Stromatolite characteristics of Mesoproterozoic Shennongjia Group in the northern margin of Yangtze Block, China

Hong-wei Kung, Zheng-xiu Fn, Yong-qing Liu, Nn Peng, Zhi-i Zhu, Zhen-rui Yng, Zhi-xin Wng, Hui-ling Yu, Qun Zhong

a Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China

b College of Resources and Environment, Yangtze University, Wuhan, 430100, China

c Administration of Shennongjia National Park, Shengnongjia, Hubei, 442421, China

Keywords:

Stromatolite

Mesoproterozoic

Shennongjia Group

Shennongjia Area

Northern Margin of Yangtze Craton

Geological survey engineering

China

A B S T R A C T

Mesoproterozoic Shennongjia Group in Shennongjia Area can be divided into three subgroups in ascender order.Of which the lower subgroup includes Yingwodong, Dayanping, Macaoyuan, Luanshigou,Dawokeng and Kuangshishan formations; the middle subgroup is formed by Yemahe, Wenshuihe and Shicaohe formations; the upper subgroup consists of Songziyuan and Wagangxi formations.Stromatolites developed very well in the carbonate rocks of each subgroup in Shennongjia Group.Based on descriptions of stromatolites macrotypes and their characteristics, this paper studied the formation environments,discussed the relationship among types, sizes, abundance of stromatolites and sedimentary environment,and established the formation and development pattern of stromatolites.As a result, this research also reveals the paleoenvironment and paleoclimate during the period of the Shennongjia Group deposited,which is beneficial to the study of paleoenvironment, paleogeography and paleoclimate, stratigraphic succession and regional correlation of the northern edge of Yangtze block.Stromatolites of Shennongjia Group are mainly conical, columnar, domal, wavy, stratiform and stromatolite reefs.The columnar and conical stromatolites are well developed.Conical stromatolites are mainly monomers, with a variety of pyramidal types, ranging in diameter from a few millimeters to several meters and formed in the high energy subtidal zone and tidal lagoon environment.Most of the columnar stromatolites are medium to small sizes implied a wide and gentle slope environment at that time.Stratiform (including wavy)stromatolites are larger scales and extends far away and distributed most widely in almost every horizon in the carbonate rocks.Stratiform stromatolites can be formed in low energy environments such as subtidal and intertidal zones and supratidal belts.Wavy stromatolites often developed in the hydrodynamic energy condition from weak energy intertidal zone gradually strengthened to the below of the high energy supratidal.Although stromatolite reefs can be a single or multiform combination, they developed mainly consisted of laminar or small walled columnar and large domal stromatolites.Shicaohe Formation also partially developed large domical stromatolites, the depositional environment is from the upper intertidal to supratidal zone.Stromatolite in Shennongjia Group usually appears as a combination of “Stratiform(wavy)-dome-columnar-coniform” or “stratiform-dome-coniform-columnar-dome-stratiform” vertieally,which represents the seawater depth from shallower to deeper or from shallow to deep and then to shallow again.These phenomenons generally reflected a stable sea level and companied with a high frequency oscillation.Comprehensive researches on the stratigraphy, sedimentary facies, sedimentary environment and the stromatolite types and their characteristics in the Shennongjia Group indicated that the Shennongjia Area is located on a gentle slope of carbonate platform in the passive continental edge,generally, i.e., one of warm and humid climate shallow water zone or/and a cold-drought climate, and had been experienced with eustatic cycles during the Shennongjia Group deposited.

1.Introduction

The study of stromatolites in Shennongjia Group began in the early 1970 s.The list of stromatolites collected in Shennongjia and named by Cao Ruiji, Zhang Luyi and Cao Renguan was first reported by the Hubei Regional Geological Survey team in 1974.In “the Upper Precambrian in the Shennongjia Region” research project, Zhu Shixing and others collected a large number of stromatolite samples,detailedly studied on the morphological characteristics of stromatolites, conducted a systematic classification and naming by binomial system of taxonomy.They determined 31 groups and 69 forms of stromatolites including 3 new groups and 27 new forms (Li Q and Leng J, 1987), which provided very valuable dates for us.The stromatolites were divided into five three-level cycles and regional correlation was conducted(Li Q and Leng J, 1987).For example, the stromatolites were correlated with the upper part of the Changcheng Group and the Jixian Group of the Jixian Section.In addition, other researchers have also compared the stromatolites of Shennongjia Group with those from other areas.For example,Cao RJ et al.(1980) believed thatConophyton lituusin the Meidang Formation of Kunyang Group in Yunnan Province was similar to the Taizi Formation of Shennongjia Group, andKussiella kussiensisfrom Luoxue Formation was basically the same as the Shicaohe Formation of Shennongjia Group.Oshania yunnanensisfrom Dalongkou Formation of Kunyang Group in Yimen, Yunnan Province, appeared in the Luanshigou Formation of Shennongjia Group, in Heiwan of Jiuchong Community, Shennongjia Forest Area, Hubei Province.Zhao WJ et al.(1989) believed stromatolites from the Dayanping Formation can be compared with the upper subgroup of Dagushi Group in Dahong Mountain area, andStratifera,Conophyton,BaicaliaandCollonnellain the lower sub-group of the Dagushi Group can be compared with the upper subgroup of the Shennongjia Group.Very few studies have been done since then.

However, the stratigraphic correlation marked by stromatolites morphology and assemblages is no longer valid after the new accurate chronological data redefined the attribution of many Mesoproterozoic and Neoproterozoic strata, such as the stromatolites in Jixian Section (1670-1400 Ma), although there are many similarities in stromatolites types and assemblages with Kunyang Group (1400-1000 Ma),Shennongjia Group (1400-1000 Ma) and Dagushi Group, but its geological age is different, the stromatolites in the Jixian Section were much older than those in Shennongjia Group.The stromatolites in the Luoxue Formation of the Dongchuan Group (＞1400 Ma) and the Shicaohe Formation of Shennongjia Group (age of intrusive rocks ＜1200 Ma), the Dalongkou Formation (＜1000 Ma) and the Luanshigou Formation (＞1200 Ma), the Meidang Formation (＜900 Ma)and the Taizi Formation (Cao RJ, 1980) are completely out of proportion in terms of their geological age.

This makes us have to think: is the stratigraphic correlation established mainly by the morphology or assemblages of stromatolites reliable? Does stromatolites have regional stratigraphic correlation significance? What are the controlling factors of stromatolites and their morphology or assemblages? In recent years, we have observed, described and collected a large number of stromatolite samples in Shennongjia area, expecting to have a thorough and comprehensive understanding of the stromatolites in Shennongjia Group through systematic description and analysis, and to discuss the significance of stromatolites in stratigraphy and sedimentology.

2.Geological background

Shennongjia is located in the border area between Sichuan and Western Hubei Province, and the northern edge of Yangtze block.During the long-term geological development and evolution process, it has undergone multi-stage deformation and metamorphism and magmatic activity transformation, the geological structure is relatively complex.It can be divided into two parts: basement structure and sedimentary caprock, Shennongjia Group belongs to the Shennongjia fold basement unit (Fig.1).And consists of a group of clastic and carbonate rocks of 1400-1000 Ma with three large sedimentary cycles.The paleotectonic background of the sedimentary period tends to be stable (Fig.1), i.e., it is the product of passive continental margin (Kuang HW et al.,2018).

Fig.1.Distribution of stratigraphy and stromatolites in Shennongjia Group.Measured sections: ① -Shennongding; ②-Guanmenshan;③-Tiechanghe; ④-Yingwodong; ⑤-Dayanping; ⑥-Macaoyuan; ⑦-Songziyuan.Chronological data: ①-1165±14 Ma (Tuff SHRIMP U-Pb,Deng Q et al., 2013); ②-1157±19 Ma (Tuff, Wang et al., 2013); ③-1139±29 Ma (Tuff, LA-ICPMS, Qiu YS et al., 2013); ④-1215.8±2.4 Ma(Tuff SHRIMP U-Pb, Li HK et al., 2013); ⑤-1180±14 Ma (Tuff, Du QD et al., 2016); NC-North China Craton; YB-Yangtze Craton.

In recent years, the stratigraphic sequence of Shennongjia Group has been redefined through detailed field and laboratory studies of sedimentary stratigraphy combined with chronostratigraphy and sedimentary geotectonics, and divided into three subgroups according to sedimentary cycles: the lower subgroup consists of Yingwodong, Dayanping,Macaoyuan, Luanshigou, Dawokeng and Kuangshishan formations, the middle subgroup consists of Taizi, Yemahe,Wenshuihe, Shicaohe formations, the upper subgroup consists of Songziyuan and Wagangxi formations.The stromatolites are developed in the carbonate rocks of Luanshigou,Dawokeng, Kuangshishan, lower Taizi, middle and upper Yemahe, the upper part of Wenshuihe Formation, Shicaohe and Wagangxi formations, their types and assemblages have different characteristics respectively.Especially in Shennongding Section, because the new highway brings convenience to observation.

Shennongding Section outcrops the middle part of Luanshigou Formation, Dawokeng, Kuangshishan and Taizi Formation.Carbonate rocks are mainly dolomite, which can account for more than 70% of the total thickness, their basic types belong to primary or syngenetic origin, but some of them have been transformed by post-diagenetic dedolomitization, silicification and recrystallization, followed by argillaceous dolomite, chert, quartz sandstone and sandy dolomite.Stromatolite dolomites, which account for more than 50% of the total thickness.Through more than 4 years of field reconnaissance and profile measurement, detailed observation and description, polished section and thin section analysis, the macroscopic and microscopic characteristics of stromatolites are described in detail.Types and characteristics of stromatolites in Shennongjia Group, especially the Luanshigou, Dawokeng and Kuangshishan formations in Shennongding Section are basically ascertained, considering the assemblages of stromatolites, the relationship between the distribution of stromatolites and sedimentary environment was analyzed, and the controlling factors of stromatolites formation were discussed.It provides a basis for further study on sedimentary, stratigraphic relations and sedimentary basin evolution process in Shennongjia Group.

3.Main types and characteristics of stromatolites in Shennongjia Group

According to its macroscopic morphology, stromatolites of Shennongjia Group can be divided into stratiform,columnar, conical, domal (spherical or hemispherical) and irregular (including stromatolite reefs) (Fig.2).

Fig.2.Main types of stromatolites in Shennongjia Group.a-Lamellar stromatolites, the basic layer is plat and is not obvious fluctuated,Kuangshishan Formation, Shennongding Section (SK3174); b-gentle-wave stromatolites, the basic layer for the gentle-wave, lamellar, Dawokeng Formation, Shennongding Section (SD3011); c-wavy stromatolites in Luanshigou Formation, Songluo-Xujiazhuang; d-unbranched columnar stromatolites, Panshui village, Songbai Town; e-laterally linked columnar stromatolites in Luanshigou Formation (SL1300), Shennongding section; f-cabbage stromatolites in Luanshigou Formation, Songluo-Xujiazhuang; g-stratiform-columnar stromatolites, the lower part is stratiform, the upper part is two columnar stromatolites, Luanshigou Formation (SL1065), Shennongding Section; h-simple branched columnar stromatolites, Wenshuihe Formation, Panshui village, Songbai Town; i-complex branched columnar stromatolites, divergent from the root,Yemahe Formation, Tiechanghe Section; j-large domal stromtolites of Kuangshishan Formation, Shennongding Section (SL3136); k-large domal stromtolites, Luanshigou Formation, Songluo; l-small irregular domal stromatolites, Luanshigou Formation, Songluo; m-conical stromatolites, the basic beds are gentle cones, Kuangshishan Formation (SK3122), Shennongding Section; n-conical stromatolites, the basic layer is conical, Dawokeng Formation of Shennongding Section (SD3097); o-conical-columnar stromatolites, the basic bed is domal, conical, silicon filled, Kuangshishan Formation (SK3202), Shennongding Section; p-stromatolite reefs consisting of a conical stromatolite at the bottom and a domal stromatolite at the top, Dawokeng Formation (SD3096), Shennongding Section; q-stromatolite reefs consisting of siliceous conical,strati-conical and strati- columnar stromatolites, Dawokeng Formation (SD3051), Shennongding Section; r-stromatolite reefs consisting of columnar (conical-columnar) with varying size and density, Kuangshishan Formation (SK3142-SK3143), Shennongding Section.

3.1.The macroscopic morphology of stromatolites

Stratiform stromatolites: stratiform stromatolites are the most common types of stromatolites in Shennongjia Group,which can be divided into lamellar, undulating (gentle wave and folded), and widely appear in Luanshigou Formation,lower part of Dawokeng Formation, lower part of Kuangshishan Formation and middle part of Shicaohe Formation.The most common types are parallel stratiform and gentle wave, dark algal layers often in the form of bands of black siliceous or chert, occasionally containing tiny domal stromatolites.Which often occurs in large sections, and some continuously extend about 5-10 m, gradually evolve into stromatolite reefs, which can also appear at the bottom or top of a single cycle.The basic layers of stratiform stromatolites are mostly wavy and flat lamellar, while the basic layers of wavy stromatolites are mainly gentle domal, domal, wavy and conical (Fig.2a-c), the symmetry and inheritance are from high to average, no walls.Both of them are found in micritic dolomite, crystal powder dolomite, fine crystal dolomite,siliceous dolomite, dolarenite, etc.The associated sedimentary structures are parallel bedding, horizontal bedding, mud fracture, bird’s eye structure, tent structure, halite pseudocrystal,some have erosion surface at the bottom.The formation environment is the upper of the intertidal zone, supratidal zone and the low energy subtidal zone.

Domal stromatolites: most of the basic layers are domal,strong domal, and gentle domal, wavy, no walls (Fig.2j-l),they are developed in siliceous fine crystal dolomite, crystal powder dolomite, sandy dolomite and siliceous dolomite.The associated sedimentary structures are parallel bedding, graded bedding, ripple marks, scour surface and wavy bedding,which mainly developed in intertidal zone.

The small domal stromatolites of Shennongjia Group are developed in all horizons, usually constitutes the transition type from stratiform to columnar in the stratiform-domalcolumnar - coniform assemblages of stromatolites.Only a few large domal stromatolites were found in Luanshigou (Fig.2k),Kuangshishan and Shicaohe formations.

Columnar stromatolites: they are the most widely developed type in Shennongjia Group.According to the diameter and height of the column, columnar stromatolites can be divided into 6 types: supergiant (column diameter ＞ 1 m, height ＞ 10 m), giant (column diameter 50-100 cm, height＞ 1-10 m), large (column diameter 10-50 cm, height ＞1-5 m),medium (column diameter 5-10 cm, height ＞ 50-100 cm),small (column diameter 1-5 cm, height ＞ 10-50 cm), micro(column diameter ＜ 1 cm, height ＜ 1 cm).(Cao RJ et al.,1991; Zhu SX et al, 1980; Shi XZ,1987; Zhang HQ, 1984;Cao RG, 1980).The shapes of the columns are narrowing upward, or /and widening upward, and spindle or dumbbell;from being laterally linked or not among columns, there are isolated or solitary column and divergent columnar and can be further divided into bifurcated columnar (one time) and complex (multiple times) branched columnar, parallel bifurcation and irregular branched columnar (Zhu SX, 1993).The columnar stromatolites of Shennongjia Group consists of regular sub-columnar and conical-columnar stromatolites and basic layers are strong domal, domal, micro domal, box-form,wavy and flat (Fig.2d-e, g-i).Most of the columnar stromatolites do not bifurcate, and those were branched mainly by one bifurcation (Fig.2d-e, g-i).And having a good medium inheritance and symmetry.They are mainly developed in siliceous dolomite, fine crystal dolomite, crystal powder dolomite, dolorudite and dolarenite with mainly grey and dark grey; the associated sedimentary structures are sand pillow structure, erosion surface structure, normal graded,fishbone cross-bedding, ripple marks and other structures.The ministromatolite and microstromatolite (Cao RJ et al., 2009)are only found locally in the Wagangxi Formation.In general,large columnar stromatolites (more than 10 cm in diameter and 1 m in height) and mini- or micro- stromatolites are rarely found in Shennongjia Group, mainly medium and small size.

Conical stromatolites: the basic layers of conicalcolumnar stromatolites are mainly conical and domal, while the rest are micro- dome, strongly domed, pointed and coneshaped and box (Fig.2m-o).Most stromatolites have no wall or local wall, a few with a single wall, no bifurcation.It is developed in micritic dolomite, fine crystal dolomite,siliceous fine crystal dolomite, crystal powder dolomite and dolarenite with mainly light gray, gray and dark gray.The associated sedimentary structures are parallel bedding, graded bedding, ripple marks, scour surface and wavy bedding.The forming environment is from intertidal zone to high-energy subtidal zone.Although conical stromatolites are not as extensive as columnar stromatolites, but conical stromatolites are undoubtedly a very significant type in Shennongjia Group.In the middle and lower part of the Luanshigou Formation, the upper part of the Dawokeng Formation, the majority of the Kuangshishan Formation and the lower part of the Taizi Formation, the size of conical stromatolites are from small to giant, and their morphologies are diversity, e.g., sharp cone,obtuse cone, columnar cone (or conical column), laminar cone are very developed.

Stromatolite reef: stromatolite reefs can be divided into biostrome and bioherm, which are macroscopically consistent with the concept of organic reefs: A dome, lenticular, or other sharply bounded masses built by sessile organism embedded in rocks of different lithologies (Cumings ER, 1932; Bogs JS,2009; Mei MX et al., 2011).Stromatolite reefs are well developed in the Luanshigou Formation, Dawokeng Formation and Kuangshishan Formation of Shennongjia Group.Some expressed horizontal lamellar stromatolites gradually change upward into gentle wavy stromatolites (Fig.2p), some are silicified domal and conical columnar stromatolites aggregates (Fig.2q), or assemblage of columnar and conical stromatolites with clear interfaces in context,narrowing or pinching out laterally (Fig.2r).Stromatolite reefs usually develop at the top of the cycle in the transition zone between intertidal to supratidal area- or in the highenergy subtidal zone where waves were resistant.

In general, there are abundant types of stromatolites in Shennongjia Group.Stromatolites are developed in different monomer types such as stratiform, domal, columnar, coniform and bioherm stromatolite.In addition to the five amentioned above and their transitional types, there also are cabbage and spherical stromatolite, and so on (Fig.2f), they are all occurred in various types of dolomites.

3.2.The microscopic morphology of stromatolites

The microstructure of stromatolites are generally considered to be a reflection of microbial community growth,decomposition and lithochemical assemblages (Golubic S,1976; Bertrand-Sarfati J, 1983; Knoll AH and Sergeev VN,1995; Kah LC and Knoll AH, 1996; Knoll AH and Semikhatov MA, 1998; Grotzinger JP and Knoll AH, 1999;Lee SJ and Golubic S, 2000; Reid RP et al., 2000), and the morphology seems to be strongly influenced by physical factors, especially water depth, hydrodynamic conditions and sediments input (Cloud PE and Semikhatov MA, 1969;Semikhatov MA et al., 1979; Semikhatov MA and Raaben ME, 1994, 1996; Andres MS and Reid RP, 2006).Therefore,stromatolites laminations which recorded the growth process of microscale and macroscale, can be regarded as the most basic aspect of the morphology (Walter MR, 1992).Each lamina represents the active degree of algae growth on the surface of algae mat during deposition, thus recording the morphology of sedimentary action surface (Hofmann PF,1969; Donaldson JA, 1976), the relative growth rate of stromatolites (Hofmann PF, 1969; Semikhatov MA et al.,1979), the effects of sediments filling, wave and/or water flow action on algae mat growth (Hoffman PF, 1967; Logan BW et al., 1974).The laminations not only record the morphology of microbial mats on the seabed, but also the geometric morphology of the continuous laminations recorded the growth of the microbial mats over time, which are reflected in the morphology of stromatolites (Fig.3).However, it is difficult to quantitatively analyze the growth process, rate and control of stromatolites based on the records of ancient stromatolites layers.According to the microstructure and structure of Shennongjia stromatolites, the laminas are mainly in discontinuous, banded, blanket and patchy (clot) forms(Fig.3).On the one hand, it is affected by diagenesis such as recrystallization, which destroys algal layers, on the other hand, stromatolites are developed in turbulent waters with high hydrodynamic energy or affected by wind and waves,which makes the algal laminas become intermittent strips or patches.

Fig.3.Microscopic characteristics of stromatolites in Shennongjia Group.a-The dark lamina is discontinuous, slightly raised and arc-shaped,and the bright lamina is relatively wide with slight recrystallization and uneven grains, and interspersed with other minerals, such as iron, appear red in the profile, showing a weak hydrodynamic condition; b-regular ribbon, dark lamina extremely developed, continuous regular shape,bright layer extremely bright, recrystallization, stromatolites should be grown in wave-washed environments; c-irregular intermittent ribbons,the column is surrounded by sparry dolomite filling without the interference of terrestrial materials; d-stromatolites are regularly ribbons, but due to the severe silicification, the original characteristics are unvisible; e-regular ribbons, similar to the characteristics of a, except that it is all micritic dolomite without recrystallization or silicification, and the bright layer is also mixed with irregular linear shape, without terrestrial source materials, and the growth of stromatolites has not been stopped; f-as a result of recrystallization the laminas of stromatolites become inconspicuous and the dark laminas are irregular patches; g-discontinuous linear, slightly recrystallized, conical stromatolite; h-wave laminas,dark lamina undulate, each ribbon consists of a series of parallel wave laminas; i-the dark lamina was irregular and clumpy, bearing quartz particles occasionally, showing that stromatolites were disturbed by storms before they were consolidated; j-it is regular linear or intermittent linear, obvious recrystallization, some lamina are intermittent, conical stromatolites; k, l-the same field of view, the left is single polarized light,the right is orthogonal polarized light, the stromatolites lamination due to the silicon effect between the light and dark lamination boundary is not very clear, it can be seen that the dark layer top boundary, the overall blanket.

4.Stromatolites assemblage types of Shennongjia Group

The stromatolites of Shennongjia Group are mainly developed in Shennongding Section and Guanmenshan Section.This is mainly own to the convenience of the newly opened highway in the scenic area.Although there are many reports about other strata, but it is difficult to find them due to weathering and leaching.Therefore, this paper mainly focuses on the Luanshigou-Dawokeng-Kuangshishan formations of Shennongding Section and Shicaohe Formation of Guanmenshan Section.The main assemblage types of stromatolites can be divided into 20 types (Fig.4):

(i) The stromatolites of the Luanshigou Formation are abundant in numbers and types.Stromatolite reefs (reefs)formed by the aggregation of stratiform, columnar and straticolumnar stromatolites are developed.The stromatolite cycles that occurred intensively for 7 times are explained as follows(Fig.4-(1)-(7)).Generally speaking, the lower part is mainly stratiform, small domal and columnar stromatolites; The middle part is stratiform and massive stromatolite reefs, and the upper part is stromatolite reefs consist of domal and columnar, which reflects the process of declining sea level,decreasing supply of terrigenous materials and improving growth environment of stromatolites.

Stromatolites are relatively few, it is characterized by a small number of disorganized small conical, domal and wavy stromatolites in the middle and thick dolomites with gravel clastic layers, accompanied by ripple marks, reflecting the turbulent water and input of terrestrial materials, sometimes there are oolitic layers.The layers in lamellar stromatolites which contain small domal, columnar, conical stromatolites,and the forms are irregular, the forms of laminated masses are mostly distorted, reflecting the strong hydrodynamic condition, the overall environment is a strongly eroded intertidal to subtidal channel (Fig.4-1).This type located in the lower part of the Luanshigou Formation (Layer 1-23).

Laminar stromatolites are common, small domal,columnar or gentle wavy stromatolites are often intercalated in the layers; some laminates are siliceous ribbons, which tend to increase upwards.The color of dolomite changes from grey to grey-black, mound appears on the top of lamellar stromatolites (giant wave domal) and single mound usually 2-3 m wide and 20-50 cm high.The thickness of a single cycle is 5-10 m, and the top shows wavy undulations, but the interior is massive, no laminate or silicification obviously(Fig.4-2).Lower part of the Luanshigou Formation (Layer 70-110).

Lamellar stromatolite transfer to small domal stromatolite and gradually evolved into columnar stromatolites with silicified algal laminations and even locally small domal stromatolites were completely silicified.Although mainly small columnar stromatolites, they are very compact and rarely branched or only simply parallel bifurcated.Stromatolites reflect typical intertidal environments (Fig.4-3).It is in the lower part of the Luanshigou Formation (Layer 111-124).

The lamellar argillaceous dolomite is interbedded with the mixed grain dolomite (mound) (Layer 125-137), and then becomes lamellar siliceous dolomite (Layer 138-157) -massive stromatolite dolomite (malachite) (Layer 158-163) -lamellar argillaceous dolomite interbedded with massive stromatolite dolomite (Layer 164-177) upward (Fig.4-4).

The addition of terrigenous materials, the formation of copper deposits and the decrease of stromatolites generally reflect the intertidal environment with relatively shallow water and enhanced evaporation (Gong L and Chen TY,2012).

Purple-red argillaceous or silty dolomite interbedded with silicified small columnar stromatolites (Layer 200-230).There are mainly single columnar, usually wavy stromatolite changed upward into wide domal, and then medium columnar and wall stromatolite, a small amount of them are thiner in the lower widening upword wide.The rhythmic laminas, which consist of 50-100 cm thick reefs and algal layers with horizontal laminas, are formed by the stratiform-domalcolumnar stromatolite assemblage (Fig.4-5; The upper part of the middle member of the Luanshigou Formation, Layer 231-290).

The upper member of the Luanshigou Formation is mainly purple-red argillaceous dolomite with horizontal and tidal bedding.And among them (Layer 299-328), there are more than 8 stromatolithic reefs composed of small- columnar and wall stromatolites, which beyond 1m in length and 10 m in width for a single bioherm.When the argillaceous content increases, the host rocks are usually red, yellow or ogrey white, so the color of stromatolites is also red or yellow.The shapes of columns are more diverse, including short columnar domal, thinning upward or the same size in vertical, and some of them are widening upward and look like a back coniform.There are both columnar and bifurcated ones, both horizontal lamellar and laminas with small domal columnar ones, both symmetrical and asymmetric columnar srtomatolite, mostly without walls.The most of the stromatolite columns have no external material filling, and asymmetrical filling materials come from in situ (Fig.4-6).It is the most prosperous period for stromatolites in the Luanshigou Formation.Tidal bedding is common in host rocks, the direction of the column always inclined with the strata suface.The morphology and occurrence of stromatolites reflect a high-energy environment in the lower part of intertidal zone in the hot climate during this period.

Lamellar siliceous algal dolomites is interbedded densely with red argillaceous dolomite.The stromatolites are horizontally lamellar large or small gentle wavy and domal.There are more siliceous intraclasts among laminas, such as algal debris, oolites and internal debris with different grain size, which reflects the high-energy intertidal environment,and gradually presents wave marks, mud cracks, halite pseudocrystals and gypsum-soluble gravel dolomite, while is a supratidal environment on the top (Layer 1-86, top Section 2 of the Luanshigou Formation) (Fig.4-7).

(ii) The lower part of Dawokeng Formation is stratiformgentle wavy-small domal or domal columnar stromatolites.The upper and middle parts are large domal and domal columnar stromatolites.And the top part is a large number of projectile conical stromatolites.

The lower part of Dawokeng Formation (Layer 1-25): The interbedding of agate conglomerates with wavy and lamellar stromatolites is in the bottom (Layer1-20).Upward mainly developed lamellar and wavy or gentle wavy stromatolites,and some of the layers are slightly silicified, reflecting the frequent fluctuation of fluvial-supratidal-intertidal zone.Further up (Layer 26-50) are mainly lamellar and gentle wavy stromatolites, with occasional small stratiform conical or stratiform columnar and cabbage stromatolites, the columns are irregular or asymmetrical with waving ripple layers,formed stromatolite reefs usually composed of gentle wavy stromatolites with stratiform columnar or conical stromatolites.Among them, Layer 44-45 are oolitic layers with bi-directional cross-bedding and Layer 50-51 are silicified stromatolites bearing abundant stromatolites with different forms and sizes (Fig.4-8).The suitable environment for such stromatolites may be the upper part of the intertidal zone.

Layer 51 to 90, the silicification of algal layers are obvious.Strong domal, stratiform columnar, stratiform conical and blunt or sharp conical stromatolites are locally found in the large gentle wavy stromatolites, which have good inheritance, symmetry and no walls.The higher the succession is, the more developed the stromatolites are, with enlarging of single stromatolites, the scale of stromatolites reef also increases.In the upper part (after Layer 90), the large gentle wavy stromatolites developed from domal and columnar gradually evolved into large stratiform conical or columnar conical and hemispherical stromatolites, and at the top (Layer 95-100) are large conical stromatolites.Dolomite also changes from laminar to thick massive (Fig.4-9).The growth of stratified algae is from mat to reef, which reflects the gradual rising of sea level.

(iii) The initial deposits of the Kuangshishan Formation were manganese and iron shale, siltstone and sandstone, and then it entered the period of unprecedented prosperity for stromatolites in the sedimentary process of Shennongjia Group.The stromatolites of the Kuangshishan Formation are mainly conical (columnar) and large domal (spherical)stromatolites.According to their assemblages on the section,the vertical variation of stromatolites can be divided into two categories and six subcategories:

In the lower part (Layer 112-118), the lamellar and domal,conical columnar and conical stromatolites are gradually developed upward in ascender order, but mainly conical stromatolites (in Layer 119-125) (Fig.4-10).

Beginning with the disordered laminated columnarconical stromatolites, upward trend gradually changes into gentle wavy, domal columnar and conical columnar stromatolites to large domal stromatolites (Layers 126-136)(Fig.4-11).

The small conical stromatolites become domal, medium conical and hemispherical stromatolites upward.Stromatolites have different morphologies in the upper and middle parts,which formed bioherm and extremely silicified stromatolites.Conical and conical-columnar stromatolites developed at the top (Layer 137-159) composed of large conical interbedded with domal (spherical) stromatolites (Fig.4-12).

Domal and gentle wavy stromatolites are mainly.Due to the influence of terrigenous matter such as collapse breccia and argillaceous dolomite breccia, stromatolites are not well developed, the stromatolites reefs composed of lamellar and large gentle wavy stromatolites (Layers 160-176) (Fig.4-13).

Gentle wavy and large spherical stromatolites with a few conical (Layer 177-187) (Fig.4-14).

Giant stromatolites reefs are composed by giant domal or spherical and conical-columnar stromatolites on the top(Layer 188-207).Especially, laminas in the large columnar conical or conical stromatolite dolomite are clearly distinguishable (Layer 196-204).Near the top, the stromatolites became smaller, eventually evolved into slender columnar and wall stromatolites that grow perpendicularly to the beds(Layer 206-207) (Fig.4-15).The assemblage types of stromatolites in the Kuangshishan Formation were characterized by conical, large domal and spherical stromatolites (Fig.5), which indicates that the sea level had been continuously rising during the period of Kuangshishan Formation deposited.Until the end of Kuangshishan Formation, the sea level was still high, but relatively stable,the large wall-columnar stromatolites with a diameter of nearly 20 cm and height of 2 m were developed.

Fig.5.Characteristics of stromatolites and their assemblages in Kuangshishan Formation of Shengnongjia Group.

(iv) The characteristics of stromatolites in Yemahe Formation are very clear.Stromatolites in the lower part of the Yemahe Formation are mainly disorderly distributed columnar or branched columnar ones, their columns are far apart from each other, unparallel and usually with walls and good inheritance.Laminas of stromatolites are unparallel too.The hydro-environment is relatively strong and a lot of terrigenous matters inputted.Stromatolites in the upper part are mainly short columnar with red argillaceous filling among columns (Fig.4-16).

(v) Slender columnar and small branched columnar stromatolites developed in the upper and top of Wenshuihe Formation (Fig.4-17).The organic and argillaceous contents in the strata are relatively high and the horizontal bedding is well developed, reflecting that the stromatolites of Wenshuihe Formation should develop in the relatively deep water and quiet subtidal environment.

(vi) The stromatolites of Shicaohe Formation are mainly lamellar-domal-oblique columnar assemblage, and locally contain large domal stromatolites, reflecting that the upper part of the intertidal zone is the main development environment, while a small amount of oblique columnar stromatolites are developed in high-energy subtidal turbulent water environment.Small stratiform or stratiform-columnar stromatolites are developed well in the lower part of Shicaohe Formation.With increasing of water energy, oblique slender columnar stromatolites are developed in red argillaceous dolomite (silicified), the dip angle is about 60 degrees.In addition, there are large domal stromatolithic bioherm and various small stratiform, wavy and columnar stromatolites developed among each parallel horizons (Fig.4-18).

(vii) The Wagangxi Formation is dominated by conical stromatolites, with a few small columnar, stratiform-columnar and stratiform stromatolites.The conical stromatolites of Wagangxi Formation are vertically developed overlying on the black carbonaceous siltstone with scour surface and small wavy bedding of Songziyuan Formation, which are 30-40 cm long and 20-30 cm wide, and some are broken and disordered upward affected by waves (Fig.4-19).The upper part contains intraclastic dolomite interbedded with thin grayish or lilac argillaceous dolomicrites.The upper is lamellar or stratiform,stratiform-columnar, conical stromatolite dolomite.The third cycle are consist of intraclastic dolomite, stratiform-conical stromatolite, purple argillaceous dolomite (Fig.4-20).The limestone or dolomite clasts filled among the stromatolites,usually several millimeters to several centimeters.There are gray chert block and ribbons in the strata, which are 7-8 cm wide and tens of centimeters long.In addition, there are stratiform and stratiform-columnar stromatolites in the strata,which are 7-8 cm high, but no common.The cycles in the stromatolites are intraclast limestone or dolomite at the bottom, stratiform or stratiform columnar stromatolites upward, and then developed stratiform stromatolites-meso or microcrystalline dolomite and argillaceous conical stromatolites, are supratidal argillaceous dolomites at the top.

5.Relationship between morphology and sedimentary environment of stromatolites

According to the characteristics of macro-, micro- and assemblage stromatolites in Shennongjia Group, macro stromatolites is small stratiform and columnar, and micro ones is dominated by blanket and banded microstructures.By comparing the single morphology with the stromatolite assemblage, the relationship between the stromatolite morphology and the environment of Shennongjia Group is revealed.

5.1.Relationship between different morphology of stromatolites and sedimentary environment

Stratiform stromatolites are the main type of stromatolites in Shennongjia Group.The layers are continuous, flat or curved in the context, where the dark layer is laminated algae,the bright layer is large size and irregular carbonate grain, but no generations and no detrital materials in stromatolites (Fig.3),which developed in lower energy intertidal zone.The second is irregular laminas, which are discontinuous and curved and contain Bird’s eye structures and detrital materials in stromatolites, formed in protected intertidal zones (for example, lamellar stromatolites in the upper part of the Luanshigou Formation).The laminas of the third type is light color and lower content in organic matters, and the lamina controlled by the grain sizes (stromatolites in the lower part of the Dawokeng Formation).It is mainly flat laminasand developed in high-energy environment eroded by waves.Stratiform stromatolites, stratiform domal stromatolites and ruffled structures are mainly developed in the supratidal environment.Stratiform conical (pointed conical),pseudopillar stromatolites, stratiform domal and partially domal stromatolites are generally associated with algal laminas, most of them are developed in the intertidal zone(mainly in the Luanshigou Formation).Therefore, the stratiform stromatolites of Shennongjia Group are mainly developed in the supratidal and intertidal zones.

Gentle wavy and small domal stromatolites often coexist with graded bedding, clastic particles such as quartzs in the layers, and even coexist with oolite.The top of stromatolites have ripple marks and the laminas are irregular, reflecting a relatively high-energy intertidal zone (Section 2, Layer 21 and 13-14 of the Luanshigou Formation, Layer 119 of the Kuangshishan Formation).There are recrystallization and cementation generations in the carbonate rocks between the laminas of gentle wavy stromatolites, and no terrigenous clasts.It reflects the intertidal hydrostatic environment on the leeward side of the bay (Layer 119 of Kuangshishan Formation, Shicaohe Formation).Gentle wavy and domal stromatolites occured between stratiform and conicalcolumnar stromatolites, which are intermediate products from stratiform conical columnar rhythm.The growing environment is also between stratiform and conical columnar stromatolite, but the characteristics tend to stratiform stromatolites.

Conical and columnar stromatolites developed in different environments, but conical and isolated columnar stromatolites are mainly developed in the subtidal zone of deeper water,such as Kuangshishan and Wenshuihe formations.Oblique growth of conical-columnar stromatolites are the products of high-energy subtidal environments (Swett K and Konll AH,1985) developed in the upper part of the subtidal zone or part of them in the intertidal zone, e.g., the oblique columnar stromatolites of Shicaohe Formation and the disorderly branched columnar stromatolites of Yemahe Formation.Hoffman PF (1976) researched the modern stromatolites in Shark Bay and considered that, large wave-resistant columnar stromatolites (30-900 cm in diameter and over 50 cm in height) developed in coastal cape, small slanting columnar stromatolites (listed in rows and inclined toward the sea)(5-15 cm in diameter and less than 10-30 cm in height)formed in the direction of the bay only affected by tidal action.The columns are elongated and perpendicular to the coastline, widened upward and branched, and some are broken off to form a receptacle on the top (calyx shape)(oblique columnar stromatolites of Shicaohe Formation).Most of the conical and columnar stromatolites without wall.The composition and color of the stromatolites are consistent with those of the host rocks, reflecting that they were formed in the subtidal zone.The walled conical and columnar stromatolites may form in the intertidal zone.

5.2.Relationship between assemblage types of stromatolites and sedimentary environment

Stromatolites are usually developed in carbonate rocks,and their assemblage morphology has a good correspondence with sedimentary microcycle sequence (Fig.4-Fig.6).The stromatolites of Shennongjia Group usually develop in the middle and the upper part of the shallowing upward cycle.For example, the stromatolite assemblage of the Luanshigou Formation reflects shallow water environment where organic matter developed or terrigenous material input, and as a result,the intertidal to supratidal zones with certain hydrodynamic forces, tidal channel and high-energy subtidal zone are existed.The stromatolite assemblage of the Dawokeng Formation reflects that it was formed from the subtidal shoal facies to the lower part of the intertidal zone with strong hydrodynamic conditions and the upper intertidal zone to the supratidal zone with lower energy.The stromatolite assemblage of the Kuangshishan Formation indicates that it was formed in the low-energy subtidal-lagoon-low-energy intertidal zone and high-energy intertidal zone.The stromatolite assemblage of the Shicaohe Formation reflects the evolution of sedimentary environment in the supratidalintertidal zone (-tidal channel) - subtidal zone (Table 1).By further summarizing 20 different vertical assemblage types of stromatolites in the above-mentioned horizons, four different types of stromatolites assemblage can be obtained as follows:

Fig.6.Successions of stromatolites in Shennongjia Group in Shennongding Section.(the stromatolite assemblages in the figure corresponds to that in Fig.4)

Table 1.Types of stromatolites and their associated sedimentary structure and formation environment

(i) Small stratiform-columnal (concial) or stratiform(columnal or concial) stromatolites formed stromatolithic reefs, which belongs to intertidal deposit.The Luanshigou Formation, the lower part of Dawokeng Formation and Wagangxi Formation are representative, Fig.4(1), (2), (7), (8),(19) and (20).

(ii) It is composed of stratiform-domal-large conical,domal and spherical stromatolites, which are not only large in size, but also contain multilayered cross-bedded oolitic dolomite.It is the product of the rapid rise of sea level in the high-energy subtidal zone, represented by the top of the Dawokeng Formation and the Kuangshishan Formation, Fig.4(3), (9), (10), (11), (12), (13), (14), (15).

(iii) Branched columnar stromatolites with different dipping directions are obviously affected by wind and waves.Dolarenite and algal debris dolomite are developed, and the algal debris laminas are distributed in clot, which are the products of storm wave near the Storm wave base.Above represented by the Yemahe Formation and the oblique columnar stromatolites in the middle of Shicaohe Formation are also similar, Fig.4 (16), (18).

(iv) The dispersed slender columnar and small simple branched columnar stromatolites, accompanied by argillaceous dolomites in the context, reflect a quiet water environment, is represented by Wenshuihe Formation, Fig.4(17).

6.Discussion: Controlling factors of stromatolites formed in Shennongjia Group

Stromatolite is not a biological entity, but a special carbonate sedimentary body that interacts with carbonate precipitation by microbial activity and binds exotic substances to consolidate diagenesis (Wen ZF, 2005; Mei MX, 2008).In recent years, accurate chronological data obtained by new dating methods have broken the pattern of stratigraphic correlation using stromatolites and their combined forms(previously mentioned), stromatolites of different ages,continents or regions can be similar in type, morphology and assemblage relationship, while the types of stromatolites in the same section can also be different.Therefore, from recent studies, the stratigraphic significance of stromatolites types and assemblages is far less than previously mentioned, on the contrary, the relationship between stromatolites and environment becomes more prominent.Compared with other sedimentary indicators, stromatolites have more definite pointing significance for water environment conditions.However, the specific structures and types of each kinds of stromatolites should be taken into account when applying stromatolites to investigate more detailed environmental conditions, such as water energy, drought conditions and climatic changes (Riding R, 2002).

In addition to blue-green algae, stromatolites can be formed by bacteria and eukaryotic algae.The conditions for the formation of stromatolites in water are as follows: ① The environment is suitable for the growth of stromatolites microorganisms; ② The growth rate of microorganisms is faster than the consumption rate; ③ The sedimentation rate is enough to preserve the structure without hindering the growth of microorganisms; ④ Stromatolites accreted faster than they were destroyed by other organisms drilling or other agents(Walter MR, 1976).This explains well why tidal flat is a good place for the growth of stromatolites, it also explains why stromatolites flourished in the Precambrian, and then decayed in phanerozoic since the development of metazoans and in a cold snowball climate.However, what factors affect the form,size and formation environment of stromatolites during their flourishing growth, which needs more detailed research and explanation.Are stromatolites biogenic or chemical, or a combination of multiple factors? The diversity of macro and micro-morphology of stromatolites leads to the problem is still inconclusive.As for the origin of stromatolites, some scholars hold the view that the morphology of Precambrian stromatolites was mainly restricted by microbial evolution and biochemical effects (Luo P et al., 2013); but other sedimentologists believe that the morphology of stromatolites is mainly controlled by environmental factors (Logan BW et.al., 1964).Suosaari et al.(2019) recently published an article in memory of the master of stromatolites, Robert n.Ginsburg,pointing out that“stromatolites can be considered both as fossils and as sedimentary structures dependent on the energy in the environment of deposition.When physical forces are strong, the environment is the main control on the morphology; however, when physical forces are weak,biological communities become the main drivers of morphology”.At present, there are two basic consensus on the morphology of stromatolites: (1) The morphological characteristics of stromatolites are influenced by environmental factors and restricted by microbial groups;(2) The macroscopic characteristics of stromatolites reefs and columnar stromatolites are greatly influenced by the environment, while the microcharacteristics of stromatolites,such as column bifurcation, side decoration, bedding and microstructure, are mainly controlled by the evolution of microbial population (Ginsburg, 1991; Cao RJ et al., 2006).This recognition of Ginsburg (1991) still shines with the brilliance of thought 30 years later.

Normally, the phototropic growth of stromatolites results in convex upward growth, which is also due to the arc or conical overlap of upward convexity in the longitudinal section, thus forming a laminated biosedimentary structure.Its growth process is affected by external forces such as water flow and wave, and its cylinder is inclined to the direction of water flow, or because of different water depths, the lamina or columns were filled with sands or detrital materials, showing different structures and shapes.On the other hand, although many scholars from a variety of ways of modern and ancient stromatolite is carried out deeply, but the biological mechanisms that control the morphology of stromatolites is still unclear, especially the discovery of some special forms of stromatolites, which adds to this uncertainty.For example,there are more than 10 morphological explanations of conical stromatolites (Mei MX et al., 2015).For columnar stromatolites, they are usually stacked by many domal raised beddings.In the profile, they show columnar structures of different shapes, these columnar structures are not single organisms, but a kind of bio-sedimentary structures formed through the life and activities of microorganisms.It is an enigma how to happen the columnar structures of different shapes and which controlled the changes of their characteristics in geological period.As Hofmann HJ (1987)said, “...We still don’t have a theory about stromatolites and a model that shows how characteristics change over time”.Grotzinger JP and Knoll AH (1999) believed that there were many factors affecting the development of stromatolites,including light intensity, salinity, nutrient supply, flow rate,distribution of the grain sizes of sediments, differentiation of microbial mat community and carbonate saturation state.In principle, however, the growth of stromatolites can be seen as dependent only on three basic evolution processes: (1) Growth and decomposition of microbial mat or biofilm; (2) Deposition of sediments; (3) Mineral precipitation.They believe that the interaction of these processes will account for most of the records of stromatolites (Cao RJ et al., 2001).

6.1.Biological effect

The formation of stromatolites is considered to be related to biological processes (He JY and Meng XH, 1987), the secretion of biofilms by cyanobacterial filaments during their growth, on the one hand, can promote the supra-saturated precipitation of CaCO3; on the other hand, fine external particles can be trapped and deposited together to form granular layers.If the granular layer is too thiner, the algaes can grow through the capture lamina.If it is too thicker, a new algae mat will grow on it.Thus, it will continue to repeat the process, and form lamellar structure.The shape of algae mat is controlled by many factors, (1) The free growth degree of algae photosynthesis; (2) Strength of expulsion gas; (3) Stirring intensity of external current (Meng XH et al., 1982).This is the common to all stromatolites.

Microflora mainly affect the microstructure of stromatolites, such as changes in microstructure and the thickness of dark laminas.When there are fewer microorganisms, the dark laminas are thinner and are sometimes discontinuous; when microorganisms flourish, the dark laminas are thicker and more continuous (Fig.3).Microbial growth will hold back sediments that pass through,as a result to determine where the bioherm may have formed;calcium carbonate sediments are trapped and barriered by organisms, which speed up the growth of bioherm.Surface encrustation forms stable crustal sediments, so that normal seabed erosion can not move the sediments, while the topographic uplift of bioherm can colonize contracture organisms and cohesive organisms, thus making the hills develop better (Fig.6 stromatolithic reefs).In the hydrodynamic high-energy zone, a large number of particles are brought.Trap capacity of algae mat is saturated.The excess debris particles accumulate and stay in the sag, which is eroded by water and filled with debris materials, inhibiting the growth of the algae mat.The algae can only continue to grow in the protuberances and are thus divided into isolated bioherm (or columns).In the soft part of the head of algae, it can still be eroded by water flow, and then will continue to grow on the top of the remaining columns to form the new branched column.If the excess clastic material covers the algal roof, the algal mat stops growing completely and cannot cross the lamina to form a new algal mat, the rock-building role is completely terminated.In strong currents and tidal channels, algal mat deposits may be strongly eroded to form ridges structure (Meng XH et al., 1982).

6.2.Hydrodynamic conditions and water depth

Hoffman PF (1976) concluded that specific stromatolites represent unique sedimentary environments and are associated with inferred paleo-water depth, water energy, and sediment supply.The characteristics of stromatolites in the upper intertidal and supratidal are that they are generally flat (with low lamina radian).The characteristics of stromatolites in intertidal zone and shallow tide (above normal wave base) are related to internal clastic and/or fine-grained clastic materials,forming domal stromatolites with lower curvature and irregular columnar stromatolites.The characteristics of stromatolites under deep subtidal (below the normal wave base) are as follows: columnar stromatolites, conical stromatolites with larger lamina curvature are developed, and there are no exotic sediments among stromatolitic columns.According to previous experience, the relationship between stromatolites morphology and relevant sedimentary parameters is quite close, and similar relationships in modern environments can be observed too.Due to local and regional changes in accommodation space, differences of hydrodynamic mechanisms and sedimentation patterns will lead to stromatolite highly potential differentiation (Andres MS and Reid RP, 2006; Bartley JK et al., 2014).

Through statistical analysis of the associated sedimentary structures and development environment of stromatolites in Shennongjia Group (Table 1), the main associated sedimentary structures of stratiform stromatolites are horizontal bedding, wavy bedding and rhythmic bedding,some wavy stromatolites have ripple marks and scour surface structures.In the host rocks of stratiform stromatolites,fragmentary materials are poorly sorted and rounded, and some contain argillaceous, which indicate that stratiform stromatolites (including lamellar, gentle wavy and wavy stromatolites) can develop in a wide environment from stable and low-energy environments, as well as in areas where terrigenous materials are abundantly supplied, to turbulent intertidal or supratidal zones.But it mainly developed in the upper intertidal zone, and a few in the supratidal zone, the lower intertidal zone and low-energy subtidal zone,occasionally developed in strong hydrodynamic conditions.The main associated sedimentary structures with stratiformcolumnar, conical- columnar, cabbage stromatolites are wave bedding and horizontal bedding, indicating that they are formed in intertidal zones with weak hydrodynamic conditions.The main associated sedimentary structures of columnar stromatolites are horizontal bedding, tent structure,scour surface, occasionally algal clastic spherules and broken algal laminas, indicating that columnar stromatolites can form not only in high-energy beaches under strong hydrodynamic conditions, but also in weak hydrodynamic conditions in the lower part of intertidal zone.Domal stromatolites are associated with horizontal bedding, wavy bedding, scour surface and other structures.It can develop either in the intertidal zone with weak hydrodynamic force or in the highenergy subtidal zone.Conical stromatolites are mainly developed in lower intertidal zone, lagoon, high-energy subtidal zone and low-energy subtidal zone.The cabbage and hemispherical stromatolites are mainly formed in the highenergy subtidal zone.The results show that the stronger the hydrodynamic conditions are, the more complex the types of stromatolites are.The deeper the water, the larger the individual stromatolites.For example, the small conical stromatolites in the lower intertidal zone and high-energy subtidal zone, while in the low-energy subtidal zone are large or huge ones.The intertidal zone is where stromatolites are most abundant (Fan ZX et al., 2018).

In addition, the supply of terrestrial materials(argillaceous) has a great influence on the deposition of carbonate rocks and the growth of stromatolites.When the supply of terrigenous materials is large, the stromatolites are usually undeveloped or small size or low abundance.While the terrigenous materials are generally low in the places where stromatolites developed in Shennongjia Group.They don’t well developed or undeveloped in the places where there are more gravel, sandy and muddy dolomites.

7.Conclusions

(i) The macroscopic morphology of stromatolites in Shennongjia Group are diversity, but they are characterized by stratiform columnar, domial and coniform and bioherm (or biostrome).Stratiform stromatolites are the most abundant,distributed widely in different formations and sedimentary environments.Small and medium-sized columnar stromatolites are mainly columnar stromatolites.They widely distributed and have the most types.Conophytons have the largest size and scale.Columnar stromatolites and Conophytons often formed stromatolites reefs.

(ii) Stromatolite assembleges in vertical in Shennongjia Group can be summarized into 4 categories: (1) Small stratiform-columnal (concial) or stratiform (columnal)stromatolites formed stromatolithic reefs, which belongs to intertidal deposit.(2) Stratiform-domal-large conical and domal stromatolites and spherical stromatolites developed in high-energy subtidal.(3) Branched columnar stromatolites with different dipping directions or oblique columnar stromatolites formed the stromatolites assemblage in a highenergy subtidal zone.(4) Slender columnar or (walled) and small simple branched columnar stromatolites formed a lowenergy quiet subtidal environment assemblage.Among them,(1) is the most common, followed by (2), less (3) and (4).The results show that intertidal zone is the main area where stromatolites develop in Shennongjia Group, followed by subtidal zone, and there are a few deep subtidal zones.

(iii) Although it is not very clear that the reason why occurred stromatolithic structures of different forms and which factors controlled the changes of their characteristics in geological period, they are extensive distribution in the time age and sedimentary environment.Stromatolites are generally considered that they can grow in different water depths.Moreover, they mainly live in the intertidal and subtidal zones, and the biomass, hydrodynamic conditions and water depth jointly control the morphology, distribution and abundance of stromatolites.

Stromatolites, serve as a recorder of the evolution of life on Earth, are active in the whole process of the Earth history.The information, including life evolution, earth history, rock record, atmospheric environment, paleocean, palaeoclimate and palaeoenvironment, provides us good materials and evidences for studying earth history and ecological environment.There are a large number of different types of stromatolites in the strata of different ages in China.As the witness of the evolution of life in the Mesoproterozoic from 1000 Ma to 1400 Ma, it is still providing us new materials with the construction and protection of Shennongjia National Park and Global Geopark.We will continue to study it and reveal its secrets, so as to serve for geological science research, popular science education and economic development of Shennongjia area.

Acknowledgement

This research was co-supported by the National Key Research and Development Program of China(2016YFC0601001), the National Natural Science Foundation of China (41472082), China Geological Survey Projects(DD20160120-01), and Globe Geopark of Shennongjia.We are grateful to the leaders of Shennongjia National Park and Mr.Zhixian Wang, Quan Zhong gave great assistances and warmly aids; the field survey was under careful direction by Mr.Lesheng Qu from Hubei Geological Survey, Mr.Yuansheng Geng from Institute of Geology, CAGS.Sincere thanks are also given Mr.Zejiu Wang, Xin Shang and Mrs.Xiulan Ma from Chinese Academy of Geological Sciences(CAGS) and All China Commission of Stratigraphy.