Sedimentary environments and unconsolidated carbonate sediments of the fringing coral reefs of Mahé, Seychelles

Abstract

Extensive fringing coral reefs occur around Mahé: the largest island in the Seychelles Archipelago. Several sedimentary environments, which are best developed on reefs exposed to the Southeast Monsoon (the dominant wind), are associated with the reefs. These are: the beach; rippled sand zone; marine grass bed: radial zone; algal ridge; reef edge; reef slope; “plateaux”; mangrove and former mangrove swamps; deltas; channels; the fore-reef environment. Skeletal carbonate sands form a veneer on the reef flats and are widespread in the fore-reef environment and locally they pass into quartz sands. Skeletal debris from sessile organisms in the reef flat deposits is only abundant landward of the areas in which they live: indicating that current transportation is dominantly landward, with little material being carried from the reef flats into the fore-reef and channel environments. It is often difficult to relate the grain-size measures of the sediments to their environment of deposition. The turbulent conditions in the beach, algal ridge and reef edge environments are reflected in the absence of fine material and the low dispersion values of the associated sediments. Grass bed samples have high dispersion values and are relatively rich in fines trapped by the grasses. Radial zone samples show much variation in grain-size measures due to the diversity of the conditions of deposition. The skewness in mangrove swamp sediments is positive, indicating normal current deposition. Sheltered fore-reef deposits tend to be rich in fines and to have positive skewness values due to the quiet conditions, whereas fine material is winnowed from windward fore-reef sediments.