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Na Ngan Formation

Na Ngan Fm

Period: 
Devonian

Age Interval: 
Lochkovian, (3)


Province: 
Viet Bac

Type Locality and Naming

Lower of two formations in Song Cao Gr version in Cao Bang Province. Stratotype in the Na Ngan section in the Ha Lang area (Cao Bang Province). On the territory of Cao Bang Province it is difficult to distinguish the sediments corresponding to the Si Ka Fm and Bac Bun Fm, because of their transitional relations and their small thickness. Pham Dinh Long (1975) described these beds under the name “Na Ngan Suite” (Na Ngan Fm entry in this Lexicon), but it is more reasonable to attribute them to the Song Cau Gr (see Bac Bun Fm entry).


Lithology and Thickness

Stratotype has 200 m of: (1) Basal conglomerate, grading upward into reddish-brown by weathering sandstone, siltstone interbedded with claystone; 50 m. (2) Violetish-brown siltstone interbedded with bluish-black shale and siltstone; 120 m; containing brachiopods.


Lithology Pattern: 
Siltstone


Relationships and Distribution

Lower contact

Unconformable on the Than Sa Fm (Late Cambrian)

Upper contact

Conformable below the Mia Le Fm.

Regional extent

Cao Bang Province. Equivalent to the Si Ka Fm and Bac Bun Fm (lower and middle formations of the Song Cao Gr) in the adjacent provinces of Ha Giang and Lang Son.


GeoJSON

null

Fossils

Brachipods of Howitia assemblage and plant remains.


Age 

Lochkovian based on collectioed fossils.

Age Span: 

    Beginning stage: 
Lochkovian

    Fraction up in beginning stage: 
0.0

    Beginning date (Ma): 
419.00

    Ending stage: 
Lochkovian

    Fraction up in the ending stage: 
1.0

    Ending date (Ma):  
412.40

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).