Grain texture (including grain size and shape) is a fundamental property of sediments and provides critical information on sediment transport mechanisms, depositional environments, and post-depositional processes. Two-dimensional digital image analysis has been widely used to quantify textural parameters of unconsolidated sands and sandstones. However, standardized analytical workflows remain lacking. In this study, ten grain size descriptors and two roundness quantification methods were applied to measure the texture of 16 sand-sized samples from fluvial, aeolian, beach, and continental shelf environments. The effects of sample pretreatment procedures (cut versus uncut) and grain shape (granular quartz versus platy lithics) on quantification results were systematically evaluated. Results show that grain sizes measured from cut samples are generally smaller than those from uncut samples. For uncut samples, the Minor, Width, Minimum Feret, and equivalent spherical diameter correlate well with sieve-derived grain sizes, whereas the Major, Length, and Maximum Feret show good correspondence for cut samples. Grain roundness and sorting are influenced by pretreatment procedures, with cut samples generally exhibiting higher roundness but poorer sorting compared to uncut samples. Grains with high aspect ratios yield relatively lower accuracy, indicating controls of grain shape on the size quantification. We propose a practical workflow for selecting appropriate grain size descriptors for different sand types and provide a correction approach for converting two-dimensional grain size into three-dimensional data. This work improves texture quantification methodology of sand-sized sediments and provides methodological support for grain texture-related research in various disciplines of geosciences.
Oceanic anoxic events (OAEs) are commonly linked to global drivers, but the role of basin configuration and tectonic instability in sustaining long-lived anoxic conditions remains insufficiently understood. In Baltoscandia, the Miaolingian–Tremadocian Alum Shale Formation (~ 513–484 Ma) records the regionally restricted Alum Oceanic Anoxic Episode (AOAE), which persisted for ~30 Myr and led to the accumulation of kerogenous black shales. This updated review integrates stratigraphic, palaeontological, sedimentological, and geochemical evidence to assess the environmental and tectonic controls on basin circulation and redox conditions during the AOAE. Deposition occurred in a semi-enclosed, flat-floored epicontinental basin developed largely on the Transscandinavian Igneous basement. Restricted circulation, high productivity, and persistent water-column stratification promoted dysoxic to anoxic bottom-water conditions, favouring exceptional preservation of organic matter and enrichment in pyrite and redox-sensitive elements. The succession is punctuated by breccias, conglomerates, shelly limestones, karstic surfaces, synsedimentary faults and Neptunian dykes, recording recurrent tectonic pulses involving local uplift and tilting. These events intermittently modified basin geometry and circulation patterns, facilitating short-term oxygenation episodes, carbonate and phosphogenic production, and localized hydrothermal activity associated with polymetallic mineralization. The AOAE started following renewed transgression after the ‘Hawke Bay unconformity’, and ended with a major uplift and regression recorded by the Bjørkåsholmen Formation, both likely related to Caledonian tectonic activity. The Alum Shale Formation provides an exceptional example of how persistent anoxia can be sustained in a semi-restricted epicontinental basin through the interplay of palaeogeographic confinement, episodic tectonic, hydrothermal fluxes, and water-column stratification, largely independent of globally synchronous OAEs.
Highly resolved stratigraphic correlations enhance our understanding of how terrestrial and marine ecosystems tracked the evolving Earth, including the development of Cretaceous Oceanic Anoxic Events (OAEs) and Red Beds. However, the interplay of climatic-paleoenvironmental factors governing major facies changes during mid-Cretaceous OAEs and ORBs remains elusive, hindering detailed cause-and-effect reconstructions. Using high-resolution carbonate carbon chemostratigraphy, we compare the early Albian dynamics of the Apennine Carbonate Platform (ApCP) and coeval basins in the Mediterranean Tethys, across the regional humid-arid belt boundary. The δ13C signature of the paralic ApCP Pietraroja section highlights that the Middle (fish- and plant-rich) and Upper (terrestrial vertebrate-rich) Lagerstätten were coeval with the OAE 1b Paquier black shales and the HN10-HN11 markers of the Vocontian Basin, respectively. Furthermore, the astrochronologic tuning of the Pietraroja δ13C signature implies that the Lagerstätten reflected short eccentricity-driven hydroclimatic shifts with a 60 kyr-long hiatus just above the ApCP Paquier equivalent. The Middle Lagerstätte was deposited in an arid wetland turning wetter, whereas the Upper Lagerstätte in a moist paralic context. They represent glacioeustatic sea-level falls during the long-term late Aptian-early Albian lowstand. The positive δ13C signature of Pietraroja Lagerstätten was determined by global rather than local paleoenvironmental and paleoecological processes, regardless of their contrasting background hydroclimatic conditions and the tendency of paralic waters for lighter δ13C values.This study demonstrates that peritidal-paralic limestones can reliably record global climatic-environmental and chemostratigraphic signals, offering a robust basis for constraining carbonate platform exposure events and enabling supra-regional, millennial-scale paleoclimatic reconstructions with near-modern temporal resolution.
The evolutionary dynamics and driving mechanisms of the Lopingian marine ecosystem are a frontier in paleoenvironmental research. Considerable global debate persists regarding the differential extinction of benthic organisms across bathymetric gradients and their underlying drivers. This study focuses on the Panjiazhuang section, a typical shallow-water microbialite profile on the northern margin of the lower Yangtze region in South China. By integrating micropaleontological analyses, pyrite framboid size distributions and terrestrial geochemical proxies, we reconstruct the succession trajectory of benthic communities across the Permian-Triassic transition in shallow platform settings and examine its coupling relationship with marine environmental evolution. Our results indicate that: (1) the end-Permian mass extinction in shallow carbonate platforms occurred as a single, catastrophic collapse that caused the rapid, irreversible breakdown of benthic ecosystems; and (2) the vertical expansion of oxygen-deficient water masses did not serve as a key environmental stressor in shallow marine settings, while enhanced terrestrial input was a major factor contributing to ecosystem collapse in this region.
Constraining the relationships between redox-dependent element redistribution and clay mineral assembly during early diagenesis remains challenging, particularly in red-bed systems which are sensitive archives of paleo-redox conditions. This study investigates relationships among redox conditions, Fe redistribution, Mg availability, and clay mineral transformations in siliciclastics of the Eocene Gercus Formation, Iraq, using an integrated approach based on global- and clay-fraction X-ray diffraction, scanning electron microscopy, electron microprobe analysis, X-ray fluorescence, Mössbauer spectroscopy, and diffuse reflectance spectroscopy. The results revealed systematic variations in clay mineral assemblages across early-diagenetic redox-defined zones, marked by progressive alteration of serpentine derived from ophiolitic sources through chlorite intermediates and subsequent transformation into vermiculite and mixed-layer vermiculite–smectite. Under suboxic conditions, limited alteration restricts Mg and Fe release, resulting in vermiculite-rich assemblages and absence of palygorskite. In contrast, the more advanced alteration observed in S2, consistent with the influence of reducing-fluid overprint conditions promotes Mg release and initial palygorskite formation, while Fe follows contrasting pathways involving partial removal and retention within the altered mineral assemblage. Under persistently oxic pore-water conditions, Mg remains available for extensive palygorskite growth, whereas Fe is predominantly retained and stabilized in ferric phases, including hematite. These observations demonstrate that redox conditions influence mineral alteration intensity and the subsequent availability of Mg, while redox conditions directly govern Fe redistribution and stabilization. In this system, Mg availability controls Mg-rich secondary mineral formation, whereas Fe undergoes redox-dependent redistribution, retention, removal, or stabilization depending on pore-water conditions.
Understanding the processes at glacier beds is crucial as they regulate ice flow, basal sediment dynamics, and meltwater routing, which collectively control glacier stability and response to climate change. Fields of glacial lineations are a spatially coherent assemblage of elongate subglacial bedforms made of sediment (e.g., soft-sediment striae, flutes, drumlins, megadrumlins, and mega-scale glacial lineations) that share a common flow-parallel orientation and are interpreted to record the geometry, kinematics and dynamics of past ice flow. These assemblages are widespread in Quaternary and modern settings, in both terrestrial and marine glacial environments. However, in terrestrial environments, these lineation fields degrade rapidly after formation, and the range of structures is much more restricted. In a marine context, they have a protective veneer of glaciomarine muds and exhibit a much greater diversity. By integrating observations from exceptionally preserved Late Ordovician (ca. 445 Ma) and late Palaeozoic (ca. 300 Ma) examples in Africa and Arabia, we find that a range of scale-independent structures are characteristic of glaciomarine fields of glacial lineations. A recurrent feature is stacked lobate sediments upon which glacial lineations are superimposed, from centimetre-scale soft-sediment striae to metre-scale flutes. These features are referred to as lobate glacial lineation sets and the largest examples are synonymous with grounding zone wedges, with self-similar examples at the scale of flutes and soft-sediment striae. A new conceptual model argues for the accumulation of these deposits at a stable, tidewater glacier margin unifying micro-, meso- and mega-scale glacial lineations in a single process interpretation.
The lower Priabonian (SBZ 18b-c) La Tossa Formation in Centelles (south-eastern margin of the Ebro Basin) records an upward transition from the siliciclastic-dominated Centelles Sandstone to the carbonate-dominated Collsuspina Limestone, with an intervening mixed siliciclastic‑carbonate interval. This long-term decline in terrigenous supply is interpreted to mark the onset of increasing aridity and seasonal climatic variability in the region during the early Priabonian. This aridification, also indicated by palaeobotanical and pedogenic records from the Iberian Peninsula, likely resulted from large-scale atmospheric reorganisation driven by Southern Hemisphere cooling and enhanced equatorial warming, which expanded subtropical dry belts. Through an integrated analysis of macro- and microfacies, combined with biostratigraphy and palaeoecology of larger foraminifera and coralline algae, this study reconstructs the facies architecture of a low-gradient homoclinal ramp. The results show a progressive ecological shift from larger foraminifera-dominated assemblages and cluster coral reefs to coralline algal-rich communities and frame-reef buildups, reflecting the influence of climate-mediated terrigenous fluxes on carbonate factory dynamics in a Priabonian shallow-marine platform.
The Mahneshan Basin in northwest Iran preserves a >6 km-thick syn-orogenic sedimentary succession that records the late Cenozoic evolution of the Arabia-Eurasia collision. We integrate sandstone petrography, detrital zircon UPb geochronology, and strontium isotope stratigraphy from the Lower Red, Qom, and Upper Red formations to reconstruct the sediment provenance history. Detrital zircon data indicate that the Lower Red Formation records continental deposition from the latest Oligocene to the earliest Miocene. Its lower part was derived from Precambrian basement rocks of the Takab Complex in the Sanandaj-Sirjan belt, whereas the upper part (22.2 ± 0.4 Ma) was mostly sourced from Eocene arc volcanic rocks, marking the first provenance shift. The overlying Qom Formation (19.6–19.4 Ma) records shallow-marine sedimentation with continued arc-related input. A second provenance shift occurred at the Qom to Upper Red Formation transition in the Burdigalian, reflecting an increased contribution from recycled crustal and sedimentary sources. The third provenance shift occurs in the middle Upper Red Formation (Serravallian), with renewed Miocene magmatic input and continued recycling of older crustal sources. Combined with published lower- and upper-plate tectono-stratigraphic data, our study documents a polyphase regional tectonic evolution involving: (i) latest Oligocene uplift and exhumation of the upper plate (Sanandaj-Sirjan Zone), near the plate suture; (ii) regional Oligo-Miocene subsidence with marine flooding reaching NW Iran in the early Burdigalian, followed by regional rapid transgression within the Burdigalian, coupled with localized upper-plate volcanism; and (iii) onset of widespread deformation, uplift, and exhumation from the middle Miocene.
Marine hardgrounds are sedimentary rock surfaces formed by early diagenesis. At the Rio Grande Rise, three expeditions documented carbonate firmgrounds and hardgrounds across the central plateaus and the Cruzeiro do Sul Rift flanks and valley. Environmental controls on their formation and preservation in the region remain poorly understood. We investigated 14 carbonate samples from water depths of 641 to 1504 m using petrographic, mineralogical, geochemical, and isotopic analyses. 87Sr/86Sr stratigraphy and microfacies analysis revealed that (i) Neogene wackestones (5.6–2.7 Ma) were deposited in the rift valley under low-energy conditions and cemented during shallow burial diagenesis, (ii) Quaternary rudstones and packstones (1.2–0.7 Ma) occur on the rift flanks, and (iii) Middle Pleistocene to Holocene grainstones (0.5 Ma–Recent) formed on the plateaus under recurrent winnowing and sediment bypass. Cement mineralogy distinguished Type 1 hardgrounds, cemented at or near the sediment–water interface by high-Mg calcite on the flanks and plateaus, from Type 2 hardgrounds, cemented under shallow burial by low-Mg calcite in the rift valley and later exhumed. Plateau samples record a continuum from incipiently cemented firmgrounds to Type 1 hardgrounds. Microfabrics, mineralogy, and carbonate isotopic compositions indicate predominantly marine-phreatic cementation. ROV imagery and hydrodynamic modelling show that bottom-current activity promotes sediment bypass, winnowing, cementation, and variable erosion and deposition on the plateaus, whereas the rift valley retains sediment. We conclude that the rift valley has been a deep-sea depocenter since at least the Late Miocene, whereas the plateaus record post-drowning deep-sea deposition since at least the Middle Pleistocene.
Fault evolution controls proximal to distal architectures and facies distribution of rift deposits. Resulting wedge-shaped syn-rift strata show thinning or thickening towards the fault as a response to breached or unbreached monocline during the sedimentation. However, the integration of seismic data and cored material to understand along-strike fault-controlled depositional variation in overall geometries and detailed sedimentary characteristics remains little studied. In this study, we reconstructed the tectono-stratigraphic evolution along the Shinan Fault System at the northern margin of the Bozhong Sag (Bohai Bay Basin, eastern China) by integrating 3D seismic data (i.e., structures and seismic stratigraphy), wireline logs (i.e., GR-log shapes), and sedimentary logs of cored material. Our results indicate that in the northern part of the study area the fault has breached the surface since the early stage of its activity generating an asymmetric syncline characterised by the main deposition loci located close to the fault. Here, base-of-scarp deposits showing up to 100 m in thickness adjacent to the fault and extending up to 2.5 km away from the fault accumulated evolving upwards and laterally into fine-grained lacustrine deposits and large-scale clinoforms prograding for up to 25 km parallel to the main fault. Towards the tip, the fault doesn't breach the monocline producing a fault propagation fold. Here, strata onlapped towards the unbreached monocline where sediments form wave-influenced deltaic systems. Our results provide insight into variations in along-strike stratigraphic architectures, thickness, lateral juxtaposition, and facies changes.
Ironstone horizons of the Chilpi Basin, Bastar Craton, India preserve finely developed FeSi microstructures within a sedimentary sequence comprising siliciclastic sedimentary rocks, stromatolitic limestone, and volcanogenic units. Intricate microstructures preserved within the ironstone, together with silicon and oxygen isotope data, provide new insights into ironstone genesis and early diagenetic processes within a Paleo-Mesoproterozoic intracratonic basin. Previously reported sulfur isotope evidence for microbial sulfate reduction (MSR) from stromatolitic limestone elsewhere in the basin further indicates spatially variable redox conditions during basin evolution. The ironstone consists of finely laminated Fe-rich and silica-rich microdomains containing diverse micron-scale fabrics, including twisted and branching filaments, reticulate networks, radial bundles of Fe-Al-Si silicates and spheroidal Fe-oxide aggregates. Electron microprobe analyses indicate that these structures contain variable proportions of Fe, Si, Al, and locally K, suggesting complex authigenic Fe-Si-Al phases rather than compositionally pure Fe-oxide, whereas silica occurs primarily as microcrystalline quartz within interlaminated bands. In situ SIMS analyses of quartz from the laminations yield dominantly negative delta Si-30 values (similar to-1.87 parts per thousand to +0.1 parts per thousand, mean -0.48 parts per thousand, n = 26), consistent with localized volcanogenic and possible hydrothermal silica contribution, restricted basin-scale silica cycling, rapid Si precipitation associated with Fe-rich sedimentation, and partial diagenetic modification. In contrast, a restricted range of delta O-18 values (similar to+13.5 parts per thousand to +16.2 parts per thousand, mean +14.3 parts per thousand, n = 17) reflects low- to moderate-temperature silica recrystallization during shallow burial diagenesis, indicating partial resetting of the oxygen isotope system without evidence of strong metamorphic overprinting. (CN-)-C-12-N-14 ion imaging of the Fe-rich microbands suggests carbon-nitrogen-bearing microdomains spatially associated with filamentous and spheroidal microstructures, suggesting preservation of organic residues, although internal cellular features are not resolved at the present scale of observation. Integration of these observations suggests that the Chilpi Basin experienced spatially and temporally variable redox conditions during basin evolution. The combined sedimentological, petrographic, microtextural, and O-Si-S isotope dataset records localized Fe enrichment, silica precipitation, organic matter preservation, and low-temperature diagenetic modification within a restricted intracratonic basin. These findings demonstrate that small, tectonically confined Proterozoic intracratonic basins can preserve detailed records of localized redox variability, possible microbial influence, authigenic mineral growth, and early diagenetic processes, providing a rare window into iron cycling outside large-scale, banded iron formation provinces.
Submarine canyons serve as conduits linking fluvial sediment sources to deep-sea depocenters. This study investigates sediment pathways within and between two contrasting canyon systems along the Colombian Caribbean margin: the Magdalena Canyons System (MCS), supplied by the Magdalena River, and La Aguja Canyon (LAC), which receives limited sediment from the Sierra Nevada de Santa Marta (SNSM), together with offshore distal (OD) sites located beyond LAC within the Magdalena Submarine Fan. Ten surface sediment samples (similar to 30-4000 m water depth) recovered by box and gravity cores across these domains (MCS, LAC, and OD) were analyzed in terms of grain-size distribution, X-ray diffraction, and detrital zircon U-Pb geochronology. Bulk and clay mineralogy reveal two provenance signatures: a quartz-rich, compositionally mature Andean signal in the MCS and a plagioclase-amphibole-rich, immature assemblage in LAC. OD sites incorporate compositional signatures of both, consistent with the convergence of multiple sediment-routing pathways. A total of similar to 1000 detrital zircon ages further support this interpretation. The MCS samples display a multimodal Andean signal including Neogene (<8 Ma), Jurassic-Cretaceous (80-150 Ma), Permian-Triassic (similar to 240-270 Ma), and Neoproterozoic (similar to 990 Ma) peaks. LAC samples show Paleogene (similar to 50 Ma) and Jurassic (similar to 180 Ma) peaks characteristic of the SNSM, while OD sites combine both age populations. The presence of <1 Ma zircons in deepmarine MCS sediments, possibly sourced from volcanic centers located >600 km inland, may reflect unusually rapid erosion and continent-to-basin sediment transfer, although contributions from currently undocumented volcanic sources cannot be excluded. These results suggest that sediment transfer along the margin is characterized by persistent Andean supply, structural confinement, multiple canyon pathways, and episodic slope reactivation and reworking. The Magdalena Submarine Fan therefore represents an illustrative natural example of provenance mixing and sediment-routing efficiency in tectonically active canyon-fan systems.
The Messinian Salinity Crisis (MSC) was preceded by profound climatic and oceanographic changes in the Mediterranean starting at ∼7.1 Myr. They influenced deep-water sedimentation during increasing basin restriction. The well-preserved sedimentary record of the Mesaoria Basin (NE Cyprus, eastern Mediterranean) is an excellent archive to study pre-evaporitic conditions at a basin-wide scale. This study analyses a ∼125 m-thick sedimentary succession that allows understanding the complex biotic-abiotic processes controlling the formation of deep-water carbonates before the MSC. The present study integrates stratigraphy, facies analysis, qualitative and quantitative micropalaeontology, petrography, and stable isotope geochemistry to reconstruct depositional and early diagenetic processes in a carbonate-dominated, deep-water setting. During the initial phase of restriction (∼7.1–6.7 Ma), the surface waters were characterized by open-marine calcareous nannoplankton and warm-oligotrophic planktonic foraminifera assemblages, while the seafloor was populated by a benthic fauna indicative of ventilated bottom-waters. As restriction intensified, the seafloor became periodically anoxic. Anoxia enhanced bacterial sulphate reduction, which induced the cyclical precipitation of pyrite and authigenic gypsum within calcareous tests. At around ∼6.7 Ma, a further step of basin restriction induced a shift in pelagic sedimentation from massive to laminated marls, followed by the cyclic deposition of wackestone-marl couplets. Carbon and oxygen stable-isotope variations and rapid shifts in nannoplankton assemblages point to salinity and primary productivity fluctuations driven by precession-controlled oscillations between water-column mixing and stratified conditions. Massive wackestones formed during arid periods, when the formation of dense saline upper waters enhanced vertical mixing and seafloor ventilation. Enhanced freshwater input during humid periods resulted in surface water eutrophication and water-column stratification favouring opportunistic nannoplankton, and coincided with the appearance of biosiliceous remains in the sedimentary record. Algal blooms boosted the export of organic matter to a dysoxic seafloor in the form of marine snow floccules, whose preservation in the laminated marls was favoured by decreased benthic scavenging. The occurrence of putative trace fossil Trichichnus, which has been attributed to giant sulphide-oxidizing bacteria, reflects short-term redox fluctuations at the seafloor, pointing to an active sulphur biogeochemical cycle in a deep environment. These results reveal how restriction, freshwater input, water-column stratification, planktonic primary productivity, and chemotrophic microbial processes shaped deep water biogenic carbonates in the eastern Mediterranean prior to the Messinian Salinity Crisis.
