A Ngo Fm
Type Locality and Naming
Vietnam-Laos border in Quang Tri Province. Holostratotype - along the road from Huong Hoa to A Luoi, from west of A Pat Village to the mouth of the La Hot Stream and farther along La Hot Stream, Quang Tri Province (x = 16°28’; y = 106°59’). Dang Tran Huyen, Nguyen Chi Huong 1995. Formerly, in geological mapping at 1:500,000 of South Viet Nam it was assigned to the Tho Lam Fm (“Suite”). However, it is composed mainly of continental red beds and its section sequence is distinguished from the Tho Lam Fm, therefore has been separated as an independent unit - the A Ngo Formation.
Synonym: - Hệ tầng A Ngo\ Đặng Trần Huycn, Nguyễn Chí Hưởng (in Tống Duy Thanh et ai. 1995); (in Vũ Khúc, Đặng Trần Huyên) 1995; Nguyễn Văn Trang et ai. 1996; Nguyễn Xuân Dương eí al. 1996; Vũ Mạnh Điền et al. 1998; Vũ Khúc {ìn Vũ Khúc et aỉ.) 2000. - Grès et terrains rouges (part.): Bourret R. 1925. - Điệp Thọ Lâm-. Vũ Khúc (in Vũ Khúc, Bùi Phú Mỹ et al.) 1990. - Hệ tầng Dộng Trúc'. Phạm Đình Trường et al. 1996.
Lithology and Thickness
Lower part of mixed continental and marine facies (with bivalves) grading upward into pure continental red beds. The stratotype succession (1215 m thick) is: (1). Mainly thick-bedded conglomerate with pebbles of greyish white quartz interbedded with pebble-bearing gritstone, 90 m thick. (2). Grey to greyish white, medium-bedded coarse polymictic sandstone with some interbeds of grey silty sandstone, 120 m thick. (3). Chocolate-colored, medium-bedded siltstone with some sandstone and silty sandstone, 90 m thick. (4). Light grey, medium-bedded, fine- to medium-grained sandstone interbedded with chocolate-colored silty sandstone and siltstonc some of whose beds are calcareous and yellowish grey, 215 m thick. (5). Brownish argillaceous shale and silty claystone with some interbeds of greenish-grey marl and calcareous siltstone, 125 m thick; in this member Vu Manh Dien et al. (1998) found the marine bivalves Cardinia cf. indochinensis, Pleuromya cf. obỉonga, and Hỉateỉỉa cf. arenicola, Ostrea sp. (6). Chocolate-colored siltstonc interbedded with fine-grained sandstone and some grey to greenish-grey calcareous siltstone and marl, 230 m thick. (7). Grey to greenish-grey, thin- to medium-bedded argillaceous limestone with some interbeds of grey limestone and shelly limestone, 40 m thick; containing bivalves, such as Cardinia indochinensis, c. orỉentaỉỉs, c. aff. latitruncata, Langvophorus vukhuci, Hiatella arenicola. (8). Chocolate-colored siltstone interbedded with yellowish-grey, cross-bedded polymictic sandstone, medium- to thick-bedded, medium- to coarse-grained quartz sandstone containing many mica flakes, 305 m thick.
Relationships and Distribution
Lower contact
The A Ngo Formation unconformably rests upon mctamorphic rocks of the Cambrian-Ordovician A Vuong Fm, the Upper Permian Cam Lo Fm and gabbro of the Upper Triassic Cha Van Complex Fm.
Upper contact
Not observed.
Regional extent
The formation occurs in western Quang Tri Province in two bands: the first, to the west of Huong Hoa District extending through Lia Village to Laotian territory; the second, narrower, in the NE-E of this district extending along the Da Krong River through Ta Rut, A Ngo to also Laotian territory.
GeoJSON
Fossils
Marine bivalves found in grey carbonate mterbeds of the formation include well-known Early Jurassic species.
Age
Depositional setting
Mixed continental and marine, grading upward into pure continental red beds
Additional Information