Six cores from the northeast reef of Enewetak Atoll reveal that 25 to 50 ft (7.5 to 15 m) of Holocene deposits unconformably overlie at least 200 ft (60 m) of Pleistocene limestone. The Holocene-Pleistocene boundary and four other unconformities within the Pleistocene section are distinguished by paleosols. The unconformities record repeated periods of subaerial emergence and death of the Pleistocene reef associated with low sea-level stands. During emergent periods, meteoric-water diagenesis altered the Pleistocene section. The resulting mineralogy and diagenetic textures are characteristic of the meteoric-vadose diagenetic environment. Aragonite generally composes from 30 to 70% by weight of the Pleistocene limestone and is absent only locally; magnesium calcite is not resent. Cementation by calcite is variable but generally increases with depth. Much of the section is poorly cemented and retains high primary and secondary porosity. Micrite and mixed micrite-microspar are the most abundant cements throughout the section. They occur as meniscus, uneven-style, and irregularly distributed ("patchy") pore-fill cement. In the

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upper parts of depositional units, paleosol-associated cements are common; caliche crusts, rhizocretions, needle-fiber cement, and lath-crystal cement are present. Phreatic-type cements are present, but generally are unimportant volumetrically. A reconstruction of the Pleistocene paleohydrology of the atoll indicates that the section has occupied the meteoric-vadose environment during emergent periods. The similarity of Pleistocene limestones beneath Enewetak and other Pacific atolls studied by drilling suggests that, for many atolls, most alteration of shallow subsurface Pleistocene limestones has occurred in the meteoric-vadose diagenetic environment.

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