Abstract: The Yidun region,western Sichuan is located in an active island-arc zone where the Upper Triassic strata are well developed. According to the regional geological map on the scale of 1:50000, the formerly Tumugou Formation may be redivided, from the base upwards, into the Dang'en, Lieyi, Qugasi and Miange Formations. With the aid of the type 1 unconformity at the base of the Qugasi Formation and the type 2 unconformity at the base of the Miange Formation, three third-order sequences have been tentatively identified, including the lower sequence (not penetrated),middle type 1 sequence and upper type 2 sequence (including the Lamaya Formation). All of the depositional systems tracts are also recognized in these sequences.