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Mo Nhai Formation

Mo Nhai Fm

Period: 
Devonian

Age Interval: 
Late Devonian, (4)


Province: 
East Bac Bo

Type Locality and Naming

No Nhai – Minh Dan section as its stratotype.


Lithology and Thickness

Mainly composed of limestone and clayey limestone, containing abundant Late Devonian corals, stromatoporoids and forams. Stratotype is 370 m of: (1) Black-grey medium-bedded limestone; 150 m thick; with abundant stromatoporoids. (2) Black-grey thick-bedded limestone and cherty limestone; 100 m thick; with abundant stromatoporoids, tabulate corals and forams. (3) White-pinkish dolomitic limestone, locally interbedded with black-grey limestone containing algae; 120 m thick.


Lithology Pattern: 
Reef limestone


Relationships and Distribution

Lower contact

Conformable upon the Tan Lap Fm.

Upper contact

Disconformity with the overlying Bac Son Fm.

Regional extent

Restricted distribution in the Bac Son and Chi Lang districts of Lang Son Province.


GeoJSON

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Fossils

Stromatoporoids include Amphipora, Tienodictyon, and other taxa. Tabulate coral of Thamnopora strelinaensis. Forams include Eogeinitzina devonica and Eonodosaria stalinogorskii.


Age 

Frasnian-Famennian(?) based on collected fossils.

Age Span: 

    Beginning stage: 
Frasnian

    Fraction up in beginning stage: 
0.5

    Beginning date (Ma): 
375.00

    Ending stage: 
Famennian

    Fraction up in the ending stage: 
0.5

    Ending date (Ma):  
365.20

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).