Abstract

The nearshore slope of Oahu consists of a shallowly dipping shelf extending from the shoreline out to the approximately -220 m contour, where there is a sharp break in slope down to approximately -230 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 5a and/or 5c. Deposition during these interglacial highstands has not previously been 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 otherwise 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.

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