Research On Characteristics Of Miaolingian Bioherms Under Sequence Stratigraphic Framework In Magou Section,Hebei Province,North China Platform

Magou section is in the west of Tang County Micheng village,west of Baoding City within Hebei Province,which is located north of North China platform.A variety of microbial carbonate dominated bioherms have been identified within the section.Based on sedimentary faces and cyclicity reflected by outcrops,8 third-level depositional sequences can be identified from Magou Cambrian Section,in which bioherms dominated by leolite,thrombolites and stromatolites are developed in the Highstand System Tracts of the DS₄ in Zhangxia formation and DS₃ in Xuzhuang formation,Miaolingian.On the macro scale,leolite dominated bioherms are composed of uniformed light-colored micrite.Bioherms in Xuzhuang formation,on a macro-scale,are thrombolites dominated,where light-color micrite and dark micrite are two general fabrics consisting the"maze-like"meso-scale characteristics.Based on its fabrics,the thrombolites in Xuzhuang formation can be classified into three types:light-color micrite blocks with interstitial dark micrite,light-color micrite blocks with interstitial mud,dark micrite blocks with light-color micrite filled interstitially.Stromatolites in Miaolingian are branched small columns,which can be identified together with leolite in the oolitic limestone bed of Xuzhuang formation and Zhangxia formation.On the micro scale,bioclastics,calcified cyanobacteria and its residues are observed within the microbial carbonate dominated bioherms of Miaolingian,with the calcified cyanobacteria mainly being Girvanella with micro-tubes calcified sheaths and Epiphyton with bush-like characteristics,and most of which are dispersed and scattered,with various states of preservation.On the macro scale,the stromatolites developed in the Miaolingian of Magou section represent the lithification of the thinner microbial mats to a certain extent,while the leolite and thrombolite with different meso-scale characteristics represent thicker ancient microbial mats.On the micro scale,various calcified cyanobacteria and residues within leolite thrombolites and stromatolites in the bioherms of Miaolingian shows that the bioherms,consisted of microbial carbonates in Magou section,are closely related to the microbial mats dominated by cyanobacteria.Based on micro-scale characteristics of microbial carbonate,the organic mineralization related to cyanobacteria as well as the binding and trapping of sediments by microbial mats play an important role in the formation of bioherms in Magou section.