Bach Tri Fm

South Song Hong Basin. The type section is located in the Bach Tri borehole drilled in the Hue Graben, at a depth of 3667 to 3936 m (N = 17° 15’; E = 107°19’Ỉ2”).

Synonym: Hệ tầng Bạch Trĩ: Đỗ Bạt 2002. Oligocene terrigenous rocks: BP 1991, 1995. Hue Gr, Da Nang Gr: Morris J. 1993; Lamcrs E. 1993; Sladen CM Hoàng Ngọc Đăng 1997; PhanTrung Điền 2000. Loạt Đà Năng: Lê Đình Thám 1988; Đô Bạt 1993.

In reference borehole, the formation is composed of clayey siltstone interbedded with grey, brown-grey, fine- to medium-grained sandstone and some thin seams of lignite. The siltstone is grey to dark grey colored containing small fragments of coal or organic matter. The sandstone consists mainly of light grey to brown-grcy, fine- to medium-grained feldspathic litharenite and litharenite, the grains of which are medium-rounded, medium- to well sorted, firmly cemented by calcareous clay. Thickness: 269 m. These beds rest unconformably upon Devonian dolomite. These are sediments formed in alluvial, lacustrine and deltaic environments.

Southwards, in lot 114 (hole 114-KT) sandstone plays the main role in the formation with intcrbcds of claystone. In lots No.118 and 119, there are rather thick beds of terrigenous sediments (> 200 m) composed mainly of medium- to coarse-grained sandstone, locally changing to gritstone and conglomerate with some interbeds of silty claystone and lignite seams (GK.118-CVX). The grains of sandstone are medium rounded, poorly to medium sorted. Sandstone consists mainly of feldspathic litharcnite, firmly cemcnted by claycy carbonate. The thickness of the formation changes from 100 to 300 m. These sediments were formed in an alluvial cone environment, coastal dcltaie plain influenced by streams. The formation unconformably underlies Miocene limestone and dolomite.

Cicatricosisporites dorogensis, Lycopodiumsporites neogenicus, Verrutricolporites pachydermus, Quercidites, Florschuetzia trilobata, Pteris, Ảlnipoỉỉeniles, Triletes, Baringtonia, Rhizophora, Verrucatosporites usmensis, Magnastriatites howardi, Acrostichum, Crassoretitnletes nanhaiensis, Rhizophora sp., that belong to the Cicatricosisporites - Lycopodiumsporites neogenicus Assemblage.

It is interpreted as a lacustrine - lagoonal environment.