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Vien Nam Formation

Vien Nam Fm

Period: 
Permian

Age Interval: 
Late Permian; P3vn, (11)


Province: 
West Bac Bo

Type Locality and Naming

Song Da Basin. Holostraỉotype: on the dirt road, from the Vien Nam Mt to Co Dong Village, Ba Vi District (N = 20°59'; E = 105°26'). Ho Trong Ty 1990.

Synonym: Hệ tầng Viên Nam: Phan Cự Tiến et al.) 1977; Bùi Phú Mỹ el al. 1978: Đinh Minh Mộng et al 1978; Nguyễn Văn Hoảnh ei aỉ. 1978; Nguyễn Xuân Bao ei al. 1978; Phan Son et al. 1978 (Permi thượng - Trias hạ); Nguyễn Đức Thắng 1994 (Trias hạ); Vũ Khúc, Đặng Trần Huyên 1995; VQ Khúc et al. 2000 (Indi); Trần Trọng Hoà 2001 (Trias hạ). Hệ Trias không xác định; Hệ Jura không phân chia: Jamoida A.I., Phạm Văn Quang (in Dovjikov A.E. el al.) 1965. Per mi thượng'. Lê Hùng (in Trần Văn Trị et al. 1977). Hệ tầng Cẩm Thùy (part.): Nguyễn Kinh Quốc (in Vũ Khúc, Bùi Phú Mỹ et al. 1990)


Lithology and Thickness

Basalt. Characteristic section is 800 m of: (1). Dark grey-green basalt, porphyritic basalt and their tuffs, 250 m thick. Basalt often contains amygdules filled with chlorite, calcite and quartz. (2). Porphyritic basalt and greenish-grey tuffaceous agglomerate, 170-200 m thick. Fragments of tuffs in agglomerate are of different sizes, cementcd by fine-grained tuff. (3). Olivine basalt, porphyritic basalt interbedded with green, green-grey irregularly amygdaloidal tuff, 150 m thick. (4). Dark green-grey porphyritic basalt interbedded with light grey, thick-bedded tuffaceous sandstone, 200 m thick.


Lithology Pattern: 
Lava


Relationships and Distribution

Lower contact

Not yet observed. Regionally, the next older is the Bac Son Fm limestone (Carboniferous-early Permian).

Upper contact

Conformably underlies the Yen Duyet Fm. However, the top member in the Phuong Lam Ferry landing of Hoa Binh Town gradually grades upwards into the beds containing Olenekian fossils of the Co Noi Fm.

Regional extent

The Vien Nam Fm is widespread in the Song Da and Ninh Binh Structural zones, from the Vien Nam - Ba Vi and Kim Boi areas (Hoa Binh Province) extending northwestward to the Van Yen, south Ta Khoa and Nam So areas (Son La Province), and scattered in some other localities, such as south Ha Tay, Ninh Son Coal mine, etc.


GeoJSON

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Fossils


Age 

Isotopic age is 260-250 Ma. [Conformable below Late Changhsingian age Van Duyet Fm of upper Changhsingian. Strat column graphics suggests late Wuchiapingian-early Changhsingian?].

Age Span: 

    Beginning stage: 
Wuchiapingian

    Fraction up in beginning stage: 
0.7

    Beginning date (Ma): 
255.83

    Ending stage: 
Changhsingian

    Fraction up in the ending stage: 
0.5

    Ending date (Ma):  
253.07

Depositional setting


Depositional pattern:  

Additional Information


Compiler:  

Tran Van Tri and Vu Khuc (Editors; Geology and Earth Resources of Viet Nam, General Dept. of Geology and Minerals of Viet Nam; completed in Dec., 2011), with additional details from Tong-Dzuy Thanh and Vu Khuc (Editors; Stratigraphic Units of Viet Nam, 2nd edition; 2011; English edited by A. Boucot).