Abstract

In the Tràng An area Triassic sediments are widely distributed, consisting of  Lower Triassic terrigenous carbonate Pa Khôm Formation (T₁o pk) and Middle Triassic carbonate Đồng Giao Formation (T₂a đg). In this area, five new fossil sites with Ammonoids, Bivalves, Brachiopods, Gastropods and traces of living activities of ancient organisms have been discovered in the Pa Khôm Formation. These fossils in  were listed and grouped into two fossil assemblages: Ammonoid and Bivalve. Based on the analysis of stratigraphic significance of these fossil assemblages, specially the Ammonoid assemblage consisting of four forms i.e. Kashmirites sp., Preflorianites sp., Clypeoceras sp. and Proptychitoides sp., the Pa Khôm Formation has been confirmed  as Olenekian in age.