Sedimentary Geology最新文献

The grain size distribution is an important property of all clastic sediments: it determines their mechanical properties and is directly related to their mode of transport and origin. Therefore, the accurate measurement and comparability of grain size data are important. The former has been studied in detail in literature and has been demonstrated to be significantly instrument-dependent, while the latter has not been given the same attention. The current study examined in detail a large set of grain size data measured on a single clay formation, the Oligocene Boom Formation, from which the large influence of sample preparation on the grain size distribution can be inferred. Especially the use of sonication to disintegrate silt-sized aggregates was found to be of a particularly big influence on the measured distributions. As a way to still be able to valorize non-comparable datasets, a statistical conversion procedure was introduced based on partial least squares regression in a compositional data space. The converted distributions follow the stratigraphical trends in grain size expected in the Boom Formation, while also being well correlated to hydraulic conductivity measurements performed on Boom Clay samples of similar depths. This is a strong indication that the conversion was successful. In the future, this approach can be used as a tool to harmonize any combination of compositional datasets, not just limited to grain size data, allowing a valorization of the maximal amount of data.

In sedimentary geoscience, the particle size distribution (PSD) of a sediment has a fundamental effect on a sediment's ability to be entrained, eroded, and deposited. Therefore, it is crucial to accurately measure the PSD of sediments. Several laboratory-based methods of particle size analysis are commonly employed in geoscience; however, each method is based on different principles and the comparison of data from one technique to another is challenging. In this study, we have compared the output of four commonly-used laboratory-based techniques: Laser Particle Size Analysis (LPSA), optical point counting, 2D automated image analysis, and X-ray Computed Tomography (XCT). Each technique has been used to measure eight samples of spherical silica particles, all prepared with known particle size ranges. Spherical particles have been used to minimise the effects of variable sorting and particle shape on data output. Here we have compared the differences between the measured PSD and descriptors of each PSD, showing that, at small particle diameters (<150 μm), all techniques agree. However, at particle diameters >150 μm, LPSA overestimates the size of particles, due to limitations in the way that particle diameter is calculated by this technique. In contrast, 2D automated image analysis and optical point counting underestimate the diameters of particles, due to stereology (e.g., the effect of slicing particles during thin section preparation). Results from XCT analyses have the lowest values of sorting (range of measured particle diameters) and are therefore the most tightly constrained. In addition, XCT is the only 3D analysis method, allowing particle shape, orientation, and intraparticle porosity to be measured for a volume of material. We therefore conclude that XCT is the most accurate way to determine a grain size distribution in sediments.

Palaeosols are common in sedimentary successions of continental origin, and notably they comprise the majority of the thickness of some accumulated successions of fluvial origin. Yet, detailed investigation of palaeosols and evaluation of their palaeoenvironmental significance are not routinely undertaken in detail in many sedimentological studies. A careful analysis of palaeosols may, however, reveal that sedimentary units, which appear similar if based solely on the facies analysis, indeed show strongly distinct palaeoenvironmental and depositional characteristics.

This is the case of the upper portion of the Bauru Group, a 100–190 m-thick Maastrichtian red sandstone unit of fluvial origin, present over an area of c. 180,000 km² in south-eastern Brazil. In this study, the palaeosols of this unit, which constitute 25–92 % of the succession by thickness, are used to decipher palaeoenvironmental climate conditions, sediment source areas, and relationships between pedogenic and depositional processes. Through the combined study of macroscopic, micromorphological, and geochemical aspects of the palaeosols and of facies analysis of the deposits, the upper portion of the Bauru Group succession is separated into three sectors: north-western, north-eastern, and south-eastern. Although these three areas are all characterised by similar lithology types and lithofacies, indicative of deposition in alluvial systems, the palaeosol analysis highlights that they were each characterised by different climate, different clastic source areas and different dynamics and interaction of the pedogenic and sedimentary processes. This research reveals the critical significance of the palaeosols for discriminating otherwise apparently similar depositional units.

Widespread calcite and quartz pseudomorphs, interpreted as originally gypsum crystals, occur within hemipelagic calci-mudstone accumulated in subtidal offshore environments in the broad marine foreland basin developed on the southern flank of the Ouachita–Alleghanian–Variscan Orogen during mid-Carboniferous times, which acted as a marine corridor connecting the Panthalassa and Palaeo-Tethys Oceans during the Mississippian, and progressively narrowed during the assembly of Pangea. In this study, 67 outcrops of radiolaria-bearing calci-mudstone deposits that contain calcite and quartz pseudomorphs located in northern Spain and southern France were studied to constrain the gypsum spatial distribution and sedimentological features. The recognised microfacies indicate intrasediment gypsum precipitation, accompanied by less abundant bottom-grown precipitates and gypsum cumulates, in extensive offshore, probably several ten to a few hundred metres deep, basinal environments. Gypsum precipitation took place during a short-lived temporal episode during the early Bashkirian time (Voznesenkian), which can be correlated, on the basis of benthic foraminifera, with the coastal (inter- to supratidal) gypsum evaporites identified in NW Africa (Tindouf and Reggan successions in Morocco and Algeria) that would represent the shallow-water counterparts. The occurrence of gypsum precipitates both in offshore hemipelagic calci-mudstones of the Variscan foreland basin and in inter- to supratidal environments of the epeiric Sahara Platform indicates that hypersaline conditions affected vast marine areas, roughly coinciding with the estimated age of closure of the Panthalassa and Palaeo-Tethys marine connection. Therefore, the studied succession represents the trace of a basin-wide evaporitic episode extending for hundreds of kilometres driven by foreland basin restriction, mid-Carboniferous sea-level fall and arid climate. This study provides new insights for the interpretation of gypsum precipitates in offshore marine environments encountered in the Phanerozoic and whose genesis is poorly understood.

Datangpo-type manganese ores which originated in the Cryogenian Datangpo Formation, are overlaid and underlaid by Nantuo diamicite and Tiesi'ao silistones, respectively. A thorough understanding of the manganese mineralization process is currently limited by the continued uncertainty regarding the metallogenic redox condition of the Mn-bearing rock series. This study employed statistical analysis of pyrite framboid sizes and iron isotope (δ⁵⁶Fe) to determine the redox conditions and constrain the mineralization process of Datangpo-type manganese ores in South China, considering the Gaolou in Chongqing, Yanglizhang in Guizhou, and Minle and Zhenxing in Hunan as examples. The results showed that Tiesi'ao siltstones mainly developed relatively larger diameter (7.29–7.68 μm) framboidal pyrites, whereas mudstones in the Datangpo Formation developed relatively smaller diameter (2.63–5.56 μm) framboidal pyrites. Furthermore, manganese ores produce non-framboidal pyrites. In three profiles, framboidal pyrite concentrations were found to be negatively correlated with Mn contents. These characteristics suggested that siltstones, mudstones, and manganese ores were deposited in oxic-anoxic, dysoxic-euxinic, and oxic-dysoxic conditions, respectively. The δ⁵⁶Fe of whole rocks (δ⁵⁶Fe_WR) exhibited a range from −0.73 ‰ to +0.48 ‰ (average of −0.32 ‰), whereas pyrite (δ⁵⁶Fe_Py) varied from +0.03 ‰ to +0.83 ‰ (average of +0.36 ‰). Furthermore, δ⁵⁶Fe_Py in manganese ore (average = +0.27 ‰) were lighter than those in mudstone (average = +0.49 ‰). The results of this study demonstrated that oxidation deposition played a role in pyrite formation, with manganese ores exhibiting more oxic conditions compared to mudstones. The findings of this study suggested that idiomorphic pyrite formation was similar to that of manganese ore, in which Mn precipitated in the form of manganese (hydro)oxides under oxic seawater, and then converted into rhodochrosite under anoxic sulfidic diagenetic conditions, with the involvement of microbes. This research has the potential to enhance comprehension regarding redox conditions and the regulation of sedimentary manganese ores via redox transformation.

The Wufeng–Longmaxi shales, which were deposited during the Ordovician–Silurian transition in the southern Sichuan Basin, exhibit distinct lithofacies transitions. The shift from organic-rich siliceous shale to organic-poor mixed shale and argillaceous shale is attributed to the influences of the Hirnantian glaciation and the Kwangsian Orogeny. Nevertheless, debates persist regarding the mechanisms underlying these significant lithofacies transformations. This study employs sequence stratigraphy, petrology, and geochemical analysis of key wells to unveil crucial insights. The findings indicate: 1) the Wufeng Formation to the Long-1 Member could be divided into two third-order sequences; 2) the waterbody underwent a transition from anoxic and strongly restricted conditions to oxic and moderately restricted conditions; and 3) the trend in formation thickness changed from an E–W orientation to a NE–SW orientation. The results suggest that considerable sediment condensation and anoxia resulting from glacier melting in the early Rhuddanian played a pivotal role in the formation of organic-rich siliceous shale. During the Middle Rhuddanian–Aeronian, the accelerated uplift caused by the Kwangsian Orogeny offset the transgression induced by glacier melting, leading to shallow water conditions and increased terrigenous influx. This diluted the organic matter content in the sediment, and subsequent turbidity currents altered sediment composition, driving the lithofacies transformation into organic-poor mixed shale and argillaceous shale. The study posits that the sedimentary differentiation in the southern Sichuan Basin during the Ordovician–Silurian transition is linked to the initiation and progression of the far-field effects of the Kwangsian Orogeny.

The Salitral de La Vidriera (SDV; 38°44′S, 62°34′W) once occupied the innermost section of a funnel-shaped estuary before its isolation due to sea level drop ∼3000 years BP. It currently is a continental saltpan detached from the Bahía Blanca estuary, having no marine connection, and acting as a hypersaline evaporitic environment. This study provides a first-hand characterization of the epibenthic microbial mats that biostabilize surface sediments in the saltpan, and the environmental dynamics and physical deformation agents behind the formation of modern Microbially Induced Sedimentary Structures (MISS). The microbial community in the hypersaline mats is dominated by cyanobacteria of filamentous (taxonomic order Oscillatoriales) and coccoid morphologies (orders Chroococcidiopsidales and Pleurocapsales). The MISS comprise rolled-up mats, flipped-over mats, folds and wrinkles, reticulate surfaces, gas domes, pinnacles, and desiccation cracks; their genesis is linked to the presence of water that either promotes microbial growth or creates hydrodynamic deformation structures. Gypsum crystals were found in surface and subsurface sediments plausibly linked to a high rate of rainwater evaporation and aided in their preservation by the microbial mat lattice. Furthermore, carbonate precipitation in the form of peloids was identified through petrographic analysis, despite the siliciclastic nature of sediments. The occurrence of modern MISS in this transitional environment, currently under continental influence, is discussed and contrasted with modern peritidal environments. Descriptions from this study, contextualized in a modern depositional setting, may be useful in the study of fossil counterparts and paleoenvironmental reconstructions.

Hydrofracture systems have been described in glacial sediments for almost a century and accelerating research since the 2000s, boosted by the advent of micromorphological techniques applied to glacial deposits, led to a significant rise of studies using paleo-hydrofractures (and their fills) as a new proxy for reconstructing glacial processes and environments. This review covers the great diversity of hydrofracturing context (subglacial, marginal, proglacial) and physical characteristics (at macro- to micro-scale) of hydrofracture systems and their fills based on a compilation of published and unpublished field-based data from both Quaternary and pre-Quaternary glacial sediments.

The text covers (1) the fundamental concepts of hydrofracturing processes including causes and triggers of overpressure in glacial environments as a preamble, (2) the physical characteristics of hydrofracture systems in glacial environments and (3) the parameters controlling these physical characteristics, (4) the characteristics of hydrofracture-fills, (5) the processes of sediment injection inferred from fill characteristics and (6) the wider implications of hydrofracturing and injection processes on paleoglaciological reconstructions. Future research perspectives, including the need for modeling of hydrofracture network in glacial environments, are finally discussed as it will certainly allow the role of ice thickness, slope and speed, meltwater input and host sediments in governing the architecture of hydrofracture systems to be untangled.

The geometry of unconformities carved by deep time ice sheets is often obscured and restricted by discontinuous exposure, or outcrop conditions that do not readily permit the examination of glacial unconformities (for example, steeply dipping strata). Here, we present new uncrewed aerial vehicle (UAV) data from selected outcrops across northern, central and southern Namibia to shed further light on the nature of the basal Dwyka Group unconformity. This includes the onlap relationship of basal diamictites onto the Gomatum palaeo-fjord system in northern Namibia, highly complex mapped ice flow orientations elsewhere in the northern Kaokoveld, previously undiscovered grooves along the Fish River area, and a set of subglacial grooves along the border with South Africa along the Orange River. In the latter two cases, photogrammetric methods integrating orthophotos and digital elevation models reveal the presence of subglacial grooves. Furthermore, subglacial grooves often show different orientations to striations and fabrics measured in overlying diamictites, raising fresh questions about the nature of small-scale flow variations beneath Late Palaeozoic ice sheets.

Many Permian desert tracks are found in formations dominated by rather homogeneous aeolian quartz arenites. This raises questions around how they got preserved. Here we test the hypothesis that strong palaeoenvironmental controls affect style and quality of footprint preservation in Permo-Triassic desert settings. To answer this, several examples of tracks and trackways from Moray, Scotland, are described in the context of their host sedimentary successions. We then discuss petrographic clues in the specific track-bearing layers with regard to taphonomy. Two key sections were logged and sampled at Hopeman Beach: (i) Hopeman Coastal Section A, being a site from which tracks have previously been recovered; and (ii) Hopeman Coastal Section B, a section still exhibiting several in-situ tracks. Tracks were also examined on the surfaces of metre-scale quarried blocks within Clashach Quarry. Logging was also undertaken at quarries in Quarrelwood near Elgin. Collected samples were examined optically and with a scanning electron microscope. Hopeman Coastal Section A exhibits convolute bedding best interpreted as dewatering structures; a pustular bed that could be linked to growth of evaporite crystals impinging on a sediment-binding microbial mat; adhesion ripples formed by dry, wind-blown sand sticking to a wet or damp surface; and laterally continuous pebble layers that are the result of ephemeral sheet floods. The oscillation-rippled layer from which NMS footprint specimen G.1997.60.1 was extracted exhibits a halite cement and petrographic evidence for re-worked halite, and these rippled sediments were most likely deposited in an interdunal lake. Hopeman Coastal Section B similarly exhibits metre-scale planar cross beds and occasional coarser-grained lag deposits that are consistent with aeolian dunes that were episodically inundated by sheet floods. Samples containing halite and lesser amounts of gypsum or anhydrite were collected from the same layer as the in-situ Hopeman Coastal Section B tracks. Metre-scale planar cross-bedded quartz arenites of Cutties Hillock quarry were clearly deposited in an aeolian dune setting. Some sands in the Cutties Hillock Sandstone with scoured bases were aeolian sediments that were reworked by fluvial processes. We conclude that this study demonstrates three different modes of track preservation in the Permian Moray area: (i) indentation of near-surface layers constituted by particles of fine silt that in many cases had infiltrated between sand grains of aeolian dunes; (ii) trackways in sediments deposited around the margins of lakes in the interdunes, with early cementation by evaporites, noting that in the studied cases the halite cement might have helped preservation of the tracks in the sense of long-term fossilisation, but probably not anatomical preservation (i.e. quality of fidelity); and (iii) indentation of clays that had been deposited in some interdunal lakes.