Van Yen Fm
Type Locality and Naming
Upper Red River Valley. The type section is located along car road No. 13A passing over the Van Yen - Van Chan area (N = 21°53’; E = 104°40’). Trịnh Dzanh, 1980. . It was established (1980) for describing coarse-grained sediments lying at the base of the Tertiary section, occurring along the Red River Trough, correlated with the Cao Bang Fm of the Viet Bac region.
Synonym: Diệp Văn Yên: Trịnh Dánh (ìn Dương Xuân Hảo và nnk.) 1980 (Mioccn giữa); Hệ tầng Văn Yên: Trịnh Dánh (in Vũ Khúc và nnk.) 2000 (Míocen giữa); Tập shl: Nguyễn Xụân Huyên và nnk. 2004 (Eocen).
Lithology and Thickness
Conglomerate. According to the description of Nguyen Xuan Huyeri et al. (2004), the main part of the Van Yen Fm consists of breccia-conglomerate having a common grain size of 15-35 cm, in some placcs up to 170 cm, especially at Trai Hut - 200-220 cm. Pebbles are generally angular, of different form, with the composition mainly from metamorphic rocks of the Song Hong Gr, among them gneiss and crystalline schists occupy about 80-90%, besides, granodiorite, granite and quartzite; the cement is commonly of polymictic sandstone and gritstone. The total thickness of the formation is about 55-250 m.
Relationships and Distribution
Lower contact
Unconformably upon the old basement in the Pho Moi and Trai Hut exposures.
Upper contact
Conformably under the Oligocene Co Phuc Fm, therefore it is attributed to the Eocene.
Regional extent
Correlated with the Cao Bang Fm of the Viet Bac region. Next older regional unit in Yen Bai province is the Nam Po Fm of early Jurassic.
GeoJSON
Fossils
Leaf imprints of Prunus sp. and Equisetum sp.
Age
Depositional setting
It is interpreted as a slope-prolúvial facies.
Additional Information