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Baguio Formation

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Baguio Formation    

Lithology

Tuff, andesite, basalt, vocanic breccia, conglomerate

Stratigraphic relations

Overlies Mirador Limestone

Distribution

Baguio District

Age

Late Miocene  – Early Pliocene

Thickness

>100 m

Named by   

Smith and Eddingfield (1911)

Synonymy

Pico Pyroclastics (Dumapit, 1966), Irisan Formation (Maleterre, 1989)

The Baguio Formation was originally defined by Smith and Eddingfield (1911) and modified by Dickerson (in Smith, 1924) but has been virtually subsequently abandoned.  This is equivalent to the Pico Pyroclastics of Dumapit (1966), which was regarded by Balce and others (1980) as a coeval member of the Klondyke Formation.  De los Santos (1982) proposes the resurrection of the term for the pyroclastic rocks around Baguio, which apparently rest above the Mirador Limestone.  Aside from exposures around Baguio City and Pico, Trinidad, the formation is also exposed on the northeast flank of mt. Santo Tomas, where it appears to rest on top of the Mirador Limestone as observed along the road going up to Mt. Santo Tomas.

The rocks constituting this formation include tuff (sometimes enclosing blocks of andesite and volcanic breccia), volcanic conglomerate, andesite and volcanic breccia as well as poorly indurated polymictic conglomerate.  Mahdi (1992) observes that in Camp 8, the tuff is overlain by basaltic flow breccias and pyroclastic flow deposit with an overall thickness of 25-40 m.  At Trinidad, the formation consists of andesitic tuff breccia and poorly indurated conglomerates.  The poorly indurated conglomerate is equivalent to the Irisan Formation of Maleterre (1989) that outcrops between Naguilian Road and Trinidad Valley and estimated to be about 100 m thick.

Maleterre (1989) reports a 3.57 Ma K/Ar dating (equivalent to Early Pliocene) of a basalt clast from a conglomerate between Zigzag Road and the Loakan airport.  This basalt is correlated by Maleterre (1989) with the basalt layer at the top of Rosario Formation.  Datings of volcanic clasts from Malaya Formation in Bontoc give values of 6.2 – 3.7 Ma, corresponding to a volcanic phase during Late Miocene to Early Pliocene time.  The Baguio Formation could be taken as the equivalent of such volcanic phase, in which case its age of formation would fall between Late Miocene and Early Pliocene time.

Formation sequenceEdit

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