Sedimentary Characterics And The Proto-basin Analysis Of The Western Sichuan Basin In Early Jurassic

The western Sichuan basin is located in the western part of the Sichuan basin.The presently boundaries of the western Sichuan basin are Longquan Shan in the east and Longmen Shan thrust belt in the west.Deposits of the Pre-Triassic is mainly composed of marine carbonate rocks.Terrestrialsedimentary environment has been occurring with huge continental deposits since the Late Triassic.Due to the present day parallel tectonic framework between the Longmen Shan thrust belt,western Sichuan basin and Longquan Shan uplift from west to east,the western Sichuan basin has been considered to be controlled by the Longmen Shan thrust belt since the Late Triassic.However,in the middle-southern segments of the Longmen Shan belt,Jurassic strata was cut by the Guanxian-Anxian fault,the easternmost top-to-the southeast thrust fault,moreover,deposits in this region mainly contain sandstone and mudstone which indicate a distant sedimentary region from the source area.Thus the western boundary of the Early Jurassic proto-basin was not controlled by the Guanxian-Anxian fault.Therefore,reconstructing the proto-basin of the Early Jurassic is important to understand the relationship of the basin and mountain during this period.This study analyzes the borehole from Sinopec and outcrops in the field.Through the sedimentation analysis of the Early Jurassic strata in western Sichuan basin,alluvial fan-fan delta-shallow lacustrine-semi-deep lacustrine facies are identified from north to south.These sedimentary facies belts are distributed with west-east striking,and alluvial fan occurs just in the northern front of the Longmen Shan belt.Distribution of the shallow lacustrine-semi-deep lacustrine facies reveal the south boundary of the proto-basin and an essential different striking of the PaleoLongmen Shan belt.The Early Jurassic sedimentation rate and ostracum beach distribution indicate that the peripheral mountains of the basin had a tectonic stability phase.The west-east distribution of the sedimentary facies demonstrate that the Early Jurassic basin was controlled by the northern Longmen Shan and Micang Shan,which were controlled by the NW extrusion of the southern Qinling thrust belt.