Abstract:
More than 1700 km of high-resolution seismic data were collected over the Marion Plateau, northeast Australia, to investigate the influence of sea level and oceanography of subtropical carbonate platforms growing on the plateau surface. Seismic data, interpreted in combination with sediments recovered during Ocean Drilling Program Leg 194 and modern oceanographic data, have enabled characterization of the parameters controlling platform growth and development in this region. Most modern carbonate platforms, such as the Bahamas Platform, have sedimentation patterns that reflect the prevailing wind direction where sediments are forced off the platform on the leeward side, leaving the windward side relatively sediment-starved. This results in platform asymmetry with steep windward and gentler leeward slopes. The seismic data presented here indicate that the carbonate platforms off northeast Australia, although similar in morphology to the Bahamas Platform in many respects, are dominated by oceanographic currents as the primary energy source creating a similar asymmetrical platform geometry where the upcurrent side of the platform is relatively sediment starved and most sediment is deposited on the downcurrent slope. Currents in the study area are dominated by the southward-flowing East Australian Current that generally flows opposite to the prevailing Southeast Trade Winds. This current likely determines not only the morphology, but also the growth potential of the platforms, as well as the volume and final location of sediment transported from the platform top. Despite the massive, tablelike structures exhibited in the seismic data, Leg 194 drilling demonstrated that the platforms are almost entirely composed of the remains of cool, subtropical organisms, such as red algae, bryozoans, and larger benthic foraminifera. Coralline algae were notably absent from most sequences. These calcite-dominated organic remains have a low diagenetic potential, resulting in uncemented and friable slope successions. Nevertheless, the platform tops are well cemented. The fact that the cool subtropical faunal assemblages produce platform geometries that are similar to tropical carbonates suggests that physical parameters, such as current flow and sea level change, may be more important than biofacies in establishing platform architectures.