9 Direction of Sustainable Development of the South Province Territories I Strolling along the waterways Water circulation 8 Strolling along the waterways Water, the founder of landscapes Shaped by time Shaped by water The geological base Shot and armour result from the accumulation of iron at the top of the profile. Iron, poorly soluble, is not leached by water and accumulates in the first centimetres of the profile. - Permeability - Rapid infiltration - Temporary flow buffer stratum with formation of surface layers in the event of rain - Weathered rock facilitates water circulation - High storage capacity which regulates flows and limits flooding - Supply of slope sources and deep drainage of massifs - Conductive layer - High porosity - Low permeability - Capacitive layer Horizon composed of gravel, coming from the dismantling of the armour. From rock to ground, where does this typical colour come from? The red colour of soils in the Great South is due to the presence of oxidized iron: rust, in a way. In our humid tropical climate (with high temperature and heavy rain), the parent rock (peridotite) weathers and leads to the formation of these so-called lateritic or iron soils topped with a crust. Also called red earth, poor in nutrients (naturally deficient in nitrogen, phosphorus, potassium, calcium), but rich in heavy metals (nickel, chromium and cobalt) potentially toxic for plants, these soils are at the origin of a drastic environmental selection which results in a high rate of endemism. The flora and habitats that develop there, resulting from a long evolution, constitute some of the most original ecosystems on the planet, including very rare, even threatened, species. Laterites are the products of more advanced alteration. We distinguish the yellow ones, exploited by the mining industry, and the red ones. They are the first phase of alteration. They retain the structure of the parent rock but are very friable. The mining industry exploits them, this level is even richer in nickel than laterite. Rocks from the earth’s mantle, called ultrabasic (green-blue in colour, rich in iron, magnesium, nickel, cobalt, chromium and manganese and poor in silica). Schematic profile showing the succession of horizons or alteration products of Peridodites (Illustration based on the Jeanpert diagrams, 2017) Schematic profile showing the principles of hydrogeological functioning The armour Aquitard semi-permeable The gravel Upper aquifer superficial 3 m 15 m 25 m 30 m 40 m The laterites Red ones Laterites Fractured saprolites & peridotites Armour Deep aquifer fractured to karstified Yellow ones The saprolites The peridotites The peridotites Essentially made up of a green mineral -olivine-, the colour of peridodites is not always easily perceptible for two main reasons: • The green of peridotites is extremely dark, almost black. • When this rock weathers, it takes on a patina and the colour of this patina is orange. The presence of these mantle rocks on the surface testifies to a geological phenomenon called obduction*. While they are rarely observed in the rest of the world, they cover more than a third of the surface of New Caledonia’s Main Island (Avias, 1967). The landscapes of the Great South offer striking contrasts created by geological history which directly influence soil characteristics and local biodiversity. Sculpting the reliefs, forming the geological profile of the subsoil, constraining movement and human settlement, water reveals itself to be a natural constituent of the great South’s landscape. A surface hydrographic network Seen from the sky, the Great South presents a dense hydrographic network, scattered with multiple bodies of water, lakes and marshes, a genuine mosaic of silvery or blue-green spots where some waterways seem to slip through, get lost, dilute and resurface. These disappearances in fact reveal a very complex hydrogeological functioning invisible on the surface. A unique example on a global scale The alteration of peridotites has led to the development of real karst* type models where water infiltrates mainly into the subsoil, instead of flowing on the surface, thus digging numerous underground paths or cavities. The Great Lakes region is therefore based on an original underground system comparable to that which models the limestone regions but here, the limestone is replaced by peridotite, which does not completely dissolve, and its coat of lateritic alteration. It is called “peridotitic pseudokarst”. Karst landscapes are characterized by the existence of: • sources and resurgences, • losses or dry valleys (openings through which a watercourse becomes underground after a course in the open air), • sinkholes, which can be defined as “more or less circular closed depressions that can be a few meters to more than a kilometre in diameter and a few meters to several hundred meters deep” 1. A fragile system that is still poorly understood Sinkholes appear to connect surface water and groundwater systems. This phenomenon therefore makes the aquifer particularly vulnerable topollution. Theundergroundcirculation of water can be rapid, and its poorly identified path without corresponding to the surface watersheds, further increases the fragility of this system. * Karst system: region of limestone formation characterized by the preponderance of underground drainage and by the development of an original topography due to the dissolution of the rock under the action of water (caves, chasms, resurgences, etc.) 1 Diagnosis of sinkholes in New Caledonia by the CNRT, 2018 * Obduction: geol. Overlapping of an oceanic plate over a continental plate.
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