Stratigraphic Division And Comparison And Sedimentary Facies Analysis Of Middle-late Permian In Dafang Area,Guizhou Province

The Permian strata in Dafang area of Guizhou are extensively distributed.On the basis of previous studies,this paper has made multiple stratigraphi-cdivisions of the study area through field surveys in the field and established sedimentary patterns of the Middle-Late Permian in the study area through the division of sedimentary facies.Eight measured sections are selected in this paper,including Qixia Formation of Zhaojiamawo in Baihe Town,Maokou Formation of Maolichong village in Baihe Town,Xianwuyan Formation of Mount Emei in Zhuchang Town,Longtan-Yelang Formation of Bainapingzi village in Baina Town,Longtan Formation of Maoli village in Wenge Town,Liangshan Formation of Huoganping village in Pudi Town,Longtan-Changxing Formation of Miaojiao village in Pengcheng Town,Xianwuyan Formation of Emeishan village in Gaodian Town.The lithostratigraphy was divided into Liangshan Formation,Qixia Formation,Maokou Formation,Xianwu Formation of Mount Emei,Longtan Formation and Changxing Formation.Then lithological characters of Stratigraphic unit,combination feature of rock,rock contact,horizontal comparison are descripted in detail.According to the species attribute of paleontological fossil,combination feature of rock and stratigraphic distribution,three Brachiopoda assemblage zones,two coralline assemblage zones four Fusulinid assemblage zones and one botanic assemblage Zone are established.Three Brachiopoda assemblage zones include Orthotichia-chekiangensis Assemblage Zone,Cryptospirifer shawanensis-Spirigerella subtriquetra Assemblage Zone and Orthotetina rubber-Oldhamina squamosa Assemblage Zone.Two coralline assemblage zones include Hayasakaia syringoporides-Protomichelinia submicrostoma Assemblage Zone and Tachylasma alternatum-Ipciphyllum ipci Assemblage Zone.Four Fusulinid assemblage zones include Nankinella hunanensis-Pseudofusulina Assemblage Zone,Neoschwagerina douvillei-Pseudodoliolina ozawai Assemblage Zone,Codonofusiella Assemblage Zone and Palaeofusulina Assemblage Zone.The one botanic assemblage Zone is Gigantopteris nicotianaefolia-Gigantopteris dictyolloides Assemblage Zone.In the study area,the Middle Permian Liangshan Formation directly covers the Cambrian and the contact relationship is disconformity.Based on the fossil species characteristics and exposed locations in the study area,chronostratigraphic divisions were made.The Liangshan Formation and the Qixia Formation were attributed to the lower part of the Middle Permian,corresponding to the Qixia Stage and XiangsuStage;the Maokou Formation was attributed to Maokou-Lengwu Stage of the Middle Permian;The Longtan Formation corresponds to Wujiaping Stage of Upper Permian;Changxing Formation corresponds to Changxing Stage of Upper Permian.Through field geological surveys about the Dafang area,combined with the types and characteristics of rock minerals,the characteristics of biological combinations,and the mechanism of sedimentary structure,etc.,the paper analyses sedimentary system of the Middle-Upper Permian in the study area.The Middle-Late Permian strata in the study area can be divided into two sedimentary system groups and four sedimentary facies.The two sedimentary system groups concluded the continental sedimentary system group and the marine sedimentary system group.The four sedimentary facies conclude the swamp,the continental volcanic eruptive,the coastal and the carbonate platform sedimentary facies.Then pattern diagram of the sedimentary facies was drawn.