PARALIC TIDAL DEPOSITS IN THE UPPER TRIASSIC XUJIAHE FORMATION IN ANYUE AREA,THE SICHUAN BASIN

It has been generally accepted for many years that the Xujiahe Formation(T3x)in the Sichuan basin is of lacustrine-delta deposits in origin,except for the first member at its basal part which is attributed to marine deposits.However,studies on several outcrop profiles and core data in 2007 revealed that the 2nd-6th members of Xujiahe Fm in the study area are all clastic deposits of shallow marine facies,dominated by estuary deposits developed in an environment with a large tidal range.Beginning with introducing the basic principles of tidal deposition,this paper analyzes in detail the lithologic and structural features of the Xujiahe Fm.Its tidal origin is evidenced by bimodal cross bedding and cross lamination,interlayered bedding of thin sand and shale,dual clay ground structure,abrupt change of flow,as well as outcrop and core data.Analyses of the sedimentary facies,microfacies and models of the Xujiahe Fm show that most of the Xujiahe Fm is of shallow marine estuary subfacies with a large tidal range consisting of various microfacies such as supratidal,intertidal,subtidal zones,except for the margin where delta facies occur.