THE JURASSIC SYSTEM IN THE VIENTIANE AREA OF LAOS

In order to understand the formation and evolution history of the Vientiane salt-bearing basin of Laos, we have carried out a regional geological survey of this area in the project of K-Mg salt deposit exploration of the Vientiane Plain of Laos. According to the investigation, the Jurassic in Vientiane area of Laos is represented by a set of red terrigenous clastic deposit with minor carbonate rocks. It can be divided into, in descending order, the Banxinhei Formation (J_2bh), Nanshe Formation (J_1-2ns) and Nanli Formation (J_1nl). The ostracod faunas in the Jurassic succesions of the Vientiane area belong to the Gomphocythere-Darwinula assemblage and Darwinula sarytirmenensi-Darwinula magna assemblage. The conchostracan fossils belong to Palaeolimnadia diannanensis fauna. The gastropod fossils are characteristics of middle Jurassic terrestrial faunas. The Jurassic in this area consists of a set of fluvial-lacustrine sediments (dominated by lacustrine sediments), which form a complete third-order sequence with recognizable of low water-table system, lake transgression system, condensation sediment, and high water-table system. The lithostratigraphic, biostratigraphic and chronostratigraphic characteristics of these formations can be compared with the Yangjiang Formation (J_1y) and Huakaizuo Formation (J_2h) in West Yunnan of China.