Dong Ho Fm
Type Locality and Naming
The type section is located along Dong Ho Stream and Troi-Bang Be Road, Hoanh Bo District, Quang Ninh Province (N = 21°03’; E = 107° 01’). Tran Dinh Nhan, Trinh Danh (1975). It was named by Pham Van Quang.
Synonym: Diệp Đồng Ho: Phạm Văn Quang et al. 1969; Trail Đinh Nhân, Trịnh Dánh 1975; Trịnh Dárih 1979, 1985, 1993 (in Dương Xuân Háo et al.) 1980; Hoàng Ngọc Kỳ et al. 1979 (part.). Hệ tầng Đồng Ho: Phạm Quaiig Trung et al. 1999; Trịnh Dánh (in Vũ Khúc et al.) 2000; Nguyễn Công Lượng et al. 2001; Hoàng Ngợc Kỷ et al. 2001. Quaternaire (part.): Patte E. 1927. Neogen (part.): Dovjikov A.E. et al. 1965. Miocen: Trần Đình Nhân (in Trần Văn Trị et al.) 1977. Hệ tầng Na Dường: Trịnh Dảnh (in Phan Cự Tiến et al. 1989; in Vũ Khúc, Bùi .Phú Mỹ et al. 1990).
Lithology and Thickness
Lacustrine-marshy sediments containing oil shale and some coal and asphalt. Characteristic sections are 140 to 430 m thick: (1) Thick-bedded polymictic conglomerate, gritstone and sandstone; 35-230 m thick. (2) Siltstone, red-brown oil shale (in Dong Ho, there are asphalt-bearing beds, and in Bai Chay are thin coal seams); 18-70 m thick, containing an abundant palynological assemblage.
Relationships and Distribution
Lower contact
Unconformably overlies Lower Jurassic Ha Coi Fm. This is a major regional disconformity in the preserved sediment succession.
Upper contact
Regional extent
Small troughs in the Quang Ninh Basin; distributed in the Dong Ho, Troi and near Bai Chay areas of Hoanh Bo District.
GeoJSON
Fossils
From the coal and asphalt bearing shale and siltstone beds, corresponding to the above-described member 2, Pham Quang Trung et al. (1999) have identified an abundant sporomorph and palynomorph assemblage. Among them Pentapollenites maomingensis, Cicatricosisporites dorogensis are not younger than Oligocene, especially Cicatricosisporites dorogensis is the index species of the Oligocene zone bearing its name. The sporomorphs Verrutricolporites pnehydermus, Gothanipollis bassensis, Liquidambarpollenites minutus, Lycopodiumsporites neogenicus have been met with only in the Oligocene; among them Ver - rutricolporites pachydermus is the index specics of the zone bearing its name, which is only met with in the Oligocene of the Bac Bo Gulf. Besides, Pham Quang Trung et al. (1999) noted the abundance of Pinaceae worldwide in the Oligocene, and Pinuspollenites sp. is the most abundant form in the Oligocene of the Bien Dong (South China Sea) continental shelf.
Age
Depositional setting
Lacustrine-marshy facies.
Additional Information