Microfabric Analysis Research Articles

Mayfly larvae preservation from the Early Cretaceous of Brazilian Gondwana: Analogies with modern mats and other Lagerstätten

The Crato Formation paleoentomofauna is noticeable for its high abundance, diversity and morphological fidelity, so the preservational approach of the Ephemeroptera larvae fossils becomes relevant, since they are aquatic insects living in a lacustrine environment of one of the major terrestrial ecosystems of the Early Cretaceous in the Gondwana supercontinent. The mayfly larvae fossils were analyzed under a stereomicroscope and a scanning electron microscope with coupled x-ray spectroscopy (SEM/EDX) for morphological, textural and geochemical purposes. The microfabric analysis confirms the most recent hypothesis that the main factor responsible for the preservation of the Crato Formation fossils was the influence of the microbial mats on the fossilization process. The microscopic signatures left by the mats occur in the insect’s cuticle and internal parts, represented by micro cracks and wrinkles, micro spheres, filaments and mineralized EPS. These features have been compared in light with deposits containing Quaternary microbial mats in the Vermelha Lagoon and its associated salt pans, in Brazil. Also, we discussed our results with taphonomic data of other mayfly larvae fossils from the Lagerstätten La Huérguina (Spain) and Yixian (China) Formations, specifically in relation to the taphonomic settings regarding these Early Cretaceous ephemeropterans. This study corroborates to the idea of the fundamental role of the microbial mats in the exquisite preservation of the Crato Formation invertebrate fossils, with an approach that permits some fundamental paleoenvironmental and paleoclimatic inferences for the Aptian of the Araripe Basin, in Brazil.

3D macro- and microfabric analyses of Neoproterozoic diamictites from the Valjean Hills, California (United States)

The Cryogenian interval (720–635 Ma) is famous for a rich archive of diamictites, many of which were deposited during glaciations. Classic examples are exposed in the Kingston Peak Formation of the Valjean Hills, near Death Valley (United States), with previous work pointing to multiple glacial cycles in other outcrop belts. Within any glacial period, diamictites are widespread, and in addition, their mechanics of deposition are highly variable. Some are massive in appearance at outcrop or in hand specimens and apparently lack any information that allows their mode of emplacement to be elucidated. Yet, the correct interpretation for deep-time successions in this area is especially important, since it is debated whether the diamictites are either associated with a tectonically driven origin, associated with rifting at the south-western Laurentian margin alongside slope-controlled gravitational mass movement, or predominantly deposited as (sub)glacial diamictites. In this paper, we demonstrate how diamictite texture can be objectively quantified based on clast orientations, at both macroscale and microscale (micromorphology), guiding interpretations. Our method is based on a technique used for Quaternary sediments, by mapping the apparent longest axes of skeleton grains (ranging from fine-grained sand to fine-grained pebbles) in oriented thin sections and reconstructing their microfabric in a 3D space coupled with macrofabric data for each diamictite. In this way, we could identify a bimodal signal in the orientation of the longest axes for each sample. Evidence for shearing and soft sediment deformation supports either subaqueous or subglacial deposition with deformation induced by basal sliding with a paleoflow directed toward the southeast. Our combined approach of micro- and macrofabric analyses can also encourage acquiring accurate fabric data for seemingly structureless diamictites from other deep-time rock archives in an objective manner.

Interplay between crystal-plasticity, fracturing and dissolution-precipitation creep in lower-crustal ultramylonite from hole U1473A, Atlantis Bank, Southwest Indian Ridge

Microfabrics of a gabbroic ultramylonitic shear zone from the Atlantis Bank oceanic core complex have been studied to investigate strain localization processes during exhumation of the lower oceanic crust through detachment faults. The ∼0.5 cm wide ultramylonite band is hosted in a coarse-grained deformed gabbro. Microfabric analysis reveal undulatory extinction, kink bands, bent exsolution lamellae or twins and healed microfractures decorated by new grains within porphyroclasts of diopside and plagioclase in the ultramylonite, as well as the host gabbro close to the border of the ultramylonite. These microstructures suggest that strain was initially accommodated by dislocation glide associated with microfracturing, i.e. high-stress crystal plasticity. Heterogenous fluid influx along microfractures is suggested to have led to localized phase transformation, as amphibole is abundant in the fine-grained matrix of the ultramylonite but not in the host gabbro. Fluid-rock interaction resulted in the development of the ultramylonite via grain-size sensitive fluid-assisted granular flow. Strain localization is concomitant to increasing fluid-rock interaction and the replacement of the former deformed anhydrous mineral assemblage into a hydrated ultramylonite composed of a plagioclase-amphibole mixture. Our study highlights the importance of fluid-assisted metamorphic reaction in decreasing the viscosity of the metastable lower oceanic crust and creating zones of extreme rheological weakening that deform by diffusion creep on the long term. Fluid infiltration plays a key role in facilitating ductile deformation and tectonic spreading through large-scale detachment faults in the hot, lower oceanic crust.

Mudrock Microstructure: A Technique for Distinguishing between Deep-Water Fine-Grained Sediments

Distinguishing among deep-water sedimentary facies has been a difficult task. This is possibly due to the process continuum in deep water, in which sediments occur in complex associations. The lack of definite sedimentological features among the different facies between hemipelagites and contourites presented a great challenge. In this study, we present detailed mudrock characteristics of the three main deep-water facies based on sedimentological characteristics, laser diffraction granulometry, high-resolution, large area scanning electron microscopy (SEM), and the synchrotron X-ray diffraction technique. Our results show that the deep-water microstructure is mainly process controlled, and that the controlling factor on their grain size is much more complex than previously envisaged. Retarding current velocity, as well as the lower carrying capacity of the current, has an impact on the mean size and sorting for the contourite and turbidite facies, whereas hemipelagite grain size is impacted by the natural heterogeneity of the system caused by bioturbation. Based on the microfabric analysis, there is a disparate pattern observed among the sedimentary facies; turbidites are generally bedding parallel due to strong currents resulting in shear flow, contourites are random to semi-random as they are impacted by a weak current, while hemipelagites are random to oblique since they are impacted by bioturbation.

Depositional controlling factors on pore distribution and structure in the lower Silurian Longmaxi shales: Insight from geochemistry and petrology

The present study investigates the lower Silurian (Rhuddanian-Aeronian) Longmaxi Formation of the Sangzhi block South China's Upper Yangtze Platform to establish the role of depositional environment as determinant of pore structure evolution. Research methods include clay grain microfabric analysis using a scanning electron microscope with cathodoluminescence, analysis of total organic carbon and major and trace element concentrations, X-ray diffraction analysis, and porosity measurements. Our results suggest that Rhuddanian shale accumulated in a suboxic environment that experienced relatively high paleoproductivity thereby enriching the sediment in organic matter (OM) and biogenic quartz. These combined to yield sediment characterized by relatively high porosity. OM appears to have been the main contributor to porosity with OM pores comprising the dominant pore type. Aeronian time was marked by an increased supply of terrigenous sediment, especially clay, and establishment of oxygenated bottom conditions. The increased supply of terrigenous sediment and decreased paleoproductivity resulted in sediment deficient in OM resulting in low porosity. Pore spaces in clay minerals are subordinate to interparticle pores, the dominant porosity of Aeronian shale. The transition of organic-rich Rhuddanian shale containing biogenic silica upward into clay-rich Aeronian shale deficient in OM is accompanied by a change in porosity and dominant pore type that reflects the changing depositional environments of these deposits. The up-section evolution of sedimentary environment reflects the effects of varying water depth and degree of water exchange with the open ocean related to sea level fluctuation. Documented changes in porosity related to depositional environment have a direct influence on shale gas reservoir capacity. Specifically, a suitable sedimentary environment creates the conditions for high-quality shale formation.

Wind erosion control using carboxymethyl cellulose: From sand bombardment performance to microfabric analysis

Wind erosion is a global environmental challenge. Its financial adverse effects on the societies as well as human health have demanded scientists to search for new solutions. The common mitigation techniques have high CO2 footprints and are not sustainable. One of the environmentally friendly techniques for this purpose is the use of biopolymers, however, their production is costly. The present study proposes the use of carboxymethyl cellulose for wind erosion control. Since carboxymethyl cellulose can be obtained from various waste sources, it is not only a sustainable treatment material but also an economical one. Therefore, a comprehensive experimental study inclusive of wind erosion experiments, wet-dry cycles durability assessment, analysis of partial treatment, and soil microfabric was conducted on treated samples. In addition to aerodynamic lift experiments in the wind tunnel, samples were exposed to sand bombardment to mimic the saltation phenomena. Furthermore, the performance of guar gum, a widely used biopolymer, was compared against carboxymethyl cellulose. Results of sand bombardment indicate that a relatively low concentration of 0.3% carboxymethyl cellulose can effectively reduce the soil loss, whereas, the sample treated with 0.3% guar gum experienced a considerable soil loss. Durability tests showed that the guar gum generated soil crust was completely destroyed at the beginning of 10th cycle while the crust produced by carboxymethyl cellulose could diminish the soil loss to less than 3% highlighting its adequate durability. Finally, to mimic the field condition, samples with untreated parts were studied which underscored the effective performance of the carboxymethyl cellulose treatments.

Stable isotope geochemistry of pedogenic carbonates in calcareous materials, Iran: a review and synthesis

Abstract The stable isotopic composition of pedogenic carbonate forms in equilibrium with environmental parameters and, thus, records palaeoenvironmental signals. The aims of this study are to synthesize available data on the stable isotopic composition of Quaternary pedogenic carbonates in calcareous parent materials of Iran and to decipher paleoenvironmental implications of the isotopic data for the country. Isotopic composition and microfabric of pedogenic carbonates in 18 pedons in both gravelly (calcareous alluvium in central Iran) and non-gravelly deposits (calcareous loess in northeastern Iran) have been investigated. The results indicate that in limestone-derived soils of central Iran in situ weathering of calcareous pebbles is a major source of Ca for genesis of the carbonates, and carbonate features consist of micritic calcite crystals. In the loessic soils of northeastern Iran, pedogenic carbonates show a dominance of nodule morphology and are classified as orthic nodules. Microfabric analysis reveals that most of the carbonates have not been altered by diagenetic processes, especially the Holocene carbonates, and are suitable for isotopic study and palaeoreconstructions. In limestone-derived soils within the arid region of central Iran, the δ 18 O and the δ 13 C values of carbonates indicate their enrichment due to the effects of evaporative water loss, a decline in plant density and the entrance of atmospheric CO 2 into the soils. In semi-arid ecosystems of central and northeastern Iran, most of the Holocene carbonates have formed in equilibrium with the ambient environment and are suitable for palaeoenvironmental reconstructions. The combination of carbon and oxygen isotopic data demonstrates the dominant role of climate in determining the δ 13 C values of carbonates. There is a strong relationship between the δ 13 C values of carbonates and rainfall, and between O isotopes and aridity indices. Stable isotope patterns in Holocene soils appear to provide data for models that can then be used to interpret the many localities where Pleistocene-aged soils and associated carbonate exist.

Tracing lake pollution, eutrophication and partial recovery from the sediments of Windermere, UK, using geochemistry and sediment microfabrics

Many lakes undergo anthropogenically driven eutrophication and pollution leading to decreased water and sediment quality. These effects can enhance seasonally changing lake redox conditions that may concentrate potentially toxic elements. Here we report the results of a multi-method geochemical and sediment microfabric analysis applied to reconstruct the history of cultural eutrophication and pollution of the North and South Basins of Windermere, UK. Eutrophication developed from the mid-19th to the earliest 20th centuries. Enhanced lake productivity is indicated by increased sedimentary δ13C, and increased pollution by a higher concentration of metals (Pb, Hg, and As) in the sediment, likely enhanced by incorporation and adsorption to settling diatom aggregates, preserved as sedimentary laminae. In the South Basin, increasing sediment δ15N values occur in step with Zn, Hg, and Cu, linking metal enrichment to isotopically heavy nitrate (N) from anthropogenic sources. From around 1930, decreases in Mn and Fe-rich laminae indicate reduced deep-water ventilation, whereas periods of sediment anoxia increased, being most severe in the deeper North Basin. Strongly reducing sediment conditions promoted Fe and Mn reduction and Pb-bearing barite formation, hitherto only described from toxic mine wastes and contaminated soils. From 1980 there was an increase in indicators of bottom water oxygenation, although not to before 1930. But in the South Basin, the continued impacts of sewage are indicated by elevated sediment δ15N. Imaging and X-ray microanalysis using scanning electron microscopy has shown seasonal-scale redox mineralisation of Mn, Fe, and Ba related to intermittent sediment anoxia. Elevated concentrations of these metals and As also occur in the surficial sediment and provide evidence for dynamic redox mobilisation of potentially toxic elements to the lake water. Concentrations of As (up to 80 ppm), exceed international Sediment Quality Standards. This process may become more prevalent in the future with climate change driving lengthened summer stratification.

Microstructural analyses of the Najd Fault System in Midyan Terrane, NW Arabian Shield, Saudi Arabia

This article discusses the microstructural indicators exposed within the Precambrian Ajjaj–Qazaz–Hanabiq (AQH) shear zones in order to determine the shear sense. The AQH shear zones are located in the southern sector of the Midyan Terrane in the northwestern part of the Arabian Shield. Structural and microfabric analyses along these shear zones indicate a top-to-the-NW sense of shear. Exposures in this area comprise elongated ridges of ophiolitic rocks, arc metavolcanics and sporadic arc-related Neoproterozoic intrusions. They are unconformably overlain by post-amalgamation molasses-type sediments and were finally intruded by late to post-tectonic intrusions. Structural field relations along with microfabric analyses and overprinting relationships provide evidence of the sense of movements and phases of deformation. There were at least four phases of deformation. The oldest (Hijaz) D1 structures are rare and are represented by foliation striking WNW to E–W and by F1 interfolial folding with an axial plane striking ENE–WSW and moderately plunging fold axes trending WSW. D2 microstructures are also scarce and represented by foliation and (F2) folding directed in NNW to N–S trends parallel to the Nabitah suture. However, they are commonly deformed and overprinted by the Najd Fault System that formed during the D3 deformational phase. The D3 deformational phase is prominent, and it developed during the propagation of the NW transpressional shearing of the Najd Fault System. These shear zones could be identified in this region with the help of the following microstructures: sigmoidal structures, mantled porphyroclastic structures, asymmetric boudins, tension gashes, fish-shaped structures, oblique foliation, bookshelf sliding structures, slickenlines, C–S tectonites and fracturing along the AQH shear zones. Shear sense analysis of microstructural markers at the microscopic scale, supported by field observations and map-scale structures, implies that both the Ajjaj and Qazaz shear zones display top-to-the-NW or top-to-the-WNW slip. The shear sense indicators along the Hanabiq shear zone show top-to-the-NNE slip. These inconsistencies in their shear sense and their orientations were caused by the exhumation of high-grade gneisses, which distorted the shear trend to the north in the Hanabiq shear zone. All the previous structures are overprinted by the brittle D4 deformational phase in the form of microfaulting and fracturing.

Tectonic implication of quantitative micro-fabric analyses of quartz c-axis development within the Tutak gneiss dome, Zagros hinterland fold-and-thrust belt

Quartz c-axis fabrics, finite strain, and kinematic vorticity number analyses were used to investigate deformation characteristics of the Tutak gneiss dome located in the eastern edge of the basement-involved Zagros hinterland fold-and-thrust belt. The opening angle of the quartz c-axis fabric patterns was used to estimate deformation temperatures, which suggest greenschist-to-amphibolite-facies conditions (430 ± 50 °C to 660 ± 50 °C). The kinematic vorticity number varies from 0.6 to 0.95 (mean kinematic vorticity number of 0.81), which indicates that the metamorphic rocks of the Tutak gneiss dome experienced a sub-simple shear regime by contribution of both pure shear (39%) and simple shear (61%) deformation components. NW–SE-striking foliations have commonly SW and NE dips accompanied by mostly NW–SE-plunging and subordinate NE–SW-plunging-stretching lineations. These indicate more extension along the NW–SE trend to produce a prolate elliptical wedge shape in map view. Structural and vorticity analysis suggests that the Tutak gneiss dome has experienced earlier dome formation followed by thrust stacking rather than the two deformations occurring synchronously, and a protracted progressive deformation through a decreasing temperature regime.

Nature of dispersed organic matter and paleoxygenation of the Campano-Maastrichtian dark mudstone unit, Benin flank, western Anambra Basin: Implications for Maastrichtian Trans-Saharan seaway paleoceanographic conditions

The Campano-Maastrichtian age is an important time in the geological evolution of Nigeria as it marked the re-establishment of the Trans-Saharan seaway that was broken due to Santonian inversion tectonics. In this paper, we conducted a high-resolution investigation of the dark mudstone unit of the Campano-Maastrichtian Mamu Formation exposed in 4-outcrops in the western segment of the Anambra basin, Nigeria, using multidisciplinary tools involving geochemistry, palynofacies, and microfabric analyses. Our objectives were to determine the nature of organic matter preserved in the sediments and the paleo-oxygenation conditions of the Trans-Saharan seaway. Our findings reveal that the Trans-Saharan seaway was of low salinity, characterized by the dominance of terrestrial organic matter in the more proximal marsh and bay sub-environments (organic facies C and CD) and mixed terrestrial – marine organic matter (organic facies BC and C) in the more distal central basin. Bottom water paleo-oxygenation was predominantly oxic. However, palynofacies and microfabric evidences as well as inferences from Fe-TS-TOC relationship suggests pyrite formation occurred in at least two phases. The first phase of syngenetic to early diagenetic pyrite formation, which was due to bacterial sulphate reduction that occurred in the anoxic zone below the sediment water interface, whereas secondary (late diagenetic) pyrite growth which formed the bulk of pyrite preserved occurred at the base of the bottom water. Furthermore, we hypothesize that pyrite formation occurred faster, and was better preserved in the central basin than in the other sub-environments. This is attributed to the presence of more reactive organic matter (marine palynomorphs), higher salinity (more sulphate), mineralogy (higher clay content), and microfabric (thinner lamination with low degree of bioturbation).

Emplacement levels and pre-existing structures control mechanisms and host rock interactions of three granitic plutons, western Arabian Shield

Field relations, structural measurements, and microfabric analyses are used to explain the emplacement mechanisms of granitic plutons in the Asir terrane. These circular-to-elliptical granitic intrusions are prominent plutons scattered across the Arabian–Nubian Shield. Various structural and tectonic fabrics affecting three plutons in the western Arabian Shield encouraged studying the effect of pre-existing structures and emplacement levels on the emplacement mechanisms and related solid-state deformation. Two of these plutons, called Al Sayl and Um Ar Raka, are circular and enveloped by ductile or brittle solid-state deformation. Pre- and post-emplacement tectonic fabrics controlled the distribution and superposition of the solid-state deformation. Lateral expansion of magma during the emplacement of these plutons induced significant radial shortening that was associated with either high or low heat transfer into the host rocks and developed brittle or ductile solid-state deformation, respectively. Heat transfer was controlled by the emplacement level and magma pressure of each pluton. The third, oval-shaped Al Hawiyyah pluton was structurally controlled, where magma was emplaced first into twofold hinges on both sides of a thrust fault under low-to-intermediate differential stress. Syn-emplacement shortening was accommodated mainly by upward and lateral magma flows. Continuous magma flow caused uplift of the pluton and its surrounding host rocks and developed stretching structures in these rocks.

Chemical, mineralogical and textural properties of the Kope Formation mudstones: How they affect its durability

The Kope Formation is a rock unit that extends over southwest Ohio and adjacent areas of Kentucky and Indiana, USA. It is characterized by meter-scale cycles of mudstones and limestones where thick mudstone is sandwiched between thin limestone layers. Kope Formation mudstones are less durable than the mudstones in the area. They cause abundant slope failures in areas of their outcrops. This study evaluates chemical, mineralogical and textural properties of Kope mudstones to determine why it is less durable. The Two-cycle slake durability index (ID2) was measured on thirty mudstones belonging to three consecutive cycles of the Kope Formation. Chemical composition and mineralogy of Kope mudstones do not show significant relationship with the ID2. Instead, a relationship to fabric was found. Kope mudstones with two main fabric types were observed in the field; laminated mudstones and non-laminated mudstones. Detailed micro-fabric analysis on Kope mudstones was done using the scanning electron microscope. Laminated mudstones with turbostratic fabric showed higher ID2 and lower moisture content than the non-laminated mudstones with matrix fabric. The difference in the ID2 of laminated mudstones and non-laminated mudstones is statistically significant. Both mudstone types exist in many cycles of the Kope Formation. Thus, core samples should be selected from both types for preliminary tests in geotechnical practice, to obtain more accurate estimations of the geotechnical properties of the Kope mudstones. The results and data interpretations of this study can be correlated with the mudstones and shale in different areas of the world. Specifically, a great attention should be paid to the macro and micro textural properties of any rock type regardless of their classification.

Deformation-induced silica redistribution in banded iron formation, Hamersley Province, Australia

The formation of banded iron formations (BIF) remains controversial despite their potential to provide key information on Precambrian atmospheres and hydrospheres. It is widely agreed that BIF are chemical sedimentary rocks comprising alternating layers of iron oxides and chert formed from poorly known precursor phases. Many models address the chemical transformation of such precursor iron oxide phases into BIF during compaction and diagenesis. However, the formation of chert and the influence of physical forces in this process have received less attention.Microstructural analysis of BIF from the Hamersley Province (Western Australia) reveals that significant amounts of silica were redistributed by dissolution-precipitation creep during both diagenesis and regional-scale deformation. This physicochemical process led to silica remobilisation and volume loss by stress-induced dissolution of microcrystalline quartz in an aqueous fluid. The dissolved solid phase was transported by diffusion and fluid flow along grain boundaries or within available porosity and then reprecipitated in low-pressure zones, leading to local volume increase. These processes were further enhanced by rheological contrasts between different minerals, resulting in significant variations of chert band thickness.Microstructural observations combined with quantitative microfabric analysis reveal domains of crystallographic preferred orientations (CPO) in quartz grains within chert layers. The CPO fabrics record strain regimes (e.g., pure and simple shear, extension and shortening) that modified quartz aggregates by dissolution-precipitation creep, providing new insights into the metamorphic and deformation history of BIF. We document microstructures that indicate that non-coaxial deformation was active during diagenesis and subsequent deformation of the Hamersley Province BIF. Further, relatively undeformed chert layers may have been similarly affected by significant amounts of dissolution-precipitation creep, providing a possible explanation for the destruction of primary features.We show that different generations of chert alternate on a micro-scale, suggesting that the role of deformation has been underestimated in many aspects of BIF research. The discovery of dissolution-precipitation creep in low-grade metamorphic BIF provides a potential link between the physical and chemical processes that occur within these rocks after deposition. In the Hamersley Province, dissolution-precipitation creep was active from the compaction or diagenesis stage and probably operated during all major extensional and compressional deformation events. We also suggest that chert layers without CPO record the least overprint by diagenesis, and as the most pristine form of BIF should be targeted for future paleo-environmental research.

Sedimentology, petrography and early diagenesis of a travertine–colluvium succession from Chusang (southern Tibet)

The Chusang travertine is situated in southern Tibet at an altitude of ~4200m asl. in a cold-arid, periglacial environment and is characterized by interbedding of hydrothermal carbonate with colluvium. Here we present sedimentological and petrographical data to elucidate the depositional environment and sedimentary processes responsible for hydrothermal carbonate precipitation and early diagenetic alteration as well as clastic sediment accumulation and provide initial 230Th/U ages to constrain the time-depth of this travertine–colluvium succession.Three main travertine lithofacies have been identified: 1) a dense laminated lithofacies, 2) a porous layered lithofacies and 3) an intraclastic lithofacies that results from erosion of pre-existing hot spring carbonate. The colluvium is composed of cohesive debris flow layers that derived from mass-wasting events from the adjacent hillslopes. Micro-fabric analyses suggest that dense laminated travertine forms via rapid calcite precipitation from hot spring water seasonally subjected to severe winter cooling, while porous layered travertine results from seasonal dilution of hot spring water with rain water during the summer monsoon months, which in turn stimulates biological productivity and gives rise to a porous summer layer. Early diagenesis in the form of recrystallization and extensive formation of pore cements is common in the Chusang travertine, but never eradicates the original crystal fabrics completely.The sedimentary architecture of the deposit is conditioned by (i) the gently dipping (~10°) pre-existing terrain on which hot spring water is discharged from multiple travertine mounds causing laterally extensive travertine sheets to precipitate, and (ii) the adjacent much steeper (up to 30°) periglacial hillslopes that are the source area of repeated debris flows that accumulate on the travertine surface. The resulting travertine–colluvium succession has a total thickness of ~24m and 230Th/U dating suggests that the base of this succession has a minimum age of ~486ka, while the upper part (top-most ~8m) of the succession started accumulating in the earliest Holocene. We hypothesize that hot spring activity (and thus travertine precipitation) and the occurrence of debris flow events has a climatic nexus, i.e. are both triggered by phases of enhanced Indian summer monsoon.

Re-examination of a supposed ‘archaeocyath’ specimen from the Hyangsanni Formation, Okcheon Basin, Korea

A supposed ‘archaeocyath’ specimen from the Hyangsanni Formation of the Okcheon Basin (Okcheon Metamorphic Belt), Korea, reported about 50 years ago, has previously been used to constrain the depositional age of the metasedimentary succession in the basin. However, a re-examination of the specimen reveals that it lacks the typical morphology of archaeocyaths. A detailed petrographic and microfabric analysis of the specimen, combined with a review of outcrop structures and microstructures of the dolomitic marble in the Hyangsanni Formation indicates that the supposed archaeocyath- like shape of the specimen was caused by deformations and metamorphic reactions. The Cambrian age of the Hyangsanni Formation should therefore be re-examined on the basis of unequivocal palaeontological evidence and/or radiometric dating.

A Comparative Analysis of the Microfabrics of Surface Horizons and Desert Varnish in Extremely Arid Soils of the Mojave (USA) and Trans-Altai Gobi (Mongolia) Deserts

The mineralogical composition of coarse fraction and characteristic features of the micro- and submicrofabrics and chemical composition of desert varnish on gravels of desert pavements and the underlying vesicular crust soil horizons were studied in the extremely arid soils of the Mojave (USA) and Trans-Altai Gobi (Mongolia) deserts. A set of common diagnostic features of elementary pedogenetic processes was identified in the automorphic desert soils developed on ancient (70–90 ka) piedmont plains composed of alluvial deposits with the high content of red-earth clay. The results of this study attest to the long and complicated history of the extremely arid soils with alternation of the humid and arid phases of pedogenesis reflected in a specific combination of textural (clay-illuvial) and carbonate pedofeatures and in the distribution patterns of iron, manganese, titanium, and barium in different layers of the desert varnish. The chemical composition of the latter did not depend on the mineralogical composition of the underlying substrates and was formed with active participation of soil microorganisms. This allowed us to conclude about the polygenetic (accretionary–microbiological) nature of desert varnish.

Comparisons between macro‐ and microfabrics in a pebble‐rich, sandy till deposited by the Cordilleran Ice Sheet

It is standard practice to measure particle fabrics in glacial studies to infer palaeo‐ice flow directions and processes of till formation but few studies examine the relationships between particle fabrics at different (i.e. the macro‐ and micro‐) scales. This knowledge is critical to inform the utility of the methods and limitations of the associated interpretations. Micro‐ (sand grain) and macro‐ (pebble) fabrics of pebble‐rich, sandy subglacial till (Kamloops Lake till) deposited by the Cordilleran Ice Sheet, south‐central British Columbia, were compared to assess their similarities and differences, and therefore their utility for understanding subglacial processes. Before comparisons were made, the data were tested for robustness by assessing various controls (e.g. sampling face orientation, number of particles measured, statistical variation resulting from sampling effects, particle shape, size and concentration) on particle fabrics. A new method of microfabric analysis was applied that involves the identification and delineation of distinct clusters of similarly orientated sand grains in order to compare them with macrofabrics and inferred ice‐flow directions. The results show that microfabrics, on their own, are an unreliable indicator of ice‐flow direction in Kamloops Lake till in the study area and should not be used as a substitute for macrofabric data, as they probably record late‐stage microscale strain patterns and pore‐water flow in addition to till deposition and deformation by overriding ice. We suspect that this would also be the case for coarse‐grained till elsewhere. Our findings suggest that till microfabric interpretations should always be made after assessing corresponding macrofabric data alongside sedimentological and structural observations.

Can hydrocarbon contamination influence clay soil grain size composition?

Microstructural (fabric, forces and composition) differences due to hydrocarbon contamination in a clayey soil (glacial till) were studied using scanning electron microscopy (microfabric analysis) and sedimentation bench test (PSD). Uncontaminated and contaminated glacial till from NE Poland (in the area of a fuel terminal) were used for the study. Because the contaminated samples exhibited lower electric charge, the electrostatic repulsive forces in the contaminated samples were much lower than in uncontaminated samples. All sedimentary-based particle size test results gave similar results and were inconclusive to find difference between contaminated and uncontaminated samples. Mastersizer PSD data show smaller aggregates in contaminated in comparison to the uncontaminated sample. The present test's finding may support earlier microstructure investigation results, where diesel-contaminated clay soil shows more open porosity, consisting consequently of looser aggregates, which were easier dispersed in water during Mastersizer measurements than the more coherent uncontaminated clay sample. This in effect increased the percentage of finer than 0.1mm grain fraction within the broad diameter range in contaminated samples results, in comparison with the uncontaminated sample results.

Micromorphological evidence of liquefaction, injection and sediment deposition during basal sliding of glaciers

The sliding of an ice mass over its bed represents one of the main mechanisms for the forward motion of glaciers and ice sheets, facilitated by the periodic introduction of meltwater along the ice-bed interface or regelation of the overriding ice. In the geological record, however, physical evidence of this process having occurred beneath former ice masses is apparently limited. This paper presents the results of a detailed micromorphological study of thinly stratified subglacial tills exposed at two sites: (i) Galmis in Switzerland and (ii) Plumpe Farm, near Gretna in SW Scotland. The stratification within these tills comprises alternating layers of massive to weakly foliated diamicton and variably deformed (folded, faulted) laminated silt and clay. Microtextural and microstructural evidence is interpreted in terms of repeated phases of basal sliding as the ice overrode a soft-sediment bed. Elevated meltwater contents/pressures encountered immediately prior to, and during basal sliding promoted localised liquefaction within the bed. Decoupling of the ice mass from its bed enabled the injection of the liquefied diamicton along the ice-bed interface and/or into the laminated sediments immediately adjacent to this boundary. The laminated silts and clays record the settling out of fines (clay, silt) from meltwater trapped along the ice-bed interface after an individual phase of basal sliding has ceased. Injection of till into the locally water saturated silts and clays resulted in partial liquefaction and incomplete mixing of these fine-grained sediments with the diamicton. Density contrasts between the two liquefied sediments led to the development of a complex ‘vinaigrette-like’ texture comprising rounded to irregular till pebbles within a matrix of variably homogenised silty clay. Recoupling of the ice with its bed led to localised folding and thrusting within the laminated silts and clays, hydrofracturing and injection of a network of sand-filled veins, and the imposition of a variably developed clast microfabric in the diamicton layers. Analysis of the clast microfabrics indicates that the intensity of these fabrics is highly variable reflecting the variation in the intensity of deformation imposed by the overriding ice.