Figure 1

The last interglacial period is recorded as emergent coral reefs on many tropical islands and coastlines. On Oahu, the distribution of emergent reef limestones (Fig. 1) was mapped by Stearns (1939, 1974, 1978). The youngest reef, limestone of the Waimanalo Formation of Lum and Stearns (1970), has been correlated with the last interglacial period based on 230Th/234U ages of corals from a number of localities by Ku et al. (1974). Their reported ages range from 112 ± 6 ka to 137 ± 11 ka. These workers did not provide a detailed stratigraphy of each locality, but described each sample as having come from either a growth-position (or at least cemented) coral or uncemented, wave-deposited coral conglomerate; the latter frequently overlie in situ reefs with corals in growth position.

THE WAIMANALO FORMATION

Figure 2</small>

Most known localities of the Waimanalo Formation were studied on Oahu, based on mapping by Stearns (1939, 1974, 1978) and locality descriptions by Ku et al. (1974). Elevations at most localities were measured by hand leveling and tape using sea level as a datum; mean tidal ranges on Oahu are less than 0.5 m, so corrections are not required. Many outcrops reported by previous workers are now obscured by new buildings, removed by construction activities or wave activity, or are modified such that detailed stratigraphy cannot be described. Other localities have survived but provide only minimum estimates of the elevation of the Waimanalo high sea stand. For example, small reefs crop out at the Makai Range pier at Kaupo Beach Park at +2.5 m, and at Alala Point near Kailua at +3.5. At Diamond Head, near the lighthouse, a small patch of coral-bearing conglomerate is exposed at +3.5 m, but is overlain by ~2 m of eolianite. At Kahuku Point, growth-position corals are exposed, but occur below sea level and up to +2.5 m above sea level, where they are overlain by eolianite. The best exposures are near Mokapu Point, southeast of Kaena Point, and directly east of Kaena Point (Fig. 2).

Well-preserved specimens of Pocillopora and Porites were collected from most exposures of the Waimanalo Formation on Oahu for dating by the uranium-series method. Uranium and thorium concentrations and isotopic activity ratios were determined by thermal ionization mass spectrometry.

Figure 3

RESULTS: THE DURATION OF THE LAST INTERGLACIAL PERIOD ON OAHU

Uranium-series ages of corals from the Waimanalo Formation on Oahu have a uniform distribution of ages from ~114,000 to ~131,000, indicating a duration for the last interglacial sea-level high stand of ~17,000 years, in contrast to a duration of ~8,000 years inferred from the orbitally tuned marine oxygen isotope record (Martinson et al., 1987) (Figure 3). Sea level on Oahu rose to >1 to 2 meters higher than present by 131,000 years ago or ~6,000 years earlier than inferred from the marine record. Although the latter record suggests a shift back to glacial conditions beginning at ~119,000 years ago, the Oahu coral ages indicate a near-present sea level until ~114,000 years ago.

REFERENCES
(*references in bold are project products)

Ku, T.-L., Kimmel, M.A., Easton, W.H., and O'Neil, T.J., 1974, Eustatic sea level 120,000 years ago on Oahu, Hawaii. Science, v. 183, p. 959-962.

Lum, D., and Stearns, H.T., 1970, Pleistocene stratigraphy and eustatic history based on cores at Waimanalo, Oahu, Hawaii: Geological Society of America Bulletin, v. 81, p. 1-16.

Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Jr., and Shackleton, N.J., 1987, Age dating and the orbital theory of the ice ages: Development of a high-resolution 0 to 300,000-year chronostratigraphy: Quaternary Research, v. 27, p. 1-29.

Muhs, D.R., and Szabo, B.J., 1994, New uranium-series ages of the Waimanalo Limestone, Oahu, Hawaii: implications for sea level during the last interglacial period: Marine Geology, v. 118, p. 315-326.

Stearns, H.T., 1939, Geologic map and guide to the island of Oahu, Hawaii: Territory of Hawaii, Division of Hydrography Bulletin, v. 2, p. 1-75.

Stearns, H.T., 1966, Geology of the State of Hawaii: Pacific Books, Palo Alto.

Stearns, H.T., 1974, Submerged shorelines and shelves in the Hawaiian Islands and a revision of some of the eustatic emerged shorelines: Geological Society of America Bulletin, v. 85, p. 795-804.

Stearns, H.T., 1978, Quaternary shorelines in the Hawaiian Islands: Bernice P. Bishop Museum Bulletin, v. 237, p. 1-57.

Szabo, B.J., Ludwig, K.R., Muhs, D.R., and Simmons, K.R., 1994, Thorium-230 ages of corals and duration of the last interglacial sea-level high stand on Oahu, Hawaii: Science, v. 266, p. 93-96.

Veeh, H.H., 1966, Th230/U238 and U234/U238 ages of Pleistocene high sea-level stand: Journal of Geophysical Research, v. 71, p. 3379-3386.