Cambrian Bioherms Dominated By Microbial Carbonates In The South-central Taihang Mountains And Their Adjacent Areas Of North China

The North China Platform began to accumulate deposition since late Cambrian Series 2,covering the "Great Unconformity" of Precambrian-Cambrian in a mega second-order transgressive-regressive cycle.The epeiric depositional sequence of mix clastic-carbonate sediments involves a diversified distribution of bioherms dominated by microbial carbonates in the Miaolingian and Furongian,forming highstand system tract or the forced regressive system tract.The characteristics of these Cambrian(particularly Miaolingian and Furongian)bioherms deposited in third order depositional sequences show a special sedimentation pattern mediated by benthic microbial communities(mainly the microbial mats dominated by cyanobacteria)in relatively high-energy waters.The higher relief of these bioherms and their dome-shaped morphology suggest higher carbonate production and slower erosion in response to the process of relative sea level decline,which do not match with the classical principles of sequence stratigraphy.This way,this study provides classical examples for future understanding of the forming mechanism of the Cambrian microbial bioherms and massive reefs.Cambrian bioherms of the South Central Taihang Mountains can be divided into five types by following the classification of microbial carbonates,including: thrombolite;leiolite;stromatolite;maze-like bioherm and bioherm complexes.The macro and micro characteristics of each type of bioherm is the scope of this study,which are described in detail.It is generally believed that microbial carbonates of Cambrian Miaolingian in the North China Platform are mainly characterized by thrombolites and dendrites.However,the present research gives the evidences of maze-like bioherms in the Miaolingian as well as leiolites which are not only rarely reported in geological records but also very less comparable modern examples.Moreover,a variety of calcified cyanobacteria,such as Girvanella,Epiphyton,Renalcis,Hedstroemia,Subtifloria and possibly Lithocodium,are recovered from these bioherms,together with the remnants of EPS and/or calcified microbial mat,suggesting that these carbonate buildings are calcification products of complex microbial mats dominated by cyanobacteria.Furthermore,the presence of numerous pyrite grains distributing in microbial carbonates also provides the evidences of sophisticated metabolic action of non-photosynthetic bacteria and heterotrophic bacteria such as sulfate reducing bacteria,which accomplished through sulfate-reducing bacteria.The micro-fabrics of microbial carbonates have a remarkable diversity and complication due to the complexity of microbial mat structure and their intricate calcification.Consequently,it can be generally summarized as two main processes: the particle trapping and binding;and the mineralization and mineral precipitation.Particle trapping and binding may be important locally,but mineralization and mineral precipitation are the most important process for the deposition of carbonates.The possible calfication mechanisms of these microbial carbonates can be attributed into three processes:(1)Alkalinity engine and degradation of extracellular polymers;(2)Photosynthesis by cyanobacteria and sulfate reduction by sulfate-reducing bacteria;(3)Production and transformation of amorphous calcium carbonate(ACC).Nevertheless,the sophisticated calcification mechanism needs to be further integrated studied.