THE OCCURRENCE AND INTERPRETATION OF CARBONATE AND SULFATE MINERALS IN A SEQUENCE OF VERTISOLS IN NEW-CALEDONIA

In the Tamoa Valley, on the western coast of New Caledonia, a Vertisol sequence shows a downslope transition from calcimagnesic Vertisols co ntaining gypsum, aragonite, and magnesium-calcite, derived from flysch colluvium, to hypermagnesic Vertisols, rich in dolomite and magnesite , and derived from peridotite and serpentinite alluvium deposited by t he Tamoa River. Gypsum, magnesium-calcite and magnesite were probably formed during the dry period of the last glaciation, between 30,000 an d 17,000 yr B.P. Magnesite nodules seem to be allochtonous, and were f ormed upstream by the weathering of serpentinite. During the more humi d period of last Flandrian transgression (5000 yr B.P.), floods of the magnesium-rich waters of the Tamoa River over the alluvial plain brou ght magnesium to the soils and lead to fluctuations of the water table . These conditions favoured the formation of manganese deposits as man gans, calcite pseudomorphs after lenticular gypsum, crystallization of authigenic barite in calcimagnesic Vertisols, and dolomite formation in magnesic and hypermagnesic Vertisols. During the recent period, app arently drier, aragonite is forming in the lower part of calcimagnesic Vertisols.