Evolution And Event Of Neoproterozoic Sedimentary Environment In Baishan Area,Jilin Province

The southern Jilin area is located in the central Korean quasi-platform area in the northeast margin of the North China plate.The Neoproterozoic strata are continuously developed and widely distributed.In recent years,many researches on Neoproterozoic sedimentary events in this area have been made,but the evolution of Neoproterozoic sedimentary environment has not been studied.Taking the Neoproterozoic in Baishan area of Jilin Province as the object,using the theory of sedimentology,event sedimentology and so on,combined with the field profile observation and thin slice analysis data,the Neoproterozoic stratigraphic sequence and sedimentary system type were carried out.Studies on sedimentary models and typical sedimentary events.The research results and knowledge are as follows:Based on the stratigraphic sequence and rock composition of the measured section,the petrological markers of various sedimentary facies of the Neoproterozoic are summarized.Two major types of clastic rocks and carbonate rocks,eight subclasses and19 basic types are identified.On this basis,five sedimentary systems including shore,platform edge,platform gentle slope and basin are identified,which are further divided into 14 subfacies and 19 microfacies.The Neoproterozoic in the study area is divided into two sedimentary sequences.The first sequence is classified as Diaoyutai-bridgehead stage,which evolved from barrier shoreline deposit to barrier-free shoreline deposit,forming a complete second cycle of Qingbaikou period;The second sequence is classified as the Wanlong-Qinggouzi period,the evolution process of the study area from the gentleslope type platform to the edge gentle slope type platform,forming a complete Sinian secondary cycle;The second-order cycle of Qingbaikou system and the second-order cycle of Sinian system constitute the large-scale first-order cycle of Neoproterozoic.The sedimentary evolution of carbonate rocks is controlled by the rise and fall of Yu Hai plane.In the early stage of the transgressive period of Wan Long formation,a gentle slope platform system was developed in the study area,which was composed of open platform and gentle slope.To the regression stage of Badaojiang formation,the open platform basin system,which is composed of open platform-margin reef-platform gentle slope,is developed in the study area,and the platform margin facies area evolves toward the basin.At the end of the Qinggouzi formation,a closed-edged platform basin system was developed in the study area,which was composed of restricted platform-margin reef-platform gentle slope,and the platform margin facies area further developed into the basin facies area.The molars structural limestone of Wan Long formation belongs to nano-grade micro-bright crystal carbonate rock,which is composed of micro-bright crystal strip and matrix.According to the morphology-genesis,it is divided into native filamentous,strip-like and tumor-like and different-place debris block-like,along with the water body from deep to shallow,presents the tumor-detritus-strip-filamentous evolution process;It is formed in the gentle slope environment of the stable Karatong platform,and mainly develops in the upper part of the middle gentle slope to the deep gentle slope.The lamellae of Badaojiang formation is characterized by slow wave,columnar and forked columnar shape,developed in the platform margin reef environment and concentrated in the lower part of reef nucleus-reef posterior beach;the stromatolith experienced the lower reef core-upper reef nucleus-reef posterior beach from the beginning to the end of sedimentation.The variation process is generally an upward shallower sedimentary sequence,indicating that at least one slow transgression occurred in the study area of the Badaojiang period,which corroborates each other with the regional tectonic evolution.