Sedimentary Features Of The Cambrian Microbial Carbonate In The Northern Margin Of The North China Platform

As a kind of calcified microbiolite,microbial carbonate preserved the fossil record of microbial mat and biofilm in the form of carbonate precipitation,represented the result of interaction between microorganisms and sedimentary environment.According to the macroscopic characteristics,microbial carbonate can be divided into six types:stromatolite,thrombolite,dendrite,leiolite,laminite and oncolite.In the study area,a variety of microbial carbonate can be observed with extensive spatiotemporal distribution in the the Miaolingian Series and Furongian Series of the Cambrian strata.Based on the research of sedimentology and stratigraphy,this study describes the characteristics of the Cambrian microbial carbonate from macro,micro and ultra-micro scale,in combination with geochemical tests.The genetic mechanism and controlling factors of microbial carbonates in the study area will be further discussed from the perspective of sedimentology.The study area located in the northern margin of the North China Platform which included 21 Cambrian sections(from the Sandaogou section in the Huludao City,Liaoning province to the Zhuozishan section in the Wuhai City,Inner Mongolia).The types of microbial carbonates were found include stromatolite,leiolite and oncolite.The observation results of macro characteristics show that the microbial carbonates in the study area have the following characteristics:1)the microbial carbonates in the study area are concentrated on the top of the third or fourth order sequence;2)the stromatolite,leiolite are in the form of thick massive bioherm;3)the occurrence of the oncolite is often associated with the ooids.The results of micro-and ultramicro-structural observation show that the microbial carbonates in the study area is composed of dark micrite and microspar,with abundant of calcified microbial fossils,representing the fossil record of microbial mat and biofilm,the builder of these microbial carbonates.However,there are different types of microbial fossils in different types of microbial carbonates,which represents the diversity of "microbail builders" of the Cambrian microbial carbonates in the study area.The results of geochemical analysis show that the formation mechanism of microbial carbonate in the study area is mainly controlled by paleogeographic and paleoenvironmental factors.The results shown that the Miaoliangian and Furongian Series of Cambrian in study area is dominated by oolitic shoal and carbonate mud deposition.During this period,the microbial mat and biofilm dominated by cyanobacteria affected the calcium carbonate precipitate by the different metabolism mechanism,which formed a spectacular deposition phenomenon in the North China platform.This also represents the sedimentary record of the first episode of the Phanerozoic cyanobacteria calcification event.The diversity of microbial carbonates and their wide spatial and temporal distribution in the study area represent the first sedimentary example of"Microbial Recovery Period" in Phanerozoic.