Marine and meteoric diagenesis of Pleistocene carbonates from a nearshore submarine terrace, Oahu, Hawaii

The nearshore slope of Oahu consists of a shallowly dipping shelf extending from the shoreline out to the similar to -20 m contour, where there is a s harp break in slope down to similar to -30 m, Limestones recovered in a ser ies 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 sugge sts that the terrace represents an in situ fossil reef complex. Th-U ages o f in situ corals are all Pleistocene and suggest that the bulk of the featu re formed during marine oxygen isotope stage 7. Later accretion along the s eaward front of the terrace occurred during marine oxygen isotope substages 5a and/or 5c. Deposition during these interglacial highstands has not prev iously been documented in the sea-level record on Oahu, Although the diagenetic record in the cored samples is incomplete, three pe riods of diagenesis are identified: early shallow marine, meteoric, and pos t-meteoric shallow marine. Early shallow-marine diagenesis includes cementa tion by aragonite and Mg calcite in an active marine phreatic zone and pred ominantly micritization in a stagnant marine phreatic zone. Meteoric proces ses occurred in the vadose zone and include precipitation of calcite (needl e fibers, meniscus cements, micritic networks), neomorphism, and dissolutio n. All limestones are now in an active marine phreatic zone. Evidence of po st-meteoric shallow-marine diagenesis is found in last-generation Mg calcit e cements and internal sediments occurring directly on limestone substrates that have otherwise been stabilized to calcite. The present seafloor is un dergoing extensive biological and physical erosion. No Holocene limestones were recovered. Petrographic and geochemical signatures of subaerial exposu re and meteoric diagenesis are recognized within the upper several centimet ers of all cores. Thus, the present seafloor in the study area is a flooded Pleistocene subaerial exposure surface.