Microfacies and paleoenvironment of microbialites of the Cambrian (Stage 4)Qingxudong Formation in the Huayuan area,northwestern Hunan Province, southern China

Zhong-Tang Su , De-Min Zhang , Jie Tang , Pei-Jie Sun ,Zhen-Feng Luo , Hui Ma

a State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology,Chengdu 610059, Sichuan Province, China

b Key Laboratory of Deep-time Geography and Environment Reconstruction and Applications of Ministry of Natural Resource, Chengdu University of Technology, Chengdu 610059, Sichuan Province, China

c Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, Sichuan Province,China

d Sinopec Petroleum Exploration and Production Research Instutite, Beijing 100083, China

Abstract After the extinction of Archaeocyatha(sponges),microbial bioherms were well developed in the lower Cambrian of the Yangtze cratonic basin,especially in the Qingxudong Formation(Cambrian(Stage 4))of the Huayuan area, northwestern Hunan Province, southern China.Herein, four sections from this area were chosen for investigating and analyzing their microfacies and depositional environment.Twelve microfacies types were recognized through petrographic analysis of textural attributes and calcimicrobes (including Epiphyton, Renalcis, Girvanella and Kenella), respectively as: laminated calcareous mudstone (MF1), algal wackestone (MF2), intraclastic grainstone (MF3), algal intrasparrudite (MF4), oolitic algal intrasparrudite(MF5), sparry oolitic grainstone (MF6), Epiphyton framestone (MF7), Renalcis bafflestone (MF8), Kenella bafflestone (MF9), Girvanella boundstone (MF10), thrombolitic boundstone (MF11), and dolomite (MF12).These microfacies represent four major depositional environments: carbonate ramp, carbonate platform,slope and tidal flat.Calcimicrobes played an important role in the transition from carbonate ramp to platform,while the evolution from ramp to tidal flat must be ascribed to sea-level fall during the Cambrian Age 4.

Keywords Microfacies, Microbialite, Carbonates, Epiphyton, Cambrian, Yangtze craton, Sea level

1.Introduction

During the early Cambrian Period, reefal metazoan communities became extinct during the microbe－metazoan transition(MMT;Chen et al.,2019)(also referred to as the microbial－archaeocyathid reef interval;Lee and Riding,2018).The microbial buildups begun to recover their predominance, e.g., Epiphyton was an important reef builder in shallow marine carbonate platforms after the extinction of archaeocyathid sponges (Woo and Chough, 2010; Zhuravlev et al., 2015; Yang et al., 2016; Cordie et al., 2020).Microbial bioherms developed well in the Qingxudong Formation of Huayuan area in northwestern Hunan Province, southern China (Adachi et al., 2014), which recorded a complete sequence of microbiota generation, flourishing, and extinction.Abundant calcareous algae and algal communities, such as Epiphyton,

Renalcis, Girvanella, Kenella and Nicholsonia, have been described by previous researchers (Sun et al.,1985), which provided an opportunity to study the microfacies and paleoenvironment of the microbialites.However, the paleoenvironment of mircrobialites has long been controversial because of different interpretations of microbial accumulation and their geological background.Some authors interpreted the microbialites as reefal sediments on a carbonate platform margin based on algal characteristics, sedimentary features, and geological background, and established a carbonate rimmed platform model(Zheng and Zeng, 1988; Xue et al., 2017).However, others suggested that they are microbial mounds on a carbonate ramp according to evidence of algal morphology, sedimentary facies distribution and the geological background of the Yangtze craton (Wang,1990; Tang et al., 2013).In this study, based on analyses of several measured sections,polished slabs,and thin sections, we described the microbial microfacies,analyzed their depositional environment and the role of algae in environmental evolution, and established a depositional model of microbialites.

2.Geological setting

During the early Cambrian, the South China Block(SCB) was located in the equatorial zone, east of Gondwana land (Scotese, 2021); SCB includes the Yangtze Craton, the Jiangnan Orogenic Belt, and the Cathaysian Land.The former two are bounded by the SW－NE-extending Xiangqian-Jiangnan fault zone(Fig.1B; Liu, 1985; Feng et al., 2001; Zhang et al.,2013).On the NW side of the fault zone, the Yangtze Craton was a carbonate platform, covering eastern Yunnan, eastern Sichuan, Guizhou, northwestern Hunan, Hubei, Jiangxi, southern Anhui, and Zhejiang provinces and Chongqing (Duan et al., 1988; Zhao et al., 2003; Steiner et al., 2007; Wang et al., 2012).Carbonate rocks, mainly grainstone and algae framestone,were deposited on the platform margin.On the SE side of the fault zone, was a basin, deposited with calcimudstone and mudstone, as well as gravity flow deposits (Duan et al., 1988).

Huayuan area is located in northwestern Hunan Province, and the Xiangqian-Jiangnan fault passes through the southeastern part of the study area(Fig.1B).The Cambrian Qingxudong Formation is well exposed here(Fig.1A;Zhu et al.,2021).Turbidites of the Qingxudong Formation were widely distributed along the fault zone (Kuang et al., 2008).Meanwhile,in southeastern Sichuan Province (northwest of the study area),gypsum is common,deposited in a lagoon during the late depositional period of the Qingxudong Formation (Liu et al., 2012).

The Qingxudong Formation is named after the Qingxudong limestone of Qingxudong village in Meitan County, Guizhou Province (Xiang et al., 1999).It is 300－500 m thick in the Huayuan area and can be divided into the first, second and third members from bottom to top (Fig.1C).The first member (40－100 m thick) is characterized by laminated calcareous mudstone and intraclastic grainstone; the second member (270－300 m thick) consists of massive calcimicrobial limestone, bioclastic limestone, and oolitic limestone;while the third member(40－80 m thick)is characterized by lamellar dolostone.

Two assemblages of trilobites, defined as Arithricocephalus chauveaui－Oryctocerella duyunensis Assemblage Zone and Protoryctocephalus arcticus Assemblage Zone, occur in the Qingxudong Formation which corresponds to the lower part of Cambrian Stage 4,based on the latest research of the Cambrian Stage 4 global stratotype section and the results of Zhao et al.(2021).Zhu et al.(2021) also analyzed the petrographic characteristics and their comparative relationships of Cambrian Stage 4 in South China, using trilobite stratigraphic data and microfacies, illustrated in Fig.2.In the study area, the Qingxudong Formation conformably overlies the Palang Formation,which is characterized by yellow-green clay shale,light-gray calcareous shale, and silty shale with intercalations of thin-bedded argillaceous carbonate(Adachi et al., 2014), and underlies the Gaotai Formation, which consists primarily of laminated dolostone.

3.Material and methods

Four stratigraphic sections near Huayuan town were measured (i.e., Limei section, 472 m thick;Yutang section, 301 m thick; Yanke section, 218 m thick; and Panshi section, 441 m thick, respectively;Fig.1A).Among them, although Limei, Yutang, and Pashi sections show similar microfacies characteristics,only Limei section exposes a complete succession of Qingxudong Formation, while the other two are partially covered.Compared to the former three sections, the Yanke section shows substantially different microfacies characteristics, with no significant thickbedded algal limestone occurring in its second member.The sections were described in the field,including their macroscopic morphology, weathering surfaces,bedding surfaces,thickness,and lithological features.A total of 385 rock samples were collected bed by bed from the four sections.

Among these samples,25 polished slabs and 245 thin sections were prepared.Thin sections were polished both sides and uncovered,and 1/3 of each thin section were stained with alizarin red.The polished slabs are observed with naked eye using a magnifying glass.The thin sections are observed under an optical microscope to collect microfacies characteristics of textures, calcimicrobes, non-skeletal components and sedimentary structures.Algal characteristics were collected from the polished slabs and thin sections of the same beds in the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology.Then, microfacies are classified according to the taxonomy proposed by Dunham (1962) and Embry and Klovan (1971).Finally, based on all the above field observations and petrographic studies,the paleoenvironment was interpreted and reconstructed.

Fig.2 Biostratigraphic and lithostratigraphic correlation of the Cambrian Stage 4 in South China(adapted from Zhao et al.,2021;Zhu et al.,2021), showing the trilobite assemblage distribution in the Qingxudong Formation.

4.Microfacies analysis

Twelve types of microfacies (MF1－MF12) in the Qingxudong Formation of Huayuan areas were identified and interpreted as below (Figs.3－5; sampling positions are shown in Fig.6).

4.1.MF1: Laminated calcareous mudstone

The laminated calcareous mudstone microfacies(MF1) was identified at the lower part of the first member of Qingxudong Formation.It is gray and thinbedded(3－5 cm)in field(Fig.3A),and consists mainly of microcrystalline calcite.It shows a laminar structure under microscope (Fig.4A), with dark layers of argillaceous microcrystalline calcite; whereas the light layers contain more angular, silt-sized terrigenous quartz grains.

Interpretation.The laminar structure and microcrystalline calcite composition indicate a low-energy open-marine environment.According to the microfacies type RMF4 described by Flügel (2010), and the thin-bedded and silt-sized quartz grains, we interpret MF1 as being deposited below the storm wave base in an outer ramp environment.

4.2.MF2: Algal wackestone

This microfacies mainly occurs in the middle of the first member of Qingxudong Formation.It is gray and thin-bedded (8－10 cm) in field.Under the microscope this microfacies is revealed as being composed of mainly microcrystalline calcite, as well as some dark-colored subangular algal intraclasts(5%－10%).Rarely, well-preserved ostracod shells are present (Fig.4B).

Interpretation.The low-content subangular algal intraclasts, and the predominant microcrystalline calcite indicate a low-energy hydrodynamic condition,which is supported by the complete ostracod fossils.Similar to the microfacies type RMF9 described by Flügel(2010),MF2 is interpreted as being deposited in a mid-ramp environment considering its thickness.

4.3.MF3: Intraclastic grainstone

Fig.3 Outcrop features of the studied sections.A)Gray thin-bedded calcareous mudstone(MF1),the first member of Qingxudong Formation,Limei section; B) Thick-bedded algal intrasparrudite (MF4), the second member of the Qingxudong Formation, Limei section; C) Epiphyton and other algae form domes (MF7), the second member of the Qingxudong Formation, Panshi section; D) Microbial carbonate (MF7) (yellow frame in C), the second member of the Qingxudong Formation, Panshi section (MF4); E) Dark-gray and thin-to medium-bedded Renalcis bafflestone(MF8),the second member of the Qingxudong Formation,Yanke section;F)Laminated dolostone(MF12),the third member of the Qingxudong Formation, Yutang section.The section locations are shown in Fig.1A.

The intraclastic grainstone microfacies (MF3) occurs in the upper part of the first member of Qingxudong Formation.It is gray and thin-bedded(8－10 cm thick per bed) in field.Under the microscope, it is identified as grainstone cemented with sparry calcite.The grains are mainly well-sorted subrounded-to-rounded intraclasts, with grain sizes of 200－250 μm and composed of microcrystalline calcites.The grainstone directly contacts the wackestone(Fig.4C).

Interpretation.The well-sorted subrounded-torounded intraclasts, as well as the sparry cements,suggest a high-energy condition; while the abrupt interchange from wackestone to grainstone indicates a change from low energy to high energy.Therefore,MF3 is interpreted to be deposited on the ramp near the shoal.

4.4.MF4: Algal intrasparrudite

The alga intrasparrudite microfacies (MF4) is the most common type in the study area.It is generally thick-bedded to massive (0.8－1.2 m thick per bed) in field (Fig.3B), and was identified in the second member of the Qingxudong Formation at Limei,Yutang, and Panshi sections, and in the third member at Yanke section.

Fig.4 Microscopic features of different microfacies.A) Laminated calcareous mudstone (MF1), containing quartz grains, LM-01b2, Limei section; B) Algal wackestone (MF2), with a well-preserved ostracod, LM-02b2, Limei section; C) Intraclastic grainstone (MF3) with sparry calcite cements, abruptly contacting with the wackestone, LM-14b2, Limei section; D) Algal intrasparrudite (MF4) with sparry calcite cements, LM-17b2, Limei section; E) Calcareous dolostone containing algal intraclasts (MF4), YQ-31b2, Yaqiao section; F) Oolitic algal intrasparrudite (MF5), containing 20% components of ooid and algal aggregate grains, YT02-03b2, Yutang section; G) Sparry oolitic grainstone(MF6),LM-51b3,Limei section;H)Dolostone(MF12),with the dolomite crystals showing euhedral planar-e fabric,LM-52b1,Limei section.LS=Laminar structure;AI=Algal intraclasts;Q=Quartz;OS=Ostracod;W=Wackestone;I=Intraclast;SC=Sparite calcite cement;O=Ooid;AA=Algal aggregate grain.This group of microscopic photos are taken under plain polarized light;and the sample positions are shown in Figs.6 and 7.

Under the microscope, MF4 shows a grainstone texture with algal intraclasts as the primary component(80%－90%),which are cemented by sparry calcite(Fig.4D).The dark-colored grains are well-to very well-sorted, fine-to coarse-grained, and range from subangular to rounded (Fig.4D).Large grains tend to be subangular.Fragments of trilobites and crinoids are occasionally found.

Interpretation.Algal intraclasts are common constituents of inner ramp and platform carbonates(Flügel,2010).The well-sorted,subangular to rounded algal intraclasts and sparry cements indicate mediumto high-energy conditions.The bioclastics of trilobites and crinoids suggest an open marine environment.Therefore, MF4 is interpreted as shoal deposition.

Fig.5 Microbial characteristics of different microfacies.A) Epiphyton (yellow arrows) forms framework; polished slab, LM-34b3, Limei section; B) Microphoto of the yellow rectangle in A, showing Epiphyton framestone(MF7) with bush-shaped thalli; C) Epiphyton framestone(MF7)with fanshaped thalli,PS-06b1,Pashi section;D)Renalcis bafflestone(MF8),with irregular,overlapping chambers forming coagulums,YQ-07b2,Yaqiao section;E)Kenella bafflestone(MF9)with a creeping growth form,and inter-thalloids spaces filled with micritic matrix,PS-13b2, Pashi section; F) Girvanella boundstone (MF10), showing Girvanella with a regular horizontal laminar distribution, PS-03b7, Pashi section; G) Thrombolitic boundstone (MF11), with dark thrombolitic clots irregularly bound to each other, LM-40b3, Limei section.Epi.= Epiphyton; Gir.= Girvanella; Re.= Renalcis; Ke.= Kenella; T = Thrombolitic clot; DC = Dissolution pore-filled calcite; SC = Sparry calcite cements.This group of microscopic photos are taken under plain polarized light; and the sample positions are shown in Figs.6 and 7.

4.5.MF5: Oolitic algal intrasparrudite

This microfacies(MF5)has similar features as MF4,but contains 5%－20% of ooids and algal aggregate grains(Fig.4F).The aggregate grain,similar to oolitic lump, has a nucleus of typical algal intraclast, and is surrounded by a thin external cortex,.

Interpretation.The presence of ooids and aggregate grains suggests high energy or more frequent waves and tidal currents.Limestones with abundant aggregate grains are common components of platform carbonates formed in open marine settings (Flügel,2010).Since it is similar to MF4, MF5 is interpreted to be deposited in a transitional shoal area.

4.6.MF6: Sparry oolitic grainstone

Fig.6 Vertical distribution and depositional environment interpretation of the microfacies (MF1－MF12) of the Qingxudong Formation at Limei section,Huayuan area,northwestern Hunan Province,southern China.The microfacies are classified according to Dunham(1962),and Embry and Klovan (1971).CM = Calcareous mudstone; MF = Microfacies; TF = Tidal flat.

The sparry oolitic grainstone microfacies (MF6) is thick-bedded in field.It occurs at the top of the second member of the Qingxudong Formation at Limei and Yutang sections.MF6 consists of ooids (0.5－1 mm;70%－75%), algal intraclasts (10%), giant ooids(1.8－2 mm;about 5%),and rare algal aggregate grains(about 2%), and is cemented by sparry calcite.The normal ooids are coarse, well-sorted, and spheroidal(Fig.4G), some of which show tangential/radial structures in their laminae.The thickness of the dark concentric laminae of ooids is 2 or 3 times that of the algal intraclast nucleus.

Fig.7 Horizontal comparison among the four sections,showing the spatial variation of microfacies at the Qingxudong Formation of Huayuan area, northwestern Hunan Province, southern China.The Yanke section develops dark-gray thin-to medium-bedded limestone (MF8) at the second member,and medium-to thick-bedded algal intrasparrudite(MF4)at the third member,which differs significantly with Limei,Yutang and Panshi sections, due to its proximity to the basin.

Interpretation.The high abundance,good sorting,and ooids suggest deposition in shallow, high-energy shoals with strong or long-lasting turbulence (Tucker and Wright,1990;Flügel,2010).The presence of algal aggregate grains and some tangential/radial ooids indicates a slightly-restricted environment (Flügel,2010).

4.7.MF7: Epiphyton framestone

The microfacies 7 (MF7) occurs in the middle member of the Qingxudong Formation at Limei,Yutang,and Panshi sections.It forms domes in the field(Fig.3C)and shows coarse structures on fresh rock surfaces(Fig.3D).MF7 consists of Epiphyton(predominant)and other algae, which are grayish white and clustered in polished slabs(Fig.5A).Epiphyton shows dichotomous(Y-shaped or forked) branching, with sparry calcite cements between the branches (Fig.5C).Of the cements,there are some bigger calcite crystals that were precipitated due to dissolution and re-crystallization(Fig.5C).Filaments grew on the branches,which have a uniform diameter or are slightly coarser in the middle part of a branch.The Epiphyton grew upwards freely,exhibiting dense uniform bush-shaped thalli(Fig.5B)or fan-shaped thalli(Fig.5C).

Interpretation.The free-growing pattern of Epiphyton, with sparry calcite cements suggesting high energy, indicates that it constructed the framework of the microbial rocks.Pb－Zn minerals are observed to occur in the inter-algal pores, also indicating the presence of a growth framework (Liu et al., 2022).Epiphyton has a unique role in providing a support structure for other microbes (Riding, 2000, 2006;Pratt et al., 2001; Woo and Chough, 2010) and is common in platform marginal environments (Pratt et al., 2001), which is consistent with the highenergy depositional environment as indicated by the sparry calcite cements.

4.8.MF8: Renalcis bafflestone

This microfacies(MF8)is mainly found at the Yanke section.It is dark gray and thin-to medium-bedded in field(Fig.3E).There are two occurrences of Renalcis.One is associated with Epiphyton, where the Renalcis chambers grew as pearl-shaped hollow syncytia, with large syncytia filled by recrystallized mortar, while small or individual coagulums commonly scattered in substrates.Another type of Renalcis consists of irregular, overlapping chambers forming clusters (Fig.5D),which are mostly filled with dark, recrystallized calcareous micrite.

Interpretation.The small clusters scattering in the substrates, and the mortar filling large syncytia, may indicate a low energy depositional environment.The morphology of Renalcis indicates that it was not resistant to strong waves and may have acted only as a baffle during deposition.Renalcis generally grows in a subtidal environment behind the platform marginal reef or at the slope in front of platform margin (Zhou and Gao, 1988; Pratt et al., 2001).The microfacies characteristics and dark-gray thin-to medium-bedded lithology suggest that Renalcis bafflestone was deposited on a slope.

4.9.MF9: Kenella bafflestone

The Kenella bafflestone microfacies (MF9) was identified in the upper second member of the Qingxudong Formation at the Panshi section.The Kenella shows a creeping growth pattern.Its thallus comprises round tubes with various diameters, which intergrew with each other randomly to form clusters, or occasionally arranged like a string of beads.The innerthallus spaces were filled with micritic matrix(Fig.5E).Dissolution pores were well developed in this microfacies, but were filled by sparry granular calcite crystals.

Interpretation.The creeping growth pattern and micritic matrix filling indicate a low-energy environment,where a substantial number of algal aggregates form a barrier.Therefore,this microfacies corresponds to a subtidal deposition.

4.10.MF10: Girvanella boundstone

This microfacies (MF10) is mainly found in the lower second member of Qingxudong Formation at the Panshi section.In the polished slad, the Girvanella filaments intergrew with each other,showing a laminar growth pattern; and under the microscope, they are single (i.e., not bifurcated), curved tubes with a uniform diameter.There are two occurrences of Girvanella: one with regular horizontal laminar filaments(Fig.5F);and the other with nearly vertical filaments.Both have their interfilament spaces filled with recrystallized micritic matrix.

Interpretation.The Girvanella filaments morphology and micritic fillings inbetween, indicate a low energy depositional environment.It shows similar sedimentary feature to that of the Cambrian intertidal－subtidal Girvanella in western Henan Province,eastern central China(Qi et al.,2017).Girvanella commonly occurs in shallow shelves below 50 m water depth(Wray,1977),ranging from subtidal to open shelf areas(Kaya and Friedman,1997;Sun et al.,1985;Zheng and Zeng, 1988).Herein, MF10 is interpreted to be deposited in subtidal or backreef environments.

4.11.MF11: Thrombolitic boundstone

The thrombolitic boundstone microfacies(MF11)is present in the middle－upper second member of the Qingxudong Formation at Limei section.It shows a thrombolitic texture, with the dark clots irregularly bonded to each other.The clots are made up of diffusely clotted micrites,while their inter spaces are filled by sparry calcite cements(Fig.5G).

Interpretation.The micritic clots indicate a relatively low-energy environment, similar to the thrombolites of Zhangxia Formation in Shandong Province, which was deposited in a low-energy subtidal environment (Yan et al., 2017).However, the inter-clot sparry cements suggest the modification by strong hydrodynamic forces after deposition.Considering its occurrence position in the section,MF11 is interpreted to be deposited in the intershoal areas.

4.12.MF12: Dolostone

This microfacies(MF12)occurs in the third member of Qingxudong Formation at Limei, Yutang and Panshi sections.It is medium-to thick-bedded, and shows lamellar structures in field (Fig.3F).This microfacies consists of fine-to medium-sized dolomite crystals(20－120 μm), with no fossils observed.Most of the dolomite crystals are semi-euhedral to euhedral rhombs with straight compromise boundaries(planar-e fabric; Sibley and Gregg, 1987, Fig.4H).

Interpretation.Euhedral planar-e fabric dolomite crystals are typically formed below 50－60°C (Sibley and Gregg, 1987; Machel, 2004), which suggest nearsurface conditions.The lamellar structure and lack of fauna indicate that the deposition occurred in a lowenergy restricted environment.Compared to modern environments such as the Persian Gulf (Friedman,1995), MF12 is interpreted to be deposited in the supratidal－intertidal zone in warm and arid areas.

4.13.Vertical and horizontal comparison of microfacies

Limei section is taken as an example to describe the vertical combination of microfacies(Fig.6).From bottom to top, the first member of Qingxudong Formation consists of calcareous mudstone (MF1),wackestone(MF2),and grainstone(MF3),respectively.The second member is characterized by abundant particles (algal intraclastics and aggregates, ooids,etc.), and sparry calcite cements.It consists of MF4,MF5, MF6, MF7, MF9, MF10, and MF11, which overlap vertically, except for MF6.MF6 mainly occurs on the top of the second member.

The third member is mainly composed of MF12,which is characterized by medium-to thick-bedded dolostone with a lamellar structure (Fig.3F).

Horizontal comparison (Fig.7) shows that the Yutang, Limei, and Panshi sections exhibit similar microfacies characteristics.However, there are slightly some differences,e.g.,Yutang section has the most prominent dolostone in the third member of Qingxudong Formation,while Panshi section preserves the most complete and abundant calcareous algal fossils in the second member.Comparing to the other three sections, the Yanke section differs significantly due to its proximity to the basin.It develops dark-gray thin-to medium-bedded limestone(MF8)at the second member, and medium-to thick-bedded algal intrasparrudite (MF4)at the third member.

5.Paleoenvironment reconstruction

The microfacies identified from the Qingxudong Formation and the association of grains and calcified algae(Wray,1977;Flügel,2010)are crucial factors for interpreting the paleoenvironmental conditions in the study area.The biotic community and the supply of clastics determine the accumulation rate of sediments and facies zonation, and control the geometry of a carbonate platform (Mutti and Hallock, 2003).

During the early depositional period of the Qingxudong Formation in the Yangtze Craton in the passive continental margin, the Archaeocyatha were extinct,and the damaged marine ecosystem had not yet recovered from the “Sinsk event” (Ishikawa et al.,2014; Babcock et al., 2015).Only very few algae were present, and frame-building organisms were missing.Micrite and calcisiltite with microcrystalline matrix (MF1 and MF2) were deposited, indicating a carbonate ramp (Flügel, 2010).Subsequently, a shoal(i.e.,MF3)was formed,providing a hard substrate for reef growth.

During the middle depositional period of the Qingxudong Formation, the Xiangqian-Jiangnan fault at the margin of the Yangtze Craton, became active,which is supported by the wide distribution of turbidites in the study area(Kuang et al.,2008).The active fault transported hydrothermal fluids from the sea bottom which supplied nutrients for the growth of microbes (Whalen et al., 2002; Tang et al., 2013).Meanwhile, various algae flourished, primarily Epiphyton, due to the lack of competitors after the Archaeocyatha extinction.When the water is strongly turbulent,algal intraclasts develop MF4,and when the turbulence is weaker,the algae bond together,forming MF11.The growth of epiphytic framework(Shen et al.,1997)resulted in the rapid accumulation of carbonate sediments and the formation of bioherms (e.g., the domes at Panshi section; Fig.3C), altering the platform geometry (Pomar et al., 2012).

Considering a clean, warm environment and the most suitable water depth for blue-green algae (less than 30 m; Riding, 1975; Wray, 1977), as well as the sparry calcite cements in the Epiphyton framestone(MF7), the microbiolites of the second member of the Qingxudong Formation should be deposited in shallow and high-energy environments.However, the collapsed breccia observed in the Yutang section(Zheng and Zeng,1988)and the turbidite identified in the study area (Kuang et al., 2008) indicate that a fracture-controlled steep slope developed during the growth of massive algae in the second member of the Qingxudong Formation.Thus, it is reasonable to interpret the microbialites as a platform marginal reef.

In summary, a rimmed platform margin developed due to microbial bioherms and a synsedimentary fault,and the platform transformed from a carbonate ramp to a rimmed platform.A comparable transformation has also been identified in the Majiang area of eastern Guizhou Province, located along the Xiangqian-Jiangnan fault zone (Ma et al., 2013).

MF8 occurs in the second member of the Qingxudong Formation in the Yanke section.Its features are significantly different from that of the reefal facies deposits of other sections formed during the same period (Fig.7).Since this section is close to the southeastern basin,Renalcis bafflestone is interpreted as a depositional environment on a slope with relatively deep water.

The gypsum deposited in the Yangtze Craton during the late stage indicates a warm and arid climate.The margin of the carbonate platform in the Limei,Yutang,and Panshi sections, evolved into a tidal flat (MF12)from ooid shoal(MF6,Fig.6),and the environment was no longer conducive to microbial growth.A significant amount of algal intrasparrudite(Fig.4F)developed on the slope of the Yanke section, confirming that the water at the carbonate margin was too shallow for algal growth.

Fig.8 Evolution of the depositional environments during the deposition of the three members of Qingxudong Formation in Huayuan area,northwestern Hunan Province, southern China.

The microbiota generation, flourishing, and extinction in the depositional period of the Qingxudong Formation indicate that water depth critically affected the development of calcareous algae.The suitable water depth and the activity of the synsedimentary fault contributed to the development of microbialites after the extinction of Archaeocyatha.

These depositional environments correspond to the three members from bottom to top of the Qingxudong Formation.The evolution of the depositional environment from the outer ramp to the tidal flat during the Qingxudong period indicates a gradual fall in the sea level, which is consistent with the global eustatic oscillations during Stage 4 in the early Cambrian (Haq and Schutter, 2008; Nielsen and Schovsbo, 2015).

Based on the analysis of microfacies and depositional environment, we discussed the depositional environment evolution and proposed a depositional model of the Qingxudong Formation in the Huayuan area (Fig.8).

6.Conclusions

Four stratigraphic sections in Huayuan area,northwestern Hunan Province, southern China, were analyzed to distinguish the microfacies types in the Qingxudong Formation and reconstruct the paleoenvironment.According to abiotic particles and calcimicrobes,twelve microfacies were identified,and a depositional evolution model from carbonate ramp to rimmed platform,and then to tidal flat,was proposed.

1) The first member of Qingxudong Formation corresponds to a ramp deposition, which is further divided into the middle ramp and outer ramp in the study area.The outer ramp consists of microcrystalline calcite and quartz grains; while the middle ramp has a gradual increase in grain size and bed thickness, containing trilobite fossils and wellsorted, subrounded to rounded intraclasts.Numerous calcareous algae suggest bioherms, and algal intraclasts and ooids indicate a fluctuating shoal environment.

2) The second member corresponds to a rimmed platform deposition, with shallow water and high energy.It is featured by microbial bioherms and a synsedimentary fault.Epiphyton framestones are predominantly developed.Slope deposits are observed at Yanke section, featured by Renalcis bafflestone.

3) The third member corresponds to a tidal flat deposition, which is near water surface and of low evergy.

4) The evolution of paleoenvironments suggests a gradual fall of sea level during the deposition of Qingxudong Formation, consistent with the global eustatic oscillations during the Age 4 of the early Cambrian.

Authors'contributions

ZTSanalyzed the microfacies,participated in field investigation and designed and drafted the manuscript.DMZconceived the study, participated in field investigation, and discussed the manuscript.JTanalyzed the microfacies.PJSparticipated in the figure design and performed the statistical analysis.ZFLparticipated in field investigation and drew the figures.HMexamined the thin section.All authors read and approved the final manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

National Natural Science Foundation of China(NSFC, No.41972104) is thanked for funding this research.We thank Jing-Juan Li and Ke-Wei Xu for their help in field investigation.We thank Fang Xiang for examining thin sections.We are very grateful to the reviews and editors for their pertinent suggestion and hard work.