准噶尔盆地南缘上侏罗统-下白垩统沉积序列及沉积环境演化

Sedimentary Sequence and Depositional Environment Evolution of Upper Jurassic-Lower Cretaceous Strata in the Southern Margin of Junggar Basin

准噶尔盆地南缘上侏罗统-下白垩统的沉积相,可以分为曲流河相、冲积扇相、湖水改造冲积扇相和滨浅湖亚相。根据构型要素分析法,划分出10种主要典型岩石相。其中Gm1为褐红色块状砾岩,复成分,粒度为细-粗砾,分选、磨圆差,主要为杂基支撑;Gm2为褐色块状砾岩,复成分,粒度为细-中砾,分选和磨圆一般-差,主要为颗粒支撑;Gm3为灰绿色、橘红色块状细砾岩,分选、磨圆较好,颗粒支撑。Gm1和Gm2沉积期,多处发育震积构造。中-晚侏罗世,气候干旱,北天山构造活动,盆地快速充填,可容纳空间减小,上侏罗统齐古组主要发育曲流河沉积。受蒙古-鄂霍次克洋快速闭合的影响,张性环境演变为局部挤压环境,北天山继续构造活动,盆地收缩,可容纳空间减小,盆地边界由早—中侏罗世的中天山退至北天山,上侏罗统喀拉扎组沉积期,准噶尔盆地南缘广泛发育褐红色冲积扇沉积,侏罗纪盆地充填结束;在下白垩统清水河组沉积期,蒙古-鄂霍次克洋闭合完成,构造相对稳定,盆地扩张,可容纳空间增大,大范围快速湖侵,气候转为湿润,准噶尔盆地南缘发育湖水改造冲积扇沉积,之后广泛发育滨浅湖沉积,发育灰绿色、橘红色滨湖砾岩及细粒沉积,白垩纪盆地充填开始。

The sedimentary facies of the Upper Jurassic-Lower Cretaceous strata in the southern margin of Junggar basin can be divided in⁃to meandering river,alluvial fan,alluvial fan transformed by lake and shallow lake facies.According to the architecture element analysis,10 main lithofacies is classified,among which polymictic Gm1 supported by matrix is massive maroon conglomerate with fine-coarse grain size,poor sorting and roundness;polymictic Gm2 supported by grain is massive brown conglomerate with fine⁃medium grain size,moderate⁃poor sorting and roundness;Gm3 supported by grain is massive greyish green or orange fine conglomerate with good sorting and roundness.Some seismite structures were developed during the depositional periods of Gm1 and Gm2.During the Middle to Late Jurassic,the climate was arid,structural activities were active in the northern Tianshan,the basin was filled rapidly and the accommodation space reduced and meandering river deposits were developed in the Upper Jurassic Qigu formation.Under the influence of the rapid closure of the Mongol-Ok⁃hotsk Ocean,the extensional tectonic setting evolved to a local compressive setting,structural activities in the northern Tianshan continued,the basin contracted,the accomodation space reduced,and the boundary of the basin withdrew from the central Tianshan to northern Tian⁃shan.During the deposition period of the Upper Jurassic Kalazha formation,brownish red fluvial fan sediments were widely developed in the southern margin of Junggar basin and the filling of the Jurassic basin ended.During the deposition period of the Lower Cretaceous Qin⁃gshuihe formation,the Mongolia⁃Okhotsk Ocean closed,the tectonic setting became stable,the basin expanded and the accommodation space increased,large⁃scale rapid lake transgression occured,the climate became humid,then fluvial fan deposits were developed in the southern margin,followed by shallow lake deposits,greyish green or orange lacustrine conglomerate and fine deposits,indicating the begin⁃ning of the filling of a Cretaceous basin.