Zircon U–Pb dating of the Shipu limestone in Zhejiang Province, coastal southeast China: Implications for the Early Cretaceous environment

The age of the Shipu limestone-bearing section in Zhejiang Province, coastal southeast China, which is commonly called the Shipu limestone, is important for understanding the depositional environment and tectonic background of the region. However, previous chronostratigraphic and chronometric studies have produced competing results from the Palaeozoic to the Cenozoic. In order to provide some constraints, 115 zircon grains from tuff interbeds within the three limestone units and of the base and the top of the section were selected for U–Pb dating. A total of 86 valid results were obtained. The results demonstrate that the base of the section is no older than 119 ± 2 Ma and the top is approximately 99 ± 3 Ma. These dates are possibly the best time constraints on the section so far. They are generally comparable to the upper part of the Lower Cretaceous Jiuliping, Guantou and the lower part of the Chaochuan formations in neighbouring areas. The zircon ages in combination with new stratigraphic correlation suggest that a transgression most likely took place widely in coastal southeast China during the Aptian–Albian. In addition, the three limestone units in the section are roughly synchronous with high stages of Pacific sea levels, according to the zircon ages (i.e., 113 ± 3 Ma, 109 ± 2 Ma and 100 ± 2 Ma). It appears, therefore, that the transgression may have been caused sea-level changes in the Pacific Ocean. These results have broad implications for regional Cretaceous geological studies in general.