Dismantling of Isolated Tropical Carbonate Platforms Through Flank Collapse and Canyon Erosion

Various isolated tropical carbonate platforms in the Indian and Pacific oceans are currently in a stage of dismantling. Past sea-level fluctuations, ocean currents, and environmental conditions control this condition. Size reduction of the banks is a consequence of two processes: (1) Reduced shallow-water carbonate production resulting in minor sediment export to the platform slopes and the toe of slopes; platform edges thus do not prograde. (2) Slopes are dismantled through gravitational processes, as reflected by a series of distinct submarine geomorphological features. This study uses multibeam, seismic and seafloor observation data from the Saya de Malha Bank (Mascarene Plateau, Indian Ocean) and the Queensland Plateau (NE Australia, Coral Sea), to characterize the slope instabilities at the flanks of these carbonate buildups. Platform slope collapse in form of submarine rockfalls and landslides occurs along the edges of the banks and within middle and lower slope positions. At the Saya de Malha Bank, scarps are 300 m high and extend laterally for at least 40 km. Other slope segments of the platforms host submarine canyons several hundreds of m wide and 400 to 500 m deep. These canyons serve as routes for calciturbiditic export with the development of small turbidite lobes at the canyon exits. The windward and leeward exposure of the carbonate platform flanks which control sediment input to the slopes, together with the ocean currents that winnow and erode slope deposits, appear as most important controlling factors of slope dynamics.