Discussion On Stratigraphic Characteristics And Age Attribution Of Shilidun Formation In Minxian Area Of Western Qinling Mountains

The northern margin of the West Qinling Orogenic Belt spans two stratigraphic units,North China and South China Blocks,with complex regional structure.The Minxian area in this study is located in the northern margin of the West Qinling Orogenic Belt.The stratigraphic research in this area is relatively low,and stratigraphic division needs to be further combed and strengthened.Based on the project of 1:50000 regional geological and mineral survey of Zhongzhaixiang and Shilipu in Minxian County,Gansu Province,this paper selects the Middle-Lower Permian Shilidun Formation(P1-2sl)stratum exposed in the Middle Qinling micro-block stratigraphic area as the research object.Based on the detailed field geological survey and sections of key locations,it uses sedimentary petrology,statistics of clastic skeleton composition,age spectrum of clastic zircon,regional stratigraphic comparison,etc.,comprehensively study the lithology,rock combination,stratigraphic sequence,sedimentary environment,source area tectonic background of the sedimentary strata of the Middle-Lower Permian Shilidun Formation in Lintan area of ??the Western Qinling Orogenic Belt.By means of regional correlation,the original cooperative Minxian-Tanchang fault in the study area is discussed the Early Carboniferous Badu Formation and the Late Carboniferous Xiajialing Formation to the south of Zhongzhaixiang area.The following progress and recognition have been made:1.The Shilidun Formation is divided into three lithologic sections,which are divided into eight basic sequence types.The main body of the Shilidun Formation is a set of gravity flow sedimentation,which is formed in the semi-deep sea deep-sea sedimentary environment.The sedimentary source of the sediment is relatively single,the sedimentary composition is relatively simple,and the corresponding sedimentary environment is relatively moderate.Based on the data of sedimentary structure,detrital zircon U-Pb age spectrum and geochemistry,it is limited the source of sediment is mainly from the northern margin of the basin.2.The Early Carboniferous Badu Formation and the Late Carboniferous Xiajialing Formation in Zhuopinggou-Yuanshangou area of Minxian are different from the Carboniferous Badu Formation and Xiajialing Formation in the region.The rock association is basically the same as the Shilidun Formation in the region.3.Microfossil conodonts were collected from the Early Carboniferous Badu Formation and the Late Carboniferous Xiajialing Formation in the north of the original cooperative Minxian Tanchang fault to the south of Zhongzhaixiang.In combination with the regional stratigraphic exposure,the former Carboniferous Badu Formation and Xiajialing Formation are both classified as Permian Shilidun Formation.Debris zircon U-Pb chronology of the Shilidun Formation indicates that the latest age of the detrital zircon is 287.9 Ma.Therefore,the sedimentary age of the Shilidun Formation should not be older than 287.9 Ma.Combined with the paleontology data of the project team,the study area is comprehensively limited.The stratigraphic age of the Shilidun Formation is Early Middle-Permian.