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Courtown Cays, Albuquerque Cays, and Roncador Bank are small atolls in the southwestern Caribbean, encompassing areas between 22 and 29 km². The shallow open lagoons contain sediments rich in Halimeda, coral, and coralline algae debris, apparently derived from peripheral reef flats and lagoonal patch reefs. Oolite and other nonskeletal carbonates are present only in small amounts.

Serrana Bank is almost an order of magnitude larger (191 km²) than the other three atolls, but has similar morphologic and hydrographic environments. Because of their relatively small area, peripheral reefs at Serrana Bank do not contribute a significant amount of sediment to the lagoon. In the eastern lagoon, sediments are derived from nearby patch reefs, but in the western lagoon, where hermatypic growth is limited, the dominant sediments are nonskeletal, and include oolite, cryptocrystalline lumps, and pelletoids. The reason that such nonskeletal carbonates can form in open lagoon conditions at Serrana Bank and yet are almost entirely absent in similar environments at the smaller atolls, may be related to the lack of biogenic carbonate sedimentation at Serrana Bank.

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