Mu Gia Fm

Viet-Laotian border in western Quang Binh Province. The type section is located on the track from Mu Gia Pass to Y Lanh Village (west Quang Binh) (N = 17°45’; E = 105°46’). Tran Due Luong, Nguyen Xuan Bao (in Vu Khuc, Bui Phu My et aỉ. 1990). Continental red beds covering all older formations with a slightly inclined attitude, named as the “Upper Red Beds” (Terrains rouges supérieur) by French geologists, were described as Mu Gia Fm during the compilation process of the Geological Map of Viet Nam at 1:500,000 (1988)..

Synonym: - Hệ tầng Mụ Giạ: Trần Đức Lưong, Nguyễn Xuân Bao et al. 1988; Vũ Khúc. Lẽ Thị Nghinh 1996; Nguyễn Quang Trung et al. 1996; Nguyễn Xuân Dương et al. 1996; Vũ Mạnh Điển et al. 1998; Vũ Khúc (in Vũ Khúc et al. 2000) (Creta); Phạm Huy Thông et al. 2001 (J₃ K₁). - Terrains rouges supérieurs: Hoffet J. 1937. - Trầm lích Creta không phân chia. Mareichev A.M. (in Dovjikov A.E. et al.) 1965; Vũ Khúc (in Trần Văn Tri et al. 1977). - Hệ tầng Nước Rụng: Phạm Huy Thông 2000 et al. (J₃). - Hệ tầng Phu Co Pi: Phạm Huy Thông 2000 et al. (K₁).

Continental red beds. In the initial description of the formation its stratotype, situated in the Mu Gia Pass area, was divided into two parts. Lower part: reddish conglomerate, greenish-grey, thick-bedded to massive, coarse- to medium- grained sandstone containing many mica flakes, 600-700 m thick. The clast composition of the conglomerate is rather varied, with quartzite, quartz, light grey limestone, sandstone, sometimes dark grey claystone too; pebbles are poorly rounded and sorted. The cement consists of chocolate-colored silty sandstone. Upper part: mainly of chocolate-colored siltstone interbedded with brownish sandstone and some chocolate-colored claystone, about 500 m thick. The total thickness of the formation in this section reaches 1100-1200 m.

In the Ca Roong area, the Co Ru Pan - Co Te Rien section was described by Ho Trong Ty (in Vu Khuc, Bui Phu My et al. 1990) is as follows. (1). Thick-bedded conglomerate and gritstone with a clast composition of quartz, limestone and chert; pebbles are poorly rounded, poorly sorted, cement of brownish sandstone, 200 m thick. (2). Greyish gritstone, brownish, thick-bedded coarse-grained sandstone grading upwards into medium- to fine-grained sandstone of the same color, 250-300 m thick. (3). Mainly chocolate-colored, medium-bedded siltstone interbeddcd with medium- to finegrained sandstone and some chocolate-colored claystone, about 200 m thick. (4). Red, brown-violet siitstone interbedded with claystone of the same color, thick- bedded, locally with interbeds of fine-grained sandstone containing muscovitc flakes, about 250 m thick. The total thickness of the formation in this section is about 950-1000 m. Later (2001) these authors reverted to the former division, and described the Mu Gia Fm with the following sequence. 1. Light violet polymictic conglomerate (2 m thick) grading upwards into violet-grey, thick- bedded, coarse- to medium-grained sandstone and some interbeds of grey siltstone, 100- 250 m thick; conglomerate pebbles consist of limestone, chert and sandstone, that are medium rounded and sorted, 0.5 to 3-4 cm in size. The cement has rather high copper content (170 ppm), which allows us to correlate these sediments with those of the Nam Phouan Fm of Late Jurassic age from the Nam Theun basin in Middle Laos. Siltstone interbeds yield some palynomorphs of land plants, with Cycadopites minutus, Bacutricolpites centricus, Classopollis sp.. 2. Light violet-grey, thick-bedded polymictic silty sandstone with some interbeds of siltstone, 80 m thick. 3. Mainly violet-grey, thick-bedded, medium-grained sandstone, locally with lenses of coarsegrained polymictic gritstone, 150-200 m thick. 4. Light violet quartz conglomerate and gritstone (5-6 m thick) grading upwards into reddish, cross-bedded sandstone, 35-45 m thick. The total thickncss of the formation in this section is about 365-570 m.

Siltstone contains pollens of land plants