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Zigzag Formation

Lithology

Conglomerate, sandstone, shale, with minor limestone and interbeds of volcanic flows and tuff

Stratigraphic relations

Unconformably overlies the Pugo Formation along Bued River in Camp 4, Tuba, Benguet and unconformably overlain by the Kennon Limestone.

Distribution

Baguio District; Cervantes-Bontoc area

Age

Late Oligocene – Early Miocene

Thickness

1,700 m and may even reach 3,000 m

Previous Name

Zigzag Series (Leith, 1938)

Renamed by

Peña (1970)

Synonymy

Bokod Formation (Maleterre, 1989), Balili Formation (Lepanto Consolidated Mining Co.)

This is a sequence of conglomerates, sandstones and shales, in places with limestone lenses and interbeds of volcanic flows and tuffs, unconformably overlying the Pugo Formation.  The type locality of this formation, previously designated as Zigzag Series by Leith (1938), is situated along Bued River past Camp 6 where the Kennon starts to zigzag upwards towards Baguio City.  The exposure of the section along the flanks of the Bued Canyon is characterized by an alternation of maroon and green beds of clastic rocks.  Dark gray limestone interbeds at the type locality contain numerous molluscan fossils.  Downstream at Camp 6, the sedimentary sequence is intruded by quartz diorite porphyry, producing mineralized skarn and marble.  Conglomerates at the type locality as well as at Camp 3 and Acop’s Place contain clasts of volcanic rocks identified with those of the Pugo Formation and much fewer clasts of medium grained diorite and quartz diorite.  Towards the top of the sedimentary sequence at Camp 3, clasts of amygdaloidal basalt in the conglomerate are notable.  The clastic rocks along this section exhibit current ripples, parallel and cross laminations and normal and reverse graded bedding.

Maleterre (1989) also describes exposures corresponding to the Zigzag Formation in the western part of the Bontoc-Cervantes area, which are folded into a north plunging anticline.  These consist of volcanic conglomerates, sandstones, red chert with limestone lenses, tuffs and volcanic flows (pillow basalts and keratophyres).

The Camp 3 to Camp 4 section of the Zigzag Formation has an estimated thickness of 1,700 m (Peña and Reyes, 1970).  Tamesis and others (1982) estimate the Bued River section to be as thick as 1,800 m, although they assume that it is thicker in other places.  Maleterre (1989) estimates a minimum thickness of 3,000 m for the Zigzag Formation in the Bontoc-Cervantes area.

On the basis of dating of samples of the Zigzag Formation in the Baguio area and the Cervantes-Bontoc area, the formation is assigned a Late Oligocene-Early Miocene age.

The Bokod Formation, exposed along the Baguio-Cagayan Basin Road, is correlated with the Zigzag Formation.  It lies above the Columbus Formation.  It outcrops between Bokod Fault to the west and Pingkian to the east.  As described by Maleterre (1989), the formation consists of red and green beds of tuffs, volcanic sandstones and andesitic conglomerates whose total thickness could exceed 1,000 m.

 Also correlated with the Zigzag Formation is the Balili Formation, which is best, exposed along Sabangan River near Bauko.  It was named by geologists of Lepanto Consolidated Mining Company for the thick sequence of sandstones, volcanic conglomerates basalt flows, andesitic pyroclastics and breccia forming the Balili Cliffs on the western flank of Mt. Data.  It was redefined by Baker (cited in Ringenbach, 1992) to include the volcaniclastic and clasticfacies from Bauko to Cervantes up north and to Buguias down south.  The contact between the Balili Formation and the Sagada Limestone has not been described but the attitudes of their bedding indicate a concordant relation.  East of Mankayan, along Payeo River, the Balili is disconformable over the volcanic substratum.

Various ages have been assigned to the Balili Formation.  Gonzales (cited in Garcia, 1991) reports a Late Oligocene dating of foraminifera from the limestone capping the Balili sediments.  Sillitoe and Angeles (1985) give a Late Oligocene – Middle Miocene age range for the basal conglomerate.  The BaliliFormation was dated by Maleterre (1989) as Late Oligocene – Early Miocene on the basis of the dating of intraformational limestone clasts in the conglomerate at the upper section of the formation.  Garcia and Bongolan (1990) assign a Middle Miocene age for the formation.  Here, the age range Late Oligocene – Early Miocene is adopted for the Balili Formation.  Middle Miocene – Late Miocene sedimentary rocks in the area is considered here as part of the Klondyke Formation.

Formation sequence

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