Thien Nhan Fm

In the Thien Nhan Mt area, along the dirt road extending from Yen Vuc to Thanh Huong villages, Huong Son District, Ha Tinh Province (N = 18° 33’45”; E = 105° 32’05”). In the first description of the formation the stratotype was not indicated, and here the lectostratotype is proposed as above presented.

Synonym: - Hệ tầng Thiên Nhẫn: Mareichev A.M., Jamoida A.I.(/rt Dovjikov A.E. et al.) 1965 (T₁.2). Hệ tầng Sông Cả (part.): Trần Vẫn Tri et al 1977 (S-D). Hệ tầng bồng Trầu (part.): Trần Tính et al. 1979, 1966 (T₂). Hệ tầng La Khê (part.): Vũ Khúc in Vũ Khúc et al. 2000) (C₁)

Siliceous shale and chert. Lower part: rhythmic intercalation (some cm to some tens of centimetres) of light-grey, dull-white, but violet-brown, dark yellow, thin-bedded clayey-siliceous shale, siliceous shale, chert; locally there are some interbeds of fine-grained silty sandstone 5-7 cm in thickness in the lower part. Thickness: about 200 m. Middle part is mainly of dark grey to light grey, ribbon-bedded chert rhythmically interbeddcd with clayey-siliceous shale; chert is usually broken into sharp-edged fragments or small thin plates. Especially, in many places there are interbeds of cherty and clayey-siliceous breccias. Thickness: about 350 m. Upper part is grey, greenish-grey, dull-white, but pinkish, yellowish-grey weathering clayey-siliceous shale rhythmically interbedded with siliceous shale, chert; locally there still are interbeds (30-40 cm) of dark grey siliceous limestone. Thickness: 250 m. The total thickncss of the formation in this section is 800 m.

Ozarkodina (1Spathognathodus) sp., Paỉmatoỉepis sp.

The striped manganiferous, thin-bedded siliceous schist and chert of turbidite type containing pelagic fauna show that the Thien Nhan Fm took shape in the accretionary prism of the Viet-Laos Paleotethys Basin in the Late Devonian