Sedimentary Facies And Paleogeographic Evolution Of Early Ordovician Lithofacies In The Dejiang-Sinan Area,Guizhou Province

The Dejiang-Sinan area is located in the northern Guizhou district of Yangzi stratigraphic area,I1 Tongzi-Yanhe District I2.From the whole Yangzi platform,in the early Ordovician Tongzi period,the Yangzi platform was mainly carbonate platform.The clastic platform is only existedin the narrow zone near the Kangdian land.However,in the Meitan period of late early Ordovician,the area of carbonate platform decreased greatly,the area of clastic rock increased greatly,and the area of carbonate platform and clastic platform were about the same.The specific reasons of the phenomenon remained to be studied.During the whole of the late early Ordovician,Dejiang-Sinan areais a clastic platform,which completely experienced the disappearance of the carbonate platform in early Ordovician..Guided by paleogeography,sequence stratigraphy,palaeobiology,sedimentary petrology,geochemistry,geotectonics and related discipline,based on the measured profiles and typical thin slices of the whole region,the composition and characteristics of each rock type were studied in detail by analyzing and testing the effective data,the sedimentary environment was analyzed and the evolution characteristics were summarized.The following points of understanding were obtained:1)According to the newly determined lithostratigraphy of the study area,the regional geological records and related paleogeography of Guizhou Province,the detailed field exploration and profile survey of the strata were accomplished in the study area,and the early Ordovician stratigraphic units in the study area were finallydetermined.According to the new to the old,it can be divided into:Meitan formation(O_1-2m),red garden group?O₁h?,Tongzi group?O₁t?.2)The rocks of Lower Ordovician in Dejiang-Sinan area can be divided into carbonate rock and clastic rock by field geological survey and laboratory test analysis.Theearly-middle stage of Lower Ordovicianis characterized by widespread distribution of carbonate rocks developed in Tongzi formation,Hongyuan formation.The late stage of Lower Ordovician is mainly characterized by widespread distribution of clastic rocks,mainly developed in Meitan formation.There is a relatively thick clastic rock in the top of Tongzi formation,Thehonghuayuan formationis sandwiched by three very thin layers of clastic rock,and a set of carbonate rocks was developed in the middleperiod of Meitan formation.3)According to the measured section data of the study area,number of thin slices,chemical data and paleontology analysis,under the high precision control of multi-disciplinary means,the sedimentary system of the study area was obtained,and two sedimentary facies and many types of microphaseand subfacieswere summarized..Early Ordovician developed continental shelf and carbonate platform deposits is a group of marine sedimentary systems.4)There are tidal flat-lagoon facies mainly still dominated by open platformin Tongzi period..Due to the transgression during the safflower garden period,the ratio of the platform margin shoal zone to the open platform area is close to 1:1.According to its structural and chemical characteristics,the edge shoal of platform can be divided into raw debris beach,oolitic beach,inner debris beach,and platform margin slope at the same time.The Meitan stage is mainly composed of mixed shelf,which can be divided into nodular gray shelf microfacies and muddy sandy shelf microfacies.5)Yangzi platform experienced a long transgression in the early Ordovician period,which led to the rise of sea level with the disappearance carbonate platform..During the process,the change was most violently in the middle-late stage of early Ordovician..The process of transgressive existed in a period of Meitan formation,showing a sand-muddy shelf in the region and a sea retreat in the later period,which led to the occurrence of a nodular gray shelf in the study area.It can be seen that the whole early Ordovician Tongzi formation-Meitan formation is a complete transgression,which can be used as a basis for stratigraphic division in the early-Middle Ordovician era from the aspect of palaeogeography.