Study On Sedimentary Facies And Trace Fossils Of The Mugagangri Formation In The Doma Area Of Shuanghu Tibet

The study on the composition and structure of Bangong Lake-Nujiang suture zone and the attributes of Banuyang basin is a hot area and key issue in the study of QinghaiTibet Plateau in recent years.The Mugagangri formation in Qiangtang basin in northern and southern Tibet is the product of the mature stage of the basin during the evolution of Banu Ocean,and the study on it is undoubtedly of great significance.Based on the regional geological survey project of 1: 50 000,on the basis of field profile measurement and laboratory test and analysis,this paper studies the age attribute,sedimentary facies,sedimentary environment and trace fossils of Mugagang formation in Doma area,Shuanghu County,Tibet.Some new achievements and understandings have been made.Biostratigraphy: through the measurement of two strata profiles and the geological survey of the route,a large number of hexagonal coral fossils have been obtained from the external collapse limestone blocks in the clastic rocks of the Mugagang formation in Doma area,and identified by laboratory sections.A total of 2 genus and 2 species(both undetermined species)were obtained,that is,Beneckastraea sp.(undetermined species of Benek corals)and Stylina sp.(undetermined species of corals)As far as these two genus are concerned,although the age distribution of the latter is wide(T3-K1),the former is a characteristic fossil of the late Triassic,and the occurrence horizon at home and abroad is limited to the Upper Triassic,which appears in the collapse limestone block.It is enough to show that the age of the Mugagang formation in the study area should be later than the late Triassic,considering that Ding Qianfu(2017)determined the Shamuluo formation,which was integrated on the Mugagangri formation according to the bivalve fossils,to be the Middle and late Jurassic.Therefore,the period of Mugagang Day formation should be between these two ages,that is,earlyMiddle Jurassic.Chronostratigraphy: the latest age data of 206 ±4.0Ma have been obtained for the dating of clastic zircon U-Pb in the sandstone of Mugagang formation,and the limited age is consistent with the results of biostratigraphy.Section sedimentary facies: according to the lithologic assemblage,sedimentary structure and fossil-bearing characteristics of the section,one sedimentary facies(sea bottom fan),three sedimentary subfacies(inner fan,middle fan and outer fan)and several sedimentary microfacies types are divided into three types in Mugagangri formation.According to the phase sequence structure of the section,it is divided into three sedimentary cycles,and the sedimentary phase model of the Mugagang formation is established.It is pointed out that the Mugagang formation is not a single seafloor fan,but a composite sector superimposed by multiple sectors.Study on relic facies: a large number of trace fossils collected from Mugagang Japanese formation were identified in detail,and 14 relic genus and 14 relic species(including 4 undetermined relic species)were obtained.According to the occurrence characteristics of these trace fossils with multiple parallel planes or small angle oblique planes,combined with the analysis of some of the characteristic molecules,they are classified as Nereites relic phases divided by Seilacher(1967).It is considered that the sedimentary environment belongs to the low energy and anoxic reduction environment under the background of semi-deep sea and deep sea,which provides evidence for the study of sedimentary facies.