AAPG ACE 2018

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Carbonate Platform Slopes: The Importance of Contour Currents

Abstract

The major isolated carbonate platforms lie in zones of the ocean with vigorous currents, either driven by winds or as part of the global ocean current system. Carbonate platforms form rigid obstacles for the flow of such currents, and the water streams along the platform flanks as contour currents. This has consequences for the sedimentation, the facies distribution, and the stratigraphic packaging along the carbonate platform slopes. Examples for current-dominated carbonate platform slopes among others are provided by the Great Bahama Bank, the atolls of the Maldives, and the Marion Plateau. Using sedimentological and geophysical data, these cases will be used to show the different effects of currents on slope deposition. Whereas the windward or leeward exposure of the flanks of carbonate platforms is the major factor controlling the potential amount of sediment which can be distributed along the slope, sedimentation is dictated by the bottom current regime. Sediment depocenters are directly linked to the exposure of the slope to currents, with greater sediment thickness in current-protected zones. The winnowing effect of bottom currents also results in unexpected trends of basinward coarsening grain-sizes. With elevated flow velocities, toe of slope erosion develops. Slope winnowing, sediment reworking and erosion by drift currents finally may be strong enough to suppress sediment deposition at the slopes of carbonate platforms, for example during sea level highstands. We therefore propose that the action of ocean currents on carbonate platform slope deposition and stratigraphy should be taken into account as a possible and even major controlling factor when interpreting outcrop and subsurface data from isolated carbonate platforms.