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Fractionation of Mineral Grains in Submarine Lobes: Evidence from Three Experimental Tank Studies

Abstract

Turbidity currents are the primary mechanism that transports sand to submarine fans in deep-water basins. Particle settling velocity in turbidity currents is primarily controlled by grain diameter, and grains are commonly deposited in decreasing size down-current. However, grain density and shape also dictate settling velocity. In this study, three sets of experiments were conducted in Tulane University's deep-water basin to document how turbidity currents spatially fractionate particles on the basis of grain density and grain shape. The basin measures 6 m L × 4 m W × 2.2 m D. The three experiments used engineered sediment with similar grain-size distributions (D50∼75μm), and turbidity currents had 2% excess density from suspended sediment. At the tank entrance, average densimetric Froude and Reynolds numbers were 0.55 and 14,000, respectively. For each experiment, 50 samples from the deposits were collected in a common grid, and 100 samples of suspended-sediment were collected by siphons at 10 locations. The “density” experiment had equal proportions by volume of spherical glass beads (?=2.50 g/cm3) and spherical alumina oxide beads (?=3.85 g/cm3), whereas the “shape” experiment had equal proportions by volume of spherical glass and angular glass (both with ?=2.50 g/cm3). The “mixed” experiment had equal proportions by volume of all three grain types. For all experiments, down-stream current velocities decreased radially away from the entrance box. Deposits were lobate in plan-form and thinned radially away from the entrance. The shape experiment maintained the highest down-current velocities, and created a deposit with an area 1.8 times greater than the density experiment. The mixed experiment had velocities and deposit areas less than the shape experiment, but more than the density experiment. Preliminary measurements indicate that high-density and spherical grains are concentrated in proximal, updip areas of the experimental lobes, whereas angular and low-density particles are concentrated in distal, downdip areas. This study uses velocity measurements and concentration profiles in the turbidity currents to relate flow processes to sedimentation. Results document that turbidity currents are effective at fractionating minerals on the basis of density and shape alone. This process can explain spatial variations in mineralogical composition described in natural submarine fans, which can have impacts on primary and secondary reservoir quality.