Study On The Genetic Mechanism Of Oolitic Limestone In The Cambrian Zhangxia Formation Of The North China

Ooids are typically coated grains in the process of carbonate deposition.The genetic mechanism of ooids has been one of the hot issues in sedimentology research.The structural characteristics of ooids can reflect environmental factors such as hydrodynamic conditions and salinity.The study of the oolitic sedimentation process can reveal the characteristics of the paleoenvironmental sedimentary environment and provide an in-depth understanding of the carbonate deposition mechanism.This paper systematically studied the characteristics of the oolitic limestone of the Cambrian Zhangxia Formation in the Chaoshuiyu section of Qinhuangdao in Hebei and the Xiaweidian section of Xishan in Beijing.Observe the oolitic limestone slices using an optical microscope,divide the oolitic types,and observe the distribution of organic matter in the stratum.Detailed scanning electron microscopy observations revealed a close relationship between the distribution of organic matter and microbial activity.The research results show that microorganisms are involved in the oolitic precipitation process,which is comparable to foreign laboratory cultivation experiments and promotes the understanding of the cause of oolitic particles.The details are as follows:There are five types of ooids in the Chaoshuiyu section of Qinhuangdao,Hebei,and Xiaweidan section of Xishan area,Beijing,mainly including concentric-radial ooids,concentric ooids,radial ooids,monocrystalline/polycrystalline ooids,and micritic ooids.The structural characteristics of different types of ooids are different.In the core of micritic ooids,pyrite particles and dark brown organic matter residues around the oolitic granules were found,indicating that the organic matter content in the original components of oolitic is relatively high.There is a microbial additive layer in the laminae,which is obviously different from the oolitic laminae,and the fluorescence of the additive layer is more intense.Therefore,the formation of oolitic particles is closely related to the deposition and construction of microorganisms.The results of high-resolution scanning electron microscopy(SEM)showed that EPS producers such as rod-shaped bacteria coated with mucous film were observed in the oolitic laminae.This structure is consistent with the confirmed EPS morphological structure found in modern ooids in the Bahama Bank,and it is speculated that it may be closely related to microbial activity.The morphology of microorganisms and theirproducts in oolitic laminae is different from that of laminae,which indicates that microorganisms are directly involved in the process of oolitic deposition and play an important role.The typical trace elements of the oolitic limestone of the Zhangxia Formation indicate that the sedimentary environment is a normal marine saline environment.According to the oxygen isotope geological temperature calculation of oolitic limestone,the average temperature of the microenvironment during oolitic deposition is 28.75??The oolitic growth process is mainly divided into two stages: active suspension and static adsorption.During the static period in the shallow low-energy area,the small energy of water is conducive to the adhesion of microorganisms on the surface of ooids and the formation of mucous EPS,and the microbial metabolic process changes the surface environmental conditions,which promotes the crystallization and precipitation of calcium carbonate in EPS generated by biofilm.Subsequently,under the strong hydrodynamic conditions in the active stage,the ooids are carried by the water into the high-energy region and are rounded and polished by the action of water erosion or rolling wear.Then the oolitic concentric layer is formed in the process of repeated cycles.However,when the water-energy cannot carry the oolites into the water flow during the static period,the oolites will stop growing and be buried,and then maybe subjected to the destructive effects of microorganisms,resulting in microbial drilling and micritization.