Holes were drilled at three sites in the Sulu Sea on Ocean Drilling Program Leg 124. Site 768 lie in the deeper part of the SE sub-basin and Sites 769 and 771 lies on the flanks of the Cagayan ridge. The results indicate that the Sulu Basin originated in the late early Miocene (c.18.8 Ma) in a backarc setting. The Cagayan Ridge was a site of early to early middle Miocene arc volcanism with the deposition of a thick sequence of andesitic to basaltic volcaniclastic deposits. In the basin center an early Miocene pelagic sequence is interrupted by a thick unit of royoltic to dacitic pyroclastic flows. Middle to late Miocene sedimentatin is more continental in character with thick quartz-rich turbidites in the basin center. Only the hemipelagic claystone related to these terrigenous turbidites were deposited on the Cagayan Ridge. A decrease in the supply of clastic detritus from arc and continental sources and a change in the level of the carbonate compensation depth in the upper Pliocene resulted in pelagic carbonate deposition throughout the late Pliocene and Pleistocene. ¿American Geophysical Union 1990