Lithofacies And Palaeogeography Of Middle Permian In Southern Sichuan

This paper takes the middle Permian in South Sichuan region as the research object,takes the method of sedimentology as the guidance,uses the field outcrop data,the field sample analysis data and the logging data to carry out the systematic research,establishes the stratigraphic framework,and reveals the stratigraphic distribution law,and the stratigraphic framework is established to reveal its stratigraphic distribution rules.Focusing on field investigation and combining with drilling data,the characteristics of sedimentary facies were studied.A lithofacies paleogeographic map of Qixia formation and Maokou formation of the middle Permian in South Sichuan was systematically drawn based on the stratigraphic and sedimentary facies characteristics,and the lithofacies paleogeographic characteristics of different periods were revealed.It laid a foundation for the further study of middle Permian in South Sichuan.The following conclusions are drawn:(1)The middle Permian in South Sichuan is mainly composed of clear water carbonate sediments,and the main rock types include micritic bioclastic limestone,sparry bioclastic limestone,marl,dolomite and siliceous rocks,etc.,which are rich in a variety of organisms,mainly normal Marine benthic organisms,such as foraminifera,ostracoda and brachiopods.(2)The sedimentary facies types of Qixia formation and Maokou formation of the middle Permian in the South of Sichuan were judged by sedimentary markers,paleontological markers and geophysical markers.Four types of sedimentary facies zones have been identified,including open platform,restricted platform,platform edge and continental shelf.Restricted platform facies can be divided into lagoons and shoals.Platform edge can be divided into platform edge beach and interbeach subfacies,the continental shelf can be divided into shallow water shelf and deep water shelf,which reveals the spatial distribution and evolution of sedimentary facies of qixia formation and maokou formation of middle Permian in south sichuan.(3)Combined with the study of stratigraphic characteristics and sedimentary facies characteristics,the lithofacies paleogeographic map of Qixia formation and Maokou formation of the middle Permian in South Sichuan was systematically drawn with the segment as the unit,revealing the lithofacies paleogeographic characteristics of South Sichuan.During Qixia formation,with the rapid expansion of ancient tethys ocean and the melting of ancient glaciers,the basin began to transgression and accept the deposition.With the acceleration of transgression,the sea level rises rapidly,and all the study areas are inundated by seawater.The restricted platform and open platform sediments are developed as the main sedimentary pattern of the basin.Maokou formation inherits the sedimentary pattern of Qixia formation as a whole,that is,the carbonate platform sedimentary system belonging to Marine sedimentary system formation is dominated by open platform deposition,restricted platform deposition,platform edge and continental shelf deposition.During the period of Maokou formation,large scale continental shelf sediments developed in the east of the study area due to continuous Marine transgression.On the scale of this sedimentary pattern,the sea level in the second and third sections of Maokou formation decreased,leading to the gradual reduction of continental shelf pattern.In the fourth period of Maokou formation,due to the influence of sea level decline and DongWu movement,some areas in the study area showed stratigraphic deficiency.The main body was open platform sedimentary pattern,and continental shelf facies belt was basically undeveloped.