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Van Xa Formation
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Van Xa Fm base reconstruction

Van Xa Fm

Period: 
Devonian, Carboniferous

Age Interval: 
D3-C1 ps, (20)


Province: 
North Trung Bo

Type Locality and Naming

Thura Thien Hue Province. Upper formation of the My Duc Gr. Corresponds to the former Van Xa Member of a Phong Son Fm, which had its holostratotype section extending from the Thanh Tan mineral water spring to the rice field situated east of Hien An Village (E = 16°29'; E = 107°22’). That formation beared the name of the Phong Son Commune, Phong Dien District, Thua Thien - Hue Province, where its stratotype is located.

Synonym: Phong Son Fm (Van Xa Member); Hệ tầng Phong Sơn: Nguyễn Hữu Hùng 1994; Nguyễn Hữu Hùng et al. 1995; Nguyễn Hữu Hùng 1996; Phạm Huy Thông eí aỉ. 1997, 2000 (D3fm – C1); Tống Duy Thanh, Nguyễn Đức Khoa (in Vũ Khúc ei ứl. 2000) (Famenn i an-Tou m a i si an). Les calcaires de ỉa region de Hue: Fontaine H. 1962 (Vise). Đá vôi Carbon họ (part.): Nguyền Văn Trang 1978. - Hê tầng Cò Bai: Nguyễn Văn Trang et al. 1985 (D2gv - D3fr). Hệ tầng La Khê'. Nguyễn Trường Giang et al. 1989 (C1)


Lithology and Thickness

The characteristic setin of the Van Xa Fm in the Phong Son area of Phong Dien District is 200-350 m thick, including ash-grey and black-grey clayey limestone regularly interbedded with black shale. Its lower part Clayey limestone and Limestone: The Van Xa Member mainly clayey limestone containing brachiopods of Famennian age, 280 m thick; and the Hien An Member of limestone and cherty limestone containing stromatoporoids, tabulates, tetracorals and foraminifera of Famennian-Tournaisian age, 350 m thick.


Lithology Pattern: 
Clayey limestone


Relationships and Distribution

Lower contact

Unclear stratigraphie relations with the older/coeval Co Bi Fm as well as the older Tam Lam Fm.

Upper contact

Conformably overlain by the Phong Nha Fm. In the northeast it is covcred by Quaternary pebbles, grit and silt.

Regional extent

The Phong Son Fm, of which the Van Xa Fm was the lower member, occurs only in Thua Thicn - Hue Province with not many natural exposures becausc of the thick Quaternary cover. Drilling information for water exploration by Geological Party No.708, as well as cement limestone and construction material exploitation data let one see that its distribution areas are large. They extend in NW-SE trending bands from south of Hue City through Van Xa, Thanh Tan to Hoa My, a distance of over 30 km and a width of 3-5 km.


GeoJSON

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Fossils

Its lower part contains the Famennian brachiopods of Yunnanella ksikwangshaensis, Yun. cf. hanburyi, Uchtospirifer sp., Athyris concéntrica,Monelasma cf. deschayesii, Tenticospirifer tenticulum, etc. Its upper part has latest Devonian-earliest Carboniferous (Tournaisian) forams of Septabrunslina cf. minuta, Sept. cf. rauserae, Sept. cf. kazakhtanica, Septatournayella cf. segmentata, and Chernyshnella sp.; stromatoporoid Rosenella miniarensis, and coral Syringopora sp.


Age 

Based on fossils and stratigraphic position, assigned as Late Devonian to earliest Tournaisian. [For graphic purposes, placed here as Famennian, with coeval Cu Bai Fm shown as Givetian-Frasnian.]

Age Span: 

    Beginning stage: 
Famennian

    Fraction up in beginning stage: 
0.0

    Beginning date (Ma): 
371.10

    Ending stage: 
Tournaisian

    Fraction up in the ending stage: 
0.1

    Ending date (Ma):  
358.04

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).