Sedimentation in western pacific backarc basins: New insights from recent ODP drilling

During the most recent phase of ocean drilling in the western Pacific, a series of Ocean Drilling Program (ODP) legs explored the history of sedimentation within several types of backarc basins: a nascent intraoceanic backarc basin (Sumisu Rift, leg 126), a young intraoceanic backarc basin (Lau Basin, leg 13S), a continental margin backarc basin (Japan Sea, legs 127 and 128), and marginal basins of uncertain origin (e.g., Sulu Sea, leg 124). The Sumisu Rift is a nascent backarc basin characterized by high sediment accumulation rates (≥4 km/yr.) and thick, coarse pumiceous units derived from periodic eruptions of nearby arc calderas and deposited by turbidity-current and mass-flow processes. The mixed volcanic provenance of the Sumisu basin fill can be attributed to syneruptive mixing or mixing during transport into the basin. Sites drilled in a series of sub-basins in the Lau backarc region show similar upward-fining sequences with basal coarse-grained, proximal deposits (volcanic sands and gravels that grade upward into nannofossil-ooze dominated sections with rare air-fall ash beds. The proximal facies record an apparent eastward shift in volcanism across the basin during the Plio-Pleistocene. Geochemical analyses of pyroclastic debris within the coarse basal sections are consistent with local intrabasinal volcanic sources. Lower Miocene volcaniclastic sections recovered in the Sulu Basin support a backarc origin for this basin. These deposits represent the basinal portion of an arc-derived volcaniclastic apron that prograded basinward soon after formation of the underlying backarc-basin crust. Subsequent influx of continental elastics into the Sulu Basin during the middle to late Miocene is attributed to a sea-level lowstand. The Japan Sea is a continental backarc basin that contains Quaternary to Miocene sand with both magmatic-arc and continental provenance. Diagenetic modification of more-deeply buried Miocene sandstones has resulted in some loss of provenance information.