SEISMIC STRATIGRAPHY OF THE BOUNTY TROUGH, SOUTH-WEST PACIFIC-OCEAN

The Bounty Trough is a late Cretaceous failed rift in the eastern New Zealand continental margin, formed at the start of the present phase of seafloor spreading in the south Pacific Ocean. The trough is bounded by continental crust of South Island New Zealand to the west, the Chatham Rise to the north and the Campbell Plateau to the south. In common with this surrounding continental hinterland, the sediments in the trough consist of Cretaceous rift-fill followed by deepening latest Cretaceous to Palaeogene marine facies and Palaeogene to Neogene biopelagic facies. The inception of the Alpine Fault plate boundary in western South Island in the mid-Cenozoic produced an influx of terrigenous sediment into the head of the Bounty Trough, forming a prograding late Oligocene to Recent piedmont-shelf prism with marginal submarine fans (Otago Fan Complex). Meanwhile, biopelagic oozes continued to accumulate in the trough seawards of the fans. A regional phase of tectonism and volcanism occurred in the late Miocene (Kaikoura Orogeny; about 15-10 Ma), marked by the eruption of alkalic volcanoes, regional faulting and folding. This tectonism was associated with a change to strongly compressive transform movement on the Alpine Fault plate boundary, and with mild folding of sediments up to early late Miocene age throughout the Bounty Trough. The modern physiography of the trough, including the present Bounty Channel, Otago Fan Complex and abyssal Bounty Fan all date from this Kaikoura event. Development of the Bounty Channel and fan was greatly accelerated after about 2.5 Ma, with the onset of global glacial/interglacial climatic cycles and the development of an icecap along the alpine region of South Island. During glacial periods, terrigenous sediment from the rising mountains of South Island was distributed throughout the Bounty Trough as hemipelagic and turbiditic sediment facies.