Tien Hung Fm

Hanoi Depression and North Song Hong Basin (north Red River Basin). The type section is located borehole GK.4 drilled in the Tien Hung District area (Thai Binh Province) from 250 to 1010 m (N = 20°34’; E =106°18’).

Synonym: Hệ tầng Tiên Hưng: Golovenok V.K., Lê Văn Chân 1966; Golovenok V.K., Kisliakov V.N. 1970; Phan Trung ĐiỀn 2000; Trịnh Dánh (in Vũ Khúc et al. 2000). Diệp Tiên Hung: Trần Đình Nhân, Trịnh Dánh 1975; Sevostianov K.M. 1977; Phạm Hồng Quế 1981; Lê Văn Cự 1987; ĐỖ Bạt, Phan Huy Quynh 1984, 1986, 1993,2000; IDEMITSU 1994. Diệp Đông Hoàng: Lê Vãn Cự 1987. Miocene: TOTAL 1992.

It is composed of clearly rhythmic sediments with the rhythms beginning with gritstone, sandstone grading upward to siltstone, claystone, coaly shale and many brown coal seams. In these rhythms, the coarse-grained part is usually thicker than the fine-grained part. There are as many as 15-18 rhythms. Sandstone and gritstone are usually feebly cemented or friable, bearing garnet, with poorly sorting and rounded grains. In the lower part of the formation, the beds are usually strongly compressed, and contain interbeds of white-grey calcareous sandstone bearing siderite concretions. The thickness of the formation in this borehole reaches 760 m.

The coal-bearing level decrease is clearly caused by the increasingly seawards marine depositional environment with the marine character. Sandstone is light grey to greenish-grey colored, fine- to coarse-grained, thick-bedded to massive structure, poor to medium sorting, with carbonate-clay cement, bearing fauna and brown coal debris. Silty claystone is greenish-grey to light grey, locally brownish-grey, black-grey colored (boreholes GK.104, 102-HD) bearing fossils and coal debris, locally, glauconite and pyrite (boreholes GK.100, 103-TH). The thickness of the formation changes from 760 to 3000 m.

Dacrydium, Ilex, Quercus, Castanea, Pinus, Liquidambar, Tsuga, Florschuetzia trilobata, FI. levlpoli, Leguminosae, Euphorbiaceae, Carya, Alnus and the spores Cyathea, Magnastriatites howardi, Acrostichum, Cleichenia, Stenochlaena of the Assemblage Dacrydium-Ilex-Quercus - FI. trilobata.

Deltaic plain environment with swamps (the coal layers).