In a 20 m deep back-reef lagoon situated along a sandy coast north of Dar es Salaam, gradual transitions from siliciclastic to carbonate sediments occur within a distance of a few kilometres from the shorcline. Carbonate is supplied to the lagoon as debris mainly derived from fringing and barrier coral reefs and by autochthonous biogenic production. Moderate rainfall of 1.1 m/a leads to a low influx of siliciclastic sediment by seasonally flowing small rivers from the 200 m high hilly coastal hinterland. The zone of siliciclastic domination is confined to a 1–2 km wide strip roughly paralleling the coastline and only extending farther out into the lagoon off river mouths and tidal channels, where sediment is carried seawards by strong ebb currents. No terrigenous sand is transported towards the outer shelf through the passages between the islands and coral reefs which protect the lagoon from the open sea.