Studies On Microbial Carbonates Of Cambrian Miaolingian In Western Henan Province And Its Adjacent Areas Of North China

Microbial carbonates developed in western Henan Provience and its adjacent area have significant diversity,including stromatolites,thrombolites,leiolites,oncolites and other microbial complex buildups.With morphology of bioherm,most of them exist in Highstand System Tract(HST)of a third-order depositional sequence with a drowning-unconformity surface and Forced Regressive System Tract(FRST),which are the sedimentary records of the relative sea-level falling stage and represents the more spectacular sedimentological phenomenon.Generally speaking,the thicker microbial mats can form thrombolites and leiolites,and stromatolites and oncoilites formed by the thinner biofilms or mats.The microbial origin of microbiolites can be inferred from the following geological records:?macroscopically,the distinctive mound-like and domical froms;?the overall sedimentary structure;?preserved microbial fossils and calcified microbial mats and its residue;?SEM images show nanospheres and honeycomb,needle,plate or sheet structures;?micritic crusts and irregular biofilms;?Captured sediments.Various types of calcified microbial fossils,calcified microbial mats and their residues are preserved in the microbial carbonates in study area,including Girvanella,Subtifloria,Hedstroemia,Epiphyton,Renalcis,Botomaella,Ortonella and Lithocodium-like residues,which indicate that these microbial carbonates are the products of complex calcifications dominated by cyanobacteria.In the dense micritic and microsparry structure of bioherms,well-preserved various biogenic particles such as spherulites,benthic radiated ooids,cortoid and accumulated particles suggest a complex microbial activity and the diversity of carbonate particles in microbial mats.The formation of benthic radiated ooids is believed to genetically related to the degradation of EPS,which are the products of precipitation of carbonate within the microbial mats in the calcite sea.Meanwhile,the micritic sheaths are associated with calcification of biofilms.On the other hand,the scattered distribution of pyrite and the locally concentrated development of dolomite crystals with fogged center and clear margin structure indicate that both the microbial precipitation or that the early lithification of these microbialites are related to the sulfate reduction which can degrade the EPS matrix,also suggesting that SRB and other non-photosynthetic bacteria heterotrophic bacteria may play an important role in facilitating precipitation of carbonate.All microbial processes need to be explained in detail,and the current study has only touched the tip of the iceberg.How to explain the calcification mechanism of microbial mats and the origin of biological particles within them will be the biggest challenge in the future.