Abstract Three-dimensional seismic data from the Torosa Field (Torosa 3D Survey) in the Browse basin of the North West Shelf, Australia display a series of well imaged isolated carbonate platforms that evolve from platform initiation to terminal drowning. The application of seismic geomorphology allows recognition of various carbonate geomorphic elements (depositional and diagenetic) including prograding margins, slope fan and gullied slope deposits, platform interior patch reefs, and karst fabrics. Mapping these features in successive horizon-slices and comparing the spatial changes with changes in seismic geometries (cross-sectional view) allow a more detailed understanding of platform response to relative changes in sea level and the relative complexity of the systems through time. Platform growth initiates as a series of kilometerscale buildups characterized by a circular to elliptical morphology (in plan view). These small-scale buildups quickly coalesce into a series of build-up complexes (2-6 km) with progradational margins. These complexes display either elliptical or rhombohedral shapes. Individual patch reefs can be observed in the platform interiors of the build-up complexes. A second phase of coalescence occurs and the build-up complexes combine into a larger platform (25 × 10 km). This large platform then backsteps and is characterized by a strongly elongate and elliptical shape. The backstepped platform progrades up to 3 km prior to final drowning and burial. The growth of elliptical and rhombohedral buildup shapes appear to be influenced by focused currents between the platforms. A distinct windward-leeward asymmetry has not been recognized in the platforms. The rhombic platform complexes are characterized by a unique “pointed promontory” morphology that develops (perpendicular to the long dimension) as the elliptical complexes evolve into more rhombic shapes through time. Sand waves are also observed in the bottom of the interplatform seaways. The backstepped platforms display prograding seismic reflectors that terminate laterally and are confined to areas of the platform margin protected from influence of currents. Coalescence of the platforms followed the growth of small buildups in the mouths of the interplatform seaways, effectively blocking the seaways and allowing sediment to accumulate in the previously current swept passage. This dataset provides an alternative growth model for platforms strongly influenced by currents, rather than the more common model of progradation with sediment flux dominated by windward-leeward sediment transport.

Experimental studies of Fe 1− x O wüstite and (Mg 0.8 Fe 0.2 )O ferropericlase were performed at room temperature and high pressure using a diamond anvil cell technique. Mössbauer spectroscopy indicated a magnetic ordering transition in FeO at 4.7 GPa, while no evidence of magnetic ordering was detected in ferropericlase up to at least 50 GPa. A structural cubic-to-rhombohedral distortion was observed in both materials using powder X-ray diffraction: at 12 GPa for FeO and at 35 GPa for ferroperi-clase. This suggests that the structural distortion is independent of magnetic ordering in the FeO-MgO system. A complete single-crystal elasticity tensor of FeO was determined up to 10 GPa, and only minor changes of elastic moduli were detected at the magnetic transition pressure, indicating relatively weak magnetoelastic coupling in FeO at high pressures. The existence of a rhombohedral distortion in ferropericlase with mantle composition at high pressures and the absence of magnetic ordering have important implications for the interpretation of seismological data with respect to lower-mantle inhomogeneity.