Nghia Lo Fm

According to its initial description (Dovjikov, A.E., et al., 1965), the Nghia Lo Fm belongs to the Phan Si Pan Structure and composed purely of limestone. However, geological mapping (Nguyen Dinh Hop et al., 1997) showed that its distributive area is not only around Nghia Lo Townlet (Yen Bai Province), but also scattered in a broad band exending from Chieng Kong – Van Ban – Lang Giang (Lao Cai Province) to Hoa Mac – Duong Quy – Nghia lo (Yen Bai Province).

Synonym: Dong Giao Fm -- Tong-Dzuy Thanh and Vu Khuc (2011; Stratigraphic Units of Viet Nam) considers the Nghia Lo Fm as a regional variant of the Dong Giao Fm, therefore "grouped by us into this formation"; whereas Tran Van Tri and Vu Khuc (2011; Geology and Earth Resources of Viet Nam) distinguished between the two; and used that terminology-description in this Lexicon. Hệ tầng Nghĩa Lộ: Dovjikov A.E. et al. 1965.

Characteristic section north of Phung Village is 400-450 m thick: (1) Conglomerate, sandstone and grey siltstone, 40 m. (2) Thin- to medium-bedded clayey limestone and light-grey, sometimes marbleized, massive limestone, 300-350 m. (3) Clayey limestone, calcareous andstone and siltstone, 60-100 m thick, containing Ladinian forams. This section has the form of the Lower-Middle section of the Son La Structure with the lower unit (1) corresponding to the Co Noi Fm, the middle part (2) to the Dong Giao Fm, and the upper part (3) with the Nam Tham Fm. Only it is much restricted and cannot be described separately.

Ladinian forams Diplotremina sp., Pseudonodosaria sp., Ophthalmidium sp.