Than Sa Fm
Type Locality and Naming
Lectostratotype: along a dry stream bed, north of Ban Chau Village, Than Sa area, Thai Nguyen Province (N = 21°49'; E = 105°57'). Hypostratotype: along the Ban Gioc - Bang Ca Road, Trung Khanh District, Cao Bang Province (N = 22°50'; N=106°54’).
Synonym: Điệp Thần Sa: Trần Văn Trị et al. 1964 (E2); Phạm Đình Long et al. 1968 (£3 - Oi); Trần Văn Tri et al. 1977) (£1); Phạm Kim Ngân, Lương Hồng Hược, Phạm Đình Long 1986 (£3); Vù Khúc, Bùi Phú Mỹ et al. 1990 (£3
O1); Vũ Khúc et al 2000 (Hệ tẩng - £ 3 - ? Ot); Cambrien; Patte E. 1927. Hệ tầng Mò Đồng-, Dovjikov A.E. et al. 1965 (£3); Phạm Kim Ngân, Lương Hồng Hược, Phạm Đình Long 1986 (E2). Sérìe de ỉịong Son (part.): Bourret R. 1922 (D1). Điệp Bồng Sơn: Dovjikov A.E. et al. 1965 (D1?-D2e); Phạm Kim Ngân 1973 (£3); Phạm Đình Long, Nguyễn Đinh Đạt 1974 (£3). Điệp Bản Cải: Phạm Đình Long et al. 1968 (£2)
Lithology and Thickness
Sandstone and Marl: Lower part: Quartz-rich silty sandstone interbedded with banded, greenish-grey, reddish-violet weathering, thin-bedded phyllitic clay shale. On the bedding surfaces there are many mica flakes and trace, fossils of worms (Planolites) forming tortuous small tubes which, sometimes, crosscut one another. Thickness: over 350 m. Upper part: (1) Grey, thin-bedded clay shale and marl, containing many mica flakes on the bedding surfaces and trilobites, 200 m thick. This member grades upwards into (2) thin-bedded pelitic clayey limestone and dolomite interbedded with banded marly shale, 120 m thick. (3) Further, there is thinly-laminated clay shale with interbeds or lenses of marl, about 100 m thick. The total thickness of the upper part locally reaches 750 m.
Relationships and Distribution
Lower contact
Its lower boundary has not yet been observed.
Upper contact
As for its upper boundary, in the Ban Cai Section the formation unconformably underlies limestone of the Da Mai Fm
Regional extent
The Than Sa Fm occurs in two areas, forming the core of the Bac Thai - Ha Lang Anticlinorium. In the territory of the Thai Nguyen and Bac Kan Provinces, it is exposed in the southwest of Na Ri, forming a continuous band extending from Mo Dong Stream, through Ban Ria, Sang Moc to Khuoi Meo, and in the Than Sa Valley, Bo Cu area. In Cao Bang Province it occurs mainly in the Bong Son Anticline, Tra Linh and Phuc Hoa areas.
GeoJSON
Fossils
Trilobites, such as Lotagnostus cf. asiaticus, Hedinaspis sp., Haniwa sp., Paraolenus? bongsonensis
Age
Depositional setting
Additional Information