Stratigraphy And Sedimentary Environment Of Late Triassic Norian In Baoji Area,tibet

The research area of this paper is located in the eastern section of the Coqen Basin,belongs to Baoji Township,Bangor County,Naqu District,Tibet Autonomous Region.The predecessors did not identify the Upper Triassic in the area,but the author identified the Upper Triassic through Paleontological and Stratigraphic studies in the area.The series of progress achieved is as follows:In terms of paleontology,the author collected rich Late Triassic Norian coral fossils and conodont fossils in two stratigraphic sections and four fossil observation points measured in Nam Co.,Baoji.Among them,there are a total of 17 genera and 34 species of coral fossils,including Distichophyllia norica(Frech),Retiophyllia?sp.,Gablonzeria?sp.,Elysastraea juliana Turnsek,Volzeia sublaevis(Munster),Guembelastraea?sp.,etc.Conodont fossils are total 1 genus and 4 species,including Epigondolella primitia,Epigondolella sp.,Epigondolella bidentata,Epigondolella postara.In terms of stratigraphy,this paper describes the two stratigraphic sections and four fossil observation points of the Upper Triassic in the study area in detail.The lithology is mainly composed of bioclastic limestone,sandstone interbedded with limestone,reef limestone,oolitic limestone and calcarenite.The Lower Cretaceous Langshan Formation(K1l)and the Lower Cretaceous Duoni Formation(K1d)were revised to the Upper Triassic Mailonggang Formation(T3m).The Triassic stratigraphic sequence was established the Upper Permian-Lower Triassic Mujiucuo Formation(P3-T1m),the Middle Triassic-Upper Triassic Camian Garezhadi Formation(T2-T3 g),the Upper Triassic Norian Mailonggang Formation(T32m),the Upper Triassic Rhaetian-Lower Jurassic Quehala Formation(T33-J1?q).In terms of lithofacies paleogeography,the discovery of the biostratigraphic profile of the study area proves that the Late Triassic sediments in this area are not missing.Through the study of the stratigraphic lithology characteristics and fossil characteristics of the two measured stratigraphic profiles and four fossil observation points in the area,it is shown that the Late Triassic Norian period in this area is carbonate platform facies.According to the comprehensive data,the Late Triassic Norian period strata are deposited in the Mailonggang of Linzhou,Jiangrang area,Yawa area,West of Namco,Bangong Co-Nujiang structural belt and Zhaxicuobu.The Bangong Co-Nujiang structural belt and the northern part of Coqen Basin is the carbonate sedimentary facies depositional environment in the Late Triassic Norian period.In terms of oil-gas exploration prospects,the Upper Triassic Mailonggang Formation of the reef flat facies has been identified in the Baoji Township area of Bangor County in the eastern part of the Coqen Basin,indicating that the Coqen Basin not only has the slope facies of the Jiangrang Formation,but also the reef flat facies Mailonggang Formation.Through the fluorescence exposure of the rock flake of the Mailongang Formation,a large number of oil-gas shows were found,indicating that the Late Triassic strata in the Baoji area have good prospects for oil-gas exploration.This further proves that the Upper combination of Guge Layer System-Upper Triassic Mailonggang Formation is also an important oil-gas exploration stratum,which enhances its oil-gas exploration value.