To provide a possible Holocene analogue for some ancient reefs, we studied the physiography, ecology, and sediments of two small reefs in Bermuda’s north lagoon. Both are rough-topped “mesas” rising 18 m from the relatively featureless lagoon floor to within 1-2 m of sea level. On the reef top, irregular coral- and algae-covered knobs are separated by sediment-filled valleys and hollows. The central area of 1 reef, believed to be in a more advanced stage of development, is an atoll-like sandy plain with only scattered knobs.
Scleractinian corals, principally Montastrea annularis and Diploria strigosa, are the most important reef-framework builders, as revealed in sections artificially constructed by blasting with dynamite. Coral growth is thought to be the major factor influencing the shape of reef cavities that are common throughout the reef mass. The walls of these cavities are covered with distinctive wall growths built by the skeletons of shade-loving organisms, principally encrusting red algae, pelecypods, a foraminifer, and ectoprocts.
Sediments on the reef top are generally coarse to very coarse sand with good to moderate sorting. The particles, dominated by Halimeda, which is also common in lagoonal sediments, are derived entirely from the breakdown of reef-top organisms. These same reef-top sands are washed down the reef face to build a steep reef-sediment slope. Fine sediments are winnowed from the reef-top and settle from suspension to the near-reef lagoon floor, where they form a “halo” of fines around the reef.