Shelf to Basin, Temperate Skeletal Carbonate Sediments, Three Kings Plateau, New Zealand

Abstract

ABSTRACT Three Kings Plateau is 10,000 km2 in area, less than 500 m deep, and includes the mainland shelf and submarine ridges and banks centered on Three Kings Islands (34°S) off northernmost New Zealand. The plateau receives little input of terrigenous sediment and lies in a zone of active nutrient upwelling and vigorous wind-wave and tidal current activity. As a consequence, surficial sediments on and about the margins of the plateau are dominantly pure skeletal carbonate sands and gravels, which lack mud to depths of 1500 to 3000 m. Plateau-top sediments include a bryozoan (-calcareous red algal-barnacle) lithofacies on coarse and rocky substrates, and a bivalve-bryozoan lithofacies associated with particulate bottoms. Plateau-edge sediments include a bryozoan-ahermatypic co al (-serpulid) lithofacies in areas of rugged topography, and a bivalve-gastropod lithofacies on sandy substrates, both grading down-slope into a planktic foraminiferal lithofacies by 1500 to 3000 m depth, depending on locality. The carbonates are dominated by a high-Mg calcite and/or low-Mg calcite mineralogy, with only subordinate aragonite. Active skeletal production is presently restricted to only about 10 percent of the plateau area, principally in regions of rough sea-floor topography, on submarine cliff faces, on bank tops, and in in-shore portions of the mainland shelf. About these areas, modern carbonates occur down to depths of 50 to 200 m, below which the sediments are mainly relict and include rare glauconite. Redeposition of plateau-derived skeletal carbonates into surrounding basins was especially active during the last-glacial low sea level(s). It is inferred that this is the ultimate fate of the bulk of the shallow-marine sediment and that the plateau-top itself preserves only a thin, laterally discontinuous, mixed-age and highly condensed sequence of Quaternary temperate skeletal carbonates.