Cao Bang Fm
Type Locality and Naming
Cao-Lang Basin. The Cao Bang Fm is the basal component of the Cao Lang Gr extending largely in the Tertiaiy troughs along highway N°4.The type section is located in northwest and southwest Cao Bang Town (N = 22°39’; E = 106°15’) . It was named by Pham Dinh Long et al. (1974), and Cao Bang Suite by Tran Dinh Nhan and Trinh Danh (1975). Later, Tertiary conglomerate in the That Khe and Na Duong trough has been also referred to this formation by Trinh Danh.
Synonym: Tằng Cao Bằng (part.): Phạm Đình Long et al. 1969 (Miocen). Hệ tầng Cao Bằng: Trịnh Dánh (in Vũ Khúc et al.) 2000 (Miocen trung). Neogen (part.):c Kitovani S.K. 1961; Dovjikov A.E. et al.1965. Diệp Cao Bằng. Trần Đình Nhân, Trịnh Dánh 1975; Trịnh Dáiih 1979, 1980,1993,1985 (Miocen trung). Miocen (part). Trần Đinh Nhân (in Trần Văn Tri et al.) 1977. Hệ iầngNa Dương (part.): Trịnh Dánh (in Phan Cự Tiến et al.) 1989; in Vũ Khúc, Bùi Phú Mỹ et al. 1990); Phạm Đình Long et al. 2001. Hệ tầng Phan Lương (part.): Trịnh Dảnh (in Vũ Khúc et al. 2000), Phạm Dinh Long et al., 1969, 2001. Hệ tầng Tuyên Quang (part.): Trần Đình Nhân, Trịnh Dánh 1975.
Lithology and Thickness
Conglomerate. Three units: (1). Polymictic conglomerate with badly rounded and sorted pebbles, weakly cemented by sandstone and clay shale, 180 m thick; pebbles consist mainly of quartz, dimensions 1-2 cm, but some rhyolite and schist pebbles 3-4 cm - 6-7 cm are also met with. (2). Granule conglomerate with lenses or thin intcrbeds of gravel stone and medium-grained, cross-bedded sandstone; 150-290 m thick. Pebbles consist mainly of quartz and chert, rare red violet sandstone, rhyolite and shale pebbles are also observed. (3). Gritstone with pebbles of red sandstone, badly rounded, interbedded with coarse- and medium-grained, cross-bedded sandstone; 50-150 m thick. The total thickness of the formation in this section is 280-620 m.
Relationships and Distribution
Lower contact
Unconformity upon Mesozoic sediments, such as on the Lower Triassic Song Hien Fm as observed at Na Roat (vicinity of Cao Bang Town) and upon the brownish-red Cretaceous deposits as observed at Loc Binh (Lang Son Province). Regionally, the next younger unit is the Ban Hang Fm of early Cretaceous.
Upper contact
Conformity to the overlying Na Duong Fm is clearly observed also in the vicinity of Cao Bang Town, on the road to Ngan Son (Bac Kan Province).
Regional extent
Cao-Lang Basin. Cao Bang, That Khe and Na Duong troughs.
GeoJSON
Fossils
No fossils
Age
Depositional setting
Considered as tectonic-derived conglomerates in the lower part, and are the beds formed during the starting activities of the Cao Bang – Tien Yen Fault, which created Paleogene trough along it. Upper part is deluvial. According to the depositional composition and tectonic setting of the East Bac Bo area, the conglomerate beds extending along the Song Lo fault can be also referred to the Cao Bang Fm. First, these are the basal beds of the “Tuyen Quang Fm” consisting of conglomerate with badly rounded pebbles, 14 m thick (Tran Dinh Nhan, Trinh Danh 1975). Conglomerate, gravelstone and interbeds of coarse sandstone, 200-300 m thick of the “Phan Luong Fm” (Pham Dinh Long et al. 1969, 2001)1, and “sll, sl2 members” consisting of breccia-conglomerate, gravelstone, 300-500 m (Nguyen Xuan Huyen et al. 2004) are also referred to the Cao Bang Fm.
Additional Information