Mixed glauconitic-carbonate-siliciclastic surficial sediments on the north Kaipara continental margin, northwestern North Island, New Zealand

Abstract

A mosaic of siliciclastic and mixed carbonate-siliciclastic sediments and authigenic minerals occurs at shelf and slope depths (30-1015 m water depth) on the open, wave-dominated north Kaipara continental margin (NKCM) off northern New Zealand. Texture and composition define five surficial sediment facies. Facies 1 (siliciclastic sand) comprises generally well-sorted fine sands that extend to outer shelf depths. Facies 2 (glauconitic sand) is composed of 30-95% authigenic glauconite grains at 150-400 m water depth in central to northern portions of NKCM. Facies 3 (mixed bryozoan-siliciclastic sand) occurs only in northernmost NKCM and involves a conspicuous (>40%) bryozoan carbonate content. Facies 4 (pelletal mud) occupies the mid shelf (100-150 m water depth) in northern NKCM and consists of muddy sediment with >30% mixed carbonate-siliciclastic pellets of probable fecal origin. Facies 5 (foraminiferal mud and sand) contains >30% foraminiferal tests at slope depths in southern NKCM and at both slope and mid-outer shelf depths towards the north. The siliciclastic mineralogy is consistent with mainly distant provenances to the south of the NKCM in central North Island and northern South Island, while much of the bryozoan material is likely reworked from the Three Kings carbonate platform to the north. No single shelf sedimentation model explains the complex facies distributions on the NKCM because the deposits record the interplay of several present and past hydrodynamic and sediment supply controls. While truly modern deposits may occur inshore, the bulk of NKCM surficial sediments are mainly palimpsest and/or relict deposits.