Ammonite fossils

Ammonite sites of Dong Dang (P3dd) and Song Hien (T1sh) formations in Chi Vien (coordinates X=609861; Y=2536500; Z=607m asl) and Noi Thon (coordinates X=622206; Y=2535902; Z=910m asl) communes, Ha Quang District; Binh Long commune (coordinates X=615589; Y=2514874; Z=206m asl), Hoa An District. At the first two sites ammonite fossils are found in the limestone of Dong Dang formation whereas at the third site ammonite fossils are found in the clayish shale and siltstone of Song Hien formation. Together with tetracorals and bivalves, the fossils indicate a change in the living marine environment, implying one of the major extinction events due to the rifting along the Cao Bang-Tien Yen fault.