Stratigraphy-Sedimentation And Tectonic Evolution Of Dunhuang Basin During The Jurassic

A series of Mesozoic-Cenozoic sedimentary basins have been developed in the northwestern part of China,which enriched oil and gas resources.It is an important carrier to study the tectonic evolution and basin-mountain coupling in northwestern China.The Dunhuang Basin is located on the northern margin of the Tibet Plateau and is controlled by the Altun fault.The basin is divided into two sub-tectonic units: the Andun depression and the Akesai depression.The river-lake sedimentary strata are thick and the lithology changes obviously.An ideal area for studying the Mesozoic-Cenozoic tectonic expansion in the northern Tibet Plateau.The Jurassic basin is an important shale gas target layer and an important material for studying the evolution of the Mesozoic basin.Based on the Jurassic strata,sedimentary environment,paleocurrent,detrital zircon geochronology and its sedimentary source changes,this paper discusses the Mesozoic-Cenozoic tectonic evolution process of the basin and its research data.In relation to the activities of the Altun fault zone,the paper achieved the following results:(1)The Jurassic in the Dunhuang Basin includes the Early Jurassic Dashankou Formation,the Middle Jurassic Zhongjiangou Formation and the Xinhe Formation,and the Late Jurassic Boluo Formation.Through the analysis of the sedimentary core sedimentary facies of the outcrop section of the Dunhuang Basin and the Dunye No.1 well,the Jurassic sedimentary system in the Dunhuang Basin was identified,including four types of sedimentary environments: alluvial fan,fan delta,braided river and lacustrine facies.In space,alluvial fans,braided river deltas and facies sedimentary environments develop at the edge of the basin,and transition to fan delta,delta and lacustrine sedimentary environments.In time,the early Jurassic alluvial fan developed,the Middle Jurassic mainly developed rivers and lakes,and the Late Jurassic was alluvial fan and river.Form a coarse-thin-thick sedimentary cycle.(2)The LA-ICPMS U-Pb chronological analysis of the Jurassic detrital zircons in the Dunye No.1 well of the Andun depression shows that the zircon age components at the bottom and top of the Middle Jurassic Zhongjiangou Formation vary greatly.The bottom detrital zircon age components are mainly in the range of 360Ma-480 Ma and 1560-2040 Ma,and a small number of age points are between 660Ma-720 Ma,indicating a wide source of sources;the top peak age is concentrated at 316Ma-469 Ma.The source of the Middle Jurassic Xinhe Formation is single,concentrated around 420Ma;the late Zircon Boluo Formation zircon age is concentrated at 284Ma-518 Ma.Compared with the formation age of rock mass in the surrounding area,in the early Middle Jurassic,the sediment source of the Andun depression in Dunhuang Basin mainly came from the northern Beishan area and the southern Sanweishan Mountain;the middle to late Middle Jurassic,the source of single source,the main source In Sanweishan.In the Late Jurassic,the sediment source again became the supply of the two directions of Beishan and Sanweishan.(3)Comprehensive stratigraphic-sedimentation and provenance analysis show that the Jurassic basin in the Dunhuang Basin is a depression basin,and the prototype basin is larger than the current basin.The basin experienced three evolution stages in the Jurassic: early Early Jurassic-early Middle Jurassic,the basin was in the Fault basin stage;the Middle Jurassic in the middle of the Middle Jurassic,the basin depression stage;the late Jurassic basin in the overall depression.The activities of the Altun Fault in the Jurassic did not affect the Dunhuang Basin.