Chihsia Formation Research Articles

Sedimentary control on diagenetic paths of dolomite reservoirs in a volcanic setting: A case study of the Permian Chihsia Formation in the Sichuan Basin, China

The formation mechanisms of multi-stage dolomites and hydrocarbon reservoirs associated with volcanic activity are complex. This study investigated the Permian Chihsia Formation in the Sichuan Basin, China, which is the location of recent discoveries of giant natural gas reserves. We undertook a petrological and geochemical study, including trace element and C–O–Sr isotopic analysis. The quality of the dolomite reservoirs associated with volcanic activity was controlled by different diagenetic pathways associated with the sediment types and fault systems. Outcrop, core, and thin-section observations identified three types of replacement dolomite: very finely to finely crystalline dolomite (Rd1), medium crystalline patchy dolomite (Rd2), and coarsely to medium crystalline dolomite (Rd3). The tidal flat sediments only experienced limited seepage reflux dolomitization, which resulted in the formation of the dense very finely to finely crystalline dolomite (Rd1). The bioclastic grain shoal sediments deposited in a high-energy shallow-water environment contain abundant intergranular and dissolution pores, and vugs, which allowed convection of seawater and dolomitization during the initial phases of volcanic activity of the late early Permian. Subsequently, the bioclastic grain shoal sediments were mostly replaced by coarsely to medium crystalline dolomite (Rd3), which formed the highest-quality reservoirs in the study area. In addition, the incompletely dolomitized medium crystalline patchy dolomite (Rd2) was formed due to the low original porosity of micritic, peloidal, grain shoal sediments during a transgression in the middle and late depositional stages. During the peak of volcanism at the end-Guadalupian, deep faults allowed seawater to mix with ascending hydrothermal fluids, which resulted in the precipitation of dolomite cement in pores near the fault fracture zones, which blocked the pores. Therefore, these high-energy shoal sediments deposited in the slope break zone far from strike-slip faults have a relatively high porosity. Our results provide new insights into the origin of dolomite reservoirs formed in association with a volcanic setting, and are applicable to deep dolomite reservoirs worldwide.

Facies and Carbon Isotope Variations during the Kungurian (Early Permian) in the Chihsia Formation in the Lower Yangtze Region of South China

The Kungurian Stage in the early Permian was a transitional glacial age between the late Paleozoic icehouse and the early Mesozoic super-greenhouse period This stage offers an excellent opportunity to study the co-evolution between global carbon cycles and environments. This study presents facies and carbon isotope variations in a new carbonate section in the Lower Yangtze region of South China in order to understand the linkage between carbon cycle fluctuation, sedimentary environment, and climate change. Based on the sedimentary facies analyses of the Chihsia Formation (Kungurian), seven facies types were identified and grouped into lower slope, upper slope, and platform facies associations. The facies analyses show that the Kungurian Stage experiences two transgressive-regressive cycles; paleoclimatic changes controlled the sedimentary records and sea level fluctuations. Early Kungurian carbonate rocks record the presence of the short-lived Kungurian carbon isotopic event (KCIE). The rapid negative carbon isotope of the KCIE was closely related to the huge CO2 emission. A warming climate could have slowed down oceanic ventilation rates and accelerated stratification of seawater. The resulting anoxic environment led to a sharp decline in biological species. In the middle Kungurian, the intensity of volcanic activity gradually weakened and the climate turned cold, which accelerated oceanic ventilation rates and led to increased oxygenation of deep-shelf water masses. The higher Δ13C values supported enhanced primary productivity and photosynthesis, which promote the prosperity of biological species. This study provides a new perspective for better understanding the links between marine carbon cycle fluctuations, climate change, and environments during the icehouse to greenhouse conversion period.

Cisuralian (Early Permian) paleogeographic evolution of South China Block and sea-level changes: Implications for the global Artinskian Warming Event

The final phase of the Late Paleozoic Ice Age (LPIA) involved a paleoclimatic transition from an icehouse to a greenhouse world during the Cisuralian (Early Permian in traditional stratigraphic correlation frameworks), which was called the Artinskian Warming Event (AWE). Paleogeography is an important tool to investigate the landform response to global sea-level changes in deep time. Stratigraphic sections gathered from published literature were digitized and stored in the Geobiodiversity Database. These biochronologically constrained sedimentary successions are used to reconstruct the paleogeographic evolution of the South China Block (SCB) throughout the Cisuralian. Four consecutive paleogeographic maps are constructed based on the features of sedimentary successions, including the Asselian – Sakmarian, the early Artinskian, the late Artinskian, and the Kungurian time intervals. These maps of the SCB may contribute to the implementation of global paleogeographic schemes. A landmass index based on spatial analysis of different elements is established further to identify the paleogeographic evolution pattern of SCB and reflect the LPIA-driven global eustatic signals. It showed two small cyclic fluctuation episodes during Asselian-Sakmarian, and Kungurian, respectively, and this trend was disrupted by a significant sea-level drop during early-middle Artinskian, which was comparable to sedimentary basins at different latitudes. The AWE and its associated marine transgression occurred no later than the late Artinskian according to rigorous biostratigraphic constraint. Large sea-level rise recorded in the Liangshan Member and Chihsia Formation is hypothesized to be the result of AWE climate change, which is regulated by the dynamic subsidence of the SCB and global concentrated volcanic eruptions since Artinskian.

VP/VS ratio anomalies in fractured rocks and implication for dolomite gas reservoir identification in the northwest Sichuan Basin

The dolomite gas reservoir of the Permian Chihsia Formation has recently become the main target of hydrocarbon exploration in the northwest Sichuan Basin. Owing to its substantially low porosity and permeability, the dolomite gas reservoir has become a great challenge for geophysicists in terms of distinguishing gas-filled dolomite thin layers. We have used limestone and dolomite core samples from the Permian in the northwest Sichuan Basin to investigate the physical characteristics of the dolomite gas reservoir in a laboratory. Dolomite has a comparatively higher permeability than limestone due to its higher fracture development. X-ray CT scanning indicates that a few large pore spaces dominate the pore space in limestone that cannot provide good pore connections, whereas small pores and fractures dominate the pore spaces in dolomite. The fracture development in the dolomites provides insignificant porosity that substantially increases the permeability. The [Formula: see text] ratio estimated from well-logging data in the northwest Sichuan Basin indicates significantly lower anomalies. These low [Formula: see text] anomalies validate the rock-physics analysis from the laboratory data and indicate that the [Formula: see text] ratio could be a good indicator of gas reservoirs in dolomite layers.

Dolomite Origins of the Carbonate Successions From the Chihsia Formation, South China: A Case Study From the Pingdingshan Section

The carbonate successions from the Chihsia Formation play an important role for hydrocarbon exploration in the South China. However, the dolomites occurring in the upper section of the Chihsia Formation have been enigmatic, which confined further understanding of the depositional and/or diagenetic fluids and hydrocarbon prediction. In this study, the Pingdingshan section was investigated as a typical outcrop of Chihsian carbonates. On the basis of lithofacies/mineralogic observation of hand specimens and thin-sections, three types of dolomites were classified according to their crystal sizes, that is, micritic dolomite, sucrosic dolomite, and saddle dolomite. Comprehensive, micro-area geochemical proxies, including homogenization temperatures of fluid-inclusions, carbon and oxygen isotopes, and trace and rare-earth elements, were analyzed on different types of dolomites and matrix calcites. The results revealed that the selected samples were representative of temporal seawater and were not contaminated by terrestrial input. The temperatures and salinities of fluid-inclusions, combined with carbon and oxygen isotopic values, revealed that sucrosic dolomites originated from penecontemporaneous micritic dolomites, which had been crystallized and altered into fine- to medium-sized dolomite clusters during the later-stage burying process. In contrast, the saddle dolomites occurring in carbonate matrix or fractures were formed in a hydrothermal environment with a dolomitizing fluid temperature higher than 170°C. This inference is consistent with evidence from rare-earth elements and carbon and oxygen isotopes. Our findings provide new clues for the origin elucidation of dolomitizing fluids in the Chihsia Formation, South China.

Early–Middle Permian strontium isotope stratigraphy of marine carbonates from the northern marginal areas of South China: Controlling factors and implications

The radiogenic strontium isotope (87Sr/86Sr) system is a powerful tool for constraining sea‐level change and palaeoclimatic variations as well as tectonic activities. The Permian is a period that records the lowest seawater Sr‐isotopic ratios in the Earth's history. The near‐primary geochemical signatures retained in the micritic carbonates of the Luduba section (uppermost lower to Middle Permian) on the northern margin of the South China Block allowed the reconstruction of reliable carbon and Sr‐isotope profiles for global chemostratigraphic correlations. The investigated interval (~300 m) spans the Chihsia and overlying Maokou formations (Kungurian to Capitanian series). The 87Sr/86Sr values decrease consistently upward from 0.70833 to 0.70684, and their trend correlates well with a long‐term increase in the carbon isotope (δ13C) values. The negative correlation between the 87Sr/86Sr and δ13C values suggests that the sea‐level change, climatic conditions, opening of the Neo‐Tethys Ocean, and accretion of the Pangean Supercontinent are among the factors that might have contributed to the consistent depletion of the 87Sr/86Sr values. The rate of upward decrease in 87Sr/86Sr values throughout the Chihsia Formation is generally higher than that of the overlying Maokou Formation, which provides a possible marker for intra‐ and interbasinal basin stratigraphic correlations. The general increase in the δ13C values of the Luduba profile peaks at ~5‰, which correlates with the distinct lowest 87Sr/86Sr value (~0.70684) near the top of the Maokou Formation (Upper Capitanian). The δ13C values decrease to 3.6‰ at the top of the Maokou Formation, that correlates with the rapid sea‐level drop in the Late Guadaualupian worldwide. Meanwhile, the87Sr/86Sr ratios remain low values, probably due to the development of cold climate, increasing input of hydrothermal flux and low terrestrial runoff.

A quantitative method of measuring roundness of outcrop gravels and its applications in the study of carbonate slope geometry

Roundness reflects the transport history of gravels, but methods of determining roundness in the field have had little progress over decades. In this study, we introduce a new method to quantify the roundness of gravels on outcrops by combining field digital photographs and laboratory image processing. The Chihsia Formation in northern Chaohu City, Anhui Province, South China deposited different types of calcirudites that were formed in carbonate slope environment, including light-color limestones gravels originated from platform and dark-color limestones matrix. Using this method, we calculate the roundness of gravels from different kinds of calcirudites and characterize the geometry of the carbonate slope of the Permian-aged Chihsia Formation in Chaohu area and conclude that the gravels in calcirudites are primarily sub-angular (0.11–0.28), sub-angular to rounded (0.21–0.43), and sub-rounded to rounded (0.36–0.58) in the upper, middle and lower slopes which have dip angles of 15–25°, 30–35° and 1–10°, respectively. These results show that this method not only can be used for quantitative studies of the roundness of gravel-size particles in carbonate slope environments, but may also has broad applications for investigations of the roundness of gravels in other settings.

The origin of bedding-parallel fibrous calcite veins in the Lower Permian Chihsia Formation in western Hubei Province, South China

Bedding-parallel fibrous veins occurring as lenticular to flattened intercalations were found in the organic-rich marlstone/calcareous shale of the upper Lower Permian Chihsia Formation in western Hubei Province, South China. They dominantly consist of fibrous calcite crystals with smooth and tight boundaries, forming fence-like inward, syntaxial growth clusters toward the vein center along which a median suture line generally occurs. Petrographic evidence indicates that these veins may form at relatively shallow burial depth, where fluid overpressures would have incrementally created the bed-parallel vein space, resulting in displacive growth of fibrous calcite. On the other hand, the C, O and S isotopic data across the vein reveal slightly depleted δ13Ccarb values (−3.32‰ to +0.19‰ VPDB) and moderately depleted δ18Ocarb values (−9.6‰ to −7.3‰ VPDB) with respect to those of coeval seawaters and slightly heavier δ34Spyrite values (−7.88% CDT) with respect to those of ambient rocks. Stable isotope evidence consistently suggests significant contribution of bacterial sulfate reduction (BSR) to the formation of the fibrous calcite cements in the vein. The BSR could have been intensive with the availabilities of residual sulfate and abundant organic matters in the Chihsia sediments during shallow burial, increasing the alkalinity of pore waters and further promoting carbonate precipitation. Thus, the bedding-parallel fibrous calcite vein in the upper Lower Permian Chihsia Formation is an important time-specific petrographic capsule, providing clues for understanding the diagenetic process in organic-rich sediments.

A Kungurian oceanic upwelling on Yangtze platform: Evidenced by δ13Corg and authigenic silica in the lower Chihsia Formation of Enshi Section in South China

The Late Paleozoic Ice Age across Carboniferous and Permian had a significant impact on the Kungurian (Upper Cisuralian series of Permian) Chihsia Formation in South China. This resulted in a unique interval with features such as the lack of reef in Chihsian limestone, widespread stinkstone and nodular/bedded chert. The Chihsia limestone (Kungurian stage) deposited during a time of cooling was resulted from oceanic upwelling. Here we present evidence for this upwelling using several geochemical analyses: bulk organic carbon isotope, biomarker molecular geochemical data, and authigenic silica of the stinkstone member in the lower Chihsia Formation of the Kuangurian stage from the Enshi Section in western Hubei Province, South China. The lower part of the stinkstone member shows a rapid organic carbon isotope excursion with a −3‰ shift triggered by the upwelling of 13C-depleted bottom water. The concurrent rapid increasing of authigenic silica content resulted from the enhanced supply of dissolved silica in the upwelling water mass. This upwelling at the Enshi Section also led to relative high TOC content, accounting for the widespread stinkstone in the lower Chihsia Formation during the Kungurian stage in Permian.

The geobiological formation process of the marine source rocks in the Middle Permian Chihsia Formation of South China

The Chihsia Formation is one of the four sets of regional marine hydrocarbon source rocks from South China. In the past two decades, detailed geochemical and sedimentological studies have been carried out to investigate its origination, which have demonstrated that the high primary productivity plays a primary role in the deposition of sediments enriched in the organic matter. However, the mechanism of this high productivity and the path of the deposition and burial of the organic matter have always been a mystery. Based on the previous studies on the Shangsi Section in Guangyuan City, Sichuan Province, we proposed that the development of the equatorial upwelling due to the sea level rise is responsible for the relatively high productivity in the Chihsia Formation. The sea waters with high nutrient were transported by the sub-surface currents along the equator. High organic carbon flux was deposited on the deeper shelf, and then decomposed by bacteria, leading to the occurrence of anaerobic respiration. The metabolism of the microorganisms consumed the dissolved oxygen in waters, which was in favor of the preservation of the organic matter. This suggested geobiological model integrating with paleoclimatology, paleoceanography and geomicrobiology will help us to understand the causes of this particular sedimentary sequence.

Organic accumulation in the lower Chihsia Formation (Middle Permian) of South China: Constraints from pyrite morphology and multiple geochemical proxies

The organic-rich Chihsia Formation (Middle Permian) on the Yangtze platform, western Hubei, South China, is considered to be the major hydrocarbon source rock in South China. The lower part of the Chihsia Formation, reported here, was deposited in an intrashelf basin and is characterised by black laminated marlstones/shales intercalated with dark-grey limestones/dolomites in which metre-scale shallowing (or cleaning)-upward cycles are ubiquitous. Each cycle is composed of basal marlstone/shale and upper limestone. The organic matter is concentrated exclusively in the shaley bases of these cycles, and shows systematic, cyclic variations in abundance up through the succession. An integrated approach of pyrite morphology and multiple geochemical proxies was used to determine the mechanism of organic accumulation. In the organic-rich sediments, many redox proxies, including pyrite framboid size distribution, C–S–Fe relationships, iron speciation, and δ34S values of pyrite suggest deposition beneath the dysoxic bottom water, which evolved into a more oxic water body when the organic-poor limestones were deposited. However, the coincidence of increased abundance of total organic carbon (TOC) with increased Ba and Mo contents indicates that organic matter accumulation was controlled mainly by primary productivity. These data indicate that increased organic accumulation resulted mainly from higher organic carbon export with increased nutrient flux during superimposed sea-level rises of different orders. This in turn could have increased the oxygen consumption by respiration of microorganisms and organic matter decay, leading to the depletion of oxygen in the water columns. Furthermore, the rapid sedimentation rate further limited bioturbation and organic matter decomposition, facilitating the preservation of buried organic matter.

APPLICATION OF PALYNOFACIES TO THE STUDY OF THE PALEOZOIC SOURCE ROCKS IN LOWER YANGTZE REGION

We discussed the origin,organic-matter type and the formation environments of the source rocks occurred in the Lower Cambrian Mufushan Formation and the Lower Permian Chihsia Formation in the lower Yangtze region upon palynofacies.High quality source rocks with high TOC occur in the Mufushan Formation.They were dominated by over 90% of amorphous organic matter in most of the cases,and predominated by the type I of organic matter mainly from aquatic organisms formed in suboxic-anoxic lagoonal or estuarine environments.The organic matters in the Chihsia Formation are also dominated by amorphous organic matters,but with obvious variations in organic components,so it is of a mixed-type.The frequent change in sedimentary environments and oxygen-deficiency level made the source rocks distributed in a heterogeneous manner with lower TOC and lower quality.

Formation and influencing factors of carbonate source rock of the Lower Permian Chihsia Formation in Chaohu region, Anhui Province

Highly-mature carbonate source rock is essential to the exploration of oil and gas in southern China. In this study, the carbonate strata in the relatively well-developed Lower Permian Chihsia Formation (located in Chaohu, Lower Yangtze) were targeted, and the formation and influencing factors of source rock were discussed based on paleoenvironment reconstruction using comprehensive sedimentology, palynofacies, and organic geochemistry data. The results demonstrate that the Chihsia Formation is oxygen-deficient biogenic carbonate sediments, with marked variations in the organic and inorganic components, formed during a period of Permian transgression. The formation of source rock is the combined result of high bioproductivity and oxygen-deficient environment generated by transgression and oxygenation events which frequently occurred during transgression periods. Source rock was affected by self-dilution effects and diagenesis, causing its heterogeneous distribution in many intervals. Source rock is over 40 m thick, and can be identified based on its different organic, biological, and mineral composition characteristics. The carbonate rock appears to require no specific clay content in order to become a source rock. The combination of sedimentology, palynofacies, and organic geochemistry has provided an effective means for evaluating and predicting high-maturity carbonate source rock in the region.

Molecular evidence for primary producers and sedimentary environmental conditions of Permian Chihsia Formation in Northeast Sichuan, China

Molecular evidence for primary producers and sedimentary environmental conditions of Permian Chihsia Formation in Northeast Sichuan, China

Subdivision of Permian Fossil Communities and Habitat Types in Northeast Sichuan, South China

Recent achievement in hydrocarbon exploration in Northeast Sichuan demonstrated that Permian calcareous and argillaceous deposits are the major contributing source rocks. Reevaluation on the hydrocarbon potential of the Permian strata over the whole Yangtze region is thus to be desired. A comprehensive corroboration was carried out at the Shangsi Section, Guangyuan, Northeast Sichuan, which is believed to be another promising area in South China. This article deals with the ecological and depositional conditions of the Permian strata in the section, including the Chihsia, Maokou, Wujiaping, and Dalong formations, which share some similarities with those occurring in the broad Yangtze carbonate platforms. Five fossil communities of Mizzia-Permocalculus, Hayasakaia, Inozoan , Ostracod, and Crinoid were identified in the Yangtze region and described in detail including their components and occurrence. On the basis of the fossil communities and depositional features, 23 habitat types, mainly occurring from inner to outer shelves, were recognized at the Shangsi Section in order to reconstruct the depositional conditions and accordingly to evaluate the paleoproductivity. In addition, the subdivision of Chihsia Formation in the section was revised, which would be of significance for the reconstruction of the Chihsian paleogeography in the study area.

Depositional and Ecological Features of Permian Oxygen Deficient Deposits at Shangsi Section, Northeast Sichuan, China

Because oxygen deficient conditions enhance the preservation of depositional organic matter, analysis on paleooxygenation conditions of depositional environments becomes a routine work in evaluations of potential hydrocarbon source rocks. The article focuses on depositional and ecological features relating to oxygen deficient shelfal environments at the Shangsi Section, as a part of multidiscipline collaboration to reevaluate the hydrocarbon potential of the Middle and Upper Permian, Guangyuan, Northeast Sichuan Province. Ichnofabric Zoophycos , sepiolite-bearing limestones (SBL) were interpreted as indicators of dysaerobic environments. Laminated calcareous and/or siliceous mudstones with pelagic ammonites and radiolarians were believed to be the deposits of anaerobic environments. When rhythmic succession was considered, average strategy was adopted for the oxygenation explanation of a given interval. The anaerobic condition in this Permian section was observed in the upper part of the latest Permian Dalong Formation, in which siliceous mudstones with ammonite, radiolarians develop in association with lower U/Mo ratio, lower biomarker ratio of Pr/Ph, and the highest TOC content. The topmost Maokou Formation, featured by thin-bedded calcareous and siliceous mudstones with ammonite, thin-shelled bivalves, and laminations would be deposits of the quasianaerobic condition. The middle part of the Members I and III of the Chihsia Formation is proposed to be dysaerobic condition as indicated by occurrences of SBL and ichnofabric features, with the Member III being the severe dysaerobic condition.

Main Controlling Factors of Organic Matter Richness in a Permian Section of Guangyuan, Northeast Sichuan

A complete Permian section in Guangyuan, Northeast Sichuan, has been investigated, to explore the variation in organic matter richness and its main controlling factors. The research results of the detailed lithological description and organic/inorganic geochemical analysis of about 325 samples indicate that a high content of total organic carbon (TOC) occurs in calcareous mudstones and laminated marls or thin-bedded limestones. In carbonate rocks, the TOC content is negatively related to the thickness of the massive beds; the thinner the bed is, the higher the TOC content is. Marine organic matter is enriched in the outer shelf and relatively shallow basin floor environments with the maximum TOC contents of 5.07% and 14.6%, respectively. The main factors that affect the quantity and quality of marine organic matter include primary productivity, depositional processes, and redox conditions during deposition and the early diagenesis stage. Three intervals of marine good quality source rocks are identified in this section; they are lower Chihsia Formation, topmost Maokou Formation, and the middle segment of the Dalong Formation. They are formed in anoxic environments, in association with high primary productivity. Among these, high productivity in the lower Chihsia interval may originate from an upwelling flow area, whereas, in the rest of the intervals, it is inferred to be related to hot fluid activity due to volcanic eruption. The results of this study suggest that good quality marine source rocks in the Permian strata offer significant hydrocarbon potential.

Biogeochemical Records at Shangsi Section, Northeast Sichuan in China: The Permian Paleoproductivity Proxies

The marine primary producers assimilate the atmospheric CO2 to form the organic carbon in surface water. The organic carbon then settles down through the water column and is removed from the oceans by final preservation in sediments in the form of petroleum or nature gases. The reconstruction of paleoproductivity will thus improve our understanding of the biological processes in the formation of fossil energy resource and help to locate new sites for future exploration. In this study, biorelated elements P, Cd, Al, Ba, as well as redox sensitive element Mo, were analyzed in the 448 rock samples collected from Permian strata at the Shangsi Section, Guangyuan, Northeast Sichuan in China. On the basis of the Ti content, the nondetrital contents of P, Ca, and Al, denoted as Pxs, Cdxs, and Alxs, were calculated and found to coincide with the TOC content throughout the whole section, with some enrichment intervals being found in the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation. This suggests that the biorelated elements could be used as proxies for the paleoproductivity here in this section. Baxs, a paleoproductivity indicator widely used in the paleoceanography, shows insignificant correlation with TOC, Pxs, Cdxs, and Alxs, probably arising from the loss of biological barium in anoxic conditions. Compiled with the data of TOC content and Pxs, Cdxs, and Alxs, three episodes of enhanced paleoproductivity were identified in Permian strata including the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation.

Geobiological interpretation of the oxygen-deficient deposits of the Middle Permian marine source rocks in South China: A working hypothesis

To decipher the origin of oxygen-deficient shelfal deposits is significant for tracing the distribution of marine source rocks and interpreting the evolution of depositional environment. The origin of the Middle Permian Chihsia Formation in South China remains a puzzle for long with its evident oxygen-deficient features but diverse benthos. This paper shows a typical Chihsian depositional rhythm composed of the massive and the laminated limestones with ecological and geochemical features. Massive bioclastic limestone from the rhythm was aerobic in paleoxygenation condition indicated by both the ecological and geochemical features. However, a contradictory oxygenation was inferred for the “laminated” counterpart from the rhythm, with the ecological signal being aerobic and the geochemical one being anoxic. The difference in ecological and geochemical indications was interpreted as the instability of paleoxygenation condition in shelf environments, caused by an enhanced paleoproductivity. Rhythmic occurrence of the oxygen-deficient condition might have been stemmed from paleo-Tethyan paleocurrents flowing across South China.

Biostratigraphy and lithofacies of the Permian System in the Laibin–Heshan area of Guangxi, South China

The Laibin–Heshan area is probably a unique area containing a suitable marine reference for the entire Permian system. Various fossils, sedimentary characteristics, lithofacies and palaeoenvironments including near shore, distal shelf, slope, and basin are developed. The Permian system in the Laibin–Heshan area is composed of the Maping, the Chihsia, the Maokou, the Heshan/Wuchiaping and the Talung formations in ascending order. The Maping Formation is characterized by restricted shoal or intertidal facies and ranges from the Late Carboniferous Kasimovian to Early Permian Artinskian. The Chihsia Formation (Artinskian–Kungurian) and the Maokou Formation (Roadian–Capitanian) represent a slope carbonate environment and a transgressive interval. By the end of the Guadalupian, the Permian sea was reduced to the confines of the Laibin area in South China and nearshore massive crinoid limestone was deposited, representing the most continuous marine deposits across the Guadalupian–Lopingian boundary. The Lopingian sequence in the Laibin–Heshan area reflects an independent transgression–regression system. During the Wuchiapingian basinal or slope phase, siliceous siltstone and chert and carbonates were deposited. During the Changhsingian, the Laibin–Heshan area subsided rapidly, accumulating about 600 m of limestone, tuffaceous sandstone, shale, siltstone before the basin was drained and terrestrial clastics with coal-seams developed. The end of the Changhsingian reflected the initial stage of the rapid transgression of the Triassic forthcoming supercycle. Limestone with latest Permian benthic faunas developed during the last stage of the Permian, which were immediately replaced by the nektic ammonoid and bivalve-dominated faunas in the earliest Triassic.