Variations in sediment texture and biota off a wave dominated coast, Peka Peka beach, New Zealand

Abstract

Peka Peka beach is near the southern end of the arc-shaped margin of the South Taranaki Bight and faces the prevailing northwesterly winds and waves from the Tasman Sea. The coast is building out from sediment introduced by rivers and distributed by waves and shore-parallel tidal currents. This thesis describes a recent study of the sea floor off Peka Peka beach carried out to investigate the causes(s) of the observed increase in mud with increasing water depth, and to see what other depth-related changes there might be in sediment texture and biota preserved in the sediment. Sea floor samples were taken in winter (June 1988) on a line normal to the coast and extending from the beach for 7 km offshore to a depth of just over 50 m. The samples showed a slight but steady increase in mud content from the beach to about 25 m (15%), followed by a rapid increase to around 40 m (70%) where the rate of increase declines. Further sets of samples were taken in the following summer and winter, and show the same trend. Sea floor features are ripples with straight crests aligned sub-parallel to the coast (continuous from 10 m to the beach face, patchy from 10 to 35 m). At 35 m and deeper the sea floor is muddy and bioturbated, and considered beyond the influence of storm waves. Sea floor features and mud content set fairweather wave base at about 10 m and storm wave base between 30 and 35 m. A survey of foraminifera and molluscs showed that they too varied substantially with depth. Foraminiferal numbers increase steadily from the beach face to 35 m and increase by a factor of five beyond this. In considering taxa, only 17 of the 103 species are restricted to a particular depth zone. The molluscan population, though, shows a strong zonation, showing almost complete assemblage changes at water depths of about 10 m and 35 m. An S4 current meter was used for short periods (up to 5 days) to measure wave period and current velocity. The S4 wave data support a more comprehensive set by shore-based observers covering a one year period and which provide a fairweather (50 percentile) wave height of 0.5 m and period of 7.0 seconds and a storm (90 percentile) wave height of 1.2 m and period of 10.3 seconds. Calculations for fairweather and storm waves so defined give values for sea floor orbital velocities just capable of moving mud (sediment finer than 4 phi or 62 microns) at depths that are consistent with the wave base determined from sea floor features. The S4 also revealed shore-parallel tidal currents which normally run at 20 to 30 cm/s but during storms may flow at more than 70 cm/s. We could find no evidence that even these high flows can entrain sediment on the sea floor, though undoubtedly they will move sediment in suspension. Comparison of offshore textural gradients reported from Monterey Bay in California, Bristol Bay in Alaska and the Roussillon Shelf in southern France indicate a consistent relationship between wave climate, sediment texture and water depth.