GEOLOGICAL SOCIETY OF THE PHILIPPINES
Malampaya Sound Group: a Jurassic-Early Cretaceous accretionary complex in Busuanga Island, North Palawan Block (Philippines)
Busuanga Island is considered an accretionary complex primarily composed of Middle Permian to Jurassic chert (Liminangcong Formation), Middle Jurassic to Early Cretaceous clastics (Guinlo Formation) and limestone units of various ages, with minor melange bodies (Bicatan Melange), collectively called as the Malampaya Sound Group. These rocks are regarded as offscraped sedimentary deposits from a subducted oceanic plate that developed imbricate structure during the Jurassic-Early Cretaceous accretion. The lithologic transition from chert to siliceous mudstone to terrigenous clastics indicates facies change from pelagic to terrigenous condition brought by a plate movement from an open ocean to the subduction zone. The siliceous mudstone units, which mark the end of chert deposition and the beginning of clastic deposition, are classified into three different ages based in Busuanga Island, and found to be younging from north to south. This provides a major basis for defining three belts composing Busuanga Island: the Northern, Middle and Southern Busuanga belts. The Northern Busuanga Belt is composed of Middle Permian to Middle Jurassic (probably Aalenian) chert followed by Bathonian-Callovian (JR4-JR5) siliceous mudstone and Callovian (JR5) terrigenous clastics. The Middle Busuanga Belt has its topmost chert deposit at Bajocian-lower Bathonian (JR4), siliceous mudstone interval from upper Bathonian to lower Oxfordian (JR5-JR6), and terrigenous clastics at Oxfordian (JR6). The Southern Busuanga Belt shows the transition from lower-middle Tithonian (JR8) chert to upper Tithonian-Berriasian (KR1) siliceous mudstone. The subsequent deposition of terrigenous clastics, supposedly overlying the siliceous mudstone, is considered occurring within the Lower Cretaceous. The offscrape accretion period of these three belts probably occurred during the Middle Jurassic for the Northern Busuanga Belt, Late Jurassic for the Middle Busuanga Belt and Early Cretaceous for the Southern Busuanga Belt. These chert-clastic sequences of the Malampaya Sound Group are also found correlative with the Togano Group of the Southern Chichibu Terrane of southwest Japan.
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