Study On The Triassic-Jurassic Boundary And Paleoenvironmental Change During The Late Triassic-Early Jurassic In Southern China

The Upper Triassic-Lower Jurassic in the Nanling region consists of continuously exposed, fossiliferous alternation of marine and nonmarine strata. This region is considered as one of the best candidate localities to establish the nonmarine Jurassic/Triassic boundary and correlate precisely the marine and nonmarine Upper Triassic-Lower Jurassic in China.This paper aims at erecting the high-resolution Late Triassic-Early Jurassic bivalve biostratigraphic sequence by reexamining the described fossils. By means of high-resolution correlation of marine and nonmarine strata, three-dimensional distribution patterns of marine and nonmarine deposits are attempted, and the Late Triassic-Early paleoenvironmenatal change are discussed. A total of 28 species, belonging to 15 genera bivalves from the Upper Triassic-Lower Jurassic in the Nanling region are systematically described. Four upper Triassic-lower Jurassic bivalve zones are identified. They are:brackish water Unionites? emeiensis peak zone, marine Retroceramus subinconditus-Hiatella assemblage zone, marine Ryderia guangdongensis-Parainoceramus jinjiensis assemblage zone, and brackish-fresh water Kija ovata-Waagenoperna mytiloides assemblage zone. Based on the analysis of the distribution of representative bivalves in each zone, this paper roughly assigns the Unionites? emeiensis peak zone to Rhaetian, the Retroceramus subinconditus-Hiatella assemblage zone to Hettangian, and the Ryderia guangdongensis-Parainoceramus jinjiensis assemblage zone to Sinemurian to Toarcian. By correlating with marine bivalve zone, the Kija ovata-Waagenoperna mytiloides assemblage zone is assigned to Hettangian to Toarcian. Based on the correlation with the major brackish water-continental Upper Triassic-Lower Jurassic in China, this paper proposes the nonmarine T-J boundary located between the Unionites? emeiensis peak zone and the Kija ovata-Waagenoperna mytiloides assemblage zone in the Nanling region, between the Modiolus weiyuanensis-Unionites? emeiensis assemblage zone and the Pseudocardinia (= Kija) kweichouensis-Cuneopsis sichuanensis assemblage zone in Sichuan Basin and between the Waagenoperna-Yananoconcha (=Kija) assemblage zone and the underlaying Haojiagou Formation in Junggar Basin.By means of analysis of sedimentary feature, palaeonecological composition, paleoenvironmental changes during the Late Triassic to Early Jurassic and the process of the early Jurassic transgression in the Nanling region are discussed. Seawater is inferred invaded from southeastern Guangdong, through northern Guangdong, into Southern Hunan, which influenced central Hunan to western Jiangxi area. During the first stage, seawater intrusion hasn’t yet begun. There existed some desalinated lagoons in northern and eastern Guangdong and eastern Hunan to central Jiangxi. Southeastern Hunan was uplifted and suffered erosion. During the second stage, transgression occurred. Shallow sea occupied northern and eastern Guangdong, and lagoons formed in southeastern Hunan, while eastern Hunan-Central Jiangxi sustained desalinated lagoon. During the third stage, seawater further invaded into southeastern Hunan.