Phong Nha Fm

Quang Binh Province. It bears the name of a well known, beautiful and lengthy cave system; and the type section is at the entrance to this Phong Nha Cave in the Phong Nha – Ke Bang area (x = 17°35’; y = 106°16’30” ) as described by Le Hung (1981). This is in the Bo Trach and Minh Hoa districts of Quang Binh Province.

Synonym: Phong Nha Member (topmost member of Tam Lam Fm = Cu Bai Fm = La Khe Fm, of which lower member is the Xe Bang Heing Mbr/Fm), as used in Tong-Dzuy Thanh and Vu Khuc (2011, Editors; Stratigraphic Units of Viet Nam). In contrast, Tran Van Tri and Vu Khuc (Editors; Geology and Earth Resources of Viet Nam) classify the Phong Nha as a distinct formation, with a transitional relationship to the Cu Bai Fm, and overlain by the La Khe Fm.

Stratotype at Phong Nha Cave entrance is about 100 m thick: (1) Black-grey bioclastic limestone and banded, thin-bedded cherty limestone with interbeds of red-brown shale, 60 m thick, containing Tournaisian fossils (see Fossils below). (2) Dark grey, thin- to medium-bedded bioclastic limestone interbedded with chert or containing small chert nests, 15 m thick. (3) Grey cherty shale and cherty limestone, 15 m thick.

Lower unit includes Brachipods: Crurityyris urii, Cr.sp.; Benthic forams: Bisphaera malevkensis, B. grandis, Parathurammina suleimanovi, Tournayellina beata, T, primitiva, Chernyshinella glomoformis, C. crassitheca, C.? ex gr. subplanispiralis, Septabrunsiina (S.) kingirica, S. {S.) grozdilovae, S. (S.) rauserae, S. (S.) compressa, S. (S.) dainae, S. (S.) minuta, S. (S.) cf. krainica, Septatournayella evoluta, S. segmentata; Corals: Rotiphyllum sp., and Fedorowskia phongnhaensis, Cobaiphyllum sp.. Middle unit has coral Pseudouralinia aff. tangpakonensis, gastropods and brachiopods. Upper unit has trilobite Pseudophillipsia sp. and ossicles of crinoids.