Physical Environment
Within this area the coastline is comprised of a diversity of landforms, sediment types and wave energies, which in turn is reflected in the diversity of flora and fauna found. The predominant landform types are dune backed beaches, but cliffs, spits, harbours and estuaries are also found.
The Hauraki Sand Facies lies to the south of the Bay of Islands Sand Facies. From Ocean Beach just north of Whangarei Heads south to Omaha Bay there is a broadening shelf covered with sandy sediment, and long stretches of beach coastline backed by dunes between rocky points. The beach and dune sands are described as clean, well-sorted feldspathic sands of fine to medium grain in size. The coarseness of the sand increases with increasing wave exposure, so that between Mangawhai Heads and Pakiri, medium grained sands occur (0.23 to 0.45 mm) while further south, at Omaha, fine sands are found (0.13 to 0.22 mm). To the north of Pakiri Beach, along Bream Bay, the sands are fine to medium (0.17 to 0.33 mm), with the finer sands occurring in the shelter of Whangarei Heads (Applied Geology Associates, 1982).
The major constituent of the sand in the Hauraki Sand Facies is feldspar (55 to 76 %) and quartz (19 to 33 %). Mafic minerals (under 2 %) and rock fragments constitute the remainder. Offshore from these sandy beaches, down to 60 m depth, the sea floor is covered with a sand bed that extends south-eastward in two arms; one into the Firth of Thames; the other between Great Barrier Island and the Coromandel Peninsula. Schofield (reported in Applied Geology Associates, 1982) states that these sandy areas are partially reworked remnants of a terrestrial plain formed when sea level was more than 60 m below present level, and when the Waikato River flowed into the Firth of Thames. This situation occurred during the last period of glaciation. The plain has since been largely coated with marine mud, although portions are sufficiently exposed to sea-current action to be kept clean. Schofield has termed the highly feldspathic sand from the outer parts of the Gulf the Hauraki (A) Sand Facies, to differentiate it from its coastal derivative, Hauraki (B) Sand Facies which forms the beach and dune sands at Mangawhai, Pakiri and Omaha. Sands in these areas are not derived from local hinterland rocks, but rather from distant volcanic sources to the south.
Estuaries on this coastline are formed from drowned river valleys and embayments, with inlets formed from a variety of barrier types. For example, Mangawhai is an example of an island enclosed estuary, with two inlets which give the estuary special flushing characteristics. A double inlet may form by wave-breaching of a barrier spit during major storms, after which the inlet slowly migrates back towards the former entrance (Hume & Herdendorf, 1988). In contrast, Pakiri estuary is an example of a “perched” estuary, in which the sea crosses the barrier beach, only entering the estuary near high tide, and with drainage occurring during the ebb tide across the beach face (Hume & Herdendorf, 1988).
In an inventory of important geological sites and landforms in the Northland region compiled by Kenny & Hayward (1993), several sites have been identified as being of national importance geologically. These include the dikes of about 1 metre across that are found along the coast on the beach at Kauri Mountain, at the north of Araarariki Beach. These are veins of metallic mineralisation that include galena, sphalerite and pyrite. In the same vicinity, at Flax Bay, hornfels of up to 2 metres wide at contact with quartz-diorite are exposed in cliffs and shore platforms. These are features of regional significance.
The “Parua Bay basal allochthon melange”, a shore platform at the eastern end of Parua Bay (Whangarei Harbour), extending 100 metres along the foreshore, has also been identified as a site of national geological significance. This is one of the classic localities in Northland showing the base of the allochthon sitting on early Miocene rocks and greywacke.
The inter-tidal and coastal rocks, and low cliffs on the south side of Reserve Point (in the Whangarei Harbour), one kilometre east of the tip, are also of geological significance nationally. These are comprised of the nephelinite flow and garnet andesite, being the only known nephelinite flow in northern New Zealand. The nephelinite flow sits unconformably on greywacke, and is intruded by garnet andesite rich in mantle xenoliths. This sequence dates from Eocene, Oligocene and Miocene times. The Eocene shelf sediments at Reserve Point (at The Nook) are of regional significance because they provide a record of a deepening shelf sequence deposited in situ in the Late Eocene.
Another coastal geological feature of national significance in this area is the natural jetty formed by rocks on the foreshore of Taurikura Bay in Whangarei Harbour. This consists of an andesite dike, 2 metres wide, intruding into Northland allochthon, and forming a 50 metre long jetty out into the bay. It represents the best natural jetty formed by a dike in New Zealand. In the inventory of important geological sites and landforms in the Northland region (Kenny & Hayward, 1993), each site has been given a "vulnerability classification”. This site is classified as being highly vulnerable to complete destruction or major modification by humans. The major hazards are listed as being hammering by geologists, vandalism, and marine development.
While not of national significance, several other sites have regional geological significance. These include the autochthonous sediments on both shores of Skull Creek and in the vicinity of Mangawhati Point, in Whangarei Harbour. These are the most complete, well-exposed autochthonous middle Tertiary sequence beneath allochthon in the Whangarei region. They consist of conglomerate, glauconitic sandstone and bioclastic limestone overlain by chaotic basal allochthon.
The “Bream Head stratovolcano”, a coastal section and foreshore rocks on a 5 kilometre length of coastline from Busby Point eastwards to Bream Head, also has geological significance regionally. This is the best exposed section through the cone facies and underlying subvolcanic intrusions in the Taurikura centre around Whangarei Heads, and dates from the Miocene age. There are cone facies, rubbly breccia and andesite flows in the east, and subvolcanic andesite, dacite and rhyolite intrusions into the underlying Northland Allochthon in the west, just beneath the volcanic outcrop which forms the upper parts of the western cliffs. The Bream Tail Arch Dome, comprising coastal cliffs and intertidal rocks 1-2 kilometres south of the tip of Bream Tail, is the best exposed Miocene dacite dome of the group around Bream Tail.
The Manaia stratovolcano breccia pinnacles, forming Mt Manaia and the ridge to the north at Whangarei Heads have regional geological significance as the most prominent exposures of Miocene volcanic breccia, and the best of two areas of ridge top tors in the Whangarei Heads area. They consist of weakly stratified andesite breccia, forming bluffs and spectacular pinnacles along Manaia ridge, remnants of cone facies of a stratovolcano. They are classified as an extremely well-defined landform of scientific/educational and scenic value (Kenny & Hayward, 1993).
The rocks on the foreshore 100-200 metres north-east of the wharf at McLeods Bay (Whangarei Heads) show an excellent exposure of deep water early Miocene flysch unconformably over Oligocene limestone, and itself overlain with erosional decollement by allochthon.
The Onemama Point allochthonous sediments, from the tip of Onemama Point (Whangarei Harbour) to 1.5 kilometres north-west, are excellent exposures documenting earliest allochthon emplacement in this region, dating from Oligocene and Miocene time.
The inter-tidal reefs running 200 metres along the south-eastern coast of Onerahi Peninsula are a well-exposed example of a weakly deformed block within the Northland Allochthon. They consist of overturned syncline with a core of sheared argillaceous limestone, and limbs of greensand, dating from Oligocene and Miocene times.
The coastal section between Waipu Cove and Laings Beach also has regional geological significance, as the sediments in the coastal low cliff and inter-tidal zone show a clear relationship between Waitemata flysch, Te Kuiti group limestone, and basement. Shallow water, bioclastic, sandy limestone unconformably sits on Waipapa greywacke, and passes abruptly and conformably up into deep water flysch. These sediments date from the Jurassic and Miocene.
Further south, Mathesons Bay (near Leigh) contains a geological exposure showing the onlap of early Miocene Waitemata sediments on the much older Waipapa Group rocks. This lies both below the tide line, and in the cliffs above. This is considered to be of national geological significance. One part exhibits an exposure of unusual chaliciform reef corals and fossilised eagle ray feeding pits. This is the richest locality in New Zealand for such fossils, and is therefore considered to be of regional importance (Auckland Regional Council, 1995).
Nearby, Ti Point (Whangateau Harbour) contains a volcanic exposure of geological significance (Auckland Regional Council, 1995).
A small geological exposure of fossils in Jurassic rocks is found on the Tawharunui Peninsula. Because this is rare in Northland, it is considered to be a site of national geological significance (Auckland Regional Council, 1995).
Flora & Fauna
The coast from Taiharuru Head to Takatu Point consists predominantly of open sandy beaches, indented by estuaries and harbours (the Whangateau, and most significantly, Whangarei Harbour), and interspersed with rocky shores and headlands (e.g. the coastline north of Ocean Beach, Bream Head, Bream Tail, the shore south of Pakiri Beach etc). These shore types provide a wide range of habitats for flora and fauna.
The flora and fauna typical of northern rocky shores have been described in some detail previously (refer to the section Cape Brett to Takatu Point – Flora and Fauna ). The rocky shore communities of this part of the coastline are similar to those found further north, and this description will not be repeated here. However, in a study of the distribution patterns of planktivorous reef fish (i.e. reef fish that eat plankton) along the coast of northeastern New Zealand by Kingsford (1989), contrasting patterns of abundance were found for 13 species along 100 kilometres of coastline, from the Poor Knights Islands in the north to Kawau Island in the south. Trends were apparent along the north-south axis in the study. Some species were more abundant at the northern end (e.g. two-spot demoiselle Chromis dispilis, and pink maomao Caprodon longimanus ) and others at the southern end (e.g. horse mackerel Trachurus novaezelandiae ). This probably relates to the geography and hydrology of the area (for example, the influence of the East Auckland current) (Kingsford, 1989).
Although more common further north, whales may also be found seasonally off the coast in this area. Recently, a large pod of Orca of a kind normally found only in Antarctica were observed in Bream Bay (INL, Stuff Northland web-site, 25 January 2001).
The soft shores of this part of the coast contain some coastal and estuarine areas of significance. Ocean Beach, Whangarei Harbour, Ruakaka estuary, Waipu estuary, and Mangawhai Harbour have been classified as habitats of high value by Ogle (1982). Pakiri River estuary, and the Whangateau Harbour have been classified as habitats of moderate to high value by the same researcher.
Ocean Beach, in the north of the area, is an exposed beach formed from a belt of semi-consolidated and consolidated sand dunes behind a mobile foredune, with associated small wetland areas and a small estuary at the northern end. The dominant vegetation includes Pingao ( Desmoschoenus spiralus ), marram and spinifex ( Spinifex hirsutus ) with coprosma, coastal herbs and small areas of wetland plants. Pingao is an unusual and primitive plant found only in New Zealand. It has no close relatives anywhere in the world. Furthermore, it is a taonga highly prized by Mäori weavers; its leaf-blades dry naturally to a beautiful and enduring yellow-gold colour.
Fauna at Ocean Beach include NZ dotterels, which are known to nest in the area, and variable oystercatchers, which have been recorded as breeding there. Caspian terns and a wide variety of other coastal and open country birds are also found.
Bream Head, Mt Lion and Mt Manaia provide a spectacular northern entrance to the Whangarei Harbour. Bream Head exhibits uninterrupted zonation from rocky seashore to up to 490 metres high, with forested slopes and rocky outcrops that provide habitats for a number of plants with restricted distribution. (This terrestrial environment is classified as a high value wildlife habitat for both flora and fauna (Ogle, 1982), as it contains the most extensive and intact coastal hardwood forest in Northland).
The Whangarei Harbour covers an area of 95 square kilometres, of which approximately 59% are uncovered as mud-flats, or sand or shell banks at low tide. The inter-tidal area consists of approximately 1379 hectares (ha.) of mangroves, 189 ha. of salt-marsh, and 3824 ha. of mud-flats (DoC, 1991). The presence of sheltered stony beaches on the northern side of the harbour provides additional habitat variation. There is a general temporal trend toward increasing area colonised by mangroves, most probably due to increased deposition of fine sediments. There have also been changes in the abundance of eelgrass ( Zostera ) over time. A survey in 1968 showed that eelgrass was extensively distributed along the Blacksmith Creek foreshore, along the channel at Marsden Point, at McLeod Bay, McDonald and Snake Banks, Motukiore Island, Parua Bay, the Tamaterau Banks (on the north side of the harbour), and One Tree Point and Takahiwai (on the south side of the harbour). A similar study in 1983 revealed only small areas of the harbour as being thinly colonised by eel-grass, with no extensive beds. However, eelgrass is now reappearing in some of these areas (DoC, 1991). The reasons for these changes in eelgrass abundance are uncertain: it has been suggested that the disappearance of eelgrass may have been due to increased sedimentation (noted in DoC, 1991). However, similar patterns of reduction and then increase in eelgrass beds have been observed in other harbours in Northland over the same time period, so it is possible that these changes relate to more widespread variables, such as climatic variation (author, pers. obs. ).
Large areas of salt-marsh are located on the southern shores of the harbour, and east of Onerahi. Dominant species in the salt-marsh include sea rushes ( Juncus maritimus ) , and jointed rushes ( Leptocarpus similis ), with some makaka ( Plagianthus divaricatus ). Large areas of the upper tidal flats behind the mangroves in the Skull Creek/Takahiwai area are covered by glasswort ( Salicornia ), with lesser numbers of shore pimpernels Samulus repens, Suaeda novaezelandiae, and Trichlochin striatum (DoC, 1991).
The Whangarei Harbour has been classified as a habitat of outstanding value (Ogle, 1982). Over 82 species of birds have been recorded in the harbour, which provides rich feeding grounds and isolated roosting areas that are of international importance. Twenty-three species of birds have been recorded as breeding in the harbour, including the rare NZ dotterel. The harbour hosts many thousands of migratory birds from the Arctic in summer and from the South Island in winter. Approximately 75% of the birds using the harbour are wading species. The most important areas for birdlife are located on the south side of the harbour, from Mangapai River through Takahiwai to Blacksmiths Creek and Marsden Point. In addition to the NZ dotterel, other rare or endangered species found in the harbour are banded rail, bittern, banded dotterel, red-necked stint, wrybill, Far-eastern curlew, Asiatic whimbrel, Eastern little tern, sharp-tailed sandpiper, curlew sandpiper, and fernbird (DoC, 1991).
The harbour also provides an important habitat for fish. The estuarine areas in the inner harbour are more highly utilised by fish than the outer or mid-harbour areas. Ten species of fish that use mangroves as nursery and feeding grounds have been recorded at the port of Whangarei, including yellow-eyed mullet, parore, kahawai, jack mackerel, juvenile anchovy, dab, yellow belly flounder, john dory, snapper, and eagle ray. Mangrove forests are migratory pathways for whitebait and inanga. Further out in the harbour, the shell banks with plentiful cockles and pipi also provide important feeding grounds for fish and birds (DoC, 1991).
To the south of Whangarei Harbour lies the long exposed beach bordering Bream Bay. Bream Bay itself is noted for its scallop beds, which are fished commercially. In the north near the harbour mouth, Mair Bank is one of the largest known pipi beds in New Zealand. This is an important feeding area for marine birds. Tuatua may also be found in shallow water along the beach (Dept. of Lands & Survey, 1982).
The extensive sandy beaches in Bream Bay are backed by sand dunes. Scattered patches of marram, spinifex and pingao colonize the dunes. Significant estuarine systems occur at Ruakaka and Waipu.
The Ruakaka estuary is dominated by mangroves, but eel-grass ( Zostera ) occurs in the low inter-tidal area. The area bordering the estuary to the north is regenerating to a coastal shrubland, and supports a wide variety of marine life. Plagianthus, Leptocarpus, and Juncus maritimus occur in a small inlet that extends into the dunes. Although comparatively small, the estuary provides excellent habitat for wading birds, including roosting and feeding areas. There is a high diversity of birds for the relatively small area - forty-five species of birds have been recorded in the mud-flat and salt-marsh areas. These include several rare or endangered species, notably NZ dotterel, banded rail, Terek sandpiper and variable oystercatchers. Fernbird probably nest in mangrove and saltmarsh areas. Other birds recorded include Caspian tern, crakes, red-necked stint, long billed curlew, banded dotterel, wrybill, common sandpiper, turnstone and fairy tern. Pipi and cockles are found within the estuary, which is noted as having an abundance of fish that come in to feed (Dept. of Lands & Survey, 1982; DoC, 1991). In addition to being recognised as a habitat of high value (Ogle, 1982), the area has been identified as an ecological area representative of a type that is becoming increasingly difficult to preserve in its natural state (DoC, 1991).
The Waipu River is a tidal river and estuary, in which salt-marsh grades into freshwater swamp in the upper reaches. Mangroves are colonising the estuary, and on the western side of the sand-spit lie large beds of Sarcocornia quinqueflora, Juncus maritimus and Scirpus. Zostera and Spartina are also present. Significant beds of pipi and cockles are also present (Dept. of Lands & survey, 1982). The estuary provides a habitat for a number of rare and endangered bird species, including NZ dotterel (known to nest in the area) red-necked stint, banded dotterel, wrybill, long-billed curlew, Asiatic whimbrel, Caspian tern, Oriental dotterel, and Siberian tattler. It is one of the few remaining areas where Fairy terns are known to nest (DoC, 1991).
Mangawhai Estuary is a sheltered tidal inlet, with shifting sands at the entrance. The estuary habitats are largely tidal sand-flats, with limited areas of salt-marsh grading to shrubland in the upper tidal reaches of streams. Shellfish in the low inter-tidal area include pipi and cockles. The estuary supports an excellent stand of mangrove and of Leptocarpus sedgeland. Urban development, farming and roads have significantly modified the margins of the estuary. The sandspit and adjacent island are classified as a habitat of national significance with at least 27 bird species recorded (Ogle, 1982). It is the only regular nesting area for the Fairy tern in New Zealand and supports the second largest colony of nesting Caspian terns in New Zealand. Other rare and endangered species include spotless crake, N.Z. dotterel, fernbird, bittern, wrybill and the N.Z. pipit (also recorded as nesting in the area) (DoC, 1991).
The pohutukawa in coastal areas at Mangawhai Heads were surveyed in 1988/89 as part of an assessment of the health of the species carried out by the Forest Institute for the Department of Conservation (Hosking et al. 1989). This survey indicated that possum damage was significantly impacting on the health of the crowns of the trees. In a recent re-survey one plot at Mangawhai Heads still showed significant possum damage (Hosking, 2000).
Scattered patches of marram, spinifex and pingao colonize the dune system that forms the Mangawhai-Pakiri barrier beach. The shore has historically been unstable with a period of erosion during 1940 to 1960, then accretion in the late 1970's. Over the last 20 years, there have been fluctuations in shore position, but an overall net build-up of sand in the foredune and beach (Hume et al., 1998).
The mouth of the Pakiri River reaches the sea through an area of mobile sand dunes. Behind the sand dunes is an area of salt marsh and reed swamp which has been classed as a moderate to high value habitat (Ogle, 1982). Species present from the lowest tidal limit to the dunes include Triglochin, Juncus maritimus, Sarcocornia quinqueflora, Samolus repens, Ammophila arenaria with Calystegia soldanella, and Spinifex. Other species recorded include Plagianthus divaricata, Leptocarpus similis, Baumea tenax, Selliera radicans, Scirpus nodosus, Carex sp., Pingao (Desmoschoenus spirilis), Scirpus pumila, Muehlenbeckia complexa, lupin and flax. Rare and endangered species recorded include fernbird, NZ dotterel (both of which have been observed to breed in the area), banded rail, and Fairy tern. Pukeko, white-faced heron, blue reef heron, pied stilt, variable oystercatcher, bittern, and a variety of gulls and shags also occur (DoC, 1991)
The Whangateau Harbour is a tidal river and estuarine area, containing mud-flats, and pockets of mangroves, with saltmarshes on the margins. It is partly continuous with forested habitat on Omaha Spit. The mud-flats support the usual assemblage of marine fauna typical of this type of habitat. Cockles are common in Lews Bay. Banded rail occur in vegetated areas, and some migratory wading birds feed on the mud-flats. It has been noted that this area is subject to heavy pressure from fishing and recreation (Ogle, 1982), and this is supported by a recent study of non-commercial harvesting of cockles in the harbour. A study was undertaken by Kearney (1999) of the ecology and management of cockles in the Whangateau Harbour. This study was conducted over a 12-month period from December 1997 to December 1998, using structured sampling regimes and interviewing all harvesters on the beach during sampling times. From the data collected in his study, Kearney estimated that the total annual harvest of cockles from Lew's Bay, Whangateau Harbour, from December 1997 to December 1998 was 27,950 kg. (It is pertinent to note that the results of a Ministry of Fisheries 1993-94 Recreational Fishing Survey (based on data from a telephone and diary survey), estimated that the total non-commercial cockle harvest in Fisheries Management Area QMA1 (Cape Reinga to Cape Runaway), was a total of 55 tonnes per year (Annala & Sullivan, 1996a). This seems low, if 27.9 tonnes are harvested from Lew's Bay alone).
References:
Applied Geology Associates. 1982. Coastal sand and shingle resources of Auckland and Northland. Unpublished report to the Auckland Regional Authority, April 1982.
Annala, J. H. and K. J. Sullivan (Comps.). 1996a. Report from the Fishery Assessment Plenary, April-May 1996: Stock assessments and yield estimates. 308 pp. Unpubl. report held in MAF Fisheries, Greta Point library, Wellington.
Auckland Regional Council. 1995 . Proposed Auckland Regional Plan: Coastal. Environmental Division, Auckland Regional Council.
Department of Conservation (DoC). 1991. Coastal wetland inventory: Volume 2: Coastal wetlands north of the Kaipara Harbour catchment (Draft). Department of Conservation, Wellington.
Department of Lands and Survey. 1982. Bream Bay management plan. Dept. of Lands & Survey, October 1982. 104pp.
Hosking, G. P., Hutcheson, D. M. A & Herbert, J. W. 1989 . Conservation of pohutukawa: Regional assessment. Report to Director Science & Research, Department of Conservation.
Hosking, G. 2000. Measuring our success: A reassessment of Pohutukawa health ten years on. Report of an assessment commissioned by the Project Crimson Trust. April 2000. Available on the Internet from: http://www.projectcrimson.org.nz .
Hume, T. M. & Herdendorf, C. E. 1988. A geomorphic classification of estuaries, and its application to coastal resource management - a New Zealand example. Ocean and Shoreline Management. 11 :249-274.
Kearney, M. B. 1999. Ecology and management of Austrovenus stutchburyi in the Whangateau Harbour. M.Sc. Thesis, School of Environmental and Marine Science, University of Auckland. 135 pp.
Kenny, J. A. & Hayward, B.W. 1993. Inventory of important geological sites and landforms in the Northland region. Geological Society of New Zealand Miscellaneous Publication No. 67 (2 nd Edition).
Kingsford, M. J. 1989. Distribution patterns of planktivorous reef fish along the coast of northeastern New Zealand. Marine Ecology Progress Series. 54 :13-24.
Ogle, C. C. 1982. Wildlife and wildlife values of Northland. Fauna Survey Unit Report No. 30. New Zealand Wildlife Service, Dept. of Internal Affairs, Wellington.
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