The skeletal composition of 273 sediment samples, collected within 14615 km2 of lagoon habitat in New Caledonia (Ouvea and Chesterfield atolls and eastern and northern lagoons of the main island), was analyzed. Major constituents were molluscs (bivalves and gastropods), foraminifers, and Halimeda plates. The quantitative examination showed that, even in a pure coralline structure such as the two atolls studied, coral debris and calcareous algae, potentially produced within the barrier reef, never constituted a dominant element in the lagoonal sediments. Distribution of coral debris showed that coral is significant only close to the barrier reef (i.e. passes and back-reef slope). From the point of view of sedimentology, this suggests that the major role of the barrier reef is to provide a physical barrier that allows the development and preservation of lagoon sediments. Sedimentation within the lagoon of grains coarser than 63 µm is the result of in situ organic production combined with low hydrodynamic control.