Lagoonal Topography of Caroline and Marshall Islands

During a two-month investigation of sea level changes in the Caroline-Marshall Island area, hundreds of miles of traverses were made across coral reef lagoons with precision depth recorders. The profiles are from lagoons inside barrier reefs, from atolls with rims consisting predominantly of islands and reefs in the tidal zone, from an atoll with a rim largely below the tidal zone (semi-drowned atoll), and from a bank with a rim that is entirely below the tidal zone (drowned atoll). Lagoonal depths of all these types are close to the 45-m average found in previous studies, suggesting that a sea-level control operated during the last glacial episode and that subsequently the platforms have been stable.

Typical lagoon profiles show a rolling topography alternating with hills and basins having a relief of 10 to 20 m. Relatively smooth floors with occasional coral knolls are common only in banks with submerged rims. Dives in one of the more irregular areas of Truk Lagoon established the nature of the basins. Almost the entire floor was covered with Halimeda, while the higher knolls are either Halimeda-covered or support active coral growth. The hills and basins may have resulted from extensive limestone solution during Pleistocene low sea-level stages, producing a karst topography, modified primarily by Halimeda growth following the last sea level rise.

Echo profiles showed few terraces in the lagoons. Steep slopes descend from surrounding reefs to lagoon floors. The much discussed 15-to 18-meter terrace could not be demonstrated in the fathograms, but was found occasionally by scuba divers on the seaward margin of atolls. Small areas of relatively flat floors were seen mostly below the rolling topography, locally as deep as 70 m.