Dong Giao Fm

The type section is located from Co Dam Village through the Tam Diep Range to the Dong Giao, Tam Diep area, Ninh Binh Province (N= 20°08’; E= 105°52’). It was named by Vu Khuc, Trinh Tho, LeTuu 1972.

Synonym: The formation mainly composed of thick-bedded to massive limestone that was formerly described as “Cuccoceras yoga Limestone,” or sometimes “Anthracolitic Limestone” lying in “chaưiage” upon Triassic sediments as shown in the Geological Map of the Van Yen area at 1:100,000 (Dussault 1929), has been correctly determined as a part of a continuous Lower-Middle Triassic scries in the Son La and Song Da Structure in the Geological Map of North Viet Nam at 1:500,000 scale (Dovjikov A.E. et al. 1965) and described as Dong Giao Fm. Hệ tầng Đồng Giao: Jamoida A.I., Phạm Văn Quang (in Dovjikov A.E. et al..) 1965 (Lađin); Vũ Khúc et al.. 1965 (Ladin); Vũ Khúc et al.. 2000 (Anisi). Điệp Dồng Giao'. Phan Cự Tiển et al. 1977; Bùi Phú Mỹ et al. 1978; Đinh Minh Mộng et al.. 1978; Nguyễn Vĩnh t al. 1978; Nguyễn Xuân Bao etal. 1978; Phan Sơn etaỉ. 1978 (Trias trung); Vũ Khúc 1989,1990, (in Vũ Khúc,Bùi Phú Mỹ et al.. 1990 (Anisi). Caỉcaires à Cuccoceras yoga: Deprat J. 1915. Caỉcaires Anthracoỉitiques: Dussault L. 1929; Fromaget J. 1937. Hệ tầng Nghĩa Lộ: Dovjikov A.E. et al. 1965. Bậc Anisi: Vũ Khúc, Trịnh Thọ, Lc Tựu 1972; Vũ Khúc, Trịnh Thọ (in Trần Văn Trị et al..) 1977, (in Dương uân Hảo et al.) 1980. Diệp Vinh Lộc: Vũ Khúc, Trịnh Thọ, Lê Tựu 1972; Vũ Khúc, Trịnh Thọ (in Dương Xuân Hào et al.. 1980)

Limestone: Dong Giao area is a total of about 1850 m, composed of two parts: (1) 350 m of grey, thin-bedded limestone, and (2) 1500 m of grey to dark grey, thick-bedded to massive limestone.

Bivalves Pseudomonotis(?) michaeli, Daonella laluensis, D. lindstroemi, and Costatoria sp. Late Anisian ammonoid Paraceratites subtrinodosus.