Tri Ton Fm

South Song Hong Basin. The type section is located borehole GK.119-CH in the Tri Ton Horst, at a depth of 1454 to 2165 m (N = 15°17’15”; E = 109°3'40”).

Synonym: Loạt Tri Tôn (part.): Morris J. 1993. Loại Đà Nằng (part.): BP 1991, 1995. Middle Miocene: Laniers E. 1992. Lower Miocene: Đỗ Bạt 1993. Hệ tầng Sông Huơng (part. LêĐinhThám 1992, 1994, 1998. Hệ tầng Đà Nang (part.): Lê Đình Thám 1992, 1994, 1998; Sladen C., Hoàng Ngọc Đang 1997;Phan Trung Điền 2000

The section at a depth of 1454 to 2165 m in borehole GK.l 19-CH has been considered the stratotype (711 m thick). It is composed of light grey, yellow, brownish-grey to dark grey limestone bearing algae, large size foraminifera and corals. The rocks are microgranular, fine-grained to cryptocrystalline, locally recrystallizcd; and of massive, rarely bedded structure. They are characterized by good porosity and absorptivity and with medium consolidation.

However, in the Hue and Quang Nghe grabens, the Fm is composed mainly of grey siltstone, claystone with some interbeds of calcareous sandstone, with a variable thickness from 300 to 1000 m.

Florschuelzia levipoli, FI. trilobata, Pinuspollenites, Dacrydium, Liquidambar, Rhizophora, Carya- pollenites, Polypodiaceisporites, Echiperiporites estelae, Triletes, Camptostemon, Crassoretitriletes vanraadshooveni, belonging to the Florchueizia levipoli - FI. trilobata Assemblage. Globigerinoides trilobus, Globorotalia foshi, Globoquadrina dehiscens, Orbulina universa and benthos Siphonodosaria, Haplophragmoides, Cydamtnina, Eponides, Cibicides, Amphistegina belonging to the CN9-N14 Foram Zones. Limestone beds yield Lepidocyclina, Miogypsina. Nannoplanktons comprise Castinaster coalitus, Coccolithus miopelagicus, Cyclococcolithus macintyrei, Discoaster broweri, D. bolli, D. exilis, D. hamatus, Helicosphaera belonging to the NN5-NN9 Nannofossil Zones.

It is interpreted as a coastal plain, shallow marine and continental shelf environments.