Sequence Stratigraphy And Sedimentology Research Of The Mesoproterozoic's Jixian Group At Jixian Section In Tianjin

The Yanshan area is the most developed region of the Meso-and Neoproterozoic stratum in north China,the whole area has continuous outcrops,complete stratum and steady deposition.The Jixian section,Tianjin,is the ideal section to study Meso-and Neoproterozoic deposition because it has the most complete strata and the stratum thickness are gigantic.The Neoproterozoic stratum in Jixian section is dominated by carbonate mixed with clastic rock and clay rock,deposited in the subtidal zone,intertidal zone and supratidal zone,which belongs to offshore facies,tidal-flat facies and submarine shelf facies respectively.The Gixian Group includes Wumishan Formation,Hongshuizhuang Formation,Tieling Formation and Xiamaling Formation.This article by a detailed study of the outcrop,the Jixian Group should be divided into 7 second order sequences and 31 third order sequences in this paper.And The Wumishan Formation is comprised of 6 second order sequences and 26 third order sequences;Hongshuizhuang Formation,Tieling Formation and Xiamaling Formation constitute 1 second order sequences and 5 third order sequences.The Mesoproterozoic Wumishan Formation in Jixian,Tianjin represents a succession of 3300m thick carbonate strata,in which stromatolite biostromes and clotted bioherms are the best developed.These biostromes and bioherms,together with tidal-flat dolomicrite and muddy dolostone,form circum tidal flat meter-scale carbonate cyclic sequences with symmetrical facies sequences and fabrics.These cyclic sequences are overlain by the lagoon dolomitic mudstones and shales.The microdigital stromatolite which developed in the lower part of the Wumishan Formation.This category of stromatolites have still been interpreted as being composed of the Proterozoic aroganite(rufa)sea floor precipitates by sedimenttologists,and have further been thought as the special products which were formed during the transitional period from the Archean stromatolites making up of the sea floor a roganite precipitates to the Neoproterozoic stromatolites made up of the clastics and micritic carbonates.Glauconite is a low-speed authigene mineral on behalf of the intermittent slow deposition,and it has also been considered as marine facies'mark.Therefore,glauconite is often called the authigene mineral marking condensed layer.But Jixian's glauconite may indicate a special submarine cementation,and its unique outputting represents that high-energy shallow sea in Precambrian is a oxygen shortage environment.