Hydraulic Sorting Research Articles

Distribution and characterization of heavy minerals in Meghna River sand deposits, Bangladesh

Heavy mineral (HM) concentrations in sand bar deposits of the Meghna River were evaluated using optical microscopy, X-Ray Diffraction (XRD), X-Ray Fluorescence spectrometry (XRF), Scanning Electron Microscopy (SEM) and Electron Probe Microanalysis (EPMA) to determine the mineralogy, geochemistry and likely source rocks of the sands, and to examine the distribution and HM potential of the valuable heavy minerals, ilmenite and garnet. The Meghna sands were dominated by the light minerals quartz and feldspar and grainsize analysis results showed that>78 wt.% of the sand population fell within the size range of fine to very fine sand (−250 + 63 μm). The HM component made up around 9 wt.% and consisted mainly of amphibole (∼17–46 wt.%), epidote (∼7–30 wt.%) and garnet (∼3–21 wt.%). Fe-Ti oxide minerals including ilmenite, rutile, and titanomagnetite were present at levels of 0.7–7 wt.% with the magnetite content at 0.9–7 wt.%. The textures (moderate sorting, sub angular) and significant feldspar (6–15 wt.%) content in the sands indicated low degrees of hydraulic sorting and low amounts of alteration/weathering. The HM assemblage indicated a likely mixed source area of varying metamorphic grade (low to high) facies and igneous rocks. EPMA results from the detrital ilmenites and garnets showed that for ilmenite, TiO2 contents were between 42.9 and 54.5 wt.% (ave. 51 wt.%), with overall low impurities, while the garnet compositions were dominated by Fe-rich, almandine types. From the chemical analysis of individual garnet and ilmenite grains, it is likely the Meghna River sands have good potential for commercial extraction of garnet and ilmenite.

River Organic Carbon Fluxes Modulated by Hydrodynamic Sorting of Particulate Organic Matter

AbstractRivers regulate the global carbon cycle by transferring particulate organic carbon (POC) from terrestrial landscapes to marine sedimentary basins, but the processes controlling the amount and composition of fluvially exported POC are poorly understood. We propose that hydrodynamic sorting processes modify POC fluxes during fluvial transit. We test this hypothesis by studying POC transported along a ∼1,200 km reach of the Rio Bermejo, Argentina. Nanoscale secondary ion mass spectrometry revealed that POC was either fine, mineral‐associated organic matter, or coarse discrete organic particles. Mineral‐associated POC is more resistant to oxidation and has a lower particle settling velocity than discrete POC. Consequently, hydraulic sorting and downstream fining amplify the proportion of fine, mineral‐associated POC from ∼55% to ∼78% over 1,220 km of downstream transit. This suggests that mineral‐associated POC has a greater probability of export and preservation in marine basins than plant detritus, which may be oxidized to CO2 during transit.

Sedimentary geochemistry response to climate change on a millennial timescale in the Qiantang River incised-valley system, eastern China

The highly controversial relationship among the chemical weathering of silicate rocks, climatic variability and human activities on various temporal scales has become a hot research topic given the booming interest in global climate change. This study of the Holocene Qiantang River incised-valley fill investigates the variability of weathering intensity and its controlling factors based on geochemical analysis. Results indicate that the variations of chemical weathering indices throughout the studied succession are chiefly climate-controlled, although partly influenced by source-rock lithology, grain-size effects, hydraulic sorting and sediment recycling by tidal bores. Over the last 12 ka, 8 periods with distinct characteristics and weakened weathering intensity are identified. The abruptly weakened weathering intensity documented by chemical weathering indices in the last 2 ka is ascribed to the intensification of human activities and/or EI-Niño activities. Noteworthy, small catchments located near land-ocean boundaries respond more rapidly to climate change and human activities on a millennial (even centennial) timescale. The climatic cooling events (e.g., Younger Dryas, 10.3 ka and 9.3 ka) are better reflected in the study interval as the sediments of units U5, U4 and U3 (before 8.2 ka BP) are sourced from the Qiantang River watershed. Slightly weakened chemical weathering related to climatic events is also documented by the coarser sediments of unit U1, which are primarily derived from the Changjiang (Yangtze River).

Geochemistry and Petrography of the Sediments From the Marginal Areas of Qinghai Lake, Northern Tibet Plateau, China: Implications for Weathering and Provenance

The provenance study of the sediments from Qinghai Lake is of great significance for the understanding of geological and climatic evolution processes of the Tibet Plateau on the one hand and for evaluating the controlling factors of the sediment components on the other hand. The samples were collected from five rivers, foreshore, beach, beach bar, and aeolian sand dune in the Qinghai Lake. The bulk geochemical composition, petrography, and mineralogy features of the samples are analyzed. The results show that: 1) Qinghai Lake sediments experienced low-intensity chemical weathering from the source areas to the deposition sites and were affected by some recycled detrital materials and 2) the source rocks for the sediments include felsic rocks (granite, granodiorite, and felsic volcanic rocks), carbonate, metamorphic rocks (marble and meta-volcanic rocks), and clastic rocks with the felsic source rocks to have the most important impact on the chemical compositions of the sediments. The geochemical indicator of Al2O3/TiO2 reflects that the provenance of fine-grained sediments from the center of Qinghai Lake is more mafic than the coarse-grained sediments from the margin of the Qinghai Lake, suggesting that the hydraulic sorting of grain size probably plays an important role in the geochemical compositions of the sediments. The mafic elements were probably preferentially enriched in muds.

River sediment geochemistry and provenance following the Mount Polley mine tailings spill, Canada: The role of hydraulic sorting and sediment dilution processes in contaminant dispersal and remediation

The failure of the Mount Polley tailings storage facility (TSF) in August 2014 was one of the largest magnitude failures on record, and released approximately 25 Mm3 of material, including c. 7.3 Mm3 of tailings into Hazeltine Creek, part of the Quesnel River watershed. This study evaluates the impact of the spill on the geochemistry of river channel and floodplain sediments and utilizes Pb isotope ratios and a multi-variate mixing model to establish sediment provenance. In comparison to sediment quality guidelines and background concentrations, Cu and V were found to be most elevated. Copper in river channel sediments ranged from 88 to 800 mg kg−1, with concentrations in sand-rich and clay/silt-rich sediments being statistically significantly different. Concentrations in river channel were believed to be influenced by hydraulic sorting during the rising and falling limbs of the flood wave caused by the tailings spill. Results highlight the importance of erosive processes, instigated by the failure, in incorporating soils and sediments into the sediment load transported and deposited within Hazeltine Creek. In this instance, these processes diluted tailings with relatively clean material that reduced metal concentrations away from the TSF failure. This does however, highlight environmental risks in similar catchments downstream of TSFs that contain metal-rich sediment within river channels and floodplain that have been contaminated by historical mining.

Isotopic compositions and geological significance of sediments from the Heihe River, North Qilian Orogen, NW China

Strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopic analyses of different size-fractions of sediments collected from the Heihe River in the North Qilian Orogen (NQO) were carried out to trace the sediment sources and to determine the relationships between the Sr–Nd–Hf isotopic behaviors, and the grain-size and hydraulic sorting effects during fluvial transport. Our results demonstrate that the sand and suspended load samples collected from the same site have different Nd isotopic compositions, while their 87Sr/86Sr ratios are only slightly different. These features indicate that contributions to the sediments from different sources with different grain sizes vary greatly. Coarse-grained sand may more intuitively reflect the variations in local sources than the suspended load. The suspended load samples are distributed along the Clay Array on the ԐHf vs. ԐNd diagram due to zircon sorting. A similar NdHf decoupling phenomenon is widespread in the global oceans and large river systems, indicating that the NdHf isotopic behavior depends on the hydrodynamic sorting of minerals during fluvial transport. Moreover, the crustal accretion of the NQO from the depleted mantle occurred in the Proterozoic based on the constraints provided by the TDM values of the river sediments. The Nd isotopic compositional characteristics of the river sediments indicate that the NQO has an affinity with the Yangtze Craton. The determination of the basement tectonic affinity of the NQO significantly contributes to our understanding of the Neoproterozoic evolution of the Gondwana continental margin.

Black sand properties in beach-dune system, Patea Beach, North Island, New Zealand

The study focuses on sand grain properties in different parts of a beach-dune system built entirely of heavy mineral particles. These properties are related to: (1) resistance of particular minerals to weathering and abrasion, (2) hydraulic sorting in the swash zone, and (3) aeolian sorting during grain transport inland from the upper beach. The main waterlain and windlain sand properties depend on settling velocity which results from grain density (reflecting grain mineralogy), size, shape and roundness. The study was performed on the beach and dunes at Patea on the west coast of the North Island of New Zealand, which are comprised of heavy minerals assemblage containing a ferromagnetic (dominated by titanomagnetite) and non-ferromagnetic (mainly pyroxene and amphibole) fractions. The result demonstrates that three zones of different sand properties can be distinguished: (1) a lower swash zone dominated by non-ferromagnetic, larger and more angular particles which are carried back from the upper swash zone down the foreshore by the backwash; (2) an uppermost swash zone and beach with almost 100% of ferromagnetic, smaller and more rounded particles deposited at the back of the beach by the uprush, which during high tide and storms can reach the cliff toe, and can be reworked by wind; and (3) a climbing dune composed of a more poorly sorted mixture of non- and ferromagnetic particles. In terms of both mineralogy and grain size and shape, the dune sand is less uniform than the beach sand. Aeolian segregation resulted here in sand textural features opposite to those found in dune sands composed of light minerals. The results highlight the density-dependent variability of grain size and shape of beach-dune deposits consisting of only heavy minerals, and broaden our understanding of mechanisms of sedimentary processes which is particularly important when reconstructing older sedimentary successions.

New sediment provenance approach based on orthonormal log ratio transformation of geochemical and heavy mineral data: Sources of eolian sands from the southeastern Adriatic archipelago

To date, much effort has been made to overcome obstacles involved in inferring the provenance of sediments based on heavy mineral assemblages and geochemical fingerprints of minerals. These efforts have been concentrated on two main avenues. First, efforts have focused on disentangling physical and chemical properties of minerals and their behavior during transport. Second, efforts have centered on processing the obtained data in a way that takes into account the properties of heavy minerals and geochemical data sets. More specifically, to consider the limitations that arise from the nature of point count and compositional data. In this manuscript, a novel workflow for provenance is proposed based on the fact that point count data, although not compositional, carry compositional information. A case study is presented to infer the provenance of late Quaternary eolian sands from the southeastern Adriatic archipelago for which two different sources have been previously proposed. Heavy mineral data were represented in orthonormal log ratio coordinates. In this way, variables which compensate for hydraulic sorting and/or selective dissolution were obtained, with the major advantage that there are no restrictions for their use with statistical methods originally developed for real Euclidian space. To support conclusions based on heavy mineral data, we analyzed the geochemical composition of pyroxenes, which were also discriminated using a novel approach. As a result, the Albania-Montenegro shelf is determined to be a sediment source for eolian sands from Korčula, Lastovo and Mljet Islands. Hvar Island has a Neretva River signature, and Vis Island has a dominantly Neretva River signature with a small portion of sediments from the Albania-Montenegro shelf.

Detrital zircon geochronology and heavy mineral analysis as complementary provenance tools in the presence of extensive weathering, reworking and recycling: the Neogene of the southern North Sea Basin

AbstractHeavy mineral analysis is a long-standing and valuable tool for sedimentary provenance analysis. Many studies have indicated that heavy mineral data can also be significantly affected by hydraulic sorting, weathering and reworking or recycling, leading to incomplete or erroneous provenance interpretations if they are used in isolation. By combining zircon U–Pb geochronology with heavy mineral data for the southern North Sea Basin, this study shows that the classic model of sediment mixing between a northern and a southern source throughout the Neogene is more complex. In contrast to the strongly variable heavy mineral composition, the zircon U–Pb age spectra are mostly constant for the studied samples. This provides a strong indication that most zircons had an initial similar northern source, yet the sediment has undergone intense chemical weathering on top of the Brabant Massif and Ardennes in the south. This weathered sediment was later recycled into the southern North Sea Basin through local rivers and the Meuse, leading to a weathered southern heavy mineral signature and a fresh northern heavy mineral signature, yet exhibiting a constant zircon U–Pb age signature. Thus, this study highlights the necessity of combining multiple provenance proxies to correctly account for weathering, reworking and recycling.

Hydraulic classification and sedimentation behaviors of iron tailings

Tailings pond is a large-scale fragmented sedimentary body formed by mining tailings, and the safety management of tailings pond affects the sustainable development of mining productions. The nature sedimentation behavior of the tailings is the initial condition to obtain the particle fractal dimension and packing structures of tailings, and is also the basis to evaluate the stability of tailings pond. To study the natural sedimentation behaviors of iron tailings, 4 large-scale simulation tests of tailings hydraulic sorting are carried out in Nanjing city, Tangshan city, and Benxi city of China, and the hydraulic sedimentation behaviors in the test process and post-test process are monitored and tested. Meanwhile, the in situ samples in the model channels are taken and tested by the soil mechanics and sedimentological theories, to analyze grain composition and spatial distribution law. This research summarizes the hydraulic sedimentation and grain composition behaviors of iron tailings, and provides data for the exploration and simulation of the natural formation process of sedimentary materials.

The influence of lithology on channel geometry and bed sediment organization in mountainous hillslope‐coupled streams

AbstractSediment transport and channel morphology in mountainous hillslope‐coupled streams reflect a mixture of hillslope and channel processes. However, the influence of lithology on channel form and adjustment and sediment transport remains poorly understood. Patterns of channel form, grain size, and transport capacity were investigated in two gravel‐bed streams with contrasting lithology (basalt and sandstone) in the Oregon Coast Range, USA, in a region in which widespread landslides and debris flows occurred in 1996. This information was used to evaluate threshold channel conditions and channel bed adjustment since 1996. Channel geometry, slope, and valley width were measured or extracted from LiDAR and sediment textures were measured in the surface and subsurface. Similar coarsening patterns in the first few kilometres of both streams indicated strong hillslope influences, but subsequent downstream fining was lithology‐dependent. Despite these differences, surface grain size was strongly related to shear stress, such that the ratio of available to critical shear stress for motion of the median surface grain size at bankfull stage was around one over most of the surveyed lengths. This indicated hydraulic sorting of supplied sediment, independent of lithology. We infer a cycle of adjustment to sediment delivered during the 1996 flooding, from threshold conditions, to non‐alluvial characteristics, to threshold conditions in both basins. The sandstone basin can also experience complete depletion of the gravel‐size alluvium to sand size, leading to bedrock exposure because of high diminution rates. Although debris flows being more frequent in a basalt basin, this system will likely display threshold‐like characteristics over a longer period, indicating that the lithologic control on channel adjustment is driven by differences in rock competence that control grain size and available gravel for bed load transport. © 2020 John Wiley & Sons, Ltd.

Paleocurrent analysis, petrographic, geochemical and statistical appraisal of Neoproterozoic siliciclastic sediments, NE Voltaian Basin, Ghana: a multidisciplinary approach to paleogeographic reconstruction

This study used sedimentary structures, facies analysis, petrographic studies, statistical techniques and geochemical data of the Bombouaka/Gambaga Group of the NE Voltaian Basin, Ghana to decipher the source of the sediments. Field measurements covered the Tossiegou, Poubogou, and Panabako Formations of the Bombouaka/Gambaga Group. The influence of hydraulic sorting was interpreted from the effects of highly resistant heavy minerals (identified in the petrographic studies) from polycyclic sources. Principal component analysis (PCA) was applied to the geochemical data to understand element associations. Two broad facies types were identified; lithofacies and ichnofacies. The lithofacies were observed as (a) asymmetric ripple marks on sandstones, (b) parallel lamination on silty/argillaceous sediments, (c) straight-crested and bifurcated ripples on shales, (d) flute casts and climbing ripples on sandy shales, (e) bipolar herringbone cross-bedding on quartz-rich sandstones, and (f) wavy lamination and cross-bedding on feldspathic sandstones. However, the ichnofacies include only Skolithos on quartzitic sandstones. The structural analyses suggest that about 85% of the paleocurrents were from NE to SW direction with subordinate directions from WNW to ESE. Based on the predominant paleocurrent directions, the sediments were probably derived from the basement Birimian rocks (metavolcanic and metasedimentary rocks with associated granitoids), previously paleogeographically located in the Amazonian Craton but now eroded, transported and transformed into the Birimian Supergroup from which the Voltaian sediments were derived. The PCA points to similar sources of granitoids and metasedimentary rocks. Therefore, the geochemical results and the PCA interpretations support the paleocurrent structures-inferred sources of the sediments in the NE Voltaian Basin.

Geochemistry and mineralogy of the shallow subsurface Red Sea coastal sediments, Rabigh, Saudi Arabia: provenance and paleoenvironmental implications

Mineralogical and geochemical characteristics of the shallow subsurface sediments retrieved from a short sediment core (2.05 m long) collected from the tidal flat south of Al-Kharrar Lagoon, Rabigh area, Saudi Arabia, are presented to determine the impact of temporal change of depositional environment and distinguish the principal control(s) on their chemical composition. The sediments are dominantly siliciclastic, consisting of two vertically stacked sedimentary facies: lagoonal (LG) gray silt-rich mud to argillaceous very fine sand at the base and intertidal flat (TF) brown mud and argillaceous very fine-grained sands at the top. The sediments of the two facies are similar in their mineralogical and chemical composition except for slight vertical variations in the relative abundance of minerals and concentrations of major oxides and trace elements. The mineralogical composition is dominated by quartz, feldspars, and plagioclase with less abundant clay minerals and hornblende, all of detrital origin. In addition, traces of high and low Mg-calcite, dolomite, and gypsum were recognized. Geochemically, the sediments are first-cycle and compositionally immature. The Al2O3/ TiO2 ratios range from 9.4 to 17 and from 13 to 15.4 for the sediments of the LG and TF facies, respectively. These values suggest an intermediate igneous source rock of the Birak group that belongs to the oceanic island arc of the Hijaz Terrane, western Arabian Shield. The average values of paleoweathering indices CIA, CIW, and PIA are <50, suggesting unweathered to poorly weathered source rock that is consistent with the late Holocene arid climate. Although the redox-sensitive V and Cr in the sediments of LG facies are slightly enriched relative to the average shale, the values of the V/Cr ratio are generally <2, suggesting deposition under oxic conditions. The chemical characteristics of the sediments are primarily controlled by the composition of source rock and to a lesser extent by hydraulic sorting during transportation and deposition.

Biogeochemistry and dynamics of particulate organic matter in a shallow-water hydrothermal field (Kueishantao Islet, NE Taiwan)

The particulate organic matter (POM) of shallow-water hydrothermal fields has been studied in the context of food web reconstruction, but the processes governing its biogeochemistry and dynamics are poorly explored. Here, we investigate the POM in the Kueishantao hydrothermal field using chemical and hydrodynamic approaches. The depletion of total suspended matter, lower C/N ratios, and higher carbon isotopic values of particulate organic carbon (δ13CPOC) in the vertical plumes relative to values derived from Si-based models were attributed to the hydraulic sorting of the vented particles, which tend to have more N and 13C in the fine fraction. The particulate organic carbon (POC), unlike the total suspended matter, was enriched in the vertical plumes and explained by physicochemical processes rather than biological addition. The POC-enriched plume-top water was found to be a better endmember than the vent fluids to explain particle mixing in the lateral plume. Physical mixing played a steering role in shaping the particle chemistry of the lateral plumes, but markedly 13C-enriched POC was still observable in several near-vent, low-to-intermediate-Si plume waters, implying locally enhanced primary production of at least 0.1–0.4 mg C/m3/h. The presence of eddies, confirmed by flow field measurements, should have contributed to the detection of biogeochemical anomalies via extending the retention time of plume water to 1–2 h. The dominating mixing process resulted in decoupling between the δ13CPOC signatures and carbonate chemistry in this shallow-water hydrothermal plume.

Weathering, recycling, hydraulic sorting and metamorphism/metasomatism implications of the NE Gondwana lower Cambrian-Lower Cretaceous siliciclastic succession of Jordan

Whole rock major elements, detrital and authigenic light, heavy, and clay minerals, and neoformed zeolite minerals of the NE Gondwana lower Cambrian-Lower Cretaceous siliciclastic succession of Jordan were investigated to evaluate the factors controlling the chemical composition of the sandstone. The provenance source rocks, controlled by the tectonic setting at the time of deposition of the subarkosic arenites and the quartzarenites were the principal mineral- and chemical-governing factor, followed by an interplay of chemical weathering, recycling, hydraulic sorting, diagenesis, and very-low grade metamorphism/hydrothermal metasomatism. The sandstone provenance was mainly granitoids of the Arabian Shield formed during the Neoproterozoic Gondwana amalgamation, and minor Paleozoic and Mesozoic siliciclastic strata. High CIA and low WIP weathering index values indicate an intensive chemical weathering event that affected the provenance during the early Cambrian-Middle Ordovician, and gave rise to mature first-cycle quartzarenites. High CIA/WIP and SiO2/Al2O3 ratios indicate a pronounced recycling in the Early Cretaceous responsible for super mature second-poly-cycled quartzarenites. The subarkosic arenites were formed either under a thorough chemical weathering, coupled with a high rate of erosion and a rapid deposition in adjacent braidplains, or an arid, glacial climate devoid of chemical weathering. Hydraulic sorting caused enrichment of Al and K in the detrital clay minerals, Ti in the silt-sized Ti-bearing minerals and Si in the sand-sized quartz. The very low-grade metamorphism/hydrothermal metasomatism, recorded here for the first time, is indicated by illite crystallinity (IC = 0.36° 2θ), illite-sericite-muscovite transformation, zeolites-apatite-authigenesis, and alteration of monazite and titanite-ilmenite into thorite and leucoxene/hematite.

Antimony and arsenic speciation, redox-cycling and contrasting mobility in a mining-impacted river system

The Macleay River in eastern Australia is severely impacted by historic stibnite- and arsenopyrite-rich mine-tailings. We explore the partitioning, speciation, redox-cycling, mineral associations and mobility of antimony and arsenic along >70 km reach of the upper Macleay River. Elevated Sb/As occur throughout the active channel-zone and in floodplain pockets up to the regolith margin, indicating broad dispersal during floods. Sb concentrations in bulk-sediments decay exponentially downstream more efficiently than As, likely reflecting sediment dilution, hydraulic sorting and comparatively greater leaching of (more mobile) Sb(V) species. However, Sb in bulk-sediments becomes proportionally more bio-available downstream. Sb(V) and As(V) species dominate stream fine-grained (<180 μm) bulk-sediments, reflecting oxidative weathering downstream. Increasing poorly-crystalline Fe(III) [Fe(III)HCl] in bulk-sediments also indicates progressive oxidative weathering of Fe(II)-bearing minerals downstream and significant (P < .05) correlations exist between PO4−3-exchangeable As and Sb fractions and Fe(III)HCl. Accumulations of poorly-crystalline Fe(III) precipitates (mainly ferrihydrite/feroxyhyte) occur intermittently in hyporheic-zone seeps and are enriched in As relative to Sb and contain some As(III) and Sb(III) (~30–40%). There is dynamic in-stream redox-cycling of both Sb and As, with localised S-coordinated As and Sb species re-forming in organic-rich, hyporheic sediments subject to contemporary sulfidogenesis. Sb [mainly Sb(V)] is comparatively more mobile in hyporheic and surface waters under oxic conditions, whereas As [mainly As(III)] is more mobile in hyporheic porewaters subject to reducing/sulfidogenic conditions. Repeat water-leaching of bulk-sediments confirms that Sb is proportionally more mobile than As. Mean concentrations of Sb in river water 168 km downstream from the mine are significantly (P < .05) higher than As, while Kd data indicate Sb is more strongly partitioned to the aqueous phase than As. Although the (mainly) oxic flow path of this river favours aqueous Sb mobility compared to As, localised redox-driven shifts in speciation of both elements strongly influence their respective mobility and partitioning.

Mineralogical and geochemical composition of the subsurface sediments at the mouth of Wadi Al- Hamd, Red Sea coast, Saudi Arabia: implication for provenance and climate

The present work is related to mineralogy and geochemistry of sedimentary successions of the modern Red Sea coastal plain at the mouth of Wadi Al-Hamd, Northern Saudi Arabia. It identifies the provenance, weathering effect, and paleoenvironment of the area. The mineralogical composition is dominated by quartz, plagioclase, and trace of feldspars, clay minerals, mica, and hornblende. Additionally, traces of non-detrital minerals including high and low Mg calcite, dolomite, halite, and anhydrite were locally detected. Four chemo-facies, dominated by three associations of element groups linked with detrital minerals, grain size properties, and biogenic products were recognized. The mineralogical and geochemical indices showed that sediments were derived from the intermediate igneous rocks at active continental margin and continental island arc settings. Weathering indices showed that the sediments were poorly weathered, consistent with the arid-to-semi-arid climate of the area. The spatial and vertical variations of mineralogical and chemical composition reflect variation depositional environments, hydraulic sorting, and the influence of coastal processes.

Sandstone-Shale Geochemistry of Miocene Surma Group in Bandarban Anticline, SE Bangladesh: Implications for Provenance, Weathering, and Tectonic Setting

The present study analyzes the geochemical composition of sandstone and shale of the Miocene Surma Group to decipher the provenance, tectonic settings and paleoweathering condition of source area in the Bandarban Anticline which is at the western margin of Indo-Burmese Hill Ranges. Statistical empirical index of chemical weathering of the sediments that have been extracted by the Principal Component Analysis (PCA) is used to understand the weathering profile of the sediments of the study area. The PCA of the geochemical composition yields three principal components (PC–1, PC–2, and PC–3), which capture total variance 52.83%, 17.58% and 6.94%, respectively. The PC–1 shows the loss of SiO₂ during weathering of preexisting source rocks; PC–2 reveals the enrichment of Na₂O, CaO, and P₂O₅ due to leeching and carried by groundwater during weathering; highest loadings with MnO and Cr shows in PC–3 due to redox environment during early diagenetic of marine sediments. The MFW and A–CN–K diagrams show an intense weathering trend, and backward trend of the MFW diagram and the major elements provenance discriminant diagram refers to the mature polycyclic quartzes provenance and originated dominantly from felsic to intermediate igneous rocks. The trend of the SiO₂/Al₂O₃–Na₂O/K₂O shows the hydraulic sorting effect and sediments were originated primarily from a recycled sedimentary provenance. The CIA (67.68–80.89), ICV (0.60–1.29, avg. 0.83) and K₂O/Na₂O ratios show a moderate to high maturity of the sediments and is derived from both weak and intensively weathered source rocks. Discriminate diagrams related to tectonic provenance refer to the deposit of the sediment dominantly under the influence of collision (active continental collision, compression) and mature sediment derived to the depositional basin after upliftment of the source areas after that collision.

Magneto‐chemical signature of the Lower‐to‐Middle Siwaliks transition in the Karnali River section (Western Nepal): Implications for Himalayan tectonics and climate

Fluviatile sediments comprising a 600‐m‐thick sequence of the Lower and Middle Siwaliks in the Karnali area in Nepal exhibit a distinct zonation revealed by magnetic and geochemical properties. Four magneto‐chemical zones (MCZ1–MCZ4), each about 150 m thick and 400 kyr in duration, provide new insights into Himalayan tectono‐climatic events during the Tortonian (Miocene) stage. They exhibit contrasting magnetic susceptibility and isothermal remanence due to differences in magnetic mineral types (magnetite, haematite, and goethite) and concentrations. Odd‐numbered zones with higher goethite/(goethite + haematite) ratio, a moisture proxy, indicate wetter conditions in the source area, while the even‐numbered zones, virtually without goethite, suggest drier conditions. Chemical indices of alteration/weathering and proxies for hydraulic sorting and mobility derived from the major element compositions also reveal contrasts among these zones. The middle of the MCZ2–MCZ3 zone, with a transitional magneto‐chemical signature, is the best candidate for the Lower‐to‐Middle Siwaliks contact, rather than the field‐based boundary placed 18 m up‐section at the base of the thick salt‐and‐pepper sandstone bed. The transition records an increase in river energy and associated accelerated erosion of the Himalayan gneiss zone as the source of coarse‐grained material. We suggest a scenario, whereby climate change from drier to wetter (with higher precipitation) conditions affects erosional processes (i.e., weathering, disaggregation and particle transport on the hillslope) prevailing in a large catchment and influencing the depositional modes.

Impact of hydraulic sorting and weathering on mica provenance studies: An example from the Yangtze River

Detrital muscovite and biotite 40Ar/39Ar analyses are useful tools for studying regional tectonic histories, sediment provenances and paleo-drainage reconstructions. During transport and recycling of detrital micas physical and chemical weathering occurs. This process effects the grain size and age populations ultimately found in river sediments, but is often ignored in provenance studies. Here, we present detrital muscovite and biotite 40Ar/39Ar results of 15 modern sediments from the Yangtze River to address the impact of grainsize on provenance age populations. The beam intensities of 39Ar, formed from 39K by neutron capture reaction during sample irradiation, have been used as an index for grain size. We found that relatively older detrital mica ages of the Yangtze River are often characterized by small 39Ar signals (i.e., grain sizes), and large grain sizes correspond to younger grains. This observation is also revealed by reanalysis of previously reported detrital mica studies in other major river systems (Red and Brahmaputra rivers) and sediments (Scotian Basin, Canada and Antarctic) and probably results from physical and chemical weathering during transport and recycling. Our Yangtze results indicate that detrital muscovite and biotite ages of grainsize ranging from 100 to 1000 μm cover all age components as identified in all dated grains (with a size of >100 μm), and thus indicate that detrital mica 40Ar/39Ar analyses should include also small grains from >100 μm to reduce the effects of hydraulic sorting and weathering. Grainsizes smaller than 100 μm have not been tested in this study, but will be more difficult to date due to both smaller beam intensities and possible recoil effects.