Carbonate platforms and their slopes are sensitive indicators of sea level variations, as they predominantly record growth during sea level highstands and shut down during sea level lowstands. Sampling these platforms records sea level change in a "dipstick" fashion. On the other hand, sediments on platform margins and slopes record sea level variations as alternations of shallowing and deepening sequences. Leg 194 drill sites on the Marion Plateau, northeast Australia (Figs. F1, F2) were positioned to establish the magnitude of the middle Miocene Zones N12-N14 sea level fall (12.5-11.4 Ma) using both of these depositional systems. This information will provide an independent assessment of an important part of the global sea level curve. An important characteristic of the stratigraphic relationships investigated during Leg 194 is that the drill sites are located in an area without intervening structural elements. Thus, subsidence of the platform is likely to have affected all sites equally, enabling the true amplitude of the sea level fall to be determined.
High-resolution seismic data collected during the Leg 194 site survey provides quasi-three-dimensional geometry of Oligocene to Pliocene depositional sequences on the Marion Plateau. The correlation of these seismic images with drill core and logging data provides a synoptic view of depositional processes in a mixed carbonate-siliciclastic carbonate platform setting. The seismic sequence geometry shows that the depositional systems of the Marion Plateau are predominantly controlled by current flow and provide insight into the genesis of a current-dominated platform system. In addition, the carbonate platforms and slopes drilled during Leg 194 preserve a record of third-order sea level variations within a mixed carbonate-siliciclastic depositional environment. Pore water geochemical records from slope sites and platform sites will provide insight into possible fluid flow processes and diagenesis. Finally, the composition and age of basement will yield crucial information on the tectonic history of this passive margin.
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