Ha Coi Fm
Type Locality and Naming
An Chau and Song Hien basins, plus Quang Ninh region. Named after Ha Coi Gulf; described by Dovzhikov et al. (1965).
Synonym: Upper Sandstone (named by French geologists for how it forms the summits of low mountain ranges).
Lithology and Thickness
Continental red beds. In Thai Nguyen City area (Au Chao Basin), it consists of up to 1850 m of mainly coarse-grained sediments, such as sandstone, gritstone and conglomerate, interbeddedwith red-brown siltstone and some claystone. Coastal section of Hao Coi Gulf has about 1200 m of: (1) Thick-bedded quartz conglomerate, white coarse-grained sandstone, red-brown siltstone with some interbeds of grey sandstone containing coall matter, and pinkish quartzitic sandstone; 300 m thick. (2). Red-brown siltstone with interbeds of cross-bedded fine-grained sandstone and lenses of marl, 480 m thick. (3). Intercalation of quartzitic sandstone and red-brown siltstone, 306 m thick.
Relationships and Distribution
Lower contact
In the Au Chau Basin, it unconformable on the coal-rich Van Lang Fm of Late Triassic. In the Song Hien Basin, it unconformably overlies the Song Hein Fm of Lower Triassic. In Quang Ninh Basin, it has an angular unconformity upon the Lower Paleozoic Tan Mai Fm schists.
Upper contact
Upper boundary not exposed. In Lang Son region, the next younger unit is the volcanics of the Tam Lung Fm. In Quang Ninh region, the next younger unit is the Dong Ho Fm.
Regional extent
Quang Ninh Basin, in Au Chao Basin (Thai Nguyen City, An Chau and Dinh Lap areas), and in Song Hien Basin scattered in adjoining areas of Lang Son and Thai Nguyen provinces.
GeoJSON
Fossils
Jurassic flora. Black shale intervals contain fresh-water fish flose to Baleiichthys, together with Early Jurassic phyllopods Euestheria minuta, Estherites ex gr. kawasakii. Also Early Jurassic fresh-water bivalves.
Age
Depositional setting
Additional Information