A structure that forms in shallow water where there is extensive deposition of carbonate that is not mixed with siliciclastic sediments. Carbonate platforms were extensive at times when large areas of the continents lay beneath shallow seas. Today they are much narrower. Changing sea levels have produced long-lived carbonate platforms with structures showing complex cyclical changes. Carbonate platforms are classified as either carbonate ramps or rimmed shelves. A carbonate ramp has a gently sloping surface extending seaward from the coastline, with no sharp break between shallow and deep water. Rimmed shelves have a fairly sharp break where the water depth increases. In some places oolitic sand has accumulated to form barrier islands. In others reef-building organisms have built reefs along the rim (e.g. the Great Barrier Reef).