National Science Foundation, United States

Two carbonate platforms on the Marion Plateau off northeast Australia were drilled during ODP Leg 194. Despite the proximity of the platforms, differences in neritic assemblages are striking and provide evidence for the sensitivity of carbonate-producing biotas to environmental changes. Neritic carbonates encountered at Site 1193 on the Northern Marion Platform were produced by a diverse bryozoan community in which larger benthic foraminifers and coralline algae were important components. Although bryozoans are commonly associated with cool-water carbonates, abundant larger foraminifers indicate at least cool-subtropical temperatures (e.g., 17-23C). The larger foraminifers represent a reduced Australian early to middle Miocene assemblage characterized by rotaliine taxa. Porcellaneous larger foraminifers are strikingly rare, as are zooxanthellate coral and calcareous green algae, indicating carbonate saturation below the threshhold for prolific aragonite production. At sites 1196 and 1199 on the Southern Marion Platform, coralline red algae, including abundant rhodoliths, were the dominant constituents of neritic carbonates. Larger benthic foraminifers were again abundant components, particularly in grainstones and packstones, and corals were common in some rudstone and boundstone intervals. A 143 m thick unit of grainstones, characterized by soritid and smaller miliolid foraminifers, abundant molluscs and frequent trace fossils that appear to be seagrass blades and roots, provide evidence for a prolonged episode of sedimentation in very shallow-water. Foraminiferal assemblages indicate that platform accretion continued through the late Miocene and possibly into the early Pliocene. Environmental factors that may be responsible for differences in biota and depositional histories of the two platforms include nutrients from terrestrial runoff or the combination of nutrients and slightly cooler water associated with topographic upwelling, either of which could have suppressed coral growth on the northern, more inshore platform. Terrigenous constituents are more common in sediments of the northern platform, while evidence for strong currents that could produce topographic upwelling was found in both platform and periplatform sediments on the Marion Plateau.