Bac Thuy Fm
Type Locality and Naming
The type section is located from the Bac Thuy Railway Station to Po Khen Mt, 12 km SE of Lang Son City (N = 21°42’; E = Ỉ06°41’).
Synonym: Hệ tầng Bắc Thủy: Vũ Khúc (in Vũ Khúc et al. 2000); Đặng Trần Huyên, Nguyễn Kinh Quốc 2000; Vũ Khúc1980,1989, 1990 (Điệp). Couches à Meekoceras: Hoffet J. 1940. Diệp Lạng Sơn (part.): Nguyễn Đình Hữu 1977; Vũ Khúc, Trịnh Thọ (in Dương Xuân Hào et al.1980); Vũ Khúc et al.1986; Đoàn Kỳ Thụy (in VQ Khúc, Bùi Phú Mỹ et al. 1990). Điệp Kỳ Cùng: Đặng Trần Huyên (in Nguyễn Kinh Quốc 1991.)
Lithology and Thickness
Clayey limestone: (1). Grey, light grey, thin-bedded clayey limestone, marl, limestone and dolomitic limestone with some interbeds of dark grey siltstone and clayey shale, 16 m thick. (2). Mainly grey clayey shale interbedded with thin-bedded siltstone, sometimes with greyish, fine sandstone, 320-330 m thick. (3). Strongly calcitized grey limestone and clayey limestone, 2 m thick. (4). Grey, thin-bedded claystone, siltstone and fine-grained sandstone, 35-40 m thick. The total thickness of the formation in this section reaches 373-388 m.
Another section: (1). Grey, thin-bedded limestone, clayey limestone, locally with interbeds of cherty limestone, 80 m thick. (2). Dark grey clayey shale interbedded with yellowish-grey, thin-bedded siltstone, 110- 120 m thick. (3). Ash-grey, thin-bedded limestone, clayey limestone and marl, 15 m thick. (4). Dark grey clayey shale interbedded with yellowish-grey, thin-bedded siltstone, 100 m thick. The total thickness of the formation in this section reaches 305-315 m.
Relationships and Distribution
Lower contact
Conformably upon the Lang Son Fm that has been observed in the Bac Thuy Railway Station and in the Ban Ru area.
Upper contact
Unconformably by volcanogenic beds of the Khon Lang Fm.
Regional extent
The Bac Thuy Fm is characterized by terrigeno-carbonate beds bearing an abundance of Olenekian ammonoids, distributed in the An Chau Structure in a narrow band to the west of Lang Son City, and another band lying between Ban Thi and Highway No.lA, extending southward to Lang Giai. Later Dang Tran Huyen and Nguyen Kinh Quoc (2000) on the basis of the section extending from Bac Thuy Station to Po Khen Mt redcscribed it.
GeoJSON
Fossils
Include 17 ammonoids (Vu Khuc 1984) and 30 conodonts (Bui Due Thang 1989). The ammonoids include Flemingites aff. flemingianus, Paranorites praestans, Meekoceras cf. yukiangense, Owenites carinatus, Pseudowenites oxynotus etc., and conodonts - Neospathodus dieneri, N. waageni, N. homeri, N. triangularis etc. Prosphingites sp., Pseudowenites sp. Columbites cf. parisianus, C. sp., Prejlorianites sp. 1) Flemingites-Owenites. 2) Tirolites-Columbites
Age
Depositional setting
Additional Information