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Ibulao Limestone

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Lithology

Limestone, calcarenites, calcirudites

Stratigraphic relations

Unconformable over the Dibuluan Formation; Unconformably overlain by the Lubuagan Formation

Distribution

Kiangan Valley, Ifugao; Maddela and Bayombong Nueva Vizcaya; Jones and Cabagan, Isabela

Age

Late Oligocene

Thickness

150 – 600 m (BMG, 1981); 200-450 m (Billedo, 1994)

Named by

Corby and others (1951)

Synonym

Sicalao Limestone

The Ibulao Limestone, named by Corby and others (1951), is a biohermal to biostromal well-bedded limestone with varying thicknesses of intercalated calcirudites and calcarenites. It rests unconformably over the Dibuluan Formation and is overlain unconformably by the Lubuagan Formation. The type locality of this limestone is at Ibulao Gate, , Ifugao. The limestone is well exposed in Sto. Domingo at the end of Kiangan Valley and in Nueva Vizcaya and Isabela, where it trends northeast from the southern portions of Maddela up to the rivers of Aburao and Tugawi, south of Jones, Isabela. It also outcrops farther north in Cabagan, Isabela. The Ibulao is primarily biohermal to biostromal in the southeastern and northeastern parts of the valley. It has a reported thickness of 150 to 600 m (BMG, 1981), although Billedo (1994) limits the thickness range to only 200-450 m for the limestone in the eastern side of the valley. Recent paleontological determinations confirm an age of Late Oligocene (Billedo, 1994). A shale sample collected at the base of the overlying Lubuagan Formation at Dibuluan River yielded nannofossils of biochronological zone NP25 or late Late Oligocene, suggesting the upper limit of the Ibulao Limestone (Billedo, 1994).

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