SHRIMP U-Pb dating of zircons from tuff of Jurassic Qigu Formation in Junggar Basin, Xinjiang

There is an unconformity between Jurassic and Cretaceous. The authors obtained a SHRIMP U-Pb zircon age of 164.6 Ma ± 1.4Ma (MSWD=1.3) for tuff from so-called upper series of Jurassic Qigu Fromation, and the age is almost the same as the bottom age of the Callovian (164.7 Ma ±4.0 Ma) in the International Geological Time Scale. According to the depositional rate of the formation, the upper boundary age should be 161.8 Ma, which is close to the upper boundary age of the Callovian (161.2 Ma ± 4.0 Ma). However, the upper boundary age of the Kalaza Formation is about 160.0Ma, which should be assigned to the lower part of the Oxfordian. The age of the lower Lower Cretaceous Qingshuihe Formation of should be early Early Cretaceous Berriasian period. So, the main age of the Qigu Fromation is Middle Jurassic Callovian period. The lower part of the formation might have entered late Late Bathonian period. The age of the Kalaza Formation is probably early Early Oxfordian period. In fact, between the Jurassic and the Cretaceous the whole Upper Jurassic is absent, suggesting that a relatively strong tectonic movement occurred in Late Jurassic period.