Cambrian Bioherms Dominated By Microbial Carbonates In The Western Areas Of Liaoning Province

During the Paleozoic period,the North China Platform was characterized by shallow Marine facies deposition,which led to the development of diverse bioherms dominated microbial carbonates.In order to explore the sedimentary reaction of the first scene of phanerozoic cyanobacteria calcification event,the microbiolites of the Furongia and Miaolingian located at the northeastern margin of the North China Platform were studied,respectively,the Baiyun Village section of Tianshifu town and the Huolianzhai section in the west of Liaoning province.The Cambrian Furongia of the North China Platform is characterized by the gently sloping platform dominated by marl,which is obviously different from the carbonate platform dominated by oolitic grains of the Miaolingian;The development and maturation of the carbonate platform were recorded with the transgressive sediments in the lower part of the Xuzhuang formation as the boundary.Based on the Cambrian stratigraphic classification and the concept of sequence stratigraphy,strata of the Miaolingian in the study area can be divided into four third order depositional sequence,strata of the Furongia can be divided into three third order depositional sequence,they contain many types of carbonate structures caused by forced regressive,illustrating the microbial mats tend to be more stable in high energy environment and depositional model.All these structures were formed in the forced regressive system corresponding to the sea level descent,thus producing a new type of carbonate platform,which is different from the classical Wilson model.Microbial deposition is most common in Cambrian strata;The benthic microbial community dominated by cyanobacteria runs through the Miaolingian and Furongia of the North China Platform and generates carbonate deposition through diverse microbial activities,such as the laminated stromatolites,the clotted thrombolites,the structureless leiolites,and the dendritic dendrolite.These bioherms preserved multiple types of calcified cyanobacteria,such as Girvanella,Epiphyton,Subtifloria,Hedstroemia,Renalsis,and the most characteristic is interpreted as shell structure,which can be analogy with Lithocodium.Tracking the evidence of calcified cyanobacteria preserved in the geological record can not only describe the microbialaction process in the microbial mats dominated by cyanobacteria,but also be the key to understanding the paleoclimatic conditions,paleo-marine environment and the formation mechanism of microbial carbonates,which has important practical value for the study of microorganisms and microbialites.The capture and binding of sedimentary particles,the calcification of cyanobacteria sheath and the precipitation of calcium carbonate minerals are the key to the genesis of microbial carbonates,among which the role of alkalinity engine and the influence of EPS degradation and sulfate reducing bacteria on the precipitation and dissolution of minerals need to be further explored.However,the current macro and micro analysis is still of great significance to the understanding of the bioherms,sedimentary pattern and paleosedimentary environment of the North China Platform.