Study On Sedimentary Facies And Lithofacies Palaeogeography Of Middle And Late Ordovician In Dejiang Of Guizhou

Dejiang is located in the northeast of Guizhou and is located in the northeast of the Yunnan-Guizhou Plateau.In recent years,with the shale gas comprehensive research in northern Guizhou and the Zhengan shale gas survey less than one hundred kilometers away from the northwest of the study area have undergone a major breakthrough and the discovery of 100 billion cubic meters of gas field.It shows that the study of paleogeography in this area still has some significance.This article takes the Middle-Late Ordovician in the Dejiang area of Guizhou as the research object,based on the measured profile and the comparative profile,and guided by the disciplines of sedimentology,paleontology and palaeogeography,combined with analysis and testing,and specifically studied the composition of various rock types.Characteristics,analysis of its depositional environment and summary of its evolutionary characteristics,specifically achieved the following understanding:1)Based on the newly determined rock formations in the region,combined with the Guizhou regional geology records and related paleogeographic studies,detailed field surveys and section surveys were conducted on the strata in the study area,and stratigraphic divisions and comparisons were made between the upper and middle Ordovician strata in the study area..It mainly consists of the Meitan Formation(O_1-2m),Shizipu Formation(O_2-3sh),Baota Formation?O₃b?and Wufeng Formation?O₃w?.2)According to field surveys and indoor analysis,the rock types of the middle-upper Ordovician in the study area can be divided into two types:carbonate rock and clastic rock.Among them,clastic rocks are widely distributed and are mainly developed in the Meitan Formation and Shizipu Formation and Wufeng Formation.Carbonate rocks are deposited in the Upper Ordovician Baota Formation,which is mostly limestone and contains various numbers of biological muddy limestones.It is also found in the middle of the Meitan Formation.3)Based on the above stratigraphic studies and rock group analysis,combined with sedimentary structure,paleontology,sedimentary geochemistry and other research methods,the phase zone signs of the study area were analyzed,and the types and characteristics of the microfacies were summarized.4)Through the comparison of the measured profile and other auxiliary profile features,comparative study and phase change analysis,the phase transition characteristics and laws were studied,and the sedimentary system characteristics of the Middle-Late Ordovician sedimentary system in the study area were divided into the carbonate platform system and land.The sedimentary facies can be further divided into deep cement limestone,shallow limestone and point beach.The continental shed system is widely developed in the study area and manifested as a mixed shelf.5)Based on the collection and organization of data,combined with the sedimentary system of Middle-Late Ordovician in the Dejiang area,a total of four lithofacies paleogeographic maps have been compiled.The paleo-geographical units in the study area have evolved from the west to the east.Shed,a limestone shelf,appears as a muddy shelf in the South East.During the Shizipu period,shale sheds and grey sheds were developed from west to east.The shady shed phase area basically inherits the characteristics of the Meitan period,and the scope has become larger.The Baota period is a carbonate platform.The paleo-geographical units from west to east are followed by shallow limestone,deep-water limestone,and partially visible beach,reflecting the overall shallowness of the water body.During the Wufeng period,there are mixed shelf sheds,and the overall is a sandy argillaceous shelf.A grey shed is visible in the eastern research area,reflecting the shrinking and disappearance of the carbonate platform.