Lung Nam Fm

Upper formation of the Trang Khang Gr (East Bac Bo version). Holostratotype: Bung O - Summit 100 Section, in the part continuing above the Toe Tat Formation (N= 22° 50'; E= I06° 54'5"). The Lung Nam Formation bears the name of a small village situated 1 km east of Toe Tat Village and occurs in the east of Cao Bang Province.

Synonym: -Hp tang Lung Nam: Doan Nhat Truang (in Tong Duy Thanh et al. 1995) (Cj); T9 Hoa Phuong, Do^n Nh§t Truong 1998, 1999 (Toumais-Vise); T6ng Duy Thanh, Nguyen Dure Khoa {in Vu Khuc et t//.2000). -Givetien (part.): Bourret R. 1922. -Tram tick Carbon ha: Ph^im Oinh Long et al. 1973, Le Hung 1973, 1975* 1977. -He tang Hdc Son: Vu Khuc, Bui Phu et al. 1990. -Hang Ga Formation: Nguyen Due Khoa (Tournais)

Cherty claystone and Limestone: The formation is characterized by an intercalation of cherty shale and crinoidal limestone. In the Toe Tat area (Trung Khanh District, Cao Bang Province) the stratotype of the Lung Nam Fm continues above the section of the Toe Tat Fm and includes 2 members. (1). Ash-grey to greyish weathering cherty shale and calcareous cherty shale, 30 m thick. Beneath, in a bed 1.5 m thick, lying 5 m beneath this basal member, Early Tournaisian fossils occur. (2). Dark grey, medium-bedded, coarse-grained, usually containing cherty limestone, 15 m thick; faunas of Middle-Late Tournaisian age in this member are rather abundant.

[Figure: Lũng Nậm Fm: Upper part: Limestone, siliceous limestone and silica (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Lũng Nậm Fm: Lower part: Cherty shale with Mn interbeds (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Fauna are fairly abundant, consisting of foraminifera, conodonts and rugose corals. At stratotype: Member 2 has Middle-Late Tournaisian age in this member are rather abundant with the foraminifera Tournayella discoidea maxima, Eoforchia moelleri, Glomospira sp., Septabrunsiina (Spinobrunsiina) sp.,, Palaeospiroplectammina tchernyshinensis, Paraendothyra cf. portentosa, P. verkhojanica₉ P. sp., Endothyra sp., Spinoendothyra cf. spinosa, Plectoendothyranopsis sp., Latiendothyranopsis sp., and Granuliferella montyi, conodonts Siphonodella sp., and Pseudopolygnathus ex gr. triangulus, Nguyen Due Khoa (in Le Hung 1975) collected the rugose corals Keyserlingophyllum sp., Dibunophyllum cf, dubium, Kueichouphyllum cf. laosense, Uralinia sp., Michelinia sp. in this member

As with the Da Nieng Fm, the Lung Nam was formed in a special, final stage of Late Devonian - Early Carboniferous time when the sedimentary basin was differentiated in carbonate intra-shelf troughs and shelf, that began since the end of the Middle Devonian and ended at the beginning of the Visean. Later, the basin became homogeneous, as expressed by the broad distribution and homogeneous composition of the Da Mai Fm.