sedimentation in the Belt basin suggests that the western source area hosted active magmatism during Belt sedimentation. The Missoula Group in the upper Belt records the sedimentologic effects of basin reorganization, which introduced sediment from a new southwestern source that, in contrast to earlier Belt history, resembles crust of the southwestern United States and lacks grains within the North American magmatic gap. Evidence for extension is widespread in the Belt basin in the form of mafic volcanic rocks, rapid rates of sedimentation, and intrabasinal syndepositional faults. The predominant contribution of detrital material from the western source area throughout Belt deposition, in concert with the large-scale facies patterns and evidence for rapid rates of subsidence, is taken to suggest that the Belt basin represents an extensional basin with a tectonically active western side. Although a wholly extensional environment has been suggested by previous workers, we suggest that an alternative model, wherein the Belt basin developed within an extensional domain along a collisional/convergent plate margin, analogous to Tethyan sedimentary basins such as the Black and Caspian Seas, is equally viable and paints a different picture of western North America at ca. 1450 Ma.

Distribution, characteristics, and genesis of high fineness gold placers, Ninemile Valley, central-western Montana

The high gold fineness, geographic distribution, and general lack of bedrock lode sources make the Ninemile district of west-central Montana one of the most interesting gold placer localities in Montana. Analysis of the distribution of the gold-bearing gravels and the petrography and geochemistry of the gold grains leads to the conclusions that the gold is detrital, of very high original purity, and derived from lode deposits hosted by Proterozoic-age Belt Supergroup rocks located within the Ninemile Valley. The limited known lode occurrences, type, and metamorphic grade of bedrock in the Ninemile Valley, and gold grain characteristics are compatible with slate hosted-type, gold-bearing quartz veins.The lode deposits, based upon relationships noted in the Ninemile Valley and elsewhere in the region, were probably emplaced during Laramide-age felsic plutonism. The gold, however, may ultimately have been derived from the Prichard Formation, the basal unit of the Belt Supergroup, which underlies the entire area and hosts gold-bearing quartz veins elsewhere in the Belt basin. The stratigraphic interval encompassing the middle Belt carbonate through the lower part of the Missoula Group may also have been the source for some of the vein gold. We speculate that the distribution of placer deposits resulted from dextral offset of lode sources by the northwest-striking Ninemile fault during the Tertiary. This fault may have displaced the northeast half of the formerly circular- to ellipically shaped lode district up to 15 km to the southeast.

UPb zircon and SmNd geochronology of the Mesoproterozoic of North Queensland: implications for a Rodinian connection with the Belt supergroup of North America

UPb zircon SHRIMP age dates and SmNd isotopic compositions are reported for a number of samples of Proterozoic metasedimentary and meta-igneous rocks from a range of geological provinces in North Queensland. The new data focus on the connection between east Australia and Laurentia (North America) in the Rodinian Supercontinent. One of the keys to the Rodinian reconstruction has been the nature and location of the source area of the Mesoproterozoic Belt Supergroup (North America). Most workers consider local derivation unlikely and Australia as the candidate. The source area requirements that are not satisfied by local derivation are: zircon populations of ca. 1590 Ma (in the Revett Formation of Lower Belt Supergroup) and ca. 1070–1200 Ma Grenvillian age [in the Deer Trail Group equivalent(?) to the top of Belt Supergroup]; and crustal residence ages ( T DM) of dominantly Palaeoproterozoic crust (1.97-2.14 Ga). Our study has shown that North Queensland contains appropriate zircon, and crustal residence times to match the ‘missing’ source area for the Belt Supergroup. The ca. 1590 Ma component of the Revett Formation is common in the Yambo Subprovince, and the 1690 Ma zircon in the Forsayth Subprovince is suitable for the Missoula Group. Both the Yambo and Forsayth Subprovinces contain a range of detrital zircons up to 2700 Ma (e.g. 1776, 1820, 1886, 2100, 2250, 2305, 2640, 2660 and 2700 Ma). For the Deer Trail Group, Grenvillian zircons occur in the Cape River Province (1238, 1145 and 1105 Ma) and in the Iron Range Province (∼1200 Ma). T DM young to the north and northeast, with the Savannah Province and Yambo Subprovince yielding ages between about 2.1 and 2.44 Ga.

Provenance and U-Pb geochronology of the Mesoproterozoic Belt Supergroup (northwestern United States): implications for age of deposition and pre-Panthalassa plate reconstructions

The Mesoproterozoic Belt-Purcell Supergroup is a thick succession of siliciclastic and carbonate sedimentary rocks in the northwestern United States and adjacent Canada. Recent stratigraphic studies provide strong evidence for an enclosed basin setting with a tectonically active western margin. The age of the western (present coordinates) landmass is unknown, but has been inferred to be different from North American basement that borders the Belt basin on the basis of whole rock Sm-Nd studies. We report U-Pb isotopic analyses of individual detrital zircon, monazite, and xenotime grains separated from westerly derived clastic units in the Belt Supergroup as well as Sm-Nd isotopic data for the same grains where appropriate. These data provide new constraints on the age and isotopic character of the western landmass. Most U-Pb data are concordant or slightly discordant; exceptions are noted below. The Revett Formation (Ravalli Group, lower Belt Supergroup) contains grains with concordant ages of 1590–1600 Ma and a grain with a discordant 207Pb/ 206Pb age of 1780 Ma: the ca. 1600 Ma age is very unusual in North America and effectively dates “Belt Island”, long postulated to be the Revett source in what are now terranes of southeast Washington and northeast Oregon that were accreted in the Mesozoic. Detrital zircons from the Missoula Group (Bonner and Mount Shields formations; upper Belt Supergroup) form a distinct group at 1670–1859 Ma with two ca. 2.6 Ga grains. U-Pb ages from monazite range from 1642 to 1786 Ma, and together with Nd isotopic data ( T DM model ages of 2.05–3.36 Ga) suggest variable interaction with previously differentiated crust. The Buffalo Hump Formation in northeast Washington may correlate with the Bonner Formation or may lie unconformably on the Belt Supergroup; it contains detrital zircons of ca. 1840 Ma and, surprisingly young, 1070–1244 Ma (several grains). Monazites have U-Pb ages of 1753–1774 Ma and Nd signatures similar to the Bonner Formation data. Several major conclusions can be drawn from these data. The ages of 1590–1600 Ma have no known possible source within western North America (although 1576 Ma augen gneiss is present in the Priest River Complex northwest of the Belt basin), and the 1642–1786 Ma ages are uncommon to rare in Precambrian rocks to the immediate south and east; together these data support previous Sm-Nd whole rock and stratigraphic conclusions of a dominantly western source for the Belt basin. The ages in the Buffalo Hump Formation can be interpreted in two ways. Either the Buffalo Hump correlates with the Bonner Formation and therefore the top of the Belt Supergroup is substantially younger (< 1070 Ma) than previously inferred (1250 Ma) and may overlap with Grenville-age tectonic activity. Alternatively, the Buffalo Hump Formation may represent a new unit that is younger than the Belt. Irrespective of this issue, these data help to constrain proposed pre-Paleozoic plate reconstructions; from the 1070–1244, 1590–1600 and 1642–1786 Ma source ages and related Nd data, we suggest that the basement blocks of south-central Australia (Gawler, Musgrave, Willyama, Arunta and possibly Mount Isa) were joined to the western side of Laurentia, adjacent to the Belt basin, prior to Neoproterozoic formation of the Panthalassa Ocean.

Tectonics and Sedimentation of Middle Proterozoic Belt Basin, and Their Influence on Cretaceous Compression and Tertiary Extension in Western Montana and Northern Idaho: ABSTRACT

The Belt Supergroup was deposited in an intracratonic basin, occupied during much of its history by alluvial aprons that sloped down to a landlocked sea. Belt rocks are here analyzed in terms of 13 types, descriptive taxa in which the metamorphic overprint is filtered out and the original sedimentary aspects are emphasized. The lower Belt records maximum transgression of the Belt sea, during which turbidite sand and pelagic mud were deposited in the central part of the basin, carbonate mud accumulated on its eastern margin, and coarse conglomerate accumulated along its fault-bounded southern side. The Ravalli Group records extensive alluvial apron/mudflat progradation from the south and west. The middle Belt carbonate, representing a second large transgressive eriod, is characterized by terrigenous-carbonate cycles in the eastern part of the basin and by turbidite sand and mud, derived from the west, in the deeper, locally slumped, western part of the basin. The Missoula Group represents a series of northwest-facing prograded alluvial aprons alternating with transgressive mudflat and shallow-water deposits. The Garnet Range Formation, near the top of the Missoula Group, represents incursion of open marine waters into the Belt basin. The Belt basin subsided as a group of at least four large crustal blocks, separated by three nearly east-west fault lines and a northwest-trending fault line. Differential subsidence of the blocks is recorded in abrupt thickness changes and soft sediment deformation along the fault lines. Cretaceous thrusting formed western and eastern thrust belts which are continuous across the blocks, but which are segmented and broken along the east-west lines. Tertiary extensional dislocations along the lines. End_of_Article - Last_Page 871------------

Geology of North End of East Pioneer Mountains, Beaverhead County, Montana: ABSTRACT

The north end of the Pioneer Mountains is underlain by the following rocks: lower Proterozoic gneiss and amphibolite; middle proterozoic clastic rocks of the Missoula Group (Mount Shields Formation?); basal Cambrian clastic unit; Cambrian-Devonian-Carboniferous shelf sequence; Permian Phosphoria Formation; and Lower Triassic Dinwoody Formation. Jurassic rocks are missing. The Lower Cretaceous Kootenai Formation is of lagoonal to fluviatile facies, overlain by a thick (^approx 2 km, 1 mi) section of fluviatile Colorado Group. An upper member of the Colorado Group has yielded a Campanian-to-Maestrichtian pollen flora. The sedimentary rocks are cut by calc-alkalic plutonic rocks (80 to 65 m.y.B.P.), the oldest of which are synchronous with upper Colorado sedimentation. The y ungest pre-Quaternary rocks are Eocene and Oligocene calc-alkalic lavas and Oligocene pumiceous tuff. The Missoula Group is entirely in thrust sheets that postdate the Colorado, so the thrusting is no older than Campanian, but the thrusts are cut by 72 to 74 m.y.B.P. plutons. The Johnson thrust of Fraser and Waldrop, on the 1972 U.S. Geological Survey Geologic Quadrangle Map 988, is part of this thrust system. A klippe of Missoula Group on Morrison Hill is an erosional remnant, but most of the overthrust rocks are west of the Wise River valley and probably overlie Phanerozoic strata. In addition to the thrust sheets, two families of high-angle faults dominate. One family trends west-northwest and displacements along these faults can be measured in kilometers. The Johnson thrust is interpreted to have been displaced by a sinistral fault of this system along the straight valley of Big Hole River between Seymour Creek and Dewey. The eastward projection of the mountain front at Maiden Rock, just south of Divide, resulted from block displacement along two strands of this fault. This particular fault is of major significance because it marks an abrupt jump in the initial strontium ratios of intrusive rocks, from values typical of the Boulder batholith (0.706 to 0.709) to those of the Pioneer batholith (0.711 to 0.716), indicating that different crustal blocks were juxta osed. The second family of high-angle faults trends north-northeast. Wise River valley is interpreted to be a graben in this system. One important fault in the system is the Fourth of July fault, with downdrop (to west) of several kilometers; the fault may be continuous with the Comet Mountain fault. The west-northwest fault system began at least before the Eocene lava flows, but complex field relations between the two high-angle fault systems indicate their growth must have overlapped in age, possibly through the late Tertiary. End_of_Article - Last_Page 1362------------

The Middle Proterozoic Purcell Supergroup in the Southwestern Rocky and Southeastern Purcell Mountains, British Columbia and the Initiation of the Cordilleran Miogeocline, Southern Canada and Adjacent United States

ABSTRACT Purcell Supergroup strata exposed in the southeastern Purcell and southwestern Rocky Mountains comprise a turbidite sequence (Aldridge Formation) gradationally overlain by shallow-water deposits. The lower member of the Kitchener Formation represents the transition from intertidal and shallow-subtidal clastic sedimentation of the Creston Formation to the shallow-subtidal shelf facies of the upper member. Varicoloured siltite and argillite of the Van Creek Formation mark the return to shallow-water clastic sedimentation. They are abruptly overlain by volcanic rocks and shallow-water clastic sediments of the Nicol Creek Formation. Stromatolitic dolomite and quartzite of the Sheppard Formation comformably to disconformably overlie the Nicol Creek Formation. The siltite and argillite comprising the Gateway Formation are interpreted as lagoonal deposits. The overlying subaerial quartzites of the Phillips Formation form a northward and westward thinning marker unit. The Roosville Formation, which forms the top of the Purcell succession, has been removed by pre-Devonian erosion over much of the area. The shallow-water deposits comprising the Creston through Phillips Formations can be subdivided into three composite stratigraphic units that are recognized throughout the northern Belt-Purcell basin. Thickness variations in the lower two units outline a north-trending basin margin that is deflected more than 200 km westward near 49°N latitude. The rectilinear shape can be ascribed to deeply rooted block faults developed during continental rifting. This shape governed the later development of a major structural re-entrant in the Rocky Mountain thrust and fold belt. Thickness variations in the upper unit record the evolution of epeirogenic structures that apparently controlled the distribution of volcanic rocks within the Belt-Purcell basin. A re-evaluation of middle and upper Belt-Purcell correlations indicates that the hypothesis of a 'dome' supposed to have formed during deposition of the lower Missoula Group was based on incorrect correlation.

Paleomagnetic poles and polarity zonation from the Middle Proterozoic Belt Supergroup, Montana and Idaho

Twelve paleomagnetic poles and a preliminary polarity zonation are reported from geologically mapped, stratigraphically controlled sections of the middle Proterozoic Belt Supergroup in western Montana and northern Idaho. Although gaps exist in the stratigraphic coverage, the lower Belt, Ravalli Group, and middle Belt carbonate appear to be largely if not entirely of normal polarity. A switch to reversed polarity takes place near the base of the overlying Missoula Group (base of Snowslip Formation), and two comparatively narrow intervals of mixed polarity containing at least 18 reversals are found in the lower and middle Snowslip. Seven reversals, mostly widely spaced in stratigraphic position, have been found in overlying strata of the Missoula Group. Poles from strata of the Ravalli Group through Missoula Group are well defined and tightly clustered. They plot in the south‐central Pacific Ocean and display only a small (∼20°) southeast to northwest shift in pole path. Poles from Belt strata in the eastern basin plot systematically east of poles from the more westerly collection sites. A counterclockwise shift in declination of 7°–10° is seen in strata from the easterly sites, which has given rise to a curving pole path that closely parallels the path for the westerly sites. This easterly path appears explainable as a consequence of a counterclockwise structural rotation of eastern Belt strata relative to western and central Belt strata produced during west to east thrust faulting of late Mesozoic and early Cenozoic age. An additional counterclockwise rotation of the same magnitude exists between the northern and southern parts of the Montana disturbed belt of the eastern Belt basin. Moreover, anomalously steep inclinations, not yet explained, have been reported from some of the strata sampled at the north end of the disturbed belt. The steep inclinations result in poles that plot distinctly east of poles reported from this study. The Belt pole path lies in a part of the North American apparent polar wander path that has been assigned an age of about 1500–1200 m.y. This is in broad agreement with isotopic ages reported for strata of the lower Belt to the middle Ravalli Group. The Belt path trends toward, and its northwest end overlaps, the beginning of the pole path for the Grand Canyon Supergroup, which formed in the interval from ≲1300 to ∼830 m.y. ago. Because polarity zonations differ between strata of the upper Belt and lower Grand Canyon Supergroups, Belt strata would appear, on paleomagnetic grounds, to be largely or entirely older than strata of the Grand Canyon Supergroup. However, isotopic ages for the Missoula Group (1100 m.y. and younger) presently require an alternative interpretation, i.e., that the Belt terrane did not participate in inferred plate motion which gave rise to a large north‐trending loop in the interval ∼1200 to ∼1100 m.y. ago and to later motion seen in the Grand Canyon path. Additional dating is needed for the Purcell Lava of the Missoula Group to support or modify the 1100 m.y. age determined by the K‐Ar method.

Stromatolites of the lower Missoula Group (Middle Proterozoic), Belt Supergroup, Glacier National Park, Montana

Abstract The mode of formation and environmental setting of stromatolites from the lower Missoula Group (ca. 1.1·109 years old) in Glacier National Park, Montana, have been determined. The stromatolite-bearing interval in the lower Missoula Group was deposited in a shallow, intermittently exposed setting of very low relief, the stromatolites forming during periods of submergence. In situ carbonate precipitation was the dominant process involved in the formation of encrusting stromatolitic laminae. This precipitate was deposited within, and probably beneath, algal mats, most likely as a result of the photosynthetic removal of carbon dioxide by the mat-building microscopic algae. Calcite also was precipitated in several types of open-space structures occurring within these stromatolites. Other laminae were produced by the organic stabilization of detrital particles; by the solely physical accumulation of terrigenous material; and probably, by bacterially induced precipitation of iron sulfide which was later oxidized to form hematite layers. Three forms of filamentous microfossils, two of which appear to be oscillatoriacean cyanophytes and the third of which is probably either a cyanophyte or filamentous bacterium, have been detected in these structures. In addition, hematitic pillar-shaped microstructures, interpreted to have been produced by filamentous bacteria, are abundant locally. In gross morphology, the lower Missoula Group stromatolites are simple, unbranched, domal structures ranging from several millimeters to several decimeters in both height and diameter. Physical conditions played a major role in determining the macrostructure of these stromatolites. Of particular importance were the shape of the positive sediment-surface irregularities upon which the stromatolites initially formed, the rate of sedimentation between stromatolite colonies, and the deposition of layers of terrigenous material on stromatolite growth surfaces. The effect of biological factors on stromatolite structure is clearly seen in those portions of stromatolites relatively free of terrigenous material; biological activity was apparently greatest on positive irregularities of the growth surface, resulting in preferential enhancement of such irregularities and development of second- and higher-order hemispheroidal structures.

Bonner Formation (Precambrian Belt of Montana) as Braided-Stream Sequence: ABSTRACT

The Bonner Formation of the Missoula Group probably represents a braided stream sequence that prograded over sea-margin mud flats from southwestern Montana, northward across western Montana. In the Pioneer Mountains close to its source, the Bonner is both crossbedded, coarse grained, feldspathic sandstone (quartzite), reflecting subaqueous dunes in low flow regime, and horizontally laminated sand-supported conglomerate, representing deposition by transitional flow. Northwestward near Missoula, the Bonner is chiefly directionally crossbedded, coarse feldspathic sandstone (quartzite), representing subaqueous dunes. Farther northwestward, near Superior, the Bonner is mostly horizontally laminated, fine to medium feldspathic sandstone with common directionally oriented, rippl -drift cross laminae and sparse crossbeds. Here the Bonner was deposited by transition flow so shallow that upon slowing to lower regime, the bed forms altered to ripples, not dunes. Here, the Bonner also contains oscillation-rippled, mud-cracked argillite and a few stromatolite beds, indicating mudflat deposition. By integrating these areas, one can interpret the Bonner to have been a giant pediplain or fan traversed by braided streams that were large enough near their sources to carry small cobbles in transition flow. Near Missoula, the streams were carrying mostly coarse sand in a lower flow regime and probably were dispersing across the pediplain. By the time they reached the vicinity of Superior, the braided streams were very shallow and were carrying medium to mostly fine sand, and were interdigitating with extensive mud flats bordering a sea. The wedge of Bonner may reflect renewed movement in the late Precambrian along the Willow Creek fault zone. End_of_Article - Last_Page 813------------

Precambrian Belt Basin of Northwestern United States: Its Geometry, Sedimentation, and Copper Occurrences

Geometry and provenances of the Belt basin at various stages of its 600-m.y. time span (between about 1,450 to 850 m.y. ago) are decipherable for the middle and upper parts of the Ravalli Group, the Helena-Wallace Formations, and the lower part of the Missoula Group. The Revett and overlying St. Regis Formations (middle and upper parts of the Ravalli Group) had a cratonic source in the south and southwest and were deposited in troughs that reflect the west-northwest trend of the Osburn fault zone (or the Lewis and Clark line whose west-central part includes the Osburn fault zone). The Spokane Formation, correlative of St. Regis, had a source in the Canadian Shield and was deposited in a north-to northwest-trending trough in northwestern Montana, west of, but approximately parallel to, the north-northwest trend of the much younger Montana disturbed belt. The Helena and Wallace Formations were deposited in a broad simple northwest-trending basin between the previous troughs, which received clastic sediments from both the east and southwest, and in which extensive carbonate deposits formed on the eastern shelf. Tectonic adjustment in early Missoula time resulted in a long low dome in the Idaho Panhandle area and in the rejuvenation of the north-northwest-trending trough in the east. Clastic sediments containing abundant hematite were deposited on Helena-Wallace carbonate rocks, and Purcell Lava was poured out in the northeast. Anomalously high amounts of copper (100 or more ppm) are scattered throughout thousands of square miles of Belt terrane. The copper occurs in almost all formations, but it is most common in green strata. This mode of occurrence suggests a syngenetic or diagenetic origin. Stratabound copper ores, however, are known only as disseminations, discrete blebs, and veinlets in white quartzites and siltites of the Revett Formation in a “copper sulfide belt” along the northwestern Montana border. This belt is perpendicular to the Revett trough but parallel to the dome formed in early Missoula time, suggesting post-Revett epigenetic reconcentration of copper. Geochronologic data as interpreted by several authors are in apparent conflict with geologic history of the basin as read from the sedimentation record. Many of the conflicts can be resolved by reinterpretation of existing geochronologic and geologic data. It is obvious, however, that the dating of many events, including those that may have caused migration of copper, is in need of refinement.

Cubic casts as indicators of 'top and bottom' in the Shields Formation (Precambrian Belt Supergroup)

Cubic salt casts are commonly observed in the Missoula Group (Precambrian Belt Supergroup) in northwestern Montana. This structure has been used as an environmental indicator and as a criterion of stratigraphic correlation. Salt casts are especially abundant in the Shields Formation, where field observations in the southern Lewis and Clark Range indicate that they occur exclusively on the bottoms of strata. Salt casts are reliable indicators of 9top and bottom9 in this area and may be useful for that purpose in other areas of Shields exposure.

Stratigraphy and Correlation of the Precambrian Belt Supergroup of the Southern Lewis and Clark Range, Montana

Several well-exposed and little-deformed Belt Supergroup sections have been studied in the southern Lewis and Clark Range. In the area studied, the Belt thins eastward or northeastward due both to primary sedimentation and to pre-Middle Cambrian erosion. These rocks can now be more precisely correlated with the well-known sections near Bonner, Helena, and Glacier National Park. In the western part of the area, the Missoula Group is thickest and lithologically intermediate between the sections at Bonner and Marias Pass. Formation names from these two sections are applied in the southern Lewis and Clark Range. The thin Belt sequence in the eastern part of the area includes the lower part of the Missoula Group and older formations that may be traced southward into the Helena, Empire, and Spokane Formations of the Helena area. Consequently, the Helena Dolomite of the Helena area, the Siyeh Formation of the Marias Pass area, and the “Newland Limestone” of the Bonner area are probably lateral equivalents.

Correlation of and Facies Changes in the Carbonaceous, Calcareous, and Dolomitic Formations of the Precambrian Belt-Purcell Supergroup

Two thick carbonate-bearing sequences are exposed in most eastern Belt-Purcell sections, whereas only one is exposed in western sections. Stratigraphic evidence suggests that the lower eastern sequence (Fort Steele, Altyn and Waterton, and Newland Formations) intertongues with or grades westward into carbonate-poor but carbonaceous strata (Aldridge and Prichard Formations). The upper sequence in the east (Siyeh and Helena Formations) is correlated with the western sequence (Kitchener and parts of Kitchener-Siyeh of British Columbia, Wallace of northern Idaho and western Montana, and “Newland” of the Philipsburg and Missoula areas, Montana) and forms the only laterally continuous carbonate-bearing sequence in the entire Belt-Purcell Supergroup. Based on these correlations, a fourfold division of the Precambrian Belt-Purcell Supergroup is recognized. In the United States these divisions are referred to as the Pre-Ravalli sequence and the Ravalli, Piegan, and Missoula Groups. Formally the classification is identical to previous classifications, but the informal term “Pre-Ravalli sequence” emphasizes the importance of the lowest Belt-Purcell subdivision. These four subdivisions are recognized throughout most of the Belt-Purcell terrain, dividing the succession into two cycles. The lower cycle comprises the Pre-Ravalli sequence, whose finer-grained fraction is characterized by black to gray, carbonaceous and/or carbonate-bearing, pyritic and commonly very evenly and finely laminated (“varved”) strata and the Ravalli Group, whose finer-grained fraction is typified by reddish to purplish (hematitic) or greenish rocks that contain abundant mud cracks and intraformational mud-chip conglomerates and are less evenly and less finely laminated. The upper cycle is composed of the Piegan and Missoula Groups, which are lithologically similar to the Pre-Ravalli sequence and the Ravalli Group, respectively. A third carbonate-bearing sequence, the Shepard Formation, is less extensive and generally much thinner than the Piegan Group. Its regional variations and correlatives are little known. Stratigraphic evidence suggests that the Shepard may be a tongue of the uppermost Piegan Group of northern Idaho and westernmost Montana (and its equivalent in the Purcell Supergroup of adjacent British Columbia) that interdigitates with the lowermost Missoula Group (and its Purcell equivalent) throughout much of the Belt-Purcell terrain. Facies belts within the Pre-Ravalli sequence (and equivalent Purcell strata) trend northwest, with shallower water strata toward the northeast. Facies belts within the Piegan Group (and equivalent Purcell strata) have a similar trend but a symmetrical disposition owing to a deeper water, calcite-bearing facies in the center of the depositional trough, flanked on both sides by shallower water, dolomite-bearing strata. The principal differences between Pre-Ravalli, Piegan, and Shepard deposition are attributed to variations in the average water depth in the Belt-Purcell basin. Although present Belt-Purcell Stratigraphic classification is confusing, only one important proposals is made for its revision—the Belt Supergroup is defined to exclude strata equivalent to the Winder-mere Group of Canada. Thus the Belt and Purcell Supergroups are to be regarded as precise stratigraphic equivalents.

Correlations and Problems in Belt Series Stratigraphy, Northern Idaho and Western Montana

A continuous strip of geologic maps has recently been completed along the Idaho-Montana state line between Clark Fork, Idaho, and Superior, Montana. New stratigraphic and petrographic information provides the basis for stratigraphic correlations and for the interpretation of facies changes in this part of the basin of deposition of the Precambrian Belt Series. Identification of facies changes is aided by the recognition of siltite (low-grade metamorphosed siltstone) as a valid rock type, in addition to quartzite and argillite, to classify most of these rocks and to establish mappable units. The older Belt rocks (Prichard through Wallace Formations) were deposited in a trough whose axis trended northwestward, perhaps approximately through Libby, Montana. Subtle facies changes suggest an ancient shore line southwest of the Coeur d9Alene district, Idaho, perhaps near the present exposed edge of the Idaho batholith. The younger Belt formations of the Missoula Group, however, thicken markedly southeastward toward Superior. Thus, the younger Belt rocks were deposited in a trough whose main axis was about at right angles to that of the older trough and perhaps was near Missoula, Montana. Deposition in the Clark Fork area was scant and often interrupted in Missoula time. The young major cross-warp in the old Belt geosyncline undoubtedly has contributed complexities to Belt correlation farther north. Detailed petrographic studies indicate that the Belt rocks from the Pend Oreille area, Idaho, are remarkably uniform in the mineralogic composition of similar rock types throughout 40,000 feet of strata. Further mineralogic studies are required to determine whether this uniformity is local or widespread.

Glauconite from the Precambrian Belt Series, Montana

Glauconite from the upper part of the Missoula Group of the Belt Series, Flathead County, Montana, has been dated at 1070 million years by potassium-argon and rubidium-strontium analyses. This is the first glauconite of Precambrian age reported in North America.