Figure 5. Fringing-reef retreat controls the transition between reef types during glacio-eustatic SL cycles.

The first SL cycle starts with a fringing reef during the glacial lowstand (1). During the rapid postglacial transgression, the reef is forced to retreat upslope due to its low accretion rate (2). Only as SL rise slows into the interglacial, can the fringing reef advance seawards producing a reef flat (3). Following glacial SL fall, the second cycle begins, as before, with a fringing reef retreating upslope with the transgression (4). This time, however, it encounters the former reef-flat platform at a lower elevation due to island subsidence. The slight reverse slope of the platform prevents further upslope retreat and fixes the reef on its rim, producing a lagoon that traps coastal sediment (5). Isolated from sediment, the reef is colonised by fast-growing acroporids which allow it to accrete vertically and keep pace with SL rise, producing a barrier reef (6). In the final cycle, subsidence and erosion displace the volcanic peak below the highstand elevation (7), so that when the fringing reef reaches the rim of the former barrier-reef flat (8), it can accrete vertically and transform into an atoll (9).