LEG 124

Celebes and Sulu Seas

Western Pacific marginal basins have had a variety of origins and histories, including entrapment from a larger ocean basin, rifting from active volcanic arcs, rifting continental margins, or a composite of events. The basins often separate collision zones and island arcs, representing islands of undisturbed stratigraphic history within an otherwise very complex geological setting.

Drilling was conducted at five sites during Leg 124 (Site 767 to Site 7771) and determined that the basement underlying the Celebes Sea is a plagioclase-olivine phyric basalt with a normal mid-ocean ridge (MORB) signature. The sea formed in open-ocean conditions in the middle Eocene. By late early Miocene, the basin came in close proximity to a continental landmass, probably Sundaland, and received significant amounts of land-derived detritus. High rates of continentally-derived turbidite deposits occurred during the late middle Miocene. The late Miocene to Pleistocene source terrain was volcanic, a result of volcanism related to increased subduction of the Mulucca Sea plate beneath the Sangihe Arc or diversion of the continental source material from the central part of the basin to its margins by the developing north Sulawesi and Cotabato trenches.

The basement underlying the Sulu Sea is composed of olivine basalts, possibly transitional between MORB and island arc tholeiites. The sea formed in the late early to early middle Miocene, almost concurrent with cessation of volcanism on the Cagayan Ridge. Outpourings of rhyolitic to dacitic pyroclastic flows mark the early formation of the basin. Thick, continentally-derived, turbidites of late middle Miocene age are contemporaneous with those in the Celebes Sea, suggesting the active mountain belt in Borneo as the likely common source. Low carbonate content and high carbonate compensation depth (CCD) in the late Miocene and early Pliocene possibly resulted from closing-off of the basin by collisions. A drop of the CCD in the Plio-Pleistocene indicates variations in depth of the sills either between Mindoro and north Palawan, or along the Sulu archipelago where a recent volcanic arc was built. This drop of the CCD corresponds to a global deepening of the CCD recognized in all major oceans at this time. Volcanogenic sedimentation dominated non-biogenic components in the Pleistocene and indicates the initiation of the Sulu Trench and volcanoes on the Sulu Ridge.