Late Quaternary development of left-bank-dominant levees in the Bounty Trough, New Zealand

Abstract

The Bounty Trough encompasses an extensive submarine channel system that arises on the Otago fan-complex off the South Island, New Zealand, and extends 1000 km eastward to feed a large fan on the 4500 m-deep Pacific Ocean floor. Channels are bordered by prominent levees which, on the steep Otago fans, are mainly narrow sandy ridges. By comparison, the gently sloping floor of the trough supports broad, low relief, muddy levees. In most cases, levees display preferential development on the left side of the channel (facing down-channel). We argue this preference results not only from the Coriolis deflection of channel-full turbidity currents, but also from bottom currents and centrifugal forces operating at channel bends. Basement relief and locally high sedimentation rates at channel confluences may also influence levee height. Piston cores reveal two distinct phases of levee growth related to Late Quaternary climatic cycles. During glacial times rivers discharged directly into the Bounty Trough. Turbidity currents travelled and overspilled the entire length of the Bounty channels thereby producing the main phase of asymmetric levee growth and terrigenous deposition. Such turbidite deposition was punctuated by bouts of hemipelagic sedimentation. In interglacial periods river-borne sediment was diverted away from the trough which became the site of predominantly pelagic sedimentation represented by a drape of foraminiferal-nannoplankton ooze over channels and levees alike. However, levees on the Otago fan-complex still received terrigenous sediment presumably reworked from the adjacent continental shelf.