Sedimentary Characteristics Of The Late Jurassic Weimei Formation In Rongbu Area,Southern Tibet

Southern Tibet is one of the most typical regions of Mesozoic Tethys marine strata in China.It is also an important region to learn about the time and space evolution of Jurassic strata,sedimentation,paleontology and geological structure in Tethys Himalaya.The study of the sedimentological records preserved in the Himalayan strata of Tethys,Tibet,will not only help us to understand the evolution characteristics of the new Tethys,but also provide direct evidence for reconstructing the Jurassic paleogeography of the region.In this paper,modern petrology,sequence stratigraphy,petrography,sedimentary geochemistry and other methods are used to study the sedimentology,sequence stratigraphy,stratigraphic age,sedimentary model and paleogeography of the Eate Jurassic Weimei Formation in Rongbu,southern Tibet.The study area experienced a relatively complete transgression and regression cycle in the Late Jurassic and the Weimei Formation is a set of medium-thick layered gravelly quartz sandstone,quartz sandstone and thin-layer siltstone,silty mudstone,shale and other terrigenous clastic rock combinations.The Weimei Formation has a middle-level sequence,which is mainly composed of two positive sequences,including lowstand systems tract,transgressive systems tract and highstand systems tract.According to the lithologic combination,sedimentary structure,thickness change and paleontological difference of the Late Jurassic strata in the study area,the identified sedimentary facies are mainly delta facies,littoral facies and neritic facies.The sedimentary subfacies include delta fluvial facies and delta plain facies in lowstand,sandstone facies in transgression stage,neritic shelf platform facies in highstand,shelf margin basin facies in highstand,and neritic platform in regression stage ground shelf facies and shallow sea shelf facies.The sedimentary geochemical characteristics show that from Late Jurassic to Early Cretaceous,the sedimentary environment gradually shifted from the oxidation-reduction transition environment with weak stratification to the reduction environment,the salinity of the sedimentary water body gradually increased,the sea level increased greatly,and the sedimentary water body had a significant increase in depth.The U-Pb ages of sandstone detrital zircons in Weimei Formation are mainly distributed in 950-1000 Ma,which matches with the age of detrital zircons in East India.It is inferred that the material source is mainly from the supply of the south side of the passive Indian continent,i.e.the stable craton basin in the Indian continent.According to the grain size analysis and test results of the clastic rocks in Weimei Formation,in the early stage of the Late Jurassic,the region was dominated by the sea land intersection delta sedimentation,with the arrival of the transgression period,with the rise of sea level,the coastline gradually extended eastward,and the study area gradually transited to the shallow sea shelf facies.In the middle of Late Jurassic,the sea level began to decline slowly,and then the progradational shallow sea shelf facies took the dominant position.At the end of Late Jurassic,the sea level in the region rose again,and its transgression scale was weaker than before,but the transgression frequency was more.In the shallow sea background,the basin was tensioned several times,accompanied by the eruption of basic volcanic rocks of fault activity,forming the grid of alternating volcanic rocks and clastic rocks.