The Bagong Formation of Upper Triassic in the northern Qiangtang Basin is distributed in the southern and northern sides of Geladandong area with a strap of northwest strike(the southern part is named as the Tumengela Formation formerly)，and tectonicly exists between the southern part of Jinshajiang suture belt and the middle uplift belt. Three assemblages have been recognized, among which one of the bivalves assemblages named  Cardium(Tulongocardium)martini—Trigonia(Kumatrigonia)huhxilensis　is recognized from the Tumengela area of the southern side, and the other two assemblages named  Halobia superbescens—Halobia　disperseinsecta　assemblage　and　Amonotis togtonheensis—Cardium(Tulongocardium)xizangensis　assemblage from the Quemocuo area of the northern side have been established in this paper. Although these assemblages are considered as to be in  the similar age as the earlymiddle Norian of Late Triassic, the predominant species of each assemblage are greatly different. Based on the investigations and analyses of the　provenance, sedimenttypes, sedimentary features of highfrequency cycles, palaeosalinity,  paleosea level, paleoclimate, lithochemical and trace element contents, as well as biotic characters, the palaeogeographical and paleoecological characteristics of the study region are discussed. It has been concluded that the bivalves with a relative lower specific diversity in the southern  region lived in shallowwater environments with a lower salinity and oxidation and a higher evaporation and alkalescence; however, those in the northern region lived in deepwater sedimentary environments with a higher salinity and deoxidization and warmhumidity and flourished during the late period of the transgression and early period of the regression. These biotic differences between the northern and southern regions were caused by different sedimentary environments in the same geologic period.