AnSTRACT: The nearshore slope of Oahu consists of a shallowly dipping shelf extending from the shoreline out to the --20 m contour, where there is a sharp break in slope down to --30 m. Limestones recovered in a series of short cores taken from this nearshore terrace are typical of shallow-marine reef environments and comprise either a branching-coral or massive-coral facies. The composition as well as shoreward zonation of facies suggests that the terrace represents an in situ fossil reef complex. Th-U ages of in situ corals are all Pleistocene and suggest that the bulk of the feature formed during marine oxygen isotope stage 7. Later accretion along the seaward front of the terrace occurred during marine oxygen isotope substages Sa and/or Sc. Deposition during these interglacial highstands has not previously beeu documented in the sea-level record on Oahu.Although the diagenetic record in the cored samples is incomplete, three periods of diagenesis are identified: early shallow marine, meteoric, and post-meteoric shallow marine. Early shallow-marine diagenesis includes cementation by aragonite and Mg calcite in an active marine phreatic zone and predominantly micritization in a stagnant marine phreatic zone. Meteoric processes occurred in the vadose zone and include precipitation of calcite (needle fibers, meniscus cements, micritic networks), neomorphism, and dissolution. All limestones are now in an active marine phreatic zone. Evidence of post-meteoric shallow-marine diagenesis is found in last-generation Mg calcite cements and internal sediments occurring directly on limestone substrates that have otberwise been stabilized to calcite. The present seafloor is undergoing extensive biological and physical erosion. No Holocene limestones were recovered. Petrographic and geochemical signatures of subaerial exposure and meteoric diagenesis are recognized within the upper several centimeters of all cores. Thus, the present seafloor in the study area is a flooded Pleistocene subaerial exposure surface.
INTRonUCTION
Giant tsunamis, generated by submarine landslides in the Hawaiian Islands, have been thought to be responsible for the deposition of chaotic gravels high on the southern coastal slopes of the islands of Lana'i and Moloka'i, Hawaii. Here we investigate this hypothesis, using uranium-thorium dating of the Hulopoe gravel (on Lana'i) and a study of stratigraphic relationships, such as facies changes and hiatuses, within the deposit. The Hulopoe gravel contains corals of two age groups, representing marine isotope stages 5e and 7 (approximately 135,000 and 240,000 years ago, respectively), with significant geographical and stratigraphic ordering. We show that the Hulopoe gravel was formed by multiple depositional events, separated by considerable periods of time, thus invalidating the main premise of the 'giant wave' hypothesis. Instead, the gravels were probably deposited during interglacial periods (when sea level was relatively high) by typical Hawaiian shoreline processes such as seasonal wave patterns, storm events and possibly 'normal' tsunamis, and reached their present height by uplift of Lana'i.
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