Sequence Stratigraphy And Sedimentary System Of Jurassic In The Northern Margin Of Qaidam Basin

Based on the drilling core,logging and geochemical data,this paper studies the sequence stratigraphy,sediment source,sedimentary environment and sedimentary facies of Jurassic in the northern margin of Qaidam Basin by using the theories and research methods of high-resolution sequence stratigraphy and sedimentology.Based on the analysis of drilling and logging data,core observation description and variation characteristics of major and trace elements,the high-precision sequence stratigraphic framework of Jurassic in the northern margin of Qaidam Basin was established.Three second-order sequences were identified,corresponding to the Lower Jurassic(JSQ1),Middle Jurassic(JSQ2)and Upper Jurassic(JSQ3),and further divided into 11 third-order sequences.The second-order sequence is used as the unit to draw the residual thickness map of each sequence,and the third-order sequence is used as the unit to carry out the sequence stratigraphic correlation.The results show that the lower Jurassic is mainly distributed in Lenghu Niudong area,the Middle Jurassic is mainly distributed in Jiulongshan area,and the upper Jurassic is mainly distributed in Yuka Jiulongshan area.The upper part of each sequence is seriously eroded,the spatial distribution of the sequence is obvious,the facies is not overlapped,and the succession is poor.Based on the analysis of the heavy and light mineral composition of the collected samples,it is found that the lower Jurassic Niudong area in the northern margin of Qaidam Basin is dominated by garnet + zircon assemblage,the Lenghu area is dominated by anatase + zircon assemblage,and the Saishiteng Piedmont is dominated by anatase + zircon + garnet assemblage.The isoline distribution characteristics of rock mineral framework components and sand land ratio also indicate the existence of three provenance areas.By the middle and Upper Jurassic,the provenance has changed obviously.The middle provenance is distributed in the front of Saishiteng mountain and Jiulongshan area,while the upper provenance mainly exists in Jiulongshan area.Based on the analysis of paleoclimate,paleosalinity,paleoredox conditions and other geochemical indicators,the sedimentary environment of the early Jurassic in the northern margin of Qaidam Basin has been accurately restored.It is considered that this area belongs to humid climate,fresh water,semi transitional and semi reductive water environment,and the climate of Niudong area is wetter than that of Lenghu area,and the reducibility of water is slightly weaker,These conditions are favorable for the development of Early Jurassic source rocks.On the basis of detailed core observation and description,combined with the analysis of logging curve characteristics,it is concluded that there are 17 lithofacies assemblages and 6 sedimentary facies types in Jurassic of northern Qaidam Basin.Fan delta and lake deposits are developed in Niudong area of Lower Jurassic,fan delta,nearshore subaqueous fan and sublacustrine fan deposits are developed in Lenghu area,and braided river delta deposits are developed in front of Saishiteng mountain;Fan delta and lacustrine deposits are developed in front of Saishiteng mountain,and Braided River Delta and lacustrine deposits are developed in Yuka Jiulongshan area.The lower Jurassic to Upper Jurassic sedimentary areas migrated from west to East,and the scale of the lake basin gradually shrunk until almost disappeared.