Muong Hinh Fm
Type Locality and Naming
Western Thanh Hoa, Nghe An and Ha Tinh provinces. The type section is located on NE slope of the Bu Quam Range, western Thanh Hoa Province (N = 19°48’; E = 105°04’). Le Duy Bach 1969.
Felsic volcanogenic rocks interbedded with continental red beds in the lower part of the section, present in the Sam Nua Depression, have been grouped into the Muong Hirih Fm during geological mapping .at 1:200,000 of the Quy Chau Map Sheet (1969). This formation is best exposed in the Thuong Xuan, Nhu Xuan areas, west of Thanh Hoa Province; in addition, it occurs to the west of Nghe An (Vinh - Thanh Chuong band, Nghia Dan) and western Ha Tinh Province.
Synonym: - Hệ tầng Mường Hình: Lê Duy Bách et al. 1969 (Cami); Nguyễn Vãn Hoành (in Vũ Khúc, Bùi Phú Mỹ et al. 1990) (J3-K1); Lê Duy Bách, Nguyễn Văn Hoành 1996 (J). - Trầm lích Jura không xác định: Jamoida A.I., Mareichev A.M. (in Dovjikov A.E. et al.) 1965; Phạm Đức Lương (in Trần Văn Trị et al.) 1977.
Lithology and Thickness
Volcano-sedimentary beds. The stratotype of the formation was designated in the northeastern slope of the Bu Quam Range (west Thanh Hoa Province) with two parts. Lower part: brownish, thick-bedded conglomerate up to tens of metres thick, with pebbles mainly of quartz, some chert, of medium roundness, and cement of greenish sandstone, grading upward to green-yellowish, chocolate-colored sandstone and siltstone, greenish- grey tuffs and porphyritic rhyolite and porphyritic rhyodacite; thickness about of 1000 m. Upper part: greenish-grey, massive porphyritic rhyolite, containing phenocrysts of feldspar and quartz, sometimes interbedded with violet-grey tuffaceous sandstone, thickness about 1000 m. The total thickness of the formation in this section reaches 2000 m.
Relationships and Distribution
Lower contact
The Muong Hinh Fm unconformably rests upon the Ladinian Quy Lang Fm. Regionally, the next older unit is the Nam Po Fm of early Triassic.
Upper contact
Quaternary.
Regional extent
Large exposures in the Thuong xuan, Nhu Xuan (Thanh Hoa Prov.), Thanh Chuong, Nghia Dan (Nghe An Prov.), and in the west of Ha Tinh Prov. The ratio between sedimentary and volcanogenic rocks in the sections of the Muong Hinh Fm changes from place to place. As in the Bu Me area, the lower part of the formation contains almost no volcanics, but includes mainly violet-brown, medium- to thick-bedded sandstone and siltstone. On the contrary, the section of the formation in the Nam Sam area (source of the Song Chu River), Huoi Cam, Ban Chieng (western Thanh Hoa Province) and west Yen Thanh areas (Nghe An) is composed almost entirely of volcanics, and especially, with hypabyssal rocks. Locally, the thickness of the formation reaches only 500-700 m. Among volcanics, the most commonly encountered consist of quartz porphyritic green- grey color dacite, of massive structure and porphyritic texture, with phenocrysts of quartz and feldspar, and microfelsitic groundmass. In addition, there is porphyritic rhyodacite very similar in appearance to porphyritic dacite and differing from it by its content of potash feldspar. Dacite and quartz porphyry occurs in the form of interbeds within the above rocks. Besides, there are tuff beds. Hypabyssal rocks usually occur in the form of dykes or veins penetrating volcanics. They are composed of white-grey porphyritic granite and granophyre with feldspar and quartz phenocrysts, of porphyritic texture with large phenocrysts and micropegmatitic, locally, aphanitic textures.
GeoJSON
Fossils
To date no fossils occur in its beds. Initially, Le Duy Bach referred the formation to the Camian on the basis of the above stratigraphic relation. But during the material synthesis for compiling the Geological map of Viet Nam at 1:500,000 (1988) the authors of this map reassigned it to the Upper Jurassic - Lower Cretaceous on the basis of correlation with the young felsic volcanic formations of North Viet Nam, which cover the Norian-Rhaetian coal-bearing formation, and were formed in the continental environment. According to the last data and correlations, this volcanic formation is attributed to the Upper Jurassic.
Age
Depositional setting
It is interpreted as a material synthesis for compiling the Geological map of Viet Nam at 1:500,000 (1988) the authors of this map reassigned it to the Upper Jurassic - Lower Cretaceous on the basis of correlation with the young felsic volcanic formations of North Viet Nam, which cover the Norian-Rhaetian coal-bearing formation, and were formed in the continental environment.
Additional Information