Abstract:

The Upper Triassic-Lower Jurassic in the Nanling region consists of continuously exposed, fossiliferous alternation of
marine and nonmarine coal-bearing strata. Bivalves, including both marine and non-marine types, are very abundant fossils in the strata.
Detailed study on these bivalves will provide important data on the correlation of marine and nonmarine Upper Triassic-Lower Jurassic
and erection of the Jurassic/Triassic boundary in this area. This paper studies the bivalve fossils from Shikang and Xiaping sections in
Hunan province and Huizhai section in Guangdong province. Four upper Triassic-lower Jurassic bivalve zones are identified, i.e.,
brackish water Unionites ? emeiensis peak zone, marine Retroceramus subinconditus-Hiatella assemblage zone, marine Ryderia

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 largely to Sinemurian, but probably to as young as Toarcian. By correlating
with marine bivalve zone, the Kija ovata-Waagenoperna mytiloides assemblage zone is assigned to Hettangian to Toarcian.
Consequently, the nonmarine T-J boundary is inferred to be located between the Unionites? emeiensis peak zone and the Kija
ovata-Waagenoperna mytiloides assemblage zone in Nanling region. It is roughly equivalent with the nonmarine T-J boundary in
Sichuan Basin (between the Modiolus weiyuanensis-Unionites? emeiensis assemblage zone and the Pseudocardinia (=Kija)
kweichouensis-Cuneopsis sichuanensis assemblage zone) and the nonmarine T-J boundary in Junggar Basin (between the
Waagenoperna-Yananoconcha (=Kija) assemblage zone and the underlaying Haojiagou Formation).