Study On Sedimentary Facies And Ammonite Fossil Of The Sewa Formation In The Doma Area Of Shuanghu Tibet

In recent years,one of the hotspots and key issues related to research on the Qinghai-Tibet Plateau is the evolution of the Banu Ocean Basin and the structure and attributes of the Bangong Lake-Nujiang suture zone.Based on the regional geological survey project of 1: 50 000 I45E022023.The research area is located at the junction of the Bangong Lake-Nujiang suture zone and the Karakorum-Nanqiangtang-Zuogong block,which is the structure of the Banu Ocean basin Part of evolution.The Sewa Formation is a product of the basin entering the depletion stage during the continuous evolution of the Banu Ocean.The study of its age and sedimentary facies is of great significance for further exploration of the evolution of the Banu Ocean.Based on the measured and the ammonite fossils collected in the field,the age of the Sewa Formation in the Shuanghu Doma area of Tibet was determined by sedimentary facies analysis,identification of ammonite fossils,and establishment of ammonite assemblage.The following achievements and understandings have been achieved:Regarding the lithostratigraphy,according to the lithological excess relationship of the measured section,the conformable contact boundary between the Sewa Formation and the overlying Shaqiaomu Formation was found,which confirmed the conformable contact relationship between the Sewa Formation and the overlying Shaqiaomu Formation in the study area.I terms of biostratigraphy and chronostratigraphy,a total of 6 genera and 7 species ammonite fossils were collected from the clastic rocks of Sewa Formation in the study area,and two ammonite assemblage were divided.After indoor identification,the ammonite fossils were divided into two groups according to the horizon,which were the first group: Ptychophylloceras cf.tatricum,Dorsetensia haydeni arkell,Dorsetensia cf.regrediens(Huag),Witchellia tibetica Arkell,and the second group: Neuqueniceras cf.Kobayashi and Fukada,Homoeoplanulites cf.acuticosat(Roemer),Oxycerites cf.subcotarius Oppel.According to the characteristics of the age indicated by the ammonite fossils,after comprehensive consideration of previous studies,the age of the Sewa Formation is from the late Early Jurassic Toarcian to the late Middle Jurassic Callovian.Regarding the sedimentary facies of the section,according to the lithologic association,sedimentary structure and fossil characteristics of the two measured sections,the Sewa Formation in the study area was classified as a submarine fan sedimentary facies.On this basis,Two sedimentary subfacies of middle-fan and outerfan,and underwater distributary channel,levee-overbank,lobe,cut channel sedimentary micro facies are divided.A total of two sedimentary cycles were identified and together formed the transgressive-regressive-transgressive sequence.As a whole,the Particle Size becomes coarser,and the sand-shale ratio increases.Although the rhythm of the water depth changes,the overall trend is regressive,which means that Banu Ocean has entered the stage of reduction.