Accretion and diagenesis of a submerged Pleistocene reef, Oahu, Hawai'i

The nearshore submerged shelf of Oahu consists of a shallowly dipping terrace 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 cores taken from this nearshore terrace are typical of shallow-marine reef environments and constitute four facies, the branching-coral, encrusting-coral, massive-coral, and encrusting-algae facies. The windward margin of the terrace is dominated by coralline algal bindstones (algal ridge) that formed in a shallow, high-energy environment within 1--2 m of paleosea level. The leeward margin of the terrace is dominated by coral framestones, bindstones, and rudstones. 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 (∼186 to 242 ka). This in situ stage 7 reef complex is herein referred to as the Waianae Reef. The position of the Waianae Reef indicates that it formed during periods when sea level was ∼9 to 20 m below present sea level. Its extensiveness and geomorphic prominence as well as the lack of an emergent stage 7 reef framework suggest that on Oahu much of isotope stage 7 was characterized by sea levels below present. This represents the first in situ reef framework correlated to stage 7 on Oahu. Later accretion along the seaward front of the terrace occurred during the latter part of isotope stage 5 (post-5e, ∼80 to 110 ka). The position of the late stage 5 reefal limestones is consistent with their formation during periods when sea level was below present. The general trend of decreasing Th-U age of stage 5 corals with distance offshore suggests that accretion along the seaward front of the terrace occurred during a period of general sea-level fall during the latter part of stage 5. These data represent the first corals on Oahu correlated to late stage 5.;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 presently 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.