Sedimentary Features Research Articles

Paleolimnological record of the Pampean plains (Argentina) as a natural archive of South American hydroclimatic variability since the LGM to the Current Warm Period

Laguna Mar Chiquita (LMC, 30°54′S – 62°51′W) is a highly variable and shallow saline lake, located in the Pampean Plains of Argentina. The paleolimnological record of LMC contains information on the environmental variability that occurred in a large area of Southern South America since the Last Glacial Maximum (LGM) until the present. As inferred from the diatom assemblages, sedimentary features and geochemical proxies, prolonged intervals of high and lowstands have caused variations in water salinity, trophic state and sedimentary processes. This is the first paleolimnological reconstruction covering the hydroclimatic variability that occurred over the last 25,000 cal yr BP in the Argentinean Pampean region. Results are in accordance with well-known global climatic phases. The Late Pleistocene record is characterized by a scenario dominated by lowstands, hypersaline, and oligotrophic lake conditions. Radiocarbon ages (25,000–19,000 cal yr BP) indicate that the onset of the record is coeval with the LGM. Later, a progressive lake water level increase was registered at ⁓17,000 cal yr BP, which can be assigned to the Heinrich Stadial 1. A shift toward comparatively drier conditions identified in the record between ⁓14,700 and 13,000 cal yr BP can be chronologically related to the Antarctic Cold Reversal. The transition from the Late Pleistocene to the Holocene (13,500–10,500 cal yr BP) is recorded by highstand phases while the Early Holocene record is characterized by high to intermediate water levels. The hydrological reconstruction corresponding to the Mid-Holocene is characterized by alternating phases of high/lowstands. The onset of the Late Holocene record is marked by the development of dry conditions and thus lowstand phases, while around 1300 cal yr BP a distinct water lake level increase is recognized. This humid phase, ascribed to the Medieval Climatic Anomaly, is followed by the record of lowstands between 500 and 1000 cal yr BP, being coeval with the Little Ice Age. The uppermost part of the record of LMC encompasses the Current Warm Period, showing a marked shift towards high lake water level conditions. The hydrological variability registered in the paleolimnological record, highlights the importance of Laguna Mar Chiquita as an outstanding paleoclimate sensor of hydroclimatic variations for a large area of South Eastern South America.

Combining implicit geological modeling, field surveys, and hydrogeological modeling to describe groundwater flow in a karst aquifer

In three-dimensional (3-D) implicit geological modeling, the bounding surfaces between geological units are automatically constructed from lithological contact data (position and orientation) and the location and orientation of potential faults. This approach was applied to conceptualize a karst aquifer in the Middle Triassic Muschelkalk Formation in southwest Germany, using digital elevation data, geological maps, borehole logs, and geological interpretation. Dip and strike measurements as well as soil-gas surveys of mantel-borne CO2 were conducted to verify the existence of an unmapped fault. Implicit geological modeling allowed the straightforward assessment of the geological framework and rapid updates with incoming data. Simultaneous 3-D visualizations of the sedimentary units, tectonic features, hydraulic heads, and tracer tests provided insights into the karst-system hydraulics and helped guide the formulation of the conceptual hydrogeological model. The 3-D geological model was automatically translated into a numerical single-continuum steady-state groundwater model that was calibrated to match measured hydraulic heads, spring discharge rates, and flow directions observed in tracer tests. This was possible only by introducing discrete karst conduits, which were implemented as high-conductivity features in the numerical model. The numerical groundwater flow model was applied to initially assess the risk from limestone quarrying to local water supply wells with the help of particle tracking.

Assessing Subaquatic Mass Movement Hazards: an Integrated Observational and Hydrodynamic Modelling Approach

High-resolution lake and reservoir bathymetric surveys can pinpoint locations that may experience underwater landslides (subaquatic sedimentary mass movements). These can pose a risk to underwater and shoreline infrastructure. This paper outlines an approach for using spatial variation in sedimentary patterns to identify areas susceptible to subaquatic mass movements in lakes and reservoirs. This study focusses on Lake Biel (Switzerland), which has experienced a protracted history of upstream alteration of river flow. Altered flow patterns increase risk of unstable sedimentary features and subaquatic mass movements. Data from sediment traps and cores, Acoustic Doppler Current Profilers and results from a 3D hydrodynamic model gave a consistent picture of spatial and temporal variation in weather-related sedimentation. Erosion caused by short-term rain events contributes the largest proportion of sediments to the lake. Strong rain events combine with typical wind patterns to drive lake circulation. The net effect results in preferential sedimentation onto a steeply sloping shelf prone to subaquatic slides. The integrated approach outlined here incorporates short- and long-term sediment dynamics to provide a systematic assessment of lake sedimentation and potential mass movement hazards. This research represents a first step in developing a risk-evaluation tool for aquatic hazard evaluation.

The Orphan Tsunami of 1524 on the Konkan Coast, Western India, and Its Implications

In comparison to the east coast, the tsunami hazard for the west coast of India remains under-recognized, despite the impact in 1945 following a Mw 8.1 earthquake in the Makran subduction zone in the northern Arabian Sea. The previous occurrences of tsunamis in the Arabian Sea that would have a bearing on the west coast of India are being debated, including the question whether the Makran region has the potential to generate greater-magnitude earthquakes. With this in the backdrop, we present here the historical and geological evidence of a tsunami impact zone from a site on the Konkan Coast of western India. Located in the village of Kelshi, the impact zone is preserved within a coastal dune complex that also reveals occupation layers. This laterally extending 30–40-cm-thick zone, coinciding with a habitation level, displays varied sedimentary structures including scour-fill features, and is inter-layered with shells, at a height of ~ 3 m from the high-tide level. We attribute these sedimentary features to a tsunami flooding event that was contemporaneous with the transportation of shells, dated at 1508–1681 CE. The geological inference matches with the description by the Portuguese fleets of a sea disturbance in 1524 CE, reported from Dabhol, not far from Kelshi, and also from the Gulf of Cambay, located about 500 km to the north. Precluding submarine landslide scenarios, the modeling results suggest that the high impact in Kelshi could have been generated by a Mw ≥ 9 earthquake sourced in the Makran subduction zone. It is, however, intriguing how a Mw ≥ 9 earthquake in the Makran region finds no mention in the historical documentation. We underscore the need for fresh efforts along the Makran coast to reconstruct the tsunami recurrence history that would generate required validating constraints on the 1524 event, if it was indeed generated by a massive earthquake among other mechanisms.

Carbonate diagenesis in fourth-order sequences: A case study of yingshan formation (Lower Ordovician) from the yubei area -Tarim Basin, NW China

The Yingshan Formation in Lower Ordovician is an important hydrocarbon reservoir of the Tarim Basin, northwest China; however, the factors controlling reservoir development in the formation are not fully understood. Data from thin-section observations, scanning electron microscopy, and gamma-ray logging were applied to produce a fourth-order sequence stratigraphic framework for the Yingshan Formation in the Yubei area of the Tarim Basin. The framework was assessed in terms of the carbonate diagenesis of the reservoir rock. The influence of diagenesis on reservoir porosity, the factors restricting reservoir quality, and the significance of the results for exploration are discussed. Diagenesis, dolomitization, and cementation by atmospheric fresh water occurred in the lowstand systems tracts. The sediments in the transgressive systems tract were consolidated by seawater, or dolomitization occurred in the landward mixed zone, or the sediments quickly entered the shallow burial marine diagenetic environment and became compacted. The highstand systems tract experienced significant diagenesis, with seafloor cementation, atmospheric water dissolution and dolomitization. These factors were related to sea-level fluctuation and sedimentary features. The porosity of some high-frequency sequences in the lower part of the Yingshan Formation was improved significantly. In the Yubei area, cementation of the middle part of the fourth-order sequence during burial reduced the porosity and permeability of the reservoir. This was the main cause of reservoir deterioration. During oil and gas exploration, attention should be paid to the higher degree of dissolution in the middle part of the fourth-order sequence and the superposition of atmospheric fresh-water dissolution in the upper part of the sequence.

Challenges to carbonate-evaporite peritidal facies models and cycles: Insights from Lower Cretaceous stromatolite-bearing deposits (Oncala Group, N Spain)

Peritidal carbonate-evaporite successions, since they are developed in the transition between continental and marine realms, provide essential keys for palaeobathymetric and palaeoclimatic interpretations. As a result, several facies models have been proposed to assist on the interpretation of ancient tidal flat deposits, and peritidal successions have been extensively used for cyclicity analyses. In this study, well-exposed, Lower Cretaceous peritidal deposits (Oncala Group, Cameros Basin, N Spain) are analysed and compared with the most commonly-used present-day analogues (from Shark Bay, the Arabian Gulf and the Bahamas) and with ancient peritidal successions, providing their palaeoenvironmental and palaeoclimatic interpretation, assessing the usefulness and limitations of the facies models, and evaluating the suitability of these deposits for analysis of decimetre to metre-scale cycles.The studied peritidal deposits consist of thinly-bedded to laminated dolostones, dolomitic stromatolites, stromatolite breccias, flat-pebble and edgewise breccias, and calcite and quartz pseudomorphs after anhydrite nodules. Abundant resemblances of the peritidal deposits of the Oncala Group with those of Shark Bay, including that they are largely composed of microbialites and intraclasts, makes the peritidal deposits of the Oncala Group one of the best fossil analogues of this present-day setting. However, the presence of anhydrite nodules indicates pervasive evaporite precipitation in the supratidal zone, which is a feature that does not occur in supratidal flats of Shark Bay, but is characteristic of arid sabkhas of the Arabian Gulf. Nevertheless, the fact that carbonate-evaporite tidal flats of the Oncala Group were laterally related with siliciclastic tidal flats with large freshwater input and broadly inhabited by dinosaurs, suggests that anhydrites precipitated under less arid climates than those of the Arabian Gulf nowadays, pointing to semiarid climatic conditions during deposition. Moreover, the fact that peritidal deposits with anhydrite nodules were exclusively formed in a low-subsidence area of the Cameros Basin suggests that the rate of accommodation space creation also played an important role in their development. Regarding the comparison with other fossil peritidal sediments, the studied deposits show more abundant similarities with Proterozoic and Cambrian successions, composed mainly of stromatolites, microbial laminites, and intraclasts, than with other Mesozoic peritidal deposits, in which bioclasts and burrowing are usually more abundant. This highlights the difficulties for assigning specific features to certain geological ages. Finally, peritidal facies of the Oncala Group may change laterally and vertically to any other facies, showing a patchy lateral distribution of facies and an unsystematic vertical stacking pattern. The sedimentary features of the stromatolite, breccia and thinly-bedded to laminated dolostone facies do not allow their assignment to a unique tidal zone. Moreover, sedimentary features indicative of subaerial exposure, such as anhydrite nodules formed in the capillary zone, occur within any of the carbonate facies and show limited lateral extent. This results in a succession that cannot be clearly subdivided into subsequent shallowing-upward cycles not even by using erosive surfaces or the anhydrite nodule layers as marker horizons of the upper part of the cycles, because their limited lateral extent prevents reliable correlations. Similar composite lateral and vertical facies relationships have been documented both in the present-day analogues and in ancient successions, which suggests that this kind of facies relationships may be common in peritidal successions and highlights the caution that must be taken when trying to perform cyclicity analysis on them.

The role of seagrass vegetation and local environmental conditions in shaping benthic bacterial and macroinvertebrate communities in a tropical coastal lagoon

We investigated the influence of seagrass canopies on the benthic biodiversity of bacteria and macroinvertebrates in a Red Sea tropical lagoon. Changes in abundance, number of taxa and assemblage structure were analyzed in response to seagrass densities (low, SLD; high, SHD; seagrasses with algae, SA), and compared with unvegetated sediments. Biological and environmental variables were examined in these four habitats (hereafter called treatments), both in the underlaying sediments and overlaying waters, at three randomly picked locations in March 2017. Differences between treatments were more apparent in the benthic habitat than in the overlaying waters. The presence of vegetation (more than its cover) and changes in sedimentary features (grain size and metals) at local scales influenced the observed biological patterns, particularly for macroinvertebrates. Of note, the highest percentage of exclusive macroinvertebrate taxa (18% of the gamma diversity) was observed in the SHD treatment peaking in the SA for bacteria. Benthic macroinvertebrates and bacteria shared a generally low number of taxa across treatments and locations; approximately, 25% of the gamma diversity was shared among all treatments and locations for macrofauna, dropping to 11% for bacteria. Given the low overlap in the species distribution across the lagoon, sustaining the connectivity among heterogeneous soft sediment habitats appears to be essential for maintaining regional biodiversity. This study addresses a current scientific gap related to the relative contributions of vegetated and unvegetated habitats to biodiversity in tropical regions.

Mineral composition and particle size distribution of river sediment and loess in the middle and lower Yellow River

Hyperconcentrated flows often occur in the middle and lower Yellow River (MLYR) and its tributaries, within which the main sediment source originates from the Loess Plateau of China due to serious water erosion. Little is known about the properties of river sediment that is transported by hyperconcentrated flows, particularly with respect to the mineral composition and size distribution. Samples of sediment and loess were collected in the northern, middle, and southern Loess Plateau and the mainstream and tributaries of the MLYR. A total of 18 loess samples and 24 river sediment samples were analyzed to determine their sediment size distribution and mineral composition. The bottom loess samples reflected the original sedimentary features of the Loess Plateau, and the median particle size reduced, and the clay content increased from the north to the south of the study region. The surface loess has been weathered under the action of wind and rainfall, and the clay particle content in the surface loess samples was higher than that in the undisturbed bottom loess. Erosion of the surface soil due to rainfall and surface runoff means that fine particles (mostly clay) have been washed away. The median diameter of surface loess particles was a little larger than that of the bottom loess particles where water erosion dominates. The particle size became coarser with increasing distance from the estuary in the MLYR, which reflects depositional sorting in the river channel. Significant logarithmic relations were found between the median diameter of the sediment particles and the i) non-clay mineral content and ii) clay mineral content. Thus, clay and non-clay mineral compositions can be conveniently estimated from the particle size distribution.

Spatial and seasonal variability of sedimentary features and nitrogen benthic metabolism in a tropical coastal area (Taganga Bay, Colombia Caribbean) impacted by a sewage outfall

The effects of anthropogenic pressures in coastal areas are extensively studied in temperate but not in tropical zones, where their impact might be amplified by high water temperatures and upwelling phenomena. Sedimentary features and benthic metabolism were studied during the non upwelling (NUPW) and upwelling (UPW) seasons in Taganga Bay (Colombia). The bay is impacted by a submarine outfall of virtually untreated, organic and nutrient-rich wastewater. Samplings were performed in November 2017 (NUPW) and in January–February 2018 (UPW) at 4 stations located in the proximity and 100, 750 and 1800 m far from the outfall, respectively, at depths between 22 and 28 m. Aerobic respiration, denitrification, dissimilative nitrate reduction to ammonium (DNRA) and nutrient fluxes were measured. The influence of the outfall was detectable 750 and 1800 m away from the point pollution source, where δ13C data suggested that ~ 40 and ~ 20% of organic inputs were terrigenous, respectively. In the proximity of the outfall benthic oxygen demand peaked and the presence of Beggiatoa mats suggested reoxidation of sulphides, that were abundant in pore water. Under sulfidic conditions, DNRA was the major driver of nitrate demand, whereas at stations far from the outfall, denitrification dominated nitrate consumption. Organic matter and nitrate inputs to the bay during the UPW season enhanced the effects of the outfall by increasing aerobic respiration and DNRA. Higher N availability during the UPW season reversed fluxes of molecular nitrogen and turned the sediments of 3 out of 4 sites from net sinks to net N2 sources. Results from this study suggest that the analysis of sediments allows tracing the impact of the outfall and that such impact is enhanced during the upwelling season. In tropical areas, marine outfalls and upwelling may act in synergy and contribute to ecosystem deterioration due to high temperatures, increase of microbial respiration, sulphide toxicity and benthic biodiversity loss.

Microstructures and Formation of Tundish Clogging Deposits in Ti-Alloyed Al-Killed Steel

The continuous casting of Ti-alloyed Al-killed steels traditionally experiences clogging in the tundish, impeding stable operation. We studied the microstructures and mode of formation of such clogging deposits in tundish skulls recovered after long cast series. We sampled the steel skull along the whole tundish bottom from under the ladle shroud (impact zone) up to the tundish well and nozzle. The microstructures imply that the steel solidifies to the refractory (gunning mass) and remains partially solid through the entire cast. Thus, a “mushy zone” of steel loaded with solid delta ferrites is present along the margins of the steel during casting. In this mushy zone, the widely observed clusters of variably sintered alumina (NMI) accumulate, along with a substantial amount of gas bubbles acting as inclusion scavengers. The clustered alumina particles show a lognormal particle size distribution (PSD) in all locations, which becomes more pronounced if correcting for the observable sintering. Sintering of alumina size distributions linearizes a PSD, and further evolution, once the deposit is locked in place, occurs by Ostwald ripening, introducing skewness to the distributions. The observed PSDs contrast with the established log-linearity of PSDs of individual floating alumina NMI in the bulk steel, and suggest that the source of the alumina in the clogging deposits is not the passing bulk steel. However, ubiquitous sedimentary features show that the alumina particles are also not grown by reoxidation in situ. An upstream source separate from the bulk stream must be inferred, which is also required by the consistent sporadic presence of oxidic materials which cannot be derived from local sidewall reaction. In summary, the development of clogging deposits in this type of steel is an example of “slurry casting” of partially solidifying steel over thermal loss surfaces of refractories in the nozzle area.

Slope destabilization provoked by dissociation of gas hydrates in the Outer Carpathian basin during the Oligocene: Sedimentological, petrographic, isotopic and biostratigraphic record

Sedimentological, biostratigraphic, petrographic, and stable C and O isotope study was carried out on chaotic complexes hosting carbonate concretions in the uppermost Oligocene successions of the Outer Carpathians. These chaotic complexes reveal a range of sedimentary features indicative of intrabasinal, submarine mass-wasting deposition, including soft-sediment deformation, brecciation, homogenization and fluidization of the parent material. Apart from carbonate concretions, they enclose blocks of rocks having lithologies typical for the underlying Oligocene strata. The concretions exhibit extremely depleted δ13C (by down to −51.3‰) indicating anaerobic oxidation of methane, whereas their δ18O values are strongly enriched relative to coeval brackish basin water indicating dissociation of gas hydrates as the main fluid source. These data and the geotectonic context imply that the chaotic complexes formed as submarine slumps on a N-inclined paleoslope of the basin in consequence to tectonically-induced dissociation of gas hydrates. Hydrates were destabilized within the sediments due to synorogenic uplift, related to northward progradation of the accretionary prism. These slumps and hosted concretions were dated by the combination of calcareous nannoplankton and organic-walled dinoflagellate cysts. The age of concretions indicated by dinoflagellate cyst assemblages always overlaps with that of the surrounding slump matrix determined by both micropaleontological groups. Together with sedimentological and petrographic evidence, this confirms that the concretions formed in situ within the sediment shortly after deposition and prior to slumping. Hydrate dissociation, seepage of methane-charged fluids and the resultant slope destabilization was not an isolated event, but a continuous process occurring diachronously in front of N-prograding accretionary prism during Oligocene, at least from early NP23 to late NP25 biozones.The stable C and O isotope analysis of carbonate concretions was crucial for this research, because the low δ13C and high δ18O values were the only preserved indicators of anaerobic oxidation of methane and dissociation of gas hydrates in the basin, respectively. Because, the concretions are composed of a mixture of dolomite and calcite, a semiautomatic method for measuring C and O isotope composition selectively for calcite and dolomite (Baudrand et al., 2012) was applied, which provided conclusive results. These measurements were combined with detailed petrographic investigations, which enabled assessment of isotopic composition for various carbonate constituents separately, e.g. sedimentary calcite (biogenic and detrital), early and late calcite cements, dolomite cement. They showed that the concretionary microcrystalline calcite was the main cement that precipitated due to hydrate dissociation and subsequent methane oxidation. This approach can provide valuable information when applied to other authigenic carbonates composed of a mixture of dolomite and calcite.

DADOS MULTI-SENSORES PARA RECONHECIMENTO E MAPEAMENTO DE AMBIENTES SEDIMENTARES NA PLANÍCIE DO RIO AMAZONAS

Este trabalho propôs utilizar imagens de Synthetic Aperture Radar (SAR) que operam no intervalo das microondas, assim como imagens ópticas que operam no Visible-Near Infrared (VNIR) para reconhecer e mapear os principais ambientes fluviais do Baixo Amazonas. A área-teste (Ilha Grande do Tapará, que correspondente ao setor central do Baixo Rio Amazonas) foi escolhida com o intuito de aumentar a acurácia e qualidade de pontos de controle usados na interpretação dos ambientes da planície. O mapeamento das feições sedimentares foi baseado em classificações não supervisionadas e intepretação visual das imagens Ópticas, SAR e da imagem SPC-SAR resultante do sinergismo entre as imagens iniciais. Os resultados permitiram a identificação de classes lama, areia, vegetação arbórea e pasto aquático, resultando na associação de ambientes de lagos, diques, canais fluviais, deltas fluviais e praia fluvial. Os índices de acurácia geral obtidos pelos classificadores foram de 93,33% e 0,90 para a imagem do sensor óptico; 93,33% e 0,89 para a imagem SAR; e 78,25% e 0,63 para o produto SPC-SAR. Os resultados indicam que o uso dos dados associados nos fornece importantes e precisas informações sobre a planície e, ainda, que o produto integrado contribuiu para o mapeamento da área estudada.

Southward migration of the austral limit of mangroves in South America

Temperature influences the global distribution of mangroves, and global warming may be causing a poleward mangrove expansion. Sedimentary features, pollen, and isotopes data from six sediment cores, as well as 14C datings, indicated a marine transgression during the Holocene, and it contributed to the expansion of tidal flats occupied by saltmarshes. Environmental conditions suitable for mangroves development occurred on the study site during the Holocene, but, according to 210Pb and 14C dating, the establishment of mangroves mainly represented by Laguncularia trees only began between ~1957 and ~1986 (AD) on the studied tidal flats. Spatial-temporal analysis, based on satellite and drone images, revealed a mangrove expansion of ~10 ha in the study area between 2003 (96.1 ha) and 2019 (106.1 ha). Nowadays, in the study area, saltmarshes, mainly characterized by Spartina and Acrostichum, are sharing tidal flats with mangroves, represented by Laguncularia (≤5 m tall) and Avicennia (≤11 m tall). Probably, the absence of mangroves during the Holocene, followed by their establishment and expansion during the Anthropocene in the subtropical zone, is associated with a migration of the austral mangrove limit into the temperate zone, caused by the gradual increase in winter temperatures. This process may be related to a poleward mangrove migration since the late Holocene, caused by a natural Holocene global warming. However, the industrial-era warming must have intensified the mangrove expansion into temperate zones.

On the dolomite reservoirs formed by dissolution: Differential eogenetic versus hydrothermal in the lower Permian Sichuan Basin, southwestern China

Dolomite reservoirs formed by dissolution are significant for hydrocarbon exploration, but their origins are poorly understood because of their complexity. This study investigated the lower Permian Qixia Formation of the Sichuan Basin (northern Longmenshan Mountains), southwestern China, which has recently been the location of giant natural gas discoveries. Petrological and geochemical analyses were undertaken, including trace elements and C, O, and Sr isotopes. The results indicate that the reservoirs were formed by differential eogenetic karstification and hydrothermal dissolution of grain dolomites. Core and thin-section observations identified three types of dolomite based on grain size: fine- to medium-crystalline dolomite (Dol-1), medium- to coarse-crystalline dolomite (Dol-2), and coarse- to megacrystalline saddle dolomite (Dol-3). The Dol-2 types can be further divided into three subtypes according to the grain structure and degree of pore development: euhedral medium- to coarse-crystalline dolomite with pores (Dol-2a), porous subhedral–anhedral medium- to coarse-crystalline dolomite (Dol-2b), and tight subhedral–anhedral medium- to coarse-crystalline dolomite (Dol-2c). The Dol-1 types appear to have formed because of the release of and subsequent metasomatism by Mg2+ in trapped seawater along stylolites at shallow burial conditions (500–1000 m). The Dol-2a types resulted mainly from eogenetic karstification of original limestone and subsequent dolomitization by marine-related fluids. These dolomites are geochemically characterized by relatively flat rare-earth element patterns and relatively low Y/Ho values (mean e 37.1). The Dol-2b and Dol-3 types were formed mainly by hydrothermal fluids and have similar rare-earth element patterns with large positive Eu anomalies (mean e 5.7). The Dol-2c types experienced metasomatism by marine-related fluids mixed with minor hydrothermal fluids. All these formation mechanisms reflect the influence of multistage diagenetic and hydrothermal fluids. The Dol-2a types form the most favorable reservoirs in the study area, followed by the Dol-2b types. The distribution of the Dol-2a types was controlled by eogenetic karstification of grain shoals, whereas that of the Dol-2b types reflects the influence of tectonically derived hydrothermal fluid dissolution of grain shoals. As such, both primary sedimentary features and later differential dissolution (eogenetic and hydrothermal) contributed to reservoir development, which involved factors such as the sedimentary facies (grain shoal), sea-level fluctuations (eogenetic karst development), and faulting (hydrothermal dissolution). Therefore, the formation of dolomite and associated reservoirs is complex. The results of this study provide an improved understanding of the origins and complexity of dolomite reservoirs formed by dissolution, which is relevant to exploration for such reservoirs.

Sedimentary transition of the Yangtze subaqueous delta during the past century: Inspiration for delta response to future decline of sediment supply

How the Yangtze delta responds to riverine sediment decline has become a hot research topic and an urgent issue to be addressed, especially after the impoundment of the Three Gorges Dam in 2003. This study investigated sedimentary features through the sediment granularity of surficial samples and short cores over the entire Yangtze subaqueous delta, aiming to better understand the spatial response of the subaqueous delta to sediment supply decline and local hydrodynamic adjustment of the estuary. The results show that four sedimentary zones of estuarine sand bar (Zone I), prodelta (Zone II), delta-shelf transition zone (Zone III) and residual sand (Zone IV) are distributed from the estuary to the shelf, characterised by first fining from zone I to zone II, and then coarsening from zone II to zone IV on grain size of surficial sediments. In the past few hundred years, the subaqueous delta has been generally prograded seaward following this sedimentary system spatially, manifested by generally fining upwards in the cores of the main subaqueous delta. However, sediment coarsening occurs on the top of the cores in the delta-shelf transition zone (Zone III), implying a recent retrogradation process. Based on the results of 210Pb and 137Cs, the sedimentary transition from progradation to retrogradation occurred since 1950s and was intensified after 1980s in the north part of Zone III, mainly caused by channel shrinking of the North Branch and riverine sediment decline. Comparatively, such a sedimentary transition is not significant in the south part of Zone III. But it is worth noting that surficial coarsening in the core (of the south part of Zone III) is probably related to drastic sediment decline since 2003 when the Three-Gorges Dam closed, which means that the Yangtze subaqueous delta has probably been in a full sedimentary transition. The sedimentary transition of the north subaqueous delta can provide an insightful reference for the sedimentary response of the Yangtze delta to further decline of sediment supply in the future.

Reappraisal of purported ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) from detailed chemical and structural analysis

The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. The structures in question are found within a suite of multiply deformed greenschist- to amphibolite-facies metamorphic rocks. To assess their premise as primary sedimentary features – as opposed to products of strain localization in layered, variably ductile rocks – we report new field mapping at the appropriate scale and resolution from the original discovery Sites reported by Nutman et al. (2016). Our new map was used to guide micro- and macro-structural investigations and comprehensive geochemical sampling. Here, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they are not deformed conical stromatolites. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, and electron backscatter diffraction (EBSD) patterns collected on fresh surfaces cut perpendicular and parallel to the ridges attest to the lack of any residual sedimentary laminae (e.g., compositional layering) within these structures' cores. Internal layering previously inferred for these features instead arises from variable weathering of outcrop surfaces that otherwise conceals granoblastic quartz ± dolomite cored boudins that sit between semi-continuous competent layers of enveloping quartzite in a calc-silicate schist. The morphology of boudins reflects viscosity contrasts of the different ductile layers during deformation. Therefore, these features are not of sedimentary origin. Furthermore, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results show that the Isua structures are the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structures and were probably never stromatolites.

Late Quaternary sedimentary processes in the central Arctic Ocean inferred from geophysical mapping

Cryospheric events in the Arctic Ocean have been largely studied through the imprints of ice sheets, ice shelves and icebergs in the seafloor morphology and sediment stratigraphy. Subglacial morphologies have been identified in the shallowest regions of the Arctic Ocean, up to 1200 m water depth, revealing the extent and dynamics of Arctic ice sheets during the last glacial periods. However, less attention has been given to sedimentary features imaged in the vicinity of the ice-grounded areas. Detailed interpretation of the sparse available swath bathymetry and sub-bottom profiles from the Lomonosov Ridge and the Amundsen Basin shows the occurrence of mass transport deposits (MTDs) and sediment waves in the central Arctic Ocean. The waxing and waning ice sheets and shelves in the Arctic Ocean have influenced the distribution of MTDs in the vicinity of grounding-ice areas, i.e. along the crest of Lomonosov Ridge. Due to the potential of Arctic sediments to hold gas hydrates, their destabilization should not be ruled out as trigger for sediment instability. Sediment waves formed by the interaction of internal waves that propagate along water mass interfaces with the bathymetric barrier of Lomonosov Ridge. This work describes the distribution and formation mechanisms of MTDs and sediment waves in the central Arctic Ocean in relation to grounding ice and internal waves between water masses, respectively. The distribution of these features provides new insight into past cryospheric and oceanographic conditions of the central Arctic Ocean.

Improved drag coefficient and settling velocity for carbonate sands

Sediment transport calculations are used globally in the numerical models that coastal managers, scientists and engineers use to assess and forecast coastal change. Most of the existing sediment transport equations were defined based on experimental results using siliciclastic sands. Yet these equations are applied to all types of sand, including carbonate sands that have different characteristics and therefore, settling behaviour. A rigorous management of the transport of carbonate sand is essential for the present and future management of sedimentary features in coral reefs such as sandy beaches or reef islands. Here we present a new approach to estimating the drag coefficient of carbonate sands that considers both friction and pressure. Based on our new method, the calculated drag coefficients explain the great variability in settling velocities of carbonate sand observed in nature (from 0.025 m/s to 0.364 m/s in our database). Using our formula, we demonstrate that even small differences in the settling velocity obtained with the new drag coefficient can lead to substantial changes in sediment transport and call for an update of numerical models.

Sandy Debris Flow Sedimentary Characteristics and Patterns in the Meishan Formation of Northern Lingshui Sag, Qiongdongnan Basin

Based on drilling core descriptions and micro sedimentary features, combined with regional paleogeographic features, imaging logging and 3D seismic attribute analysis, the sedimentary characteristics and representative identification mark of sand debris flow sandstone in Meishan formation of northern Lingshui Sag, Qiongdongnan basin are discussed in detail, and the sand debris flow sedimentary pattern in the study area is established. Meishan formation sandy debris flow sandstone in the study area is mainly consist of 2 lithological associations, thick massive siltstone to very fine sandstone without sedimentary structure on the upper part and silty mudstone, mudstone and siltstone interbeds with abundant contemporaneous deformation structure on the lower part, and the “mud-encased sand” structure in massive sandstone can be used as an important symbolic evidence to identify the sand debris flow deposits. The distribution of sandy debris flow sandstone is controlled by shelf slope break, and the area with relatively gentle, constructive slope, where the slope break line is characterized by “bulge” form to the sea, is the most favorable area for the development of sandy debris flow sandstone reservoirs.

Marginal basins of the NW Pacific and Eastern Eurasia

ABSTRACT The broad zone between old oceanic lithosphere of the NW Pacific and Eastern Eurasian continental lithosphere is home to a chain of marginal basins. Different from oceans, marginal basins are more influenced by the underlying subduction zone both geophysically and geochemically and are more likely to be blanketed by sediments from the nearby continent. This special issue collects 19 papers that explore the tectonic, magmatic, sedimentary and fluid activity features of marginal basins during rifting, spreading and post-spreading stages. Most papers in this special issue focus on South China Sea marginal basins, where abundant research provides interesting insights into how marginal sea basins evolve. Because South China Sea basins are fully evolved and their key features have not been overprinted by younger deformation, the results of this special issue are very useful for understanding the evolution of other marginal basins.