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Submarine Slope Stability: Advances in Modelling the Anatomy of Carbonate Slope Systems


Slopes systems are important in controlling the progradation, aggradation and retrogradation of carbonate platforms, and can also have considerable reservoir potential. There is increasing evidence that in many slope systems failure has produced volumetrically significant geobodies of reworked material, which can extend tens of km into the adjacent basins. However, forward models of slope systems currently rely on diffusive processes to redistribute sediments, with slope geometry controlled by sediment texture, and cannot simulate failure of margins and slopes. Where accumulation of debris on the slope is considered, it requires external specification of key properties, such as the fraction of reef within the breccias. Here we present a novel numerical approach to prediction of the 3D distribution and magnitude of slope failure which is both physically-based and sufficiently computationally efficient to allow incorporation into the forward models of carbonate platform evolution. Engineering geologists have a long tradition of modelling the stability of heterogeneous slope systems, but prediction of failure requires exhaustive iterative analysis, limiting its applicability to 2D. To extend analysis to 3D slopes and predict the failed volume, we have developed a novel method using multiple trial slip circles that start instead from points on the surface of the slope. The basis for this novel approach is the integration of recent improvements in theoretical understanding of plasticity, with empirical observations relating the area and volume of failures. In plasticity theory, an upper and a lower bound for failure surfaces (those with the lowest factor of safety) is established and this can now be applied to any section of the slope and the corresponding volume of the failure for these bounds ascertained. Analysis using a 2D limit equilibrium model of many different slopes has shown that the most likely failure surface has always been between these bounds. This allows application of the analysis in 3D, using the distribution of precursor properties – such as pore water pressures, sediment density, and shear strength – which are already simulated at high vertical resolution within the forward sediment model CARB3D+. In addition to incorporation of slope failure generate more meaningful predictions of carbonate platform evolution and slope facies distribution, this methodology also has a broader application to 3D slope stability modelling.