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A Method for Detrital Corundum Characterization in Sediments: Case Study of the Gem Mountain Mine Placer Sapphire Deposit (Rock Creek, Montana, USA)
Rock surface luminescence dating of gravel determines the age of a glacial outburst megaflood, Glacial Lake Missoula, Montana, USA
Abstract Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques. In the absence of valid strain markers, it can be difficult to identify the presence of an early tectonic fabric development and resulting layer parallel shortening (LPS). In order to identify early LPS, we carried out anisotropy of magnetic susceptibility (AMS) analyses on Mississippian limestones from the Sawtooth Range of Montana. The Sawtooth Range is an arcuate zone of north-trending, closely spaced, west-dipping, imbricate thrust sheets that place Mississippian Madison Group carbonates above Cretaceous shales and sandstones. This structural regime is part of the cordilleran mountain belt of North America, which resulted from accretion of allochthonous terrains to the western edge of the North American continent. Although the region has a general east–west increase in thrust displacement and related brittle deformation, a similar trend in penetrative deformation or the distribution of tectonic fabrics is not observed in the field or in the AMS results. The range of magnetic fabrics identified in each thrust sheet ranges from bedding controlled depositional fabrics to tectonic fabrics at a high angle to bedding.
White Mica Geochemistry of the Copper Cliff Porphyry Cu Deposit: Insights from a Vectoring Tool Applied to Exploration
Sheetflood sedimentology of the Mesoproterozoic Revett Formation, Belt Supergroup, northwestern Montana, USA
The ca. 1.460 Ga Revett Formation is a gray and purple quartzite lithosome in northwestern Montana, and it interfingers eastward into red argillite of the Grinnell Formation in Glacier National Park. The Revett Formation was analyzed in northwestern Montana by identifying sedimentary structures in stratigraphic sections and by interpreting flow processes of the structures using the standard flow regime model (e.g., Simons et al., 1965). The sedimentary structures and thicknesses of the event beds were then organized into eight sediment types (lithofacies) that were grouped into three sediment complexes: the playa complex, the antidune complex, and the sheet sand complex. The arrangements of the sediment types and complexes within the stratigraphic framework of the lower informal Revett member indicated the configurations of the depositional environments in space, and the vertical configurations of the sediment types revealed the depositional history of the lower Revett member. The lower Revett member lithosome interfingers eastward into the red argillite of the Grinnell Formation lithosome, and has eight through-going descriptive, stacked, lithic units, called lithostromes. Lithostromes 2, 4, 6, and 8 (from the bottom up) are composed of the sheet sand complex and extend into playa complexes of the Grinnell Formation. They were deposited by sandy sheetfloods that flowed at grade and terminated as the water sank into the sand substrate. Between lithostromes 2, 4, 6, and 8 are lithostromes marked by playa lakes of the playa complex that spread from the east across western Montana during humid periods. They were overlain by sheetfloods of the antidune complex that built eastward over the playa complex as the playa lakes retreated with increasing aridity. The antidune complex was overlain by the sheet sand complex of a vast sand plain deposited by sheetfloods from the southwest that flowed at grade level across western Montana during arid periods. The sheetflood deposits of the Revett Formation were mostly deposited by the upper-flow regime element of the established fluvial facies model.
Crinkle cracks are sand-filled cracks up to 5 mm wide in plan view that pinch at their ends. In cross section, they are canted and crinkled. They cut mudstone beds that underlie hummocky cross-laminated sandstone lenses. They are here described from the Piegan Group, Proterozoic Belt Supergroup, but they are widespread in Proterozoic and Phanerozoic marine and lacustrine rocks. However, they represent a distinctive, descriptive style of mudcracks, not attributed to inferred syneresis processes, although they have been commonly attributed to syneresis. In plan view, crinkle cracks closely resemble cracks formed where oscillatory waves striking viscous mud banks are transformed into fluid solitary-like waves that open surface cracks on their trailing limbs and close the cracks on their leading limbs as they pass through the viscous mud. Crinkle cracks preserved in rocks are hypothetically attributed to oscillatory waves moving sand over viscous mud. The oscillatory waves are transformed into solitary-like waves as they pass down into the mud, forming the cracks. The surface sand falls down into the cracks, preserving them. With burial, the water escapes, and the viscous mud compacts, crinkling the sand-filled cracks.
The Chamberlain Formation, one of the lower members of the early Mesoproterozoic Belt Supergroup, has previously yielded low-diversity assemblages of microfossils but the reported fossils were of limited utility for inferring paleoenvironmental conditions. Here, we describe substantially more diverse microfossil assemblages from drill core of the Chamberlain Formation obtained from the Black Butte mine locality near White Sulphur Springs, Montana. The Chamberlain Formation biota contains abundant Valeria , Leiosphaeridia , Synsphaeridium , and Lineaforma , with lesser amounts of Satka , Symplassosphaeridium , and Coniunctiophycus. The assemblages partially overlap with, but are distinct from, microfossils recently reported from the Greyson Formation, another unit from the Helena embayment of the Belt Supergroup. Since the overlapping taxa exhibit similar states of preservation but dissimilar relative abundances, we interpret the assemblages as reflective of distinct paleoenvironmental conditions of the sampled sections of the Chamberlain and Greyson Formations. The Chamberlain Formation assemblages are most comparable to microfossil groupings reported from the Bylot Supergroup of Canada and the Roper Group of Australia from sediments from very shallow-water (supratidal to lower shoreface) marine environments. This comparison corroborates previous hypotheses on the basis of sedimentological data that the lower Chamberlain Formation sediments were formed in a lagoonal or mud-flat environment. By contrast, the Greyson Formation assemblages are most comparable to microfossil groupings associated with sediments from shallow-shelf marine environments. The fidelity of comparisons among the 1.2 Ga Bylot Supergroup, 1.49 Ga Roper Group, and 1.45 Ga Belt Supergroup assemblages indicates that the groups of microorganisms that produced these assemblages, and their associations with the paleoenvironments that they inhabited, may have been characteristic of the littoral marine biosphere throughout much of the Mesoproterozoic.
A recent 1:24,000 scale mapping project within the northern Beaverhead Mountains along the Idaho-Montana border has resulted in a reinterpretation of both the Mesoproterozoic stratigraphy and the regional structural framework. A 15-km-thick stratigraphic section of the Mesoproterozoic Lemhi subbasin was initially deformed by northeast-southwest shortening into giant northwest-striking, northeast-verging folds, probably during Cretaceous Sevier orogenesis. These initial folds were then dissected by a system of subparallel and anastomosing, oblique-slip reverse, thrust, and normal faults that generally strike northwest, but that exhibit east-west–oriented lineations, suggesting components of strike-slip displacement. Contractional faulting appears to have been followed by Eocene to Miocene extensional faulting, with many normal faults following the preexisting fabrics. Extension opened Tertiary basins along some of these faults, including the Salmon Basin along the southwestern side of the Beaverhead Range. Subparallel faults in the surrounding region appear to have a similar complex history, and all appear to be part of a major northwest-striking Cretaceous fold-and-thrust belt that was later dissected by Tertiary extension. Although the faults of the Beaverhead Mountains are significant and long-lived, they are not terrane-bounding structures separating the Belt and Lemhi sedimentary sequences. Instead, Lemhi strata extend across the range and northward to Missoula, where they grade into correlative Missoula Group strata.
We investigated the crustal structure of the central Mesoproterozoic Belt Basin in northwestern Montana and northern Idaho using a crustal resistivity section derived from a transect of new short- and long-period magnetotelluric (MT) stations. Two- and three-dimensional resistivity models were generated from these data in combination with data collected previously along three parallel short-period MT profiles and from EarthScope MT stations. The models were interpreted together with coincident deep seismic-reflection data collected during the Consortium for Continental Reflection Profiling (COCORP) program. The upper-crustal portion of the resistivity model correlates well with the mapped surface geology and reveals a three-layer resistivity stratigraphy, best expressed beneath the axis of the Libby syncline. Prominent features in the resistivity models are thick conductive horizons that serve as markers in reconstructing the disrupted basin stratigraphy. The uppermost unit (up to 5 km thick), consisting of all of the Belt Supergroup above the Prichard Formation, is highly resistive (1000–10,000 Ω·m) and has relatively low seismic layer velocities. The intermediate unit (up to 7 km thick) consists of the exposed Prichard Formation and 3+ km of stratigraphy below the deepest stratigraphic exposures of the unit. The intermediate unit has low to moderate resistivity (30–200 Ω·m), relatively high seismic velocities, and high seismic reflectivity, with the latter two characteristics resulting from an abundance of thick syndepositional mafic sills. The lowest unit (5–10 km thick) is nowhere exposed but underlies the intermediate unit and has very high conductivity (4–8 Ω·m) and intermediate seismic velocities. This 17–22-km-thick three-layer stratigraphy is repeated below the Libby syncline, with a base at ~37 km depth. Seismic layer velocities indicate high mantle-like velocities below 37 km beneath the Libby syncline. The continuous high-conductivity layer in the lower repeated section is apparently displaced ~26 km to the east above a low-angle normal fault inferred to be the downdip continuation of the Eocene, east-dipping Purcell Trench detachment fault. Reversal of that and other Eocene displacements reveals a >50-km-thick crustal section at late Paleocene time. Further reversal of apparent thrust displacements of the three-layer stratigraphy along the Lewis, Pinkham, Libby, and Moyie thrusts allows construction of a restored cross section prior to the onset of Cordilleran thrusting in the Jurassic. A total of ~220 km of Jurassic–Paleocene shortening along these faults is indicated. The enhanced conductivity within the lowest (unexposed) Belt stratigraphic unit is primarily attributed to one or more horizons of laminated metallic sulfides; graphite, though not described within the Belt Supergroup, may also contribute to the enhanced conductivity of the lowest stratigraphic unit. A narrow conductive horizon observed within the Prichard Formation in the eastern part of the transect correlates with the stratigraphic position of the world-class Sullivan sedimentary exhalative massive sulfide deposit in southern British Columbia, and it may represent a distal sulfide blanket deposit broadly dispersed across the Belt Basin. By analogy, the thick conductive sub–Prichard Formation unit may represent repeated sulfide depositional events within the early rift history of the basin, potentially driven by hydrothermal fluids released during basaltic underplating of attenuated continental crust.
Unkinking the Lewis and Clark tectonic zone, Belt Basin, Idaho and Montana
A succession of separate tectonic events affected Mesoproterozoic Belt Supergroup strata of NW Montana, just as in the better-displayed Coeur d'Alene Mining District of Idaho. A series of these established a tectonic zone historically known as the Lewis and Clark line, here re-identified as the Lewis and Clark tectonic zone, an apparent product of periodic reactivation of fundamental basement structures and physical constraint of a growth fault on developing folds. Six events identify a partial tectonic history of the west-central Belt Basin. The oldest produced growth faults concentrated along at least two structural lineaments, one of which, the Jocko line, substantially controlled the distribution of subsequent structures; the other, the north-trending Noxon line, is implicated in creation of metal-enriched source rock for Coeur d'Alene veins and provides a marker for right-lateral faulting within the Lewis and Clark tectonic zone. Subsequent deformation produced (1) west-northwest–trending folds, mostly confined to the Lewis and Clark tectonic zone and terminating northward against the Jocko line as the likely result of their having been compressed against this pre–Belt Basin structure; (2) north-trending regional folds, which extend southward from Canada and cross the ultimate Lewis and Clark tectonic zone; (3) foliated shear zones in the Lewis and Clark tectonic zone and associated Coeur d'Alene veins and reverse faults; (4) right-lateral, transcurrent faults, so identified by offsets of the Noxon line, north-trending regional folds, and the Montana overthrust belt and its associated foredeep basin; and, last, (5) Lewis and Clark tectonic zone normal faults and associated kink folds, which extensively reached gigantic, “megakink” proportions. These megakinks locally disrupted all prior structures, greatly confusing local structure; these need to be “unkinked,” so that structures resulting from prior tectonism may be fully recognized and correctly interpreted. Many faults of the Lewis and Clark tectonic zone trend southeasterly in its easterly part, tracking pre–Belt Basin structures separate from those associated with the Jocko line.
Belt-Purcell Basin: Template for the Cordilleran magmatic arc and its detached carapace, Idaho and Montana
The bedding-plane anisotropy and structural configuration of the Mesoproterozoic Belt-Purcell Supergroup guided a narrow magmatic salient >350 km eastward from the Salmon River suture of Idaho to the foreland basin of central Montana, along a deep graben within the southern part of the Belt-Purcell Basin. The magma assimilated anatectic melt from the lower Belt-Purcell Supergroup in the western half of the graben, where the Lemhi subbasin had intersected and deepened the graben by several kilometers. The magma stepped across the stratigraphic section as it intruded eastward along the graben, spread laterally as it climbed into the overlying Paleozoic and Mesozoic strata, and eventually erupted into the foreland basin. This paper develops a model in which the magma formed a thick, east-tapering wedge beneath the Belt-Purcell carapace. The wedge elevated and tilted its lid, which failed along the trend of the graben to a terminus in the Crazy Mountains basin of the Helena structural salient, much like a tectonic-scale landslide. The carapace failed in two main phases between ca. 100 and 75 Ma. It slid ~100 km during the first failure phase, and ~40 km during the second, when the Boulder batholith and its volcanic cover filled a large pull-apart structure within the carapace. Slaty cleavage, tectonic slides that omit strata, and a nested series of hairpin-shaped allochthons characterize the failed carapace. Shear zones and nappes bound the carapace; the sinistral Lewis and Clark line bounds it on the north, and the dextral southwest Montana transverse zone bounds it on the south. The Lewis thrust fault and associated structures of the Rocky Mountain fold-and-thrust belt overprinted and displaced the magmatic salient and its carapace from ca. 74 to 59 Ma. The magma crystallized, cooled, and generated hydrothermal ore deposits from Late Cretaceous to middle Eocene time. Eocene extension overprinted the system from 53 to 39 Ma and exhumed its infrastructure in core complexes. Those exposures, together with regional structural tilt, enable reconstruction of a deep cross section of the magmatic wedge and its carapace.
SHRIMP U–Pb and REE data pertaining to the origins of xenotime in Belt Supergroup rocks: evidence for ages of deposition, hydrothermal alteration, and metamorphism
Abstract Glacial Lake Missoula was repeatedly dammed by the Purcell Trench Lobe of the Cordilleran ice sheet during the last glaciation to maximum altitudes near 4200 ft (1280 m). Studies from outside of the lake basin suggest that the lake filled and drained multiple times in the late Pleistocene. Deposits and landforms within the former glacial lake basin provide evidence for a complex lake-level history that is not well understood for this famous impoundment. At least two general lake phases are evident in the stratigraphy: an earlier phase of catastrophic drainage that was responsible for large-scale dramatic erosional and depositional features, and a later, less-catastrophic, phase responsible for the preservation of fine-grained glaciolacustrine sediments. Features of the earlier lake phase include giant gravel dunes and openwork gravel with anomalously large clasts (erratics). Deposits from the later phase are mostly low-energy glaciolacustrine sediments that record a history of lake-bottom sedimentation and repeated lake-floor exposure. A focus of this field trip is to look at evidence for the two lake phases as well as evaluate the record of exposure surfaces, and therefore lake-level lowerings, during the second phase at multiple locations in the lake basin. One of the second phase sites is close to a highstand, full basin position in the lake (near Garden Gulch), representing a maximum water depth at this site of ~100 m, whereas others (Rail line and Ninemile) are at lower altitudes in regions that may have been under as much as 300 m of water. Fine-grained glaciolacustrine sediments are rippled very fine sandy silt and fining-upward sequences of laminated silt and clayey silt of glaciolacustrine origin. Periglacial features, contorted bedding, desiccation, and paleosols in outcrop provide clear evidence of multiple exposure surfaces; each represent a lake-lowering event. Optically stimulated luminescence (OSL or “optical dating”) ages on quartz from the three sections (Ninemile, Rail line, and Garden Gulch) allow for preliminary correlations that suggest approximately the same phase of glacial Lake Missoula sedimentation. The exposure surfaces suggest that the glacial-lake level rose and fell at least 8–12 times to elevations above and below the sections (936–1180 m), filling to within 100 m of full pool (1280 m). Optical dating shows that this occurred after 20 ka and the last inundation of the lake before 13.5 ka. Correlation of specific exposure surfaces throughout the basin will be required to develop a lake-level history.
Near infrared spectra of white mica in the Belt Supergroup and implications for metamorphism
Negative correlations between Mg:Ca and total dissolved solids in lakes: False aridity signals and decoupling mechanism for paleohydrologic proxies
Lewis and Clark Line, Montana: Tectonic evolution of a crustal-scale flower structure in the Rocky Mountains
Abstract The Lewis and Clark line (LCL) is a major transverse structure that crosses the North American Cordillera from northeastern Washington to central Montana. It initiated as a rift structure within the Mesoproterozoic Belt basin and reactivated several times during the Phanerozoic. This field trip examines the internal structure of the LCL along a transect in central-western Montana. Structural plunge permits examination of a 25-km-thick crustal section of a flower structure that formed along the LCL during Late Cretaceous-late Paleocene sinistral transpression. We will observe changes in structural style from the deepest parts of the Belt Supergroup upward to the syntectonic depositional surface.
Belt-Purcell Basin: Keystone of the Rocky Mountain fold-and-thrust belt, United States and Canada
The Mesoproterozoic Belt-Purcell Basin of the United States–Canadian Rocky Mountains formed in a complex intracontinental-rift system. The basin contained three main fault blocks: a northern half-graben, a central horst, and a southern graben. Each had distinct internal stratigraphy and mineralization that influenced Phanerozoic sedimentation; the northern half-graben and horst formed a platform with a condensed section, whereas the southern graben formed the subsiding Central Montana trough. They formed major crustal blocks that rotated clockwise during Cordilleran thrust displacement, with transpressional shear zones deforming their edges. The northern half-graben was deepest and filled with a structurally strong prism of quartz-rich sedimentary rocks and thick mafic sills that tapered toward the northeast from >15-km-thick near the basin-bounding fault. This strong, dense prism was driven into the foreland basin as a readymade, critically tapered tectonic wedge and was inverted into the Purcell anticlinorium. Erosion did not breech the Belt-Purcell Supergroup in this prism during thrusting. The southern graben was thinner, weaker, lacked mafic sills, and was engorged with sheets of granite during thrusting. It was internally deformed to achieve critical taper and shed thick deposits of syntectonic Belt-Purcell–clast conglomerate into the foreland basin. A palinspastic map of the basin combined with a detailed paleocontinental map that juxtaposes the northeastern corner of the Siberian craton against western North America indicates that the basin formed at the complicated junction of three continental-scale rift zones.