Abstract
The sediments of the Suoi Bang Group (T3n-r sb) are present in many parts of the northwest region of Vietnam; they are distributed in the sedimentary basins of Song Da, Tu Le, Muong Te and Ninh Binh. This Triassic Norian- Rhaetian coal-bearing sediments consist of two parts and are mainly composed of sandstones, siltstones, coaly shales interbedded with coal seams. The lower part is the sediments of Suoi Loi Fm. with rich marine fauna fossils and rare plant fossils. Meanwhile the upper part is the continental swamp coal-bearing sediments of Muong Vo Fm. containing some marine - brackish water fauna fossils and the abundant plant fossils. The fauna and plant fossil assemblages collected in these sediments are important for the geological dating determination and the stratigraphic correlation. The remains of the representatives of the genus Equisetites are abundantly found in the sediment of seven Late Triassic coal bearing formations (they are: Hon Gai, Van Lang, An Diem, Suon Giua, Dong Do, Suoi Loi and Muong Vo). However, this is the first time two species of Equisetites with the large stem are collected in the coal-bearing sediments of the Muong Vo Fm., which is also the new data of its diverse morphogenous development. This finding has the significance to implication of the climatic conditions of the Late Triassic-Norian-Rhaetian coal-bearing sedimentary basin in the northwestern region of Vietnam: The large- sized Equisetites thrived in the swamps of hot and humid tropics, they contributed to the formation of thick coal seams.