STRATIGRAPHIC DEVELOPMENT OF FORE-REEF AND MARGINALSHELF ENVIRONMENTS SOUTH OF MOLOKA'I, HAWAI'I

摘要

High-resolution seismic-reflection profiles provide insight into the laternQuaternary development of the largest coral-reef complex in the main Hawaiian Islands,rnthe fringing reef which borders southern Moloka'I. Terrigenous sediment derived fromrnthe steep slopes of Moloka'I mixes with reefal material on the semi-protected southernrnmargin of the volcanic island. Approximately 100 km of seismic-reflection profiles wererncollected in water depths of 3-140 m, using a 50-tip mini-sparker source, to examine therninternal structure, distribution, and thickness of coral reef deposits and associatedrnsediment of mixed carbonate-silicilastic composition. Six seismic units were identifiedrnon the basis of reflection characteristics and lithosome geometry: antecedent substrate,rnfore-reef wedge, channel fill, transgressive sheet, nearshore wedge, and coral reef. Twornhighly reflective surfaces (R1 and R2) underlie the modern reef, including a continuousrnreflection that was traced in seismic data for nearly the entire 60-km length of the island.rnThe prominent R1 surface dips gently seaward (1-2°) to ~130 m depth and locally cropsrnout on the seafloor, and probably represents either a wave-cut platform or ancient reefrnflat. The uniformly low relief of the antecedent substrate contrasts with the irregularrnshape of the overlying, modern reef. We propose two evolutionary models, in which sealevelrnchanges have modulated the development of Hawaiian reef systems, to explain thernobserved stratigraphic relationships.