Van Xa Fm
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.
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
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
Depositional setting
Additional Information