Abstract

Unconsolidated carbonate sands and cobbles on Kapapa Island, windward Oahu, are 1.4-2.8 (+ or - 0.25) m above present mean sea level (msl). Agreeing with Stearns (1935), we interpret the deposit to be a fossil beach or shoreline representing a highstand of relative sea level during middle to late Holocene time. Calibrated radiocarbon dates of coral and mollusc samples, and a consideration of the effect of wave energy setup, indicate that paleo-msl was at least 1.6 (+ or - 0.45) m above present msl prior to 3889-3665 cal. yr B.P, possibly as early as 5532-5294 cal. yr B.P., and lasted until at least 2239-1940 cal. yr B.P. Hence, the main phase of deposition on Kapapa Island lasted a minimum of c. 1400 yr and possibly as long as c. 3400 yr. No modern samples have been recovered from the fossil beach on Kapapa Island, and samples from potential source sites offshore of the island show modern ages, indicating that sediments on the island are not deposited by modern-era storm and tsunami overwash. Because antecedent sediments are uncommon offshore but common on the island, deposition must have been time-transgressive rather than related to a single event. Radiocarbon ages of coral and mollusc clasts from a breccia lining an emerged (1.4 + or - 0.25 m msl) intertidal notch, cut into emerged coralline-algal carbonate of presumed last interglacial age, on south Mokulua Island (15 km to the southeast of Kapapa Island) correlate to the history recorded on Kapapa Island. Calibrated ages range from 2755-2671 to 3757-3580 cal. yr B.P. (averaging c. 3100 cal. yr B.P.) suggesting that a higher than present sea level formed the notch prior to 3757-3580 cal. yr B.P. A storm or tsunami origin for the features on Kapapa and south Mokulua islands is highly unlikely. Their age and elevation indicate, instead, a history of higher relative sea level (and subsequent fall) on windward Oahu during the middle to late Holocene. This history is consistent with geophysical models of postglacial geoid subsidence over the equatorial ocean first predicted by Walcott (1972) and later refined by Clark et al. (1978) and Mitrovica and Peltier (1991).

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