Abstract
On the basis of field investigations from April to June 2012, combined with the previous geological data, the Lower and Middle Triassic sediments in Tràng An area can be divided into two Formations:
1/ Olenekian Pa Khôm Formation (T1o pk) is composed of terrigenous-carbonate sediments, including four members, containing Ammonoids, Bivalves, Brachiopods, Gastropods with a thickness of 295-305 m.
2/ Anisian Đồng Giao Formation (T2a đg) is composed of carbonate beds with two members: a/ Phố Ngọc member (member 1) includes dolomite, calcareous dolomite, dolomitic limestone, 83-90 m thick; b/ Member 2 contains grey to dark grey limestone, mainly thin- to medium-bedded, with a thickness of less than 300 m.
The light-colored, thick-bedded to massive limestone (member 3) is barely present in the Tràng An area.