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1. Landform evolution in New Caledonia
New Caledonia is an island group in the south-west Pacific consisting of one large mountainous island (known as La Grande Terre; area: 16,750 sq km) and over a hundred small associated islands, including the lies Belep to the north-west and the Ile des Pins to the south-east. Its coastlines, about 1,000 km long, are bordered by extensive coral reefs, including major barrier reef systems off the east and west coasts (fig. 1). The east coast is generally steep and mountainous, while the west coast has foothills and wider coastal plains. Major river systems are shown in figure 2.
FIG. 1 Geomorphology of New Caledonia
FIG. 2. Major River Systems in New Caledonia
Lying just north of the Tropic of Capricorn, New Caledonia has a generally warm and equable climate, with cooler conditions in the interior uplands; south-easterly trade winds prevail for much of the year, producing annual rainfalls of more than 2,000 mm on much of the east coast, rising to more than 3,000 mm in mountain ranges, while the west coast, in rain shadow, receives less than 1,000 mm. There is a winter dry season which lengthens and intensifies northwards, especially along the west coast, where some areas have warm semi-arid climates and are subject to prolonged droughts. Tables 1 and 2 give the mean monthly temperature and rainfall data for selected stations. In the summer months recurrent depressions bring hot and wet conditions, and occasionally tropical cyclones move from the north or north-east across New Caledonia, bringing violent winds and torrential rains followed by extensive river flooding. The most recent of these was Cyclone Gyan, which crossed northern New Caledonia at Christmas 1981, bringing winds of up to 170 km per hour at Koumac, where 429.7 mm of rain fell in one day; at Kaala-Gomen, to the southeast, this cyclone brought over 500 mm of rainfall; and heavy downpours were registered at many stations in the northern half of the island, followed by extensive river flooding.
It is likely that the natural vegetation of New Caledonia was largely forest (rain forest on the mountain ranges and along the wetter east coast; sclerophyllous forest on the west coast) giving place over about 30 per cent of the land surface to maquis (scrub) on ultrabasic massifs covered with ferruginous lateritic soils and on micaschist and chert outcrops with acidic soils. These soils are poor in lime, potash, and phosphates; on ultrabasic massifs they are locally rich in nickel, cobalt, and chromium compounds. Such soils set edaphic limits to tree growth, notably on high plateaux such as the Dome de Tiébaghi in the north and in interior basins such as the Plaine des Lacs in the south. West of the ranges the drier foothills probably sustained woodland communities, grading into savanna and shrubland on the coastal plains.
TABLE 1. Mean Monthly Temperatures (°C) for Selected Stations, 1956-1975
Source: National Meteorological Office
TABLE 2. Mean Monthly Rainfall (mm) for Selected Stations, 1956-1975
Source: National Meteorological Office
Of botanical interest because of the numerous endemic species as well as those with Australian and Melanesian affinities, the existing vegetation has been profoundly modified by human activities (plate 1). Extensive areas were cleared and burned by Melanesian farmers, whose cultivation terraces are widespread, especially in the foothills west of the main ranges. There was further cleaning and burning as well as the introduction of grazing animals and the development of mining activities after the arrival of the first settlers in the 1860s. The outcome is that forests now occupy only about 18 per cent of the land surface of New Caledonia, persisting mainly in the uplands, including the higher parts of some of the river valleys. Large areas that formerly carried forest or woodland have been converted to savanna dominated by swamp paperbark trees known as niaouli (Melaleuca quinquenervia) or by mimosa (Leucaena leucocephala), gaiac (Acacia spirorbis), or lantana (Lantana camera) scrub. or to open grassland, by repeated burning and by stock grazing.
Moreover, opencast mining of nickel and iron ores on hill crests and plateaux has inevitably resulted in destruction of the vegetation cover, not only in the quarried areas but also on surrounding hill slopes mantled by quarry waste and downwash (plate 2), and on the floors of neighbouring valleys (Jaffre et al. 1977). In addition, the existing vegetation cover has been extensively disrupted by landslides, gulleying, and associated fan deposition in areas where erosion was initiated or accelerated by such activities as road-making (plate 3), mineral prospecting, and the depletion of natural vegetation by clearing, grazing, and burning.
It is possible that some of the hill-slope erosion so widely evident in New Caledonia is the outcome of natural geomorphological processes, especially in the Massif du Sud and in the northwest, where the climate is relatively dry and tropical cyclones have generated torrential runoff and locally severe erosion at infrequent intervals, and on some steep slopes on deeply weathered formations. But the onset of widespread erosion during the past century and a half has been largely the outcome of human activities in an environment where the relative stability of steep hillsides mantled by deep weathering materials evidently depended on the persistence of a retentive natural vegetation (cf. Iltis 1979).
An assessment of the direct and indirect effects of opencast mining and other activities on the landscapes of New Caledonia, and in particular on river mouths and coastal systems, is best preceded by an account of the evolution of these landforms.
New Caledonia is geologically complex (Paris 1981), with areas of strongly folded sedimentary and metamorphic rocks and massifs of ultrabasic formations, mainly peridotites and serpentines, with overlying gabbros and intrusions of granodiorite in the southern regions. In addition, there are dissected hilly areas of Tertiary basalt, and extensive Quaternary piedmont fans and alluvial deposits, especially on the west coast, which has a lowland fringe of variable width in contrast with the generally steep east coast.
Evolution of the landforms of New Caledonia has been related partly to geological structure and component formations, and partly to a historical sequence of tectonic movements as well as climatic and sea - level fluctuations (IItis 1981). Figure 1 shows the pattern of mountain relief, hill relief, depositional landforms, and coral reefs that make up New Caledonia. The basement rocks, which outcrop in the central and northern high-lands, are mainly Palaeozoic schists, Mesozoic sandstones, greywackes, and limestones; they were uplifted, folded, and faulted by Lower Cretaceous earth movements, then deeply dissected by erosion. They were overlaid during Eocene times by ultrabasic formations, the origin and emplacement of which are still not fully understood. It is now widely thought that they originated as crustal material on the ocean floor and were pushed up and over the basement rocks by overthrusting due to collision of the Indo-Australian and Pacific plates; such overthrust rock masses are known as "nappes." Basalts which form hilly country between Bourail and Koumac on the west coast were evidently extruded in Eocene times as this overthrusting took place. The ultrabasic formations were then disrupted by a network of faulting due to revived tectonic
movements. They have subsequently been deeply weathered and dissected by river valley incision, largely guided by the fault pattern, leaving residual steep-sided massifs. The resulting distribution of ultrabasic formations is shown in figure 3.
Ultrabasic rocks are ferromagnesian silicates, often with associated metallic compounds, which originally formed deep in the earth's crust. They include peridotites, which are olivine-rich nonfelspathic rocks, often modified into serpentine, a hydrated magnesium silicate, by processes of metamorphism due to intense heat or pressure. In New Caledonia these formations, emplaced by overthrusting, have persisted as upland areas. Since Oligocene times the uplands of New Caledonia have remained above sea level, and the ultrabasic rocks show subaerial weathering to depths of 20 to 30 metres with the formation of thick lateritic profiles in which are found the nickel, chromium, cobalt, and iron ores that have been widely mined. The plateaux which cap these massifs at various levels between 500 and 1,600 metres above sea level show peridotitic karst features, including enclosed depressions (pseudo-dolines) formed by the leaching of magnesium silicate beneath a porous ironstone duricrust (Wirthmann 1970). This ferruginous duricrust (50 to 80 per cent iron hydroxides) is often several metres thick, and rainfall percolating rapidly through it is responsible for solution and karst development. These features are well displayed in the lowland region of the Plaine des Lacs in southern New Caledonia, where solutional lowering of the land surface within faultbounded depressions has produced almost enclosed basins that resemble limestone poljes.
FIG. 3. Peridotites and Associated Formations in New Caledonia (from Atlas of New Caledonia)
Stages in the denudation of the New Caledonian highlands are marked by successive partial planation surfaces formed during episodes of stability in Tertiary times. Prolonged subaerial denudation during the Quaternary has led to the deposition of extensive terraces and colluvial piedmont fans in front of steep slopes, particularly in regions inland from the west coast of New Caledonia. The sequence of sediments deposited in these piedmonts indicates alternations of relatively rapid slope wastage, when torrential streams washed down broad fans of relatively coarse material ranging from sand gravel to cobbles and boulders, and episodes of gentler activity, when downwashed sediment was mainly sand, silt, and clay and weathering profiles developed on relatively stable depositional surfaces on the piedmont fans, with some subsurface accumulation of salts precipitated from groundwater, notably gypsum (calcium sulphate) and magnesium compounds. Phases of relatively rapid erosion in the uplands may have been initiated by tectonic uplift, but in general these alternations seem to have been mainly the outcome of climatic fluctuations. The most intensive erosion occurred as the result of increased runoff, either in periods of higher annual rainfall or, more probably, in brief episodes of torrential discharge in a more arid environment, where the vegetation cover was sparser, and soils and surface weathered materials more readily eroded. The intervening quieter episodes imply a gentler runoff, probably with a more luxuriant vegetation, sufficient to impede rainfall and runoff, and retain the soil and weathering mantle on all but the steepest slopes.
It is likely that under natural conditions (i.e., without any modification of the land surface and vegetation cover by human activity) these piedmont fans would have remained relatively stable in the present climatic environment. Runoff from forested uplands would now be carrying only small quantities of relatively fine-grained sediment down the narrow watercourses across the piedmont fans and into the sea. Such features are still seen locally on west coast piedmont fans, where the upper catchments of streams remain undisturbed by mining activities and fairly well vegetated, and it is these watercourses that provide a standard of reference against which the widespread modifications of channel form and sediment yield in recent decades can be gauged.
The coastline of New Caledonia has small tide ranges, generally less than a metre, and wave action related to local, mainly easterly or southeasterly trade winds in coastal waters protected by extensive reef formations (Bird and Iltis 1983). It is dominated by features produced by submergence during the Holocene marine transgression, the last of a series of major sea-level oscillations that took place in the world's oceans during Quaternary times. Each time the sea level fell, New Caledonia was enlarged by the emergence of bordering sea floor areas; rivers extended their courses seaward and incised their valleys towards the lowered base levels (Launay 1972); coral reefs were exposed as limestone ridges subject to karstic dissection of the kind that is now seen on the uplifted atolls of Lifou and Mare in the Loyalty Isles to the north (Davis 1925). When the sea level rose, these features were submerged; some higher areas persisted as promontories and offshore islands, while intervening lowlands became embayments (e.g., the Baie de St. Vincent on the west coast) and the mouths of incised valleys became rias (e.g., the Baie de Prony in the far south and the fault-bounded Baie de Canala on the east coast) or estuarine inlets (e.g., the Nimbaye estuary at Ponerihouen and several other river mouths on the east coast). Some of these drained areas have subsequently been infilled with sediment to form deltaic plains and protruding deltas, especially on the west coast (e.g., the Dumbéa, Ouenghi, louanga, and Koumac deltas), but also on the southern half of the east coast (e.g., the Kouakoué, Ni, Ngoye, Comboui, Thio, Nakéty, Canala, Kouaoua, Koua, and Houailou-Neaoua deltas). As a generalization, rivers that drain catchments which include deeply weathered ultrabasic formations had built deltas, whereas those draining Mesozoic and Tertiary sedimentary and volcanic regions, where the weathered mantle is relatively thin, flowed into estuarine mouths or rias, indicating a natural contrast in the regional sedimentyield regime, based on geological features, prior to any impact of mining activities. As will be seen, human activities have accelerated sediment yields in many areas, but the acceleration has been much more emphatic where the highly erodible weathered mantles of ultrabasic massifs have been disturbed by opencast nickel and iron ore extraction, augmenting sediment yields to rivers and their deltas.
The Holocene sea-level rise has been followed by revival and renewed growth of coralline barrier and patch reefs offshore, thereby augmenting structures that had been developing intermittently through Pleistocene times, and the initiation of Holocene fringing reefs along the present coastline (Chevalier 1973). However, the south-east coast near Yaté is bordered by an emerged Pleistocene fringing reef up to 10 metres above sea level, and similar emerged reefs up to 20 metres above sea level encircle the Ile des Pins to the south. According to Launay (1982), these fringing reefs originated during interglacial phases of the late Pleistocene when the sea stood close to its present level, and have been raised to their present positions by subsequent tectonic movements; the reefs on the Ile des Pins have been dated at 120,000, 165,000, and more than 200,000 years B.P. by isotopic analyses (Launay 1982). It is thus evident that the south-east coast and the adjacent Ile des Pins have had a different tectonic history from the rest of New Caledonia, subsiding during the phase of general Neogene uplift and emerging during the phase of general Late Quaternary subsidence.
Much of the east coast is steep, particularly where there are fault scarps 300 to 400 metres high bounding the ultrabasic massifs, which descend abruptly to the sea with gradients of more than 50 degrees. Basal marine cliffing is limited, except on a few promontories exposed to stronger oceanic waves entering through gaps in the barrier reefs (e.g., on the seaward flank of Cap Bocage). The boldest cliffs, up to 40 metres high, are on Cap Ndoua in the far south, where a broad gap in the outlying reefs permits the prevailing south-easterly ocean waves to reach the coastline. The steep coasts bordering the ultrabasic massifs are interrupted by inlets and branching rias at the mouths of river valleys between Yate and Ponérihouen. Some of the valley mouths have deltaic features produced by fluvial deposition (e.g., Thio) or spits and beach ridges where wave action has reworked fluvially supplied sands and gravels (e.g., Kousoua). Further north a sector of hilly coastal country on strongly folded Mesozoic sandstones and volcanic rocks has gentler coastal slopes bordered by a low depositional terrace with fringing coral reefs, prominent on the Cap Bayes peninsula and in the Touho district. Beaches of coralline sand and gravel extend along the reeffringed coastline, and beaches of fluvial sand and gravel occur near river mouths. The rivers widen to estuaries fringed by mangroves, and their mouths are often encumbered by sandy spits that have locally been prograded as beach-ridge plains (e.g., Nimbaye, Tchamba, Amoa, Tiwaka). The adjacent sea floor is also dominated by terrigenous sediments, mainly silt and clay washed into the sea by rivers draining a steep and rainy hinterland; but further out, towards the barrier reefs, calcareous coralline sediments prevail (Guilcher 1963). To the north, alignments of steep-sided karstic pinnacles and towers of Eocene limestone, with basal notches and caves cut by marine erosion, distinguish the Hienghène coast, where rivers enter the sea by way of deep winding rias formed by the Holocene marine submergence of previously incised river valleys. North of Hienghène the coast is again steep along the flanks of the metamorphic Panié massif, a high range of metamorphic rocks. Bold spurs are separated by deep valleys, several of which have spectacular waterfalls where streams flow across massive rock outcrops. The spurs have been truncated by steep coastal slopes, undercut at the base by active cliffs a few metres high, especially near Ouaième where a broad gap in the barrier reefs allows stronger oceanic waves to reach the coastline. Further north the Massif du Panié declines, and its seaward slopes are bordered by a coastal plain between Pouébo and Balade, a depositional lowland partly covered by mangrove forests that are protected from the open sea by fringing and barrier reefs. The ranges then decline further through the Pam peninsula, their trend being continued by the outlying high island of Balabio. Fringing and nearshore reefs, and shoals in the lee of the barrier reef (Récif de Cook) are extensively exposed at low tide.
In northern New Caledonia, the country's longest river, the Diahot (length: 90 km), flows into a gulf that occupies a structural trough between the Balabio and Arama ranges. Wide swampy plains have been built by deltaic deposition from the Diahot, which has extended its levees of silt and sand seaward in such a way as to enclose depressions occupied by extensive mangroves backed by salt pans that are subject to intensive evaporation in the long dry season. Associated depositional features extend along the eastern shores of the Arama peninsula, which gradually declines and narrows to Pointe Narian. Ile Baaba (mainly sedimentary) and the lies Belep (entirely of ultrabasic rocks) are outlying high islands in the same structural alignment.
In spite of the proximity of steep-sided ultrabasic massifs, the northern half of the west coast of New Caledonia is low-lying, consisting of gently sloping Pleistocene piedmont fans that have been partially submerged by the sea, and alluvial valley floors built by fluvial deposition. Rivers such as the Néhoué and the Poya have built intricate mangrove-fringed deltas bordered by tidal mudflats, and mangroves also occur in sheltered inlets and embayments, in the lee of headlands and offshore islands, and alongside estuaries and shallow lagoons down the west coast. About 20 species of mangroves are present in New Caledonia, but Rhizophora and Avicennia are the most common on this coast, with large stands near Poum and in the Baie de Néhoué. Behind the mangroves there are sparse salt marshes and extensive unvegetated salt pans, submerged only at the highest tides or after widespread river flooding and dessicated during the dry season that here extends from April to December. Beaches occur intermittently, some consisting mainly of coral sand derived from nearby reefs, others of fluvial sand and gravel or material washed out of the piedmont deposits. The diversity of coastal sediments around New Caledonia is due partly to geological variety in the hinterland, partly to intricacy of coastal configuration, and partly to the sheltering effects of the surrounding reefs, which generally exclude strong wave action and have prevented shoreward drifting of sea floor sediments (cf. Bird 1972).
The Koumac River has delivered substantial quantities of chert (phthanite) gravel, derived from Cenozoic outcrops and Pleistocene terraces and piedmont fans in its upper catchment, to its deltaic coastline, and beaches and spits of this material have grown northwards from there in response to longshore drifting by the prevailing southerly waves in coastal waters. By contrast, the Néra delta has a gently-curving sandy shoreline behind the Baie de Gouaro, shaped by ocean swell arriving through a broad gap in the barrier reefs.
Further south the hinterland becomes hilly, and some higher areas run out as promontories, such as the limestone plateaux of the Muéo and Népoui peninsulas, separated by shoaly bays into which rivers are carrying sediments and protected to seaward by coral reef formations. South of Bourail the promontories become more numerous (Mare, Lebris, Ouano, Bouraké), and there is local cliffing on sectors exposed to stormy wave action, especially when there are gaps in the barrier reef, as at Roche Percée near Gouaro. Broad intervening bays have wide intertidal zones occupied by extensive mangroves and salt marshes, as at Mara (Baltzer 1965). Some of these are threaded by wavebuilt cheniers of sandy material. The Baie de St. Vincent has been formed by marine submergence of an undulating plain, the higher parts of which persist as promontories and hilly islands; it is well protected by outlying barrier reefs, and marine cliffing is limited to small sectors on the seaward margins of steep-sided promontories and islands. The Ouaméni, Ouenghi, and Tontouta rivers are building mangrove-fringed deltas into bordering embayments, and mangroves are also present on the lee shores of some of the islands; but comparison of outlines shown on Banare's chart of the Baie de St. Vincent, made in 1863, with modern air photographs (1954, 1976) shows only minor changes in coastal features, chiefly around the river mouths. Beaches of fluvially supplied sand occur locally, as do beaches of cliff-derived material and, adjacent to reef areas, coralline sands. Bay-floor sediments show related patterns of terrigenous and reefal material (Dugas 1974). On some of the outer islands there are deposits of Pleistocene dune calcarenite, formed from wind-blown calcareous sands that accumulated during the late Pleistocene low-sea-level phase, when the climate was drier than it is now (Coudray 1976). Dunes are otherwise very rare in New Caledonia.
South from the Baie de St. Vincent the coast is generally hilly and indented by bays and inlets, notably the intricate Baie de Dumbéa, which is flanked by long narrow peninsulas, the delta of the Dumbéa River developing at its head. Here Baltzer (1969) has identified zonations of mangrove and salt marsh vegetation related to the depth and duration of intertidal submergence (see fig. 19). The hilly Nouméa; peninsula is reef-fringed, its outer embayment shaving sandy beaches of tourist importance culminating in the cuspate foreland south of the Anse Vata, while the inner embayments, more sheltered, have sectors of mangrove shore. Beyond the Baie de Boulari rise the steep slopes of Mont Dore, an outlier of ultrabasic rocks associated with the Grand Massif du Sud. This marks the commencement of bolder coasts, with valley-mouth inlets (Rivière de Pirogues) and branching rias (Baie de Prony). Ile Ouen is a steep-sided high island, also of ultrabasic rock, beyond which the barrier reefs (Grand Récif du Sud) loop seawards to enclose a sea area dotted by patch reefs, some of which carry low sandy cays (Guilcher 1965). As has been noted, the cliffed coast of Cap Ndoua faces deeper and stormier seas, but to the east a chain of reefs with intervening deep channels extends to the Ile des Pins, where emerged fringing reefs form a lowland encircling the hilly area of Pic Nga, another outlier of ultrabasic rocks.
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