Study on Sedimentary Facies of the Upper Triassic in Central and South Sichuan Province

Based on comprehensive investigation of a lot of coring data,outcrops and well logging data from Upper Triassic in central and south Sichuan Basin,delta facies and lacustrine facies are identified.Depending on the sedimentary environment and sedimentary developing features,the delta facies can be subdivided into three subfacies,namely,delta plain,delta front and predelta,and the lacustrine facies into shore lake subfacies and shallow lake subfacies.The distribution ranges of both facies fluctuated.During the sedimentary period of member 1,member 3,member 5 and member 6 of Xujiahe Formation,lacustrine facies predominated,with the water deepening increasingly from east to west.During the sedimentary period of member 2 and member 4 of Xujiahe Formation,delta facies predominated and the distribution range of lake was limited.During the sedimentary period of member 2 of Xujiahe Formation,because of strong tectonic action of source areas in north,delta spread mainly from north to south.During the sedimentary period of member 4 of Xujiahe Formation,tectonic action of source area in south-east enhanced,producing increasingly more deposit which formed delta spreading from south-east to north-west.Along with the fading tectonic action of source areas,depositional area in the region of interest extended.During the early stage,the depositional area was limited to the north of Yibin-Luzhou,while during the middle and late stage,the depositional area extended to the south of Yibin-Luzhou.