Channel-axis, overbank and drift sediment waves in the southern Hikurangi Trough, New Zealand

Abstract

In the southern apex of the Hikurangi Trough east of New Zealand, swath bathymetry and backscatter, seismic profiles and cores from the proximal part of the 2000 km long Hikurangi Channel show different types of sediment waves in the channel axis and on the levee backslopes. These represent different responses to large turbidity currents. Locally, backslope sediment waves merge into sediment waves formed by a deep geostrophic flow. In the channel axis and on channel walls, sediment waves that have wavelengths of 1–4 km and heights of less than 6 m occur where cores show gravelly and coarse sandy turbidites. They are not apparent in layering below the channel axis. They are inferred to be thin, transitory, surface sand bodies migrating across coarser substrates and may have formed in a recent, perhaps the last, turbidity current. Their geometry suggests that they probably formed as antidunes in high-velocity flows that were much thicker than the channel depth. Overbank sediment waves also have similar wavelengths of 1–4 km but heights of up to 45 m. They have conspicuous sub-seabed layering to a depth of at least 400 m beneath the seabed. Seismic stratigraphy indicates that they have developed for most of the last 2 Ma, during near-vertical aggradation of the channel, its levees and overbank slopes, at average rates of 0.4–0.7 m/ka. The sediment waves migrate up levee backslopes towards the levee crest at rates of 3–20 m/ka, and are inferred to be formed beneath lee waves initiated in overbank flows by the linear perturbation of the levee. They are best developed on the outsides of bends, where centrifugal outflow of upper, ‘overspill’ parts of large turbidity currents might be concentrated. They are particularly well developed at right-hand bends on the left bank, where centrifugal and southern hemisphere Coriolis effects combine. Poorly developed sediment waves on the inside of some left-hand bends may be the result of Coriolis effects only. Overbank sediment waves decrease in height and wavelength away from the channel and continue into a depression along the slope-toe deformation front at the landward edge of the turbidite plain. Where the channel intersects the slope-toe depression, the absence of a left-bank levee suggests that overbank flows re-enter the channel. Along the toe of the Chatham Rise, sediment waves up to 5 m high and 2–5 km in wavelength migrate away from the channel and towards a slope-toe ‘moat’. They are inferred to be part of a drift deposit, perhaps associated with a branch of the Pacific Deep Western Boundary Current.