Abstract
In this paper, some sections in the lower part of the Đồng Giao Formation (T2a đg) in west Bắc Bộ are described. The dolomitic member in the bottom of this formation is proposed to be named as “Phố Ngọc Dolomite member”. These new lithostratigraphic data demonstrate that from the latest Olenekian to the earliest Anisian, the paleogeographic conditions in this region changed, resulted in the deposition of Phố Ngọc Dolomite member in the earliest Anisian.