Abstract

Reflection profiles across the 500-meter shelf south of Oahu provide precise information on the location, configuration, and morphology of reefs on the shelf. New charts of bathymetry, basement topography, and buried-reef topography reveal shelf morphology and structure, along with reef growth patterns and relationships with antecedent topography. The reefs are commonly overlain by sediment, which can be up to 100 m (328 ft) thick. Individual reefs vary in size and shape, ranging from small cylindrical pinnacle reefs 0.35 km (0.22 mi) across and 0.19 km (0.12 mi) high to large, elongate patch reefs up to 2 km (1.24 mi) across, 5 km (3.1 mi) long, and 0.29 km (0.18 mi) high, to even larger barrier and shelf margin reefs. Fault control is evident; many reefs have grown on the upthrown side of tilted basement fault blocks, usually directly on elevated basement, but in some places on a stratified sediment layer overlying the elevated basement. Reef construction appears to have begun shortly after initiation of the block faulting, following formation of the terrace. Reef growth apparently kept pace with the early stages of shelf subsidence. More rapid subsidence, perhaps involving the entire volcanic edifice or coincident with eustatic changes in sea level, or both, may have resulted in the drowning of the entire reef complex, thus accounting for its location in waters presently more than 500 m (1,640 ft) deep.

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