Ages of Tuchengzi Formation in western Liaoning-northern Hebei area in correlation with those of international strata

Various fossils such as plants, spore-pollen, ostracods, conchostracans, insects and dinosaurs are well developed in the Tuchengzi Formation (Houcheng Formation), and fossils of each kind has their own evolutional orbit and ages. According to the study and correlation of different assemblages in the whole globe，mainly biostratigraphic correlation of spore-pollen and ostracods both in China and abroad, the Tuchengzi Formation should belong to the early-middle Late Jurassic Oxfordian-Kimmeridgian, identical with the age obtained by paleomagnetism and geochronology. The authors obtained some SHRIMP U-Pb zircon ages of tuff, such as 140.2Ma±1.3Ma from Horizon 22 and 139.6Ma±1.9Ma from Horizon 30 of the Tuchengzi Formation in Jinlingsi-Yangshan basin of western Lioaning Province, and 139.6Ma±0.7Ma from the upper part of the Tuchengzi Formation and 137.3Ma±1.0Ma from Horizon 83 along the Xiaoliujia valley section (about 6km north of Sanbaoying Town). Because Horizon 22 is 369m from the bottom of the Tuchengzi Formation and, according to the depositional rate, the authors hold through calculation that its deposition took place at about 1.5Ma. Therefore, the boundary age should be 141.7Ma. In addition, Horizon 83 is close to the top boundary, so the SHRIMP U-Pb zircon age 137.3Ma±1.0Ma should be regarded as the top boundary age of the Tuchengzi Formation. However, the bottom boundary age of Oxfordian is nearly 141.7Ma±1.3Ma, and that of the top boundary of Kimmeridgian is 137.3Ma±1.0Ma; nevertheless, Oxfordian age is 163.5Ma±1.0Ma and Kimmeridgian age is 152.1Ma±0.9Ma in International Geological Time Scale, so there exists the age difference of 21Ma and 15Ma between the authors' calculation and the age indicated by International Geological Time Scale for Oxfordian and Kimmeridgian respectively.