Study on Depositional Fabric and Environmental Characteristics of Dendrolites in the Cambrian Zhangxia Formation: An Example from the Houziyu Section of Ezhuang Town, Zibo, Shandong Province, Eastern China

. The dendrolites composed of light gray massive micrite limestone occur at the top of the Cambrian Zhangxia Formation in Houziyu Section, Zibo, Shandong Province. Because it is very di ﬃ cult to study the genesis of microbial rocks through “ the function of diagenesis ﬁ lter, ” this paper analyzes its genetic mechanism by characterizing the sedimentary fabric and formation environment of dendrolites and it is of great signi ﬁ cance to study microbial carbonate deposited in geological history in oil exploration. The Cambrian Zhangxia Formation in the Houziyu section is a marine deposit. The macroscopic shrub-like fabric of centimeter scale can be seen on the surface of the tree-shaped stone, with small biological burrows and remnants of calcareous mud components after ﬁ lling. The microstructure observation shows that the main fabric of the dendrolites is a dark micrite matrix, bright calcite cement, clastic particles, and a calci ﬁ ed cyanobacteria biological community. Detailed supplementary information on the diversity of calci ﬁ ed microorganisms in dendrolites has been made through the study of various types of calci ﬁ ed cyanobacteria and calci ﬁ ed residual substances in the microbial membrane of calci ﬁ ed cyanobacteria in the dendrolites of the study area. It is clear that dendrolites are the product of the calci ﬁ cation of bio ﬁ lms or microbial mats dominated by cyanobacteria. With the deepening of this kind of research, it is bound to raise the research of microbial carbonate to a new level in China and provide great help to ﬁ nd oil and gas resources in marine strata of microbial origin in China.


Introduction
Microbialites refer to the carbonate sediments formed in situ by benthic microbial communities through capturing and binding clastic sediments, as well as various inorganic-or organic-induced mineralization related to microbial metabolic activities, and preserved after solid diagenesis [1].Microbial carbonate rocks are mainly calcified microbial rocks [2].The research on microbial rocks is difficult because the evidence that its genesis is controlled by microbial metabolism is difficult to preserve after diagenesis filtration [3].The sedimentary fabric characteristics of microbialites and the possible preserved microbial fossil records reflect the influence of seawater chemical conditions and atmospheric CO 2 concentration on microbial metabolism during this period [4,5].Therefore, understanding the microbialites in ancient strata, especially the exquisitely preserved microbial fossil records and special sedimentary fabrics within them, has become the key to understanding the oceanic and atmospheric environment in geological history [6].In recent years, unconventional oil and gas resources such as shale gas and tight oil have been gradually industrialized [7,8], and calcified microbial rocks are also an important oil-and gas-producing layer in the world, which is widely distributed, Many areas at home and abroad are dominated by carbonate reservoirs [9][10][11].Microbial carbonate rocks are mainly developed in older strata and are often buried deep.In view of its potential source rocks, reservoirs, and abundant oil and gas resources, deep oil and gas exploration has great potential.Through the classification of microbial rock types, it is of great significance to study microbial carbonates deposited in geological history in petroleum exploration [12][13][14][15].
The first reported dendrolites are preserved in the middle lower Cambrian [16,17] The Cambrian and Ordovician dendritic-like rocks are described as specific microbial carbonate dendrolites [16]; later, dendrolites were accurately defined as microbial carbonate rocks formed by calcified microorganisms with centimeter-scale macroscopic shrubbery texture that can be distinguished by naked eyes [17].As an important part of microbial carbonate rocks, dendrolites have limitations in distribution, which is rarely studied by scholars domestically and abroad.Most scholars believe that dendrolites are the products of calcification microbial processes [3,[17][18][19][20][21][22].However, the discovery and study of finger dendrolites at the Permian-Triassic boundary in Xiushui, Jiangxi Province, and Laolongdong, Chongqing City, provide another possibility for the genesis of dendrolites [23,24].The research progress and understanding of scholars and the modern dendrite analogues found in Shark Bay, Australia, in recent years provide a high reference value for this study [25].On the one hand, through field investigation and microscopic identification of the Cambrian Zhangxia Formation in the Houziyu section, this study aims to clarify the calcified cyanobacteria fossils and calcified cyanobacteria microbial membrane residues contained in this section and make a more accurate and detailed supplement for the diversity of calcified microbial fossils in dendrolites.Pyrite particles can be seen between internals.It also provides a typical example of the complex calcification and metabolic effects of microbial membrane or microbial mat dominated by calcifying microorganisms.In this paper, the study of the special sedimentary fabric and formation environment of dendrolites provides more important practical materials for the in-depth understanding of the complex formation mechanism of dendritic stone.On the other hand, with the depletion of conventional oil and gas resources, the oil and gas industry is developing towards unconventional oil and gas resources or more challenging resources.At the same time, the current technological development enables people to explore deeper strata [7,8,[26][27][28].Deep exploration has become an important field of oil and gas exploration in China.The reservoirs of these oil and gas reservoirs are mostly characterized by deeply buried and old microbial carbonate rocks.Therefore, the study of microbial carbonate oil and gas exploration potential will also be put on the agenda.The systematic theoretical research results of microbial rocks will provide important theoretical support for exploring the formation of microbial carbonate source rocks and their reservoirs.

Geological Background
The Cambrian strata with a thickness of about 700 m were deposited in the North China platform, which was composed of mixed red beds and carbonate rocks from the late second to the early Miaolinian.Among them, the Miaolinian carbonate platform is characterized by more oolitic shoals (Figure 1), while the Furong series is mainly characterized by argillaceous carbonate rocks [29,30].From the middle and late Cambrian to the Middle Ordovician, the western Shandong area has been continuously deposited, forming a set of mixed deposits of clastic rocks and carbonate rocks with a thickness of more than 1000 km, which belongs to the typical products of shallow sea sedimentary environment [31].
The profile studied in this paper starts from Houziyu village, about 6 km southeast of Ezhuang Town, Zibo City, and extends northwest along the highway to Ezhuang town.The dip angle of the stratum is about 10 to 15 degrees.The strata from Xuzhuang Formation to Zhangxia Formation are well exposed near the highway, and from Gushan Formation to Furong series of the third series, there is no outcrop.
The Zhangxia Formation is roughly equivalent to the tertiary Cambrian Gushan stage [32,33].From bottom to top, the strata are thick massive limestone and calcareous shale, striped medium-thin layer marl and mediumthick layer micritic limestone, strip thin-layer marl and thick-layer micritic limestone, and thick-layer massive tree limestone and thin strip marl interbedded (Figure 2).The sedimentary facies sequence of Zhangxia Formation is a third-order sequence, which can be further divided into three fourth-order subsequences as follows: (1) The thick block dark gray and grayish green shelf calcareous shale constitutes the CS unit of the third-order sequence and the fourth-order subsequence, with a thickness of about 35 m.The banded medium thin-layer marl and the medium-thick layer micritic limestone form an l m-type cycle sequence with a thickness of about 20 m, which constitutes the rhst (relative high water level system unit) of the fourth-order subsequence.The L-M cycle is mainly composed of pelagic calcareous mudstone and semipelagic limestone.It is characterized by antigrain sequence and occurs in deep water environment above storm wave base level and below normal wave base level [34,35] (2) The middle thin-bedded marl and medium-thick micritic limestone form an L-M-type cycle sequence 2 Geofluids with a thickness of about 40 m, which constitutes the RLST (relative low water level system unit) of the fourth-order subsequence.The thin strip marl and thick-layer micritic limestone constitute a 20 m thick subtidal cycle sequence, which constitutes the rhst (relative high water level system unit) of the fourth-order subsequence.The subtidal cycle developed in the normal shallow water environment is characterized by shallowing upward, coarsening grain upward, and thickening upward [34,35] (3) The thin-banded marl and thick micritic limestone constitute a 20 m thick subtidal cycle sequence, which constitutes the RLST (relative low water level system unit) of the fourth-order subsequence.The interbedding of light gray to grayish white thick massive tree-shaped limestone and thin-banded marl with a thickness of about 50 meters constitutes the meter scale cycle sequence of subtidal carbonate and constitutes the rhst (relative high water level system unit) of the fourth-order subsequence According to the general sedimentary environment, the shallow sedimentary environment of the upper Triassic is represented by the shallow sedimentary environment.

Materials and Methods
Dendrolites in the massive limestones are most abundant in the upper part of the Miaolingian Zhangxia Formation at the Houziyu section.The macro-and mesostructures of these massive limestones were observed on pale-weathered surfaces.Detailed observations were conducted on the wellexposed outcrops, where 33 samples were collected from massive limestones.Microfabrics were observed in thin sections with a ZEISS Axio Scope A1 microscope.Highresolution images were made for dendrolites under the microscope, and the limestone samples were analyzed by XRD mineral composition and carbon and oxygen isotope.The dendrolites in the Houziyu section are mainly produced in the third-and fourth-order sequences of the Zhangxia Formation.Dendrolites are composed of micritic fabric and dendritic microfabric with irregular morphology and lack of lamina (Figure 3(b)), reflecting its irregular accretion [16].The macroscopic shrubby fabric of centimeter scale can be seen on the surface of the dendrite (Figures 3(b)-3(d)), with gray and gray-black color, a width of 0.5-1 cm, a length of 1-2 cm, and clear boundary.The branches forminter connections, most of which are 2branch connections.The width between branches is 0.3~1 cm, and the width between most branches is 0.5 cm.There are dark argillaceous crystals between the shrubby fabrics, some interbedded with light yellow calcareous mudstone, and some small biological burrows and calcareous mud component residues after filling (Figure 3(b)).
In the field, the dendrolites of the Zhangxia Formation were sampled, and the limestone samples were analyzed by whole-rock XRD for mineral composition and whole-rock carbon and oxygen isotopes.XRD analysis results show that the main mineral of dendrolites is calcite, containing a small amount of dolomite, quartz, pyrite, and clay minerals (Table 1 and Figure 4).The carbon and oxygen isotope test results show that the δ 13 C PDB value is low positive and the δ 18 O PDB value is medium negative, which is consistent with the water environment required for the formation of dendrolites: the dendrolites were formed in the shallow slope environment, and the light carbon and oxygen isotopes were evaporated in the seawater, and the heavy isotopes were enriched, thus increasing the δ 13 C and δ 18 O isotope values in the limestone.Previous studies have shown that the carbon and oxygen isotope values of limestone are correlated with the salinity of the forming environment.Some scholars combined δ 13 C PDB and δ 18  O PDB of carbonate rocks to indicate the paleosalinity and distinguish marine limestone from freshwater limestone by Z value [36].Z value more than 120 is marine facies, less than 120 is continental facies, where Z = 2:048 ðδ 13 C + 50Þ + 0:498 ðδ 18 O + 50Þ, δ 13 C PDB and δ 18 O PDB adopt PDB standard.As can be seen from the results (Table 2), the Z values of the dendrolite samples in the Zhangxia Formation are all greater than 120, which is consistent with their marine origin.
During the formation of limestone, the temperature has a great influence on the δ 18 O PDB value, while the δ 13 C PDB value is insensitive to the temperature change.In the case of no drastic change in salinity, the δ 18 O PDB value decreases with the increase of temperature.Therefore, many scholars have proposed the application of the δ 18  O PDB value to determine the temperature of its formation [37].In this paper, t = −258:4 − 5:41δ 18 O PDB is used to calculate the formation temperature of calcite in homogeneous stone biodomes.The results showed that the calculated maximum temperature was 58.69 °C and the calculated minimum temperature was 51.11 °C (Table 2).The δ 18 O PDB value in carbonate rocks has changed greatly with the geological history.The older the age and the longer the diagenesis time, the stronger the oxygen isotope exchange, and the lower the δ 18 O PDB value.Therefore, the application of the above method is greatly limited, and the calculated temperature does not represent the temperature of the original sedimentary water body.Nevertheless, this value still has a certain reference value, especially for reflecting the strength of diagenesis, and has qualitative significance [38].

Microscopic Characteristics. Microscope observations
show that the main fabric of dendrolites in the Houziyu section of Ezhuang Town, Zibo, is dark mud crystal matrix, bright crystal calcite cement, clastic particles, and calcified cyanobacteria, including Epiphyton, Hedstroemia, and Bacinella Lithocodium-like fabric.Notably, cements in dendrolites may be classified as dolomite deposits formed in organic-rich microenvironments in microbial mats (or biofilms) [39,40], which may represent organic mineralized residues resulting from the formation of microbial mats (or biofilms) of amorphous magnesium silicate prior to EPS calcification [1,[41][42][43].In addition, the possible pyrite residues in dendrites (as shown in Figures 5-8) may be the result of microbial precipitation [16] or early petrification [3,44], and sulfate-reducing reactions by sulfate-reducing bacteria [45,46] are causatively linked, further suggesting that the alkalinity engine within microbial mats [47] and extracellular.The two key elements, the extracellular organic matrix [48] composed of polymeric substances [49][50][51][52], are tightly coupled.The calcification of these dendrites is formed and facilitated by in situ precipitation of carbonate minerals.The formation of bacterial calcification in sediments [21], and thus the formation of bacterial calcification in the sediments, is still possible.4.2.1.Epiphyton.Epiphyton is composed of dense mud crystals, which may be rod-shaped, shrubby, or tubular dendritic filaments, and grow upright or suspended [54].In the study area, the length of a single branch hyphae of dendrolites from the Zhangxia Formation of Cambrian is 40~100 μm.In addition, the crystal calcite occasionally replaced the Epiphyton.According to the previous classification [18,19,21,53] by characteristics of branching size, cell-like segments, and tubular structures, the Epiphyton in the study area were divided into five types according to their morphological differences: muddy crystal coarse branched, mud crystal segmented branching, mud crystal fine branching, microcrystalline tubular branching, and bright crystal coarse branching.There are four types of microbial communities: mud crystal dendrite (also known as a shrub), single chamber, cluster ball, and bright crystal dendrite (as shown in Figure 5).
Microscopic image shows that the Epiphyton is  6 Geofluids thallus less than 40 μm.The branching density is smaller than that of coarse branches of argillaceous crystals.This type usually consists of a small shrubby thallus with a semicircular cross-section and a fanshaped longitudinal section with branches radiating from a single point It is further speculated that they are similar to microcrystalline tubular branching, and they are partially hollow rod-shaped (tubular) forms.However, the morphology of their colonies is quite different from that of microcrystalline tubular branches, and most of them are bush-like.Because of the strong recrystallization effect, it is difficult to see more details The study area is characterized by the development of four microbial communities.The mud crystal dendrimer community is mainly composed of coarse-branched argillaceous and segmented argillaceous Epiphyton, which can grow alone or coexist with each other.Microcrystalline dendrimers occasionally form mud crystal dendrite communities.The chamber community is mainly formed by the interaction of argillaceous-segmented branching and micritic branching, the cluster spherical community is mainly composed of argillaceous branching and microcrystalline tubular branching, and the dendrimer community is formed by Epiphyton bifidum.8 Geofluids crusts [56].This fabric has reticular crusts with obvious thickness and extended dark mud crystal walls, and vertical tubules and irregular spaces can be observed inside, which often coexist with Hedstroemia (Figures 7(c) and 9).
4.2.4.Lithocodium.Under the microscope, there are oval spots or tubular structures characterized by dark mud crystal and micro bright crystal calcite, which are similar to the calcified cyanobacteria residues in the form of "Spirulina."Lithocodium was once defined as "crusty foraminifera" and is also considered a fossil of a kind of Spirulina (belonging to green algae) with a skeletal structure [57,58].The dark mud crystals forming the network are more likely to be interpreted as the product of cyanobacterial calcification.

Discussion
The top of the Cambrian Zhangxia Formation in Ezhuang Town, Zibo, Shandong Province, develops dendritic lime-stone composed of light gray massive micrite.The Zhangxia Formation in the profile constitutes a third-order sequence bounded by the submerged unconformity.The sequence evolution of the Zhangxia Formation sedimentary facies from bottom to top is as follows: shelf facies-deep gentle slope facies-medium gentle slope facies-shallow gentle slope facies.This indicates that the sedimentary environment in the Zhangxia Formation has changed from overall submersion to overall shallower upward, and the sea level has declined relatively.The dendrolites in the study area are composed of argillaceous texture and dendrite microfabric, and the macroshrubbery fabric with centimeter scale can be seen by the naked eye.The results show that there are many kinds of calcification from bacteria and clinopyrite, which are more complex than those of bioclastic calcite.
The dendrolites in the study area are characterized by the complex of four microbial communities dominated by Epiphyton.Other cyanobacteria calcifications coexist with the communities, including Epiphyton, Hedstroemia, Bacinella, and Lithocodium-like fabric.The number of other cyanobacteria fossils found in the study area is much less than that of the accessory mycobacteria, or the colonies formed do not 9 Geofluids have typical dendritic morphology.Therefore, it is considered that cyanobacteria such as Bacinella may play an auxiliary role in the dendrite formation process, and it is uncertain whether they occupy a dominant position.

Analysis of Sedimentary Environment and Genetic
Mechanism.The results of geochemical test and analysis of limestone samples from Zhangxia Formation in Houziyu Section, Zibo, Shandong Province, show that the main mineral composition of dendrolites is calcite, with a small amount of quartz and clay minerals (Table 1).The carbon and oxygen isotope test results of homogeneous stone biomounds show that the δ 13 C PDB value is low positive and the δ 18 O PDB value is medium negative.The low positive values of δ 13 C PDB in biomound samples are different from the high positive values of global Cambrian carbon isotopes [59].This difference in data can be explained by the fact that the low positive value of δ 13 C PDB in biomound samples indicates that during the formation of dendritic rocks, microorganisms absorb a large amount of light isotope 12 C, which increases the proportion of heavy isotope 13 C in water [60].Previous studies have shown that the variation of δ 18 O PDB value is mainly affected by temperature and salinity.The oxygen isotopic composition of some carbonate rocks at the time of formation is dependent on temperature, and the δ 18 O PDB value in carbonate rocks still fluctuates during the evolution of geological records.Therefore, the calculation of diagenetic temperature based on the oxygen isotope data of limestone does not represent the temperature of the original sedimentary water, but it is still of qualitative significance to reflect the strength of diagenesis.
Microbial carbonate rocks in ancient strata and their internal fossil records represent the construction achievements of microbial communities in sedimentary environments [16,17,44].Inferring the possible microbial metabolism during the formation of microbial carbonates in ancient strata is an important means to explain its genetic mechanism.
EPS (extracellular polymer) is a type of viscous and protective matrix created by microorganisms and accumulated outside the cell [61].EPS contains a microbial membrane composed of microbial communities with tens to hundreds of microns in thickness.The microbial membrane grows 10 Geofluids continuously and is defined as the microbial mat when the thickness reaches the millimeter level [62].According to results from research on a large number of modern microbial carbonate rocks, especially on dendrolites, it is considered that the microbial-dominated deposition is a process in which different kinds of microorganisms in microbial mats or biofilms produce alkalinity engines and promote mineral crystallization precipitation [25,44,63].The dendrolites of Zhangxia Formation in the study area have preserved a lot of bioclasts (Figures 6(a During the formation of dendrolites, the microbial mat binds and captures particles, and the captured particles enter into the microbial mat resulting in accumulation.Thus, the particles become the substrate for further growth of microorganisms [17].For dendrolites, particle capture is important locally, but the key process is precipitation [44], and the sediment deposited by dendrolites is dominated by calcified microorganisms.A modern example of dendrolite found in Hamelin pool, Shark Bay, Australia, is the first known example of living shrubs formed by microbial activities in the marine environment [25].In the study area, we can observe the cross-section of the colonies.The bacteria are filled with black mud crystal, and the outer layer of the bacteria is wrapped by the bright crystal calcite.The whole colony is cemented in the bright crystal calcite (as shown in Figures 9(b)-9(d)), which is similar to the simulation results in the modern dendrite laboratory in Hamelin pool, which may represent the evidence of EPS calcification.
Based on the study of the more typical bush-like fabric in the microbial rocks of modern lacustrine facies, the Epiphytons are likely to be caused by heterotrophic bacteria causing filamentous cyanobacteria (such as Oscillatoria and Calothrix) The calcification results in colonies in microbial mats [21,31,64], so it should be a more reasonable explanation to classify Epiphytons as cyanobacterial calcification colonies.There are still uncertainties, but it is relatively reasonable to suggest that the coarse-branched mud crystals and the segmented and branched mud crystals are both analogous to modern true bifurcated filamentous cyanobacteria such as Stigonem [16,17,54,65].It is also possible that these two fossils belong to the products of different growth stages of the same type of cyanobacteria.The calcification remnants of the sheaths of cyanobacteria such as Tolypothrix, composed of bifurcated and segmented filaments, and E. dendrocystis is more definitely analogous to modern, more advanced, nitrogen-fixing glues-Rivulariahaem atites [63].11 Geofluids It can be explained by the fact that it is similar to Hedstroemia in the Epiphyton, such as Rivularia [38,66,67].Two possible calcification microbial fabrics also increase the complexity of dendrolites: Bacinella-like fabric and Lithocodium algal fabric.The former has reticular crusts with discernable thickness and extended dark mud crystal walls, which can be interpreted as filamentous cyanobacteria and their stomatal crusts.On the other hand, the latter is characterized by dark argillaceous membrane-separated microstructure.The dark mud crystals forming the network are more likely to be interpreted as the product of cyanobacterial calcification.In particular, it is found that the calcified cyanobacteria residues in the form of "Lithocodium" are symbiotic with the accessory mycobacteria.These results suggest that these networks are more likely to be the remains of microbial community calcification [54,56,57].Second, in the high-density preservation of filamentous cyanobacterial fossils, clots, or dark dense micrite compositions, there are always sparsely distributed black dots that represent remnants of pyrite crystals [45,46,[68][69][70], suggesting that organic matter degradation by sulfate-reduction reactions by heterotrophic sulfate-reducing bacteria promotes these photosynthesis early rapid calcification of biofilms.Notably, dolomite cements in dendrites may be classified as dolomite deposits formed in organic-rich microenvironments in microbial mats (or biofilms) [39,40], which may represent organic mineralized residues from amorphous magnesium silicates that form microbial mats (or biofilms) prior to EPS calcification [1,[41][42][43].The diversity of calcifying microorganisms in the study area proves that dendrolites are the building materials of complex calcification of biofilms or microbial mats dominated by calcification microorganisms dominated by accessory mycobacteria.
Therefore, it is considered that EPS plays a key role in the development of dendrolites and mineral precipitation.Cyanobacteria such as Epiphyton dominate the biofilm or microbial mat, and different kinds of microorganisms in the biofilm or microbial mat undergo complex calcification through metabolism.After the release of Mg 2+ , Ca 2+ , and H + from EPS in the biofilm, amorphous carbonate minerals are formed on the calcified cyanobacteria sheath or EPS surface [71], it can produce an alkalinity engine which can satisfy carbonate ion precipitation and promote crystallization precipitation of minerals.

Research Significance.
Oil and gas reservoirs dominated by microbial carbonate enclaves have been found in Jurassic reef formations on the Gulf coast of the United States, Kazakhstan, and Turkmenistan [72].It is directly proved that oil and gas reservoirs can exist in microbial carbonate strata, which leads to an upsurge in exploring the law of oil and gas enrichment in microbial carbonate strata.The research on microbial carbonate has the following three important meanings: 5.2.1.Microbial Carbonate Development Zone Is Conducive to the Preservation of Organic Matter.Here are different opinions on the hydrocarbon-generating capacity of marine carbonate rocks [73,74].Scholar pointed out that the hydrocarbon-generating potential of marine carbonate rocks mainly depends on the organic facies [75], that is, the oxidation-reduction degree of the sedimentary environment.In mudstone and silt with low permeability, due to the restriction of oxygen and sulfate, the metabolism of microorganisms during the early burial of organic matter is reduced, which is conducive to the preservation of organic matter.Because the enrichment of nutrients (eutrophication or even hypernutrition) can cause an increase in microbial carbonate productivity, it is speculated that the areas where the ancient microbial carbonate is very developed generally belong to the sedimentary environment with high organic matter productivity, and the main control factor of the organic-rich sedimentary interval is the anaerobic underwater environment during deposition.However, the development of microbial carbonates is controlled by the oxygen content of seawater.Most of them are developed in the deep-water environment.Only when the oxygen content of surrounding seawater is lost, microbial carbonates can be developed in a large number at the bottom of shallower waters and restrict or even occupy coral habitats [17].In the medium shallow water environment, the bottom anoxic water environment caused by the photosynthesis of benthic algae and cyanobacteria is also conducive to the development of microbial rocks.It can be seen that there is a good consistency between the sedimentary background of organic-rich sediments and the development background of large-scale microbial carbonate deposits [76].Therefore, microbial carbonate deposits can be used as a contributor to the source rocks of petroleum strata.

Microbial Carbonate Can
Be Used as a Scale for Paleogeographic Reconstruction.Microbial communities and microbial carbonate deposits are closely related to paleoclimate, paleoenvironment, and major biological events.So far, studies generally believe that microbial carbonate deposition is common in most Paleozoic carbonate formations and that the prosperity of microbial communities and microbial carbonate deposits are generally related to environmental and climate changes during major changes in the earth [14,77].
The development of various microbial carbonates is determined by the deposition rate of sediments in the sedimentary environment and the level of water energy.During the sea level rise period, low background sedimentation rate and medium water energy can accelerate the proliferation rate of microorganisms forming dendritic microbial carbonate in deep water and then develop a large number of dendritic microbial rocks.Moreover, dendritic microbial carbonate represents a low-energy hydrodynamic environment with a slightly increased sedimentation rate [17].Therefore, microbial carbonates deposited in geological history can be used as a good indicator of paleoenvironment and paleoclimate, which is conducive to the paleogeographic reconstruction of sedimentary basins [13,14,77].

Microbial Carbonate Can Indicate the Type of Organic
Matter.Combined with the study of modern microbial communities and biochemical sedimentation processes, the characteristics of biomarkers of microbial carbonate deposits in the strata can be compared to reveal the types of organic matter related to the formation of oil and natural gas in various geological periods.In the primary era, microbial organic matter was the dominant factor of microbial rocks.During Phanerozoic, although biological disturbance and biological eating destroyed a large number of microbial records, it still could not cover up the great role of microorganisms in biological deposition and organic matter evolution.According to the research, the two high-yield periods of phytoplankton in the geological history period are the Precambrian to early Paleozoic and Late Jurassic to Cretaceous.The Precambrian to Paleozoic are mainly controlled by plankton characterized by organic walls, such as bluegreen algae and green algae; however, from Late Jurassic to Cretaceous, it was mainly composed of microplankton producing calcareous products, including small single-celled plants with calcareous skeleton (such as coccolithophyta) and siliceous plankton.From Cambrian to Devonian, the oil-generating parent materials were mainly phytoplankton and bacteria, and to a certain extent, benthic algae and zooplankton [78].Among them, the wavy or folded oilgenerating parent material layer may be an indication of the occupying microbial mat.In addition, through the analysis of biomarkers of Cambrian-and Silurian-tufted stromatolites in the Sichuan Basin of China, the distribution of n-alkanes mainly shows the even carbon number advantage of low carbon number, and the distribution of pentacyclic triterpanes is characterized by the main peak of chopane, revealing that bacteria and algae are the main contributors of organic matter ( [77]., et al.).However, the parent material of the organic matter of the Middle and Upper Proterozoic carbonate rocks in the northern part of North China is mainly sapropelic mud-type dominated by low biological algae and bacteria [79].Therefore, the study of microbial carbonate plays a very important role in evaluating the hydrocarbon generation potential of carbonate rocks.

Summary and Conclusions
Based on the study of various types of calcified cyanobacteria fossils and the bioclasts contained in the dendrolites of the Zhangxia Formation in the Houziyu section, this paper provides a more detailed supplement to the diversity of microbial fossils in the dendrolites and also provides a typical 13 Geofluids example for the fabric formed by the complex calcification of microbial membrane or microbial mat dominated by calcification microorganisms.However, the mechanism of interaction between microbial activities and the water environment during the formation of dendrolites needs further study.
(1) In the dendrolites of the study area, there are mainly Epiphyton microbial community complexes.Dendrolites may be the complex calcification products of biofilms or microbial mats dominated by cyanobacteria such as Epiphyton.The calcified cyanobacteria fossils or the remains of calcified cyanobacteria fossils, such as Epiphyton, Hedstroemia, Bacinella, and Lithocodium-like fabrics, coexist with them and participate in the formation of dendrolites (2) Geochemical analysis of dendritic stone samples shows that its main mineral composition is calcite; the values of δ 13 C PDB and δ 18 O PDB are low-positive values and medium-negative values, indicating that the environmental output of the formation is consistent with the judgment of macroscopic sedimentary facies change (3) The study of microbial carbonates deposited in geological history has three important meanings in petroleum With the deepening of this kind of research, it is bound to raise the research of microbial carbonate to a new level in China and provide great help to find oil and gas resources in marine strata of microbial origin in China

4 Geofluids 4 .
Fundamental Features of the Dendrolite 4.1.Macrocharacteristics and Geochemical Analysis.The top of the Xuzhuang Formation in the Houziyu section of the study area is a thick massive oolitic limestone (Figure 3(c)), which constitutes an oolitic beach facies stratum.Large-scale cross-bedding and frequent scour surfaces (Figures 3(a) and 3(d)) are developed in the oolitic limestone, which is characterized by high-energy deposition with a thickness of about 60 m.Thick massive limestone and shelf facies calcareous shale are developed at the bottom of Zhangxia Formation, covering the thick massive oolitic limestone of Xuzhuang Formation (Figures 2 and 3(a)); the middle part is developed with a middle-thin layer marlstone and medium-thick layer micritic limestone with deep gentle slope facies to shallow gentle slope facies.The top is developed with dendrolites composed of light gray and gray-white massive micritic limestone with a total thickness of about 50 m.

Figure 3 :
Figure 3: Macroscopic characteristics of Houziyu Section in Ezhuang Town, Zibo, Shandong Province.(a) The boundary between the Zhangxia Formation and Xuzhuang Formation (as shown by white curve).(b) The centimeter-scale shrubby fabric in the dendrolites of Zhangxia Formation is partly replaced by yellow calcareous mudstone.(c) The oolitic limestone of the Xuzhuang Formation develops scouring surface (as shown by red arrow).(d) Shrubby fabric composed of dark argillaceous crystals in dendrolites of Zhangxia Formation.

( 1 )
Coarse-branched mud crystals (Figures 6(a) and 6(b) and 7(a)) with diameters ranging from 80 to 90 μm.The cross-section shows a round spherical aggregate with two branches, and the outermost part of the dense branches has a space-time tubular shape, which belongs to a typical Epiphyton (2) Segmented and branched mud crystals (Figure 6(c)) whose branches are transversely divided.The outline of the thallus and the size direction of the branches are similar to those of the coarse branches of the argillaceous crystals, and the branches are slightly thicker (3) Argillaceous fine branching (Figures 6(d) and 7(b)) is in the shape of twigs and with a diameter of the

( 4 )
Microcrystalline tubular branches (Figure 7(b)), occasionally branching and occasionally hollow tubes with a diameter of 40 μm.The thallus is hemispherical, shrub-shaped, growing in layers and irregularly gathering, and the edge of mycelium is very sharp (5) Bifurcated rod-shaped dendrolites (Figures 7(d) and 8(a) and 8(b)) are suggested to be filled with calcite.

Figure 6 :
Figure 6: Micrographs of different types of Epiphyton in dendrolite of Zhangxia Formation, Houziyu Section, Shandong Province.(a, b) Typical large shrubby thallus micritic coarse branches Epiphyton (E; as shown by red arrows) forming micritic dendrolite communities.(b) A magnification of the boxed area in (a), and most of the branches of Epiphyton are two branches.(c) Typical large shrubby fungi micritic-segmented branching Epiphyton (E) forming micritic dendrolite communities.(d) Typical micritic fine-branching Epiphyton (E) form micritic dendrolite communities with Hedstroemia (H; as indicated by the red arrow), possibly with a Lithocodium-like fabric (L).

Figure 8 :
Figure 8: Micrograph of symbiosis of calcified cyanobacteria fossils in dendrolite rocks of Zhangxia Formation in Houziyu section.(a, b) Sparry coarse Epiphyton (E) forming sparry dendrolite communities.(b) The enlarged photo of the box area in (a), and a clear hollow tube wall can be seen.(c, d) Typical Hedstroemia (H).(d) The enlarged photo of the boxed area in (c).It can be seen that the branches diverge from one point and grow outwards, and the branches are long and tapering at the tip, basically without branching.

Figure 9 :
Figure 9: Micrographs of Hedstroemia in dendrolites of Zhangxia Formation in Houziyu Section, Shandong Province.(a) Symbiosis between a typical Hedstroemia (H) community and a possible Bacinella-like fabric (B); (b-d) enlarged image of the boxed area in (a); (b) atrioventricular Hedstroemia (H) with short branches spreading to all sides; (c) cluster globular Hedstroemia (H) symbiosis with possible Bacinella-like fabric (B), branching outwards in a fan pattern; and (d) small shrubby Hedstroemia (H) grew outwards on the base of nitrogen-fixing cells and tapered at the tip of the branch.

Table 1 :
XRD results of carbonate rocks forming the dendrolites.