The soft bottoms of the lagoons of New Caledonia are made up of sediments of two origins : on the one hand the terrigenous contributions due to the rain erosion of the laterite crust of Grande-Terre, on the other hand the degradation by the swell of the coral reefs composed of carbonates. This double origin of the particles leads to a triple zonation of the lagoon bottoms with a silted coastal area, an area of white carbonate bottoms and, in-between, a mixing area of gray bottoms.