Abstract

Sedimentologic, stratigraphic, and geochronologic analyses of a previously undescribed carbonate section on Oahu, Hawaii, provide new evidence for two distinct sea-level highstands on Oahu during the last interglacial period (oxygen isotope substage 5e). Whereas electron-spin-resonance and uranium-series ages (122 ±8 ka to 152 ±25 ka, and 115 ±10 ka to 160 ±15 ka, respectively) of in situ corals place the age of the deposits within substage 5e, it is the unique sequence of strata found in these exposures that reveals the two transgressions. A highstand lagoonal deposit of coral-algal bafflestone is overlain by large seaward-dipping slabs of beach-rock. The beachrock, deposited during a mid-5e regression, is in turn overlain by a second highstand lagoonal deposit. This sequence was deposited in a broad, shallow, back-reef embayment that was very sensitive to fluctuations in sea level. Elsewhere, along much of the shoreline of Oahu, an in situ coral-algal framestone (Waimanalo Formation), representing the initial 5e highstand, is erosionally truncated on its upper surface. This erosional unconformity represents the mid-5e lowstand and separates the framestone from overlying, seaward-dipping, planar-bedded grainstone and rudstone (Leahi Formation) that accumulated during the second 5e highstand.

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