STEPWISE REEF GROWTH, EMERGENCE AND DROWNING RECORDED ON A RAPIDLY SUBSIDING MARGIN, OFFSHORE HAWAII: THE LAST 600,000 YEARS

Eight progressively deeper reefal terraces off the northwest coast of Hawaii have terrace tops at about –150, -400, -585, -690, -900, -1150, -1225, and –1300 m depth. The terraces formed over the past 600,000 years on a flank of Hawaii that is subsiding at about 2.4-2.6 mm/yr; the higher rate closer to the island, so the terraces are tipped slightly towards the southeast. The stratigraphy and morphology of the terraces should constitute a high-fidelity record of sea-level fluctuations over this time period. Terraces at about –150, -400, and –1150 m formed robust complexes with barrier reefs and lagoons whereas those at –585, -690, and -900 m are mainly lava flows and volcaniclastic rocks capped by thin coral reefs. The–1150 m reef has a narrow shelf at -1175 m and a broader terrace at about –1150 m. Corals collected with the ROV Tiburon include Porites lobata, P. compressa, P.evermanni, Lepastrea purpurea, and species of Pocillipora, Montipora, and Fungia suggesting shallow water environments. The corals occur on the reef surfaces in growth position and as outcrops of bafflestone and boundstone. Along the ROV traverses, most of the carbonate rocks are sandstone and fine-grained algal and bioclastic limestone. Outcrops are deeply pocked, probably by a combination of bio-erosion and dissolution.

Computer models of the formation of these reefal platforms on a subsiding slope using the OJ96 sea level curve broadly agree with multibeam bathymetric and ROV sample data but fail explain the terrace at –585 m and the lower step in the –1150-m terrace. The models suggest reef construction during periods when eustatic sea level was falling at, or a little more slowly than, island subsidence. Coral samples recovered from the tops of the shallowest 6 terraces have variable amounts of secondary cement that may have formed during brief episodes of emergence when eustatic sea level was falling more rapidly than subsidence.. Emergence is also suggested by the set of longitudinal and transverse dune- or beach-ridge forms topping the –400 terrace and enclosing box-like (karstic) depressions about 30 to 50 m deep. The final drowning of each platform occurred as sea level rose rapidly at the onset of interglacial periods.