Research About The Formative Mechanisms Of Oncoids In The Second Series Of Cambrian System At The Fuzhou-Bay Section In Dalian City,Liaodong Peninsula

For the past few years,an increasing number of geologists have paid extensive attention to microbial carbonate that is the largest one of microbialites family,for the special macrofabrics and microfabrics as well as intricate formative mechanisms inherent in it.Many geologists have engaged detailed macroscopic and microscopic research and systematic taxonomy to microbialite,so far as to elemental scale.Among them the stromatolite has mostly been researched while oncoid remains to be elaborated at extend and quantity.The origin of oncoid is closely associated with the kinetic energy of ambient water for it,s distinctively larger size and unsual occurrence from ooid,which make it a good proxy for palaeoenvironmental and palaeogeographical interpretation.Simultaneously,we compared the various microbial information that oncois contained in different timescales and extracted the evolutional information of microbe systems as well as the putative variation in physiochemical environment,then correlated them with the closest geological events(for example,the biological extinction).So,the research conducted for oncoid can not only provide reference for the labeling of biostratigraphyical position,but for the consideration of the orgin and evolution of life.Most primarily,the calcification processes and mechanisms of microbes participated in the formation of oncoid are of significant implication to interpretation of the origin and evolution of mineralized skeletons of life.Oncoid is one of the coated grains that contain both sides of properties of stromatolite and ooid,which tender it a preference for the research of microbial carbonite sedimentology.Oncoid ubiquitously occurred at the top of Jianchang Formation and Mantou Formation in Second Series,Maozhuang Formation,Xuzhuang Formation and Zhangxia Formation in Miaoligian of Cambrian,and make it a time-specialized sedimentological facies.By means of field observation and logs of the targeted oncoid in study area and polarized electronic microscopically checking out and analysis of their microfabrics as well as microbial compositions,we may gain some evidences for further research of what role that microbes played during the growth of oncoid.Oncoid from the Second Series of Cambrian is the research object of this article,which can be divided roughly into two classes: one with nucleus,another without.The former which can be subdivided into oncoid with nucleus concsisting of trilobite skeletons or other bioclasts and oncoid with nucleus of bacterially adhesive aggregates as well as oncoid with detrital rock nucleus.Abundant of intensively crowded Girvanella filaments and Hedstroemia,Apophoretella,Acuasiphonoria and Ortonella etc.,which can find analogues in modern species as well as some biologically unconstrained Renacils piles were founded in oncolite and cortexes of oncoids at Fuzhou-Bay Section.These diversely cyanobacterial composition and unsmooth envelopes combine with abundant black dots which may prove to be residues of pyrite suggest that the formation process of oncoids were closely associated with microbial metabolisms or physical activities.The evidence of abundant fossils of microbes,especially calcified cyanobacterial sheeth contained in the oncoids from Fuzhou-Bay Section confidently suggest that oncoids there were built by complex calcification in non-layered photosynthetic biofilms.