Numerical Modeling of Submarine Minibasin Flow and Morphodynamics

Intraslope basins, or minibasins, are important morphological features of the continental slope in both modern and ancient sedimentary systems. Minibasins have an elliptical or spherical shape with a steep inlet or proximal zone followed by an almost horizontal basin floor, and an overflow zone near the downstream basin lip. These basins are filled with sediment transported by successive events of turbidity currents and other types of submarine flows. The work presented here focuses on turbidity current sedimentation in intraslope minibasins, which is often described in terms of the ‘fill-and- spill’ conceptual model. The ‘fill-and-spill’ model has been used previously to describe the process by which the minibasins are filled creating sand rich deposits that constitute prime targets for hydrocarbon exploration. The objective of this study is to understand how the shape and the grain size distribution of the deposit changes with the characteristics of the turbidity current and the morphology of the basin-floor topography of the area. The flow characteristics, i.e., velocity and concentration in both the vertical and longitudinal directions, and the grain size distribution of the deposit are investigated at the laboratory and field scale in two and three dimensions. A stratigraphic model was implemented within a 3D numerical modeling system of turbidity currents by accounting for storage and access to the grain size distribution of the deposit. A Morphological Acceleration Factor (MORFAC) for morphodynamic upscaling was also implemented to reduce computational costs. The model was validated at laboratory scale against available 2D and 3D data from experiments conducted in constant width flumes and 3D minibasins. The model prediction showed a satisfactory agreement between measured and simulated deposit geometries, vertical profiles of suspended sediment concentration and spatial distributions of sediment sizes in the deposit. The field-scale application involving the present seafloor bathymetry of the Brazos-Trinity system in the Gulf of Mexico showed that deposition in intraslope minibasins varies depending on the current magnitude and duration, and the geometric configuration. The field-scale simulations were performed on two linked minibasins. Most of the sediment deposited in the upstream basin and the rest was transported to the downstream basin through submarine canyons. The deposition in the downstream basin was characterized by current reflection of the downstream flank and preferential deposition of sediment in the proximal and central parts of the basin. Model runs conducted to study the effect of MORFAC demonstrated small values of MORFAC from two to five could capture bed evolution and stratigraphic architecture of the deposit, while large values of MORFAC could only capture the overall trend of the deposition process. Thus, caution should be exercised in the choice of the proper MORFAC value