North China Platform Research Articles

Tracking depositional and geochemical variations in the Cambrian North China Platform: Insights from sedimentology, geochemistry, and C-O isotopic records

The Cambrian carbonate sequence in North China is of significant global and regional scientific importance due to its continuous and thick exposure and the data on the major environmental changes and biological events of the early Phanerozoic, which can be gleaned from it. While the well-exposed Cambrian outcrops have been utilized to unravel the processes governing these events, few studies have focused on the outcrops' spatio-temporal depositional patterns and geochemical variations. Therefore, our study address, these issues by integrating fieldwork with petrographic and geochemical analyses. In the North China Platform (NCP), the Cambrian carbonate sequence is predominantly composed of six third-order depositional sequences (~370–400 m thick) that can be divided into nine microfacies, representing three primary microfacies associations, namely tidal flat, lagoon, and shoal. In contrast to the standard carbonate sequence stratigraphy model and many other examples of Cambrian platforms, the deposition of oolitic grainstones in the NCP are overlain by the deep-water calcareous mudstone and shale during a period of slow fall of sea-levels. Such a particular style shows a similar trend to the Schlager model of a type III sequence boundary that formed between a highstand systems tract (HST) and a transgressive system tract (TST). Scanning electron microscope (SEM) observations show an abundance of microbial fossils, a nanosphere, and remnants of extracellular polymeric substances (EPS) inside the dark micrite of normal regressive deposits (oolitic grainstone), suggesting the significant role of microbial activity in the formation of ooids. The δ18O isotopic values range from −4.15 to −11.75‰ VPDB, portraying pervasive diagenetic alterations with a variable degree of diagenesis at different localities, and both marine- and burial-related diagenesis. In contrast, the δ13C VPDB values have a very narrow range, from −2 to +2‰ VPDB, suggesting little to no influence of meteoric diagenesis. At the same time, the slight depletion of δ13C isotope values may reflect the role of microbial processes during the genesis of ooids. This study highlights the importance of microbial activity and marine-related diagenetic overprints during the formation of carbonate facies in the Cambrian Period and provides evidence that carbon isotopic excursions observed during different major environmental and biological events in the NCP are related to primary signals and not altered by intensive meteoric diagenesis.

Possible Continuous Vertical Water Leakage of Deep Aquifer: Records from a Deep Well in Tianjin Province, North China

It is well known that the storage of the valuable helium and other natural gases needs thick well-confined aquitards. Meanwhile, the assumption that deep aquifers are confined is most evident from the wide practices of targeting deep aquifers as the storage of toxic wastes and CO2. With the negative phase shifts of the M2 tidal wave, previously, deep Gaocun well (~3500 m) in the North China Platform is assumed to be confined fairly well. However, water level of the Gaocun well has been continuously decreasing for ~40 years without coseismic variations and without explainable mechanism. In this study, innovatively, we find that even buried in ~3500 meters deep, and covered by thick compact mudstones, the aquifer of well Gaocun is calculated to be continuously leaking for ~40 years probably induced by the vertical leakage incurred by aquitard fractures, which is under the assumptions of the none-leaking bottom layer and none direct pumping or exploiting in the observation aquifer layer. Meanwhile, for the first time, we did a detailed systematic comparison between the leaky models of the tidal response and the barometric response of water level, which indicate a consistency leaky result, and the minor difference mainly induced by the frequency differences. Merits and demerits of both models are also analyzed. Last but not least, underground fluid leakages might frequently occur, which are becoming increasingly worldwide urgent since that might also be related with surface sourced contaminations, toxic waste burials, and burial and exploration of the natural gas reservoirs.

Structural Control of Gold Deposits in the Northern Margin of the North China Platform

The North China Platform is one of the oldest continental crusts in China. The northern margin of the North China platform is one of the top gold mining concentration areas in China. Studying its ore-controlling factors is of great practical significance for finding similar gold deposits in areas with similar geological backgrounds. The authors of this paper have concluded that the gold deposits in the northern margin of the North China platform are mainly controlled by various faults. In the long geological history, these fault structures have undergone many stages of activation and transformation almost without exception. Generally, it has experienced the following three stages of development and evolution: 1. Beginning in the early stage of ductile deformation at the end of the Archean period, it is characterized by mylonite. The rock is transformed from the original high amphibolite facies-granulite facies into greenschist facies through ductile shearing. The typical mineral combination is sericite + chlorite + quartz ± feldspar. 2. The brittle-ductile deformation stage occurred in the Hercynian period, that is, the middle activation stage. Due to the different lithology of activation in different sections, the uneven strain of the mining area is caused. Some of the rocks are of continuous inhomogeneous strain, while others are of discontinuous inhomogeneous strain. As a result, hardening in plastic deformation occurs in some areas, and two different types of ore would appear in some mine. 3. The late transformation stage occurred in the early Yanshanian orogeny. It is mainly manifested in the strong fragmentation and transformation of the existing ore bodies, resulting in the dislocation of the ore bodies and complicating their occurrence. This late transformation stage has created favorable conditions for the later oxidative secondary enrichment of ore.

The Paragenetic Stage of the Dongping Gold Deposit in the Northern Margin of the North China Platform

The Dongping gold mine, located at the northern margin of the North China Platform, is one of the few large-scale gold deposits in China related to alkaline complex intrusions. In view of this, in-depth dissection of its causes is not only of geological theoretical significance, but also helps to find more similar gold resources under similar geological background, so as to benefit local people. To achieve the above goals, it is necessary to accurately grasp the paragenetic process of the deposit, that is, its paragenetic period and paragenetic stage. Through detailed field and laboratory research, the authors of this paper concluded that the formation of the Dongping gold deposit was the result of multi-stage paragenetic process. Specifically, the formation of the Dongping gold deposit has gone through the following four paragenetic stages, namely (from early to late): Milky white quartz (Q1)-potassium feldspar (K1)-coarse grained pyrite (cgPy) stage, Grey white quartz (Q2)-brick-red potassium feldspar (K2) stage, Potassium feldspar (K3)-quartz (Q3)-polymetallic sulfide stage, and Calcite-chert (Q4)-barite stage. Among them, the third stage, that is, the Potassium feldspar (K3)-quartz (Q3)-polymetallic sulfide stage, is the most important paragenetic stage, marking the climax of the gold mineralization of the deposit.

Changes of Hydraulic Transmissivity Orientation Induced by Tele‐Seismic Waves

AbstractWater level monitoring data from 5 deep wells on the North China Platform are used to study how two large earthquakes impact deep aquifers. After the passage of seismic waves from earthquakes, a subset of these wells shows changes in the phase shifts of water level responses to lunar diurnal (O1) and semidiurnal (M2) tides. We fit a model for the tidal responses of hydraulically conductive fractures that intersect the wells to explain the phase and amplitude of responses to both O1 and M2 tides. To explain changes after the earthquakes with this model, the apparent orientation of fractures must change. Because the stresses are small, we propose that changes in tidal response occur when the passage of seismic waves modifies the hydraulic connectivity of a network of fractures by unclogging or clogging flow paths, thus changing the apparent orientation of the fractures controlling tidal responses. The seismic energy density that results in changes appears to decrease as the dominant frequency of seismic waves increases.

Occurrence State and Properties of Gold Minerals from the Gold Deposits on the North China Platform

By studying both the microscopic physical and chemical typomorphic characteristics of gold minerals associated with representative gold deposits on the north-central margin of the North China Platform, this paper seeks to identify macroscopic metallogenic mechanisms of gold deposits. The majority of the gold minerals from the gold deposits are native gold with minor electrum and calaverite. The form and grain size of gold minerals from the representative gold deposits reflect the metallogenic properties of medium-deep and medium-high temperature deposits. The projected gold fineness of the Xiaoyingpan gold mine all falls in the medium-temperature area in the figure and tends to be in the high-temperature area, while that of the Zhangquanzhuang gold mine mostly falls in the middle-temperature area, showing the difference in the metallogenic conditions of the two deposits. Both Xiaoyingpan and Zhangquanzhuang gold deposits show the following rules: poor in Zn, Pb, Sb and Se, but rich in Cu, Fe without Mn, indicating that both deposits are of medium-deep origin. The three representative gold deposits all show high gold fineness characteristics, and the general rules are as follows: Dongping-type gold deposit (934.8-969.7) > Xiaoyingpan-type gold deposit (900.0) > Zhangquanzhuang type gold deposit (824.0), showing that the representative gold deposits were formed at higher temperature and deeper depth, and the gold in the ores inherited the characteristics of high gold fineness originating from the Archean metamorphic rock series, which implying that the metallogenic material originated from the ancient metamorphic rock series; the long metallogenic period and multiple metallogenic stages made gold fully transformed, purified and enriched. There are three main occurrence states of gold minerals in the study area, from primary to secondary, they are respectively inclusion gold, gold minerals between other mineral crystals and those filled in the fractures of other minerals.

Nitrogen Isotopes from the Neoproterozoic Liulaobei Formation, North China: Implications for Nitrogen Cycling and Eukaryotic Evolution

The nitrogen isotope compositions (δ15N) of sedimentary rocks can provide information about the nutrient N cycling and redox conditions that may have played important roles in biological evolution in Earth’s history. Although considerable δ15N data for the Precambrian have been published, there is a large gap during the early Neoproterozoic that restrains our understanding of the linkages among N cycling, ocean redox changes and biological evolution during this key period. Here, we report bulk δ15N and organic carbon isotope (δ13Corg) compositions as well as the total nitrogen (TN) and total organic carbon (TOC) contents from the Tonian fossiliferous Liulaobei Formation in the southern part of the North China Platform. The δ15N in the study section is dominated by very stable values centering around +4.3‰, which is moderately lower than in modern sediments (~ +6‰). These positive δ15N values were attributed to partial denitrification under low primary productivity (scenario 1) and/or denitrification coupled with dissimilatory nitrate reduction to ammonium (DNRA) (scenario 2). In either case, the availability of fixed nitrogen may have provided the nutrient N required to facilitate facilitated eukaryotic growth. Our study highlights the pivotal role of nutrient N in the evolution of eukaryotes.

Diagenetic implications for the oolitic limestone in the Miaolingian Zhangxia Formation, Beijing (North China Platform)

Diagenetic implications for the oolitic limestone in the Miaolingian Zhangxia Formation, Beijing (North China Platform)

Mesoproterozoic oxygenation event: From shallow marine to atmosphere

AbstractOxygen availability is crucial for the evolution of eukaryotes in geological history, but the redox conditions of the mid-Proterozoic atmospheric-oceanic system remain vigorously debated. Previous geochemical studies suggested a very low mid-Proterozoic atmospheric oxygen level of probably below 0.1%–1% of the present atmospheric level (PAL) with weakly oxygenated surface ocean, which may have suppressed the early evolution of eukaryotes in the ocean. A recent study, however, revealed a pulsed oxygenation event that was recorded in shallow marine carbonate of the middle Gaoyuzhuang Formation (at ca. 1.57 Ga, North China) with atmospheric pO2 of up to ≥4% PAL based on iodine speciation. This event is associated with the earliest known multi-cellular eukaryotic fossils. To elucidate whether this pulsed oxygenation was limited to shallow local seawaters or could also reflect an increase in atmospheric oxygen, chromium (Cr), and carbonate-associated sulfate (CAS), sulfur isotopes in the middle Gaoyuzhuang Formation were studied in two sections of the North China Platform. The results show a positive shift in authigenic Cr isotope from −0.18‰ to +0.66‰, which was followed by a decline to −0.16‰. This suggests a rise in atmospheric oxygen sufficient to initiate oxic Cr cycling and isotope fractionation (pO2 > 0.1%–1% PAL). This positive Cr isotope excursion was associated with a transient positive shift in δ34SCAS from ~20‰ to ~50‰ and a subsequent decline to ~10‰, which provides independent evidence of atmospheric oxygenation. This oxygenation may have enhanced oxidative terrestrial weathering and increased sulfate input to the ocean, thus stimulating bacterial sulfate reduction and pyrite burial in deep anoxic seawaters. This is broadly consistent with previous results of carbonate I/(Ca + Mg) and Ce anomaly but also reveals a lag between shallow seawater and atmospheric oxygenation. Fluctuated redox conditions and decoupled oxygenation between shallow seawater and atmosphere during the mid-Proterozoic may help to account for the current debate.

Deep origin of the crossed‐lamellar microstructure in early Cambrian molluscs

AbstractAragonitic crossed‐lamellar (CL) is one of the most commonly formed and extensively studied molluscan shell microstructures, yet its origin and early evolution within the Mollusca remains poorly understood. Here, a primitive CL microstructure from one of the oldest gastropods, Pelagiella madianensis, and the problematic hyolith Cupitheca sp. of the Cambrian Series 2 Xinji Formation on the North China Platform, was investigated. In P. madianensis, detailed characterization has revealed a typical four‐ordered hierarchical organization of aragonitic crystallites, and a thick layer of organic membranes surrounding its first‐order lamellae. A transitional fibrous microstructure was observed between the outer CL and inner foliated aragonite structural layers. In Cupitheca sp., only the first and second‐order lamellae were visible due to preservation limitations, and the first‐order lamellae were extremely irregular in shape and size, which is consistent with modern representatives. This study demonstrates that the capability to construct highly‐mineralized intricate shells was acquired in early Cambrian stem‐group gastropods. The CL microstructure first emerged in the early Cambrian and as a basal synapomorphic trait in total‐group molluscs. Moreover, presence of the CL microstructure in problematic lophotrochozoans (i.e. hyoliths) is confirmed. This study contributes to a more complete picture of the evolutionary origin and architectural diversity of biomineralized mollusc shells during the Cambrian explosion, and strengthens the phylogenetic links between hyoliths and molluscs.

Linkage of the late Cambrian microbe-metazoan transition (MMT) to shallow-marine oxygenation during the SPICE event

Microbe-metazoan transitions (MMTs), representing a switch from microbe-mediated to metazoan-mediated carbonate production, have been linked to major changes in Earth-surface conditions. The ‘late Cambrian MMT’ (nomen novum), during which microbial reefs were replaced by maceriate and lithistid sponge reefs, coincided with a sharp rise in atmospheric O2 levels attributed to the Steptoean Positive Carbon Isotope Excursion (SPICE) at ~497–494 Ma. However, relationships between atmospheric oxygenation, marine redox conditions, and the MMT have not been thoroughly investigated to date. Here, we conducted paired analyses of carbonate carbon isotopes (δ13Ccarb), sulfur isotopes of carbonate-associated sulfate (δ34SCAS) and pyrite (δ34SPy), pyrite morphologies, and fossil assemblages of two upper Cambrian shallow-marine sections on the North China Platform (Tangwangzhai and Huangyangshan). The sulfur isotopic composition of framboidal pyrite (δ34SFrPy) was calculated from that of total pyrite (δ34STPy) assuming a varied fractionation with euhedral pyrite based on pyrite morphological analyses. Modelling of S-isotope fractionations (Δ34SCAS-FrPy) indicates that contemporaneous seawater had low sulfate concentrations (i.e., [SO42––]sw ~4–6 mM) with locally high spatial heterogeneity during the Rising SPICE (early to middle Paibian), which were the product of massive burial of pyrite (together with organic matter) linked to extensive oceanic anoxia. Immediately following the SPICE, a reduced pyrite burial rate and an increased flux of terrestrially sourced, 34S-depleted sulfate to shallow-marine environments resulted in lower δ34SCAS values. [SO42–]sw rose to a maximum of ~9–16 mM and became more uniform during the Post-SPICE (middle to late Jiangshanian), suggesting a more oxidized condition of seawater and the atmosphere. Rising O2 levels in both the atmosphere and marine environments triggered the late Cambrian MMT and set the stage for the subsequent Great Ordovician Biodiversity Event.

Low level of phosphorous concentration in terminal Paleoproterozoic shallow seawater: Evidence from Chuanlinggou ironstone on North China Platform

Phosphorus (P) is a bio-limiting nutrient element, which through modulating marine primary productivity can influence ocean redox conditions. Precambrian, especially the mid-Proterozoic (1.8–0.8 Ga), seawater phosphorous concentrations have been poorly constrained. The well-preserved ∼1.65 Ga ironstones in North China provide a nice opportunity to explore the phosphorous concentrations of shallow seawater at that time. To avoid potential contamination we performed fabric-specific analysis of dense laminasets in ooid cortices without cracks, and in stromatolitic columns. These data revealed a low P/Fe molar ratio of 0.0007 ± 0.0003 (n = 12), pointing to an extremely low phosphorus concentration (<0.002–0.016 μM) in ∼1.65 Ga shallow seawater. If such a low phosphorus concentration is an universal phenomenon during terminal Paleoproterozoic, this is likely the significant reason for limited primary productivity and therefore a low atmospheric oxygen level during that time.

A transient oxygen increase in the Mesoproterozoic ocean at ∼1.44 Ga: Geochemical evidence from the Tieling Formation, North China Platform

Oxygen availability is crucial for the evolution of eukaryotes in geological history, yet detailed Mesoproterozoic oceanic-atmospheric redox conditions remain enigmatic. In contrast to the generally accepted hypothesis of an anoxic mid-Proterozoic ocean and atmosphere, several transient oxygenation events may occur at the Earth’s surface during the Mesoproterozoic, especially for the period around 1.4 Ga. The North China Platform develops one of the most complete and continuous Mesoproterozoic stratigraphic successions globally, preserving key information on the redox state of the surface ocean–atmosphere system during the mid-Proterozoic. In this study, I/(Ca + Mg), rare earth elements (REE) plus Y (REY), and paired stable isotopes (δ34SCAS and δ13Ccarb) data are reported for the carbonate rocks of ∼1.44 Ga Tieling Formation in the Jixian section, North China Platform. The lower Tieling Formation (i.e., the Daizhuangzi Member) exhibits middle-REE-enriched patterns with several relatively high I/(Ca + Mg) values (0.56–0.83 μmol/mol) suggesting a possible oxic or suboxic seawater condition. The upper Tieling Formation (i.e., the Laohuding Member) is characterized by modern seawater-like REY patterns with negative Ce anomalies (0.67–0.91) and variable I/(Ca + Mg) values (0.02–1.77 μmol/mol). The peak I/(Ca + Mg) ratios (up to 1.77 μmol/mol), negative shifts in Ce/Ce* and δ13Ccarb, and previously reported positive Cr isotope fractionation in the middle Laohuding Member indicate a significant oxygen increase in both ocean and atmosphere. A long-term increasing trend in δ34SCAS below the middle Laohuding Member outlines roles of pyrite burial in contributions of rising oxygen levels during this period.

Records of organic carbon isotopic composition (δ13Corg) and volcanism linked to changes in atmospheric pCO2 and climate during the Late Paleozoic Icehouse

The Late Paleozoic Icehouse (LPI) provides a deep-time perspective for climate-glaciation-environment coevolution and offers potential insights into future climatic and environmental predictions. Most previous studies attributed climatic and environmental changes during the LPI to perturbations of atmospheric p CO 2 , yet the driving mechanism for p CO 2 changes remains controversial. Although the environmental and climatic effects of volcanism have been recognized, its high-resolution geological records and links with changes in atmospheric p CO 2 and climate are rarely reported. We address this by investigating volcanic records (indicating by Hg/TOC ratio) and organic carbon isotope composition (δ 13 C org ) of paralic strata from the latest Gzhelian to Roadian stages in the North China Platform (NCP), China. Four δ 13 C org negative excursions (latest Gzhelian, middle Artinskian, middle Kungurian and latest Kungurian stages) and two positive isotope plateaus (Asselian and Roadian stages) are identified. Each of the four negative isotope excursions coincides with peaks in Hg/TOC ratio and rising periods of atmospheric p CO 2 . The co-occurrence of the negative isotope excursions and Hg/TOC peaks suggests volcanic origins for both the Hg and perturbations in the carbon cycle. The two positive isotope plateaus correspond to the glacial P1 and the climatic transition period to glacial P3 in Australia and to weak periods of volcanism. Our results provide clear insights into volcanism driving perturbations in environment, climate and the carbon cycle in deep time, and support the assertion that volcanism was sufficient to shift global climates from glacial to interglacial through greenhouse gas emissions during the LPI. • Early Permian continental δ 13 C org record presented from North China Platform. • Negative δ 13 C org excursions and Hg/TOC ratios identify episodes of volcanism. • Volcanism is important driver for changes in atmospheric p CO 2 and climate. • Two positive δ 13 C org plateaus correspond to P1 and P3 glaciations in Australia.

Are Deep Aquifers Really Confined? Insights From Deep Groundwater Tidal Responses in the North China Platform

AbstractMany deep aquifers overlain by barrier formations in the continental US are used as geological repositories for wastewaters coproduced from hydrocarbon exploration. This practice is to protect shallow groundwater following the US Environmental Protection Agency’s regulation. Implicit in such practice is the assumption that deep aquifers overlain by mudstones or shales are confined so that the injected fluids will not migrate upward to contaminate shallow groundwater. However, no systematic test of this hypothesis has been made. Here we invert the groundwater response to both the M2 and the O1 tides and to the barometric pressure across a large (2.046 × 106 km2) geologic regime, the North China Platform, to systematically evaluate the hydraulic parameters as functions of depth and time without a priori assumption. Our result, the first of such inversion, shows no depth dependence of aquifer confinement to a depth of 3,400 m and that deep confined aquifers overlain by barrier formations may become leaky after distant earthquakes. It suggests that monitoring of aquifer confinement may be needed to ensure if the targeted deep aquifer for wastewater injection is really confined. The results may be timely and of global significance because the practice of hydrofracking for natural gas and the deep injection for the disposal of the coproduced wastewaters in the US may soon be adopted by other countries, such as China.

First Report of Small Shelly Fossils from the Cambrian Miaolingian Limestones (Zhangxia and Hsuzhuang Formations) in Yiyang County, Henan Province of North China

Small Shelly Fossils (SSFs) from the Cambrian are widely distributed and well known across different paleocontinents of the world. However, middle Cambrian SSFs from North China Platform have only rarely been documented until now. In this paper, we presented the first report on SSFs from bioclastic and oolitic limestones of the Zhangxia and Hsuzhuang formations of Henan province, North China. The carbonate-hosted fauna includes brachiopods (Micromitra sp., M. modesta, Eoobolus sp., and Schizopholis sp.), helcionellids (Oelandiellaaccordionata and O. aliciae), hyolithids, Hyolithellus sp., Chancelloriaeros, sponge spicules, echinoderm ossicles, and chancelloriid sclerites. In terms of preservation, the brachiopod shell valves of M. modesta appeared to be homogeneous, consisting of tightly packed phosphate grains. Eoobolus sp. is composed of primary layer and secondary baculate, both of which consist of tightly compacted phosphate grains. Schizopholis sp. has multiple-lamellar phosphatized microstructures that distinctly differ from the other brachiopods recovered from the Longwanggou section. A similar multiple-lamellar microstructure was also revealed in conchs of Hyolithellus, with tightly compacted phosphate grains. The argillaceous shell of Oelandiellaaccordionata and O. aliciae, and the calcitic inner molds of hyolith did not preserve any shell structure. The helcionellids O. accordionata and O. aliciae and the brachiopod M. modesta were reported for the first time from North China. The fauna is most similar to the middle Cambrian faunas of Australia, in the brachiopod and mollusk components; it is also similar in composition of brachiopods and mollusks to coeval faunas from South China. The new fauna of SSFs in the Yiyang Longwanggou Section indicated that the Hsuzhuang and Zhangxia formations are late Drumian to middle Guzhuangian in age, most likely correlating with the Murrawong Creek Formation of Australia.

Ooid distribution and fabric in the Miaolingian of Xiaweidian Section, Beijing (North China Platform)

This study is an attempt to describe the distribution and fabric of the Cambrian ooids from the Xuzhuang, Zhangxia and Gushan formations at the Xiaweidian section. The oolitic banks occupy the upper parts of the 3rd order depositional sequences recognized for these formations. Petrographical techniques were applied to describe the sedimentary features of ooid grains. Different characteristics of ooids including size distribution, composition, morphology and the internal and external cortical architecture were taken into consideration. Radial-concentric, micritic, superficial, composite, pseudo-oids, neomorphosed and geopetal ooids are properly studied under the microscope. Different fabrics of ooids have been linked to their different depositional settings, and a variety of sub-environments has been established. The oolitic grain banks are composed mainly of calcite, with noteworthy presence of aragonite and dolomite. The two-fold role of microorganisms during and after the formation of ooids can be recognized under the microscope. The mechanism of ooids fabric modification has been elaborated in detail. Firstly, the dark laminae in several ooids most probably show the remains of filamentous cyanobacteria taking part in the construction of ooids. Secondly, they destroy the cortex through boring, which is then subsequently filled by aragonite. In order to apprehend the sedimentological features of the Miaolingian strata in the Xiaweidian section, this research highlights the distribution of oolites and their resultant fabric in response to relative sea-level variations. The Miaolingian ooids in the Xiaweidian section provide a good reference example of the depositional pattern of oolitic grain banks.

Chemical and physical characteristics of quartz from gold deposits in the North China platform: relationship to gold mineralization

Chemical and physical characteristics of quartz from gold deposits in the North China platform: relationship to gold mineralization

Cambrian marine radial cerebroid ooids: Participatory products of microbial processes

The Cambrian Miaolingian Series strata at the Kouquan section, North China, are primarily divided into the Xuzhuang, Zhangxia, and Gushan formations. This particular series comprises oolitic limestone in the middle to upper intervals. Thin section observations of carbonate rock samples from the Zhangxia Formation show that the well‐preserved and most remarkable grains among them are cerebroid ooids. Although these ooids have characteristic serrated margins, they are distinct from ooids reported in previous publications due to inconspicuous concentric laminae and radial structures composed of micrite and microspar. To understand the origin of these ooids, we employed petrographic examination, X‐ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and isotope analyses to explore their compositions and conditions of development. In addition, the occurrence of these cerebroid ooids is closely related to that of dark‐coloured micrite: most cerebroid ooids and quasi‐cerebroid ooids float inside dark micrite; dark micrite appears in the interior of cerebroid ooids as a radial micrite structure. EPMA and energy‐dispersive X‐ray (EDX) spectroscopy analyses of micrite reveal that the dark micrite consists of lithomicrobial mats. Fine‐scale characterization of the dark micrite inside these cerebroid ooids shows the presence of filamentous and bowl‐shaped calcified microbial fossils and extracellular polymeric substance (EPS) calcified remnants. Isotopic data indicate a variation of δ13C isotopic values between −4.92‰ and −0.11‰ (VPDB) and −9.89‰ and −4.93‰ for δ18O values (VPDB). The negative values of δ13C in the studied ooid suggest the genesis of carbon from the high organic‐rich zone such as the microbial zone. Also, the 13C‐depleted carbon values in the ooids advocate that the carbon is mainly derived from organic sources associated with the sulphate reduction bacterial activity. Thus, isotopic data of this study also provide consistent clues in favour of microbial participation during the genesis of cerebroid ooids. In conclusion, our new findings strongly confirm that the cerebroid ooids of the North China Platform are genetically related to microbial processes.

Cambrian ooids, their genesis and relationship to sea-level rise and fall: A case study of the Qingshuihe section, Inner Mongolia, China

ABSTRACT: TheCambrian strata at the northwestern margin of the North China Platform in InnerMongolia hold thick oolitic-grain bank deposits.Generally, the strata are dominated by calcareous mudstone of shelf facies in the lower part, micritic limestone consisting of deep to middle ramp facies in the middle part, and oolitic limestone encompassing shallow ramp to grain bank facies in the upper part of each formation. The shelf and deep ramp facies are the result of relative sea-level rise, while oolitic limestones developed in response to relative sea-level fall. Microscopically, the studied ooids are represented by radial crystal structures and concentric laminations with or without cores, single crystal or neomorphosed ooids, and highly bored ooids. The size andmorphology of the ooids indicate a two-fold mechanical influence of microbes; constructive in the Miaolingian and destructive in the Furongian ooids. Based on these observations, it can be inferred that microbes (predominantly composed of filamentous fossils of cyanobacteria) excreted extracellular polymeric substances (EPS) to develop multiple bacterial biofilms microbial mats. The subsequent decay of the EPS through sulfate reducing bacteria most likely caused precipitation around these ooids. The depositional style of ooids occupying the upper parts of the formations, and their possible genesis from microbes provide clue for regional correlation, as well as affirm biological control in the formation of ooids.