川西坳陷中段中侏罗统上沙溪庙组层序岩相古地理及砂体展布特征

Sedimentary facies and sand body distribution of the Middle Jurassic Upper Shaximiao Formation in the middle segment of western Sichuan depression

为了更为精细地刻画出川西坳陷中段中侏罗统上沙溪庙组的古地理和砂体展布特征,笔者以高分辨率层序地层学理论为指导,建立了研究区上沙溪庙组高分辨率层序地层学划分方案,并进行了高分辨率层序地层格架的搭建。在此基础上以中期旋回层序为编图单元,进行了系统的古地理和砂体展布特征研究。发现川西坳陷中段中侏罗统上沙溪庙组可以划分为1个长期和4个中期(M1,M2,M3,M4)沉积期。M1和M2沉积期,研究区以发育曲流河三角洲—湖泊沉积体系为特征;M3和M4沉积期,研究区转变为以发育冲积扇—冲积平原—辫状河三角洲—湖泊沉积体系为特征,砂体的展布主要分布于两个地区:大邑—温江地区和中江地区,前者砂体主要发育于M1,M3,M4 3个中期旋回,后者砂体主要发育于M1,M2,M3 3个中期旋回。

Guided by the theory of high-resolution sequence stratigraphy,based on the classification of base level cycle sequence,and aimed at the distribution characteristics of sand bodies,this paper has divided Upper Shaximiao Formation into base level cyclic sequences of one long period（L1） and four moderate periods（M1,M2,M3,M4）.Sequence-based lithofacies and paleogeographic maps of Upper Shaximiao Formation were compiled on the basis of mapping units of middle period base level cyclic sequence.The result shows that meandering river delta facies and lake facies were developed during the stage of M1 and M2.During the stage of M3 and M4,the depositional setting was changed and characterized by the development of alluvial fan facies,alluvial plain facies,braided river delta facies and lake facies.The sand bodies were distributed in two areas,i.e.,Dayi-Wenjiang area and Zhongjiang area： the former occurred during the stage of M1,M3 and M4,whereas the latter came into existance during the stage of M1,M2 and M3.