Dinh Cao Fm

North Song Hong Basin (north Red River Basin). The type section is located borehole GK.104 drilled in the DỈnh Cao Commune area, Phu Tien District (Hung Yen) at 2396 to 3544 m (N = 20°42’; E = 106°05’). . It was named in accordance with the name of the locality, where the borehole was drilled.

Synonym: Điệp Dinh Cao: Phạm Hồng Quế 1981; Skorduli V.D. 1983; IDEMITSU 1994. Hệ lẩng Dinh Cao: Lê Văn Cự 1982; Trịnh Dảnh (in Vù Khúc và nnk. 2000). Hệ tầng Phan Lưong: Palustovitch B. 1972. Hệ iầng Thụy Anh: Lê Văn Cự 1987. Điệp Dinh Cao-Phong Châu: Đỗ Bạt, Phan Huy Quynh 1984, Ị986, 1993,2000.

Oligocen: TOTAL 1991. Hệ tầng Đình Cao: Phan Trung Điền 2000. Hệ tầng Thụy Anh: Phan Trung Điền 2000.

Dominated by sandstone. The Dinh Cao Fm has been named in accordance with the name of the locality, where the borehole was drilled. There, within the borehole it is composed mainly of light to dark grey, locally violet-grey sandstone interbedded with puddingstone, gritstone grading upward to grey, black-grey siltstone and claystone with interbeds of conglomerate and gritstone. The thickness of the formation in this borehole reaches 1148 m.

The Dinh Cao Fm is strongly developed in the Dong Quan, Thai Thuy, Tien Hai and Bac Bo Gulf areas, including light to dark grey, fine- to medium-grained sandstone, locally with ịnterbeds of medium to well sorted conglomerate and gritstone. These sediments are well cemented by carbonate, clay and some ferrous oxide. The sandstone sometimes contains glauconite (boreholes GK.104-QN, 107-PA). The claystone is light to dark grey with lustrous glide surfaces, locally containing coal lenses (borehole GK.200) or thin interbeds of marl and fauna. The thickness of the formation changes from 300 to 1148 m.

These sediments yield palynomorphs, such as Florschuetzia trilobata, Verrucatosporites usmensis, etc. Quercus neriifolia, Laurus cubeba, Pỉstacỉa miocenica, Q. ỉameỉỉosa, Cercidiphyllum, Acer hilgendorfi, Zelkova ungeri, Laurus similis, Sapindus, Diospyros brachycepala, Platanus, Eucalyptus, Juglans, Ulmus, Liisea belonging to typical temperate Fagaccae, Lauraceae, Betulaceac, Ulmaceae and rare forms of marshy plants, such as GraminiphyUum, Phragmites, Arundo. Alnipollenites, Quercidites, Liquidambarpollemtes, Tricolpites longicolpus, Magnastriatites howardi, Triletes, Pinuspollenites, Verrucatosporites usmensỉs, Trìporocolptíes, Tricolpopolỉenites, Corsinỉpoỉỉenites, LycopodiumsporUes neogenicus, Gothanipoỉỉỉs bassensis, Cicatricosisporites dorogensis, Verrucatopoilenites pachydermus, Florschuetzia ỉrílobata, Tsugapollenites, Dacrydium (very little) belonging to the Cicatricosisporừes - Verrucatopoỉỉeniíes pachydermus - Cothanipoilis bassensis - Florchuetzia trỉỉobăta Assemblage. Cicatricosisporites dorogensis (LAD in Late Oligocene), Lycopodiumsporites neogenicus (known only in the Oligocene), Gothanipoỉỉis bassensis (known only in the Late Oligocene), Florschuelzia tnlobaia (FAD in Eocene/Oligocene).

It is interpreted as fluvial-deltaic facies.