Sediment texture of the continental shelf and upper slope off the west coast of South Island, New Zealand

Abstract

Surface sediments on the continental shelf and upper slope off the west coast of the South Island, New Zealand, from north of Karamea Bight (40°45'S) to the Whataroa River mouth (43°07'S) consist almost entirely of sands and muds. Inshore (< 50 m depth) sediment is predominantly fine sand to coarse silt. Silt‐clay content increases towards mid‐shelf depths, and areas of mud occur at depths of about 90–140 m. Sediments tend to become coarser again on the outer shelf, and sandy mud or muddy sand covers the outer shelf and upper slope. The west coast shelf experiences a high input of fluvial sediment, probably c. 12–26 × 106tonnes y‐1for the study area. Sedimentation rate on the shelf is estimated to average at least 1–2 mm y‐1. Sedimentation is probably appreciably lower than this in the north of the area and higher in the south, and this is reflected in shelf grain‐size parameters and sediment carbonate content. There are north‐to‐south decreases in mean grain size and in the ranges of sorting and skewness: shelf sediments tend to become finer and more uniform in texture towards the south. Carbonate content ranges from 0–70%, generally increasing with depth, but with a marked north‐south contrast. In the north, carbonate values increase to c. 15% by 200 m depth and to 60–70% by 650–700 m, whereas off the southernmost transect, carbonate values are < 5% by 500 m and reach only c. 40% by 1100 m. Shelf and upper‐slope sediments (< 935 m) contain lig‐nin (mostly at levels of < 5 mggdry sediment‐1), also indicating an input of land‐derived material. © Crown 1985.