Na Tuong Fm
Type Locality and Naming
The section exposed 4.5 km southeast of Cho Moi District Centre, along the road on the right bank of the Song Cau River and on a rut in the left bank extending from the center of Quang Chu Commune to Den Tham Temple (Cho Moi District; Bac Can Province) (N= 21°45"; E= 105°47").
Synonym: In the Na Ri area, Bac Kan Province, cherty shale beds were named by Patte E. the “Na Lii pteropod-bearing Shale”. Na Tuong Fm: Racheboeuf p. and Tong-Dzuy Thanh 2000 (Upper Emsian) Hệ tầng Nà Tuồng'. Nguyễn Ilữu Hùng, Nguyễn Thị Thủy, Đào Trọng Toàn 2009 (D1em-D3fr). Schỉstes près du Quang Co: Patte E. 1927 (Eifelien-Givetien); Saurin E. 1956 (Devonien). Schistes de Cho Moi: Pattc E. 1927 (Frasnien); Saurin E. 1956 (Devonien). Schistes à Píéropodes de Na Lau (Eifelien); Saurin E. 1956 (Devonien). Bậc Ei/eìi (part.): Trần Văn Trị et al. 1963, 1964. Các bậc Eifel - Giveí (part.): Vasilcvskaia E.D. (in Dovjikov A.E. et al. 1965, 1972). Schistes silic antédévonien?: Tong-Dzuy Thanh 1967. Phụ điệp Sông cầu trên: Phạm Đình Long et al. 1968 (D2e). Đá phiến sét silic chứa Tentacuỉites-. Dương Xuân Hảo et al. 1968 (D2gv). Điệp Mia Lé (part.): Phạm Đình Long el al. 1974 (D1-D2e). Trầm tích Coblen 'ỉ- Eifeli: Dương Xuân Hảo, Nguyễn Thơm, Nguyễn Đức Khoa 1975. Trầm tích Eife1 - Givet: Dương Xuân Hào et al. 1976. Trầm tích Eifdi: Dương Xuân Mào (in Trần Vẫn Trị et al. 1977).
Lithology and Thickness
Chert and Claystone. In general, the Na Tuong Formation is composed of cherty shale, black-grey intercalated with white-grey, with thin parallel bedding. About 450 m thick, containing in abundance tentaculites, trilobites and brachiopods that were described by researchers in different units of the Devonian. Patte E. (1927) described this cherty shale in the “Bac Son Geoblock” as follows: (1). Shale containing Spirifer speciosus (= Euryspirifer tonkinensis); (2). Black limestone containing rugose corals and brachiopods; (3). Chert with limestone bands; (4). Shale containing the pteropod Styliola clavulus and Tentaculites elegans of Eifelian age and maybe extending to the Early Givetian.
Relationships and Distribution
Lower contact
Rests unconformably upon limestone of the Na Quan Fm. Regionally, the next older unit is also the Mia Le Fm of Pragian age.
Upper contact
The stratigraphic relations and age of the Na Tuong Fm in the Na Ri - Yen Lac area still are in discussion. (corresponding to the Si Ka Fm and Bac Bun Fm according to the present viewpoint).
Regional extent
Distributed in Lang Son, Bac Can and Thai Nguyen provinces.
GeoJSON
Fossils
The characteristic faunal assemblages of the Na Tuong Formation as follows: Ductina vietnamica - Plagiolaria orientalis (Late Emsian); Chlupacina nariensis - Perichonetes mirabilis (Late Emsian); Nowakia richteri (Eifelian); Nowakia otomari (Givetian); and Striatostyliolina loufuensis (Frasnian).
Age
Depositional setting
In the Viet Bac Region (Ha Giang, Cao Bang, Lang Son, Bac Kan and Thai Nguyen Provinces) Devonian sediments begin with sub-continental beds of the estuarine-coastal environment of the Si Ka Fm, then occurs clay shale containing the brachiopod Howittia wangi Assemblage of the near-shore facies Bac Bun Fm. These beds grade upwards into the Mia Le Fm with clay shale and interbeds or lenses of limestone containing the brachiopod Euryspirifer tonkinensis Assemblage of the neritic continental shelf facies.
Additional Information