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Lithology

Sandstone, mudstone, shale, claystone, conglomerate

Stratigraphic relations

Unconformable over the Ibulao Limestone

Distribution

Conner, Kalinga-Apayao; Magat River, Isabela

Age

Late Oligocene (NP-25)– Early Miocene (NN2-NN3)

Thickness

2,700 m

Previous name

Lubuagan Coal Measures (Corby and others, 1951)

Renamed by

Gonzales and others (1978)

Synonym

Mabaca River Group (Durkee and Pederson, 1961)

Correlation

Upper Zigzag Formation (Caagusan, 1978)

The Lubuagan Formation, originally described by Corby and others (1951) as Lubuagan Coal Measures, is exposed along the west side of Cagayan Valley from Conner, Kalinga-Apayao to Magat River, Isabela. It is primarily a thick sequence of clastic sediments with minor pyroclastic intercalations. The formation rests unconformably over the Ibulao Limestone east of Jones, Isabela up to the southern extremities of the Cagayan Valley Basin.

A tripartite subdivision was recognized and was mapped under the Mabaca River Group by Durkee and Pederson (1961) based on varying sandstone-shale ratio. These were given formational rank but were reduced to member status by Gonzales and others (1978). These members are the Asiga, Balbalan and the Buluan.

The Lower Asiga Member was named after barrio Asiga along the Mabaca River west of Pinukpok, Kalinga-Apayao. It consists mainly of interbedded shale and graywacke. The member has a thickness of about 1500 m.

The Balbalan Sandstone Member was named after Balbalan, a barrio along Mabaca River between Saltan and Pasil rivers in Kalinga- Apayao. It is composed dominantly of fine to coarse grained sandstone and conglomerate. It measures 1165 m thick along the Mabaca River east of Asiga.

The Upper Buluan Member is characterized by the predominance of dark gray silty claystone with occasional thin graywacke beds. It was named after the exposures along Buluan Creek near Buluan, Kalinga-Apayao. As measured along the Tuao-Conner Road, the thickness is 1036 meters.

On the other hand, Maac (1988) subdivides the Lubuagan Formation into a Sicalao Limestone member and a Cañao Turbidite member.

Recent paleontological dating of samples of the Lubuagan Formation indicates an age range of late Late Oligocene (nannofossil zone NP25) to Early Miocene (nannofossil zones NN2- NN3) as reported by Billedo (1994). The Lubuagan Formation of BED-WB (1986) and Caagusan (1978) is assigned an age range of Early Miocene to Middle Miocene.

The Upper Zigzag Formation of BED (1986a) and Caagusan (1978) spanning the age range of Late Oligocene to Early Miocene may be considered equivalent to the Lubuagan Formation. However, BED (1986a) and Caagusan (1978) regards this sequence of clastic rocks as coeval with the Ibulao Limestone.

Equivalent formation(s):

·        Mabaca River Group - The Mabaca River Group of Durkee and Pederson (1961) refers to a thick sequence of clastic rocks along the western margin of the Cagayan Valley subdivided into three sub-units based on varying sandstone-shale ratio. From the bottom, these are: Asiga Formation, Balbalan Sandstone and Buluan Formation. The Mabaca River Group is considered equivalent to the Lubuagan Formation (Gonzales, 1978), which was previously designated by Corby and others (1951) as Lubuagan Coal Measures. The sub-units of the Group were demoted to member status by Gonzales and others (1978).

·        Asiga Member - The Asiga constitutes the lower member of the Lubuagan Formation in Cagayan Valley. Previously, it was mapped as a formation by Durkee and Pederson (1961) who subdivided the Mabaca River Group into three formational units. These units were reduced to member status by Gonzales and others (1978). The Asiga was named after Barrio Asiga along Mabaca River west of Pinukpok, Apayao. It consists mainly of interbedded shale and greywacke and has an estimated thickness of 1500 m. The other two members comprising the Lubuagan are the middle Balbalan Sandstone and upper Buluan member.

·        Balbalan Sandstone - The Balbalan Sandstone, which constitutes the middle member of the Lubuagan Formation, was named after Balbalan, a barangay along Mabaca River between Saltan and Pasil rivers in Kalinga-Apayao. It is composed dominantly of fine to coarse grained sandstone and conglomerate. It has a thickness of 1165 m along the Mabaca River east of Asiga.

·        Buluan Member - The Buluan represents the uppermost member of the Lubuagan Formation. It is characterized by the predominance of dark gray silty claystone with occasional thin graywacke beds. It was named after the exposures along Buluan Creek near Buluan, Kalinga-Apayao. As measured along the Tuao-Conner Road, the thickness is 1,036 m.

·        Cañao Turbidite - The Cañao Turbidite was designated by Maac (1988) as one of two members of the Lubuagan Formation in the Cagayan Valley Basin. The other member designated by Maac (1988) is the Sicalao Limestone.

·        Upper Zigzag Formation - The Upper Zigzag Formation of BED (1986a) and Caagusan (1978) for the sequence in that spans the age range of Late Oligocene to Early Miocene may be considered equivalent to the Lubuagan Formation. However, BED (1986a) and Caagusan (1978) regards this sequence of clastic rocks as coeval with the Ibulao Limestone.

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