The Transgression-Regression Cycles And Sedimentary Evolution Of Evaporites Of Paleocene In Yarkand Basin,Xinjiang

Yarkand basin lies in the southwestern Tarim basin of Xinjiang,since the Mesozoic it experienced tectonic evolution,transgression and regression cycles and evaporites sedimentation process,which is similar to the evolution of Central Asia basin.This is demonstrated in widely sylvite in Central Asia basin in Jurassic and widely salt in Yarkand basin in Cenozoic were deposited.The unique geological background and favorable ore-forming conditions of this region make it has greater potential and prospect of Potassium enrichment.However,due to the lack of technology and drilling data,it is still a weak point in the line of studying basin evaporites.Based on field investigation,mineral and chemical analysis of drilling cuttings,sedimentary rhythm division of drilling profile and sedimentary cycle identification,this paper determines output layers of evaporites strata,distribution and mineral characteristics etc.and also discusses sedimentary characteristics of regional evaporites,concentrated trend and evolutionary environment of ancient salt lake in the background of transgression,so as to make conclusion of migration rule of evaporites and controlling factors,providing geological basis for further analysis of potash forming environment.Through this research,this paper has the following preliminary understanding:The sedimentary framework of the Paleocene in Yarkand basin was built according to field geological profile measure and drilling profile data.It was preliminary identified that 5 transgression-regression cycle and evaporites sedimentary cycle happened in Yarkand basin from Cretaceous period to Paleocene period,among which the Aertashen-Qimugen sedimentary period was one of most important sedimentary period of evaporites.Through field outcrop investigation,profile measure,drilling profile and previous studies,this study determined that the main storage layers of the Paleocene evaporites in Yarkand basin located in Aertashi Formation,which was greatly characterized by development of gypsum with a stable and sustainable distribution in the entire basin.There is a bigger difference on the lateral thickness of gypsum,and gypsum was more developed in front part of western Kunlun Mountain.The main species of saline mineral in basin are halite,gypsum and secondary Glauberite,potassium-bearing minerals are potassium-bearing gypsum,magnesium-bearing gypsum.It can be seen from the mineral assemblage that evaporation intensityof Yarkand basin wasquite strong during the sedimentary periodof Aertashi salt lake and the ancient lake has at least evolved to the stage of sylvine-precipitation.It is worthy of further attention and research.Sedimentary rhythm partition and sedimentary cycle identification were undertaken on 8 evaporites sections of drilling holes,and 1stage of sedimentary cycles of evaporites are identifiedin early Paleocene.4 stages of secondary cycles which respectively indicate 4 transgression stages of different period were also identified during period of intermittent transgression.It assumed that the distribution of evaporites in the basin tends to develop from west to east.Transgression stages and transgression scope of each stage during sedimentary period of Aertashi Formation were preliminary speculated.Chemical analysis of drilling debris was applied to determine 4 concentrated trends of the ancient lake happened during sedimentary period of Aertashi Formation of the Paleocene,including 1 period of concentrating into salt and 3 concentrated trends.The bottom of Aertashi Formation is rich in potassium and the trend is obvious,therefore it is favorable potassium-rich layer.Based on sedimentary characteristics of regional evaporites,saline mineral assemblage and evolutive environment of the ancient salt lake in the Yarkand basin,it can be concluded that the favorable salt forming and potashforminglayer is the lower member of Aertashen formation.