Dong Dang Fm

Holostratotype: Ban Long Section lying along former Highway 1, 2 to 5 km from Lang Son City, Cao Loc District, Lang Son Province (N= 21°50'; E= 106°54'). Hypostratotype: Ba Xa Section, along the track crossing the Phia En (Khua Thum) Village, Van Quan District, Lang Son Province (N= 21° 48'; E= 106° 37'). In the holostratotype the formation is not completely exposed, while it is fully exposed in the Ba Xa section, so the last has been proposed as the hypostratotype pf the Dong Dang Formation. The Dong Dang Formation was established by Phan Cu Tien (1978) based on the bauxite bearing Dong Dang Beds and the beds bearing Palaeofusulina described by Nguyen van Liem (1966).

Synonym: -Tang chua bauxit Dong Dang: Nguyen V5n Liem 1966 (P2). -Di4p Ddng Ddng: Phan C\r Tien 1978; Nguydn VSn Liem 1985 (H〇 tang); Le Hung, Tong Duy Thanh el al. 1987 (P₂); Nguyln Van Liem, Le Hung (in Duong XuSn Hao et al 1980 - Hq tSng P₂); Vu Khuc, Bui Phu My et al. 1990 (P); Tong Duy Thanh, Nguyen Due Khoa (in Vu Khuc et al. - H〇 tang) 2000. Calcaires noirs de Lang Nac a Palaeofusulina prtsca: Dcprat J. 1913 (Dinanticn). Roche ferrugineuse de massif Ourah - Permlen: Bourrct R. 1922, Laterites anciennes (part.): Patte E. 1927 Facies schisfo-greseux terminal du Permien: PaUe E. 1927. Kazanien infetieur superieur (part,): Saurin E. 1956. Tdng siiic luc nguyen Permi&dai Song Hiem: Dov\ikov A.E, et ai 1965, 1972. -Tang Palaeofusulina: Nguyen Van Liem 1966, 1978. -Tdng than Pho Bang: L6 Hung 1971 (P₂). -Tang Bauxite Tap Nd: Nguyen Tudn Anh 1971, 1974 (P₂). Tang Nhi Tdo (p^rt.): Ph^m Dinh Long 1976 (P₂). H4 Permi thong thu<?ng: Doan Thyy 1976; Lc Himg (in Trin Van Trj et a!.) 1977. Non Di$p Ddng Dang a Mien Dong Bac Bac Bq: Lc Hung (in Vu Khuc, Bui Phu My et al. 1990) (P) (= tang B5i Chay).

Clayey limestone with basal bauxite: There are two types of sections. The Ba Xa section with a total thickness of 110-120 m was described by Doan Ky Thuy et al, (1976) as follows: (1). Bauxite and allite ore seam, 20 m thick. (2). Brown-grey, yellow-brown weathering clayey shale, limestone, black-grey chert interbedded with siltstone, 4-5 m thick. (3). Black-grey, thin- to medium-bedded clayey limestone containing lenses or nests of chert, 38-40 m thick; containing in abundance Nankinella inflata, Reichelina sp. (4). Light-grey, variegated, thick-bedded to massive limestone, 16-20 m thick; containing Neoendothyra dongdangensis, Palaeofusulina sp._s Reichelina sp., Pachyphloia sp. (5). Mainly of greenish-grey, brittle cherty-clayey shale, 28-30 m thick

The second type of section are characterized by dark grey, thin- to medium-bedded limestone, calcareous clayey chert containing a Nankinella-Staffella Assemblage. In these areas, the lower part of the formation may begin by a bauxite seam (Nhi Tao section) or without a bauxite seam (Lung Cam section), but the following consists of clayey and siliceous shale, and sometimes clayey coaly shale is met with, and in a especial case a thin seam of coal is observed. The upper part of the formation is homogenous in all its distribution areas and consists of limestone, cherty, marly limestone only.

The Dong Dang Formation contains abundant foraminifera, corals, brachiopods, bivalves, gastropods, ammonoids, trilobites and bryozoans. It was dated mainly on the basis of foraminifera belonging to the genera Reichelina, Codonofusiella, Colaniella and especially, Paiaeofusulina. In the Lang Nac section Palaeojusulina was found by Deprat J. (1913) for the first time in black-grcy limestone lying at the base of the formation.

Following bauxite onto a karstic surface, the main Dong Dang Fm was deposited in a marine environment, at a transgressive time corresponding to the Changhsingian