STRATIGRAPHIC SEQUENCE OF THE YIXIAN FORMATION OF YIXIAN—BEIPIAO REGION,LIAONING——A study and establishment of stratotype of the Yixian Stage

The Yixian Formation (s.l.) can be subdivided into the Wangjiatun (temporary name) and Yixian Formations in Yixian region, Liaoning. All of them belong to the Wangjiatun and Yixian volcanic cycles respectively. The former is the acidic and alkalescent volcanic rocks. The main part of the latter is intermediate basic volcanic rocks. The rocks of the final phase turned into intermediate acidity with widespread subvolcanic facies, including besaltic porphyrite, andesitic porphyrite and agglomeratic brecciated lava pipe. There are seven major sedimentary layers in the Yixian region. They ( from bottom to top) are the Mashenmiao Bed of Wangjiatun Formation, Laogonggou, Yenangou, Zhuanchengzi, Dakangpu, Zhujiagou and Jingangshan Beds of Yixian Formation. It is confirmed that the abundant fossil vertebrates, invertebrates and plants occur just in these strata. The Yixian volcanic cycle can be subdivided into four sub cycles, bounded by Zhuanchengzi, Dakangpu and Jingangshan Beds, representing the initial, principal, late and end volcanic active stages. The Yixian Formation in Sihetun area of Beipiao may be corresponded to the first and second sub cycles of Yixian Formation in the Yixian region. The main depositional layers (Jianshangou and Shangyuan Beds) containing primitive birds in the Sihetun area of Beipiao may be correlated with the Zhuanchengzi Bed in the Yixian region.