Unconformable over the Ilocos Peridotite; overlain discordantly by the Megabobo Limestone.
Bangui, Baruyen and Lammin area, Ilocos Norte
Late Eocene – Late Oligocene (P17)
Probably exceeds 2,000 m
The name Bangui was first used by Smith (1907) for the sandstone unit which constitutes the upper member of his Baruyen Series. It is here called Bangui Formation to include not only the sandstone but also the associated conglomerate and shale of Fernandez and Pulanco (1967) southwest of Pasaleng in northeastern Ilocos Norte. These rocks are also seen along the road between Baruyen and Pasaleng. In the Lammin area, a similar sequence is intercalated with marble. However, the upper and lower contacts of this formation have not been described.
According to Pinet (1990), the Bangui Formation consists mainly of volcanic sandstones interbedded with varying amounts of conglomerates and mudstones. In places, the sandstones and mudstones are characterized by alternating red and green beds.
Pinet and Stephan (1990) have noted an olistostrome unit in the Vintar River section containing serpentinite, radiolarian chert, greywacke, basalt and gabbroic clasts. It is 200 m thick and exposed over a distance of 20 km. This unit is regarded as part of the Bangui Formation. This is apparently equivalent to the Baruyen Formation of Smith (1907) with type locality in the Dungan-Dungan estate along the Baruyen River in Ilocos Norte. It also crops out along Caruan River in Pasuquin. The chert is dirty red, fine grained, hard and easily breaks into slabs. Irving and Quema (1948) described the chert as intensely folded, strongly fractured and brecciated.
The marble intercalated with the clastic rocks in Lammin area has been dated Late Eocene (BMG, 1982). Pinet (1990) reports that recent dating of planktonic foraminifera in samples from Pasaleng area and elsewhere indicate ages of Late Eocene to Late Oligocene (P17). The thickness of the Bangui Formation probably exceeds 2000 m.