Study On Microbial Carbonates Of The Cambrian Furongian In Jinzhouwan,dalian Of The Liaodong Peninsula

Microbialites span over 80%of the fossil records of earth's history,Characteristic microbialite mesostructures are divided into the following broad divisions:leiolite(aphanitic),stromatolite(laminated),thrombolite(clotted),and dendrolite(dendritric).The Cambrian strata of North China platform are well exposed and continuous,and widely deposited ramp facies carbonate platform dominated by lime mud,which favors the reproduction of microbial carbonates.In this paper,microbial carbonates of Cambrian Furongian in Jinzhouwan section,Dalian are systematically studied macroscopically and microscopically,the genetic mechanism and control factors are further discussed.Furongian can be roughly divided into Changshan Formation and Fengshan Formation from bottom to top,sedimentary facies changed from shelf facies at the bottom to deep ramp facies,then to shallow ramp facies and tidal flat facies at the top.These formation are divided into three third-order depositional sequences.The sequence changes upward from the condensed section at the bottom to the highstand system tract and then to the forced regressive system tract at the top,microbial carbonates developed in massive limestone of forced regressive system tract.Leiolite and thrombolite were deposited in Changshan Formation,besides,leiolite and dendrolite were deposited in Fengshan Formation.Microbial sedimentation is most common in skeleton-deficient stormy ocean of Cambrian Miaolingian,diversified calcified cyanobacteria are recorded herein.The microscopic characteristics of microbial carbonates in the Furongian of the study area,in addition to the widely developed matrix composed of dark dense micrites and matrix made up of microspars,can also identify the Hedstroemia,Epiphyton,Renalsis,Girvanella,Subtifloria,Bacinella-like fabric,which confirms the first episode of calcification of Phanerozoic cyanobacteria.The pyrite crystals,benthic oolitics,dolomite crystals and agglutinated clots show that microbial carbonates are the product of complex calcification in microbial mats.Global warming and increased oxygen levels in the late Neoproterozoic restarted the CO₂Concentrating Mechanisms(CCMs),which stimulated the calcification of the sheath of living cyanobacteria widely distributed in the early Cambrian.During the growth process,cyanobacteria can absorb CO₂ and/or HCO₃^-through photosynthesis,and release OH^-,which prompting the accumulation and precipitation of carbonate minerals in the sheath.Heterotrophic bacteria such as sulfate-reducing bacteria can degrade extracellular polymeric substances(EPS)secreted by microorganisms,release Ca²⁺and increase the alkalinity of the microenvironment to cause calcium carbonate precipitation.Furthermore,the capture and adhesion of EPS promote the upward growth of microbial mats,forming a top-down complex layered structure containing photosynthetic cyanobacteria,purple photosynthetic sulfur bacteria and sulfate-reducing bacteria,leiolite thrombolite and dendrolite in study area are product in such thick microbial mats.After complex diagenesis,many original characteristics of microbial mats have been destroyed,and the interpretation of the formation mechanism of microbial carbonates has become a great challenge,this study on microbial carbonates in the Furongian Formation of Jinzhouwan section in North China Platform provides a research clue and possible idea.