Dong Do Fm

Tinh Gia, north Nghe An and Ha Tinh provinces (SE Sam Nura basin). Lectostratotype - along the Ngan Sau River, situated. 15 km northeast of the Dai Loi Ferry landing (x = 18° 13', y = 105°46'). The formation bears the name of Dong Do Mt, Huong Khe District, Ha Tinh Province, where there is a coal mine.

Synonym: - Hệ tầng Dồng Đỏ'. Mareichev A.M. (in Dovjikov A.E. et al. 1965 - Jura sớm); Nguyễn Chí Hường (if? Tống Duy Tlianh et ai. 1995 - Nori-Ret); Lê Duy Bách, Đặng Trần Quân et aỉ. 1996 (Nori-Ret); Đặng Trần Quân, Lê Duy Bách, Nguyễn Văn Hoành et al. 1996 (Nori-Ret); Lê Duy Bách, Nguyễn Văn Hoảnh et aỉ. 1996 (Nori-Ret); Nguyễn Quang Trung et ai. 1996 (Nori-Ret); Nguyễn Văn Hoành et al. 1996 (Nori-Ret); Trần Tính eí al. 1996 (Nori-Ret); Vũ Khúc (in Vũ Khúc ei al. 2000 - Nori-Ret).. - Grès supérieur (part.): Jacob c. 1921 (Ret). - Điệp Đồng Đò: Vũ Khúc. Trịnh Thọ (in Trần Văn Trị eí al. ỉ 977 - Nori-Ret), (in Dương Xuân Hảo et al. 1980- Nori-Rct); Vũ Khúc 1980, 1984, 1990, (in Vũ Khúc, Bùi Phú Mỹ et aì. 1989 - Nori-Ret).

Coal-bearing beds. It includes polymictic sandstone interbeddcd with grey, but changing into red or chocolate-colourcd in the upper part, siltstone, claystone, conglomerate and gritstone, containing some coal seams. The lectostratotype lying in the valley of the Ngan Sau River situated 1.5 km northeast of the Dai Loi ferry landing. This section includes 6 members as follows: (1). Fine-grained conglomerate, gritstone with some interbeds of siltstone and claystone, 70 m thick. The grain composition of the conglomerate consists mainly of white quartz, some greyish to black chert; the cement consists of fine-grained sandstone containing many grains of dull white feldspar. (2). White-grey to greyish, coarse-grained quartz sandstone, polymictic sandstone interbedded with grey, dark grey siltstone and claystone, 45 m thick. (3). Light coloured quartz gritstone and gravelstone interbeddcd with greyish siltstone and argillaceous shale, 65 m thick. (4). Black-grey or reddish, thin-bedded siltstone, claystone, carbonaceous shale and thin lenses of coal, 60 m thick. The siltstone contains many leaf impressions of the Hon Gai Flora, such as: Taeniopteris jourdyi, Clathropteris meniscioides, Cladophlebis raciborskii, Pterophyllum halinense and some poorly preserved fauna: Gervillial sp. and Songdaellal sp. (5). Black-grey or reddish, thin-bedded siltstone and claystone interbedded with reddish medium-grained sandstone, 35 m thick. (6). Reddish weathering siltstone and claystone, black to black-grey, thin-bedded carbonaceous shale, 50 m thick. The total thickness of the formation in this section is about 350 m.

Fossils of the Dong Do Formation consist mainly of plant remains with a limited assemblage, but clearly belonging to the Hon Gai Flora with the species Clathropteris meniscioides, Cladophlebis {Todites) shensiensis, Taeniopteris jourdyi, Cycadites saỉadini, Cladophiebỉs raciborskii, Pterophyllum haíỉnense, Czekanowskia cf. rigida), and Equisetites sp. Fauna are rarer with the marine bivalve Gervillia sp. (aff. G. praecursor), the brackish-watcr bivalve Modiolus sp., and some brackish-, fresh-water phyllopods Euestheria minuta and Estheria sp.