The Songkan section in Tongzi County of Guizhou Province， with a complete development of the Lower and Middle Triassic， abundant fossils and favorable exposure is an ideal section for the study of marine strata in the  Upper Yangtze area. According to the stratigraphic division and correlation， the Lower Triassic is divided into the Yelang and Maocaopu Formations， and the Middle Triassic comprises the Guanling Formation. Carbonate rocks and clastic rocks dominate the Lower and Middle Triassic with the former one making up 67％ of the entire section and the latter one  24％ respectively. Carbonates are distributed in the Yelang， Maocaopu and Guanling Formations， while clastics are confined to the Yelang Formation. The research results show that the Lower and Middle Triassic was characterized by a multicycle deposition. The Yelang Formation includes two transgressiveregressive cycles with sedimentary environments of tidal flat， open platform and shoal， and the Maocaopu Formation includes three cycles with sedimentary environments of open platform and restricted platform. The Guanling Formation is mainly a regressive succession， with sedimentary environments of restricted platform and tidal flat. Upon these researches， the sedimentary model of the Lower and Middle Triassic is established， ie., tidal flat-restricted platform-shoal-open platform pattern.