|dc.description.abstract||The northeastern Northland continental shelf (NNCS) includes the coastal, shelf and uppermost slope seafloor out to depths of 210 m and over a distance of 150 km between North Cape and Ngurauru Bay slightly north of Whangarei. The nature of the surficial sediments covering NNCS is poorly known and this study uses archived and recently collected samples to begin to address this situation, with special attention to the nature and distribution of the skeletal carbonate fraction. The sediments are mainly neither carbonate-dominated as occurs in the Three Kings area immediately to the north, nor overwhelmingly terrigenous in nature as typifies more southern shelf sectors. Instead, mixed terrigenous-carbonate deposits prevail in an often patchwork mosaic distribution of facies. Carbonate content is often highest nearshore and in bays and harbours (50-80%), and generally decreases into deeper water (few-50%). The NNCS sediments are almost completely dominated by sand-sized components (60-90%) comprising gravelly sand, sand, muddy sand and gravelly muddy sand textures.
A sedimentary facies model is developed for the NNCS identifying five different facies. Facies 1 is siliciclastic quartzofeldspathic sand with minor bivalve contributions, occurring only in the northern sector. Subfacies 1a (quartz>feldspar) occurs at inner- to mid-shelf depths and is mainly derived from reworked Quaternary Karioitahi Group sand deposits and/or podzolisation and reworking of local soils. Subfacies 1b (feldspar>quartz) occurs in mid- to outer-shelf depths and is derived from the erosion of a combination of the Quaternary sand deposits (Katioitahi Group), Mesozoic basement rocks and Northland volcanic and sedimentary rocks. Facies 2 comprises mixed barnacle-siliciclastic gravel and sand at inner-shelf to uppermost slope depths in the northern sector and includes submarine basaltic and type 1 sedimentary rock fragments derived from Mesozoic basement rocks and the Tangihua Complex of the Northland Allochthon. Facies 3 consists of mixed calcareous red algae and bivalve-siliciclastic gravel and sand at inner- to mid-shelf depths in the southern sector and involves a mix of relict and modern skeletal material and sedimentary rock fragments type 2 sourced from Tertiary deposits. Facies 4 is mixed foraminiferal-siliciclastic sand and mud and occurs only in the very southern sector in two subfacies. Subfacies 4a, a mixed benthic foraminiferal-siliciclastic sand, occurs in mid-shelf depths and comprises plagioclase dominated siliciclastic material sourced from Mesozoic basement rocks. Subfacies 4b, a mixed planktic foraminiferal-siliciclastic sand and mud, occurs in mid- to outer-shelf depths and comprises rock fragments derived from Tertiary deposits which could also be a local source of the reworked planktic foraminifera at these depths. Facies 5 consists of mixed bryozoan-siliciclastic sand at mid-shelf depths and in scattered locations across the NNCS associated with rocky outcrops for bryozoan attachment. Siliciclastic material comprises plagioclase sand sourced from reworking of Mesozoic basement rocks, and sedimentary rock fragments type 2 sourced from Tertiary sediments.
The NNCS sedimentary facies model is compared with other temperate carbonate shelf models and carbonate-rich North Island shelf sectors, including Hauraki Gulf, Three Kings platform, north Kaipara continental margin and Wanganui shelf, which serves to both test and expand scenarios of cool-water carbonate sedimentation established for New Zealand. Overall, the mixed terrigenous-carbonate material on the NNCS comprises an often mosaic facies distribution with the skeletal fraction dominated by bivalve and barnacle fragments and a mixture of relict and modern material. These characteristics reflect several environmental controls, including (a) diversified substrate types (rock, shell, sand, mud); (b) variable supply and dispersal routes of terrigenous material linked to positions of hinterland estuaries/harbours and rocky versus sandy coastlines; (c) scattered and discrete areas of primary carbonate generation (the “carbonate factories”) and the subsequent selective transportation of skeletal grains; (d) local effects of nutrient-rich upwelling; and (e) the variable mix of relict and modern sediment associated with the post-glacial rise of sea level since about 20 ka.||