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Lithology

Calcareous shale and sandstone; limestone; siltstone; conglomerate

Stratigraphic relations

Unconformable over the Callao Formation and older rocks

Distribution

Cabagan, Isabela; Kalinga-Apayao

Age

Late Miocene – Early Pliocene

Thickness

750-1,000 m

Named by

Geophoto Exploration Ltd (1966)

The formational name was introduced by Geophoto Exploration Ltd (in BM Petroleum Division, 1966) referring to the sedimentary section along Cabagan River in Cabagan, eastern Isabela and similar deposits throughout the Cagayan Valley. It is distributed at the margins and at the core of the Pangul Anticline in the center of the valley. From south to north of the Cagayan Valley Basin, the Cabagan Formation covers unconformably the Lubuagan Formation, the Ibulao Limestone, Callao Limestone and older dioritic units. Caagusan (1978), however, describes the relation between the Callao Limestone and Cabagan Formation as intertonguing. Three lithologic entities are represented in the formation. The lower consists chiefly of gray silty to sandy calcareous shale with interbeds of calcareous sandstone and limestone; the middle, essentially dark gray shale with thin beds of nodular limestone; the upper, dominantly siltstone and fine grained sandstone. Maac (1988) describes a conglomeratic facies of this formation exposed along the Tabuk-Batong Buhay route in Kalinga-Apayao. The formation is 750 m thick at its type locality but could reach an overall thickness of 1,000 m (Billedo, 1994). Calcareous sandstone sampled in the lower portion of the formation yielded large foraminifera indicating a Late Miocene age, while nannofossils from a shale sample from the upper portion were dated Late Miocene to late Early Pliocene (NN7 and NN11) as reported by Aurelio and Billedo (1987).

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