Terrigenous Clastic Sediments Research Articles

Deep learning‐based grain‐size decomposition model: A feasible solution for dealing with methodological uncertainty

ABSTRACTTerrigenous clastic sediments cover a large area of the Earth's surface and provide valuable insights into the Earth's evolution and environmental change. Sediment grain‐size decomposition has been widely used as an effective approach to inferring changes in sediment sources, transport processes and depositional environments. Several algorithms, such as single sample unmixing, end‐member modelling analysis and the universal decomposition model, have been developed for grain‐size decomposition. The performance of these algorithms is highly dependent on parameter selections, introducing subjective uncertainty. This uncertainty could undermine the reliability of decomposition results, limit the application of grain‐size decomposition techniques and reduce comparability across different studies. To mitigate the methodological uncertainty, a novel deep learning‐based framework for grain‐size decomposition of terrigenous clastic sediments is proposed. First, an improved universal decomposition model is used to analyse the collected grain‐size data, in order to provide training sets for the end‐to‐end decomposers. To meet the data size requirements of supervised learning, generative adversarial networks are also trained for data augmentation. The performance of the new framework is then evaluated using a small‐scale dataset (73 393 samples from 18 sites) of three sedimentary types (loess, fluvial and lake delta deposits). The decomposed grain‐size results demonstrate high feasibility and great potential of the framework in constructing a robust grain‐size decomposition model. Finally, it is proposed that future grain‐size research should aim to establish guidelines for grain‐size data sharing and produce a big grain‐size database for deep learning.

Evolution of Cenozoic sedimentary architecture in Central and Southern South China Sea basins

The Central and Southern South China Sea (CSSCS) has a complex tectonic dynamic background and abundant oil and gas resources, which has always been a hot topic of academic and industrial attention. However, systematic analyses are still lacking regarding its sediment filling structure and evolution, mostly due to limited borehole penetration and poor quality of seismic reflection data for deeply buried sequences. No consensus has been reached yet on the sedimentary infilling processes, which impeded the reconstruction of the palaeogeography of Southeast Asia and the oil-and-gas exploration undertakings. Here, we illustrate the Cenozoic sedimentary evolution of the CSSCS region by synthesizing relevant data from previous literature and our own observations and displaying the evolution of depositional systems in sequential reconstructions. Besides, the controlling factors of preferred sedimentary scenarios in the CSSCS incorporate the latest interpretations of the spreading of South China Sea (SCS) as well as the demise of the hypothetical Proto-South China Sea (PSCS). The results show that there are three types of sedimentary basins in the CSSCS (foreland, strike-slip, and rift basins) with different sedimentary filling structures. The foreland basins formed a depositional pattern of ‘transition from deep water to shallow water environments’, dominated by deep-water depositional systems which were formed before the Early Oligocene with submarine fans developed. Later, the foreland basins were gradually dominated by shallow-water depositional systems with deltas and shallow marine facies. The strike-slip basins showed the depositional architecture of ‘transition from lake to marine environments’, i.e. the basins were dominated by lacustrine deposits during the Eocene and evolved into the marine depositional environment since Oligocene with delta developed in the western part of the basin. The depositional evolution of rift basins illustrated the characteristics of ‘transition from clastic to carbonate deposits’, i.e., the rift basins were dominated by Eocene–Oligocene shallow marine clastic depositional systems, while carbonate platforms started to develop since the Early Oligocene from east to west. The above-mentioned differences of depositional architecture in the CSSCS were controlled by the scissor-style closure of the PSCS and the progressive-style expansion of the SCS. Specifically, the early-period deep-water sedimentary environment of CSSCS basins was controlled by the distribution of PSCS in the Eocene. As the scissor-style closure of PSCS progressed from west to east during the Oligocene to Early Miocene, the northwest of Borneo continued to rise, providing a great number of clastic materials to the basins and gradually developing large-scale deltas from west to east. The distribution of early-period lacustrine sedimentation of strike-slip basins was affected by paleo uplift, and the basins transgressed from the northeast and gradually evolved into marine sedimentary environment due to the expansion of SCS. The expansion of SCS also controlled the sedimentary filling evolution of the rift basins, which broke away from the South China continent and drifted southward. Thus, the rift basins lacked the supply of terrigenous clastic sediments which hindered the development of large-scale deltas and formed a clear water environment conducive to the development of carbonate platforms from east to west.

Organic matter enrichment in shale deposited proximal to paleo-uplifts and its impact on shale gas exploration

Much of the global exploration and development of shale gas is focused on organic-rich shale deposited in basin or slope/basin margin settings. There is a clear need to assess the degree to which organic-rich shale deposited in other settings, notably proximal to paleo-uplifts, are promising shale gas prospects. Organic-rich sediments that accumulated proximal to paleo-uplifts are recognized globally and include the Cambrian Niutitang Formation, Ordovician-Silurian transition “hot shales”, and modern sediments of the Black Sea. Though the subject of recent research efforts, much remains to be learned of the mechanisms of organic matter enrichment of fine-grained sediment deposited adjacent to paleo-uplifts. Our study focuses on the Ordovician Wufeng Formation that accumulated proximal to uplifted areas of the Upper Yangtze platform. Though the TOC content of the analyzed Wufeng Formation samples recovered from near the paleo-uplifts is less than that of Wufeng samples collected from basin or slope/basin margin settings, average TOC value of the former can still reach more than 2%. Wufeng Formation shale deposited proximal to paleo-uplifts accumulated under conditions of medium to high paleoproductivity, elevated terrigenous clastic input and sedimentation rate, and oxic-suboxic water column conditions. Principal component analysis suggests that the factor most critical to organic matter enrichment of Wufeng deposits proximal paleo-uplifts was paleo-redox conditions. The TOC abundance in Wufeng Formation shale deposited during late Katian stage is higher than that of shale deposited during Hirnantian stage. It was at the Hirnantian stage that the paleo-uplifts became most active in association with the Kwangsian Orogeny resulting in diminished paleoproductivity and preservation conditions as reflected in the reduced organic matter abundance of associated shale. Uplift of the paleo-uplift of the Yangtze Platform, a manifestation of the Kwangsian Orogeny during the Ordovician-Silurian transition, appears to have been largely responsible for spatial and temporal distribution differentiation of organic matter abundance of the Wufeng Formation of the Upper Yangtze Platform. Results of the present study contribute supplementary information regarding the mechanism of organic matter enrichment in shale and can provide guidance for shale gas exploration and development in geologically similar regions.

Sand-rich Pleistocene deep-water channels and their implications for gas hydrate accumulation: Evidence from the Qiongdongnan Basin, northern South China Sea

Deep-water turbidite channels are conduits for transporting terrigenous clastic sediments into the ocean, and channel deposits can also act as high-quality hydrocarbon reservoirs. In this study, based on high-resolution 3D seismic data and logging-while-drilling data, four Pleistocene turbidite channels are identified and characterized in the deep-water area. These deep-water channels are sinuous with V- or U-shaped on the seismic profiles, and their evolutionary history can be divided into Unit 1 (Erosion-dominated stage), Unit 2 (Erosion-Deposition stage), Unit 3 (Erosion-Deposition stage), and Unit 3 (MTDs-dominated stage). The internal fillings of these channels are almost basal lags with high-amplitude reflections at the bottom, pelagic sediments with low-amplitude reflections, and mass transport deposits with translucent/transparent seismic facies. Since sand-rich sediments dominate the deep-water channel-levee systems in Unit 3 in the study area, they could act as shallow gas reservoirs and facilitate the accumulation of gas hydrates. By analyzing the source-to-sink systems around the basin, the source of the sediments within the deep-water channels is discussed, which are mainly from the Truong Son Belt. Floods, sea level fluctuations, and earthquakes are the most likely triggering mechanisms of turbidity currents in the Qiongdongnan Basin. This study provides new insights into the Quaternary deep-water sedimentary process, gas hydrate exploration, and source-to-sink systems in the northwestern South China Sea.

Fe-Si-C isotope constraints on the genesis of iron ores in Gongchangling iron deposit in northeastern China

The Gongchangling iron deposit is an oxide-facies Algoma-type banded iron formation (BIF) and one of the most representative iron deposits in northeastern China. We present new data from petrological, whole-rock geochemical, and Fe-Si-O isotope compositional data from the Gongchangling BIFs to constrain the origin of ore-forming materials and mineralization processes. The low Al2O3 and TiO2 contents in iron ores indicate that the sources of the BIFs were not contaminated by terrigenous clastic sediments. The Gongchangling BIFs are characterized by depletions in LREEs and 30Si and enrichments in HREEs, with positive Eu and Y anomalies. Given these and the Fe isotope results, we conclude that both high-temperature hydrothermal fluid and seawater contributed to the formation of the BIFs and that a mixture of seawater and ~ 0.1 % hydrothermal fluid, which percolated and dissolved the submarine volcanic rocks, provided the ore-forming materials. Combining the Ce anomalies, enrichments in heavy Fe isotopes and low δ13C compositions, we conclude that the fO2 of Archean seawater was limited. Integrated petrographic and geochemical evidence indicates that the high-grade iron ores from the Gongchangling BIFs experienced incomplete oxidation and low amounts of Fe precipitation in a marine environment and that dissimilatory iron reduction and metamorphic hydrothermal fluid played important roles.

Multi-Proxies Analysis of Organic Matter Accumulation of the Late Ordovician–Early Silurian Black Shale in the Lower Yangtze Region, South China

The evolutional process of palaeoceanic environment and its effect on the accumulation of organic matter during the Ordovician–Silurian transition in Lower Yangtze region has been overlooked compared to that in Upper Yangtze region of South China, although their paleogeographic settings were expected to be discrepant. This paper documents the marine depositional environment, paleoclimate, and sediment supply changes, and discusses their roles in controlling the organic matter enrichment in sedimentary rocks within the Ordovician–Silurian transition of the Lower Yangtze region, using the latest geochemical data of the continuous drilling core. The stratigraphic framework of the Ordovician–Silurian transition in the Lower Yangtze region is composed of two third-order sequences, each of which can be subdivided into a lower TST (transgressive systems tract) and an upper RST (regressive systems tract). TST1 represented an evident depositional transition stage which was marked by the ending of the underlying carbonate sediments and the initiation of the terrigenous clastic-dominated sediments. Geochemical proxies indicate that the relatively low productivity, dysoxic water column condition, and high sediment supply flux collectively resulted in inadequate organic matter hosted in deposits of the TST1. During the depositional period of RST1, the global sea level declined due to the Hirnantian glaciation age. The icehouse also caused the decrease in overall river flux and, thus, the terrigenous clastic sediment supply. The icehouse also strengthened the upwelling that occurred in the Lower Yangtze sea. The upwelling boosted the marine algae explosion through the delivery of abundant nutrients, which not only enhanced paleoproductivity but also led to an anoxic environment by oxygen consumption. Such high paleoproductivity, anoxic water column environment, and low sediment supply flux caused the deposition of organic-rich shale. The sea level rose during the TST2 due to the ending of an ice age. The relatively large water depth and high paleoproductivity associated with volcanic eruptions are the main factors that caused the enrichment of organic matter during this stage. During the deposition of RTS2, the increase of sediment supply flux resulted in a decrease in accommodation space and water depth and the dilution of organic matter in deposits, which was the primary constraint of organic matter accumulation.

Contrasting shale oil accumulation in the upper and lower sweet spots of the lacustrine Permian Lucaogou Formation, Junggar Basin, China

Shale oil enrichment and accumulation in lacustrine strata is rather heterogeneous (unlike that in marine strata), which is a challenging issue to study. Here we carried out a case study in the Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwestern China. Based on data from drill cores, thin sections, rock extracts, and crude oils (including well logging, petrophysics, nuclear magnetic resonance, organic geochemistry, and oil test), the differences of lithology, reservoir physical property, oil generation potential, shale oil content, and physical property and geochemistry of oil between the lower and upper sweet spots were comprehensively compared. Results show that a mixed sedimentary system comprising interbedded carbonate, siltstone, and mudstone was developed owing to terrigenous clastic sedimentation, volcanism, and carbonate deposition. The lower sweet spot has a relatively higher content of silt compared with the upper sweet spot. Due to spatial changes in the depositional environment, only the lower sweet spot occurs at the northeastern margin of the study area and only the upper sweet spot occurs at its southeastern margin, but both occur in the central part of the study area. The porosity and permeability of the sweet spots are highly heterogeneous, due to the complex sedimentary–diagenetic processes, and dissolution pores are common in the lower sweet spot. The hydrocarbon generation potential and shale oil content of the sweet spots are both excellent. Comparatively, the shale oil in the lower sweet spot has higher densities and lower wax contents than those in the upper sweet spot. Thus, the shale oil is more mobile in the upper sweet spot. This implies a high-salinity depositional environment for the lower sweet spot and the oils are generated from salt-tolerant planktonic algae. The oil saturation index (OSI) values are not entirely consistent with the test production results. This indicates that the shale oil productivity is comprehensively controlled by multiple factors, e.g., the hydrocarbon generation potential, reservoir physical properties, and shale oil mobility. These are key features that distinguish lacustrine from marine shale oil systems. The exploration and exploitation strategies of shale oil in lacustrine systems need to be carefully developed.

Characteristics of terrigenous components of Amerasian Arctic Ocean surface sediments: Implications for reconstructing provenance and transport modes

We investigated the petrographic composition of coarse-grained clasts (>250 μm) in 117 core-top sediment samples from the western Arctic Ocean (the Amerasian Basin, Chukchi Sea, Alaskan/Beaufort margin and East Siberian Sea). Combined with analyses of bulk sediment mineralogy and grain size, we use the results to elucidate the terrigenous clastic sediment distribution patterns and transport modes in four different regions: (1) In the Chukchi Sea, the Bering Strait Inflow (BSI) contributes most of the sediments, with an additional terrigenous clastic contribution from Northern Alaska and Eastern Siberia. The highest coarse clastic content and a northward-fining of the grain-size distributions are attributed to the winnowing by the BSI. (2) In the Alaskan margin, terrigenous clasts are enriched in sea ice/anchor ice-transported quartz-rich sediments from Northern Alaska, and in addition the Alaska Coastal Current (ACC) contributes sediments originating from the Yukon River. Coarse-grained sediments accumulate near offshore Alaska with the finer fractions being transported further into the Chukchi Sea Shelf. (3) In the East Siberian Sea, coarse clasts are sparse and even absent in most study sites. Combined with the results of previous studies, we conclude that the terrigenous components are derived from the Laptev Sea and the adjacent Siberian Platform. They are transported by the Siberian Coastal Current and sea ice. (4) In the Canada Basin, terrigenous clasts are characterized by carbonate and quartz-rich sediments derived from the Canadian Arctic Archipelago (CAA). Sediments incorporated in sea ice/anchor ice drift from the CAA to the Beaufort Sea, where they combine with sediments from the Mackenzie River and are then transported into the Canada Basin by the Beaufort Gyre. Sediments and grain size in northern and southern Canada Basin also involve the early Holocene iceberg-entrained IRD from CAA and winnowing/nepheloid transported mica from marginal areas, respectively.

Factors that controlled deposition of lacustrine, mixed siliciclastic-carbonate sediments in the upper fourth member of the Eocene Shahejie Formation in the Zhanhua Sag, East China

The Zhanhua Sag is located in the east-central Jiyang Depression, southern Bohai Bay, northeast China, and is a half-graben, lacustrine basin. Mixed siliciclastic-carbonate sediments are quite common in Eocene deposits of the Zhanhua Sag. We established characteristics of the lithofacies, facies associations, and depositional environments of the upper fourth member of the Eocene Shahejie Formation in the Zhanhua Sag, using core descriptions, thin sections and well logging data. Mixed grainstones and mixed wackestones are two lithofacies of mixed sediments identified in the study area. Paleogeomorphology, sediment supply and gravity flows jointly controlled development of the mixed sediments in this study. Subaqueous highs, formed by terrigenous clastic sediments, provided substrates for carbonate production. The Ordovician carbonate strata exposed on both sides of the rise provided the material for carbonate production on the north and south sides. The clastic sediment supply, however, developed mainly on the south side. Less siliciclastic influx in the north was probably associated with weakening of tectonic activity and less topographic difference. Massive mixed sediments were the product of gravity flows developed under low-energy hydrodynamic conditions between the carbonate shoals and a deltaic system. Most of the siliciclastic material in the mixed sediments was transported by gravity flows from the fan delta to lows in a shallow lake.

Ammonium and nitrate sources and transformation mechanism in the Quaternary sediments of Jianghan Plain, China

Excessive inorganic nitrogen (IN) compound content in groundwater is generally attributed to anthropogenic activities. Here, natural nitrogen sources in Quaternary sediments from aquifers and aquitards of Jianghan Plain (JHP), China were identified. Ammonium and nitrate content in groundwater samples collected from 129 well sites were determined through chemical analysis. Subsequent 4 boreholes were drilled at areas with high nitrogen concentration in the Quaternary aquifer. Indicators from hydrochemistry and soil geochemistry analysis, as well as optically stimulated luminescence dating and various of radioactive isotope δ14C-CO2 and stable isotopes including δ15N-NH4+, δ15N-total organic nitrogen (TON), δ15N-NO3−, δ18O-NO3−, δ18O-H2O, δD-H2O, and δ13C-total organic carbon (TOC) were used to identify high-concentration N compound sources and transformation mechanisms (NO3−: 0.02–770 mg L−1; NH4-N: 0–30.5 mg L−1) in the porous media. The thick clay layer protected the underlying media. Paleo-precipitation characteristics were preserved in the porewater; that is, it had not been affected by anthropogenic activities. The high nitrate concentration in the shallow oxidized aquifer was mostly attributed to manure and sewage (δ15N-NO3− was 14‰). The ammonium-N in the deep strata and part of ammonium-N in the shallow strata (aquifers and aquitards) were from natural sources, mainly from natural TON mineralization. Adsorption was an auxiliary factor for ammonium enrichment in the shallow strata, as were dissimilatory nitrate reduction to ammonium (DNRA) and low ammonia volatilization. Organic matter (OM) involved in mineralization was a mixture of lacustrine algae and terrigenous clastic sediments (from river upstream). The algae were traced to lake formation and frequent evolutionary changes in river environments, as indicated by alterations in sedimentary facies. The present findings may encourage researchers to consider natural IN sources' contribution to N contamination using quantitative models. They also serve as a valuable reference for understanding other pollutants' transformation mechanism in similar environments and provide research ideas for similar areas.

Forward numerical modelling constraining environmental parameters (Aptian carbonate system, E Iberia)

The facies distribution in time and space of sedimentary successions is controlled by a complex interplay between physical, chemical and biological processes, which are nowadays difficult to construe from the geological record. Numerical models constitute a valuable tool to identify and quantify such controlling factors permitting a reliable 3D extrapolation and prediction of stratigraphic and facies architectures beyond outcropping rock strata. This study assesses the roles of three controlling parameters being carbonate production rate, relative sea-level changes and terrigenous clastic sediment supply, on the evolution of an Aptian carbonate system. The SIMSAFADIM-CLASTIC, a 3D process-based sedimentary-stratigraphic forward model, was used for this evaluation. The carbonate succession modelled crops out in the western Maestrat Basin (E Iberia), and corresponded to a platform-to-basin transition comprising three depositional environment-related facies assemblages: platform top, slope and basin. Testing of geological parameters in forward modelling results in a wide range of possible 3D geological scenarios. The documented distribution of facies and sequence-stratigraphic framework combined with a virtual outcrop model were used as a reference to perform geometric (quantitative) and architectural and stacking pattern (qualitative) research by model-data comparison. The time interval modelled spans 1450 ky. The best-fit simulation run characterizes and quantifies (1) relative sea-level fluctuations recording five different genetic types of deposit (systems tracts) belonging to two depositional sequences as expected from field-data analysis, (2) a rate of terrigenous clastic sediment input ranging between 0.5 and 2.5 gr/s, and (3) a mean autochthonous carbonate production maximum rate of 0.08 m/ky. Furthermore, the quantitative and qualitative sensitivity tests carried out highlight that the fluctuation of relative sea level exerted the main control on the resulting stratigraphic and facies architectures, whereas the effect of inflowing terrigenous clastic sediment is less pronounced. Facies assemblages show different sensitivities to each parameter, being the slope carbonates more sensitive than the platform top facies to inflowing fine terrigenous sediments. On slope depositional settings, siliciclastic input also controls stratal stacking patterns and the dimensions of the carbonate bodies formed. The final 3D model allows to spot architectural features such as stacking patterns that can be misinterpreted by looking at the resulting record in the outcrop or by using other 2D approaches, and facilitates the comprehension of reservoir connectivity highlighting the occurrence of initial disconnected regressive platforms, which were later connected during a transgressive stage.

The Sr/Ba ratio response to salinity in clastic sediments of the Yangtze River Delta

The Sr/Ba ratio is a geochemical indicator that distinguishes between terrestrial and marine sedimentary environments in terrigenous clastic sediments. However, conclusions drawn from the Sr/Ba ratio is often inconsistent with those drawn from other sedimentary environment indicators during practical application; therefore, the usability of this indicator is not widely acknowledged. First, to verify this theory and accurately determine the differences between the sedimentary geochemical behaviours of Sr and Ba under different salinity conditions, 14 artificial deltaic sediments were prepared by mixing clean seawater and muddy river water in varying proportions to produce different constant salinities, ranging from 0.0 to 31.2 psu. The Sr and Ba concentrations in the different fractions were then examined via sequential extraction. The experimental results show that, on average, 87% of the total Ba and 65% of the total Sr in the 14 artificial deltaic sediments existed in the residual fraction that is independent of the geochemical conditions of the depositional environment. The experiment also revealed that the decrease in Ba in the sediments was caused by the desorption of adsorbed Ba, owing to the addition of seawater with increasing salinity. Furthermore, the Sr/Ba ratio in the exchangeable fraction yielded a strong linear correlation with salinity, and the Sr/Ba ratio in the carbonate-bound fraction tended to increase slightly with increasing salinity, the Sr/Ba ratios in the FeMn oxide-bound, organic and reductive, and residual fractions, and the total Sr/Ba ratio were not significantly correlated with salinity. Therefore, the Sr/Ba ratio (Sr/Ba-HAc) of exchangeable and carbonate-bound Sr and Ba extracted using dilute acetic acid (HAc) and the Sr/Ba ratio (Sr/Ba-NH4Ac) of exchangeable Sr and Ba extracted using ammonium acetate (NH4Ac) can be used to distinguish between marine and terrestrial sedimentary environments. If the Sr/Ba-HAc or Sr/Ba-NH4Ac ratio in the sediment is >1.0, it can be considered to have been deposited in a sedimentary environment affected by seawater with a salinity >5.0 psu. Second, to verify the aforementioned conclusions, the Sr and Ba in 24 field samples from the Yangtze River Delta were extracted using 10% HAc or 1.0 M NH4Ac. The results indicate that the increase in the amount of Sr in the estuarine terrigenous clastic sediments from the river to the marine environments is caused by the addition of Sr-rich, Ba-poor, marine carbonate shells. The extracted Sr/Ba-HAc ratios were < 1.0 in freshwater (fluvial), 1.0–3.0 in brackish water (delta front), 3.0–8.0 in saltwater (prodelta), and > 8.0 in normal seawater (neritic environments and relict sand); however, in the traditional bulk analysis, total Sr/Ba ratio did not change significantly with increasing salinity. Therefore, it is suggested that the selective extraction of sedimentogenic Sr and Ba is the only effective means of using the Sr/Ba ratio to discriminate between marine and terrestrial sedimentary environments in terrigenous clastic sediments, while the traditional method of using the total Sr/Ba ratio of whole samples is unsuitable.

Deciphering multiple controls on mesopore structural heterogeneity of paralic organic-rich shales: Pennsylvanian–Lower Permian Taiyuan and Shanxi formations, Weibei Coalfield, southwestern North China

This study investigates the various factors that affect the mesopore structural heterogeneity of paralic organic-rich shales in the Pennsylvanian–Lower Permian Taiyuan and Shanxi formations, in the Weibei Coalfield of southwestern North China. This assessment is based on an integrated analysis of facies, mineralogical, petrographic, geochemical, and low-temperature N2 adsorption data. Facies analysis leads to the recognition of five facies associations that record the following depositional systems: medial epeiric shelf (FA1), proximal epeiric shelf (FA2), open-coast tidal flat (FA3), river-dominated delta (FA4), and coastal lake (FA5). Shales from the different depositional facies are classified into five different lithotypes: fossiliferous calcareous shale (FCS), fossiliferous mixed shale (FMS), argillaceous siliceous shale (ASS), illite-dominated argillaceous shale (IAS), and kaolinite-dominated argillaceous shale (KAS). FCS appears to be confined to the carbonate shelf in the medial epeiric shelf. Its mesopore structure is severely damaged by extensive micritization and local micrite recrystallization, and only a few plate-like large mesopores (4.34–50 nm) between the carbonate grains survived. FMS is typical of seaward flank deposits of the proximal epeiric shelf. Although abundant calcite cements are formed by the calcification of aragonite shells, FMS still has a few inkbottle-shaped mesopores due to the input of terrigenous clastic sediment. IAS is widely developed in the open-coast supratidal marsh due to volcanic ash kaolinization and kaolinite illitization. Illite-smectite group clays determine the presence of fine mesopores (3–4.34 nm), which are the main contributors to the specific surface area. Thus, IAS has the most abundant homogeneous fine mesopores, as well as the largest specific surface area and pore volume. Conversely, KAS is formed in the delta-plain interdistributary swamp and has a dense structure. ASS is mainly composed of terrigenous clasts and can be formed in multiple environments. Silt-sized brittle minerals determine the number of large mesopores, which are conducive to the total pore volume. Therefore, ASS has a bimodal pore size distribution, as well as medium pore structure parameters and fractal dimensions. This study contributes to a better understanding and prediction of the mesopore structure evolution of organic-rich shales that formed in various paralic sub-environments.

Depositional environments in an arid, closed basin and their implications for oil and gas exploration: The lower Permian Fengcheng Formation in the Junggar Basin, China

The Fengcheng Formation is a nonmarine, carbonate-dominated succession that formed under arid climatic conditions in a hydrologically closed basin. Two transects and two seismic profiles were examined, the characteristics and environmental significance of different lithofacies were studied, and a model of depositional environment divisions was proposed. The sedimentary model involved an alkaline lake in which the depositional environments consisted of a shallow saline lake margin, slope, saline lake center, and steep lake margin from northeast to southwest. The perennial central salty lake was located in the southwestern part of the study area, whereas there were widespread, low-gradient lake margins in the northeast, east, southeast, and southern parts of the study area. Lake-level fluctuations had a major influence on the shallow saline lake system and complicated the depositional environments in these areas. The deposits are derived from bedrock reworking, volcanic eruptions, and authigenic minerals that precipitated from brine during the hypersaline phase. Fine-grained terrigenous clastic sediments, volcanic ashes and dusts, and authigenic minerals mixed in the depocenter (concentration center of the brine pool), which was covered by high-salinity brines, and the depositional environment was anoxic as a result of salinity-based brine stratification. A thick sodium carbonate succession occurred in the depocenter of the ancient Mahu lake, where bedded sodium carbonate alternated with fine-grained, organic-rich tuff or tuffaceous hydrocarbon source rocks. Microorganisms bloomed in the alkaline, high-salinity brine, and the organic matter was well preserved, which is similar to those modern alkaline saline lakes in eastern Africa and western North America. Thus, the Permian Fengcheng Formation contains source rocks that formed in an alkaline saline lake.

Climate and tectonic-driven deposition of sandwiched continental shale units: New insights from petrology, geochemistry, and integrated provenance analyses (the western Sichuan subsiding Basin, Southwest China)

How the depositional setting efficiently governs the characteristic sandwiched continental shale units remains uncertain, which restricts an integrated assessment of organic-rich fluvio-lacustrine shale reservoirs, and accurate estimation of potential natural gas resources. Here, we present new results from petrological observations, element geochemical fingerprinting, and integrated analyses of heavy mineral, basinal subsidence history, and sandstone / stratum ratio on typical and terrigenous sandwiched-like depositional systems of the targeted Upper Triassic Xujiahe Formation in the western Sichuan subsiding Bain, Southwest China. In view of the representative and environment-sensitive indices, we suggest that passive continental margin and continental island arc dominated by granite to granodiorite source are the major tectonic settings of the provenance, and both a more warm-humid climate characterized by intensified chemical weathering conditions and a calm tectonically quiescent setting are identified as two major drivers forcing the accumulation and preservation of organic matter in organic-rich continental shale units. Finally, a comprehensive depositional model is established for providing new insights into the linkage between palaeoclimatic conditions, tectonic pulses and terrigenous clastic sedimentation. Both the cyclic palaeoclimate fluctuations and episodic tectonic activities are believed to have exerted a very considerable force on development of the unique sandwiched-like stratigraphic framework, and the coupling interactions between the tectono-climatic evolution and fine-grained sedimentation are thus also stressed.

Weathering geochemistry and palaeoclimate implication of the Early Permian mudstones from eastern Henan Province, North China

Terrigenous clastic sediments are generated by the integration of the Earth surface processes and their deep-time counterparts provide a valuable archive for regional/global climatic, geographic and landscape evolution. It is thus important to read and interpret these deep-time sedimentary records, especially for reconstructing continent climate. Previous studies on the Early Permian sequences from the North China document a dominant control of source chemical weathering on mudstone compositions and its linkage with continent climate conditions. Based on the weathering geochemical data of these mudstones, element mobility during weathering can be ordered as Ca > Na ≥ Mg > Sr > K ≥ Ba > Rb. The weathering regime in the source area is inferred to be supply-limited according to the estimated continent physical erosion rate and regional tectonic evolution, sedimentation in North China. Further exploration of palaeoclimate implication is presented in terms of variation of high-to-low latitudinal temperature gradient across the Early Permian glacial to post-glacial climate transition.

A unique lacustrine mixed dolomitic-clastic sequence for tight oil reservoir within the middle Permian Lucaogou Formation of the Junggar Basin, NW China: Reservoir characteristics and origin

The middle Permian Lucaogou Formation in the Jimusaer Sag of the southeastern Junggar Basin, NW China, was the site of a recent discovery of a giant tight oil reservoir. This reservoir is unusual as it is hosted by lacustrine mixed dolomitic-clastic rocks, significantly differing from other tight reservoirs that are generally hosted by marine/lacustrine siliciclastic–calcitic sequences. Here, we improve our understanding of this relatively new type of tight oil reservoir by presenting the results of a preliminarily investigation into the basic characteristics and origin of this reservoir using field, petrological, geophysical (including seismic and logging), and geochemical data. Field and well core observations indicate that the Lucaogou Formation is a sequence of mixed carbonate (mainly dolomites) and terrigenous clastic (mainly feldspars) sediments that were deposited in a highly saline environment. The formation is divided into upper and lower cycles based on lithological variations between coarse- and fine-grained rocks; in particular, dolomites and siltstones are interbedded with organic-rich mudstones in the lower part of each cycle, whereas the upper part of each cycle contains few dolomites and siltstones. Tight oil accumulations are generally present in the lower part of each cycle, and dolomites and dolomite-bearing rocks are the main reservoir rocks in these cycles, including sandy dolomite, dolarenite, dolomicrite, and a few dolomitic siltstones. Optical microscope, back scattered electron, and scanning electron microscope imaging indicate that the main oil reservoir spaces are secondary pores that were generated by the dissolution of clastics and dolomite by highly acidic and corrosive hydrocarbon-related fluids.

Origin of the North Qinling Microcontinent and Proterozoic geotectonic evolution of the Kuanping Ocean, Central China

The high Pb isotopic ratios among the North Qinling Microcontinent (NQM), the western margin of the Yangtze Craton (WYC) and the western part of South Qinling (WSQ) indicate an identical or very similar origin of their basement. In addition, the analogical peak values of Paleoproterozoic detrital zircons also agree that the NQM originated from the WYC. The evolution of the NQM and the Kuanping Ocean is closely related to the aggregation and break-up of the Columbia supercontinent. According to the formation of the Supercontinent Columbia in the Paleoproterozoic (ca. 2.0–1.8Ga), the debris from the WYC accreted vertically to form the NQM. Then the Kuanping Ocean began its subduction when the Supercontinent Columbia was rifted after the Paleoproterozoic (ca. 1.8Ga). As a result, the passive continental margin of the North China Craton (NCC) and the Yangtze Craton (YZC) turned into an active continental margin. The continental arc (the Xiong’er Group) at the southern margin of the North China Craton (SNCC) and separation of the Qinling Group from the WYC in the Paleoproterozoic to Mesoproterozoic (ca. 1.8–1.0Ga) are also the consequence of this subduction. In the context of the amalgamation of the Neoproterozoic (ca. 1.0–0.9Ga) Supercontinent Rodinia, the NQM and the YZC collided with each other, thereby generating the closure of the Songshugou back-arc basin. Moreover, the terrigenous clastic sediments from some cratons of the Rodinia Supercontinent and the remnant oceanic crust of the Kuanping Ocean constituted the protolith of the present-day Kuanping Group.

Aragonite bias, and lack of bias, in the fossil record: lithological, environmental, and ecological controls

AbstractMacroevolutionary and macroecological studies must account for biases in the fossil record, especially when questions concern the relative abundance and diversity of taxa that differ in preservation and sampling potential. Using Cenozoic marine mollusks from a temperate setting (New Zealand), we find that much of the long-term temporal variation in gastropod versus bivalve occurrences is correlated with the stage-level sampling probabilities of aragonitic versus calcitic taxa. Average sampling probabilities are higher for calcitic species, but this contrast is time-varying in a predictable way, being concentrated in stages with widespread carbonate deposition.To understand these results fully, we link them with analyses at the level of individual point occurrences. Doing so reveals that aragonite bias is effectively absent in terrigenous clastic sediments. In limestones, by contrast, calcitic species have at least twice the odds of sampling as aragonitic species. This result is most pronounced during times of widespread carbonate deposition, where the difference in the per-collection odds of sampling species is a factor of eight. During carbonate-rich intervals, calcitic taxa also have higher odds of sampling in clastics. At first glance this result may suggest simple preservational bias against aragonite. However, comparing relative odds of aragonitic versus calcitic sampling with absolute sampling rates shows that the positive calcite bias during carbonate-rich times reflects higher than average occurrence rates for calcitic taxa (rather than lower rates for aragonitic taxa) and that the negative aragonite bias in limestones reflects lower than average occurrence rates for aragonitic taxa (rather than higher rates for calcitic taxa).Our results therefore indicate a time-varying interplay of two main factors: (1) taphonomic loss of aragonitic species in carbonate sediments, with no substantial bias in terrigenous clastics; and (2) an ecological preference of calcitic taxa for environments characteristic of periods with pervasive carbonate deposition, irrespective of lithology per se.

Temporal, environmental and tectonic significance of the Huoqiu BIF, southeastern North China Craton: Geochemical and geochronological constraints

The Huoqiu iron deposit with a typical banded iron formation, located in the southeastern margin of the North China Craton (NCC), and hosted by Precambrian high-grade metamorphic rocks, mostly occurred as metasedimentary rocks. Eighty-eight detrital zircons have been collected from three BIF samples, and they displayed clear oscillatory zoning, high Th/U ratio and low to variable luminescence, indicating magmatic origin. There are two peaks of 2753 Ma and 2970 Ma in concordant 207Pb/206Pb ages distribution plot, which is well consistent with the protolith ages of wall rocks (e.g., TTG gneiss and amphibolite) in this region. These appearances in association with the relatively good linear correlations between Al2O3 and TiO2 for BIF and their wall rocks, suggest that at least some terrigenous clastic sediments contributed to BIF formation, which thus constrained the upper limited age of BIF deposition at 2.75 Ga. In addition, no detrital zircons record 2.56 Ga magmatic event, or recently reported 2.71 Ga magmatic event in this region and neighboring blocks, probably constrained the lower limited age of BIF formation at 2.56 Ga or 2.71 Ga. The flysch rhythmic structure occurrence in BIF-bearing strata, abundant carbonate minerals such as primary breunnerite association with magnetite, and no volcanic record present within BIF ore body or adjacent rocks inferred that the Huoqiu BIF deposited in continental marginal sea or back-arc basin environment, thus belonged to Superior-type. They show similar REE patterns to seawater, however, the positive Eu anomalies and reduced Y/Ho ratios relative to seawater indicates a possible mixture of hydrothermal fluids.