Bach Ho Fm

Cuu Long Basin. The type section is located in borehole BH.l drilled by the Mobil Company in 1974, at a depth of 2037 - 2960 111 (N = 9°46’; E = 108°).

Synonym: Hệ tầng lìạch Hổ + Hệ tầng Côn Sơn (part.): Ngô Thường San 1981, 1988; Đồ Bạt 1986, 1993,2000. Hệ tầng lìạch nổ + Hệ lảng Tiền Giang: Lê Văn Cự 1982.

Black-grey, greenish-grey, thin-bedded claỵstone and sandstone grading upwards into sandstone and grey to brown siltstone. The upper part includes mainly brown-grey claystone grading upwards into greenish-grey, homogeneous claystone containing foraminifera of the Rotalia group (mainly Ammonia of 1/10 mm size) called the Rotalia Claystone. The total thickness of the formation in this borehole is 923 m.

The formation is developed largely in the basin. Its two parts are clearly maintained.

1. Lower part comprises sandstone interbedded with siltstone and claystone, locally containing coal fragments formed in the alluvial to flooded deltaic plain environment in conditions of strongly changing energy from place to place.

2. Upper part in almost all areas includes relatively clean claystone containing many shallow- marine foraminifcra, Rotalia, interbedded with siltstone, some beds of greenish-grey, finegrained sandstone containing much glauconite. In general, the marine and shallow-marine environments tend to gradually increase from the southwestern margin of the basin (lots 16 and 17 of the Dragon Structure) through the central part to the northeastern margin (lots 01 and 15).

Collected fossils are relatively abundant including palynomorphs, nannofossils and foraminifera belonging to the Magnastriatites howardi - Pediastrum - Botryococcus Assemblage; Florschuetzia levipoli Zone; and Rotalia Zone. Spores and pollen arc abundant occurring usually in the upper part of the formation, mainly in the beds of claystone and siltstone. Characteristic forms consist of: Florschuetzia levipoli, FI. trilobata (very abundant), Retimonocolpites, Magnastriatites howardi and Echiperiporites estaela, fresh-water Pediastrum. Besides, there still are Tripollenites, Crudia, Leguminosae, Crassoreii- triletes nanhaiensis, Alnipollenites, Carya, DCipterocarpidites, Brownlowia, Pterospermum, ỉllexpolỉenites, Durio, Lycopodium, Getnmamonoles, Palmae, Perfotricolpites digitatus, Retitri- colpites, TricolporopoHenites, etc. and many freshwater and fresh- to slightly brackish water algae Botryococcus. The lower part. of the formation contains much sandstone, therefore the palynomorphs are rarer and yield only a few Magnastriatites howardi and algae Botryococcus. Ammonia and ostracods

The Lower Miocene sediments of the Bach Ho Fm reflect a process of transgression.