Malampaya Sound Group : a Jurassic-Early Cretaceous accretionary complex in Busuanga Island, North Palawan Block (Philippines)

ブスアンガ島は，ペルム紀からジュラ紀にわたるチャート（リミナンコン層）とジュラ紀中世から白亜紀古世にわたる砕屑岩類（ギンロ層）から主として構成される付加体（マランパヤサウンド層群）よりなる．マランパヤサウンド層群は，全体として北に急角度で傾斜する覆瓦構造をなしている．同層群はチャート・砕屑岩シーケンスの剥ぎ取り付加によって形成されたと考えられる．放散虫の生層序学的検討により，チャートから珪質泥岩に移化する層準は，北から南に向かって系統的に若くなる年代極性をもつことが明らかになった．この年代の違いに基づき，ブスアンガ島のマランパヤサウンド層群分布域を，北帯，中帯，南帯の3帯に区分した．同層群の付加年代は，ジュラ中世から白亜紀古世にわたる．マランパヤサウンド層群は，西南日本南部秩父テレーンの斗賀野層群に対比される． / 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 off scraped 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 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 Batho-nian-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 off scrape 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 SW Japan