Sedimentary Strata Research Articles

Correlation of the Upper Pliocene sediments of the Dnipro-Donetsk Depression based on palynological data

The problems of intra- and inter-regional correlations have always been the most difficult in stratigraphic studies. Particular difficulties arise when comparing same-age subaerial continental sediments that are depleted in palaeontological remains. The spore-pollen analysis, which is a component of the palynological method, is most effective in stratigraphic dissection and correlation of subaerial sediments, since pollen and spores, with few exceptions, are almost the only paleontological remains contained in fossil soils and intersoil clays and loess. The author has developed the methodological approaches to stratification and correlation of continental Pliocene and Pleistocene sediments of the Ukraine based on palynological data (Sirenko, 2017). These methods are applied in the presented work. Due to the changes in the International Stratigraphic Scale, including a change in the position of the lower boundary of the Quaternary period, it became necessary to modify the existing stratigraphic schemes of Cenozoic sediments. Therefore, there is a need to obtain new representative materials on sediment characteristics, as well as to solve the problems of intra- and interregional correlations. At the present stage of research on the Upper Cenozoic continental sediments of Ukraine, the least studied by the palynological and palaeopedological methods are Pliocene fossil soils and intersoil clays of the central and southeastern parts of the Dnipro-Donetsk depression. The presented paper is devoted to solving the issues of correlation of Upper Pliocene sediments of the Central Graben of the Dnipro-Donetsk Depression and its junction with the North-Western part of the Donetsk folded structure. Two sections (well 11 and the outcrop near Kamianka village) are located within these areas. The analysis of materials on the palynological characterization of the Kyzylyr and Bogdanivka climatoliths of these sections allowed us to identify the general patterns of the spore-pollen complexes that characterize them. The identified common patterns of the complexes from the Kyzylyr and Bogdanivka climatoliths can serve as a basis for comparing sediments of the same age located in different parts of the study area and for interregional correlations. The individual features of the described spore-pollen complexes are associated with different geomorphological and geographical locations of the compared sections. It is concluded that it is necessary to take into account the geographical and geomorphological location of the sections when performing stratigraphic and correlation constructions based on the palynological data. The efficiency of the palynological method for stratigraphic and correlation constructions is proved by comparing the sediments of two sections. The obtained conclusions on the correlation of the sediments of these sections are confirmed by the materials of palaeopedological studies. Key words: Upper Pliocene; fossil soils; stratigraphy; palynological method; spore-pollen complex; Ukraine.

Stratigraphic chronology and mechanisms of formation of bottom sediments at the mouth of the Grøndalen River (Grøn-Fjord, West Spitsbergen) during the period of climatic changes

The results of the study of bottom sediments and water of the estuary of the River Grøndalen (Grøn-fjord, West Spitsbergen) are presented. The stratigraphic features of the sedimentary strata are determined. The chronology of changes in geoecological environmental conditions has been established using radioisotope dating methods. The main factors influencing sedimentation in the study area are air temperature and precipitation during the period of predominance of low temperatures. It is shown that climate fluctuations determine the chronological sequence of the sedimentation rate and the change of the lithotype of the bottom sediment towards fine fractions.

Genesis of Cu deposits hosted by volcanic rocks in Yunxian-Jinggu Arc, Western Yunnan: Constraints from fluid inclusions and C-O-S isotopes

A series of vein-type Cu deposits have been preserved in the Triassic volcanic rocks of the Yunxian-Jinggu area. The Guanfang, Bazijie, and Minleyichang deposits are typical examples of regional Cu mineralization. The deposits are preserved in the intermediate-basic volcanic rocks, and the ore bodies are controlled by NNE-trending structures with similar alteration features and mineral assemblages. The fluid inclusions are recognized in quartz and calcite at the three deposits, dominated by aqueous inclusions (L) and liquid phase-rich vapor–liquid two-phase inclusions (L-V) with similar temperature (270-140℃) and salinity (14.0–20.2 wt% NaCl eq.). The three deposits have a medium–low temperature, moderate salinity, and higher oxidation rate than the volcanic host rocks, confirmed by the fluid inclusion, alteration, and mineral assemblage. The O isotope values for early quartz associated with sulfides in the three deposits are comparable (δ18OSMOW = 15.23 to 20.01 ‰). Late quartz in the Guanfang and Bazijie deposits shows a deficit in O isotope values compared to early quartz (δ18OSMOW = 9.89 to 13.98 ‰), indicating the incorporation of meteoric water or basin fluid during the late stage of mineralization. The early calcite in the Guanfang and Bazijie deposits have similar C-O isotopic signatures (δ13CV-PDB = −10.51 to −4.75 ‰, δ18OV-SMOW = 11.29 ‰ to 12.63 ‰), and δ18Ofluid (3.84 to 5.18 ‰) calculated from calcite is approximately the same as the δ18Ofluid (2.08 to 3.95 ‰) calculated from early quartz. The early calcite and early quartz in the Guanfang and Bazijie deposits may be connected to the same fluid source based on the similar δ18Ofluid signatures and salinities. The calcite C-O isotopic compositions of the Minleyichang deposit are 13CV-PDB = −23.03 to −17.74 ‰ and δ18OV-SMOW = 19.72 to 28.32 ‰, which exhibit the characteristics of biogenic carbon and indicate that the material originated from sedimentary layers in the Simao Basin. The sulfide δ34SV-CDT ranges from −17.2 to 3.2 ‰ in the three deposits, which is related to the addition of reduced S from the sedimentary strata in the Simao Basin. The fluid characteristics of medium to low temperature, moderate salinity, and the S-C isotope signatures indicate that the ore-forming fluids are the basin brine from the Simao Basin. The Cu mineralization in the Yunxian-Jinggu area is closely related to the thrust-fold structure system formed during the Late Cretaceous to the Cenozoic period. The ore-forming fluids may have come from the brine in the central part of the Simao Basin, which migrated through the thrust fault and assimilated the materials in the regional volcanic rocks and sedimentary rocks, forming Cu deposits in the Triassic volcanic rocks of Yunxian-Jinggu area.

Exhumation response to oceanic plateau accretion and oroclinal bending: Low-temperature thermochronology study of Wrangellia terrane on southern Vancouver Island, Canada

Approximately 50 Myr ago, the triple junction of the Kula-Farallon-North America plates converged with the continental margin, causing ridge subduction and the formation, accretion and translation of two oceanic plateaus. We investigate the effects of this tectonic configuration on the exhumation of southern Wrangellia terrane on southern Vancouver Island since the Eocene. We report late Cretaceous to late Oligocene (85.4 to 23.3 Ma) apatite fission track ages (AFT) and, for the first time, Oligocene to early Miocene (36.6 to 14.0 Ma) apatite (UTh)/He ages (AHe) for 16 bedrock samples of Wrangellia. The thermal history modelling of these ages for 13 samples reveals variable cooling patterns between regions. Samples close to the major faults of a fold and thrust belt show accelerated cooling (4–5 °C/Myr) during the Eocene. In the central area, the modelled cooling rates have been slow and generally uniform throughout the Cenozoic (0.5–1.5 °C/Myr), whereas samples from the west coast yielded very slow cooling (<0.5 °C/Myr) from 70 to 30 Ma, followed by moderate cooling (1.5–3 °C/Myr) since. Combining ages, fission track length and thermal history models in this and previous studies, we interpret the moderate-accelerated exhumation of the fold and thrust belt in the Eocene to be a response to oroclinal bending following oceanic plateau accretion. The exhumation pattern of the western side of southern Wrangellia is linked to the ongoing Cascadian Subduction zone ca.30 Ma. This exhumation pattern also supports a hypothesis that all crust of southern Wrangellia was all overlain by sedimentary strata in Eocene before ∼50 Ma, and that an accretionary complex of the Pacific Rim terrane was partly the outboard equivalent of these strata. In the southern Wrangellia, no exhumation response to the Miocene oroclinal bending associated with formation of the Olympic mountains is observed.

The source of lithium in connate fluids: Evidence from the geothermal reservoir at Soultz-sous-Forêts, Upper Rhine Graben, France

Geothermal brines in the Upper Rhine Graben have been used as a spa or for salt production since Roman times. Heat and power are generated in geothermal power plants since 2007. Recently, their elevated Li-content has additionally attracted economic interest. This increased interest is in contrast with our understanding of the geological-hydrothermal evolution. We use petrology, major and trace element mineral chemistry and mass balance calculation from drill cores that intersect granitic geothermal reservoir rocks at Soultz-sous-Forêts between Strasbourg and Karlsruhe to shed light on fluid-rock interaction in a reservoir that is actively used for heat and power generation. The alkali feldspar and the two-mica granite in the reservoir have a typical plagioclase, K-feldspar, quartz, biotite and muscovite assemblage with some accessories of titanite, apatite and zircon. Two hydrothermal alteration events are distinguished: (1) albitization of the feldspars; (2) distal replacement of feldspars by sericite and calcite, of biotite by chlorite and titanite; and proximal to hydrothermal veins replacement of the feldspars by sericite and kaolinite. Event 2 feldspar alteration quantitatively releases Pb and Ba to the fluid, whereas Rb, Cs, Sr and Zn show different behaviour depending on whole-rock and mineral composition. Event 2 biotite-chlorite alteration releases Li, Rb, Cs, Sr, Ba, Zn and Pb to the fluid. Mass balance calculation indicates that Si, Fe, Ca, K, Rb, Sr, Zn and Pb contents of the Soultz-sous-Forêts geothermal brine may be explained by fluid-rock interaction in the reservoir. However, the reservoir rock volume that needs to be leached in order to reach recent brine composition varies by several orders of magnitude between the different elements. Many of the elements may be leached during hydrothermal alteration, however in particular Li and Cs require unrealistic fluid-rock ratios of >1/300. These considerations indicate that Na, Ca, Li, Cs and Ba need an additional external source. Based on this, we propose a model where Middle Triassic bittern brines already enriched in Li, Rb, and Cs reacted with the reservoir rocks during hydrothermal event 2 and subsequently mixed with Jurassic-Cretaceous marine water that dissolved evaporites during downward migration. This agrees with Jurassic-Cretaceous illite ages from various sites in the Black Forest and indicates a complex ~150 m.y. hydrothermal evolution for the brines. There is likely no single source of Li, and it is likely derived from complex fluid-rock interaction with the sedimentary (evaporite) and, less importantly, the crystalline strata of the Upper Rhine Graben. Critical for Li-resource development is the complex hydrothermal history of connate fluids that interacted with sedimentary strata and the preservation in deep-seated reservoirs.

Geoelectric heterogeneities of the Pre-Dobrudga Trough a zone of hydrocarbon manifestations

For the first time, a geological and geoelectrical interpretation of the three-dimensional model of the Earth’s crust and upper mantle was carried out for the Pre-Dobrudga Trough and adjacent areas. It was based on observations of the Earth’s low-frequency electromagnetic field conducted in 2009―2012 by the National Academy of Sciences of Ukraine. The material confidently indicates that hydrocarbon occurrences are confined to the detected anomalies of high electrical conductivity. These are characterized by subvertical channels (from the surface to 10 km deep) galvanically connected with sediments or sub-vertical contact zones of different resistivity observed not only in the Earth’s crust (10―40 km) but also at crust-mantle depths (40―60 km) and in the upper mantle (110―160 km).The channels may determine the flow of ultra-deep fluids. The geoelectric criteria included maximum thickness of the sedimentary stratum, subvertical rise of electrical conductivity anomalies from the crust-mantle depths or column to the entire thickness of the Earth’s crust, subvertical boundaries of heterogeneities (contacts of different resistivity) in the consolidated Earth’s crust and upper mantle,presence of a highly conductive asthenospheric layer. We identified a significant number of local areas that can be considered promising for hydrocarbon occurrence. The majority of them spatially coincided with the already well-known oil and gas fields ― Skhidnosaratske and Zhovtoyarske, with the explored oil and gas prospective areas (Izmailska, Kyslytska, Prymorska, Shyrokivska), and with those that correspond to the areas of special permits for industrial exploration. However, the newly discovered areas that meet most of the geoelectric criteria, but for which no no information on oil and gas content is publicly available, clearly require additional geological and geophysical studies. It has been shown that zones of high electrical conductivity caused by the presence of deep fluids should be considered as deep centers of hydrocarbon generation and ways of their migration to the upper horizons of the Earth’s crust.

Eolian-fluvial succession in the Early Cretaceous from the Ordos Basin

Cretaceous eolian deposits are widely developed in North China, among which the Lower Cretaceous eolian (erg) sedimentary strata in the Ordos Basin are the most representative. Many studies have been conducted on eolian deposits in the Ordos Basin, especially the Luohe Formation; however, there is a lack of systematic sedimentology research and the corresponding research on the evolution of sedimentary facies. Therefore, to explore the Mesozoic drought events and the systematic evolution of the sub-tropical erg zone, this study reports the fluvial–eolian sedimentary sequences of the Early Cretaceous Luohe and Yijinhuoluo Formations in the Ordos Basin. Spatially, through detailed sedimentary lithofacies and structural analysis of different outcrops, the combination law of sedimentary facies can be determined. Based on this, a three-dimensional sedimentary model of erg is reconstructed. The erg environment is divided into three sub-environments: erg margin, erg margin–centre transition and erg centre. According to the evolution of the sedimentary facies, the sedimentary evolution from the Latef Jurassic to the Early Cretaceous in the Ordos Basin is divided into five stages. The Ordos Basin has experienced the delta, erg formation, erg deposition, erg shrinkage and evaporating salt-lake stages. The vertical sedimentary evolution indicates climatic oscillation in North China from the Late Jurassic to the Early Cretaceous. Under the control of the sub-tropical high belt, the Ordos Basin experienced a hot and arid climate during the Cretaceous, forming a widely distributed erg belt in the region. During this period, a small-scale climate shock occurred under the hot and arid background, changing the sedimentary environment.

Identifying the Vertical Stratification of Sediment Samples by Visible and Near-Infrared Spectroscopy

Vertical stratification in marine sediment profiles indicates physical and chemical sedimentary processes and, thus, is the first step in sedimentary research and in studying their relationship with global climate change. Traditional technologies for studying vertical stratification have low efficiency; thus, new technologies are highly needed. Recently, visible and near-infrared spectroscopy (VNIR) has been explored to rapidly determine sediment parameters, such as clay content, particle size, total carbon (TC), total nitrogen (TN), and so on. Here, we explored vertical stratification in a sediment column in the South China Sea using VNIR. The sediment column was 160 cm and divided into 160 samples by 1 cm intervals. All samples were classified into three layers by depth, that is, 0–50 cm (the upper layer), 50–100 cm (the middle layer), and 100–160 cm (the bottom layer). Concentrations of TC and TN in each sample were measured by Elementa Vario EL III. Visible and near-infrared reflectance spectra of each sample were collected by Agilent Cary 5000. A global model and several classification models for vertical stratification in sediments were established by a Support Vector Machine (SVM) after the characteristic spectra were identified using Competitive Adaptive Reweighted Sampling. In the classification models, K-means clustering and Density Peak Clustering (DPC) were employed as the unsupervised clustering algorithms. The results showed that the stratification was successful by VNIR, especially when using the combination of unsupervised clustering and machine learning algorithms. The correct classification rate (CCR) was much higher in the classification models than in the global model. And the classification models had a higher CCR using K-means combined with SVM (94.8%) and using DPC combined with SVM (96.0%). The higher CCR might be derived from the chemical classification. Indeed, similar results were also found in the chemical stratification. This study provided a theoretical basis for the rapid and synchronous measurement of chemical and physical parameters in sediment profiles by VNIR.

Oil and Gas Content of the South Caspian Basin

Abstract —We have compiled maps of areal isotope composition changes and constructed graphs of gas HC distribution depending on the stratigraphic age of the host rocks and graphs of isotope composition changes depending on the depth of sampling to study the hydrocarbon potential of the deep-seated oil and gas pools of the South Caspian Basin. The genesis of gas was studied using the techniques elaborated by M. Schoell and A. James. Two stages of hydrocarbon formation have been established by study of various kinds of gas manifestation. The first stage (Miocene–Eocene) began in the deposits underlying the Productive Series and continued up to the deposits of the latter. This stage was characterized by a frequent change in the directions of both downward and upward movements of gases. The second stage of hydrocarbon formation (Anthropogene–Pliocene) began in the deposits of the Productive Series and was characterized by a change in the regional geodynamic setting. Avalanche sedimentation and the predominance of downward movements over upward ones favored the accumulation of thick sediments at the time when the Productive Series formed. The sedimentation and tectonic processes (downwarping) in the deep-water zone of the basin led to harsh thermobaric conditions in the sedimentary strata. A detailed analysis of the results of gas survey in the deep-water zone of the South Caspian Basin has shown gas generation with a predominance of two components, methane and ethane. Study of the trends of temporal and areal hydrocarbon migration and of the areas of oil and gas generation makes it possible to reveal structures with evidence for large and giant hydrocarbon reserves. We have established that gas hydrocarbons in the bottom and upper-section sediments of the southern Caspian Sea are intimately related to the sources of hydrocarbons, their migration, and other processes running in the deep-seated sediments and in the upper section.

The Implications of Seeping Hydrocarbon Gases in the Gunsan Basin, Central Yellow Sea, off the Southwest of Korea

A detailed analysis of high-resolution (3.5 kHz) chirp seismic profiles acquired in the Gunsan Basin of the central Yellow Sea revealed that hydrocarbon gases are actively seeping via the formation of many plumes. The uppermost sedimentary layer was acoustically confirmed to be fully or partially charged with gases. Somewhat favored by the low-tide period, episodic gas seepage is mainly associated with the underlying fault systems of Cretaceous-Cenozoic sedimentary strata in the southwestern part of the basin. Catastrophic gas expulsion seems to have formed a crater at the sidewall of a sedimentary ridge and two diapirs. Here, methane is poorly concentrated but rich in the heavy carbon isotope (δ13C, −52.6‰ to −44.7‰ The Vienna Peedee Belemnite [VPDB]), indicating that methane formed mainly through biodegradation of heavy oils at depth remains in the shallow sediments following its expulsion. Episodic rapid upward advection of porewater is also manifest by unmixed heavy methane trapped in the upper part of the primary biogenic methane (δ13C, about −90‰ VPDB)-filled sediment core. These findings imply that the Gunsan Basin fulfills the requirements for possible generation and preservation of oil and gas, like the petroliferous basins of eastern China and the Yellow Sea.

Analysis of Factors Influencing Tight Sandstone Gas Production and Identification of Favorable Gas Layers in the Shan 23 Sub-Member of the Daning-Jixian Block, Eastern Ordos Basin

The Daning-Jixian Block harbors abundant tight sandstone gas resources. However, significant variations in gas production exist among the different wells within the block. A comprehensive study was conducted on key factors such as sedimentary strata and petrophysical characteristics to elucidate their impact on gas reservoir productivity. Linear regression equations were employed to classify the favorable reservoirs within the study area. The analysis revealed that within the first 6 months of production from the Shan 23 gas layer, daily gas production ranged from 2576.19 to 156,078.17 m3/d, averaging 24,037.9 m3/d. Over the first year, average daily production varied from 2185.05 to 136,806.99 m3/d, averaging 23,469.23 m3/d, indicating relatively stable production from the Shan 23 layer alone. In the dominant central area of the underwater distributary channel delta front in Shan23, the sand body exhibits a superimposed cutting type, resulting in high production rates. Conversely, the sand bodies on the periphery gradually transition to superimposed and isolated types, leading to decreased production. Through a correlation analysis of gas layer thickness, porosity, permeability, and initial gas well production, it was determined that gas production from the wells within the same layer is primarily influenced by gas layer thickness, porosity, and permeability. Gas saturation demonstrates a minimal impact on production according to single-factor analysis. The evaluated factors such as the gas productivity coefficient, energy storage coefficient, and enrichment coefficient exhibited similar distribution patterns across the study area. The high-value areas for the gas productivity coefficient, energy storage coefficient, and enrichment coefficient are concentrated in distributary channel zones and delta lobes. In contrast, regions with underdeveloped skeletal sand bodies generally display lower values for these parameters. The linear relationships between these parameters and the average gas production were calculated to further classify the favorable reservoirs in the study area. This study aimed to establish a scientific basis for the efficient development of the tight sandstone gas reservoirs within the Daning-Jixian Block.

Carbon isotope stratigraphy and biostratigraphy, and associated organic carbon deposition during the Late Cenomanian to Early Turonian in the Tarfaya Basin, Morocco

The Cenomanian-Turonian (C/T) boundary event, also known as Oceanic Anoxic Event 2 (OAE2), is characterised by significant organic carbon preservation and a positive carbon isotope excursion that is recorded globally in sedimentary strata. In this study of the Tarfaya Basin, Morocco, three outcrop sections were logged - at Lagguing East, Tazra, and En-Naila, dated from Late Cenomanian to Early Turonian. New dating, based on ammonites and planktic foraminifera, combined with high-resolution δ13C isotope stratigraphy, yields a more precise delineation of the C/T boundary in the studied section. A total of 99 samples were collected for combined sedimentological, mineralogical, and geochemical analyses to allow a better understand of the palaeoenvironments of deposition and interpret controlling factors on organic enrichment. Five lithofacies have been identified, all indicative of a relative deep marine setting. Variations in lithofacies can be correlated with relative sea level changes, which align with the global eustatic curve.The OAE2 interval comprises black mudstones that signal anoxic conditions, leading to significant organic matter (OM) preservation, even though they appear weathered in the studied section. Notably, organic matter-rich mudstones were evident during the post-OAE2 Early Turonian maximum marine transgression. Trace element enrichment in these organic-rick mudstones indicates that organic matter accumulation was accompanied with increased sea surface productivity and associated with oxygen-depleted bottom water conditions, which facilitated organic matter preservation and enrichment. Upwelling has also been previously described as a process to deliver nutrients to the shelf, enhancing productivity. Some units with depleted organic content are the result of dilution from continental derived clastics. Others may originally have had high TOC, but were influenced by weathering or diagenetic process, changing from the typical black mudstones to yellow in colour. The identified carbon isotopic values and organic carbon enrichment in the C/T strata of the Tarfaya Basin reflect signals of global perturbations in the carbon cycles.

A preliminary exploration of waste red-bed as a supplementary cementitious material

A red-bed is a type of continental clastic sedimentary stratum, with red serving as the main color. This type of stratum accounts for 20% of the land area in China. In the process of infrastructure construction, a large amount of waste red-bed is generated. Waste red-bed exacerbates the shortage of land resources and is a misuse of resources. In this study, a preliminary exploration of the recycling of waste red-bed in central Yunnan Province was conducted. The studied red-bed contains approximately 30% clay minerals and was used as supplementary cementitious material (SCM) instead of clay after calcination. Through laboratory characterization, environmental impact assessment, and cost analyses, the feasibility of using the red-bed as a type of SCM was demonstrated. The results show that the dehydroxylation reaction of clay minerals in the red-bed was almost complete after calcination at 800 °C. A pozzolanic reaction occurred, and additional heat was released after the addition of the calcined red-bed (CRB) to the cement mortar. When the cement replacement ratio increased from 10% to 30%, the strength activity index decreased from 98.2% to 82.2%, which met the relevant requirements. However, compared with the control samples, the samples with replacement ratios of 40% and 50% had lower mortar fluidities by 57.3% and 75.5% and higher porosities by 33.3% and 20.0%, respectively. Owing to the dilution effect of the cement and the decrease in the mortar fluidity, the internal structure of the sample porosity increased, and the strength decreased dramatically. In addition, the environmental impacts, energy consumption, and cost of using waste red-bed as a SCM were quantitatively analyzed via a carbon emission index and a cost index.

Chronostratigraphy of the Late Glacial Żabinko site (western Poland) and investigation of the dose rate variability

The Żabinko exposure (western Poland) reveals the classic fluvio-aeolian succession known from studies in the European Sand Belt. Previous chronostratigraphic studies were mainly based on uncalibrated radiocarbon dates from organic sediments and thermoluminescence dating. The picture visible from these studies indicated a number of discrepancies between these methods. The new research in this exposure was based on optically stimulated luminescence (OSL) dating and calibrated radiocarbon dates. The results obtained indicate a general discrepancy between the results achieved by these two methods. While the radiocarbon dates provide some meaningful picture and allow correlation with previous studies, the results of OSL dating do not allow for a chronological model of sedimentary processes. The OSL dates show large inversions of the results and are clearly younger than the other dating results. Detailed analysis of OSL measurements shows radioactive disequilibrium and variability linked to differential stratification of sediments, significantly impacting the assessment of environmental dose rates. We believe that this atypical variability is presumably the result of postdepositional processes, such as changes in groundwater levels, chemical weathering and radionuclide migration.

East Siberian Sea Composite Tectono-Sedimentary Element, Siberian Arctic

The East Siberian Sea Composite Tectono-Sedimentary Element (ESS CTSE) occupies most of the central and southern parts of the East Siberian Sea (ESS) and extends onshore into the Chukchi Peninsula. The first multichannel seismic reflection data, acquired in this region in 1989 and 1991 revealed its structural complexity and the presence of a deep sedimentary depocentre in its central part. In the 2010s, the first long-offset seismic data provided a much better understanding of the geology of this remote segment of the Arctic shelf. It includes a large extensional basin that started to form in the Albian (?)-Cenomanian, postdating the Barremian-Aptian Chukotkan orogeny and formation of the New Siberian-Chukchi Fold-and-Thrust Belt (FTB). The latter experienced extensional collapse and developed into the tectonic basement that underlies the CTSE over its entire extent. The ESS CTSE is mainly composed of Upper Cretaceous-Holocene siliciclastic sedimentary strata with a total thickness up to 14 km. This succession has not been drilled deep enough to test its hydrocarbon potential since the beginning of exploration around 2013. However, the strata contain abundant terrestrial organic matter and are considered as potential source rocks sufficient for generating gas accumulations.

Factors responsible for Witwatersrand gold mineralisation

Abstract The exceptional gold mineralisation in quartz pebble conglomerates of the Witwatersrand Basin is attributed to a combination of factors. These factors are linked to the co-evolution of the atmosphere, hydrosphere, lithosphere and biosphere, at a very specific time in Archaean geological history and the evolution of the Kaapvaal Craton. Following craton stabilisation and its subaerial emergence, intense chemical weathering and erosion of large volumes of granitoid-greenstone basement released detrital and dissolved gold. Shallow-marine reworking in a long-lived and slowly subsiding basin subjected to episodic compressional deformation and relative sea-level oscillations led to sedimentary concentration of detrital gold. The interaction between acidic, anoxic, and sulfurous surface runoff and more oxidizing marine waters in a near-coastal oxygen oasis supported microbially mediated diagenetic pyrite formation and incorporation of dissolved gold in the pyrite crystal lattice. Erosion and reworking of diagenetic pyrite gave rise to detrital pyrite that characterise most reefs. Abundance of detrital uraninite in conglomerates, derived from erosion of Mesoarchaean granites, and episodes of hydrocarbon migration through sedimentary strata during deep burial set the scene for further enhancement of gold grades in the reefs. Granular and seam pyro-bitumen formed by radiation-induced polymerisation of hydrocarbons around detrital uraninite. Gold dissolved in migrating hydrothermal fluids was then reduced and precipitated upon interaction with the reef pyro-bitumen during hydrothermal placer modification.

Noble gas characteristics of microbial gas in the Qaidam Basin, China: Implications for helium enrichment processes

Helium is a critical, strategic and nonrenewable resource with limited reserves and unstable supplies. However, the helium sources and migration processes in petroliferous basins remain unclear. The Qaidam Basin features multiple reservoir layers, multiple genetic types of natural gas, and varying helium concentrations, providing an ideal natural laboratory for understanding the helium enrichment processes. Here, we present the original noble gas isotopic compositions and major gas components of 11 microbial gas samples from three gas layers in the Sebei No.1 gas field, eastern Qaidam Basin, China. The helium isotopic ratios range from 0.007 to 0.057 Ra, reflecting a predominantly crustal source. The Quaternary Sebei microbial gas features much lower helium concentration (0.0003–0.001%), 21Ne/22Ne ratio (0.031–0.034), and 40Ar/36Ar ratio (292–380) than thermogenic natural gases in the Qaidam Basin. The middle gas layer (II) of the Sebei gas field features higher 21Ne/22Ne and 40Ar/36Ar ratios than the top and bottom gas layers, displaying positive correlations with higher 4He groundwater concentrations before degassing, as calculated from 4He/20Ne ratios and 20Ne concentrations in air-saturated water. However, the correlations between the 4He contents in natural gas and the 21Ne/22Ne and 40Ar/36Ar ratios are unclear, suggesting that 4He dissolves into the groundwater system before degassing into a gas reservoir and that the variations in the 4He concentration in the gas phase are caused by the different levels of hydrocarbon dilution. Quantitative calculations of helium production revealed an external helium flux from the crustal basement except for the in-situ production from sedimentary strata even in this broadly-defined self-generating and self-preserving natural gas system. The helium resources in the Qaidam Basin are mainly distributed in tectonically active zones with uplifted pre-Jurassic basement rocks (providing sufficient helium sources), ancient groundwater systems (accumulating more helium) and suitable hydrocarbon production (acting as a carrier gas but diluting helium as slightly as possible).

Centimeter-scale sedimentary structures in a lacustrine delta front, northern China: Ripples or Froude supercritical-flow bedforms?

Sedimentary strata are a significant record of the Earth and planetary history, and accurate recognition of sedimentary structures and their link to environmental conditions is a key component in deciphering past surface processes. Centimeter-scale sedimentary structures are commonly acknowledged as ripple cross-laminations, but here we document ambiguous centimeter-scale structures from a lacustrine delta front in northern China, that at careful look do not seem to fit with the known ripple cross-lamination criteria. The here documented sedimentary structures range from scour-and-fill, irregular lenses with structureless or low- and high-angle, up- and downstream dipping, concave and convex laminations. These centimeter-scale sedimentary structures thus considerably differ from ripple cross-laminations in their outer shape, internal organization, and morphometric parameters. Detailed comparison of these centimeter-scale structures with Froude supercritical-flow structures suggests that they were likely produced by Froude supercritical flows. Such centimeter-scale supercritical structures are not unique in the Bantanzi delta, as they have been also documented in a variety of settings ranging from rivers to deepwater turbidites. In light of this finding, we expand the Froude supercritical-flow sedimentary structures to centimeter scale, and advocate caution in interpreting centimeter-scale sedimentary structures axiomatically as ripple laminations.

Exploring the formation of Tangará sill: A single-pulse intrusion feeding CAMP lava flows in Parecis Basin, Brazil

The Central Atlantic Magmatic Province (CAMP) stands out as a distinctive large igneous province on the Earth. Due to the extensive outcrop area of CAMP and its complex geological setting, the links between the extrusive and intrusive units are not fully understood. In the intracratonic Parecis Basin located in Central Brazil, the CAMP rocks are represented by mafic lava flows at its south (Tapirapuã Formation) and western edges (Anari Formation). Despite the availability of substantial geophysical, petrological and geochronological data, the understanding of CAMP feeder system in this region remains unclear. In this study, our focus is on investigating the Tangará sill, a dolerite intruded into Triassic sedimentary rocks in the South region of the Parecis Basin. Our aim is to contribute to a better understanding of the plumbing system in this area by investigating the Tangará sill and its potential links to the lava flows at the surface. The sill is identified as a tabular feature in seismic data, with a discernible base and top that extend for about 100 km from northeast to southwest. The Tangará sill was drilled by the 6-ANP-MT-02 stratigraphic well, revealing a continuous igneous intrusion with a thickness of 180 m (at depths ranging from 470 to 650 m). The dolerite intrusion is composed by plagioclase, clinopyroxene, opaque minerals and mesostasis (quartz + feldspar intergrowth and apatite). It can be divided into five segments: the upper and basal margins consist of fine-grained dolerite, the central portion comprises coarse-grained gabbro, which includes pegmatoid zones. The central portion is separated from the margins by two sections of medium-grained dolerite. The dolerite is tholeiitic, with low TiO2 content (<2 wt% TiO2) and low La/Yb ratio (average of 4.4). Geochemical and petrographic data indicate that the intrusion was formed by a single magma pulse and underwent differentiation through plagioclase and pyroxene dominated crystal fractionation. Two samples dated by 40Ar/39Ar geochronology using laser step-heating technique, yield plateau ages ranging from 204.0 ± 0.4 Ma to 202.0 ± 0.5 Ma. Sr-Nd-Pb isotopic data suggest a lithospheric mantle source with possible crustal contamination. Our findings suggest an emplacement model in which a thick single-pulse magma intruded the Triassic red beds. Along a NE-SW section, the sill is truncated by faults along the southern border of the basin, near the mapped lava flows of the Tapirapuã Formation. This suggests that the magma propagated from the northeast to the southwest along the sedimentary strata and fed the Tapirapuã volcanism. This example highlights a single-pulse sill feeding lava flows of the Central Atlantic Magmatic Province (CAMP).

Magnetostratigraphy of the Late Cretaceous to Early Eocene sediments from the Tulsipur section, western Nepal: Tectonic implications for the Indian northern passive margin

The sedimentary strata from the Late Cretaceous to Early Eocene in the Himalayan foothills offer detailed insights into land collision processes. Unfortunately, a comprehensive dating study on these strata has been lacking, limiting our understanding of the collision and splicing history of the Indian and Lhasa plates. To address this question, we conducted a paleomagnetic study on the Late Cretaceous to Early Eocene succession in the Tulsipur area, western Nepal. The paleomagnetic data obtained from a 357 m thick succession were used to establish a new magnetostratigraphic framework for this region. The observed magnetic polarity sequence contains six normal and six reversed polarity zones, spanning the age range of 68–54 Ma (Maastrichtian to Early Eocene). In addition, two major and two stable sediment accumulation rates (SARs) were identified which were likely controlled by tectonics. The rifting and subsidence events caused by continuous northward movement of the Indian plate might have modulated the shifting of distal offshore to tidal flat settings in this region. We document fifteen (Unit I) and seven (Unit II) transgressive-regressive cycles, indicative of passive margin instability influenced by near-field effects of the India-Asian convergence. This new magnetostratigraphic result is expected to serve as a basic age framework in reconstructing the Late Mesozoic and Early Cenozoic tectonic evolution. Furthermore, it will also facilitate reliable cross-regional correlation between lithostratigraphic units across passive margins in the southern Himalaya and similar strata in the Tethyan Himalaya and active continental margins of the southern Tibetan Plateau.