Lang Son Fm

The type section is located on the road from Phai Xe to Highway No.1, in the section between Bac Ca and Con Sang villages, Lang Son Province (N = 21°47’; E = 106°42’). It was named by

Synonym: Điệp Lọng Sơn: Jamoida A.I., Phạm Văn Quaag (in Dovjikov A.E. et al. 1965.); Vũ ccKhúc 1984, 1990; Vũ Khúc (in Vũ Khúc et al. 1965, 1986); Vũ Khúc, Trịnh Thọ (in Trần Văn Tri et al. 1977, in Dương Xuân Hão et al. 1980), Đặng Trần I ỉuỵên (ìn Nguyễn Kinh Quốc et ai. 1991); Nguyễn Đình Hữu 1997 (part.); Nguyễn KinhQuốc (in Vù Khúc, Bùi Phú Mỹ ei ai. 1990). Hệ tầng Lạng Sơn: Vfl Khúc 2000; Đặng Trần Huyên, Nguyễn Kinh Quốc 2000 (Trias hạ). Schistes à Pseudomonotis griesbachì: Man suy H. 1908.

Verfenien: Patte E. 1927; Saurin E. 1949. Non: Điệp Lạng Sơn: Hoàng Xuân Tình (in Vũ Khúc, Bùi Phú Mỹ et al. 1990) = Hệ tầng Hồng Ngài.

Flysch-like Claystone and fine-grained sandstone: (1). Pinkish, violet, yellowish-grey banded claystone, cherty clayey shale with some thin-bedded siltstone, 20-25 m thick. (2). Grey claystone interbedded with greenish-grey marly shale in the lower part, and clayey siltstone, siltstone in the upper part, 45-50 m thick; these sediments are thin- to medium-bedded. (3). Regular intercalation of flyschoid form fine-grained sandstone, siltstone and clayey shale, 130 m thick. Sandstone usually medium-bedded, siltstone and clayey shale thin-bedded . The total thickness of the formation in this section reaches about 200 m.

Lytophiceras sp. and the bivalves Claraia kiparisovae, C. vietnamica, C. aurita, bearing Reichelina pulehra, Paraorbuloides sp. Lytophiceras sp., Claraia aurita, C. cf. gervilliaeformis, C. kiparisovae, C. stachei,

Characterized by terrigenous sediments which have, in some parts of the section, flyschoid structure.