An Phu Fm
Type Locality and Naming
The type section is located near An Phu Village, Luc Yen District, Yen Bai Province (N = 21° 47', E= 105° 00'). It was named by Hoàng Thải Sơn et al. 1997.
Synonym: Hệ tầng An Phủ (Neoproterozoic- Lower Cambrian): Hoàng Thải Sơn et al. 1997. Hệ tầng Nà Hang (Proterozoic) (part): Dovjikov A.E. et al. 1965. Hệ tầng Thái Ninh (Precambrian): Phạm Đình Long et al. 1969. Hệ tầng Sông Chảy (Neoproterozoic - Lower Cambrian) (part.): Trẩn Văn Tri et al. 1977; Trần Xuyên et al. 1987.
Lithology and Thickness
Its initial composition consisted of white, clean, thick-bedded limestone with some interbeds of claystone in lower part. Its relatively homogeneous rocks were subjected to metamorphism varying from greenschist facies to lower epidote amphibolite facies transforming the rocks into pure marble, phlogopite-bearing marble, and muscovite-chlorite schist and tremolite-epidote schist. Total of 550 m.
Relationships and Distribution
Lower contact
Conformably upon the Thac Ba Fm, and has tectonic contact with Paleozoic sediments.
Upper contact
Uncertain. Tran Van Tri and Vu Khuc (2011) indicate a "pseudo-conformity" to the regionally next younger unit of the Ha Giang Fm containing Middle Cambrian fossils, however the contact was not observed.
Regional extent
Distributed in the Lo-Gam Anticline, situated in the external south margin of the Song Chay Granite Massif, in the Thanh Thuy and Bac Quang areas (Ha Giang Province), Luc Yen Anticlinate (Yen Bai Province), and in a band lying between the Song Chay (Chay River) and the Song Lo (Lo River) faults (which are just north and parallel to the Red River Fault) extending southeastward to Hanoi, then plunging beneath Quaternary deposits of the Red River Delta.
GeoJSON
Fossils
No fossils.
Age
Depositional setting
The An Phu Fm and Thac Ba Fm were formed in a basin of terrigenous and carbonate sediments having similar composition with the Sa Pa Gr in West Back Bo and equivalent formations in the West of South China.
Additional Information