Cambrian Quartzite Research Articles

The cambrian gog quartzite and deadwood formations of alberta: lessons for subsurface carbon sequestration

Injecting carbon dioxide into reservoirs has been undertaken since the 1940s, mostly as a method to enhance oil recovery. With the recognition of the impact of climate change, interest in carbon sequestration has increased exponentially, with hundreds of projects in progress around the globe. Many of these projects focus on Cambrian quartzites, which meet the requirements for carbon dioxide injection (CCS). Several CCS projects in Alberta, Canada, currently focus on using these reservoirs, including the Basal Cambrian Sandstone and Deadwood Formations, for injection and storage.A series of spectacular outcrops of the Cambrian Gog Quartzite are available for study in the area around Lake Louise in the Canadian Rockies of Alberta. These outcrops provide compelling analogues to other clastic Cambrian formations in the province that have been utilized for CCS. As an example, the Gog Quartzite was used by Shell as a direct reservoir analogue when modeling CO2 injection into the Basal Cambrian Sandstone in the QUEST Project. The Gog Quartzite also has potential to act as a CCS reservoir in its own right and may reach 2 kilometres in thickness.Outcrops of the Gog Quartzite were studied at Sink Lake, Spiral Tunnels and in the vicinity of Lake Louise in Alberta. They include fluvial and shallow marine, high net:gross sandstone beds with stacked trough cross-bedded sandstones. These are commonly interbedded with thin mudstone beds. Sedimentary structures include planar and trough cross-beds, wave ripples and contorted horizons interpreted as seismites with associated dewatering. Some units of the Gog Quartzite are heavily bioturbated with burrows including Arenicolites and Skolithos as well as other marine traces such as trilobite related ichnofacies.The Cambrian Deadwood Formation is also a target for carbon dioxide sequestration and provides the main reservoir for the Aquistore Project in Saskatchewan. It has been targeted by exploration wells in eastern Alberta and a new core study has demonstrated its suitability as a CO2 reservoir. Facies ranging from regolith overlying igneous basement, through alluvial fan and tidal deposits, and out into shallow marine settings with well developed parasequences, are all seen in the cores.The outcrop and core studies provide new data on Cambrian reservoirs which can be applied directly to potential Cambrian CCS targets in the subsurface of Alberta. The data can also be used to identify other potential Cambrian CO2 sinks around the world.

Structure of the Assynt window, Moine Thrust Zone and relationship of thrusts to alkaline igneous complexes, Caledonian orogeny, NW Scotland

AbstractThe Moine Thrust Zone forms the Caledonian orogenic thrust front where the Moine Supergroup metamorphic rocks have been thrust westward across the Laurentia plate stable foreland, comprising Archean-Proterozoic granulite and amphibolite facies rocks (Lewisian gneisses), with unconformably overlying Mesoproterozoic and Neoproterozoic Torridonian clastic sediments and Cambrian-Ordovician passive margin sedimentary rocks. Four major thrusts beneath the Moine thrust in the Assynt window include the (i) Ben More Thrust, which places the Loch Ailsh syenite intruded into Lewisian basement and Cambrian-Ordovician sedimentary rocks over the Sole thrust sheet, (ii) Glencoul thrust, which places Lewisian basement and folded cover rocks over Cambrian-Ordovician sedimentary rocks, (iii) Borralan thrust, which carries a large alkaline syenite intrusion beneath the Ben More roof thrust and (iv) the Sole thrust sheet, which carries imbricated Cambrian-Ordovician sedimentary rocks and lamprophyre sills over the stable foreland. Three further thrust sheets within the Lewisian basement gneisses are now recognised through restoration of balanced cross-sections, which were responsible for doming of the Assynt window. Although the Moine thrust is mapped as a single line on the map it encompasses, (a) deep ductile shear zone formed of mylonites derived from hangingwall Moine schists, footwall Cambrian quartzites and Ordovician limestones, and basement Lewisian gneisses, (b) roof thrust of the Glencoul and Ben More Thrust sheets and (c) brittle out-of-sequence motion where the Moine schists have been thrust over mylonites, which directly overlie the stable foreland (Knockan Crag).

Large‐Volume and Shallow Magma Intrusions in the Blackfoot Reservoir Volcanic Field (Idaho, USA)

AbstractThe Blackfoot Reservoir volcanic field (BRVF), Idaho, USA, is a bimodal volcanic field that has hosted silicic eruptions during at least two episodes, as recently as 58 ka. Using newly collected ground and boat‐based gravity data, two large negative anomalies ( mGal) are modeled as shallow ( km) intrusions beneath a NE‐trending alignment of BRVF rhyolite domes and tuff rings. Given the trade‐off between density contrast and model volume, best‐fit gravity inversion models yield a total intrusion volume of ; a density contrast of kg results in two intrusions, each km km and about 0.5 km thick, with cumulative volume of 100 . A network of trending faults lies directly above and on the margins of the mapped gravity anomalies. Most of these faults have m throw; one has throw up to m. We suggest that the emplacement of shallow sill‐like intrusions produced this fault zone and also created a ENE‐trending fault set, indicating widespread ground deformation during intrusion emplacement. The intrusions and silicic domes are located km E of a regional, 20 mGal step in gravity. We interpret this step in gravity as thickening of the Upper Precambrian to lowermost Cambrian quartzites in the Meade thrust sheet, part of the Idaho‐Wyoming Thrust Belt. Silicic volcanism in the BRVF is a classic example of volcanotectonic interaction, influenced by regional structure and creating widespread deformation. We suggest volcanic hazard assessments should consider the possibility of large‐volume silicic eruptions in the future.

Neoproterozoic Windermere Supergroup Near Bayhorse, Idaho: Late‐Stage Rodinian Rifting Was Deflected West Around the Belt Basin

AbstractConflicting models of Rodinian rifting have been proposed to explain the recognized variation in the Neoproterozoic and early Cambrian tectonostratigraphic architecture of the western Laurentian margin. However, discrimination among rift models is hampered by limited exposure and metamorphism of the rocks. Southeastern Idaho preserves more than 6 km of Neoproterozoic and Cambrian strata. In contrast, along the inferred continuation of the margin in east central Idaho, correlative rocks are missing across the Lemhi arch. Our field mapping and U‐Pb dating studies, located approximately 50 km west of the Lemhi arch unconformity, focused on a succession of regionally extensive rocks that were previously assigned an Ordovician age. We show that ~1.5 km of strata here overlies a ~667 Ma reworked felsic tuff and was intruded by a 601 ± 27 Ma gabbro sill; we thus redesignate these rocks as Cryogenian and Ediacaran in age. These rocks are overlain by a ~1 km thick Ediacaran to middle Cambrian quartzite. Middle Ordovician quartzites overlie these middle Cambrian strata, indicating that though Neoproterozoic and lower Cambrian rocks are present west of the Lemhi arch, upper Cambrian and Lower Ordovician rocks are thin or absent. Comparison of this redesignated section to the closest correlative sections suggests an initial stage of symmetric rifting followed by later asymmetric rifting. We suggest that prerifting ~1,370 Ma magmatism within the Belt basin produced lithospheric rigidity that influenced the final stage of rifting and produced heterogeneity in the geometries of structural domains similar to those documented in other well‐defined, modern rift margins.

Rheology of a coaxial shear zone in the Virginia Blue Ridge: Wet quartzite dislocation creep at ~250–280 °C

Cambrian quartzite along the western margin of the Blue Ridge near Front Royal, Virginia records significant penetrative strain under subgreenschist-facies conditions. Strain analyses of the quartzite indicate general flattening and octahedral shear strain values up to 1.25 (X:Z up to 5.7:1). Samples with X:Z as low as 1.5:1 have crystallographic preferred orientations that record a dominance of basal <a> slip and patterns consistent with coaxial flattening. In most samples quartz has undergone minor amounts of dynamic recrystallization with mean grain sizes ~4–5 μm. Locally, the quartzite is brittlely deformed, and subgrains and recrystallized bulges are present along fractures and extinction bands. Fluid-inclusion planes and fluid inclusions along deformation lamellae are abundant, and quartz FTIR analyses yield average H:106Si values of 1100–3300, indicating fluid flow and high intragranular water content during deformation. Higher strain samples generally record lower water contents, which may indicate that fluids migrated to grain boundaries during deformation. Dissolution microstructures are rare in high strain samples, and we interpret most strain to have been accommodated by dislocation creep. Thermal modeling of zircon low-temperature thermochronology data coupled with quartz c-axis opening angle thermometry suggest deformation temperatures were likely ~250–280 °C. These deformation temperatures are lower than what is typically considered the brittle-plastic transition in quartz. We attribute localization of this high strain zone and dislocation creep at these temperatures to hydrolytic weakening and a relatively low strain rate estimated to be ~10−15 s−1.

The properties of reservoir water in post-mining excavations of Cambrian and Devonian quartzite sandstones (Holy Cross Mountains)

In the Holy Cross Mountains (southern Poland), there are numerous disused quarries. Some of these are filled with water, and some, despite their nearby location, have extremely diverse physicochemical and chemical properties of their waters. One such object is the Wiśniówka Mała reservoir. Its waters contain large amounts of sulfates (> 700 mg/L) and iron (24 mg/L), which are weathering products of rocks containing metal sulfides (mainly pyrite) in the direct drainage zone. As a consequence, there is an increase in the electrolytic conductivity of the water supplying the reservoir, resulting in very low pH values (< 4). This article presents the detailed limnological characteristics of this reservoir and explains the process that led to its water acidification. A control reservoir, Barcza, was also selected for the tests. Although it is also a post-mining excavation, it has a neutral pH and a low concentration of sulfates and iron. The examined reservoirs differ in the hydrochemical type of waters. The Wiśniówka Mała reservoir represents a sulfate–calcium double-ion type (SO42−–Ca2+) and the Barcza reservoir, a bicarbonate–calcium type (HCO3−–Ca2+). The concentrations of the main ions occurring here are shaped by the lithological features of the surrounding rocks (respectively: Upper Cambrian quartzite sandstones with pyrite mineralization zones, Lower Devonian sandstones cut by mudstones and claystones with tuffite inserts).

Water loss during dynamic recrystallization of Moine thrust quartzites, northwest Scotland

AbstractInfrared absorption measurements of molecular water in sheared Cambrian quartzites in the footwall to the Moine thrust reveal a decrease in water content from 4080 to 1570 ppm with increasing recrystallization traced toward the overlying thrust at the Stack of Glencoul in northwest Scotland. These results are contrary to the expected correlation between shear strain and water content for quartz deformed by dislocation creep and water-weakening processes. The observed inverse correlation indicates that fluid inclusions and hydrous defects within grains were lost by mobile grain boundary sweeping and grain boundary diffusion. Although reduced water contents might lead to hardening as chemical weakening is diminished, quartz mylonites in the immediate footwall (5 mm) to the thrust are characterized by intense strain localization and contain the least water, and there is little evidence of shear zone widening. Water weakening appears to have been important throughout the quartz mylonites, controlled by the presence of water, not by water concentration. Fluids present within relict inclusions and at grain boundaries may have governed the high water fugacities critical for water weakening.

Structure of the Northern Moine thrust zone, Loch Eriboll, Scottish Caledonides

This paper reviews the geology of the Late Ordovician - Silurian Caledonian Moine Thrust zone in the Loch Eriboll region, NW Scotland. We present new detailed mapping and balanced/restored cross-sections to examine its structural evolution. This thrust zone comprises four major thrust slices, carried by the Moine, Lochan Riabhach, Arnaboll and Sole thrusts. Ductile mylonites were derived from lithologies in both the hangingwall (Moine schists) and the footwall (Lewisian basement gneisses, Cambrian quartzites, Ordovician dolomites) of the Moine Thrust. The mylonites are bounded by the Lochan Riabhach thrust below and the Moine thrust above. The Arnaboll thrust sheet comprises Lewisian basement gneisses and Cambrian-Ordovician sedimentary rocks over the Sole Thrust sheet, which exposes a spectacular imbricate sequence of Cambrian-Ordovician sedimentary rocks. Although thrusts sequentially propagated in-sequence from higher (Moine) to lower (Sole) thrusts with time, there is evidence for minor out-of-sequence thrusting and breakback thrusting. Thrusts evolve from deep ductile shears where strain is concentrated along mylonite zones up into the brittle field where they become discrete planar thrust faults. The Early Silurian Scandian metamorphism (~435–415 Ma) is related to a Himalayan style structure with SSE-directed subduction and WNW directed extrusion of a migmatitic core, which we liken to channel flow. We suggest that two major crustal-scale thrusts that extend down into the upper mantle imaged on seismic profiles across the foreland, the Outer Isles and Flannan thrusts, are unrelated spatially or temporally to the Moine thrust sequence.

Some aspects of lithological and exogenic control of sandstone morphology, the Świętokrzyskie (Holy Cross) Mts. case study, Poland

Various morphologies of cliffs built of different quartzose rocks in the Świętokrzyskie (Holy Cross) Mts. (upland region, central Poland) – from Cambrian quartzites and Devonian quartzitic sandstones to Triassic and Jurassic porous sandstones – were described in order to examine the constraints of their lithological and spatial occurrence. The quantitative study of the occurrence of these morphologies on cliffs makes it possible to distinguish two principal groups of morphologies: angular relief produced by rock splitting (crumbling), typical of quartzites indurated in silica and of open porosity less than 1.5%, and morphologies developed due to granular disintegration and exfoliation of sandstones of open porosity higher than 1.5%. Among the relief types of this second group, morphology reflecting sedimentary and diagenetic structures as well as smooth surfaces are the most common and are referred to sandstones of a wide range of porosity, whereas honeycombs and surfaces suffering fast granular decay and scaling are characteristic of rocks of specific porosity (respectively: 5–8% and 3.5–8%). The occurrence of honeycombs on rock surfaces is also conditioned by exogenic factors: sun, wind and rain, since this morphology tends to occur on cliffs with aspects ranging from south-east, through south, to west-north-west.

The geological constraints of the development of sandstone landforms in Central Europe, a case study of the Świętokrzyskie (Holy Cross) Mountains, Poland

Eighty sites of crag groups and individual crags occurring in the Świętokrzyskie (Holy Cross) Mts., upland situated in central Poland, were described in detail in order to examine the lithological, structural, mechanic properties and the tectonic features of sandstones and quartzites representing six crag-forming lithostratigraphic units: Cambrian quartzites, Devonian quartzitic sandstones, Triassic and Jurassic sandstones. Specific features of these rocks are: their occurrence within the sequences consisting of different rock series, high energy depositional environments and siliceous composition. The cragforming rocks differ in the amount of cement (from strongly cemented quartzites to very porous sandstones with poor cement), which determines the diverse mechanical properties (from very strong to friable rocks). The crucial feature enabling formation of crags built of porous and friable sandstones is very dense grain packing due to chemical and mechanical compaction. Regarding the principal role of the gravitational disintegration of rock massifs under the periglacial conditions in the Late Pleistocene, other factors constraining the crag formation and shaping are: the tectonic situation of rocks (the orientation of strata and joints) as well as adequate joint spacing and bed thickness. Petrographic, structural and tectonic features are interrelated.

Enhanced development of lacustrine microbialites on gravity flow deposits, Great Salt Lake, Utah, USA

The Great Salt Lake, Utah, USA is a shallow, hypersaline, intracontinental lake hosting extensive microbial deposits. At a large spatial scale, the distribution of these deposits is driven by environmental and geodynamical factors (i.e. water-level fluctuations and a fault-related framework). A detailed mapping of the Buffalo Point area, in the north-western part of Antelope Island, indicates the presence of an anomalous concentration of microbial deposits dated ca. 5.8ka BP and distributed along a lobe-shaped geometry. This uncommon microbial deposit geometry results from an extensive colonization of a conglomerate substrate exhibiting an accumulation of m-sized rounded Cambrian quartzite boulders. We suggest that this conglomerate substrate provides a stable nucleation point that promotes the development and preservation of the lobe-shaped microbial deposits. Microbial deposits may also have protected the conglomerate substrate from erosional processes and thereby increased the preservation potential of the lobe-shaped structure. Based on the characteristics of the conglomerate (e.g. grain size, texture) and its location (i.e. 200m beyond the average shoreline), this lobe-shaped structure likely results from subaqueous debris or a hyperconcentrated density flow that transports sedimentary material from the Buffalo Point slopes downward to the shore. We estimate the age of the conglomerate deposition to be between 21 and 12ka BP. The initiation of the flow may have been triggered by various mechanisms, but the existence of a major active normal fault in the vicinity of these deposits suggests that an earthquake could have destabilized the accumulated sediments and resulted in conglomerate emplacement. The catastrophic 15ka BP Bonneville Flood, which led to a drop in the lake level (approximately 110m), may also provide an explanation for the initiation of the flow.

Catastrophic Rock-Slope Failures in NW Scotland: Quantitative Analysis and Implications

ABSTRACT126 postglacial catastrophic rock-slope failures (PRSFs) and 66 debris-free failure scarps (DFFSs) were identified using Google Earth™ imagery on Lewisian Gneiss (LG), Torridon Sandstone (TS), Cambrian Quartzite (CQ) and Moine Schist (MS) terrains in NW Scotland. Normalisation of these data for area and gradient shows that RSF density on slopes ≥25° decreases in the sequence MS > LG ≈ CQ > TS. Most are apparently discordant with structure, suggesting that failure along slope-(sub)parallel joints was more common than failure along bedding or foliation planes. DFFSs represent failure scars from which runout debris has been removed by later glaciation. Eighteen were recorded outside the limits of the Loch Lomond Readvance, implying that they survived the last ice-sheet glaciation, and support the view that many steep upper rock slopes have been dominated by gravity-driven rock detachment rather than glacial erosion. We argue that many RSFs were probably triggered by Lateglacial earthquakes due to fault (re)activation by crustal rebound, as 74% of recorded PRSFs and DFFSs are located at or near slope crests. 44% of recorded PRSFs and DFFSs occur in cirques, suggesting that cirque enlargement during successive glacial-interglacial cycles involved deepening by glacial erosion, headwall retreat by rock-slope failure and removal of RSF debris during subsequent glaciation.

Detrital zircon signatures of the Baltoscandian margin along the Arctic Circle Caledonides in Sweden: The Sveconorwegian connection

New evidence is presented here that the Sveconorwegian Orogen continued northwards from type areas in southwestern Scandinavia along the Baltoscandian outer margin into the high Arctic. The Silver Road (Silvervägen) profile through the Scandinavian Caledonides, located in Sweden along the Arctic Circle at 66–67° N, provides a full section through the tectonostratigraphy of the Baltoscandian margin from the Autochthon, via the Lower Allochthon to the upperment parts of the Middle Allochthon. Metamorphic grade increases upwards through the nappes, being low greenschist facies at lowest levels and increasing to eclogite grade in the highest parts of the Seve Nappe Complex, the latter being related to early Ordovician subduction of the Baltoscandian outermost margin. The sedimentary rocks range in age from Neoproterozoic to Ordovician and provide evidence of the changes of environment from the Baltoscandian platform, westwards out over the Cryogenian rifted margin to the continent-ocean transition zone; also the Ordovician foreland basin. Twelve samples of psammites from the different tectonostratigraphic levels have yielded U/Pb detrital zircon age-signatures that reflect the changing character of their provenance. Autochthonous sandstones are derived from late Paleoproterozoic (1800–1950Ma) crystalline rocks in the vicinity to the east of the thrust front. Ediacaran-early Cambrian quartzites of the Lower Allochthon also yield mainly late Paleoproterozoic zircon signatures, but with subordinate Mesoproterozoic and late Archaean populations, whilst mid Ordovician, W-derived foreland basin turbidites are dominated by Sveconorwegian (950–1100Ma) signatures, with subordinate older Mesoproterozoic to latest Paleoproterozoic populations. All samples from the lower parts of the Middle Allochthon (lacking dolerite dykes) have signatures that are dominated by latest Paleoproterozoic and early Mesoproterozoic ages, with subordinate populations down to Sveconorwegian ages; the latter dominate the overlying Särv nappes and also the Seve Nappe Complex, where c. 945Ma rhyodacites have been previously reported. This evidence of Sveconorwegian source rocks in the hinterland, taken together with previously published detrital zircon data farther south and north of the Arctic Circle, clearly favours the interpretation that the Sveconorwegian Orogen, during the Neoproterozoic, extended along the entire Baltoscandian outer margin into the high Arctic.

Crustal growth and tectonic evolution of the Mojave crustal province: Insights from hafnium isotope systematics in zircons

Coupled U-Pb ages and Hf isotopic ratios in zircons from Proterozoic basement and three siliciclastic cover sequences in southern California provide important insights into the formation of the southern Mojave crustal province and its incorporation into southwestern Laurentia. Hafnium isotopic ratios measured in >800 zircons, coupled with new and previously reported U-Pb ages, suggest that the crystalline basement of the Mojave crustal province formed from four main components: 1) mantle components ranging from depleted to moderately enriched; 2) metasedimentary framework rocks derived from 2.6 to 2.4 Ga and 2.0 to 1.8 Ga crust; 3) 1.79 to 1.64 Ga intrusive rocks that reflect mixing of mantle-derived melts and crust; and 4) Mesoproterozoic (1.4 to 1.2 Ga) anorthosite and granitic to syenitic intrusive rocks. Initial Hf isotopic ratios of detrital zircons in siliciclastic cover sequences suggest varying degrees of insularity of the Mojave province during assembly of southwestern Laurentia. The Mesoproterozoic Pinto Mountain Group appears entirely derived from Mojave province basement. In contrast, Neoproterozoic quartzites of the Big Bear Group had a distal provenance, either an unexposed, older, western subprovince of the Mojave crustal province lacking ca. 1.7 Ga magmatic rocks or from a distinctive Paleo- and Mesoproterozoic basement province far to the east within Laurentia. Zircons in latest Neoproterozoic to Cambrian quartzites reflect provenance from an integrated transcontinental drainage network delivering sediment to the craton edge and westward into the Cordilleran miogeocline.

Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

Subglacial erosion beneath glaciers occurs predominantly by abrasion and plucking, producing distinct erosional forms. The controls on the relative importance of abrasion vs. plucking are poorly understood. On the one hand, glacial conditions that favour or suppress cavity formation (ice velocity, ice thickness, and water pressure) are thought to favour plucking or abrasion, respectively. Conversely, bedrock properties are also known to control landforms, but this has rarely been analysed quantitatively. In this study we compare landforms and bedrock properties of sandstone and quartzite at the bed of a palaeo-ice stream near Ullapool in NW Scotland. The boundary between the rock types is at right angles to the westward palaeo-ice flow, and palaeoglacial conditions on both rock types were similar. We report quantitative parameters for bedrock properties (Schmidt hammer hardness and joint spacing) and use morphometric parameters to analyse the landforms. Torridon sandstone is soft but thick-bedded and with a wide joint spacing. Erosional bedforms include roche moutonnées with smoothed tops and concave stoss sides, whalebacks, and elongate p-forms, indicating a high proportion of abrasion over plucking. Cambrian quartzite is hard but thin-bedded with narrow joint spacing. Erosional landforms are angular to subangular with abundant plucked lee faces, suggesting a high proportion of plucking over abrasion. Hardness and joint spacing thus exert a strong control on subglacial erosional landforms and the mechanisms that formed them. Thus glacial conditions (ice velocity, ice thickness) can only be inferred from glacial erosional landforms if the effects of bedrock properties of the substrate are considered.

Moine Thrust zone mylonites at the Stack of Glencoul: I – microstructures, strain and influence of recrystallization on quartz crystal fabric development

Abstract Since the early descriptions published by Callaway in 1884, the gently dipping mylonites exposed along the Moine Thrust at the Stack of Glencoul have drawn generations of geologists to the northern part of the Assynt district. These mylonites, derived from Cambrian quartzites (footwall) and Moine pelites and psammites (hanging wall), have figured prominently in: a) early research into the influence of crystal plastic deformation and recrystallization on microstructural and crystal fabric evolution; b) debates on the kinematic interpretation of macro- and micro-structures and crystal fabrics; and c) debates on the tectonic significance of such kinematic data. In this paper first we briefly review the historical aspects of this research and then, using both previously published and unpublished data, document the finite strain and quartz fabric development at this classic mylonite locality. A tectonic interpretation of these data, together with quantitative estimates of flow vorticities associated with mylonite formation at the Stack of Glencoul, are presented in a companion paper by Law (2010) .

Predicting seismic properties from three-dimensional microstructures: a new look at an old quartzite

Abstract The mylonitic Cambrian quartzites, Moine Thrust Zone, NW Scotland, have long been used to study microstructural and petrofabric evolution and to develop understanding of grain-scale processes accommodating large-scale displacements. Today, structural geology is entering a new age of understanding of the basic processes involved in microstructural evolution due to the emergence of novel instrumental techniques and theoretical models. It seems apposite therefore to re-evaluate the microstructure of one example of this classic quartz mylonite from the Stack of Glencoul, Assynt, using arguably the most important of these new techniques, electron backscattered diffraction (EBSD). The three-dimensional (3D) microstructure and petrofabric of this rock was analysed using EBSD, to: 1) corroborate previous optical and X-ray texture goniometry measurements; 2) investigate the potential for sampling and/or tectonic sectioning bias that may be introduced inadvertently into any petrofabric analysis; and 3) predict its seismic properties. It is found that microstructures do differ between orthogonal structural sections, leading to variations in strengths of different components in the overall petrofabric that might impact on seismic properties. The results emphasize the true 3D nature of microstructures and petrofabrics, which can be recognized and accommodated more readily by this new generation of analytical techniques.

Detrital zircon analysis of Mesoproterozoic and Neoproterozoic metasedimentary rocks of north-central Idaho: implications for development of the Belt–Purcell basin

The authors analyzed detrital zircon grains from 10 metasedimentary rock samples of the Priest River complex and three other amphibolite-facies metamorphic sequences in north-central Idaho to test the previous assignment of these rocks to the Mesoproterozoic Belt–Purcell Supergroup. Zircon grains from two samples of the Prichard Formation (lower Belt) and one sample of Cambrian quartzite were also analyzed as controls with known depositional ages. U–Pb zircon analysis by laser ablation — inductively coupled plasma — mass spectrometry reveals that 6 of the 10 samples contain multiple age populations between 1900 and 1400 Ma and a scatter of older ages, similar to results reported from the Belt–Purcell Supergroup to the north and east. Results from the Priest River metamorphic complex confirm previous correlations with the Prichard Formation. Samples from the Golden and Elk City sequences have significant numbers of 1500–1380 Ma grains, which indicates that they do not predate the Belt. Rather, they are probably from a relatively young, southwestern part of the Belt Supergroup (Lemhi subbasin). Non-North American (1610–1490 Ma) grains are rare in these rocks. Three samples of quartzite from the Syringa metamorphic sequence northwest of the Idaho batholith contain zircon grains younger than the Belt Supergroup and support a Neoproterozoic age. A single Cambrian sample has abundant 1780 Ma grains and none younger than ∼1750 Ma. These results indicate that the likely protoliths of many high-grade metamorphic rocks in northern Idaho were strata of the Belt–Purcell Supergroup or overlying rocks of the Neoproterozoic Windermere Supergroup and not basement rocks.

Moine Thrust zone mylonites at the Stack of Glencoul: II - results of vorticity analyses and their tectonic significance

Abstract This paper presents quantitative data from the Stack of Glencoul on flow vorticities associated with mylonite generation in the hanging wall and footwall of the Moine Thrust, using samples collected in a vertical traverse from 80 m above the thrust plane to 8.5 m beneath the thrust. Estimated vorticity numbers (Wm) in Moine pelites and psammites above the thrust range from 0.775–0.725 ( c . 43–47% pure shear component) increasing downwards to 0.83–0.75 (35–45% pure shear) at 10 cm above the thrust. Wm values in dynamically recrystallized Cambrian quartzites at 0.5–14.5 cm beneath the thrust range from 0.99–0.90 (10–30% pure shear). At 3.0–8.5 m beneath the thrust estimated Wm values are less than 0.75 in the quartzites, although there is some thin section-scale partitioning with Wm values of 0.75–0.65 (45–55% pure shear) in domains of dynamically recrystallized quartz and Wm values &lt;0.65 (&gt;55% pure shear) in domains of relict detrital quartz grains. Integration of strain and vorticity analyses indicates a vertical shortening of 50–75% in these gently dipping mylonites located at the base of the Moine Nappe. The tectonic implications of vertical shortening (thinning) and transport-parallel stretching at the base of the Moine Nappe are discussed.

Gorceixite from the Upper Cambrian Rocks of the podwiśniówka Mine Pit, Holy Cross Mountains (South-Central Poland)

Gorceixite from the Upper Cambrian Rocks of the podwiśniówka Mine Pit, Holy Cross Mountains (South-Central Poland)This report presents the results of a petrographical, mineralogical (SEM/EDS, XRD) and geochemical (XRF, CV-AAS, ICP-MS) study of gorceixite (barium aluminophosphate) from the abandoned Podwiśniówka mine pit. This site is highlighted by the presence of highly acidic pit pond whose chemistry is strongly affected by the exposed pyrite-bearing zone. The gorceixite occurs in the Upper Cambrian carbonaceous clayey shales, quartzites and tuffs in form of minute accumulations varying from about 0.5 to 100 μm in diameter. These accumulations infill voids, cavities, cracks and partly fissures in the rocks examined. The other minerals of the crandallite series, i.e. florencite and goyazite, can be found only in trace amounts. The gorceixite-bearing rocks, especially carbonaceous clayey shales, are characterized by the highest concentrations of REE reaching 455.09 mg·kg-1. In addition, these rocks are distinctly enriched in light rare earth elements (LREE), with the La/Yb ratio ranging from 24.44 through 36.30. Some of the examined gorceixite accumulations are paragenetically linked to the veined pyrite and nacrite. The latter mineral is indicative of crystallization temperatures of about 200 to 300°C. The coexistence of gorceixite with the veined nacrite or pyrite mineralization and the volcaniclastic rocks, as well as the microtextural features and high concentrations of REE in the gorceixite-bearing parent rocks suggest that this mineral formed as a result of both hydrothermal and volcanic activity in a shallow-marine basin during the late Cambrian.