与那国島のサンゴ礁段丘および段丘構成層からのウラン系列年代--それらの海面・地殻変動史への意義

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Coral reef terraces on Yonaguni Island are morpho-stratigraphically divided into four units, from Terrace I to IV, in descending order. Among them, the lower two terraces (Terraces III and IV) were determined by α-spectrometric 230Th/234U dating to be correlative to oxygen isotope stages 7 and 5e, respectively. On the basis of the present height of Terrace IV, the amount of uplift since the last interglacial maximum, isotope stage 5e, is different for each small tectonic block of the island. For instance, the amount of uplift during the last 130, 000 years varies from 12m for the Sonai areas to 24m for the Kita-Bokujo area; these are adjacent to each other on the north coast.The limestone that unconformably underlies Terraces IV is composed of two upward-shallowing sequences, each of which is typically built up with four types of limestone, (rhodolith limestone, Halimeda limestone, bioclastic limestone, and coral limestone, in ascending order). The give-up type of reef (Neumann and MacIntyre, 1985) can be recognized in such sequences. That is, coral limestone in the uppermost part of the lower sequence is formed as a give-up type of reef, because it could not catch up with a rapid rise of sealevel at the beginning of isotope stage 7. During the time of high sealevel stand in the penultimate interglacial, the coral reef may have grown laterally seaward. Such lateral growth of the reef deposited fore-reef sediments like Halimeda limestone and rhodolith limestone, on the bottom of the nearby insular shelf. As a result, the upper sequence is correlated with the reef limestone in the lower of the two sequences and may be assigned to the high sealevel stand in the oxygen isotope stage 9.The last intergalcial reefal (coral) limestone generally occurs as thinner beds than those of the penultimate interglacial. Bioclastic limestone, Halimeda limestone, and rhodolith limestone deposited during isotope stage 5 are rare on Yonaguni Island. Terrace IV may be mainly depositional but partly erosional in origin, because typical examples of karst (e.g., pinnacles and dolines) are observed, and because its surface is partly collapsed to expose the underlying limestone, which is bioclastic limestone of the upper sequence formed during isotope stage 7.

与那国島のサンゴ礁段丘および段丘構成層からのウラン系列年代--それらの海面・地殻変動史への意義

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