RESEARCH ON SEDIMENTARY EVOLUTION CHARACTERISTICS OF THE JURASSIC SHAXIMIAO FORMATION IN THE SICHUAN BASIN

The main depositional environment in the Sichuan Basin during the Jurassic Shaximiao Formation included fluvial,delta,lacustrine facies and so on,which based on the analysis of field outcrop and stratigraphic characteristics. The climate changed from wet to dry resulted in lake shrinkage of the Sichuan Basin during the J₂s¹ to J₂s².The deposition setting was changed from underwater to subaerial environment in the central Sichuan Basin.

From the analysis of sedimentary characteristics in the central Sichuan Basin,it is found that within the Shaximiao Formation,form bottom to top,the thickness of sandbodies was gradually thinning,the wave-derived sedimentary structures were gradually decreasing,and the color of mudstone and sandstone was gradually turning red. All these indicated that the climate gradually turned arid from early to late,and the supply of the detrital material was weakening.Combining the sedimentary distribution characteristics of the Shaximiao Formation in the Sichuan Basin and the sandbodies evolution characteristics in the central Sichuan Basin,it was believed that the sedimentary evolution of the Shaximiao Formation in the Sichuan Basin had gone through four stages.In the J₂s¹ and J₂s^2-1,the river-normal delta-lacustrine sedimentary system mainly developed.In the J₂s^2-2, it gradually transformed into a rivershallow water delta-lacustrine sedimentary system.In the J₂s^2-3, it turned to the fluvial sedimentary system. Finally, it evolved into the distributive fluvial system in the J₂s^2-4.