A Study On Sedimentary Facies And Sedimentary Environment Of The Upper Jurassic Qigu Formation In The Southern Margin Of Junggar Basin,Xinjiang

The Jurassic in the southern margin of the Junggar Basin is an important oil and gas reservoir.The increase in demand for oil and gas has made Jurassic stratigraphic oil and gas exploration a hot spot.Jurassic sandstone has always been one of the focuses of geologists.Predecessors have different views on the sedimentary facies and sedimentary environment of the Upper Jurassic Qigu Formation in the study area.In this paper,based on the measured profiles of Jurassic outcrops in the Hutubi area on the southern margin of the Junggar Basin,through a combination of field geological survey and indoor analysis,the rock assemblage,lithological characteristics,grain size characteristics,provenance characteristics and carbon and oxygen isotopic composition of the Upper Jurassic Qigu Formation in the study area have been systematically studied.The sedimentary facies of the Qigu Formation have been divided,and the sedimentary environment and environmental evolution characteristics of the Qigu Formation have been determined.The results are as follows:(1)The rock assemblages of Qigu Formation in the study area are purplish red and reddish brown mudstone,medium-fine grained sandstone and argillaceous siltstone,and grey-green medium-fine sandstone,fine-grained sandstone and siltstone appear as interbeds or interbeds,with micrite limestone locally contained.Under the thin section microscope,the sandstone is feldspar lithic sandstone,and the interstitial material is clayey heterogroup,calcareous cement or tuff.The Qigu Formation develops parallel bedding,horizontal bedding,trough cross bedding,and plate cross bedding.It has an angle-unconformable contact relationship with the Kalazha Formation of the Upper Jurassic,and an angle-conformable contact relationship with the Toutunhe Formation of the Middle Jurassic.(2)The analysis of grain size characteristics,grain size diagram and provenance characteristics show that:in the sedimentary period of Qigu formation,the grain size of sandstone is mainly coarse components,which are mainly suspended particles,with good sorting ability,general grinding roundness,low maturity of composition and structure,strong hydrodynamic condition and single component.The material source of Qigu Formation is relatively concentrated,and the sandstone has not been transported for a long distance,and the provenance area is close to the sedimentary area,which is mainly provided by the North Tianshan Mountains,and it has the tectonic background of continental plate magmatic arc orogenic belt.The sedimentary environment of Qigu Formation is relatively stable,and it is the sedimentary environment of coastal shallow lake.(3)The carbon and oxygen isotopic composition of carbonate in the Qigu Formation in the study area shows that the diagenetic process was reformed by atmospheric fresh water leaching,the salinity of diagenetic fluid was low,and the sedimentary period of the Qigu formation was in an open freshwater lake environment.The negative?¹⁸O of carbonate indicates that the alteration occurred in the diagenetic process,so that the calculated temperature can only reflect the diagenetic intensity.However,no alteration phenomenon was found under the thin section microscope.It may also be that the buried depth causes the negative?¹⁸O and the high temperature.(4)According to the lithology,sedimentary structure,sedimentary color,mineral combination characteristics,paleontological characteristics,sedimentary sequence,grain size and other sedimentary facies signs of the Qigu Formation in the study area,there are three types of sedimentary facies in Qigu Formation:lacustrine facies,fluvial facies and alluvial fan facies,which can be divided into four subfacies:lakeside subfacies,shallow lake subfacies,channel subfacies and alluvial fan root subfacies.The depositional age of the Qigu Formation is the Late Jurassic,and the main sedimentary period is dry shore shallow lake environment,and local hot and dry river environment.Due to the continuous and intense tectonic activity in the northern Tianshan Mountains,the Tianshan Mountains uplifted and eroded,the lake basin area decreased.From the lower section to the upper section,oxidation is enhanced,the climate is gradually hot and dry,and the water body gradually becomes shallower.(5)The coupling relationship between basin and mountain shows that from the deposition period of Toutunhe Formation to the deposition period of Qigu Formation,due to the continuous intense tectonic activity of North Tianshan,the uplift and denudation occurred in the North Tianshan,and the transformation of the southern margin of Junggar Basin from the extensional environment to the local compression environment resulted in the lake regression of the basin,the deposition filling occurred in the basin,and the lake basin area decreased.the rapid uplift in the northern Tianshan during the Karazha Formation deposition period,which resulted in the complete lake regression of the basin and the end of filling in the Jurassic basin.