Bac Son Fm

Widespread in Viet Nam and Southeast Asia. Named after Bac Son – Kim Hi massif (East Bac Bo Basin). Nguyen Van Liem, 1978.

Synonym: Da Mai Fm (preferred name by Tong-Dzuy Thanh and Vu Khuc (2011, Editors; Stratigraphic Units of Viet Nam): "The described Carboniferous-Perniian light grey limestones were described by geologists under different names, such as: Bac Son Group, Muong Long Formation, Middle-Upper Carboniferous - Permian, Anthracolitic, Uralo-Permian, Fusulines Limestone, Producius Limestone, Middle Limestone, etc.. The most widespread name is Bac Son Formation first used by Nguyen Cong Luong (1980); it has been used largely in geological mapping at different scales, especially at 1:200.000 scale. … The term Bac Son was used a long time ago by Mansuy H. and Colani M. (1925) for a Quaternary (Holoccne) stratigraphic unit and has been becoming well known in regional stratigraphy and world archaeology (Saurin E. 1956. … Furthermore, the carbonate stratigraphic body named as “Bac Son Formation” (Nguyen Van Liem 1978) was later described than the Da Mai Fm (Dussault L. 1929; Nguyen Xuan Bao 1970). Therefore the name Da Mai can be considered a valid name, and the name Bac Son is its synonym." [However, this Lexicon retains the "Bac Son Fm" as used by Tran Van Tri and Vu Khuc (2011, Editors; Geology and Earth Resources of Viet Nam), which as indicated, is the most widespread name.]

[Figure: Bắc Sơn Fm: Type section - Limestone of Bắc Sơn Fm in the Hạ Long bay (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Geological map of Halong bay area (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

In general, it consists of light grey and grey, mainly thick-bedded to massive dolomitised limestone, argillaceous limestone, and organic limestone. The thickness of this stratigraphic body varies from 1000 to 1500 m. In Long Son an Ha Giang provinces, the formation is about 700-1000 m thick, usually composed of two parts: (1) White-gray, thick-bedded in lower part, but massive in upper part, dolomitic limestone about 300 m thick; (2) Gray, sometimes pinkish in the upper part, coarse-grained, thick-bedded limestone about 400 m thick, containing Early to Middle Permian forams, such as Pseudodoliolina ozawai, Triticites paramontiparus, Occidentoschwagerina sp., Cancellina sp. In some places, the formation begins with organic limestone, composed mainly of crinoid ossicles.

In West Bac Bo, the formation is 800-1000 m thick, including light grey, thick-bedded limestone, sometimes there are cherty limestone beds or chert nests. Among fossils, apart from forams, there are corals giving the age of Visean to Middle Permian. Forams from the upper part of the formation include Zellia muongthensis, Neoschwagerina craticulifera, Verbeekina aff. verbecki, Neofusulinella sp., Pisolina sp., Pseudofusulina sp., Parafusulina sp., etec.

[Figure: Bắc Sơn Fm: Type section - Limestone of Bắc Sơn Fm in Đồng Văn Highland (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Bắc Sơn Fm: Type section - Limestone of Bắc Sơn Fm in Phong Nha – Kẻ Bàng World heritage (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Bắc Sơn Fm: Landscape image (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

See lithologic descriptions. Benthic forams, corals, crinoids.

Carbonate platform