Sedimentary Sequences And Environmental Evolution Of Lower-Middle Triassic In Maqin County,Qinghai Province

This paper is based on the northern mountainous area of Qinghai province 1:5 million Youyun Township,Changma River six regional geological survey project,by field geological survey,select the Maqin County of Qinghai Province in three side of underground strata as the research object,the field occurrence,rock combination,spatial distribution characteristics and sedimentary environment do a systematic Research on the study area in three Triassic strata,Based on the comparison of rock strata between Gande Formation and Changmahe Formation in the region,the statistical analysis of grain(gravel)degree of glutenite in the region,the summary of structural characteristics and the analysis of sedimentary facies,the stratigraphic characteristics of the Lower-Middle Triassic in the study area,the geological age of formation and the sedimentary environment in the region are determined.1.in the course of the study,further determined the side area under the three Triassic Changma River group,gander group stratigraphic unit,a detailed discussion of the lithology,paleontology and age groups,rock assemblages and other issues.The2.The Permian Triassic strata are continuous and fossils are continuous,so they are integrated contacts.But there is a transition between them,from shallow sea to deep-sea and semi deep sea.3.Arnautoceltites sp.and Leiophyilites sp.brachiopods Neophricodothyris cf.atiatica(Chao)coral Iranophyllum sp.are found in the bioclastic limestone of Western Changma River.Leiotriletes sp.,Lund-bladispora sp.,Kracuselisporites sp.and Neocalamites Mariani(Brongniart)Halle were collected from the lower strata of Gande Formation.4.Changmahe Formation and Gande Formation are a set of deep-sea to semi-deep-sea continental slope turbidite deposits,and the lower part of Changmahe Formation is shallow-sea clastic rock deposits gradually developed into slope turbidite fan.The gander formation is mainly sub deep-deep sea sand plate facies.5.The Paleo-sedimentary environment in the study area is high in the South and low in the north,and deep-water sedimentary environment in the north and coastal land environment in the South.It is inferred that the southern part is a craton plain,which is the source of the material supply.The weathered material from the craton plain is transported to the north with flowing water,and the sedimentary strata of Changmahe formation and Gande formation are gradually formed.The depositional environment of the three Permianis characterized by a swirling depositional model.At first,it is transgression,then regression,and then transgression.When transgression,it reflects the sub-deep-sea basin facies sedimentary model;when transgression,it shows the shallow shelf facies sedimentary model and the slope turbidite fan sedimentary model.