North China Platform Research Articles

Intense intrusion of low-oxygen waters into mid-Cambrian surface ocean carbonate factories

The Phanerozoic surface ocean is characterized by its high dissolved oxygen content owing to mixing with the atmosphere. However, atmospheric oxygen levels varied in the early Paleozoic and it remains unclear whether the surface ocean was susceptible to significant redox fluctuations in response to extreme environmental events. In this study, we probed the redox structures of shallow middle Cambrian marine depositional environments across the North China Platform, ranging from open tidal flats to relatively deep subtidal environments. We utilized a combination of least diagenetically altered carbonate materials (such as ooid cortices, calcimicrobes, and their fringing cements), as well as in situ element measurement and imaging techniques. By analyzing a set of redox-related elements (e.g., Ce anomaly, Zn/Fe molar ratio, Mn and Cr) and mineralogical proxies (hydrogenetic Fe oxides), we revealed a stratified redox structure in the Drumian surface oceans. Compared to earlier Drumian conditions, late Drumian surface oceans experienced significant intrusions of ferruginous waters, probably reaching into shallow subtidal environments with water depths less than 10 m. Furthermore, we identified shallow subtidal microbial O2-producing factories, characterized by dendritic Epiphyton thalli. These calcimicrobes exhibited more oxygenated signatures (negative Ce anomalies and enrichment of hydrogenetic Fe oxides) relative to contemporaneous less oxic shallower and deeper environments. This finding indicates that they produced oxygen oases or refuges during periods of both normal and poor dissolved O2 conditions. This study has the potential to broaden our understanding of redox conditions and microbial oxygen-producing mechanisms in the surface ocean, particularly during intervals characterized by low atmospheric oxygen levels or episodic anoxic events.

Shallow ocean deoxygenation drove trilobite turnover during the late Cambrian SPICE event

The spread of marine anoxia is believed to have played a key role in the development of the SPICE (Steptoean Positive Carbon Isotope Excursion) event and the end-Marjuman extinction in the late Cambrian (∼497.5 m.y. ago), but their cause-and-effect relationship is poorly constrained. Here we present an integrated analysis of carbonate δ13C, cerium anomalies (Ce/Ce*), and genus-level diversity data of trilobites from the North China Platform. Our results show tightly coupled changes between the SPICE, an increase in Ce/Ce*, and a trilobite turnover event, which we interpret as indicating enhanced productivity and organic remineralization, leading to the development of low-oxygen conditions in shallow-water settings. This study therefore establishes a direct link between local ecological stress and trilobite turnover during the global SPICE event. Furthermore, the presence of low-oxygen rather than fully anoxic conditions during the peak of the SPICE event could explain the nature of the end-Marjuman crisis, which was characterized by the replacement of shallow-water fauna by deeper-water counterparts that were potentially more tolerant of hypoxia.

Long‐ and Short‐Term Effects of Seismic Waves and Coseismic Pressure Changes on Fractured Aquifers

AbstractTwo adjacent groundwater wells on the North China Platform are used to study how earthquakes impacted aquifers. We use the response of water level to solid Earth tides to document changes after earthquakes and how aquifer and fracture properties recovered to pre‐earthquake properties. We consider two models for the phase and amplitude of water level response to the lunar diurnal (O1) and semidiurnal (M2) tides: a leaky aquifer model, and a model in which fracture orientation determines the response. In the leaky aquifer model, changes arise from changes in permeability and storage; in the fracture model, changes are due to changes in apparent orientation of transmissive fractures. Responses in one well are best explained by the leaky aquifer model, and can explain the large amplitude coseismic water level and permeability changes and the non‐recoverable changes after the largest earthquake. Responses in the other well are consistent with the fracture model and show little coseismic change in water level but changes in apparent fracture orientation. Larger ground motions lead to larger coseismic water level changes and longer recovery times. We propose that the well in the more permeable and shallow aquifer has less variable pore‐pressures around the well. Larger coseismic strains from water level changes may enable longer‐lasting changes in aquifer properties. We conclude that relatively high permeability aquifers are less susceptible to impacts from seismic waves, and thus have small changes in water levels and hydrogeological properties.

Upper Cambrian and lower Ordovician conodont and carbon isotope bio-chemostratigraphy of the eastern North China Platform: Implications for global correlation and palaeoenvironments

Revised conodont biostratigraphy and new carbon isotope data through the Cambrian/Ordovician transition are presented for the Yaowangshan section in the eastern North China Platform. Eight conodont zones are recognized, including the Proconodontus muelleri Zone, Eoconodontus Zone, Cordylodus proavus Zone, C. caboti Zone, C. intermedius Zone, C. lindstromi Zone, C. angulatus Zone, and Rossodus manitouensis Zone, in ascending order. The Eoconodontus Zone can be subdivided into the E. notchpeakensis Subzone and the overlying Cambrooistodus minutus Subzone. The new δ13Ccarb data from the Chaomidian Formation are well constrained by the conodont biozonation and indicate pronounced carbon excursions and spike, Top of Cambrian Excursion (TOCE), Cordylodus proavus spike (CPS), and Cambrian–Ordovician Transition Isotopic Carbon Excursion (COTICE). The isotopic excursions are generally consistent with those of other sections in North China, South China, Tarim, North America, Argentina, and Baltica.

Sr-ISOTOPIC COMPOSITION OF PALEOOCEAN WATER ON THE BORDER OF 960 Ma (DATA FOR NIZHNY TUNGUSKA FORMATION OF TURUKHAN UPLIFT OF SIBERIAN PLATFORM)

At the boundary of the Meso- and Neoproterozoic, Grenville collision events were widely manifested on the margins of ancient continental blocks. Most of the sedimentary sequences that had accumulated by that time had undergone significant thermal–metamorphic changes. In many ways, this is the main reason for the lack of isotopic data for carbonate deposits in the interval of 1200–900 Ma in world practice. Sr–isotopic composition in carbonate rocks with an age of 980–920 Ma was determined in the only section of the World – the Huainan Formation of the North China Platform with accumulation time determined by U–Pb dating of detrital zircon grains. In the upper part of the Nizhny Tunguska suite of the Turukhansk uplift, among carbonate deposits, we established the presence of altered volcanic rocks, as well as weathering crust products along them – poor bauxite and chamosite ores. Based on U–Pb zircon isotope dating, the age of the volcanic rocks is 964 Ma. For the least altered limestones of the upper part of the Nizhny Tungusska Formation, received 87Sr/86Sr values are 0.70532–0.70578, which are close enough to those found in the rocks of the Huainan Formation. Geochronological age of the studied limestones gives us more correct data. These data can be used to refine the previously proposed configuration of the Sr–isotopic composition variation curve in the Early Neoproterozoic. The Nizhny Tunguska Formation is the only carbonate section of the Early Neoproterozoic in the world with correctly geochronologically substantiated dating (based on zircon from subsynchronous volcanic rocks). This compares favorably with the Huainan formation.

Microbial Reefs Predominated by Leiolites in the Zhanxia Formation of Cambrian from Qinhuangdao City,Hebei Province

Stromatolites, thrombolites, leiolites and dendrites are juxtaposed and classified as typical microbial carbonates, with leiolite characterized by cryptocrystalline lithology, undeveloped agglomerate structure, lacking obvious macroscopic structure and clot. Since its designation in 1995, leiolites have been rarely observed and recognized in strata and modern analogies are scarce, whoch makes the report of a leiolite example extremely important. In order to research the characteristics of leiolites in Cambrian of North China Platform, combine the methods of field sampling observation and observation of indoor microscopic characteristics under the microscope and selected microbial reefs predominated by leiolites with thickness of tens of meters develop intensively at the top of the Forced Regression system domain of the Cambrian Zhangxia Formation in Qinhuangdao, Hebei Province was selected, which constitutes a three-order sequence as research object. Leiolite consists of dense mud crystals and a small amount of microlithophores, and the Filamental Girvanella similar to the modern Plectonema is visible in the dense mud crystals that make up leiolites, while Epiphyton and Hedstroemia can be seen locally in the bushy mud crystals. It is indicated that microbial reefs predominated by leiolites are influenced by complex organic mineralization processes or by early cyanobacteria-dominated lithification occurring within microbial membranes or microbial mats. Therefore, in understanding the formation of ancient microbial reefs, the microbial reef predominated by leiolites at the Cambrian Zhangxia Formation in Qinhuangdao, formed by the calcification of microbial mats, are an important window.

Development characteristics and genetic analysis of dense limestone reservoirs in the Taiyuan Formation of the Ordos Basin, China

Tight limestone reservoirs in the Permian Taiyuan Formation of the Ordos Basin have garnered increasing attention in recent years, emerging as a pivotal domain in the quest for natural gas exploration. This research delves into the comprehensive investigation of petrology, reservoir space, reservoir physical properties, and microscopic characteristics through the examination of field outcrops, core observations, thin section identification, scanning electron microscopy, stable carbon and oxygen isotope analysis, and testing. The systematic analysis focused on elucidating the development characteristics of these reservoirs and identifying the controlling factors governing favorable reservoir conditions. Based on our research analysis, the specific rock types with the potential to serve as excellent reservoirs, includes bioclastic silty limestone, bioclastic micrite limestone, and algal-rich limestone. The predominant reservoir spaces within these formations were found to consist of dissolved pores, residual bioclastic cavity pores, intercrystalline pores, and microfractures. These reservoirs exhibit an average porosity of 2.1%, and an average permeability of 0.22 × 10−3 μm2, indicating their classification as low porosity and low permeability reservoirs. The formation of favorable reservoirs in the Taiyuan Formation limestone was determined to be influenced by many factors; notably, the favorable sedimentary microfacies associated with bioclastic shoals and bioherms provided the foundational material basis for the formation of reservoirs, influencing the type of reservoir space and its extensive planar distribution. Penecontemporaneous karstification, guided by high-frequency cycles, was favorable for the formation of dissolution holes, effectively improving reservoir performance and facilitating the development of thick limestone reservoirs. Furthermore, fractures were identified as crucial agents in improving the seepage capacity of these tight limestone reservoirs. Drawing from our research results, this study offers valuable guidance for the future exploration of limestone formations in the Taiyuan Formation in the Ordos Basin. Additionally, these findings hold considerable significance as a reference point for research and exploration endeavors focused on bioclastic limestone reservoirs in the North China Platform.

Sr-Isotopic Composition of Paleoocean Water 960 Ma Ago (according to the Data for the Nizhny Tunguska Formation of the Turukhansk Uplift, Siberian Platform)

The Sr-isotopic composition in carbonate rocks aged 980–920 Ma has been defined in only one place in the world, the Huainan Formation of the North China Platform, with the accumulation time limited by U–Pb data of detrital zircon grains. The upper part of the Nizhny Tunguska Formation in the Turukhansk uplift is found to contain altered volcanic rocks, as well as weathering crust products along them, such as poor bauxite and chamosite ores, among carbonate deposits. Based on U–Pb zircon isotope dating, the age of the volcanic rocks is 963 Ma. For the least altered limestones in the upper part of the Nizhny Tunguska Formation, the 87Sr/86Sr values were obtained equal to 0.70532–0.70578, which are close to those found in the rocks of the Huainan Formation. These data about the geochronological age of the studied limestones make it possible to obtain more accurate data, which allows refining the previously proposed configuration of the Sr-isotopic composition variation curve in the Early Neoproterozoic. The Nizhny Tunguska Formation compares favorably with the Huainan Formation in being the world’s only Early Neoproterozoic carbonate sequence that is correctly geochronologically substantiated (based on zircon from subsynchronous volcanic rocks).

Horizontal well path design with seismic inversion for a shale oil reservoir in the Ordos Basin, China

The Ordos Basin is located in the western part of the North-China Platform, where China’s largest shale oil field was discovered. Most of the production originates from relatively thin and tight sand bodies within the Chang 7 member of the Triassic Yanchang Formation. This consequently requires accurate and high-resolution inversion data to characterize the reservoirs and define the horizontal well paths. In this paper, we adopt full-waveform inversion (FWI) to obtain the velocity model for time-to-depth inversion. Compared with conventional migration velocity analysis, FWI can produce velocity models with more high-wavenumber variations that reflect thin and localized geologic features. Geologic statistical inversion is also applied to resolve the thin beds beyond seismic resolution. A comprehensive index is then computed with the inverted parameters to evaluate the reservoir and build a sweet-spot map. We define the coordinates, direction, and length of the horizontal well on the maps. The well path is subsequently carefully designed on the sections of the time-to-depth converted inversion data. The effectiveness of the workflow is verified using the application case of the Qingcheng oil field at the Ordos Basin.

The Role of Cyanobacteria In the Microfabric Heterogeneity of Cambrian Leiolite, North China

ABSTRACT Based on their mesostructures, carbonate microbialites are divided into four major groups: laminated stromatolite, clotted thrombolites, structureless leiolites, and dendritic dendrolite. Among the various types of microbialites, leiolites, also known as “cryptomicrobial deposits,” are uncommon in the stratigraphic records. This is exacerbated by leiolites relatively structureless and aphanitic structure, making their identification and characterization more challenging. Previous studies have revealed that leiolites have contrasting fabrics at different scale, heterogeneous microscopic fabrics and structureless mesoscopic to macroscopic fabrics. While it is widely assumed that cyanobacteria play a significant role in the formation of carbonate microbialites in marine environments, the origin and controlling mechanisms of variations in leiolites microstructures and macrostructures remain enigmatic. In the central part of the North China Platform, two excellent exposures of massive, structureless Cambrian bioherms (Furongian) dominated by leiolites were observed (Qijayu section). The Qijayu-section leiolites offer a unique opportunity to investigate the role of cyanobacterial input in governing the formation and textural heterogeneities of such microbialites. In addition, the significance of the Furongian leiolites stems from their association with a period of global development of microbial-dominated carbonate platforms. Our findings suggest that these Furongian leiolites developed during the forced-regressive systems tract. At the microscopic level, these Furongian leiolites exhibit a high degree of heterogeneity which is controlled by the presence of various types of cyanobacteria (Hedstroemia, Subtifioria, Girvanella), sponge mummy, and benthic ooids. The Hedstroemia, Subtifioria and Girvanella excreted extracellular polymeric substances that form multiple biofilms in cyanobacterial microbial mats where Furongian leiolites grew. The role of cyanobacteria in the microscale heterogeneity of Cambrian leiolites is evident, and this provides new insights into the development of microbial-dominated carbonate platforms in similar settings elsewhere. As a result, this study provides not only a reference example for global correlation but also some solid clues for further understanding the growth style of leiolites in the geologic record.

Correlations between trace elements in pyrite and gold mineralization of gold deposits on the North China platform

Correlations between trace elements in pyrite and gold mineralization of gold deposits on the North China platform

The Cambrian sequence filling process and sedimentary evolution of the Kuruktag area in the northeastern margin of the Tarim basin

The Kuruktag area in Xinjiang is located between the northeastern margin of the Tarim Basin and the South Tianshan orogenic belt, and its complex tectonic background has been extensively studied to assess the tectonic evolution of the Central Asian orogenic belt and the Tarim Craton. Although the Cambrian strata in the study area are well developed and continuous, due to the limitations of the outcrop environment and drilling and seismic data, studies of the sequence stratigraphy and sedimentary evolution have been limited and have yielded results with low accuracy, leading to limited knowledge of the local structure. In this paper, field observations and actual measurements of the stratigraphic profile, supplemented by perimeter logging and seismic data for stratigraphic sequence comparison, are collected, and sea level changes and controlling factors are discussed. Studies have shown that various types of sequence interfaces are present in the Kuruktag area, and three grade II and six grade III type-II sequences can be identified. The grade II and III sequences are composed of a transgressive systems tract (TST) and a highstand systems tract (HST), and the depositional thickness of the TST is less than that of the HST. The northern area has experienced an evolutionary process from a deep-sea basin to a deep-sea shelf, a shallow-water shelf, and a platform edge slope, while the southern area has evolved from a deep-sea basin to a deep-water shelf and shallow-water shelf before again transitioning to a deep-water shelf during the ЄSQ1-ЄSQ6 peroid. The sedimentary facies evolution and sequence structure indicate that the Kuruktag area has experienced a process of rapid large-magnitude transgression and slow regression in terms of sea level. In addition, the curve of sea level change for the Kuruktag area is quite different from the corresponding curves for the Tarim platform facies area and the North China platform. This difference is associated with the evolution of the Kuruktag rift basin.

Multiphase dolomitization mechanisms of the Cambrian upper Changping Formation, North China Platform, China

The Cambrian dolomite reservoirs of the North China Platform offer good exploration prospects, but their occurrence and the impacts of dolomitization on reservoir quality are not yet clearly understood. The study herein assesses outcrop samples in the Cambrian upper Changping Formation, Dingjiatan area, and highlights the important role of multiphase dolomitization in the formation of paleo-reservoirs of acceptable porosity, where oil pools and fields may be discovered. A combination of petrology, fluid inclusion microthermometry, isotopes, and trace element compositions has been used to explain multiphase dolomitization mechanisms and their impacts on dolomite reservoirs. Five types of dolomites are identified through outcrop observation, thin section identification via transmitted light, and cathodoluminescence. The following geochemical analyses reveal various dolomitization mechanisms. In the (pene) contemporaneous stage, microbial dolomite is commonly related to microbial metabolic activities with significant carbon isotopic depletion compared to the Cambrian seawater values. With the influence of microbial dolomitization, dolomicrite corresponds to the sabkha dolomitization mode in a low-salinity seawater environment and early-stage dolomitization. The structureless dolomite (ssD) in the early highstand systems tract (EHST), characterized by elevated 87Sr/88Sr ratios and low oxygen isotopic values, forms from stratum brine water in the shallow-middle burial stage rather than in the (pene) contemporaneous stage. In contrast, ssD in the late highstand systems tract (LHST) undergoes (pene) contemporaneous dolomitization at salinities between 6% and 28% before later pore water transformation, with the participation of atmospheric freshwater through faults and unconformities exhibiting the lowest inclusion temperature and salinity values. The medium to coarse crystalline dolomite (MCD) in the LHST and the saddle dolomite (SD) in the EHST with low REE values are atypical hydrothermal dolomites caused by a combined superposition of middle-deep burial hydrothermal fluids at temperatures >150°C and stratigraphic brines. The MCD is also influenced by terrigenous water characterized by relatively low Eu anomaly values. Finally, the mechanisms of porosity increase are investigated, and it is concluded that the pore increase caused by the (pene) contemporaneous reflux interaction and the later pore retention both lead to better ssD reservoirs in the LHST than in the EHST.

Spatio-temporal distribution of the Cambrian maceriate reefs across the North China Platform

Maceriate reefs, characterized by centimeter- to decimeter-scale branching structures, flourished during the Cambrian to Early Ordovician on Laurentia, and Gondwana (the North China Platform). Dendritic and columnar maceriate reefs were newly discovered from the northwestern and central parts of the North China Platform. The maceria structures are composed of sponges (e.g., keratosans and lithistids) and microbial components, including Girvanella, Renalcis, and Tarthinia, whereas the interstitial material consists of lime mud with bioclasts and intraclasts. The first appearance of maceriate reefs was diachronous across the North China Platform, younging from the northwest (Drumian Age, Miaolingian) to the southeast (Jiangshanian Age, Furongian). In general, thrombolites and dendrolites were dominant in the microbialite-dominated reefs of the Miaolingian and microbe–sponge maceriate reefs were dominant in the Furongian. This dramatic shift to microbe–metazoan reef assemblages resulted from a combination of factors associated with the Miaolingian–Furongian boundary interval, including global transgression and a shift from widespread anoxia of the SPICE event to more oxygenated post-SPICE conditions. The North China examples suggest that sea level and paleoenvironmental conditions may have played a major role in this transition and its timing.

Anisotropy and Heterogeneity Induced by Shale in Aquifer lithology—Influence of Aquifer Shale on the Leaky Model With Tidal Response Analysis

AbstractTidal and barometric water‐level responses in wells have been widely used to calculate the hydraulic properties of aquifer systems. The effect of anisotropy induced by shale content on such responses has not received significant attention. In this study, we examine how the presence of shale (anisotropy, extremely low porosity/permeability; approximately 10−4 to 10−3 mD), which occurs as interlayers in aquifers, affects the tidal responses of the leaky model. Our findings show that the number of wells with shale in the screened section of the study aquifer is limited. Here, we focus on the study of the limited number of wells in the North China Platform to ensure a similar geological background for each well. Calculations indicate that wells, even with a small amount of shale (≥∼5%) in the observation aquifer, may exhibit strong anisotropy and heterogeneity, deviating from the theoretical analytical solutions obtained using isotropic and homogeneous assumption models. Generally, the higher the shale content in aquifer lithology, the greater the phase shifts deviated. Thus, theoretical ideal models with isotropic and homogeneous assumptions may only obtain a rough estimation for wells with shale in aquifer lithology, suggesting that we avoid setting observation aquifers onto shale layers.

A Case Study on the Co-Occurrence of Oncoids and Ooids in the Cambrian Miaolingian Series, North China Platform

A Case Study on the Co-Occurrence of Oncoids and Ooids in the Cambrian Miaolingian Series, North China Platform

Late Ordovician gastropods from the Zhaolaoyu Formation in the southwestern margin of the North China Platform

Late Ordovician gastropods from the Zhaolaoyu Formation in the southwestern margin of the North China Platform

Deposition of sandstone (early Cambrian) above the Great Unconformity in the Sino-Korean Block, Gondwana: Control of antecedent topography

The global transition between the Precambrian and Phanerozoic eons in concert with rise in eustatic sea-level caused an important change in paleoenvironments, represented by sedimentary records above the Great Unconformity. In the Sino-Korean Block, Gondwana, the unconformity has been recognized by a boundary between various basement rocks and the overlying distinctive suite of sandstone (including conglomerate, conglomeratic sandstone, and mudstone) of the early Cambrian age. A plate tectonic reconstruction of the Cambrian-Ordovician succession in the northeast Asian continent reveals that the sandstone deposited largely in the eastern part of the platform (North China Platform), including the Taebaeksan and Pyeongnam basins. In the western part of the platform, the basal sandstone is correlated to basal carbonate rocks of middle Cambrian age, forming a time-transgressive or diachronous unconformity. The disparity in sedimentary facies of the basal deposits overlying the Great Unconformity was primarily due to antecedent topography of the bounding continent: in the eastern platform, the drainage area comprised high-relief Precambrian massifs, whereas in the western part, it comprised low-relief massifs. During the initial stage of sea-level rise, the eastern part of the platform formed progressively open shoreface and foreshore environments with localized embayments in which relatively coarse-grained gravelly sandstone deposited in fan-delta and tide-dominated environments. In the western platform, however, the supply of siliciclastic sediments was limited and the sandstone formed a relatively narrow linear belt of delta and distributary mouth bars and foreshore deposits. In the rest of the platform farther west, accommodation created by subsequent sea-level rise was filled with carbonate sediments. This study demonstrates that antecedent topography of the unsubmerged drainage area played a major role for the deposition of sandstone in the initial transgression stage, forming the Great Unconformity.

Cambrian Series 2 (Stage 4) calcimicrobial reefs in Shandong Province, North China: Calcimicrobial diversity and contribution to reef construction

Calcimicrobes together with archaeocyaths became significant in reef construction during the early Cambrian; however, purely calcimicrobial reefs have not been as well documented as archaeocyath-bearing reefs. Here we describe the age-specific features of calcimicrobial reefs of the Zhushadong Formation (Cambrian Series 2, Stage 4) at the Sunmayu and Jinhe sections in Shandong Province, North China. The calcimicrobial reefs of the Sunmayu section are constructed mainly by Epiphyton, Kordephyton, and tubiform microbes. The reefs are in some cases dominated by Amgaina, passing from a framework of Kordephyton, Epiphyton, and tubiform microbes to dendritic growth of Epiphyton, and finally to aggregations of Amgaina. Calcimicrobial reefs in the Jinhe section are likewise constructed mainly by Epiphyton, Kordephyton, and tubiform microbes, but with subordinate Tarthinia and Bija. Epiphyton and Kordephyton as well as Amgaina were thus the main framework builders in the Zhushadong calcimicrobial reefs. Tubiform microbes contributed as encrusters, enhancing the framework rigidity, whereas Tarthinia and Bija played subordinate roles as encrusters and consolidators. Interreef deposits of the Sunmayu reefs are of peloidal or ooidal grainstone; these reefs are inferred to have formed in a high-energy shoal setting. In contrast, interreef deposits of the Jinhe reefs are of peloidal and intraclastic wackestone or grainstone with bioclasts such as trilobites and hyoliths. Tubular cavities, possibly representing burrows, are common in the spaces among frameworks. These calcimicrobial reefs are interpreted to have formed in a subtidal setting, where the water energy was lower than that in the Sunmayu section. In North China, reefs of Cambrian Epoch 2 are limited, as development of the carbonate platform began only toward the end of that epoch. The reefs of the Zhushadong Formation are therefore among the earliest on the North China Platform. The calcimicrobial reefs contain various kinds of calcimicrobes, and the calcimicrobial assemblages are similar to or more diverse than those of coeval archaeocyath-bearing reefs during this period of archaeocyath decline. Calcimicrobes thus flourished at that time and contributed greatly to the construction of reefs. These purely calcimicrobial reefs provide further information on the spatial and temporal changes in calcimicrobial diversity and reef construction during the period before and after the extinction of archaeocyaths.

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.