Structural elements of Mindanao include a north-trending median volcanic belt (central cordillera) flanked by side deeps (Agusan-Davao trough and Cotabato basin), and three mountainous coastal belts fronting respectively on the Philippine deep (Pacific cordillera) and the deepest parts of the Celebes and Sulu seas (Tiruray highland and Zamboanga cordillera). The mountainous areas are composed essentially of pre-Tertiary basement and Tertiary volcanic rocks; Tertiary sediments, mainly marine Miocene-Pliocene deposits, occur in intervening lowlands. The geosynclinal sequences in the Agusan-Davao trough and Cotabato basin include probable petroleum source and reservoir beds: potential stratigraphic and structural traps are present.

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