Distribution of surficial sediments in the ocean around New Zealand/Aotearoa. Part B: continental shelf

ABSTRACT

This paper provides new maps of the surficial sediment distribution on the continental shelf (0∼150 m water depth) of New Zealand based on a new database – nzSEABED. The maps of percent mud, sand, gravel and carbonate, are compared with previous research to provide a comprehensive update of the surficial sediment distributions on the continental shelf, together with a review of the main environmental (oceanographic and climatic), geomorphological and geological processes and human activities that have influenced sediment deposition. Continental shelves are dynamic regions that are in a constant state of flux from floods, storms, tides, waves, earthquakes and volcanic activity. While some of these events may be captured by individual samples, the compilation of >23,000 samples collected and analysed over 60 years provides a long-term average distribution of sediments on the continental shelf that can inform future research and coastal management.

KEYWORDS:

• Sediment
• continental shelf
• New Zealand
• grainsize
• carbonate
• oceanography

Introduction

The current study is Part B of two-companion papers that reviews and updates the distribution of surficial-marine sediment around New Zealand. Part A deals with the open ocean systems of the slope and deep basins within the New Zealand exclusive economic zone (EEZ) and extended continental shelf (ECS). It also summarises the history of sediment collection and previous maps, along with the main methods that were used in this study and the sediment distributions in the open ocean. Here, in Part B, we focus on the grainsize and carbonate content of the continental shelf around New Zealand, (defined as a near flat plain that descends from the shore to the shelf edge which averages at 130 m water depth, but may be as deep as 250 m), comparing them to our current knowledge of the environmental parameters such as: terrigenous fluxes, tectonic and volcanic influences, oceanographic currents, waves, tides and storms and human influences. This paper builds on the partially completed New Zealand coastal sediment chart series (Part A; Table 1 and Figure 2) and updates a previous overview of marine sediments around the New Zealand continental ‘terrace’, undertaken by Carter (1975) and its sister publication where these sediment patterns were interpreted in concert with knowledge of the oceanographic currents, waves, tides and storms at that time (Carter and Heath 1975), and a subsequent review of coastal oceanography by Hume et al. (1992). Understanding the surficial sediment distribution of the continental shelf will aid with the management and protection of New Zealand's marine realm, by providing information about the physical character of the seabed, helping to determine biological habitats, favourable places to locate offshore marine infrastructure, as well as potential resources (e.g. sand and gravel, ironsands).

Table 1. Characterisation of the continental shelf sediments for each region and the dominant processes influencing sediment transport.

	Region	Dominant sediment	Terrigenous input	Oceanographic regime	Tectonic/Volcanic influence
3.1	East Coast Northland	Relict, modern biogenic, terrigenous	Very low	Moderate – storms, currents, tides	Low (minor volcanic ash)
3.2	Auckland/Hauraki Gulf/ Tīkapa Moana	Modern terrigenous, biogenic, relict	Moderate	Moderate – tidal and storms	Moderate – volcanism (ash and gravel/cobbles)
3.3	Coromandel/Bay of Plenty	Modern terrigenous, biogenic, volcanic	Moderate	Low – occasional subtropical storms	High – volcanism (ash and gravel/cobbles)
3.4	East Coast North Island
a	East Cape	Modern terrigenous	Extreme	Moderate – storms	High – tectonic (minor volcanic ash from TVZ)
b	Hawke Bay	Modern terrigenous, relict	Moderate	(including floods and waves)	High – tectonic (minor volcanic ash from TVZ)
c	Wairarapa	Modern terrigenous	Low		High – tectonic
3.5	West Coast Northland	Relict, biogenic, authigenic	Very low	Moderate – storms	Low
3.6	West coast North Island/North Taranaki	Modern terrigenous	Low	Moderate – storms	Moderate volcanism (influx of ironsands from Mt Taranaki and volcanic boulders)
3.7	South Taranaki/ Wanganui/Manawatu/ Kapiti Coast	Modern terrigenous, biogenic, relict	Moderate	Moderate – tidal and storms	Moderate volcanism (influx of ironsands from Mt Taranaki). Some tectonic influenced bathymetry.
3.8	Cook Strait	Modern terrigenous (within bays), biogenic, relict gravel and cobbles (in the strait)	Low	High – tidal and storms	High – tectonic
3.9	Marlborough Sounds	Modern terrigenous, biogenic (relict gravels)	Very low	Low – tidal currents, storms and estuarine flows	Low
3.10	East Coast South Island
A	North Canterbury	Modern terrigenous	Moderate	Moderate – currents and storms	Moderate – tectonic
B	South Canterbury	Modern terrigenous	Moderate	Moderate – currents and storms	Low
c	Otago	Biogenic, relict	Low	Tides and storms	Low
3.11	Southern South Island
a	Foveaux Strait	Relict, biogenic	Very low	High – tidal	Low
b	Stewart/Rakiura and Snares Islands/ Tini Heke	Relict, biogenic	Very low	High – currents and storms	Low
c	Puysegur	Relict, biogenic	Very low	High – storms	Moderate – earthquakes
3.12	West Coast South Island
A	Fiordland	Relict terrigenous	Very low	Moderate – currents and storms	High – earthquakes
B	Westland	Modern terrigenous	High	Moderate – currents and storms	Moderate – earthquakes
C	Kahurangi	Modern terrigenous	Moderate	Moderate – currents and storms	Low
3.13	Nelson Bays	Modern terrigenous, biogenic	Very low	Low – currents, storms	Low

Data and methods

The main methods used in this paper are briefly summarised below; we refer the reader to Part A for more detailed methods. More than 30,000 surface sediment data from the New Zealand Oceanographic Institute (NZOI, now the National Institute of Water and Atmospheric Research (NIWA)) and other international databases within the New Zealand Exclusive Economic Zone (EEZ) and extended continental shelf (ECS) were integrated using the dbSEABED software tools (see Jenkins et al. 2008), to generate a new database – nzSEABED. The coverage of the surface sediment samples around the New Zealand region is highly variable (See Part A: Figure 1B). Approximately 76% (∼23,000) of these samples were collected on the continental shelf from water depths <150 m (Figure 1A, B). Sample density is patchy, and more concentrated in regions where previous sediment charts were produced (Part A; Table 1, Figure 2) and in regions of high scientific interest, such as the Hawke Bay shelf (Pantin 1966), Canterbury shelf (Herzer 1977) and Cook Strait (Carter 1992; Lucieer and Lamarche 2011). The database was imported in to ArcGIS and we used the ordinary kriging method to interpolate between the data-points on a 1 × 1 km grid. The predicted standard error (standard deviation for any point) for percent mud for the kriging interpolation is low in well sampled regions such as the continental shelf (8–16%). We have not used other environmental parameters to enhance the interpolation as we found no correlation between grainsize and bathymetry and water depth, or current oceanographic conditions as some regions display relict sediments (sediments deposited under different conditions >7000 yrs ago). Several caveats should be noted; that we have not included rocks in this database, thus the interpolated sediments will superimpose over rock outcrops; and the traditional methods for sampling sediments struggle to admit shell, pebble and cobbles (Ferguson and Paola 1997) and thus the methods are inherently biased against these larger grainsizes. Thus gravel percent may be less reliable than the mud and sand percent.

Figure 1. Map of New Zealand with the sediment stations included in nzSEABED. A) North Island, B) South Island. Isobaths are shown at 50 m, 100 m, 150 m and 200 m (also 500 m, 1000 m, 2000 m and 3000 m). The edge of the continental shelf is shown in a dashed pink line. The area shaded light brown was exposed when sea level was -120 m during the last glacial. The main ocean currents are shown in blue and labelled: EAUC – East Auckland Current, ECC – East Cape Current, WCC – Wairarapa Coastal Current, SC – Southland Current, WC – Westland Current, DUC – D’Urville Current. The strong tidal currents in Cook Strait and Foveaux Strait are shown by a double-headed arrow.

Figure 2. Map of the surficial sediments of the North Island continental shelf. A) Mud %, B) Sand %, C) Gravel %; D) Carbonate %. We have used the dbSEABED colour legends for the representation of the different sediment properties (http://instaar.colorado.edu/~jenkinsc/dbseabed/helpful/index.html). Station locations are not shown as they obscure the information in regions that are densely sampled. Place names are labelled: TK – Three Kings, NC – North Cape, PH – Parengarenga Harbour, BOI – Bay of Islands, PK – Poor Knights Islands, WhH – Whangarei Harbour, Auc – Auckland, HG – Hauraki Gulf/Tīkapa Moana, FT – Firth of Thames, CC – Colville Channel, C – Coromandel, BOP – Bay of Plenty, T – Tauranga, Wh – Whakatane, M Is. – Mayor Island, Wha Is. – Moutohora/Whale Island, Whi Is. – Whakaari/White Island, EC – East Cape, RB – Ranfurly Bank, MP – Mahia Peninsular, PB – Poverty Bay/Turanganui-a-Kiwa, HB – Hawke Bay, CK – Cape Kidnappers, Cape Turnagain, Wai – Wairarapa, KH – Kaipara Harbour, HH – Hokianga Harbour, R – Raglan, M – Mokau, MtT – Mt Taranaki, CE – Cape Egmont, FS – Farewell Spit, Whan – Whanganui, Man – Manawatu, K Is. – Kapiti Island, DUS – D’Urville Seavalley, W – Wellington, CP – Cape Palliser.

The distribution of surface sediment grainsize/texture (percent mud, sand and gravel = total 100%) and carbonate contents (CaCO₃ percent, versus non-carbonate material (not shown) = total 100%) from the nzSEABED database are presented geospatially as maps to show the large scale spatial patterns on the continental shelves of the North Island (Figure 2) and South Island (Figure 3).The carbonate content provides information about the amount of biological material produced in the water column by plankton, or on the seafloor by marine organisms and is typically carbonate-rich. The inverse non-carbonate component it primarily derived from the land, termed terrigenous material, due to reworking by wind and water (rainfall and rivers) into the oceans. Authigenic minerals are also present in some regions, these are precipitated at the sea floor, under specific environmental conditions and typically over long periods of time and on the continental shelf are usually glauconite. The sediments are also referred to as modern, which indicates that they have been deposited under modern (interglacial) environmental conditions over the last ∼7000 years when sea level (Clement et al. 2016) and ocean currents have been similar to today. In other regions sediments are referred to as relict, which implies they were deposited >7000 years ago when sea level was lower and or other environmental conditions were different. Sea level fluctuates over glacial/interglacial cycles due to changes in the global ice volume. During the last glacial at ∼20,000 years ago, sea level was ∼120 m lower than the present (Lambeck et al. 2014) and vast areas of the continental shelf were exposed (Figure 1), or very shallow and influenced by waves and storms. These nzSEABED maps, therefore, plot each component of texture (percent mud, sand and gravel) and carbonate (percent calcium carbonate) independently, in contrast to the previous NZOI-NIWA coastal sediment charts, which used a ternary and hierarchical sediment classification approach (e.g. Folk 1954; Folk et al. 1970), combining textural information with carbonate data. For example, some regions of the coastal charts are colour- and pattern-coded as ‘calcareous-gravelly sandy mud’ (e.g. Cook Strait Sediment Chart, Lewis and Mitchell 1980), indicating that there is a polymodal distribution of sediment grainsizes, both gravel sand and mud, with the gravel dominated by (>50%) biogenic carbonate shell material. These historical coastal sediment charts, therefore, provide significantly more sediment textural information on one map. However, they can be complicated to interpret for non-specialists, especially as each chart was completed by different individuals and used different classifications and nomenclature i.e. compare the classification scheme used for the 1966 Foveaux chart (Cullen and Gibb 1965) with the last coastal sediment chart published by NZOI-NIWA (Patea Sediment Chart, Nodder et al. 1992). The new nzSEABED provides a seamless and consistent dataset of the surficial sediments on the entire continental shelf of the North and South islands of New Zealand for use by other researchers and policy makers. To this end we have included a glossary of some of the sedimentological terms that may not be familiar for a more general audience. It should also be noted that all locations referred to in the text are official or recognised unofficial names according to the New Zealand gazetteer (https://gazetteer.linz.govt.nz/).

Figure 3. Map of the surficial sediments of the South Island continental shelf. A) Mud %, B) Sand %, C) Gravel %, D) Carbonate %. Station locations are not shown as they obscure the information in regions that are densely sampled. Place names are labelled: KC – Kaikoura Canyon, CantP – Canterbury Plain, PB – Pegasus Bay, BP – Banks Peninsula, LE – Lake Ellesmere, O – Oamaru, OP – Otago Peninsula, MB – Molyneux Bay, NP – Nugget Point, FS – Foveaux Strait, S Is. – Stewart Island, Sn Is. – Snares Islands/Tini Heke, PuyB – Puysegur Bank, Fiord – Fiordland, MF – Milford Sound, HC – Hokitika Canyon, KS – Kahurangi Shoals, FS – Farewell Spit, GB – Golden Bay, TB – Tasman Bay.

Continental shelf surficial sediments

The definition of the continental shelf is a near-flat plain that descends from shore to the shelf edge, where the seabed then descends more steeply to form the continental slope (IHO 2008; Harris et al. 2014). Around New Zealand, the edge of the continental shelf is typically ∼130 m below sea level, but can be as deep as 200 m off Hawke Bay and 250 m off Taranaki-Manawatu. The shelf width varies considerably around New Zealand from just a few kilometres off Fiordland and Kaikoura, to more than 160 km wide in the Taranaki Region. The shelf edge can be curvilinear, but in some regions it is irregular due to incisions by submarine canyons or landslides (e.g. Cook Strait, the East Coast North Island, the West Coast South Island, and Otago).

In the presentation of the results below the continental shelf surficial sediments are discussed by region with accompanying texture and carbonate maps (Figure 2; Figure 3 and Supplementary Figures). The new maps are discussed and compared to previous sedimentological work undertaken in the area and related to the local influx of terrigenous sediments via rivers, volcanic and tectonic influences, and the regional hydrodynamic regime (waves, tides, currents, as recently reviewed by Stevens et al., submitted). The dominant sediment and drivers for each region are summarised in Table 1 and Figure 4.

Figure 4. Summary of the dominant sediments in each region and the main environmental and geological factors influencing the distribution of the sediments. See Table 1 for more details. Numbers refer to the sections in the paper that discuss the regions.

East Coast Northland

The Three Kings region sits at the northernmost tip of Northland and projects out into the Tasman Sea. No detailed coastal sediment charts have previously been made of this region, although a larger chart was (McDougall 1979). The sediments in this region have low mud percent (<20%; Figure 2A; S1A), with this region dominated by sand and gravel (Figure 2B; 2C; S1B; S1C). The carbonate content is variable ranging from 0–80% (Figure 2D; S1D), and some of the gravel around the coast is volcanic, eroded from the local headlands (Cryer et al. 2000). The sediments in this region are affected by high energy subtropical storms and waves; with strong tidal and current flows (0.5–2 m s⁻¹) that redistribute the fine-grained sediments (Nelson et al. 1982; Cryer et al. 2000). This is the region where the Tasman Front, which transports subtropical water from the East Australian Current (EAC) eastwards across the Tasman Sea, re-attaches to the northern tip of New Zealand to form the East Auckland Current (EAUC; Stanton et al. 1997).

The EAUC flows down the east coast of Northland past Auckland with surface flows as strong as ∼0.6 m s⁻¹ (Stanton et al. 1997), although, the strength and location of the core of the EAUC varies seasonally and interannually (Roemmich and Sutton 1998). The EAUC primarily bathes the edge of the continental shelf, but during Austral summer there are occasional incursions onto the shelf, initiated by strong southeasterly winds, potentially influencing biological productivity on the shelf (Sharples 1997; Sharples and Grieg 1998; Zeldis et al. 2004). Shelf tidal currents on the northeast shelf are low (0.05–0.1 m s⁻¹; Sharples and Grieg 1998), although strong internal tides have been observed along this shelf (Sharples et al. 2001), with speeds of 0.3 m s⁻¹ on the shelf around the Poor Knights Islands (Stevens et al. 2005). These internal tides are highly variable, but at these speeds they could potentially transport up to medium sand sized sediments on the shelf. Despite their potential vigour there have been no studies of sediment transport by internal waves on the continental shelf of New Zealand.

Along the east coast of Northland from North Cape to the Bay of Islands the muds are largely restricted to the inner shelf, but the mid- and outer shelf are dominated by sand (60–100%; Figure 2B; S1B), with small amounts of gravel (0–60%; Figure 2C; S1C). Apart from a small area off North Cape, the region becomes increasingly carbonate-rich to the south along the shelf (Figure 2D; S1D). The low mud content for most of this region is not particularly surprising, as the numerous small catchment streams and rivers only deliver a total of 1.2 Mt y⁻¹ of suspended sediments (mud and fine sand) to the entire northeast coast (Hicks and Shankar 2003; Hicks et al. 2011). The modern terrigenous sediments are primarily trapped in estuaries and harbours (Pickrill 1985) with small amounts being transported and deposited on the inner shelf, while the majority of the sediments on the mid- and outer shelf are considered relict (Summerhayes 1969; Carter 1975). Just offshore from the mouth of the Parengarenga Harbour, there are well-sorted, fine-grained sands that extend as far out as the mid-shelf (Summerhayes 1969). These clean, well-sorted, silica-rich (95%) sands are known as the ‘Parengarenga Sand Facies’ (Schofield 1970; Hayward et al. 2001). Their presence on the mid-shelf probably reflects a relict origin, derived from deposits formed during a period of lower sea-level (Carter and Heath 1975). These observations are supported by the common occurrence of glauconite in mid- to outer shelf sediments (Summerhayes 1969). This green mineral is formed by the marine diagenetic alteration (chemical break-down and alteration) of clays and micas under reducing conditions, and is considered diagnostic of depositional environments with slow rates of sediment accumulation (Bentor and Kastner 1965; McRae 1972; McConchie and Lewis 1980). Pockets of gravel (Figure 2C; S1C) occur nearshore around headlands due to localised cliff erosion along this coast.

South of the Bay of Islands, the sand content on the shelf declines with increasing mud content (40–60%; Figure 2A; S1A). The mud is sourced from the rivers flowing in to the Bay of Islands which have a total annual suspended sediment load of 0.6 Mt y⁻¹ (Hicks and Shankar 2003). Muddy sand and sandy mud are the most dominant sediment types within the bay (Bostock et al. 2010). The muddy sediments are transported by the tidal currents from the bay out to the adjacent shelf (Pritchard et al. 2010). This is supported by compound specific isotopes on biomarkers which show that sediments from the Bay of Island river catchments are present on the adjacent shelf (Gibbs and Olsen 2010). The mud is then transported south by the EAUC, although a previous coastal sediment chart of the Poor Knights region (Eade 1974) shows little mud and the dominance of biogenic gravel immediately around the islands and headlands, suggesting that most of this mud is deposited upstream of the Poor Knights region. Or alternatively the strong internal tides in this region are resuspending the fine sediments, which continue to be transported south. South of Whangarei Harbour there is a small amount of mud nearshore (Figure 2A; S1A), while sand dominates on the inner shelf (Figure 2B; S1B).

Auckland-Hauraki Gulf/Tīkapa Moana

A coastal sediment chart of the Hauraki Gulf/Tīkapa Moana region was published by Carter and Eade (1980). The outer Hauraki Gulf/Tīkapa Moana is dominated by sand and gravel (Figure 2B; S1B), with small amounts of mud (Figure 2A; S1A). This is due to the strong tidal currents between the islands of up to 0.8 m s⁻¹ (Greig 1990), which winnow away the fine sediments to leave coarser carbonate debris/gravel (Manighetti and Carter 1999). Bathymetric mapping of this region also shows a series of sand waves of different orientations that are possibly relict (Manighetti and Carter 1999; Mackay et al. 2012). Carbonate contents in this region are moderate, ranging from 20–60% but reaching 80% in the outer Hauraki Gulf/Tīkapa Moana southwest of Great Barrier Island (Aotea Island) (Figure 2D; S1D), where carbonate bivalve and bryozoan fragments prevail (Smith and Nelson 1994; Manighetti and Carter 1999). It has been suggested that the central Hauraki Gulf/Tīkapa Moana is a large depocentre of modern terrigenous sediment, contributed from the river run-off to the region, but also sediment transported shoreward from the outer shelf during large storm events (Manighetti and Carter 1999). However, this region may be just a conveyance zone as little sand sized sediment gets in to the Firth of Thames from the outer shelf, and most of it is transported offshore to the southeast through Colville Channel (Manighetti and Carter 1999).

Mud dominates in the inner Hauraki Gulf/Tīkapa Moana and Firth of Thames (80–100%; Figure 2A; S1A). Previous studies have found that this region is characterised by sandy or calcareous gravelly muds (Carter and Eade 1980; Manighetti and Carter 1999), with very high organic matter, especially in the firth (Giles et al. 2007). The suspended sediment load of rivers in this region is 0.35 Mt y⁻¹ (Hicks and Shankar 2003). While there are strong tidal currents in the outer Hauraki Gulf/Tīkapa Moana, the circulation in the Firth and Gulf responds predominantly to winds from the northwest or southeast (i.e. parallel to the coast; Proctor and Greig 1989; Zeldis et al. 2004). The inner Firth of Thames circulation is also strongly influenced by freshwater influx from the Waihou and Piako rivers (Zeldis et al. 2004).

Coromandel Peninsula-Bay of Plenty Region

The first compiled sediment map of the Bay of Plenty Region was produced by Doyle et al. (1979). The map shows a sand-dominated coast, with small amounts of mud. The nzSEABED data shows there are moderate amounts of gravel (<40%) off Tauranga City, and associated with a patch reef just west of Whakatane (Figure 2C; S2C). These reefs are also associated with moderate amounts of carbonate (Figure 2D; S2D), but carbonate is generally low on the rest of the shelf. Detailed sampling by Longdill et al. (2005, 2007) along the shelf displays a band of fine silty sand nearshore, interspersed with some cobble and patch reefs, some of which are associated with islands. A small patch of more sandy sediment is found at 100 m water depth off Whakatane (Figure 2B, S2B). On the eastern side of the bay there is a large wedge of mud on the mid-outer shelf (Figure 2A; S2A). Some of the fine muds show significant amounts of total organic matter (Longdill et al. 2005, 2007). This suggests that most of the fine sediment is likely sourced from the local rivers that flow into the bay, which contribute an average of 14 Mt y⁻¹ of terrigenous suspended sediment to the region (Hicks and Shankar 2003). There is also a large amount of volcanic ash (tephra or ash flow deposits) and other volcanic products (lava flows, bombs, volcaniclastic flows) found in this region, with volcanic sediments especially dominant adjacent to volcanic edifices, such as Moutohora/Whale Island and Whakaari/White Island (Kohn and Glasby 1978; Propp et al. 1994).

Water movement within the Bay of Plenty is relatively minor with a general flow from west to east, dominated by the EAUC, which is found immediately off the shelf (Ridgway and Greig 1986). Longdill et al. (2008) showed that the general surface circulation in the bay is strongly responsive to wind events, which are predominantly from the northwest, but are quite variable. Sediment transport and distribution, however, are predominantly influenced by large storms, which originate in the mid-latitudes (Tasman Sea) or from extratropical cyclones (Hay et al. 1991; Gibb 1997). The frequency and strength of these extratropical storms appear to be related to climate oscillations such as the El Niño Southern Oscillation (de Lange and Gibb 2000).

East coast North Island

Several charts have been published for the east coast of the North Island including Mahia Peninsula (Pantin and Gibb 1968) and Cape Turnagain (Lewis and Gibb 1970). The texture data from nzSEABED show the narrow continental shelf is dominated by mud (60–80%; especially along the northern edge of the East Cape; Figure 2A; S2A), with only a small amount of sand and gravel (Figure 2B; 2C; S2B; S2C). In contrast, there is a small patch of gravel, with high carbonate content immediately offshore from East Cape, associated with Ranfurly Bank (McDougall 1975).

The east coast North Island is part of the Hikurangi Subduction Margin and sedimentation along this coast is strongly controlled by active tectonics (Gerber et al. 2010). Active upper plate thrust faults breach the continental shelf along the length of the east coast and have formed discontinuous rocky outcrops along the length of the shelf (e.g. Ariel Bank; Lachlan Banks). These reefs influence modern sediment transport processes and commonly define the outer edge of sediment depocentres (Mountjoy and Barnes 2011).

On top or this tectonic control, the eastern region of the North Island has the highest suspended sediment discharge in New Zealand today with 69 Mt y⁻¹ between East Cape and Mahia Peninsula (Hicks and Shankar 2003). The fluvial sediment load is dominated by three river catchments; in the north by the Waiapu (35 Mt y⁻¹) just south of East Cape, the Uawa-Hikuwai (5 Mt y⁻¹) in Tolaga Bay, and the Waipaoa (15 Mt y⁻¹) that drains into Poverty Bay/Turanganui-a-Kiwa (Hicks and Shankar 2003). The high terrigenous loads in these rivers are due to the tectonic uplift and rapid weathering of highly erodible Tertiary mudstone- and sandstone-dominated catchment during severe storms (Hicks et al. 2000). Sediment discharge and shelf deposition increased significantly with human arrival and deforestation (Gomez et al. 2007). A considerable amount of work has been undertaken on the continental shelf and adjacent slope of Poverty Bay/Turanganui-a-Kiwa, immediately offshore of the Waipaoa River, through the US-NZ Margins Source to Sink programme (reviewed in Kuehl et al. 2016), and the shelf off East Cape and the Waiapu River (e.g. Ma et al. 2008, 2010; Kniskern et al. 2010). This work has shown that flood events deliver considerable quantities of sediment to the shelf, but these flood deposits are then reworked on the inner-shelf by waves and currents (and in gravity driven turbidity currents), which move the sediment in the nearshore bays out on to the shelf depocentres and beyond (Ma et al. 2008, 2010; Wadman and McNinch 2008; Kniskern et al. 2010; Bever et al. 2011). Wave heights average ∼1 m, although they can exceed 6 m during large storms (Orpin 2004).

Along the shelf, the relatively weak Wairarapa Coastal Current flows northwards, from Cape Palliser to Mahia Peninsula, although it is highly variable. Further offshore the East Cape Current flows southwards (Chiswell and Roemmich 1998; Chiswell 2002, 2005). Both these currents, reinforced by storm-forced water motions, result in northeast-ward alongshore transport as well as an across shelf transport of sediment (e.g. Stephens et al. 2001; Wood 2006; Hale et al. 2014).

Thus the long-term sediment accumulation on the Poverty Bay/Turanganui-a-Kiwa shelf is primarily driven by re-dispersal by oceanographic events (storms and currents) and gravity driven flows, rather than the initial flood-derived sediment delivery (Hale et al. 2014; Moriarty et al. 2015). Centennial accumulation rates reach a maximum of 1 cm y⁻¹ on the outer shelf (Orpin et al. 2006; Gerber et al. 2010). Sediment dispersal over millennial time periods has trapped a thick, post-glacial mud deposit on the mid- and outermost shelf, with sediment depocentres reaching a maximum thickness of >45 m. However, half of the sediment from this dispersal system today, is lost to the wider margin, either advected offshore to the upper slope (Alexander et al. 2010), or redistributed northeast along the shelf through sediment reworking by combined current and wave activity (Hale et al. 2014).

Further south, the narrow continental shelf off Hawke Bay and along the Wairarapa coast are dominated by mud (20–100%; Figure 2A; S3A) and sand (20–80%; Figure 2B; S3B) with low concentrations of carbonate (0–40%; Figure 2D; S3D). The terrigenous sediment flux into Hawke Bay is 11 Mt y⁻¹ (Hicks and Shankar 2003). A detailed study of Hawke Bay showed that there was an inshore sand belt with two gravel zones in the centre of the bay and just north of Cape Kidnappers (Pantin 1966). There are additional patches of sand along the edge of the shelf, which extend down from the Mahia Peninsula, atop the Lachlan Ridge (Barnes and Nicol 2004; Paquet et al. 2009). The transition from sand to mud in the bay is related to increasing water depth and the decrease in wave power. However, this does not explain the presence of the sand and gravel patch in the centre of the bay and along Lachlan Ridge. These sand and gravel zones are considered to be relict due to the presence of glauconite, pebbles and exposed rock on the ridge (Pantin 1966; McDougall 1975). However, there must be sufficient bottom current or wave action to sweep these regions free from the accumulation of muddy sediments which dominates along the rest of this coast (Pantin 1966; Pickrill and Mitchell 1979).

The terrigenous sediment discharge along the Wairarapa coast is 3 Mt y⁻¹ (Hicks and Shankar 2003). Here, the continental shelf is relatively narrow and the nearshore inner shelf is dominated by sand, which transitions to muddy sand on the outer shelf (Figure 2A, 2B; S3A; S3B). There is one minor patch of gravel in the south and some evidence of glauconite in a couple of locations suggesting low sedimentation rates and only minor terrigenous flux, possibly due to the sediment transport being restricted by a ridge feature. The transition from sand on the inner shelf to mud on the outer shelf is likely controlled by wave grading on this exposed coast. Ocean swell at Cape Palliser are predominantly from the south and occasionally exceed 5 m during a large southwesterly storm (Pickrill and Mitchell 1979; Matthews 1980; Gorman et al. 2003).

West coast Northland

No detailed coastal sediment charts were previously made for this region, but a detailed study of this region was undertaken by Payne et al. (2010). The nzSEABED maps display small amounts of mud (0–40%; Figure 2A; S1A) on the nearshore inner shelf of the northwest shelf, but the inner, mid- and outer shelf of this region is dominated by sand (40–100%; Figure 2B; S1B), with minor amounts of gravel as one heads further south (0–40%; Figure 2C; S1C). High carbonate content (40–80%) is prevalent in the north and decreases (0–40%) in the central region, before increasing slightly towards the south (0–60%). The carbonate content in the north is mainly bryozoan fragments, most likely sourced locally and transported in from the Three Kings region at the northern tip of Northland (Nelson et al. 1982; section 3.1), while further south in shallow waters (<80 m) the carbonate is predominantly gastropods and molluscs (Payne et al. 2010). Sediments on the outer shelf of the Northland west coast are considered to be relict due to the high glauconite content, especially in the north (Payne et al. 2010). The terrigenous discharge to this coast is 2.9 Mt y⁻¹ (Hicks and Shankar 2003), but the majority of this fluvial sourced sediment appears to be delivered to the coast via Kaipara and Hokianga harbours and is likely trapped in these expansive tidal embayments aided by the mangrove forests in some areas (Reeve et al. 2008). Thus the high glauconite content is the result of the paucity of modern terrigenous sediment on the shelf.

Low mud content on the continental shelf in this region is due to the prevailing southwesterly wind systems and high-energy storm waves (Carter 1975; Heath 1982, 1985; Nelson et al. 1988). These storms enhance the dominant alongshore current that flows northeast along the margin (Brodie 1960; Heath 1973; Stanton 1973; Sutton and Bowen 2011). Waves resuspend fine sediments and transport sand along the shelf (Schofield 1975) and offshore (Payne et al. 2010). Tidal currents on the open shelf of west Northland are weak (Stanton 1973) and unlikely to result in sediment transport per se, but reinforce the mean flow and storm-forced currents to affect transport.

West coast North Island-Northern Taranaki

Two coastal sediment charts were produced for the west coast region north of Taranaki: Raglan (Doyle and Arron 1982) and Mokau (Arron and Doyle 1983). The nzSEABED data show a similar trend in the sediment grainsize as the west coast of Northland, with a small amount of mud in the nearshore (<60%; Figure 2A; Figure S4A) and then a sand-dominated mid-shelf (up to 80%; Figure 2B; S4B), with increased gravel content (up to 40%; Figure 2C; S4C).

Barotropic tidal currents on the Taranaki continental shelf are relatively weak with speeds of around 0.07 m s⁻¹ (Stanton 1977), although internal tides may generate phase speeds of up to 0.3–0.5 m s⁻¹ at the shelf edge (Proctor and Carter 1989). The barotropic tides do not transport sediment, but the potential influence of the internal tides on the sediment transport has not been studied. The prevailing westerly winds result in a wave-dominated coast with wave heights of 1–3 m and period of 6–9 s, which is sufficient to resuspend the sediment at the sea bed down to 40 m (Carter and Lewis 1995; Chiswell and Kibblewhite 2010). During storms, when winds can exceed 100 km h⁻¹, large wave heights of 4–6 m have been measured and are able to rework sand down to depths of >70 m (Carter and Heath 1975; Lewis 1979; Pickrill and Mitchell 1979; Carter 1980; Gorman et al. 2003). The presence of abundant bedforms orientated orthogonal to the prevailing westerly swell (Orpin et al. 2009) suggests that waves and near-bed currents frequently transport coarse sand-sized detritus on the inner shelf.

There is also evidence for northward transport of sediments along this coast driven by the combination of the along shore current and storms (section 3.5). This is particularly evident from the distribution of ironsands on the continental shelf, which decrease in concentration northwards (Carter 1980). The ironsands are rich in titanomagnetite, sourced from Mt Taranaki and older volcanic centres (Christie 1975; Brathwaite et al. 2017), and are present on the inner and mid shelf in concentrations >5% as far north as Auckland (Carter 1980). Elsewhere, ironsand contents are lower (<2%), with a slight increase in concentrations immediately offshore river mouths such as the Mokau River (36%) and Waikato River (39%), due to new influxes of reworked ironsand from the land (Carter 1980). The rivers in this region deliver 4 Mt y⁻¹ of suspended sediment onto the coast (Hicks and Shankar 2003), with an additional bedload of cobbles and pebbles prevalent nearshore in the southern section of this region (McDougall and Brodie 1967). Around the Taranaki peninsula, large areas of the seabed are covered in coarse gravel sediments from cobbles through to boulders. Diver observations indicate boulders in excess of 2 m in diameter that are volcanic in origin and related to reworking of lava flows from Mt Taranaki (Nodder 1995). In this southern section, just to the north of Taranaki there is also increased mud content on the mid-shelf (up to 100%; Figure 2A; S4A), reduced sand content (<40%; Figure 2B; S4B) and up to 60% gravel (Figure 2C; S4C). This section of shelf is very wide (up to >160 km) and the outer shelf is up to 300 m deep (Figure 1A; 2). The high mud and low sand content likely reflects the deeper depths in this zone, with lower current speeds, allowing the mud and fine sediments to settle out. There is a small area of the outer shelf with higher carbonate content (40–80%) (Figure 2D; S4D). These carbonate sands and gravels are considered relict from previous lower sea-levels (McDougall and Brodie 1967; Carter and Heath 1975).

South Taranaki-Whanganui Bight and Manawatu-Kapiti

Sediment charts were compiled for the Patea area, south of Cape Egmont by Nodder et al. (1992) and a significant amount of work has been done on the surface sediments and bedforms of the South Taranaki-Whanganui Bight region (e.g. Norris and Grant-Taylor 1989; Gillespie and Nelson 1996; Gillespie et al. 1998), especially as it has recently become the focal area for potential offshore ironsand extraction (Lewis 1979; Gillespie and Nelson 1996; Orpin et al. 2009; Brown 2013). South of Taranaki the sediment distribution on the inner shelf is dominated by sand (up to 80%; Figure 2B; S5B) and gravel (up to 100%, especially offshore Whanganui; Figure 1C; S5C). Further offshore on the mid-shelf there is slightly more mud (Figure 2A; S5A), less sand (Figure 2B; S5B), but still up to 40% gravel (Figure 2C; S5C). The gravel is predominantly shelly carbonate material (bivalves, bryozoans) that is a mix of modern and relict shells. Volcanic pebbles are also common. The continental shelf in this region merges with the continental shelf of the South Island and there is a tongue of sand that extends off the end of the Farewell Spit at the northern tip of the South Island (section 3.12; 3.13; Figure S5) and attaches to the Taranaki shelf as the low-lying Farewell Rise (Nodder and Baldwin 1992), potentially influenced by the D’Urville Current, which is an extension of the Westland Current, and flows into the South Taranaki-Whanganui Bight and through Cook Strait (Chiswell et al. 2017).

This region receives significant quantities of terrigenous sediment with up to 6.8 Mt y⁻¹ from rivers south of Taranaki; the largest suspended sediment load of 4.7 Mt y⁻¹ comes from the Whanganui River (Hicks and Shankar 2003). Despite the high sediment supply, there is evidence for erosion down to 30 m water depth on the inner shelf, such as rock outcrops and ancient buried river valleys incised into the underlying mudstone (Lewis 1979). The mid-shelf itself has also been relatively starved of terrigenous sediment, and storm-generated depositional bedforms occur including ironsand ridges, sand ribbons, symmetrical megaripples (1–3 m wavelength), and sand waves (Lewis 1979; Carter 1980). These are formed by strong oscillatory currents today, approaching 1 m s⁻¹, which occur during the passage of large southerly storms (Lewis 1979; Nodder 1995), and the D’Urville Current that flows southeast along the shelf (Chiswell et al. 2017). The largest sediment bedforms are dunes situated immediately southeast of the mouth of the Whanganui River, and are 4–12 m high, several hundred metres wide, and several kilometres long. They are aligned subparallel to the coastline (Lewis 1979; Carter 1980). Based on the hydrodynamic motions required to create bedforms of this size at 30–50 m water depth, Lewis (1979) inferred that these bedforms are relict, with radiocarbon evidence showing that they formed 9000–12,000 years ago (Gillespie et al. 1998), when sea level was ∼30 m below present sea-level (Clement et al. 2016).

Mud is dominant on the shelf of the Manawatu-Kapiti coast, although there is still 40% sand (Figure 2B; S5B) and up to 40% gravel (Figure 1C; S5C) in this region. On the easternmost Whanganui-Manawatu shelf, under normal wave conditions (significant wave height of 0.5 m and period of 7 s; Perrett 1990) an analysis of modelled bed-shear stress by Dunbar and Barrett (2005) suggests that mud resuspension occurs regularly down to ∼15 m water depth. As such muddy deposits are better developed at >50 m water depth, typical of a wave-graded coastline where mud content broadly increases seaward as a function of wave energy. At around 100 m water depth, well-developed stacked reflectors mark the development of a thick muddy depocentre of modern sediment (Nodder 1995; Gillespie et al. 1998) associated with the D’Urville Seavalley aligned along the middle of greater Cook Strait (Nodder and Baldwin 1992). On the sheltered inshore side of Motu Kapiti Island the dominant flow of currents and tides is to the south along the Kapiti coast into Cook Strait (Chiswell and Stevens 2010; Chiswell et al. 2017), although there is sediment currently building up between Paraparaumu Beach and Motu Kapiti Island.

Cook Strait

Cook Strait separates the North and South Islands and is a highly active sedimentary environment that has been well studied by a number of different researchers. The first comprehensive map of Cook Strait sediment was produced by Lewis and Mitchell (1980). This maps show a complex pattern of sediments in the strait that is likely related to a combination of modern transport processes involving strong tidal- and wind-mixing (Heath 1978; Harris 1990; Stevens et al. 2012), and frequent earthquakes (Mountjoy et al. 2009). There are also relict sedimentary features from before Cook Strait opened during the last glacial c. 20,000 years ago (Lewis et al. 1994). The centre of Cook Strait, the 22 km-wide ‘Narrows’, has very low mud and sand and very high gravel content (Figure 2A-C; S5A-C; Lewis and Mitchell 1980), with moderate carbonate percentages (Figure 2D; S5D). The Narrows experiences strong tidal flows and bottom speeds of >1 m s⁻¹ (Black 1986; Carter et al. 1991; Walters et al. 2010; Mountjoy et al. 2014), which ensure that there is daily transport of sand and gravel down to depths of at least 225 m (Pantin 1961; Black 1986). Outside the main tidal stream in protected bays, such as Wellington Harbour and Palliser Bay, there is more mud, with sand on the inner and middle shelf derived from the local fluvial input (Carter 1992; Carter and Lewis 1995; Hicks and Shankar 2003). Slightly more sand is present on the southern and eastern ends of the strait, due to local fluvial input from the northeast South Island (Hicks and Shankar 2003), but this may also be the result of a northerly flow on the western side of the strait recirculating and redepositing some of the sediment (Stevens 2014). Southern Cook Strait, however, is exposed to Southern Ocean swell with significant storm surge currents, especially on the south coast of the North Island around Wellington (Carter and Lewis 1995). These southerly swells help to transport sediments northwards into the entrance of Wellington Harbour (van der Linden 1967; Matthews 1980).

There have been multiple campaigns that used marine geophysical surveys (bathymetry, sub-bottom profiles, backscatter) to characterise seafloor substrates in Cook Strait (Carter 1992; Lamarche et al. 2011; Lucieer and Lamarche 2011). These methods highlight evidence of sediment transport, dunes and sediment waves, present in the centre of Cook Strait, along with and erosional features, such as scours, seafloor abrasion, scarps and lineaments, and submarine gullies and canyons (Mountjoy et al. 2009; Micallef and Mountjoy 2011). These remotely sensed methods provide a much more complete picture of the sediment cover and seafloor transport processes and deposits than discrete sediment samples (Lucieer and Lamarche 2010).

Marlborough Sounds

The Marlborough Sounds are a series of drowned river valleys caused by differential subsidence, accompanied by rising sea-levels (Cotton 1974; Singh 2001; Nicol 2011). The sediments are dominated by mud (60–100%; Figure 3A; S5A) with little sand in the outer sounds (20–40%; Figure 3B; S5B), but large amounts of gravel (20–60%; Figure 3C; S5C). This gravel correlates with regions of high carbonate content at the heads of the sounds (40–80%; Figure 3D; S5D). Sediment samples from this region are typically described as shell and gravel in a mud matrix (Estcourt 1967). Sampling of the mud in the sounds has found abundant benthic organisms including ophiuroids, bivalves, gastropods, crustacea and holothurians (Estcourt 1967) and benthic foraminifera (Hayward et al. 1997), which all produce carbonate shells or skeletons that contribute to the high carbonate and gravel content of the sediments (Figure 3D; Figure S5D). The Marlborough Sounds is also an important area for aquaculture with abundant mussel and salmon farms (e.g. Hartstein and Stevens 2005; Zeldis et al. 2013). The high mud content is surprising given that there are few major rivers that flow into the heads of the sounds, with the region receiving an estimated 0.5 Mt y⁻¹ of suspended sediment, with the Pelorus River the major contributor with 0.24 Mt y⁻¹ (Hicks and Shankar 2003), suggesting that that mud may be more locally sourced from the eroding landscape, rather than just from the main rivers.

The oceanographic circulation of the region is dominated by tides (Heath 1974, 1976), although flows are altered during large freshwater inflows due to stratification of the water column, producing an estuarine circulation (Carter 1976). The asymmetric tidal flow has a stronger ebb tide (which produces more turbulence) than the flood tide (Sutton and Hadfield 1997; Stevens and Smith 2016) that resuspends the fine sediments and transports them seaward (Carter 1976). However, at the entrances of the sounds the flood tide dominates and transports sediment from Cook Strait into the sounds. This results in slightly more sand at the entrances of the sounds, especially in the southern sounds (up to 40%; Estcourt 1967). The mean thickness of the modern mud is 5 m with some areas as thick as 10 m, especially in the river deltas (Singh 2001). However, at several locations in the sounds the upper modern mud layer is absent and the underlying relict gravels are exposed. This typically occurs in narrow sections of the sounds where the tidal currents are strong (e.g. Stevens and Smith 2016).

East coast South Island – Canterbury to Otago

A series of NZOI/NIWA coastal sediment charts were published that span the entire length of the coastline of the east coast of the South Island including (from north to south): Pegasus (Carter and Herzer 1986), Banks (Herzer 1979b), Ellesmere, (Herzer 1981), and Oamaru (Arron 1986). As the rivers of the east coast of the South Island transport large amounts of terrigenous sediment to the coast (17.23 Mt y⁻¹ total suspended sediment from all the rivers from Otago to Kaikoura; Hicks et al. 2011), the majority of previous publications for this region have focussed on terrigenous sediment depocentres and sediment budgets for the continental shelf (Andrews 1973; Herzer 1977; Carter and Herzer 1979; Gibb and Adams 1982; Carter 1986; Carter and Carter 1986, 1986; Nokes 2014).

Most of this riverine sediment discharged at the coast is fine sand and mud originating from the rapidly eroding Southern Alps (Adams 1980; Koons 1989; Herman et al. 2010). Despite the large amounts of mud discharged at the coast, mud is generally absent across large regions of this shelf (Figure 3A; S6A; S7A). The exceptions to this are a nearshore mud in the northern lee of the Otago Peninsula, and several depocentres on the mid-outer shelf north off Nugget Point in Molyneux Bay (Carter 1986; Carter and Carter 1986) and north of Banks Peninsula (Herzer 1977; Figure 3A; S6A; S7A). As with the the higher mud contents occur north of significant promontories due to the shelter provided from the prevailing northward currents and southerly storms (Herzer 1977; Carter and Herzer 1979). The primary sink for this terrigenous sediment has been the nearshore/beach zone, which has resulted in the progradation of the Canterbury plains by as much as 4–5 km since the mid-Holocene (6000 years ago; Barnes 1993; Duns 1995; Browne and Naish 2003) with an additional small amount of sediment accumulating in harbours, bays and inlets of Banks Peninsula (Gibb and Adams 1982; Carter 1986). The rest of the fine sand and mud is entrained in the northeast along shore current system and is transported as far as Kaikoura Canyon, driven by both the Southland Current (Chiswell 1996; Sutton 2003) and the prevailing storm direction from the southeast (Andrews 1973; Carter and Heath 1975; Carter and Herzer 1979; Nokes 2014). Most of these fine sands and muds are eventually transported off the shelf down canyons and over the continental slope, forming a mud veneer on the continental slope and infilling the upper canyons (Herzer 1979a; Lewis and Barnes 1999).

On the inner shelf there are large areas dominated by gravel (Figure 3C; S6C; S7C), deposited at the river mouths and from local coastal cliff erosion (Herzer 1977; Gibb and Adams 1982; Lewis and Barnes 1999). These gravel deposits are typically in high energy, wave-dominated inner shelf and sediments are not transported far due to their coarse-grained nature (Nokes 2014). Cobbles and carbonate gravels are also found on the mid- and outer shelf around Banks Peninsula (Herzer 1977; Carter and Herzer 1979; Gibb and Adams 1982), with recent sediment transport modelling confirming that these cobbles are relict, deposited during lower sea-levels (Nokes 2014). The biogenic carbonate contribution on this shelf ranges from 10–40% (Figure 3D; S6D; S7D). The highest carbonate contents are typically found on the outer shelf east of Otago, that is characterised as a relict biogenic sand/gravel facies (Andrews 1973; Carter et al. 1985; Carter 1986). The carbonate on the outer shelf is primarily comprised of temperate carbonate organisms such as bryozoans (Nelson et al. 1988; Steger and Smith 2005; Wood and Probert 2013).

Foveaux Strait, Stewart Island/Rakiura and the Southern South Island

The Foveaux sediment chart was the first coastal sediment chart produced by NZOI (Cullen and Gibb 1965). Foveaux Strait was of high interest as it supported a flourishing oyster industry, but over time the effects of dredging on the shelf have had a significant impact on the sediment distribution (Cranfield et al. 2003). The original sediment maps of this region show a large area of cobbles and gravel northwest of Stewart Island, with fine to medium sand and pebbles to the southeast, with the latter being the region that hosted the oyster beds (Cullen 1967). These pebbles are interpreted to have been deposited by fluvial processes during lower sea-levels (Cullen 1966), whilst areas of elevated bathymetry are interpreted as paleo-shorelines (Cullen 1976). There was some debate about whether the pebbles and gravel were relict (Cullen 1976; Schofield 1976, 1977). The strong tides (0.8–1.2 m s^−1, and even higher through some of the gaps between the islands; Cranfield 1968), and large southerly storms and swell may be sufficient to move pebble and cobble grainsizes. Bryozoan reefs once dominated the seafloor habitat of Foveaux Strait. In 1977–1979 they formed elevated structures parallel to the strong tidal current direction that were between 300 m and 13 km in length, up to 500 m wide and found to be sitting on top of the relict gravel waves (Cranfield et al. 1999, 2003). Smaller patch reefs were approximately 1 m high, 50 m in length and 5 m wide and transverse to the current. These reefs formed in regions where there were strong currents that removed fine sediments from the region (Carter et al. 1985). However, by 1998 large bryozoan reefs had mostly disappeared and most of the area was flat with occasional patch reefs, representing a significant reduction from those surveyed in 1977–1979. It appears that once the main bryozoan and oyster reefs were disrupted by dredging, the biogenic sediments were redistributed by the strong currents leaving bryozoan debris in large dunes (Cranfield, et al. 2003). There is now evidence that small bryozoan reefs are reforming (Cranfield et al. 2004).

The nzSEABED maps show high gravel (20–60%; Figure 3C; S8C), with some mud in the northwest (20–40%, with up to 80% nearshore to the South Island; Figure 2A; S8A) and up to 80% sand in the southeast (Figure 3B; S8B). The limited mud is likely due to the very low terrigenous supply to the region (2.1 Mt yr⁻¹ from the South Island and 0.2 Mt yr⁻¹ from Stewart Island; Hicks and Shankar 2003; Hicks et al. 2011), with most of this material trapped prior to reaching the continental shelf by a series of lagoons on the South Island (Cullen 1976) and Paterson Inlet on Stewart Island. However, the sand, gravel and carbonate maps include sediment samples from the 1960s to the 1990s and therefore the maps are probably a combination of before and after dredging, which has significantly altered the seafloor sediment distribution.

To the south of Stewart Island are the Snares Islands/Tini Heke. The shelf around the islands is dominated by carbonate-rich sand and gravel (Figure 3B; 3C; 3D; S8B; S8C; S8D). The carbonate material is probably a combination of modern and relict material. There is very little terrigenous sediment that makes it out to this area of the shelf. Further west of Foveaux Strait there is also little mud, except nearshore, and the region is dominated by sand (Figure 3B; S8B) and gravel (Figure 3C; S8C), especially on the wide Puysegur Bank, south of Fiordland. Some of the coarse sediments are biogenic with up to 60% carbonate (Figure 3D; S8D) found on Puysegur Bank. There has been little investigation of the surficial sediments of this region. We suggest that the gravels and coarse sands are most likely relict from previous lower sea-levels. Biogenic input may be a combination of modern and relict, while the low mud content is most likely the result of limited terrigenous input into the region (section 3.11), and the result of the vigorous ocean climate in this region, resuspending and transporting fine sediment offshore during the frequent Southern Ocean storms (e.g. Gorman et al. 2003).

West coast South Island

Only a few studies have been undertaken on the surficial sediments of this region (Carter 1980; Stoffers et al. 1984; Probert and Swanson 1985; Radford 2012) and there are no NIWA/NZOI coastal sediment charts. In general, the nzSEABED maps show the northern part of the west coast is dominated by sand (Figure 2B; S10B), the central part by mud (Figure 3A; S9A; S10A) and the southern part by gravel (Figure 3C; S9C). Carbonate concentrations are low across the whole region (0–40%; Figure 3D; S9D; S10D).

In the southern Fiordland region little modern terrigenous sediment makes it to the open coast (<1.3 Mt y⁻¹; Hicks et al. 2011), with most of the eroded terrigenous sediment captured in the fiords (Pickrill 1987, 1993). The very narrow shelf of Fiordland (Figure 1B) is characterised by a mixture of sands and silts (but very low clay content), with minor gravel, primarily from local coastal cliff erosion and transported along the shelf by large storms (Barnes et al. 2013). The shelf sediments are composed of up to 30% locally produced carbonate, comprised of benthic foraminifera, molluscs, bryozoans and shell fragments (Barnes et al. 2013). Sedimentation rates are generally very low and the region is a relict landscape with little post-glacial marine sediment overlying the glacial gravel outwash fans that formed when the Fiordland glaciers extended to the ocean ∼20,000 years ago at lower sea levels (Barnes 2009).

Sediments on the central west coast South Island shelf are composed of mud, with minor amounts of sand (Figure 3A; 3B; S9A; S9B; S10A; S10B). This reflects locally high discharge from the Hokitika – 6.2 Mt y⁻¹, Arawata – 7.2 Mt y⁻¹, Haast – 5.9 Mt y⁻¹, and several other small rivers delivering a total of 62.3 Mt y⁻¹ suspended sediment to central west coast (Hicks and Shankar 2003). Those high suspended sediment loads reflect some of the highest erosion rates in the world measured on the western side of the Southern Alps (Larsen et al. 2014). The inner shelf is composed of fine, clean, well-sorted sand in the nearshore zone (<40 m water depth), which grades into muddy sand on the mid- and outer shelf (Carter 1980; Probert and Swanson 1985; Radford 2012), typical of a wave graded shelf. The mean flow of the Westland Current is 0.05 m s⁻¹ to the northeast along the coast, driven by prevailing southwest winds (Heath 1982). However, during periods of calm weather this is replaced by a weak southwesterly flow (Heath 1969; Stanton 1976). Little sediment is transported under these conditions. Net sediment transport on the shelf is to the north during southwesterly storm conditions (Carter 1980; Radford 2012). Several large canyon systems including the Hokitika Canyon incise the continental shelf and intercept some of this mixed muddy sandy sediment being transported north. On the northern rims of the canyon heads there is evidence for relict coarse sands and gravels with glauconitic grains, especially prevalent on the Hokitika Canyon rim at 90–150 m water depth (Stoffers et al. 1984; Radford 2012). Carbonate content is very low (0–20%) on the west coast shelf, with the main modern components coming from foraminifera tests, molluscs, gastropods and bryozoan fragments (Carter 1975).

On the northern part of the shelf from Kahurangi Shoals to Farewell Spit there is very little mud (0–40%; Figure 3A; Figure S10A; S5A). The low mud content is due to the relatively low terrigenous suspended sediment input to this northern west coast shelf (a total of 6.5 Mt y⁻¹, predominantly from the Grey and Buller Rivers in the south of this region; Hicks and Shankar 2003). The low suspended fluvial discharge is presumably a reflection of the highly indurated, metamorphic and igneous rocks in catchments and along the coast in this region (Hicks et al. 2011), although there are also come Cretaceous sediments along some of the coast. This section of the continental shelf is dominated by sand (Figure 3B; Figure S10B; S5B), with small amounts of gravel (Figure 3C; S10C; S5C) present on the Kahurangi Shoals (Van der Linden 1969). These coarse sands and gravels are considered to be relict, as they are unable to be transported under the moderate modern oceanographic conditions (Chiswell and Bowman 1985; Barnes and Ghisetti 2016). The modern alongshore flows, however, transport fine sands to the northeast, with some of this sand eventually depositing on the sandspit at Farewell Spit. The spit is 25 km long (in 2010) and has expanded in area by 75% since 1950 (Tribe and Kennedy 2010). It is thought that the extension of the spit is related to the development of shallow shell banks, which then get overrun by the longshore transported sands and are sequentially added on to the main spit (Tribe and Kennedy 2010) (section 3.13).

Nelson Bays – Golden and Tasman Bays

The very wide, shallow, continental shelf in this region merges with the continental shelf of the Taranaki Bight and western North Island (Figure 2; 3; S5; section 3.6). A sediment map was previously produced for the Tasman region (Mitchell 1987). The nzSEABED maps for the Tasman region show it is dominated by mud, with most of Nelson Bays (Golden and Tasman Bays) displaying >60% mud (Figure 3A; S5A). The high mud content is surprising given that there is very little terrigenous suspended sediment input from the rivers in this region (0.8 Mt y⁻¹; Hicks and Shankar 2003). While these may be average suspended sedimentation rates delivered to the coasts, it has been suggested that the suspended sediment flux may be four-times greater during flood events (Basher et al. 2011; Gillespie et al. 2011). However, a large amount of the fine sediment is probably derived from west coast South Island rivers and transported into the general Cook Strait region by the Westland Current and D’Urville Current (Furkert 1947; Van der Linden 1969).

The D’Urville Current passes round the edge of Farewell Spit and to the north of the Nelson Bays (Chiswell et al. 2017). From limited observations and a numerical model, Heath (1976) suggested the residual weak circulation in Golden Bay is in a clockwise direction and an anticlockwise direction in Tasman Bay. More recent higher resolution modelling by Tuckey et al. (2006) showed that the flow that enters Tasman Bay splits, with part going clockwise to Golden Bay and part moving towards the Marlborough Sounds. The combination of the easterly flow of the D’Urville Current around Farewell Spit, and the residual clockwise circulation in Golden Bay results in the extension of the spit (Van der Linden 1969; Heath 1976). The maps show high carbonate content (20–60%; Figure 3D; S5D) and the dominance of sand on the inner and mid-shelf around Farewell Spit (Figure 3B; S5B). Golden Bay, which is subaerially exposed during low tide, is potentially also influenced by the influx of wind-blown sand from the dunes on the spit being blown eastwards into the bay (van der Linden 1969).

The fate of the suspended sediment from the four major rivers in the region (Aorere, Takaka, Motueka and Waimea rivers) was tested in a simple hydrodynamic model (Tuckey et al. 2006). The results show that the sediments >20 μm diameter (silt size) do not travel far from the estuary and river mouth, even under flood conditions (Tuckey et al. 2006). However, waves were not included in the model and likely rework and redistribute the sediment over time. This process of wave reworking is apparent from sediment tracing work using trace metal concentrations which showed that the Motueka River plume influenced a region of 180 km² (Gillespie et al. 2011). Carbonate concentrations in the Nelson Bays is up to 40%, which is not surprising given that this region is well known for its scallop fishery (Williams et al. 2014). Boulders and cobbles found in the Tasman Bay (which form the famous Nelson Boulder Bank), were originally considered to be transported to their location by longshore drift (Johnston 2001), but more recent work has suggested that they are actually an erosional feature, sculptured from the underlying bedrock outcrops (Dickinson et al. 2003).

Discussion and conclusions

The surficial sediments from the different regions of the New Zealand continental shelf are influenced by a wide range of geological, oceanographic, climatic and human activities that are summarised in Table 1 and Figure 4. Some regions are strongly influenced by the local geological activity, such as the accumulation of volcanic gravels around active volcanoes in the Bay of Plenty (e.g. Whakaari/White Island) and strong, active tectonic controls on sedimentation along the east coast of the North Island. In this region the onshore geology, highly erodible Tertiary marine mudstones, also plays a strong control on the offshore surficial sediments, both type and quantity. While the Miocene volcanics of the west coast of the North Island also influence the offshore sediments by producing significant quantities of ironsand. Other regions are predominantly influenced by oceanographic currents that do not allow sediments to deposit, such as the strong tidal currents in Cook and Foveaux straits. Several areas exhibit relict sediments due to low terrigenous influx and/or strong currents at these locations such as the east and west Northland shelves, Otago shelf and Kahurangi Shoals off northwest Nelson. Yet other regions are loaded with modern terrigenous muds supplied by large muddy rivers, such as the northern East Coast of the North Island and the central west coast of the South Island. The ∼200 Mt/y total suspended sediment yield from New Zealand amounts to just under 2% of the global sediment delivery to the oceans, making New Zealand a major contributor (Hicks et al. 2011). In many regions the deposition of sediments on the continental shelf is often temporary, with this material being resuspended and transported by subsequent storms and strong currents alongshore and offshore to the adjacent slope (Part B). In some cases, humans have also had a significant impact on the distribution of surface sediments, such as the region of Foveaux Strait which has changed dramatically due to the impact of dredging for the wild oyster fishery (Cranfield et al. 2003). Anthropogenic influences are also evident on the east coast of the North Island, with dramatic increases in the sediment load delivered to the coast and continental shelf due to deforestation and a change to agriculture since Māori and European arrivals (Gomez et al. 2007). The influence of trawling-based fisheries on continental shelf sediment processes has been shown to be significant in other places around the world, but has only been quantified for the Hauraki Gulf/Tīkapa Moana in New Zealand (e.g. Thrush et al. 1998).

The sediment records now compiled in the new nzSEABED database for the New Zealand region were collected and analysed over 60 years, thus these maps provide a long-term average of the substrate characteristics. It should therefore be noted that in some regions there may have been changes in the sediment rates and sources (e.g. Foveaux Strait), or changes in the amount of storms over the last 60 years and future work should consider this aspect. However, the surficial sediments in many regions of the continental shelf will always be in a constant state of change due to recent floods, storms, earthquakes, volcanic and human activities. The continental shelf has many conveyance zones that act as temporary depocentres, and the sediment is subsequently reworked and transported alongshore, and offshore, as well as onshore. Nonetheless, these long-term average sediment maps developed from large numbers of samples over long time periods provide an insight into the dominant processes.

It is clear from this review that there are still many gaps in our knowledge of the surficial sediment distribution on the New Zealand continental shelf. Some regions are still relatively under-sampled and less well characterised from both a sedimentological and oceanographic perspective (Stevens et al., submitted). These include the continental shelf regions of East Cape, southern Wairarapa, Fiordland and the west coast of the South Island. We suggest that future work should combine oceanographic models with the sediment distribution to understand the main sediment transport pathways for each region of the continental shelf (e.g. Nokes 2014). Another major gap in our understanding is the question whether internal tides or other physical processes (e.g. coastal trapped waves, sub-mesoscale eddies) can transport sediment across the New Zealand continental shelf, as no work has been done to investigate these possible sediment transport modes.

These surficial sediment maps for the New Zealand continental shelf provide useful data to assist coastal use and management. The database and maps are available for others to use (on request) and will provide background information for a wide range of future research and policy decision-making on sediment sources and transport, sediment budgets, benthic habitat modelling, and marine energy and mineral resources. It is anticipated that the nzSEABED database will be updated with sample data as new information is made available in the future.

Glossary

Anthropogenic	=	human influences on the environment e.g. dredging of the sediments
Benthic	=	living on the bottom of the ocean e.g. benthic organisms live on the seafloor, or within the seafloor sediments
Biogenic	=	produced by marine organisms e.g. shells or skeletal remains. Most commonly this is composed of calcium carbonate, but may be composed of biogenic silica. Mud sized biogenic shells are commonly dominated by phytoplankton such as coccolithophore, while sand sized biogenic material is produced by planktonic and benthic foraminifera, gravel sized material is from shells, corals and bryozoans.
CaCO3	=	calcium carbonate, the mineral used by most marine organisms to produce their shells and skeletons
Carbonate	=	calcium carbonate or CaCO3, see CaCO3 definition
Continental shelf	=	a relatively flat plain from the shore to the slope, where the edge of the shelf is defined by an increase in the slope gradient, typically at water depths >150 m
Depocentre	=	a region of sediment accumulation – short for ‘depositional centre’
Diagenetic	=	alteration of the sediments in the water column or sediments, e.g. due to dissolution via fluids, or pressure changes, or heat
Extratropical	=	originating in the tropics and transported into the subtropics, e.g. extratropical cyclones.
Glauconite	=	green mineral is formed by the marine diagenetic alteration (chemical break-down and alteration) of clays and micas under reducing conditions, and is considered diagnostic of depositional environments with slow rates of sediment accumulation.
Hydrodynamic	=	oceanographic conditions such as waves, tides, currents.
Ironsands	=	black sediments rich in titanomagnetite (and of course iron)
Modern	=	sediments transported and deposited under modern environmental conditions, <7000 yrs, when sea level and ocean currents were similar to today
Relict	=	sediments that are not forming or depositing under modern environmental conditions, >7000 yrs ago when sea level and ocean currents were different from today
Tectonic	=	movement of the earth's crust causing earthquakes
Tephra	=	volcanic ash that is expelled during volcanic eruptions in a large plume that is transported via wind and rivers into the ocean
Terrigenous	=	sediment that is derived from weathering of rocks on land and then transported via rivers to the ocean
Tertiary	=	geological time period from 65 million years to 2.58 million years
Texture	=	the grainsize of the sediments
Turbidity currents	=	gravity current of rapidly moving, sediment-laden water, which transports large amounts of mud and sand downslopes, producing distinct sediment deposits called turbidites.

Acknowledgments

The authors would like to acknowledge all the scientists, technicians, students and crew from all the research voyages that have collected, documented and analysed sediment samples from the southwest Pacific Ocean over the past six decades. Digitisation of the NIWA surficial sediment data and the write up of this work was funded by NIWA Core and Strategic Science Investment Fund funding, provided by the Ministry of Business, Innovation and Employment, to the Marine Geological Processes and Resources programme. We would like to acknowledge Steve Chiswell for his review of the oceanography in this series of papers. The sediment layers (% mud, % sand, % gravel and % carbonate) are available as GIS rasters from the New Zealand Ocean Data Portal (https://nzodn.nz/).

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Scott Nodder http://orcid.org/0000-0002-1963-8907

Alan Orpin http://orcid.org/0000-0003-2054-7081