Nam Ma Fm

The type section is located along the Nam Ma Stream, Muong Te District, Lai ChauP rovince (N = 21°52’; E = 101°52’). Tran Dang Tuyet et al. 1994. The formation consists of continental red beds, lying subhorizontally upon all older sediments and occurring largely in the Vietnamese-Laotian border area of the Muong Te District, was defined as the Nam Ma Fm during geological mapping at 1:200,000 of the Muong Te area (1994). It is exposed mainly in large bands along the Pu Den Dinh Range on the border, and in some other small areas, such as Nam Cang, Muong Toong, Quang Lam and Nam Bay . It was named by the following sequence of the formation stratotype was described by Tran Dang Tuyet et al. (1994) along the Nam Ma Stream.

Synonym: - Hệ tầng Nậm Ma: Trần Đăng Tuyết et al. 1994. - Hệ Creta: Dovjikov A.E. et al. 1965. - Hệ tầng Mụ Giạ (part,): Bùi Phú Mỹ (in Vũ Khúc, Bùi Phú Mỹ et al. 1990).

Continental red beds. The following sequence of the formation stratotype was described by Tran Dang Tuyet et al. (1994) along the Nam Ma Stream. (1). Thick-bedded to massive conglomerate and gritstone grading upwards into coarse-grained sandstone, gritstone and conglomerate with some interbeds of chocolate-colored siltstone and claystone, 1500-1600 m thick. Pebbles and gravel consist mainly of quartz, sandstone, black-grey chert and calcareous chert; their size varies from 2-3 to 5 cm. This member rests unconformably upon Paleozoic sediments. (2). Thick-bedded to massive conglomerate with interbeds of brownish sandstone and chocolate-colored silty sandstone and siltstone, 600-700 m. (3). Coarse-grained sandstone, gritstone interbedded with chocolate-colored, thick- to medium-bedded siltstone and claystone, 300-400 m thick; containing palynomorphs Camptotriletes sp., Latipollis sp., Tricolpites sp., Classopollis sp., Coniopteris sp., and Gleicheniidites sp. of Jurassic-Cretaceous age. The total thickness of the formation in this section was reported as about 2400-2700 m, but this thickness seems to be very great; in further survey no section having this thickness has been found.

Duong Binh Soan et al. (2002) described the following sequence of the formation section extending from Long Dao Village to Nam Chim Stream. (1). Conglomerate, gritstone with well rounded clasts mainly of quartz and quartzitc, some chert, and cement of chocolate-colored sandstone, then thick-bedded, medium- to coarsegrained sandstone and some chocolate-colored siltstone, 280 m thick. (2). Brownish to light grey, thick-beddcd, medium- to coarse-grained sandstone with some interbeds of chocolate-colored siltstonc, 60-80 m thick. (3). Reddish thick-bcddcd quartz conglomerate and gritstone interbedded with chocolate- colored, vermilion weathering siltstone, 230 m thick. These beds form high cliffs in a mountain range on the border. (4). Chocolate-colored, thick-beddeđ siltstone and silty sandstone with interbeds of thin- bedded conglomerate and gritstone in the upper part, 180 m thick. The total thickness of the formation in this section is 760 m.

Jurassic-Cretaceous spores and pollen of Camptotriletes sp., Latipollis sp., Tricolpites sp., Classopollis sp., Coniopteris sp., and Gleicheniidites sp. of Jurassic-Cretaceous age.