Nam Po Fm

Widespread in western North Vietnam. Holostratotype - along the Nam Po Stream, in the section where the stream cuts across the track from Si Pa Phin to Muong Nhe (x = 21°02’; y = 102°55’) (Son La Province). Dovjikov A.E. et al. 1965. Describing this formation, Dovjikov A.E. et al. (1965) touched only upon the lower continental red beds present in the Muong Te Structure of extreme West Bac Bo. On the basis of lithostratigraphic principle, this formation should includes all continental red beds developed continuously from the upper Triassic Suoi Bang Group and distributed largely in West Bac Bo and North Trung Bo, together with some small areas of Jurassic continental red beds lying unconformably upon older rocks in North Trung Bo. Therefore, it also has a large distribution area around the Xuoc Mt (north of Nghe An Province).

Synonym: - Hệ tầng Nậm Pô: Dovjikov A.E. et al. (1965); Bùi Phú Mỹ et ai. 1978; Vù Khúc (in Tống Duy Thanh el aỉ.) 1995, (in Vũ Khúc et ai.) 2000 (J,). - Indosinias supérieur: Fromaget J. 1934; Fromaget J., Saurín E. 1952; Saurín E. 1956 (J-K). - Jura khôngphân chia: Vũ Khúc (in Trần Văn Trị et al.) 1977 (J). - Điệp Nậm Thếp: Nguyễn Vĩnh (in Phan Cự Tiến và nnk.) 1977 (Tjr-J|); Nguyễn Vĩnh et al. 1978 (J|). - Hệ tầng Hà Cối (part.): Bùi PhúMỹ(i« Vù Khúc, Bùi Phú Mỹ et a i) 1990 (J|.ỉ). - Hệ tầng Núi Xước: Nguyễn Chí Huởngl 998 (J 1.2). - Nui Xuoc Formation: Nguyen Chi Huong 1998 (J|.2).

Continental red beds. The following sequence represents the Nam Po section described by Dovjikov A.E. et al. (1965). (1). Dark grey siltstone interbedded with chocolate-coloured silty claystone containing small flakes of mica on the bedding surface, 30-50 m thick. (2). Light grey, fine- to medium-grained quartz sandstone containing black-grey plant debris interbedded with grey siltstone; this intercalation grades upwards into chocolate-colored siltstone and reddish sandstone; total thickness 250-300 m. (3). Reddish, fíne- to medium-grained, sometimes cross-bedded polymictic sandstone interbedded with chocolate-colored siltstone and claystone, 300-350 m thick. The total thickness of the formation in this section is 580-700 m.

In the Nui Xuoc Mt area, North Trung Bo the Nam Po Formation is characterized by the following sequence described by Nguyen Chi Huong (1998). (1). Light grey, medium- to coarse-grained, medium- to thick-bedded sandstone interbedded with gritstone and conglomerate, 230-270 m thick. (2). Light grey, thick-bedded coarse-grained sandstone interbedded with lenses of chocolate-colored siltstone and light grey gritstone and conglomerate, 100-130 m thick. (3). Light grey, medium- to coarse-grained, usually thick-bedded sandstone, locally containing fossilized tree trunks or poorly preserved leaf impressions, interbedded with some white-grey gritstone or violet-brown siltstone, 250-300 m thick. (4). Light chocolate-coloured, medium- to coarse-grained, thick-bedded to massive sandstone interbedded with chocolate-coloured siltstone and claystone, with a thin lens of carbonaceous shale (0.2 m) containing leaf impressions of Coniopteris sp., Radicites sp., Podozamites sp., and Equisetites sp.; 180-220 m thick. (5). Light-brown, thick-bcdded gritstone and conglomerate, medium- to coarse-grained, locally cross-bedded sandstone containing pinkish quartz pebbles, with some interbeds of chocolate-colored siltstone and claystone in the upper part, 110-130 m thick. The total thickness of the formation in this section reaches 870-1050 m.

Rare; only flora leaf impressions, of which Coniiopteris commonly characterizes the Lower Jurassic.

Continental red beds