Redefinition of the Lower Cretaceous terrestrial Yixianian Stage and its stratotype candidate in the Luanping basin, northern Hebei, China

The Liying-Wangjiagou section and Majuan section of the Xiguayuan Formation in Luanping County have been studied, and based on a multidisciplinary systematic study, the authors divided the section into four lithologic members, erected the conchostracan biostratigraphic succession (five zones), performed an isochronous correlation between the Yixian Formation of western Liaoning and the Xiguayuan Formation of northern Hebei, analyzed the features of superimposition of sedimentary facies and proved the continuity and cyclic variation of sedimentation. Four third-order sequences have been recognized in the section. Therefore it is proposed that the “Yixianian Stage” and its stratotype be redefined: the Liying-Wangjiagou section be taken as the unit stratotype candidate of the Lower Cretaceous Yixianian Stage, the boundary section of the Dadianzi and Xiguayuan formations as the lower boundary stratotype of Yixianian Stage and the time range of the Xiguayuan Formation of Luanping as the Yixianian Stage. The original lower substage of Yixian defined by the time range of the Dadianzi Formation of Zhangjiagou should be abolished and changed to the Zhangjiagou′an Stage.