A Study On Jurassic Bivalves Biostratigraphy Of Duoma Area In Shuanghu,Northern Tibet

Based on the field profile and fossil collection,the bivalve assemblages were established through the fossil chamber identification and analysis,which were compared with other bivalve animals in China.And make a discussion about the era ownership of the Mugagangri Formation and Shamuluo Formation in the Duoma area,Shuanghu area of Tibet,we made the following conclusions:(1)26 species referred to 17 genera(including the uncertain species)were obtained by the detailed chamber evaluation,after collecting rich bivalve fossils in the Jurassic region of Duoma,Shuanghu area of Tibet.It was not only greatly enriched the information of the fossils,but also laid foundation for classifying the era of the Mugagangri Formation and Shamuluo Formation.(2)The bivalve assemblages of Pergamidia irregularis-Waagenoperna aff.planata(PW)and Astarte amdoensis-Protocardia lamellose(AP)were established in the study area according to the natural production sequence and combination characteristics of fossils in the section.(3)The combination of PW and AP belong to the Late Triassic Norian and the Middle Jurassic Bajocian to Callovian respectively,based on the discussion of the above two combinations of constituent structures,the distribution of each species,especially the characteristic molecules and the dominant molecular history,and the detailed comparison between the local bivalve assemblage with other related strata.(4)By analyzing the formation and output characteristics of upper and lower strata,we concluded that the AP assemblage comes from the limestone interlayer of the Shamuluo Formation lower-middle member,which the era of the Shamuluo Formation lower-middle member,that is,from Middle Jurassic Bajocian to Callovian.Combined with the era of the Shamuluo Formation upper member in regional,the age of the Shamuluo Formation was from the Middle Jurassic Bajocian to Late Jurassic Kimmeridgian.The combination of PW is come from the isolated limestone blocks(collapsed limestone blocks)in the Mugagangri Formation of siltstone and mudstone rhythmic interbreeds beneath the Shamuluo Formation.As the Mugagangri Formation is the deposition of the main body of sandstone and mudstone which may be later than the isolated limestone block but earlier than the overlying Shamuluo Formation,the age of the Mugagangri Formation was from the Early Jurassic to Middle Jurassic Aalenian period.