The results of six two-ship seismic refraction profiles are presented in a schematic structure section between Palawan Island and the northern arm of Celebes Island. Along the line of section the northwest part of the Sulu Sea basin has seismic structure typical of submarine ridges and is believed to have been a part of the Palawan-Calamianes archipelago ridge that has subsided. A basement depression is filled with thick sediments whose surface is a terrace at a depth of 2000 meters. The deeper southeast part of the Sulu Sea basin and the Celebes Sea basin have oceanic crust. The separating Sulu archipelago ridge has seismic structure characteristic of an island arc, upon which sedimentation has produced a flat sea floor in most places. Linear troughs with a probable synclinal cross section occur in the basins adjacent to the northwest sides of Sulu ridge and Celebes Island.
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