Sediments in the New Zealand Area
New Zealand Shelf sediments consist basically of gravel and sand near the open coast, sand and mud in various proportions on the central part of the Shelf and near river mouths, and gravel, sand, and mud in various proportions near the margin of the Shelf and in constricted channels where strong currents are active, for instance, Cook Strait and Foveaux Strait. The near-shore sediments and those on the central part of the Shelf are evidently being deposited at the present day, but the relatively coarse sediments near the Shelf margin and in constricted channels were deposited during an earlier phase of sedimentation, being late Pleistocene to early Flandrian in age. The areas where these older sediments are found are zones where no present-day deposition is taking place.
The derived pebbles and sand which make up the Shelf sediments of New Zealand consist mainly either of very resistant rocks like greywacke, or of types whose great abundance outweighs their relatively non-resistant character, such as the Taupo rhyolitic pumice. Metamorphic fragments are abundant off some parts of the South Island coast. Formations of Tertiary age consist mostly of non-resistant rocks which are rapidly broken up by erosion and transport, and are rarely found as recognisable fragments on the Shelf. In a few areas, for example, Foveaux Strait, shells and shell fragments make up a considerable proportion of the sediment.
The sediments on the New Zealand Slope, over most parts of the continental borderlands, and in the nearby ocean basins, consist mainly of terrigenous mud and foraminiferal ooze in various proportions. The foraminiferal component predominates at a considerable distance from land, and here the sediment is whitish in colour; near the coast, on the other hand, the percentage of terrigenous mud rises, and the sediment takes on a pink, brown, or green tinge, depending on the colour of the terrigenous admixture. South of the latitude of Auckland the colour is usually pale green, but north of this latitude pale pink or brown colours are typical. On the higher parts of the continental borderlands, particularly in the case of isolated banks, coarse shell-fragment sand tends to occur instead of foraminiferal muds.
The sediments of the Chatham Rise are unusual in several respects. The typical sediment is a foraminiferal sand or silt, but there occur places where cobbles and pebbles of igneous, metamorphic, and sedimentary rocks are present in and below the finer material. These pebbles may have been rafted on to the Rise by icebergs during the Pleistocene. There also occur pebbles of phosphatised limestone, and small granules of phosphatic glauconite.
by Henry Moir Pantin, B.A.(CANTAB.), PH.D.(CANTAB.), New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, Wellington.