Depositional history of the Celebes Sea from ODP sites 767 and 770

Leg 124 of the Ocean Drilling Project drilled Sites 767 and 770 in the northern Celebes Sea, reaching late middle Eocene basaltic basement at both sites. Major shifts in sediment provenance record the changing tectonic setting of the basin. From late middle Eocene into early Miocene time pelagic sedimentation prevailed, with little influence from continental or volcanic arc sources. A major continental influence is first documented in middle Miocene time as a thick sequence of quartzose, mud-rich turbidites accumulated on the deeper basin floor, possibly in response to middle Miocene orogeny in northern Borneo. Terrigenous turbidite deposition waned during the late Miocene as active arc volcanism began to contribute significant amounts of hemipelagic sediment and ash layers, which have remained the dominant basinal sediment to the present. Although the Celebes Sea is now nearly surrounded by volcanic arc terranes, the absence of volcaniclastic sediment in the Eocene to early Miocene section suggests that the basin did not form by back-arc spreading.