Abstract

We used a numerical model to investigate the formation processes of coast-parallel magnetite enrichments observed in the western Bay of Plenty, New Zealand. The model simulates differential transport of two separate grain fractions with different sizes and densities on an inclined surface, using the average hydrodynamic and geological conditions of the study area as input. In the observed and modeled structures, magnetite concentrations increase coastwards from the depth where quartz grains are first entrained until the critical shear stress of magnetite is surpassed. Beyond this point, the concentration decreases. The dominant process for structure formation is grain-size-selective entrainment. The degree of enrichment is further controlled by differential transport of the fractions. Additional tests show the influence of hydrodynamic (wave height and period, current velocities) and geological parameters (grain size and density, bed slope). Parameters that change the bed shear stress shift the enrichment into deeper or shallower water depths. The model shows the importance of cross-shelf sediment transport for effective enrichment, demonstrates the mechanisms for the formation of this type of structure, and should be able to predict the location of enrichment zones in other coastal environments for a variety of heavy minerals.

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