Abstract

The Late Paleozoic – Early Mesozoic sedimentary formations in the South – eastern region have exhumed incomplete, and they have more complex geological characteristics, related to the closure process of the Paleo-Tethys ocean and the Indosinian orogeny occured from late Permian to late Trias. Sedimentary sequences in this period were classifed to Ta Not, Hon Quan, Song Sai Gon and Chau Thoi formations. The results from researches on petrography at areas of Chau Thoi formation such as Chau Thoi, Buu Long, Minh Duc, Loc Thanh have indicated that the pyroclasic rocks such as tuff, agglomerate, tuffaceous conglomerate, tufaceous sandstone conglomerate, sandstone, siltstone… are also present with the mainly lithological component of formation as conglomerate, sandstone, siltstone... Reseach on the lithological composition and geochemical content from the number of rock samples of the Chau Thoi formation show that the Chau Thoi formation is formed in rift setting as backarc basin type.