Hong Ngai Fm
Type Locality and Naming
The type section is located in Hong Ngai - Pho Bang area, Dong Van District, Ha Giang Province, on the dirt road from Ta Lung northeastward to Sin Thau villages (N = 23° 15’; E = 105°12’). It was named by
Synonym: Hệ tầng Hồng Ngài (part.); Tạ Thành Trung 1972 (Permi thượng - Trias hạ), Vũ Khúc, Đặng Trần Huyên 1995; Đặng Trần Huyên 1996; VQ Khúc (in Vũ Khúc et al. 2000). Điệp Nhị Tảo (part.): Phạm Đình Long 1975 (Permi thượng). Điệp Lạng Sơn: Hoàng Xuân Tình (in Vũ Khúc et al. 2000)
Lithology and Thickness
Shallow-marine marl to oolitic limestone. Hong Ngai area: (1). Black-grey thin-bedded marl and clayey limestone with interbeds of dark-grey thin-bedded limestone; 205 m thick, with Induan bivalves. (2). Oolitic limestone, dolomitic limestone; then black-grey thin- to medium-bedded limestone; 120 m thick, with Olenekian forams and bivalve. The total thickness of the formation in this section reaches 325 m.
Another section: 1. Grey, yellowish-grey or pinkish-grey weathering, clayey limestone interbedded with dark grey limestone, sometimes thin-bedded (some to 25-30 cm) marl, 302 m thick. 2. Grey to light grey, thin-bedded (20-30 cm) limestone, sometimes with interbeds of clayey limestone of the same color, 105 m thick. 3. Grey to white-grey, mainly medium-bedded (30-40 cm), sometimes thin- (8-10 cm) to thick-bedded (up to 80 cm) limestone interbedded with oolitic limestone and some clayey limestone, 195 m thick. 4. Mainly grey to light grey, medium- to thick-bedded (30-40 cm, sometimes 80-150 cm) limestone, 40 m thick. 5. White-grey to pinkish-grey, medium-bedded (30-40 cm) oolitic limestone, dolomitizcd limestone, 70 m thick;
Relationships and Distribution
Lower contact
Conformably upon Upper Permian limestone of the Dong Dang Fm bearing foraminifcra of the Palaeofusulina Zone.
However, in the interval of this boundary no specimens of the conodont parva Zone Isarcieỉia parva — Hindeodus parvus), which is the zone proposed by the Working Group on the Permian/Triassic boundary of the International Stratigraphic Committee as the lowermost zone of the Triassic, have been found. Therefore, the above conformable attitude is a pseudoconformity, and there is a small depositional gap between the Permian and Triassic in die area. This problem needs specialized study in the future.
Upper contact
Locally, unconformably overlain by the Anisian-age Yen Binh Fm.
Regional extent
Extending from Lung Pu to Luong Le villages, Meo Vac District, Ha Giang Province; and in Tap Na and Nhi Tao (Cao Bang Province), situated in the north of the Song Hein Basin.
GeoJSON
Fossils
Lower marl member has two assemblages: 1) Claraia wangi, C. griesbachi, C. yunnanensis, Lingula sp.; 2) Claraia aurita, C. pkobangensis, C. hunanica, C. stachei, Pteria ussurica, Eumorphotis cf. multiformis. Second oolitic limestone member has Olenekian forams Ammodiscus incertus and Glomospirella irregularis; and bivalve Entolium discites mirotis.
In the interval of this boundary no specimens of the conodont parva Zone (Isarcieỉia parva — Hindeodus parvus), which is the zone proposed by the Working Gr on the Permian/Triassic boundary of the International Stratigraphic Committee as the lowermost zone of the Triassic,
Age
Depositional setting
Additional Information