Lower Cambrian Black Shales Research Articles

Enrichment of Platinum Group Elements in Lower Cambrian Polymetallic Black Shale, SE Yangtze Block, China

Platinum group elements (PGEs) occur mainly in basic–ultrabasic igneous rocks and are concentrated by the differentiation and crystallization of magma. Thin polymetallic layers including Ni, Mo, V, PGEs, and rare-earth elements are widely developed in lower Cambrian black shales in southern China. The PGE contents in such layers may not be economically significant but are still unusually enriched. PGE enrichment mechanisms have not been widely studied, but here the PGE compositions of polymetallic layers in the Shuidong (Nayong) and Niuchang (Weng’an) sections of the SE Yangtze block in China were determined, and results compared with published data for the region. Results indicate that PGEs are enriched in the polymetallic layers, whereas the surrounding country rocks are barren. The ΣPGE contents in the Shuidong Ni–Mo deposits are generally lower than in the Niuchang V deposits. PGE distribution patterns in the polymetallic layers are similar to those in basic–ultrabasic rocks, indicating that such rocks played a role in the PGE enrichment. Oceanic hypoxia during the Sinian–Cambrian transition resulted in the production of large amounts of organic matter and H2S in the ocean. When high-salinity brine reached the ocean bottom, rapid changes in Eh–pH conditions caused enrichment of metals at the sediment–seawater interface, and this enrichment was later enhanced during diagenesis.

Control of V accumulation in organic-rich shales by clay-organic nanocomposites

Organic-rich shales exhibit a wide range of vanadium (V) concentrations in deep-time successions. Although V accumulation is generally acknowledged to be related to redox state, organic matter and clay mineral content, previous studies have failed to explain the mechanism of variable V enrichment in organic-rich shales. Here, we hypothesize that clay-organic nanocomposites control authigenic V uptake in organic-rich shales. Specifically, V is scavenged by dissolved organic matter in seawater, which is then taken up by smectite during transport or deposition, followed by V incorporation into illite through illitization of smectite and pyrolysis of organic matter during diagenesis. To test this hypothesis, lower Cambrian black shales from South China were investigated by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) at a micro- and nanoscale. Results show that only 1M/1Md-type illite, generated through transformation from a smectite precursor, has significant V concentrations (0.7–4.8 wt%), whereas 2M1-type illite and other minerals contain little or no V. All 1M/1Md-type illite was closely associated with organic matter, occurring as clay-organic nanocomposites. Our results are significant in (i) showing clay-organic nanocomposites play a key role in V cycling in the marine system, (ii) providing a mechanism for understanding variations in the V concentrations of deep-time organic-rich shales, and (iii) improving strategies for prospecting and extraction of V ores.

Different accumulation mechanisms of organic matter in Cambrian sedimentary successions in the western and northeastern margins of the Tarim Basin, NW China

Lower Cambrian black shales are widely distributed in the Tarim Basin, but those in the Keping area in the west have a high total organic carbon (TOC) content different from those in the Kuluketage area in the northeast of the basin. The Cambrian Yuertusi Formation in the Shiairike section (Keping) and the Xishanbulake Formation in the Yaerdangshan section (Kuluketage) are time-equivalent and can be correlated. Black shales of the Yuertusi Formation have lower δ13CKer values (<−34‰) than the Xishanbulake Formations (δ13CKer values > − 4‰). These values suggest that black shales in the Yuertusi Formation were dominated by benthic algae, whereas rocks in the Xishanbulake Formation contain substantial amounts of planktonic algae. Analyses of trace elements demonstrate an anoxic depositional environment, with high productivity in both the Yuertusi and Xishanbulake Formations. The enhanced productivity was promoted by upwelling fluids and hydrothermal events, leading to the enrichment of nutrient elements such as barium, cadmium, copper, phosphorus, nickel, vanadium, and zinc. More terrigenous inorganic detritus input to the Xishanbulake Formation, which led to less amounts of organic matter and degradation of planktonic algae during sedimentation, resulted in a lower TOC content than the Yuertusi Formation.

Climate-ocean control on the depositional watermass conditions and organic matter enrichment in lower cambrian black shale in the upper Yangtze Platform

The lower Cambrian sediments including the Qiongzhusi and Maidiping formations in the Sichuan Basin are generally considered as the main source rocks of Precambrian-Cambrian petroleum system. A comprehensive study integrated with multiple geochemical methods was conducted on 119 lower Cambrian drillcore samples collected from the upper Yangtze Platform. The results show that the fluctuation of sea level and climate during the deposition of the Qiongzhusi and Maidiping formations act as the critical factors controlling the watermass conditions and thus OM accumulation. During the deposition of upper Maidiping Formation (Є1m), the paleoclimate was arid and sea level was relatively low in the study area. The depositional environment indexes (UEF-MoEF, gammacerane index, β-carotane and VEF) suggested that the upper Є1m was deposited in a strong restricted marine setting with high water salinity and anoxic water conditions in Ziyang-Weiyuan area. However, during the early Qiongzhusi Formation period, the paleoclimate became humid and sea level rose in the study area. High paleoproductiviy caused by high concentrations of nutrients brought by seawater and freshwater plays an important role in OM accumulation in the basal part of the Qiongzhusi Formation (Є1q). Furthermore, the model of paleoproductivity promotion caused by the coordinated control of climate and extensive transgression in the lower part of Є1q may also be applicable to interpret the origin of high organic carbon abundance in the strata deposited in the same period with the Є1q (such as Shiyantou, Shuijingtuo and Niutitang formations) in other parts of the upper Yangtze Platform.

The world-class Koushk Zn-Pb deposit, Central Iran: A genetic model for vent-proximal shale-hosted massive sulfide (SHMS) deposits – Based on paragenesis and stable isotope geochemistry

The Koushk Zn-Pb deposit is the largest known and least deformed and non-metamorphosed Early Cambrian shale-hosted massive sulfide (SHMS) deposit at Central Iran. The current remaining reserves are estimated to be greater than 14 Mt ore, averaging 7% Zn and 1.5% Pb; the primary resources ore of the deposit is estimated to be more than 60 Mt.At this deposit, different hydrothermal ore styles (bedded ore, vent complex, and feeder zone) are well preserved within the Lower Cambrian black siltstones and shales. According to fluid-rock interaction and different ore-forming processes in SHMS systems, these ore facies with extensive hydrothermal alteration provide unique conditions to understand critical textural and geochemical frameworks to present a genetic model. In this research, we focus on different paragenetic stages of sulfide mineralization and fluid-rock interactions in different ore styles from the Koushk SHMS deposit. The paragenetic relationship provides the context for the interpretation of stable isotopes (S, C, and O) in hydrothermal sulfides and carbonates.Detailed petrography and paragenetic studies represent three major generations of sulfide mineralizations at different ore zones: (1) stage I includes very fine-grained (<6 μm) framboids, spherulite pyrite (py1), associated with minor fine-grained disseminated sphalerite (sp1), and galena (gn1); (2) Stage II is composed of a diagenetic intergrowth of coarse-grained framboids and spherulite pyrite, packed polyspherulite aggregates and pyrite nodules (py2) replacing diagenetic barite and carbonate nodules, and are followed with coarse-grained sphalerite (sp2) and galena (gn2) that replace former sulfides and barite, deposited as disseminated, laminated and sulfide-rich banded textures; (3) stage III of sulfide mineralization is characterized by vent complex development (VCD) over the feeder zone, hydrothermal brecciation, dissolution of rock-forming minerals, and extensive replacement of earlier sulfides and barite, leading to deposition of stage III of ore sulfides. The oxygen and carbon isotopes values, for fluid in equilibrium with hydrothermal calcite and dolomite in this deposit range from δ18O +8 to +16.7‰ and δ13C from −8.3 to −4.3‰, are generally compatible with basinal brines and formation water as fluid sources. In addition, highly positive δ34S values of hydrothermal sulfides (+6.5 to +36.7‰) in different ore stages of the Koushk deposit are consonant with other SHMS deposits.Textural relationships and S isotope data reveal that the contribution of bacterial sulfate reduction (BSR) in the Zn-Pb mineralization is not so significant, but the thermochemical sulfate reduction (TSR) nd direct barite replacement could provide sufficient sulfur for the main sulfide mineralization in the SHMS deposits. Also, the data presented in this paper are against a syngenetic, purely synsedimentary-exhalative model, and give prominence to that vent-proximal SHMS deposits formed predominantly during the diagenesis in the uppermost sediment pile and replacement of host rocks during vent complex development (VCD) processes.

Evidence for high organic carbon export to the early Cambrian seafloor

Oxygenation of the early Cambrian ocean is commonly ascribed to high organic export to the sediment due to the rise of algae and filter-feeding animals, but direct evidence of elevated export fluxes has been lacking to date. Here, we report an integrated proxy dataset (U-Mo isotopes, Fe speciation, and major and trace elements) for lower Cambrian black shales at Yuanjia on the Yangtze Platform (South China). These shales are characterized by high iron speciation ratios and Mo and U enrichments, indicating persistently euxinic water conditions during their deposition, but a broad range of δ98Mo values (−0.10‰ to +1.94‰) implies strongly variable watermass sulfidity. High and variable nutrient element (Cu, Zn, Ni, Cd) enrichments at Yuanjia are consistent with elevated and variable organic productivity and export fluxes from the euphotic zone to the early Cambrian seafloor. The Yuanjia shales also exhibit super-heavy δ238U values (to +0.84‰) that cannot be explained by U reduction in sediment porewaters. We hypothesize that these values record a U reduction process within the water column, where U isotope fractionation can be much larger than in sediment porewaters. Such processes may have operated within a benthic organic flocculent layer at the sediment-water interface, a feature that is characteristic of high productivity systems with anoxic bottom waters. Furthermore, the coupling between δ238U, nutrient elements, and δ98Mo suggests that elevated productivity levels were driven likely by episodic upwelling in early Cambrian oceans, supplying abundant nutrients for productivity and influencing H2S concentrations in the water column. These observations, along with high organic carbon accumulation rates (5600 mg cm−2 kyr−1), are direct evidence of high organic export fluxes to the early Cambrian seafloor, thus providing key support for the hypothesis that enhanced organic export and burial triggered a major atmospheric-oceanic oxygenation event that stimulated the subsequent explosion of Cambrian life.

Formation and evolution of the Ediacaran to Lower Cambrian black shales in the Yangtze Platform, South China

Black shales in the Ediacaran Doushantuo Formation (predominantly from Member II, IV and equivalent strata) and the Lower Cambrian Niutitang Formation (or equivalents) occur widely in the Yangtze Platform, South China. These black shales could provide sufficient hydrocarbons for the petroleum system in this region. However, biomarker parameters have proven invalid in the assessment of petroleum resources because of the high thermal maturity. Therefore, it is necessary to study the characteristics and formation mechanisms of these two black shales. This study analyzed redox-sensitive elements, total organic carbon (TOC) contents, and carbon isotopic compositions of organic matter in four continuous sedimentary successions including the two investigated shales in the Yangtze Platform (namely Jiulongwan on the inner shelf, Songlin in an intra-shelf lagoon, Dongkanshang on the upper slope, and Fengtan in the basin). Combined with other previously reported five sections, the redox conditions of sedimentary waters and marine paleoproductivity of the Ediacaran to Lower Cambrian black shales in the Yangtze Platform were discussed and compared. The results showed that the redox conditions of the Early Cambrian in the study area were similar to those of the Ediacaran Doushantuo period, which were oxic-suboxic at surface water, and anoxic in the deep with the occurrence of euxinic conditions. However, the euxinic conditions in the deep waters were distributed more widely in the Early Cambrian than in the Ediacaran period. The source of organic matter in the Doushantuo Formation was dominated by algae in the shallow-water platform and the shelf-margin areas, while chemoautotrophic/methanotrophic biomass may also have contributed in the intra-shelf lagoon and deep-water basin areas. In contrast, the organic matter of the Niutitang Formation was mainly contributed by the chemoautotrophic/methanotrophic biomass, with a minor contribution by photosynthetic algae and cyanobacteria. During the Doushantuo period, the input of terrigenous clastic materials was low, resulting in the enrichment of organic matter and formation of organic-rich black shale in the Yangtze Platform over a long geological time period. Compared to the Doushantuo period, the shorter sedimentation time and the higher deposition rate during the Early Cambrian produced a thicker and higher-quality Niutitang Formation source rock widely distributed in the Yangtze Platform.

Pore structure characteristics of the Lower Cambrian black shale in the Cengong block, southeastern Guizhou area

Pore structure characteristics of the Lower Cambrian black shale in the Cengong block, southeastern Guizhou area

Interpretation of regional-scale distribution of high Hg in soils of karst area in southwest China

The abundance and spatial distribution of mercury in soil samples covering nearly the whole of China ( c . 9 million km 2 ) were obtained by the China Geochemical Baseline (CGB) project from 2008 to 2015. From the data and maps, it was found that the Hg contents both in the top (0–25 cm) and deep soil (below 100 cm or C-horizon) of the karst area in southwestern China are richer than that of any other geographical landscape areas. Median Hg values for this karst area are 113 and 90 µg/kg respectively ( n = 126), in the top and bottom soils, much higher than the corresponding data of 26 and 18 µg/kg for the entire country ( n = 3380). This regional-scale high Hg distribution is due mainly to high Hg geochemical backgrounds in bedrock and soil formation on limestone. A large-scale epithermal metallogenic domain developed in southwestern China and the extensive distribution of Lower Cambrian black shale in southwest China led to a high Hg geochemical background for parent rocks of soils. The chemical weathering of carbonate minerals and the geochemical behaviour of Hg in the epigenetic environment result in Hg enrichment by ferric oxides, iron hydroxide and organic carbon in limestone soils. Human activities mostly result in local-scale Hg anomalies, but have little effect on this regional-scale anomaly.

Controls on the organic carbon content of the lower Cambrian black shale in the southeastern margin of Upper Yangtze

Control of various factors, including mineral components, primary productivity and redox level, on the total organic carbon (TOC) in the lower Cambrian black shale from southeastern margin of Upper Yangtze (Taozichong, Longbizui and Yanbei areas) is discussed in detail in this article. Mineral components in the study strata are dominated by quartz and clay minerals. Quartz in the Niutitang Formation is mainly of biogenic origin, and the content is in positive correlation with TOC, while the content of clay minerals is negatively correlated with TOC. Primary productivity, represented by the content of Mobio (biogenic molybdenum), Babio (biogenic barium) and phosphorus, is positively correlated with TOC. The main alkanes in studied samples are nC18–nC25, and odd–even priority values are closed to 1 (0.73–1.13), which suggest the organic matter source was marine plankton. Element content ratios of U/Th and Ni/Co and compound ratio Pr/Ph indicate dysoxic–anoxic bottom water, with weak positive relative with TOC. In total, three main points can be drawn to explain the relationship between data and the factors affecting organic accumulation: (1) quartz-rich and clay-mineral-poor deep shelf–slope–basin environment was favorable for living organisms; (2) high productivity provided the material foundation for organic generation; (3) the redox conditions impact slightly on the content of organic matter under high productivity and dysoxic–anoxic condition.

Evolution of porosity and clay mineralogy associated with chemical weathering of black shale: A case study of Lower Cambrian black shale in Chongqing, China

This work aimed to evaluate the evolution of pore networks and clay mineral in black shale weathering systems in Chongqing, China. The pore-size distributions, relationships between fractal dimensions and minerals, and clay mineral weathering pathways were evaluated for three weathering profiles, A, B, and C, which were characterized as weak, weak to moderate, and moderate to intense based on the Chemical Index of Alteration. It was found as the shale density decreased, the porosity and specific surface area increased with decreasing depth during weathering. The largest pore diameter peak in a pore-size distribution graph shifted further right as the degree of weathering increased. Micro-transition pores (<100 nm) dominated the parent shale, while mesopores (0.1–1 μm) and macropores (>1 μm) dominated the saprock and regolith zones. The surface fractal dimension of the micro-transition pores was positively correlated with the pyrite and carbonate content, but negatively correlated with secondary clay mineral content. By contrast, the surface fractal dimension of the macropores was negatively correlated with pyrite and carbonate content and positively correlated with secondary clay mineral content. These relationships indicate that micro-transition pores at the surface became smoother and macropores became rougher during weathering. Inherited micas, including illite, predominated the clay fractions along transects as weathering proceeded. The pedogenic minerals (i.e., smectite, vermiculite, kaolinite, and mixed-layer minerals) were minor components in the saprock and regolith zones. Smectite formation was likely due to direct transformation from mica or illite under strongly acidic conditions. A second possibility is transformation proceeded through mixed-layer mineral intermediates, i.e., mica-smectite/illite-smectite and mica-vermiculite/illite-vermiculite. The weathering of clay minerals in black shale from weak to intense weathering stages was mica/illite → mica-smectite/illite-smectite or mica-vermiculite/illite-vermiculite → “chlorite” (including vermiculite, hydroxy-interlayered vermiculite?, and chlorite?) → smectite → kaolinite → gibbsite. This study revealed pore geometry evolution depends on transport-limited solutions that facilitate mineral breakdown and formation. The clay mineral weathering pathways were controlled by the weathering environment and duration.

Geochemical investigation of the lower Cambrian mineralised black shales of South China and the late Devonian Nick deposit, Canada

Lower Cambrian (Tommotian) black shales of the Niutitang Formation, South China, host a thin accumulation (5–20 cm) of Ni, Mo, platinum group elements (PGE)-Au, Ni, As, Zn, Cu, V and rare earth elements (REE). Of all known deposits, the late Devonian Ni-Mo-PGE sulphide horizon in black shales of the Nick deposit, Selwyn Basin in Canadian Yukon, presents the strongest similarity with the Niutitang Formation polymetallic layer. In the present study, samples of the mineralised layer and host shales in South China from two mine sites (Zunyi and Sancha) were investigated in order to further characterise the prevailing redox conditions and mechanisms involved in the metal concentration process at both sites. Additional comparison with the Nick deposit mineralised layer was also undertaken. Bulk geochemical analysis (including PGE analysis) were conducted on all samples, along with μm-scale investigation of metal distributions and associations by X-ray fluorescence (XRF) mapping and Synchrotron XRF mapping. The comparison of the two sites from South China highlights strong similarities in metal enrichment factors. PGE and REE distribution patterns are also highly similar and suggest a source from seawater. Strong variations in elemental distributions at µm-scale were observed at both sites, most likely related to intense variations in redox conditions in the sediment. The comparison with the Nick deposit highlights a stronger enrichment in Ni and Zn in the Nick deposit and a greater variety of minerals in the Niutitang shales. The PGE and REE distributions of both the Nick deposit and the Niutitang shales, however, present highly similar patterns, and support an origin from seawater. The present study provides further insight into mineralisation style and processes in these mineralised black shales, highlighting the importance of redox conditions and re-emphasizing the role of organic matter in the formation of these mineralised layers.

Characteristics and formation processes of (Ba, K, NH4)-feldspar and cymrite from a lower Cambrian black shale sequence in Anhui Province, South China

ABSTRACTBarium-rich silicates were recently found in the lower Cambrian black shale sequence of Anhui Province, South China. The Ba silicates can be divided into Ba-poor K-feldspar, Ba-rich K-feldspar and cymrite, on the basis of occurrence and composition. The Ba-poor K-feldspar have low BaO contents (&lt;1.00 wt.%) and occur as sporadic grains in mudstone. In contrast, the Ba-rich K-feldspar have higher BaO contents (1.36–20.51 wt.%) and occur mainly as rims around grains of Ba-poor K-feldspar in mudstone, as void fillings in dolostone, and as grains dispersed in chert. The cymrite is observed only in chert and occurs as dispersed euhedral tabular crystals. Analyses of the Ba-poor K-feldspar and Ba-rich K-feldspar provide compositions that total &lt;100 wt.% and show non-stoichiometry characterized by (Na + K + Ba + Ca)apfu less than 1.00 and (Al + Si)apfu slightly higher than 4.00. Raman and infrared (IR) spectroscopy has revealed the presence of ${\rm NH}_{\rm 4}^{\rm +} $ and H2O in the Ba-rich K-feldspar, which accounts for the unusual composition of these feldspars. We suggest that the formation processes of the Ba-poor K-feldspar, Ba-rich K-feldspar and cymrite were closely associated with multi-stage hydrothermal fluids enriched in Ba and NH4, consistent with an extensional tectonic setting during the early Cambrian. The Ba-poor K-feldspars might have formed through interactions between pre-existing K-feldspars and hydrothermal fluids. The Ba-rich K-feldspars in mudstone and dolostone formed mainly by precipitation from hydrothermal fluids that infiltrated the sediments, and we suggest the Ba-rich K-feldspars and cymrite in the chert are of exhalative hydrothermal origin.

Mineral composition of the Lower Cambrian black shale in the Upper Yangtze region and its significance in oil and gas exploration

The Lower Cambrian is one of the important exploration strata of marine shale gas reservoirs in the Upper Yangtze region, but its exploration is geologically impacted by multiple factors such as deep burial depth, fewer wells and unsatisfactory exploration results. In this paper, the mineral composition of the Lower Cambrian black shale in the Upper Yangtze region was investigated using the data of X-ray diffraction, rock thin section, scanning electron microscope (SEM) and argon ion polishing-electron microscope, and its significance in oil and gas exploration was also analyzed. The following findings were made. First, shale in this region is mainly composed of clay minerals and quartz, and secondarily of carbonate minerals and feldspar. The average content of quartz in shale is 44.6%, and two types of quartz are developed, i.e. terrigenous quartz and biogenic silica. The average content of clay minerals is 33.32%, with illite (I) content being the highest, illite/smectite (I/S) layer content uneven, the mixed layer ratio lower, generally 5%, and chlorite (C) and kaolinite (K) locally distributed. Second, four types of clay mineral assemblages are identified, including I + I/S + C, transition from I + I/S + C to I + C, I + C, and I, indicating that diagenetic environments of shale are different in different areas. Third, the disordered arrangement of mineral grains is favorable for the survival of intergranular pores, indicating that in the diagenetic alteration process of feldspar into clay minerals, linear pores can be generated. It is concluded that the brittleness index of shale in this region is generally above 40%, indicating a good fragility. The fragility of shale in SE Guizhou is better than that in SW Sichuan Basin.

Application of low-pressure gas adsorption to nanopore structure characterisation of organic-rich lower Cambrian shale in the Upper Yangtze Platform, South China

ABSTRACTThe pores in shales are mainly on a nanometer scale, and the pore-size distribution is vital with regard to the preservation and exploitation of shale gas. This study focuses on the organic-rich lower Cambrian black shale in the Upper Yangtze Platform, South China and investigates their TOC, mineralogical composition and nanopore structure. Low-pressure N2 and CO2 adsorption experiments were conducted at 77.35 K and 273.15 K, respectively, and the nanopore structures were characterised by the modified Brunauer–Emmett–Teller, Dubinin–Radushkevich, t-plot, Barrett–Joyner–Halenda and density functional theory (DFT) methods. The results indicate the following. (1) The lower Cambrian shale has a high TOC content (1.77–7.23 wt%) and a high quartz content (27.7–51.6 vol%). The total specific surface area varies from 12.02 to 28.87 m2/g. Both the total specific surface area and quartz content are positively associated with the TOC content. (2) Shale samples with a higher TOC content have a greater number of micropores, resulting in more complicated nanopore structures. Micropore volumes/surface areas and non-micropore surface areas all increase with increasing TOC content, indicating that TOC is the key factor determining the nanopore structure of the lower Cambrian shale. (3) A combination of N2 and CO2 adsorption provides the most suitable detection range (∼0.3–60 nm) and is both highly reliable and accurate with regard to nanopore structure characterisation.

SIMS U-Pb zircon ages and Ni-Mo-PGE geochemistry of the lower Cambrian Niutitang Formation in South China: Constraints on Ni-Mo-PGE mineralization and stratigraphic correlations

In comparison with previous studies on the Ni-Mo-PGE ore deposits within the deep water Lower Cambrian black shales of the Niutitang Formation around Hunan and Guizhou provinces, this study focused on the shallow water Ni-Mo-PGE enriched layer of the Niutitang Formation around Hubei Province by means of an integrated geochronological, PGE, REE and trace element geochemical study. The results suggest the Ni-Mo-PGE enriched layer has a dominant seawater origin, whereas the Ni-Mo-PGE ore deposits have mixed seawater, hydrothermal and terrigenous origins. Trace metals precipitated from ambient seawater under anoxic-suboxic settings at exceedingly slow sedimentation rates with episodic injections of fresh oxidized seawater.Organic matter played an important role in sourcing, transporting and precipitating trace metals. However, decay of organic material by means of sulfate reducing reactions and/or fluid reworking overprinted primary relationships with trace metal contents. However, the positive linear relationships are clearly shown between organic carbon and trace metal contents. New SIMS U-Pb ages constrained the depositional age of the Ni-Mo-PGE enriched layer to be 532–527Ma, which argues against previous assumption that the anoxic event recorded in the Ni-Mo-PGE took place at the Ediacaran–Cambrian boundary. Furthermore, new radiometric ages suggest the Shuijingtuo/Niutitang Formation could be correlated with Shiyantou Formation, Dahai and upper Zhongyicun members, whereas Yanjiahe Formation correlated with Daibu and lower Zhongyicun members in Yunnan Province, respectively.

Mineralogy and geochemistry of three weathered Lower Cambrian black shale profiles in Northeast Chongqing, China

This paper reports a geochemical study on the mineralogy and major elements of mid-ridge (A), near mountaintop (B), and valley (C) profiles developed in the Lower Cambrian black shale in Northeast Chongqing, China. The primary objective was to understand the elemental mobility, mineralogical transformation, and weathering progression during black shale chemical weathering in a subtropical climate. Profiles A, B, and C are characterized as weak, weak to moderate, and moderate to intense in terms of weathering intensity, respectively, by the Chemical Index of Alteration (CIA). Results indicate that most elements were mobilized by acidic solutions produced during the oxidation of pyrite and organic matter (OM). Among the major elements, Si was slightly enriched in profile A, but depleted through desilication in profile B and C. Al was enriched in the regolith zone in profile A and C, and Fe was enriched at the oxic front because of secondary clay and sesquioxide formation. The addition and depletion of major elements and the depth of the oxic front increased with the degree of weathering. Gypsum and Fe- (hydro-) oxides were observed to form and re-precipitate in the saprock and fractured shale zones. Clay minerals formed from dissolution of plagioclase and the transformation of other labile clay minerals during weathering. The progressive changes in mineralogical composition of weathered material from profile A to C showed the sequence of mineral decomposition with degree of weathering: first, oxidation of pyrite and OM; then Ca and Mg were removed during dissolution of carbonatite; followed by removal of Na from dissolution of plagioclase; lastly, transformation of clay minerals during weathering; meanwhile, desilication occurs at moderate to intense weathering stages.

Swelling characteristics of soils derived from black shales heightened by cations in Northern Chongqing, China

Black shale is widely distributed in southern and western China. The swelling and breakdown characteristics of black shale are important physicomechanical properties in engineering activities. Aqueous solutions with different compositions exert various influences on the swelling characteristic of black shale because of the complexity of its composition. Soils derived from black shale are most commonly developed on black shales with bedded horizons that generally have cutting and weathering profiles. This paper reports an axial free swelling study for soils derived from black shales and developed in the Lower Cambrian black shales in Chengkou County, Chongqing Municipality, Southwest China. To discuss the swelling characteristics of black shale under the influence of cations, an axial free swelling test was conducted on black shale samples while considering the initial water content, weathering stage of black shale, and different cationic solutions with various concentrations. Results showed that the swelling deformation curve of black shale could be divided into three phases: acceleration, deceleration, and stability. The axial swelling ratio eventually decreased with increasing water content, and the axial free swelling rate changed with the free expansion model of the exponential relationship. Under a constant initial water content, the axial swelling rate increased with the intensity of black shale weathering in the same immersion solution. When different types of immersion solutions were used, the cationic price was high and the axial swelling rate increased with rising cation valence. The changes in the swelling ratio were significant at a valence of one or two. The results of this study provide further understanding of the engineering geological characteristics of black shale and the geological hazards caused by the swelling characteristics of black shale.

Multiple controls on the paleoenvironment of the Early Cambrian marine black shales in the Sichuan Basin, SW China: Geochemical and organic carbon isotopic evidence

In order to understand the paleoenvironment of the Early Cambrian black shale deposition in the western part of the Yangtze Block, geochemical and organic carbon isotopic studies have been performed on two wells that have drilled through the Qiongzhusi Formation in the central and southeastern parts of Sichuan Basin. It shows that the lowest part of the Qiongzhusi Formation has high TOC abundance, while the middle and upper parts display relative low TOC content. Redox-sensitive element (Mo) and trace elemental redox indices (e.g., Ni/Co, V/Cr, U/Th and V/(V + Ni)) suggest that the high-TOC layers were deposited under anoxic conditions, whereas the low-TOC layers under relatively dysoxic/oxic conditions. The relationship of the enrichment factors of Mo and U further shows a transition from suboxic low-TOC layers to euxinic high-TOC layers. On the basis of the Mo-TOC relationship, the Qiongzhusi Formation black shales were deposited in a basin under moderately restricted conditions. Organic carbon isotopes display temporal variations in the Qiongzhusi Formation, with a positive excursion of δ13Corg values in the lower part and a continuous positive shift in the middle and upper parts. All these geochemical and isotopic criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Qiongzhusi Formation black shales. The correlation of organic carbon isotopic data for the Lower Cambrian black shales in different regions of the Yangtze Block shows consistent positive excursion of δ13Corg values in the lower part for each section. This excursion can be ascribed to the widespread Early Cambrian transgression in the Yangtze Block, under which black shales were deposited.

Characteristics of Hydrocarbon Fluid Inclusions and Their Significance for Evolution of Petroleum Systems in the Dabashan Foreland, Central China

Field investigation combined with detailed petrographic observation indicate that abundant oil, gas, and solid bitumen inclusions were entrapped in veins and cements of sedimentary rocks in the Dabashan foreland, which were used to reconstruct the oil and gas migration history in the context of tectonic evolution. Three stages of veins were recognized and related to the collision between the North China block and the Yangtze block during the Indosinian orogeny from Late Triassic to Early Jurassic (D1), the southwest thrusting of the Qinling erogenic belt towards the Sichuan basin during the Yanshanian orogeny from Late Jurassic to Early Cretaceous (D2), and extensional tectonics during Late Cretaceous to Paleogene (D3), respectively. The occurrences of hydrocarbon inclusions in these veins and their homogenization temperatures suggest that oil was generated in the early stage of tectonic evolution, and gas was generated later, whereas solid bitumen was the result of pyrolysis of previously accumulated hydrocarbons. Three stages of hydrocarbon fluid inclusions were also identified in cements of carbonates and sandstones of gas beds in the Dabashan foreland belt and the Dabashan foreland depression (northeastern Sichuan basin), which recorded oil/gas formation, migration, accumulation and destruction of paleo‐reservoirs during the D2. Isotopic analysis of hydrocarbon fluid inclusions contained in vein minerals shows that δ13C1 of gas in fluid inclusions ranges from –17.0‰ to –30.4‰ (PDB) and δD from –107.7‰ to –156.7‰ (SMOW), which indicates that the gas captured in the veins was migrated natural gas which may be correlated with gas from the gas‐fields in northern Sichuan basin. Organic geochemical comparison between bitumen and potential source rocks indicates that the Lower Cambrian black shale and the Lower Permian black limestone were the most possible source rocks of the bitumen. Combined with tectonic evolution history of the Dabashan foreland, the results of this study suggest that oil was generated from the Paleozoic source rocks in the Dabashan area under normal burial thermal conditions before Indosinian tectonics and accumulated to form paleo‐reservoirs during Indosinian collision between the North China block and the Yangtz block. The paleo‐reservoirs were destroyed during the Yanshanian tectonic movement when the Dabashan foreland was formed. At the same time, oil in the paleo‐reservoirs in the Dabashan foreland depression was pyrolyzed to transform to dry gas and the residues became solid bitumen.