Fine-grained Sediments Research Articles

Genesis of gray sandstone within the red beds in HLJ-DL uranium deposit, southwest Songliao Basin and its relationship with uranium mineralization

The search for uranium deposits in the red beds is currently a hotspot in China's sandstone-type uranium exploration efforts. The typical occurrence of uranium deposits is within gray sandstone layers amidst the red beds, where the distribution of these gray sand bodies dictates the presence of uranium deposits. Hence, delving into the genesis of gray sandstone within the red beds proves beneficial for both uranium exploration and unraveling the ore-forming mechanisms. However, there is currently significant controversy surrounding the genesis of gray sandstone in the red beds. This paper focuses on the typical uranium deposit in the red beds—the HLJ-DL uranium deposit in the southwestern part of the Songliao Basin. It investigates the genesis of gray sandstone within uranium-bearing rock series and its relationship with uranium mineralization. The lithology of representative drill cores, including the red sandstone for comparative study, is methodically examined, along with their elemental geochemistry as well as carbon and oxygen isotope characteristics. The petrological characteristics suggest that both the gray and red sandstone share similar compositions of clastic particles, indicating minimal influence of the source rock on the sandstone color. The gray sandstone displays comparable TOC content (averaging around 0.05 %) and δ18OPDB values (averaging around −16 ‰) to the red sandstone. However, it exhibits relatively higher δ13CPDB values (averaging −0.26 ‰), whereas the average δ13CPDB value for the red beds is −1.61 ‰. The data indicate that the gray sandstone is less affected by hydrothermal or hydrocarbon fluids. Compared to the red sandstone, which has relatively higher Cu/Al values (averaging 1.09) and Fe3+/Fe2+ ratios (averaging 3.37), the gray sandstone exhibits low Cu/Al values (averaging 0.44) and a low Fe3+/Fe2+ ratio (averaging 0.36), suggesting its formation in a relatively reducing environment. The reducing conditions necessary for the formation of gray sandstone may stem from the climate becoming relatively warm and humid, irrespective of whether it originates from the original sedimentary environment or is induced by the reducing pore water discharged from dark mudstones. The chemical weathering index of fine-grained sediments indicates that, the gray sandstones were commonly configured in relatively humid-warm climate with fairly low dissolved oxygen content in the bottom water. Additionally, the deposition of the uranium-bearing rock series experienced alternating climatic changes, rather than consistently being in a humid-warm climate. This resulted in the sand bodies with low TOC content, insufficient reducing potential, and uranium mineralization primarily occurring in the interior of the basin. Organic matter has a relatively weak control over uranium mineralization, whereas uranium mineralization shows a certain correlation with iron reduction. The ore-forming fluid was uranium-rich oxygenated atmospheric water. Dark-colored mudstones, formed under humid-warm climatic condition, contain relatively high TOC contents (avg. 0.43 %) and exhibit a strong reducing capacity. The reducing pore water in dark-colored mudstones excreted into the adjacent sandstones under compaction during the early diagenesis, is able to serve as a vital reducing agent in later-stage uranium mineralization. Additionally, certain dark-colored mudstones with high uranium content (up to 944 ppm) undergo significant uranium pre-enrichment, providing a partial uranium source for uranium mineralization.

Enrichment and Bioavailability of Phosphate in a Highly Urbanized Zone of Cochin Estuary, South India

ABSTRACT Sequential extraction (SEDEX method) was employed to explore the distribution of sediment-bound phosphorus (P) fractions and the bioavailability of P in Cochin estuary. Concentration of P fractions in surface sediments of the study area ranged as follows: Detrital P > Organic P > Authigenic P > Exchangeable P > Iron-bound P. Generally increasing trend was displayed by all the phosphate fractions toward the downstream sites due to effluent discharges from industrial, domestic, municipal, agricultural sectors and destruction of mangrove forests. The elevated levels of inorganic phosphorus (133.55–918.73 mg/kg) reflected the large-scale delivery of sediment loads associated with great floods during 2018 & 2019. Significant correlations of fine-grained sediment particles with all phosphate fractions indicated surface adsorption as a dominant distribution mechanism. Concentration of iron-bound phosphorus was controlled by adsorption on iron oxy-hydroxides. The buildup of bioavailable phosphate (range: 23.76 to 49.97%) indicate the fact that the estuarine sediments function as a sink for the nutrient. The upper and middle part of the study area recorded more severe contamination and eutrophication risk as indicated by phosphorous pollution index (range: 0.31 to 2.56).

Influence of tidal regime and shelf circulation on sedimentary provenance changes in the middle Okinawa Trough of the East China Sea since the last deglaciation

Deciphering sedimentary provenance in the Okinawa Trough (OT) is vital for understanding the dispersal system from source to sink and the paleoenvironmental evolution in the East China Sea (ECS). Sources include the Huanghe, Changjiang, and Taiwan rivers, as well as the ECS shelf; however, there is controversy over which source has dominated sediment supply since the last deglaciation, and how the tidal currents, shelf circulation, and Kuroshio Current (KC) has controlled the sediment transport. Here, we present a high-resolution record of grain size and clay minerals over the last 14.3 kyrs in an AMS 14C dated piston core from the middle OT. We use these data to reconstruct changes in the KC, identify the provenance of fine-grained sediment, and unravel the impact of paleoenvironmental factors on these provenance changes. A grain size component of 15.6–31.3 μm was extracted to reconstruct the KC changes. These data show that the KC strengthened sharply until 10.2 ka, then kept strong and stable during the Holocene. Clay mineral assemblages indicate that the sediment was mainly derived from the ECS shelf between 14.3 and 8.6 ka, then from the Taiwan rivers after 8.6 ka. The shelf sediment was eroded and transported by intense tidal currents. Taiwan river sediment became the dominant source due to the development of Taiwan Warm Current rather than the strengthening of KC. The ratio of smectite and kaolinite to illite and chlorite content indicates that the sediment contribution from the Huanghe (old Huanghe delta) and Changjiang rivers increased from 7.6 to 5.0 ka and from 3.2 ka to the present. This increase is attributed to the strengthening of East Asian winter monsoon and the consequent intensification of cross-shelf circulation.

Sedimentological characterization of earthquake-generated turbidites in fault-proximal glacial lakes: a case study from Jenny Lake, Teton range, Wyoming

Lakes in seismically active regions preserve valuable sedimentary archives of paleoseismic activity within their catchment and beyond. A series of glacially-excavated lakes positioned directly along the surface trace of the Teton normal fault at the base of the Teton Range, WY, are ideally situated to record past fault activity since their formation approximately 15,000 years ago. Here, we focus on the sediment fill contained in Jenny Lake (5 km2; approximately 73 m max depth) located at the bottom of Cascade Canyon, in the central Tetons, where postglacial slip rates are greatest. Past earthquakes that generated slope failures in and around Jenny Lake are expressed stratigraphically as coseismic turbidite deposits. These deposits were previously identified and dated in sub-bottom profiles and in sediment cores taken from multiple locations around the basin. In this study, we focus on the six thickest turbidites (ranging from 6 to 34 cm thick) present in multiple cores recovered from the central depositional basin and analyze them at sub-centimeter resolution for changes in physical, biological, and geochemical parameters, including sediment density, magnetic susceptibility, grain size distributions, organic content, and elemental composition. Results reveal each deposit contains a well-defined, three-component sedimentary sequence composed of a relatively homogenous and thick basal sandy unit with a sharp bottom contact, a similarly thick silt-rich middle unit, and a thin top unit of very fine-grained sediments. The characteristics of these components and their similarity between individual deposits suggest consistent sediment sources and transport pathways during successive earthquake events. Based on our analyses and the unique local geomorphic setting, we create a mechanistic model of coseismic turbidite formation in Jenny Lake, which may provide an improved framework for identifying, characterizing, and correlating earthquake-generated disturbance deposits in other Teton lakes and lakes in similar tectonic-geomorphological settings, for example, in the Basin and Range Province, USA or beyond.

Paleoenvironmental and Paleoecological Dynamics of the U.S. Atlantic Coastal Plain Prior to and During the Paleocene-Eocene Thermal Maximum

Abstract We studied the rapid paleo-environmental changes and the corresponding biotic responses of benthic foraminifera of a shallow shelf site during the late Paleocene and the Paleocene-Eocene Thermal Maximum (PETM). The PETM is globally characterized by a negative δ13C excursion in marine and terrestrial sediments. Isotope data from the Atlantic Coastal Plain from the South Dover Bridge core, Maryland, show an additional small δ13C excursion just below the base of the PETM: the “pre-onset excursion” (POE). The benthic foraminiferal and coupled grain-size record of the late Paleocene indicates a well-oxygenated, current-dominated environment with a stable, high food supply. During the POE, bottom currents become subdued and finer-grained sediment accumulation increased. These changes are partially reversed after the end of the POE. Before the PETM the river influence increases again, food supply becomes more pulsed and the benthic taxa, typically connected to the PETM, start to appear in those gradually warming conditions. During the PETM, the environment shifts to a river-dominated one, with strongly reduced currents. The low-diversity PETM fauna thrives under episodic low-oxygen conditions, caused by river-induced stratification, while the Paleocene assemblage nearly vanishes from the record. Gradually the environment begins to recover, the grain size shows an uptick in bottom currents and pre-PETM foraminifera become more abundant again, indicating increased oxygen levels and a more stable food supply. While the overall environmental shifts at South Dover Bridge fit within the observations across the shelf, the POE related insights are so far unique. Our bathymetric reconstructions show an outer neritic paleodepth (∼100 m) during the Paleocene, with a modest sea level rise in the core phase of the PETM, which is subsequently reversed during the recovery phase.

Detrital signatures of clastic serpentinite in tectonically diverse settings and interpretation of an example from the Northern Apennines

Abstract Serpentine-bearing sediment, a rare sediment type that is formed and deposited in divergent, convergent, transform, and collisional plate-tectonic settings, carries important evidence of sediment provenance. Specific sources of serpentine-rich sediment display grain assemblages of distinct character that can be used to infer the serpentinization condition and sediment formation. This study reports quantitative and qualitative results on serpentine components in sandstones from Ocean Drilling Program Legs 149 (Iberia), 210 (Newfoundland), and 125 (Mariana and Izu–Bonin regions), and from serpentine-rich debris flows and arenitic breccias in deep-water successions in the Northern Apennine fold–thrust belt. We propose a textural scheme that offers a visual guidance for evaluating serpentinite grains that can be broadly adopted, is easily reproducible, and reduces user bias in determining compositional modes that allow comparison of serpentinite grain populations in arenites from different depositional environments, provenance, and associated tectonic settings. These data allow us to define a scheme for serpentine-dominated deposits that demonstrates the presence of two main groups of grain textures (pseudomorphic and non-pesudomorphic) with specific mineralogy and crystal shape as a function of temperature and pressure in the source rocks. The quantitative analysis of the serpentine-rich arenites and fine-grained sediments derived from forearc and rifted continental-margin settings shows that the studied samples are characterized by high percentages (c. ≥ 80%) of serpentine detritus and subordinate dense minerals and other lithic fragments, including basalt. In rifted continental-margin settings, the prevalent textures in serpentinite sandstones consist of polygonal mesh, mesh-core, and hourglass that all belong to the pseudomorphic category, which preserves the pre-serpentine features and mineralogy. These textures are typically formed in low-temperature conditions (< 390°C); lizardite is the most common mineral, along with minor chrysotile and, in rare cases, antigorite. In contrast, in forearc settings, serpentine-rich grain assemblages exhibit dominantly non-pseudomorphic, interlocking, and interpenetrating textures, dominantly composed of lizardite and recrystallization of lizardite by antigorite. Minor preserved ultramafic minerals related to dynamic recrystallization might be associated with the diapiric rise and protrusion of serpentine bodies. The Northern Apennines case study adopted to test this model indicates that the relationship of detrital serpentine texture to setting can be employed in provenance studies. Firstly, serpentine-bearing sediments derived from ophiolites deformed in fold–thrust belts have more variable serpentinite content, ranging from a few percent to < 10% for samples from deep marine environments, to typically c. 20 to ≤ 50% for stream and beach samples. This compositional variation arises from mixing of sediments derived from deeper to shallower oceanic lithosphere (peridotites and serpentinites) with material from overlying volcanic rocks and sedimentary cover. The deep-water serpentine-rich sands of the Northern Apennines display variable compositions with intermediate characteristics. The source of the serpentine-bearing deposits is interpreted to be a residual oceanic lithosphere characterized by subcontinental mantle-lherzolite originated in the Middle–Late Jurassic by mantle delamination. The serpentinite-dominated debris flows and sand beds contain serpentine grains that exhibit compositional and textural transitions from pseudomorphic to non-pseudomorphic categories, along with changes in mineralogy from lizardite to antigorite. Serpentinite with pseudomorphic texture is observed in the mantle section away from the deformed area. On the contrary, the presence of serpentine-rich arenites with dominant non-pseudomorphic textures suggests derivation from tectonized serpentine along fault scarps and or as products of serpentine diapirism. The detailed serpentinite texture scheme used to classify sand grains in this study includes pseudomorphic (often lizardite, minor crysotile) and non-pseudomorphic textures, with the latter attributed to temperature- and pressure-controlled recrystallization (often to antigorite) or shearing during or after serpentinization. For comparison of different detrital-serpentinite populations, a new ternary plot is proposed where counted parameters are grouped into three end members: undeformed, deformed, and recrystallized. This plot appears to discriminate different sources of detrital serpentine by tectonic setting (e.g., Iberia and Newfoundland margins vs. Mariana forearc) and shows the potential complexity of serpentinite sources in the Apennine basin example. Additional texturally based petrographic data sets are needed to determine the usefulness of this plot in provenance studies.

Onset of aridification in mid-latitude Asia at 41 Ma linked to the proto-Paratethys Sea retreat

The initial aridification of mid-latitude Asia is thought to be one of the most prominent Cenozoic environmental changes in Asia. However, precisely dated, fine-grained sediments documenting initial aridity beyond central Asia are extremely rare. Here, we reconstruct the history of Middle-Late Eocene (47.8–37.8 Ma) hydroclimate change in northeast China using continuous palynological records from a lacustrine sedimentary sequence in the Fushun Basin. Our results indicate that the onset of aridification occurred concurrently with increased climatic variability at 41.2 Ma, as expressed by a sharp increase in xerophytic Ephedra and mountainous coniferous forest at the expense of an abrupt decrease in broadleaved forest, along with frequent alternations between semi-desert and swamp forest vegetation after 41.2 Ma. The climate changed markedly from a stable, warm-wet climate during the Middle Eocene to a cooler, drier climate with pronounced warm-dry and cold-wet oscillations during the Late Eocene. This finding challenges the previously prevailing idea that the East Asian monsoon was significantly enhanced in northeast China after 41–40 Ma. A comprehensive comparison of records from eastern and central Asia demonstrates that the onset of aridification and enhanced climate variability after 41 Ma were widespread across mid-latitude Asia. This may be explained by reduced moisture transport by the westerlies and strengthening of high- and low-latitude forcing linked to the coeval retreat of the proto-Paratethys Sea.

System-scale dynamics of long-term sediment and phosphorus accumulation in contrasting valley-bottom wetlands in dryland landscapes of South Africa

Anthropogenic-induced acceleration of sediment and associated nutrients presents a serious challenge to water resource management across the globe. While wetlands are potential natural solutions, little is known of how well key wetland types attenuate downstream sediment fluxes and how these relate to local- and catchment-scale factors. This research aimed to develop an understanding of patterns and rates of sediment and phosphorus retention in valley-bottom wetlands in South Africa. The geomorphology, sedimentology and historical accretion rates of three wetlands which offered diversity in size, catchment position and degree of organic accumulation were compared to ascertain the impact of local- and catchment-scale factors. Estimates of sediment and associated phosphorus accumulation suggest higher rates in clastic sediment-dominated wetlands (1089–7655 g sediment m−2 yr−1, 0.4–1.6 g-P m−2 yr−1) compared to an organic sediment-dominated system (601–1308 g-sediment m−2 yr−1, 0.2–0.3 g-P m−2 yr−1). Sediment accretion rates generally increased longitudinally, indicating that distal wetland reaches act as sediment and phosphorus retention hotspots. Phosphorus distribution patterns were primarily attributed to variations in organic matter content and associated aluminium and iron complexing agents in fine-grained sediment deposits, while particle size distributions were less important. However, the relevance of individual complexing agents in phosphorus dynamics varied among wetlands with differing landscape conditions. Contrasts between these systems reflect variations in catchment lithology and hydroclimatic regime, which influence the relative magnitude of sediment supply and vegetation productivity. We postulate that intermediate-sized wetlands characterised by catchments around 900 ha in size, gentle longitudinal slopes (∼2 %), a high proportion of fine sediment deposition, limited organic matter breakdown, and a long vegetation growing season provide optimal conditions for sediment and phosphorus accumulation. This system-scale information is essential to guide management decisions to identify and prioritize within-catchment ‘hotspots’ of sediment and phosphorus retention.

Multiproxy approach to characterize the sedimentary process of Cape Darnley Bottom Water flow through the Wild Canyon, East Antarctica

To reconstruct the interaction between Antarctic Bottom Water (AABW) and global climate change, extracting information about past AABW formation from the sedimentary record is crucial. The Prydz Bay region, East Antarctica, has high glacier outflow and is a region of vigorous formation of AABW associated with sea ice formation. The Cape Darnley Bottom Water (CDBW), a primary AABW precursor, flows into Wild Canyon west of Prydz Bay. Thus, the sedimentary record of Wild Canyon can serve as an archive for CDBW flow. In this study, to identify the sedimentary processes and mineral compositions characteristic of CDBW flow, we evaluate deep-sea camera imagery, X-ray computed tomography images, grain-size data, and optical and chemical mineralogical analyses. Heavy minerals in the surface sediments are characterized by pyrope-rich almandine garnets (up to 80% of the heavy-mineral assemblage, 0.2%–3.7% of the dry bulk sediment by weight) typical of nearshore sediment derived from Mac. Robertson Land. In addition, ripples on the channel floor indicate sediment transport occurred as tractional bedload, whereas settling of suspended sediment prevailed on the canyon levee. We propose that modern CDBW flow is the dominant process of sediment reworking through sustained high flow velocities, distinctly different from turbidity currents triggered by ice sheet expansion and rapid retreat. Our results further suggest that fine-grained sediments are transported to the canyon levee as CDBW flows through the submarine canyon. These findings will be a key for reconstructing past AABW formation, which is crucial for understanding the response of AABW to future climate change.

Sedimentary characteristics of mixed source fine-grained gravity-flow and its significance for shale oil exploration in a lacustrine depression basin: A case study of the Chang 73 Sub-member of the Triassic Yanchang Formation in Ordos Basin, NW China

This study investigates the transport and evolution of fine-grained gravity-flow deposits in lacustrine basins, focusing on the relationship between fine-grained deposits and their respective source–reservoir combinations. Such knowledge is vital for the exploration and development of unconventional oil and gas resources within the source-rock series of continental lake basins in China. We investigated the fine-grained gravity-flow deposits of the Late Triassic Chang73 Sub-member within the Yanchang Formation in the Ordos Basin. Our approach combined core observations, field outcrop evaluations, thin section analysis, scanning electron microscopy, geochemical data, and laboratory measurements. The sedimentary facies, characteristics, controlling factors, and sedimentary model of the fine-grained gravity-flow deposits are investigated. The primary sources of fine-grained sediments are terrestrial clastic and volcanic–hydrothermal fine-grained sediments. These sediments are mainly developed in warm, humid environments with abundant precipitation, favoring a highly reducible hypoxic terrestrial brackish–freshwater environment. Ten lithofacies and eight bed types of fine-grained gravity-flow deposits are categorized. Bed types represent deposits of muddy slide deposits, muddy slump deposits, muddy debris flow deposits, muddy flow deposits, hybrid event beds, fine-grained transitional flow deposits, surge-like turbidity flow deposits, and fine-grained hyperpycnal flow deposits. As a result of paleo-seismicity, volcanic events, and paleogeomorphology, instability and subsequent collapse of the delta front sediments led to the formation of gravity-flow deposits. These deposits formed the “channel–lobe complex” and “sublacustrine fan” sedimentary systems in both the southwestern and northeastern parts of the basin, respectively. Meanwhile, extraordinary flood events formed during the Carnian Pluvial Episode (CPE) played a significant role. It prompted increased fluvial drainage that drove fine-grained hyperpycnal flows through pre-existing valley systems, resulting in the establishment of the “channel–lobe complex” sedimentary system. The Chang 73 Sub-member contains high-quality hydrocarbon source rocks, laying the foundation for shale oil. The introduction of volcanic ash and the influence of hydrothermal activities amplify the enrichment of organic matter within the lacustrine basin. Fine-grained gravity-flow deposits yield siltstone-rich sweet intervals, primarily seen in surge-like turbidity and hyperpycnal flows. The interbedding of sandy and muddy sediments, influenced by hybrid event beds and fine-grained transitional flows, establishes a stratigraphic arrangement in which the upper layer nourishes the underlying reservoir. This structure is particularly beneficial for the development of shale oil and gas sweet intervals.

A Quaternary carbonate contourite drift system on the Eyre Terrace, Great Australian Bight: implications for the origin of prograding shelf-edge–upper slope clinoforms, southern Australian passive margin

An appraisal of the Quaternary shelf-margin succession in the western Great Australian Bight has yielded new insights into the development of this prograding passive margin. Our analysis of seismic reflection and sedimentary data from the Eyre Terrace and adjacent outer shelf challenges the established view that this shelf-margin wedge represents a prograding carbonate ramp. Instead, we identify separate outer shelf and upper slope depositional systems: the outer shelf comprises an aggrading to prograding succession whereas the upper slope was constructed and shaped predominantly by alongslope processes driven by the major boundary currents that form the Southern Australia Current System. The latter resulted in the formation of a spectacular 500 m thick, basin-scale, elongate-mounded carbonate contourite drift (the ‘Eyre Terrace Drift’) and associated sediment waves. Sheeted drift and infill drift geometries are also observed. Key sedimentary attributes include fine-grained sediment, multi-scale gradational bed contacts and pervasive bioturbation. The upper flank of the drift provided a platform across which the prograding outermost-shelf facies progressively migrated. The resulting stratigraphic pattern of the Quaternary shelf-margin clinoforms was formed by a combination of ‘conventional’ clinoform progradation from the outer shelf and ‘contouritic clinoform’ progradation and drift accretion on the upper slope.

Experimental study of reservoir flushing through a bottom tunnel initially covered by cohesive sediment

Reservoir sediment flushing, one of the most effective strategies for alleviating reservoir sedimentation, involves discharging sediment-laden flows downstream through bottom tunnels. However, whether flushing can be accomplished if the intake of a bottom tunnel is initially covered by cohesive sediment remains poorly understood. Here, flume experiments were done to investigate cohesive sediment flushing in a reservoir. It is demonstrated that cohesive sediment in a reservoir is harder to flush than non-cohesive sediment. A higher water level in the reservoir, initially smaller cover layer thickness, and lower dry density of the sediment favor the occurrence of sediment flushing. The flushing process of cohesive sediment is significantly affected by seepage. Under the combined action of gravity erosion and water erosion, the scour hole upstream of the dam is characterized by angular and broken edges. The threshold conditions for flushing of non-cohesive and cohesive sediments are evaluated. Empirical formulas applicable to both non-cohesive and cohesive sediment are proposed to estimate the equilibrium scour depth immediately upstream of the bottom tunnel intake. Also, empirical models are proposed for the time variation of sediment position in the bottom tunnel. The current findings are significant for informing the design and operation of reservoirs on rivers carrying fine-grained cohesive sediment in support of reservoir benefits and capacity preservation.

The Safi boulders (Morocco): Evidence of past extreme wave events

Tsunamis and storm surges are among several hazards threatening the Atlantic coast of Morocco. During the last two decades, sedimentological deposits left by these events, in the form of fine-grained sediments (washover) and boulders, have been studied along the Moroccan shores to determine the return period of these events and how they fluctuate in terms of intensity. The present work investigates two new boulder fields located along the Safi coast. Field surveys were accomplished in the two areas to collect the position/elevation and dimensions of 164 boulders using a GPS/DGPS and a traditional meter, respectively. The field data was combined with hydrodynamic equations to estimate the minimum velocity and storm/tsunami wave height to move the measured boulders. In site 1, the boulders are situated on a rocky intertidal platform, while in the second, they are on the top of a cliff. For both sites, the size of the boulders ranges from 0.6 to 5.60 m, 0.54 to 3.6 m, and 0.27 to 1.5 m on the major (a), medium (b), and minor (c) axes, respectively. They reach a volume of up to 25.20 m3 and a weight of 55.44 t. The maximum distance and elevation recorded are around 41.77 m and 13 m, respectively. According to hydrodynamic equations, the coastal platform boulders (site 1) can be moved by tsunami waves with heights between 0.10 and 0.59 m, and storm waves from 0.42 to 2.37 m. To be displaced, the cliff-top boulders (site 2) need tsunami and storm waves with at least 5.91 and 8.64 m, respectively. The results from the field surveys and hydrodynamic equations suggest that both tsunami and storm waves can move the boulders in site 1. For the cliff top-boulders, the data favours tsunamis as a source for this deposit.

Circulation of hydraulically ponded turbidity currents and the filling of continental slope minibasins

Natural depressions on continental margins termed minibasins trap turbidity currents, a class of sediment-laden seafloor density driven flow. These currents are the primary downslope vectors for clastic sediment, particulate organic carbon, and microplastics. Here, we establish a method that facilitates long-distance self-suspension of dilute sediment-laden flows, enabling study of turbidity currents with appropriately scaled natural topography. We show that flow dynamics in three-dimensional minibasins are dominated by circulation cell structures. While fluid rotation is mainly along a horizontal plane, inwards spiraling flow results in strong upwelling jets that reduce the ability of minibasins to trap particulate organic carbon, microplastics, and fine-grained clastic sediment. Circulation cells are the prime mechanism for distributing particulates in minibasins and set the geometry of deposits, which are often intricate and below the resolution of geophysical surveys. Fluid and sediment are delivered to circulation cells by turbidity currents that runup the distal wall of minibasins. The magnitude of runup increases with the discharge rate of currents entering minibasins, which influences the amount of sediment that is either trapped in minibasins or spills to downslope environs and determines the height that deposits onlap against minibasin walls.

Source fingerprinting sediment loss from sub-catchments and topographic zones using geochemical tracers and weathering indices

Application of sediment source fingerprinting techniques to generate reliable information on sediment sources is growing, because of the global environmental importance of elevated fine-grained sediment mobilisation and delivery to rivers. Despite the application of fingerprinting at different spatial scales (e.g., land use types, geological units, and tributary sub-catchments), it has rarely been used to investigate the sediment contribution of topographic zones. The application of sediment sourcing techniques to topographic zones can be helpful in better understanding the spatial distribution of sediment sources within a catchment and the contribution of different topographic features to sediment production and transport. Against this background, this study assessed both topographic zone and tributary sub-catchment spatial sediment sources in the Kan River catchment, Iran. For this purpose, targeted sampling was performed for each classification of spatial sediment sources, and different tracers (geochemical, elemental ratios, and weathering indices) were measured for 80 surface soil samples collected in topographic zones, 20 bed sediment samples collected in sub-catchments, and 11 suspended sediment samples retrieved at the outlet of the main catchment. Three new statistical approaches were applied to identify composite signatures for spatial sediment source discrimination. These approaches combined the Kruskal-Wallis H-test with Particle Swarm Optimization (KW-PSO), Multi-Layer Perceptron (KW-MLP), and Radial Basis Function (KW-RBF) algorithms. A Bayesian un-mixing model was used to apportion the spatial sediment sources. The three composite signatures identified for elucidating the contributions from either classification of spatial sources generated consistent results. The results clarified that the median contribution from topographic zone 1 equated to it being the major spatial sediment source on the basis of topographic zones, with the contributions estimated at 57%, 49%, and 52% (5%-95% uncertainty range) based on the KW-PSO, KW-MLP and KW-RBF signatures, respectively. In terms of the tributary sub-catchments, the Sangan sub-catchment was identified as the dominant spatial sediment source, with corresponding estimated median contributions of 84%, 83%, and 97%, respectively. A combination of weathering indices and elemental fingerprint properties generated statistically robust composite signatures, providing further evidence that the former can augment more conventional tracers. The predicted source proportions can be explained by topographic characteristics from the digital elevation model of the different spatial topographic zones. Therefore, we conclude that topographic features can be an effective predictor of sediment sources.

Variable downcanyon morphology controlling the recent activity of shelf-incised submarine canyons (Alboran Sea, western Mediterranean)

This research aims to distinguish genetic sedimentary processes building canyon geomorphological patterns and the factors driving different sedimentary activities in two nearby Mediterranean shelf-incised submarine canyons (Carchuna and Motril) that exhibit different degrees of incision on the narrow margin of the northern Alboran Sea.The straight Carchuna Canyon incises the shelf up to 200 m off the coastline and exhibit steep canyon walls featuring narrow terraces, muddy sands with high contents of organic matter along the thalweg, and transported shelf benthic foraminifera in distal settings. The Motril Canyon head is wider and incises the shelf edge, ca. 2 km off the coastline. It exhibits a sinuous morphology and less steep walls, wider terraces, and higher sedimentation rates with muddy sediments along the thalweg. In both canyons, cross-section relief, width, incision, and area decrease downslope, although these parameters increase locally.The downslope variations of geomorphological parameters are attributed to enhanced erosional/depositional processes promoted by tectonically controlled abrupt changes of the axial channel orientation. The degree of shelf incision, the location of the canyon heads in relation with the local sediment sources, and the seasonally variable hydrodynamic regimes determine the different degrees of recent canyon activity. The Motril Canyon is interpreted as a mature system that reflects episodic activity, collecting fine-grained sediments from the nearby Guadalfeo River. The Carchuna Canyon exhibits a youthful developmental stage whose activity is more continuous and involves sediment trapping of littoral cells and continuous downslope sand transport.

Serpula? alicecooperi sp. nov. – a new serpulid from the Lower Jurassic (Pliensbachian) Hasle Formation of Bornholm, Denmark

Serpulid remains are very rare in the Lower Jurassic Hasle Formation of Bornholm, Denmark. A historical specimen mentioned, but not figured by Malling & Grön wall (1909) was reexamined and attributed to Pentaditrupa quinquesculcata and here f igured for the first time. New finds of additional well-preserved serpulid tubes are described as Serpula? alicecooperi sp. nov, which show adaptations for a lifestyle on fine-grained sediment in a nearshore environment.

Overcoming inter-dataset discrepancies in the grain size distributions of fine-grained sediments by partial least squares regression: A case study of the Belgian Boom Formation

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.

The effect of microfabric heterogeneity on shear wave properties in fine-grained sediments

The water-sediment interface is highly susceptible to physical, biological, and chemical processes, which can create significant levels of heterogeneity in sediment biogeoacoustic properties at various spatial scales that affect transmission and scattering of elastic waves. Using a suite of microscopy, spectroscopy, and medical sonoelastography techniques, relationships between heterogeneity in the sediment microfabric and shear wave speed can be investigated at spatial scales ranging from centimeters to micrometers. Here, we study fine-grained sediments extracted from two unique intertidal mudflats in the Great Bay Estuary, New Hampshire, USA. Shear waves were excited via acoustic radiation force and external vibration in the 50–200 Hz frequency band, and particle velocity was sensed in the top 2 centimeters of sediment at approximately 0.5-micrometer spatial resolution using a research-grade medical imaging array. By directly imaging the propagating shear wave, spatial heterogeneity of shear wave speed was directly measured. Next, subsamples were extracted and prepared for microscopy, where volume fraction and orientation of organic, mineral, and aqueous components were studied at various spatial scales. A comparison of between shear wave speed and microfabric parameters will be presented. [Work sponsored by ONR.]

Reactive transport modeling constraints on the complex genesis of a lacustrine dolomite reservoir: A case from the Eocene Qaidam Basin, China

Reactive transport modeling (RTM) is an emerging method used to address geological issues in diagenesis research. However, the extrapolation of RTM results to practical reservoir prediction is not sufficiently understood. This paper presents a case study of the Eocene Qaidam Basin that combines RTM results with petrological and mineralogical evidence. The results show that the Eocene Xiaganchaigou Formation is characterized by mixed siliciclastic-carbonate-evaporite sedimentation in a semiclosed saline lacustrine environment. Periodic evaporation and salinization processes during the syngenetic-penecontemporaneous stage gave rise to the replacive genesis of dolomites and the cyclic enrichment of dolomite in the middle-upper parts of the meter-scale depositional sequences. The successive change in mineral paragenesis from terrigenous clastics to carbonates to evaporites was reconstructed using RTM simulations. Parametric uncertainty analyses further suggest that the evaporation intensity (brine salinity) and particle size of sediments (reactive surface area) were important rate-determining factors in the dolomitization, as shown by the relatively higher reaction rates under conditions of higher brine salinity and fine-grained sediments. Combining the simulation results with measured mineralogical and reservoir physical property data indicates that the preservation of original intergranular pores and the generation of porosity via replacive dolomitization were the major formation mechanisms of the distinctive lacustrine dolomite reservoirs (widespread submicron intercrystalline micropores) in the Eocene Qaidam Basin. The results confirm that RTM can be effectively used in geological studies, can provide a better general understanding of the dolomitizing fluid-rock interactions, and can shed light on the spatiotemporal evolution of mineralogy and porosity during dolomitization and the formation of lacustrine dolomite reservoirs.