Ocean Drilling Program, United States

Cretaceous rifting in the western Coral Sea (offshore northeast Australia) formed continental fragments that are now capped by carbonate platforms. The Marion Plateau carbonate platform, which has grown on one of these fragments, provides a natural laboratory to study the causes, magnitudes, and effects of sea-level change on continental margin sediments. One of the fundamental controls on the nature and geometry of continental margin sediment deposition is sea level; however, much of the information on the relationship between sea-level and depositional facies is qualitative. Leg 194 coring will provide a superb and unique opportunity to determine the absolute magnitude of the major Cenozoic sea-level falls. The drilling strategy outlined for Leg 194 utilizes the stratigraphic relationship between a lower to middle Miocene second-order highstand carbonate platform complex and an upper Miocene second order lowstand platform complex to establish the magnitude of the middle Miocene N12-N14 sea level fall. An important characteristic of this platform relationship is that the proposed sites are essentially located along a single strike line without intervening structural elements. Thus, subsidence of the platform will have affected all sites equally, enabling determination of the true amplitude of the sea-level fall that caused a shift in the locus of carbonate platform deposition. The carbonate platforms and adjacent slopes of the Marion Plateau also preserve a superb record of third-order sea-level variations within a mixed carbonate-siliciclastic depositional environment. High-resolution seismic data collected for the Leg 194 site surveys provide quasi-three-dimensional images of Oligocene-Pliocene depositional geometries. The correlation of these seismic images with drill core and logging data will provide a synoptic view of depositional processes in a mixed carbonate-siliciclastic carbonate platform setting.