The Indosinian Movement resulting in the large- scale sea- continent transition deeply changed the paleogeograpical framework of the South China Contient and East Asia. The Badong Formation and related strata, which consist of carmine clastic rocks, are well developed in the north margin of the Yangtze block and respresent a transition process from marine carbonate- dominated marine sedimentation to clastic rock- dominated continental deposit. The Badong Formation can be divided into three members, with member 2 marlstone sandwhiched by member 1 siltstone and member 3 mudstone. Because there are few fossils found in the third member of the Baodong Formation, its stratigraphic age has been controversial. This study carried out LA- ICP- MS dating for zircons from the tuff in the 2nd Member of the Baodong Formation in the Zigui area. The reults show that two tuff samples from the middle and top sections of the 2nd Member in the Badong Formation formed at 241.7±1.7 Ma和237.5±2 Ma, respectively. The stratigraphic age of the Badong Formation, combined with emplacement age and fossil record of tuff layer, has been constrainted between the Middle Triassic Anisian and the Late Triassic Carnian. Zircons in the Badong tuffs occur in elongated crystals without rounding, show obvious oscillatory zones and the similar trace elements characteristic as orogenic magmatic zicons. The top marine stratum, which is represented by the Badong Formation limestone, is distributed mainly along southeastern Hubei to western Sichuan provinces, reflecting the sea- land changing of the Upper- Middle Yangtze regions due to westward regression of seawater during the period from Middle Triassic Ainsian Stage to Late Triassic Carnian Stage.