THE LINJIA FORMATION IN BENXI, EAST LIAONING PROVINCE ASSIGNED TO MIDDLE TRIASSIC: Evidences from Zircon, Fossil and Stratigraphy

The Linjia Formation is outcropped in Linjiawaizi area, Benxi, eastern Liaoning Province. Since recorded by Kobayashi in 1942, its age has long been a great controversy. In the early 1980s, Zhang et al. suggested that the Linjia Formation should be assigned to Middle Triassic on the basis of the plant fossils fish scales, insect wings, spores and pollens, as well as stratigraphic correlation. As the components of Linjia flora are very complicated with both Paleozoic and Mesozoic elements but without evidences of zircon ages, the age of Linjia Formation has not yet been determined. Recently, the metamorphic tuffaceous siltstones containing lots of zircons were found at the bottom of Linjia Formation. According to the dating results of 30 grains of zircon crystal, the average ²⁰⁶Pb/²³⁸U age of 4 newest zircons was 239.5±4.1 Ma. In addition, Pleuromeia shaolinii was lately discovered from Linjia Formation in Qiandianzi area. Although some significant fossil plants of Linjia flora are distributed striding cross Permian and Triassic, combined with the zircon dating, stratigraphic correlation, lithologic characteristics, parallel unconformity relation, typical Triassic fossil plants and the Middle Triassic-Early Cretaceous insect fossil Sogdoblatta, it is confirmed that the Linjia Formation belongs to Middle Triassic.