Geochemistry of shales from the Upper Miocene Samh Formation,north Marsa Alam,Red Sea,Egypt: implications for source area weathering,provenance, and tectonic setting

The Upper Miocene shales of the Samh Formation, North Marsa Alam along the Egyptian Red Sea coastal plain were analyzed for major and selected trace elements to infer their provenance, weathering intensity, and tectonic setting. The Samh Formation consists of sandstone underlies by shale and marl intercalations. The Samh shales are texturally classified as mudstones. Mineralogically, these shales consist mainly of smectite and kaolinite, associated with non clay minerals (abundant quartz and trace of plagioclase, microcline, and halite). Compared to post-Archaean Australian shales (PAAS), the Samh shales are highly enriched in SiO₂, Al₂O₃, and Fe₂O₃ and depleted in TiO₂, P₂O₅, Na₂O, MgO, and K₂O contents. The K₂O/Al₂O₃ ratio values indicate predominance of clay minerals over K-bearing minerals. Trace elements like zirconium (Zr), Cr, Pb, Sc, Rb, and Cs are positively correlated with Al₂O₃ indicating that these elements are likely fixed in K-feldspars and clays. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and Chemical Index of Weathering (CIW) values indicate moderate to intense weathering of the source material in a semiarid climate. The geochemistry results suggest that the Samh shales were deposited in a passive margin of a synrift basin and derived from felsic (granitic) source rocks. The inferred tectonic setting for the Upper Miocene Samh shales in Marsa Alam is in agreement with the tectonic evolutionary history of the Eastern Desert of Egypt during the Upper Miocene.     

Geochemical Signatures of Karlad Lake Sediments,North Kerala: Source Area Weathering and Provenance

In the present study, the lake floor sediments of the Karlad lake, located at higher elevation in Wayanad region of north Kerala, were analyzed for textural characteristics, organic matter, calcium carbonate, major oxides and trace elements. This study was carried out to infer the chemical composition, provenance and intensity of chemical weathering of the source rocks in the lake catchment area. Textural studies signify that lake floor sediments are predominantly as clays (38.75%) followed by sand (36.36%) and silt (25.19%) fractions. The C/N ratio of the lake sediments signify that the sediments are both autochthonous and allochthonous in origin. The major oxides average content reveals the order of abundance as follows; SiO₂ > Al₂O₃ > Fe₂O₃ > TiO₂ > MgO > CaO > K₂O > P₂O₅ > Na₂O > MnO. Moreover, the various weathering indices such as Chemical Index of Alteration (CIAAvg. 93.5%), PlagioclaseIndex of Alteration (PIA- Avg. 95.6%) and Chemical Index of Weathering (CIW- Avg. 95.76%) suggest an intense chemical weathering of the source area. The A-CN-K diagram is also corroborating the same. Various provenance discrimination diagrams reveal that the sediments are derived from the mafic source rocks.     

Major element geochemistry of purple soils/rocks in the red Sichuan Basin, China: implications of their diagenesis and pedogenesis

Major element compositions and chemical weathering features of the purple soils and their corresponding rocks in the Red Sichuan Basin, China were studied in this paper to infer the diagenetic features of the purple rocks and the pedogenetic features of the purple soils. The results showed that all of the rock and soil samples, except those from Yibin, have strongly similar major element compositions and the distribution patterns of their major element compositions are similar to those of the upper continental crust. Chemical index of alteration, chemical index of weathering and Na₂O/K₂O indicate that the purple soils and rocks have similar chemical weathering intensities, whereas the Al₂O₃–CaO* + Na₂O–K₂O (A–CN–K) relationship, suggests that chemical weathering was strong during diagenesis of the purple rocks, but weak during pedogenesis of the purple soils. Variations in major element compositions of the purplish rock samples, except that from Yibin, do not primarily reflect differences in compositions of the sediment source, sediment recycling and potassium metasomatism, and therefore the effects of chemical weathering on the major element compositions were mainly dependent on climate condition and the residence time of material exposed to chemical weathering. Chemical weathering of the purple rocks reached only moderate degrees under the general influence of warm and humid climate during Jurassic and Cretaceous. Warmer and more humid climate conditions partly resulted in stronger chemical weathering of rocks in the southern region of the basin than in the central and western region, whereas shorter residence time of material in upslope position partly resulted in weaker chemical weathering of rocks in the eastern region than in the central and western region. The same climate and stable tectonic setting led to comparable chemical weathering intensities of rocks in the Ziliujing and Jiaguan Formations. Gradually colder and dryer climate from Early Jurassic to Middle Jurassic then to Late Jurassic and shorter residence time in tectonically active setting of Middle Jurassic and late Late Jurassic resulted in the following order of chemical weathering intensity of the purple Jurassic rocks: Ziliujing Formation ? Shaximiao Formation ≈ Suining Formation > Penglaizhen Formation.     

Provenance and weathering history of the maastrichtian shale member of the Patti Formation,southern Bida Basin,Nigeria

The present study is focused on the geochemical characterization and provenance of the shale member of the Maastrichtian Patti Formation which is well represented at the centre of the southern Bida Basin in Nigeria. The major element composition of the studied shales and clays compare favourably with the reference shales, however, the trace elements vary slightly. Application of binary plot of TiO₂ versus Al₂O₃ and Al₂O₃/TiO₂ ratios of the studied shale samples show mixed mafic to felsic igneous sources. The chemical composition (plot of SiO₂ versus Log (K₂O/Na₂O) indicates active to passive continental margin province, however, a minor derivation from recycled provenance is probable. Intense chemical weathering based on the high values of Chemical Index Alteration (CIA), Chemical Index of Weathering (CIW) and Zr values obtained from the shale and clay samples a warm humid condition is suggested. The high values also probably suggest predominance of clay minerals and low feldspar. This is supported by the XRD (X-Ray Diffractometer) data which indicates predominance of kaolinite in the samples.     

甘肃景泰崔家墩下奥陶统阴沟组砂岩化学组分特征及物源区构造背景判别

甘肃景泰一带早奥陶世沉积了以浊流砂岩夹板岩为主的阴沟组。崔家墩剖面阴沟组较为发育,总体分为两个岩性段,但东西向逆冲断层造成地层垂向不连续。北段（下段）为粉-细砂岩、板岩夹硅质岩;南段（上段）为岩屑含量较多的岩屑砂岩夹板岩,硅质岩少或无。对阴沟组砂岩进行主化学组分特征分析,其特征指标：Fe₂O*3+MgO值为503%～17.63%,Al₂O₃/SiO₂值为0.12%～0.28%,K₂O/Na₂O值为0.25%～1.26%及 Al₂O₃/（CaO+Na₂O）值为1.01%～3.53%。利用砂岩的化学组分特征结合岩相学分析,可以判断物源区特征及大地构造背景。研究表明:该套浊积砂岩下部物源主要来自北部的阿拉善地块及该地块上的古老造山带,为被动大陆边缘背景;上部碎屑主要来自南侧岛弧中基性火山岩,为活动大陆边缘初始弧后盆地构造背景。     

东营凹陷牛38井沙河街组烃源岩生排烃评价

东营凹陷牛38井沙河街组三段烃源岩从沙三下亚段向沙三中-上亚段为一从高水位深湖相向低水位前三角洲相发展的沉积旋回,该沉积环境控制从下向上发育不同类型的有机质。相应地,该段烃源岩品质从下向上变差,其中沙三中亚段上部及沙三中亚段中部烃源岩产烃能力小,沙三下亚段烃源岩整体品质好,局部存在优质烃源岩。其优劣性体现了在该段烃源岩中,Ⅰ型干酪根具有高的生产力,而Ⅲ型、Ⅱ₂型干酪根生烃潜力很低,基本为无效有机质。计算结果显示不同数据点的烃源岩单位质量原始生烃潜量可相差3个数量级,体现了烃源岩存在强烈的非均质性。有机碳的非均质性分布为其提供了物质基础,而湖盆的旋回式沉积及湖平面的波动性变化是其内在原因。同时,本文也提供一种计算生排烃量的新方法,结果表明该方法较为有效可行。     

Subaerial weathering of sedimentary organic matter

Small diameter core samples were taken from outcrops of the Permian Phosphoria Formation and the Cretaceous Pierre Shale of the Western United States to determine the effects of weathering on organic matter in shale outcrops. While the Pierre Shale core showed no evidence of weathering, the Phosphoria Formation showed significant reduction of overall organic content and pronounced changes in organic composition over the near-surface interval of the core. Total organic carbon is lower by as much as 60% over the upper 2 ft of the core. Chloroform-soluble organic matter and total hydrocarbon (C₁₅₊) concentrations are 50% lower over this same interval. The ratio of saturated to aromatic hydrocarbons decreases steadily with core depth over the upper 2.6 ft of the core. Aromatic hydrocarbons are enriched in the stable carbon-13 isotope by an average of 1.7%. over this same interval. Shallow core samples also show a loss of n-paraffins relative to branched/cyclic compounds in the saturated C¹⁵⁺ fraction.Although the extent of weathering is variable, certain characteristic effects are recognizable and can be applied to the interpretation of outcrop data in organic geochemical studies.     

The role of kerogen constituents in oil generation in the Gevar'am Formation (Lower Cretaceous), Israel

Kerogen from various samples of the Gevar'am Formation (Early Cretaceous, Israel) were analyzed for their elemental composition, and by I.R. and ESR spectroscopy. The results were compared with other organochemical characteristics of the basin (Amit, 1978, Bull. Am. Assoc. Pet. Geol., V. 62 p. 827–836). The types of organic material and their role in the formation of oil were investigated. It appears that the kerogen is mostly constituted of terrestrially derived organic matter. A small amount of marine-derived organic matter is also inferred. The kerogen of terrestrial origin is a poor oil producer and is expected to generate oil only at a higher stage of maturation than the Gevar'am Formation has attained. Any hydrocarbons that have been generated in this formation were probably generated from that part of the kerogen thought to be marine derived. Thus the Gevar'am Formation is considered as a potential source rock which has generated only a small amount of oil.     

西藏北羌塘盆地光明湖地区上侏罗统白龙冰河组烃源岩有机质特征

通过对研究区露头烃源岩样品进行Rock-Eval热解模拟实验、干酪根镜鉴、镜质体反射率、族组分分离、单体烃碳同位素等分析,对北羌塘盆地光明湖地区侏罗系白龙冰河组烃源岩有机质特征进行研究。结果表明,该区白龙冰河组烃源岩以碳酸盐岩为主,有机碳含量0.01%~1.21%,平均值为0.39%,有机质丰度较高;干酪根显微组分以腐泥组为主,含量70%~92%,平均值为80%,以Ⅱ₁型为主,主峰碳、(nC_21-/ nC₂₂₊)轻/重比及Pr/nC₁₇-Ph/nC₁₈的相关图均表明有机质来自低等水生生物,有机质类型较好;镜质体反射率最小值1.409%,最大值2.423%,平均值为1.909%,有机质的热演化程度总体较高,达到高成熟-过成熟阶段。     

羌塘盆地沃若山地区上三叠统土门格拉组烃源岩沉积环境分析

岩石学、沉积相、元素地球化学的综合分析结果表明,羌塘盆地沃若山地区上三叠统土门格拉组(T_3t)地层为三角洲沉积;整体富含水解性元素、Fe族元素以及上部砂岩Fe元素富集(高Fe/Mn比值)的特征表明古水体环境为较浅的河口-滨岸环境;CIA值(63.8~79)、ICV值(0.52~1.4)和Ga/Rb-K2O/Al2O3关系图解分析结果显示当时为风化强度中等、较温暖的半干旱—半湿润气候;Sr/Ba比值全部小于1(0.09~0.27),表明沉积水体为盐度较低的陆相淡水;根据V、U、Mo的含量比值,判断其为氧化-弱氧化的富氧水体环境。较为温湿的气候有利于生物的繁殖,但氧化-弱氧化的环境不利于有机质的富集,限制了该地区烃源岩发育:土门格拉组烃源岩的TOC含量为0.76%~1.46%(均值为1.04%),属中等品质的烃源岩。     

Geochemical characteristics of modern river sediments in Myanmar and Thailand: Implications for provenance and weathering

The elemental composition of organic matter and the major and trace element compositions of stream sediments from Myanmar (Ayeyarwady and Sittaung rivers) and Thailand (Mekong and Chao Phraya rivers, and their tributaries) were determined to examine their distributions, provenance, and chemical weathering processes. Higher total organic carbon (TOC) and total nitrogen (TN) contents in the finer grained sediments indicate hydrodynamic energy may control their distributions. TOC/TN ratios indicate inputs of both aquatic macrophyte and higher vascular plant material to the river sediments. The major element abundances of the sediments are characterized by predominance of SiO₂ in coarser fractions and a marked negative correlation with Al₂O₃, representing primary grain size primarily control on SiO₂ content. Marked depletion of most labile elements (Na₂O, CaO, K₂O, Ba and Sr) relative to UCC (upper continental crust), indicate destruction of feldspar during chemical weathering in the source area or during transport. However, enrichment of some high field strength elements (Zr, Th, Ce and Y) relative to UCC and higher Zr/Sc ratios indicate moderate concentration of resistant heavy minerals in finer-grained samples. Discriminant diagrams and immobile trace element characteristics indicate that the Mekong, and Chao Phraya river sediments were largely derived from felsic sources with compositions close to typical rhyolite, dacite/granodiorite, UCC, I- and S-type granites. Relative enrichment of ferromagnesian elements (e.g. MgO, Cr, Ni) and high Cr/V and low Y/Ni ratios in Ayeyarwady and Sittaung sediments indicate the presence of a mafic or ultramafic component in their sources. The ICV (Index of Compositional Variability), CIA (Chemical Index of Alteration), PIA (Plagioclase Index of Alteration), α^Al, Rb/Sr and K₂O/Rb ratios indicate that the Ayeyarwady and Sittaung sediments record low to moderate degrees of chemical weathering in their source, compared to moderate to intense chemical weathering in the Mekong and Chao Phraya river basins. These results are compatible with existing major ion data for river waters collected at the same locations.     

Geochemical characteristics of sandstones from Cretaceous Garudamangalam area of Ariyalur,Tamilnadu, India: Implications of provenance and tectonic setting

The Trichinopoly Group (later redesignated as Garudamangalam) has unconformable relationship with underlying Uttatur Group and is divided into lower Kulakanattam Formation and upper Anaipadi Formation. These calcareous sandstones are analysed major, trace and rare earth elements (REEs) to find out CIA, CIW, provenance and tectonic setting. The silica content of fossiliferous calcareous sandstone show wide variation ranging from 12.93 to 42.56%. Alumina content ranged from 3.49 to 8.47%. Higher values of Fe₂O₃ (2.29–22.02%) and low MgO content (0.75–2.44%) are observed in the Garudamangalam Formation. CaO (23.53–45.90) is high in these sandstones due to the presence of calcite as cementing material. Major element geochemistry of clastic rocks (Al₂O₃ vs. Na₂O) plot and trace elemental ratio (Th/U) reveal the moderate to intense weathering of the source rocks. The Cr/Zr ratio of clastic rocks reveal with an average of 1.74 suggesting of felsic provenance. In clastic rocks, high ratios of \(\sum \)LREE/\(\sum \)HREE, La/Sc, Th/Sc, Th/Co, La/Co and low ratios of Cr/Zr, and positive Eu anomaly ranges from (Eu/Eu* = 1.87–5.30) reveal felsic nature of the source rocks.     

Measuring and adjusting the weathering and hydraulic sorting effects for rigorous provenance analysis of sedimentary rocks: a case study from the Jurassic Ashikita Group,south‐west Japan

The present study examines the provenance of the Jurassic Ashikita Group distributed in south‐west Japan, which is composed of the Idenohana, Kyodomari and Sakamoto Formations. Two geochemical diagrams for provenance analysis were utilized, which incorporate full consideration of compositional modifications resulting from weathering (MFW diagram) and hydraulic sorting processes (SiO₂/Al₂O₃–Na₂O/K₂O diagram). The MFW diagram delineates weathering trends of sedimentary rocks and allows estimation of the original source rock composition by tracing the weathering trends backwards to an unweathered domain. Weathering trends of the Idenohana and Kyodomari Formations extend backward to the domain of intermediate and felsic igneous rocks. In contrast, sediments of the Sakamoto Formation do not fit into a linear weathering trend, indicating that the source rock cannot be approximated to igneous rocks. On the SiO₂/Al₂O₃–Na₂O/K₂O diagram, sediments are organized into compositional trends, in which the range reflects compositional variations induced by the hydraulic sorting effect. On this diagram, sediments derived from the igneous and recycled sedimentary provenances can be distinguished by reading the inclination of the trend. By utilizing this principle, source rocks of the Idenohana and Kyodomari Formations are interpreted as igneous rocks and those of the Sakamoto Formation are interpreted as recycled sedimentary rocks. Therefore, these diagrams concurrently estimate the source rock composition through quantifying and adjusting the weathering and sorting effects, and reveal a systematic transition in the provenance of the Ashikita Group. The Idenohana and Kyodomari Formations were supplied chiefly from an igneous provenance, which shifted from intermediate to felsic compositions in stratigraphic order. Whereas, sediments of the Sakamoto Formation were sourced primarily from a recycled sedimentary provenance.     

Provenance,tectonic setting and source-area weathering of the lower Cambrian sediments of the Parahio valley in the Spiti basin,India

The geochemical study of siliciclastic rocks from the Lower Cambrian of Parahio Valley has been studied to describe the provenance, chemical weathering and tectonic setting. The K₂O/Al₂O₃ ratio and positive correlation of Co (r=0.85), Ni (r=0.86), Zn (r=0.82), Rb (r=0.98) with K₂O reflects that the presence of clay minerals control the abundances of these elements and suggests a warm and humid climate for this region. The chondrite normalized REE pattern of the samples is equivalent to upper continental crust, which reflects enriched LREE and flat HREE with negative Eu anomaly. The tectonic setting discriminant diagram log[K₂O/Na₂O] vs. SiO₂; [SiO₂/Al₂O₃] vs. log[K₂O/Na₂O]; [SiO₂/20] – [K₂O+Na₂O] – [TiO₂+Fe₂O₃+MgO] indicates transitional tectonic setting from an active continental margin to a passive margin. The discriminant function plot indicates quartzose sedimentary provenance, and to some extent, the felsic igneous provenance, derived from weathered granite, gneissic terrain and/or from pre-existing sedimentary terrain. The CIA value indicates low to moderate degree of chemical weathering and the average ICV values suggests immature sediments deposited in tectonically active settings. The A–CN–K diagram indicates that these sediments were generated from source rocks of the upper continental crust.     

Regional geology, depositional environment and maturity of organic matter of Early to Middle Jurassic coals, coaly shales, shales and claystones from the Eastern Pontides, NE Turkey

Jurassic coals, coaly shales, shales and claystones from the Eastern Pontides in NE Turkey have been investigated using microscopical, petrophysical and detailed organic geochemical methods in order to determine their catagenetic stage, to reveal the composition of the organic matter and to discuss the depositional environment. The Liassic–Dogger period in the Eastern Pontides was characterised by the presence of a rift system which resulted in rock units of very variable lithology and facies. Coal seams, ranging from a few centimetres to several decimetres and intercalated with shales, claystones and sandstones occur within the basal part (the Aggi Formation) as well as in the uppermost part (the Hamurkesen Formation) of the rift deposits. All coal seams investigated are at a catagenetic maturation stage corresponding to subbituminous B up to low volatile bituminous ranks. They represent a depositional environment of short-lived swamp areas with intense aerobic reworking of the higher plant detritus by heterotrophic bacteria, but with possible anaerobic microenvironments at deeper levels. At least some coal seams were influenced temporarily by marine ingressions. Most samples are impure humic coals with highly variable chemical compositions as indicated by the broad range of their hydrogen contents. This variation in hydrogen content is partly attributed to variable contributions of algal material. On the other side, considering several analytical results, the hydrogen variation is primarily due to bacterial reworking that affected the composition of the organic matter to variable extent and resulted especially in an enrichment of bacterial lipids. Bacterial reworking by an active microbial community within the upper layer of the peat swamp is inferred from elevated concentrations of iso-alkanes even exceeding those of the corresponding n-alkanes in several samples which, to the best of our knowledge, have not been observed with coals before.     

青海南部沱沱河冻土水合物潜在区火山岩地球化学特征及地质意义

沱沱河盆地是冻土天然气水合物潜在分布区之一,其内发育下—中二叠统开心岭群九十道班组、上二叠统乌丽群那益雄组、上三叠统结扎群巴贡组、中—渐新统雅西措组4套烃源岩以及不同类型的火山岩。研究表明,火山岩岩石类型主要为玄武岩、玄武安山岩、安山岩、玄武质粗面安山岩和粗面安山岩。火山岩主量元素低TiO_2,Al_2O_3含量较高,K_2O含量较低,K_2ONa_2O;火山岩的稀土元素配分模式为轻稀土富集型;微量元素配分模式呈锯齿状,Ta、Nb、P_2O_5、TiO_2、Y、Yb以及铁族元素Sc、Cr、Ni亏损,综合判断青海南部沱沱河地区火山岩形成于岛弧环境。结合火山岩的地球化学特征,推断火山活动可以加快烃源岩的热成熟,进而促使有机质裂解,产生热解气,为水合物的生成提供气源条件。同时,火山岩的气孔构造发育,连通性较好,有可能成为水合物的储层。火山热液具有的高压使周围的烃源岩产生裂隙,这些裂隙不仅是水合物气源的运移通道,还可以为水合物提供形成场所和储存空间。     

The geochemistry of primary and weathered oil shale and coquina across the Julia Creek vanadium deposit (Queensland, Australia)

A significant resource of vanadium and molybdenum exists near Julia Creek, Australia, where the middle Cretaceous organic-rich Toolebuc Formation lies between 0 and 25 m of the surface. We present and discuss a comprehensive geochemical study of the Toolebuc Formation and its enclosing stratigraphy near Julia Creek to understand this ore deposit. V and Mo contents in fresh facies are strongly associated with total organic carbon (TOC) contents, but not with Al or CaCO₃; this suggests that V and Mo were originally concentrated in the organic fraction. However, chemical extractions using H₂O₂ indicate that Mo was originally concentrated in pyrite. The data also suggest that V was mobilised from organic matter during early diagenesis and became associated with clays as little V was extracted by H₂O₂ in the fresh samples. TOC contents in the Toolebuc Formation were removed during weathering, residually enriching trace metals including V and Mo, and as a result, the TOC relationship with V and Mo disintegrates. With weathering, both V and Mo predominantly became associated with iron oxide/hydroxide phases (and possibly other unidentified phases) as these elements in the weathered facies were highly soluble in the sodium citrate–sodium dithionite digestion. Large shale-hosted V and Mo deposits such as Julia Creek offer a potentially viable alternative to the currently mined magnetite-hosted deposits. A thorough understanding of the formation and host mineral phases for V and Mo of these shale deposits, however, is critical to ensure that these valuable metals can be feasibly extracted.     

Geochemistry of major and trace elements and Pb–Sr isotopes of a weathering profile developed on the Lower Cambrian black shales in central Hunan,China

This paper reports a geochemical study on the major and trace elements and Pb–Sr isotopes of a weathering profile developed in the Lower Cambrian black shales in central Hunan (China). Six weathering horizons were identified and sampled vertically throughout the profile. The chemical composition of the profile consists of variable concentrations of the major elements Fe₂O₃, FeO, MnO, MgO, CaO, Na₂O, and P₂O₅ and of less variable concentrations of SiO₂, TiO₂, Al₂O₃, and K₂O. The chemical change caused by weathering is estimated by mass-balance calculations, and the results show that the element mobility is characterised by substantial loss of SiO₂, FeO, CaO, K₂O, Na₂O, LOI, Cr, V, Ba, Cs, Rb, Sr, U, and Th, and moderate loss of Al₂O₃, MgO, Fe₂O₃, Ni, Cu, Pb, Tl, Sn, Sc, Ge and REE (Y). The high field strength elements TiO₂, Sn, Sc, U, Ga, Ge, Zr, Hf, Nb, and Ta were immobile during weathering. The chemical changes and the Pb–Sr isotopic data suggest that four types of chemical reactions occurred: the oxidation of sulphide minerals (e.g., pyrite) and organic carbon (OS), the dissolution of less resistant clinochlore-Ia, calcite, and P-bearing minerals (DL), the dissolution of detrital albite and microcline (DA), and the transformation of clay (TC) minerals (e.g., muscovite and illite–smectite). These chemical reactions then led to two stages of geochemical processes, an early stage of chemical differentiation and a later stage of chemical homogenisation. The chemical differentiation dominated by the OS, DL, and DA reactions, led to the leaching of mobile elements (e.g., MgO, Na₂O, K₂O, P₂O₅, Sr, and REE) and the redistribution of some less mobile elements (e.g., SiO₂ and Al₂O₃). In contrast, the chemical homogenisation, which was caused by TC reactions, led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil. Soils derived from black shales in South China might result from the above two geochemical processes.     

Grain size and geochemistry of surface sediments in northwestern continental shelf of the South China Sea

Eighty-six surface sediments collected from the northwestern continental shelf of the South China Sea (SCS) were analyzed for grain size distribution, calcium carbonate, organic carbon, and major and trace element compositions to investigate sediment provenance and factors controlling their geochemical composition. Sediments from the eastern continental shelf of Hainan Island have higher sand and lower clay content, while the samples from the nearshore Hainan Island have higher contents of gravel and clay. Calcium carbonate contents in samples show a positive correlation with water depth in northwestern shelf of SCS, suggesting that it is related to biological factors. However, the nearshore sediments have higher contents of organic carbon compared to those of the outer shelf, possibly suggesting that the terrigenous organic matter usually deposited in nearshore environments such as bays and estuaries. Compared with the upper continental crust, the samples have relatively lower contents of SiO₂ and Al₂O₃, higher than those of the Pearl and Red river sediments. The low contents of K₂O and Na₂O in sediments from the northwestern continental shelf are consistent with intense chemical weathering in the river basin due to the seasonally hot and humid climate regime. The sediments mainly consist of three components, including the gravel fraction composed of calcareous debris, the sand fraction composed of quartz, and the silt and clay fractions mainly composed of clay minerals. The content of each component depends on grain size, sediment source, biogenesis, and hydrodynamic conditions, which finally controls the chemical composition of the sediments. The distributions of Co/Al₂O₃, Cr/Al₂O₃, and Zr/Sc ratios for sediments in the northwestern continental shelf suggest that source rocks are mainly composed of felsic rocks rather than mafic rocks. There is a distinct difference in sediment source between eastern and western shelf sediments; the eastern shelf sediments are characterized by high Zr/Sc ratios mainly derived from the Pearl River, while the western shelf sediments have relatively low values of Zr/Sc indicating a main contribution possibly sourced from the Red River Basin. Terrigenous materials from Hainan Island usually influence the geochemistry of sediments deposited in the nearshore area.     

Paleocene-Eocene potential source rocks in the Avengco Basin,Tibet: Organic geochemical characteristics and their implication for the paleoenvironment

The Avengco Basin is located in the western part of the Tibetan Plateau and is similar to the Nima Basin in the central part of the plateau and the Lunpola Basin in the eastern part in terms of sedimentary characteristics and tectonic settings, which are well known to provide a good source rock potential. However, the organic geochemical characteristics of the Paleocene-Eocene potential source rocks in the Avengco Basin have been under debate. Thirty-four marl and mudstone outcrop samples of the Niubao Formation in the Avengco Basin were collected and subjected to the following analyses: total organic carbon (TOC), Rock–Eval pyrolysis, stable carbon isotopes of kerogen, gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). Here, we present the results indicating the organic matter of the upper Niubao Formation is mainly composed of Type II kerogen with a mixed source, which is dominated by algae. However, the lower Niubao Formation has the less oil-prone Type II–III kerogen, and the sources of the organic matter are mainly terrestrial plants with less plankton. In addition, the samples are thermally immature to marginally mature. The Niubao Formation was deposited in an anoxic–oxic environment which was brackish with an imperceptible stratified water column. The upper Niubao Formation has a medium to good hydrocarbon-generating potential. However, the lower Niubao Formation has a zero to poor hydrocarbon-generating potential.