NEW URANIUM-SERIES AGES OF THE WAIMANALO LIMESTONE, OAHU, HAWAII - IMPLICATIONS FOR SEA-LEVEL DURING THE LAST INTERGLACIAL PERIOD

The Waimanalo Formation (limestone) of Oahu has been correlated with t he last interglacial period based on U-series dating of corals by T.-L . Ku and colleagues. The limestone consists of growth-position corals and overlying coral conglomerate. An apparent bimodal distribution of ages for the growth-position corals (mean age = 133 ka) and the overly ing coral conglomerate (mean age= 119 ka) has been interpreted to repr esent two distinct high stands of sea that occurred within the last in terglacial period. Both growth-position corals and overlying, conglome rate coral occur in an outcrop east of Kaena Point and consist mainly of Pocillopora and Porites. U-series ages of growth-position corals th at show closed-system conditons are 120 +/- 3 ka and 127 +/- 4 ka; ove rlying conglomerate corals have U-series ages that range from 120 +/- 3 ka to 138 +/- 4 ka. At Kahe Point, conglomerate corals have ages of 120 +/- 3 ka and 134 +/- 4 ka. These data show that the growth positio n corals are not systematically older than the conglomerate corals; th us, there is no evidence for two distinct high stands of sea. Waimanal o deposits at Kahe Point and Mokapu Point (new U-series ages of 134 +/ - 4 ka and 127 +/- 3 ka) have beach deposits as high as 12.5 m and, at Mokapu Point, growth-position corals as high as 8.5 m. A last-intergl acial sea-level stand of + 8.5 to + 12.5 m conflicts with estimates of + 6 m from a number of tectonically stable coastlines and islands in the western Atlantic Ocean. We infer, therefore, that Oahu may be unde rgoing uplift at a low rate. This uplift may be due to compensatory li thospheric flexure, because the island of Hawaii has been subsiding th roughout much of the Quaternary from volcanic loading. Because of this possible uplift, Oahu and islands like it elsewhere in the Pacific ca nnot be used as reference points for sea level during the, last interg lacial period.