Correlation between shelf sediments near cape farewell and a numerical simulation of the Westland current

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Abstract
The sediments on the north‐west shelf of the South Island, New Zealand, were mapped by van der Linden (1979: New Zealand Journal of Geology and Geophysics 12). He found that the underlying rocks of the Kahurangi Shoals date back at least 65 million years, but the sediments are generally in equilibrium with the present hydrological regime. The sediments are generally sand, although a belt of coarse elastics extends north‐east of the shoals which van der Linden suggested is associated with high currents. We have applied a numerical model to the north‐west shelf to simulate a mean Westland Current driven by shelf scale pressure gradients. No current‐meter data were available for this model and it was calibrated with limited drifter measurements. The isotachs derived from the model are very similar in shape to the contours of sediment mean grain size given by van der Linden (1979). Over the Kahurangi Shoals, the currents are high and grain size is also high. Conversely, a lobe of low grain size over the Paturau Bank appears in the same position as a minimum in the isotach map. The thread of maximum current tracks the belt of coarse elastics north‐east of the Kahurangi Shoals. A correlation of grain size against current speed gives a line close to that for the threshold speed required to resuspend quartz. We suggest that generally the mean grain size is a good indicator of the minimum grain size of the static bed load, and that the threshold current associated with this distribution is very similar to our numerically derived currents. We conclude that the close correspondence between the sediment distribution and isotachs is a verification of our model. We also suggest that a minimum in the current speed seen downstream of the Kahurangi Shoals may have been instrumental in forming the Paturau Bank. In addition, negative vorticity in the numerical velocity field coincides well with the flanks of the Paturau Bank, and bottom convergence may contribute to the maintenance of this bank.

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