Sediment distribution and composition on the shallow water carbonate basin of the Zanzibar channel
Sediments of the shallow water carbonate basin in Zanzibar channel were investigated for composition and grain size distribution. The surface sediment composition was dominated by carbonate sands (with CaCO3 > 30%), except in the area adjacent to mainland coastline and a thin lobe which projects from Ruvu River to the middle of the channel. Mean grain size distribution closely resembles that of the carbonate content, where Tidally Dominated Reef Platform Sediments (TDRPS) located east of the Zanzibar channel were characterised by medium to coarse sands and the siliciclastic sediments adjacent to the mainland were characterised by fine sand. The TDRPS are the most poorly sorted sediments with sorting values between 1.2 and 1.6 phi. The present study highlights existence of major differences between the eastern and western sides of the channel. Sediments on the eastern side of the channel, that are predominantly biogenic, were characterized by grain size frequency curves without any prominent mode. Sediments on the western side of the channel are composed of both biogenic and terrigenous material. The grain size frequency curves of these sediments have a fine mode and usually a coarse tail.