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Dong Son Formation
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Dong Son Fm base reconstruction

Dong Son Fm

Period: 
Ordovician

Age Interval: 
Early Ordovician; O1 ds, (9, 11)


Province: 
West Bac Bo

Type Locality and Naming


Lithology and Thickness


Lithology Pattern: 
Fine-grained sandstone


Relationships and Distribution

Lower contact

Conformable upon the Ham Rong Gr.

Upper contact

The upper boundary of the Dong Son Fm is irregularly eroded. Overlain by the Nam Pia Fm in type area. [Regionally the next younger unit is the Sinh Vinh Fm or Ket Hay Fm or Tay Trang Fm.]

Regional extent

The Dong Son Fm is distributed on the two limbs of the Cam Thuy - Ba Thuoc and Song Ma Anticlinoria. The NE limb extends from south of Pha Din Pass (Dien Bien Province) through Ban Mo, Ban Pha, Muong Chanh (Son La Province) and Laotian territory to the Ham Rong Bridge area (Thanh Hoa Province). The SW limb - from Co Ma (Son La Province) to the Song Luong and Song Lo rivers area (Thanh Hoa Province).


GeoJSON

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Fossils

The Early Ordovician age of the Dong Son Fm has been determined on the basis of a characteristic trilobite collection including different species of Hysterolenus, Asaphellus, Bienvillia, Asaphopsis, and Annamitella. These fossils can be distinguished in four levels, (in ascending order): 1) Asaphellus trinodosus, or Hysterolenus sp.; 2) Asaphopsis jacobi; 3) Annamitella asiatica; and 4) crinoids.


Age 

Early Ordovician [The underlying Ham Rong Gr also includes some earliest Ordovician.]

Age Span: 

    Beginning stage: 
Tremadocian

    Fraction up in beginning stage: 
0.5

    Beginning date (Ma): 
481.96

    Ending stage: 
Floian

    Fraction up in the ending stage: 
1.0

    Ending date (Ma):  
471.26

Depositional setting

Light-colored, coarse-grained terrigenous sediments and benthic faunas (trilobites, brachiopods, bivalves and crinoids) show that the Dong Son Fm is a marine shallow littoral facies.


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).