Distribution and origin of clay minerals in surficial shelf sediments, western North Island, New Zealand

Abstract

Western shelf surficial sediments are mainly terrigenous muds and variably muddy fine and very fine sands of mixed modern/relict heritage. Overall their clay fraction is dominated by illite and smectite, lesser amounts of chlorite and mixed-layer clays, and some kaolinite. The close similarity of shelf clay-mineral assemblages with those in adjacent river and harbour sediments demonstrates that offshore north—south regional trends in clay mineralogy parallel regional changes in onshore geology, involving both western North Island and more distant northwestern South Island sources. The same detrital clay-mineral patterns on the shelf have persisted during the sea-level rise following the Last Glaciation and are complicated only by local concentrations of authigenic smectite from volcanogenic material. On the basis of the distribution and origin of the major clay minerals, five north—south clay petrologic zones occur on the shelf: (1) Hamilton Shelf — abundant smectite and common poorly crystalline illite; primarily detrital from Oligocene mudstones, some smectite authigenic from Quaternary volcanogenic material; (2) North Taranaki Shelf — abundant crystalline illite and common mixed-layer clays and crystalline chlorite; primarily detrital from Miocene mudstones; (3) Central Taranaki Shelf — abundant crystalline illite and common smectite and chlorite; smectite is authigenic from Egmont-derived andesitic material, contemporaneous with deposition of detrital illite and some chlorite derived from northwestern South Island via the Westland Current and from adjacent shelf zones; (4) South Taranaki Shelf — abundant crystalline illite and chlorite; detrital via the Westland Current from northwestern South Island rivers that drain an actively eroding granitic-schistose provenance; and (5) North Cook Strait Basin — abundant crystalline illite and common mixed-layer clays and crystalline chlorite; detrital from Pliocene—Quaternary mudstones of the South Taranaki—Wanganui region as well as from northwestern South Island sources via the D'Urville Current.