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Lithology

Volcanic flows, breccias, pyroclastics, sandstone, conglomerate, siltstone, mudstone

Stratigraphic relations

Unconformable over the Abuan Formation; Unconformably overlain by the Ibulao Limestone

Distribution

Western flank of northern Sierra Madre

Age

Early Oligocene

Previous name

Dibuluan River Formation (MMAJ-JICA, 1989)

Renamed by

MGB (2004)

Synonym

Dumatata Formation (Huth, 1962)

Correlation

Masipi Green Tuff (MMAJ-JICA, 1989), Mamparang Formation (MMAJ-JICA, 1977), Lower Zigzag Formation (Caagusan, 1978)

This formation, named by MMAJ-JICA (1989) as Dibuluan River Formation, is found  along the western flanks of the Northern Sierra Madre Range. It embodies the principal position of the westward-dipping monoclinal structure of the Cagayan Basin. It unconformably overlies the Abuan Formation and is unconformably overlain by the Ibulao Limestone along Dibuluan River and elsewhere in the southeastern end of the Cagayan Valley Basin (Aurelio and Billedo, 1987). The Dibuluan Formation consists mainly of basic volcanic flows, volcanic breccias and pyroclastics, with interbeds of clastic rocks. The clastic rocks in the lower portions generally consist of well indurated brownish gray to greenish gray feldspathic wacke with minor intercalated intraformational conglomerate, while the upper portions are marked by thin to medium beds of green siltstone and light green to red, well indurated mudstone.

Radiometric K-Ar dating of a sample of basic lava flow of the Dibuluan Formation gave an age of 29 Ma, equivalent to late Early Oligocene (Billedo, 1994).

This formation is partly equivalent to the Dumatata Formation of Huth (1962) in the southwestern part of the Cagayan Valley Basin.

The Dibuluan could be correlated with the Oligocene Masipi Green Tuff of MMAJ-JICA (1989) in Northern Sierra Madre. The Masipi Green Tuff represents a sequence of parallel-bedded greenish tuff, tuffaceous sandstone and some pyroclastics found at the type locality, Masipi River, in Cabagan, Isabela. The nannofossils contained in tuffaceous sandstone indicate a Middle to Late Oligocene age (MMAJ-JICA, 1987). Likewise, it could be correlated with the Mamparang Formation of MMAJ-JICA (1977) in the eastern fringe of the Northern Sierra Madre Range. The Dibuluan Formation may also be considered as partly equivalent to the Lower Zigzag Formation of BED (1986a) and Caagusan (1978), which is estimated to be around 1,800 m thick.

Equivalent formation(s):

  • Dumatata Formation (see Abuan Formation)
  • Masipi Green Tuff - The Masipi green Tuff of MMAJ-JICA (1989) represents a sequence of parallel-bedded greenish tuff, tuffaceous sandstone and some pyroclastic rocks at , Cabagan, Isabela.

Nannofossils contained in tuffaceous sandstone indicate a Middle to Late Oligocene age (MMAJ-JICA, 1987). It may be correlated with the Dibuluan Formation on the western flanks of the Sierra Madre Range and the Mamparang Formation of MMAJ-JICA (1977) in the eastern fringe of the Range.

  • Lower Zigzag Formation - The Lower Zigzag Formation was named by Caagusan (1978) for the Oligocene red and green clastic beds in Cagayan Valley. It may be correlated with the Dibuluan Formation. The estimated thickness of the Lower Zigzag Formation is 1,800 m (BED, 1986a).

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