Lower To Middle Triassic Chang Landmark Group Formation Characteristics And Its Analysis Of Sedimentary Facies In Youyun Area,North Bayankala,Qinghai Province

The study area located in the east to south of Qinghai province,the Bayankala tectonic belt and the south slope of the animaqing mountain range.The structure of the area first started from the Variscan,and formed the Maerzheng formation volcanic rocks and Carbonatite intercalated clastic rock formation,These rocks formed the basement of the Triassic turbidite basins in this area,it lay the foundation for the formation of the lower to middle Triassic chang landmark group formation in the study area.This article is based on lithostratigraphy and biostratigraphic data obtained from the measured stratigraphic profile in the study area,combined with previous data,this paper makes the multi-stratigraphic subdivision towards the stratum in the study area and sedimentary facies analysis on the lower and middle Triassic,estimate the distribution of sedimentary facies and evolution in the study area,and achieves the depositional model of the interim and Lower to middle Triassic in the study area.There are six measured sections in this paper,according to the results of the section,we concluded the stratigraphic lithology and rock assemblage characteristics in this research area.The study area has a huge thickness of strata,almost entirely composed by sandstone strata,hardly to see the bottom;The main lithology is gray,grayish green,brownish coarse-fine grained sand stone,feldspar quartz sandstone,quartz sandstone sandwich siltstone,slate and thin layer limestone,sandstone can contain gravel.The sedimentary thickness is large,lithological monotonicity,biological poor,with turbidity flow sedimentary characteristics.Based on the types and characteristics of the fossil,outcropped strata and distribution of ancient organisms in the study area's layer,two groups of ammonites were established:Wordiceras-Flemingites,Subcolumses-Procarnites-Prohungarites,a bivalve assemblage zone: Claraia aurita peak,abundant sporopollen and a large number of new calamite plant fossils and two conodont combination belt:Parachirognathus-Pachycladina,Neospathidus belt,also contains brachiopoda,snails,fusulinida fossils,which provide a basis for the next step of sedimentary facies analysis.Sedimentary facies analysis has been done based on the lithostratigraphy and biostratigraphy in the study area and The North Bayankala lower and middle Triassic is divided into ocean sedimentary system and turbidity flow sedimentary system.The Marine sedimentary system mainly includes shallow sea shelf facies,semi-deep-sea-deep-sea sedimentary facies;According to the sedimentary lithology,it can be divided into inner continental shelf subfacies and outer continental shelf subfacies.Turbidite sedimentary system is mainly slope turbidite fan phase.The turbidite fan facies of the continental slope are subdivided into the slopes down-turbidite fan,Mid-turbidite fan and the up-turbidity fan.According to the study of the middle and lower Triassic sedimentary facies in NorthBayankala,this paper established the sedimentary facies model of the early and middle Triassic in North Bayankala and believes that sedimentary model in the study area is the sedimentary facies mode pattern has been summarized in this paper.