The Reef Flat and ‘Two-Meter Eustatic Terrace’ of Some Pacific Atolls

Characteristics of Indo-Pacific coral reefs long cited as evidence of a recent decline in sea level include: (1) the reef “flat” or pavement, (2) intertidal and supratidal flat erosion remnants of cemented coral rubble, and (3) erosion of the shores of reef islets.

Our examination of 25 low islands and 8 high islands in the Caroline and Marshall Islands during the 1967 Scripps Institution of Oceanography Expedition CARMARSEL leads us to conclude that the reef flats in the western Pacific generally are not simply erosional platforms but represent an equilibrium surface between upward accretion by reef-building organisms and erosion at the mean level of low tides.

Intertidal and supratidal coral rubble and calcareous sand accumulate above low-tide level during storms forming all of the visible land in the visited low islands. This rubble is currently being cemented and welded to the reef flats between tidal limits, and probably is also being cemented as beachrock and island conglomerate below the fluctuating water table of the cays.

Topographically highest cemented carbonate rocks of the Caroline and Marshall Islands are uniformly less than 2 m above the adjacent reef flat, and entirely in the intertidal zone, hence, within the tidal range of present sea level.

Flat horizontal surfaces of small extent occur on shore rocks, locally truncating inclined beds of beach rock. These surfaces roughly correspond to the upper limit of contemporary cementation somewhat modified by gravel scour. Comparison with rock platforms of a slightly elevated atoll of the Tuamotus, Raroia, indicates that the Marshall and Caroline Islands do not display expectable features of uplift or of subsiding sea level.