Knowledge of the geometry, dynamics and ecology of carbonate-siliciclastic transitions is a useful tool for prediction of the impact of sediment runoff on carbonate production. After heavy rains at Barbados, fresh water containing sand and mud is debouched on to the east coast insular shelf. The coarse-grained material is transported on to a sand ramp, over the shelf break, or incorporated into the adjacent beaches. The fine-grained sediment is transported along the shore within a sediment plume and is deposited on to the spur and groove system and deep water reefs of the northeast coast insular shelf. The shallow water facies transition is abrupt (within 30m laterally), is located on a submerged barrier reef, and is defined by a change in lithology, textural properties of the sediment and benthic biota. The deepwater transition is gradual (over 200m laterally). The main controls on the configuration of the facies and the vigor of the reefs are: 1) the prevailing hydrodynamic regime, 2) shelf floor topography, 3) textural properties of the sediments, and 4) distance from the source of terrigenous sediment. The sil-icielastic facies and the reefs of the carbonate facies accumulate as adjacent, contemporaneous and compatible facies.