サンゴ礁段丘の地形層序と構成層の230Th/234U年代測定からみた過去30万年間の古海面変化 (黒潮圏の第4紀古環境特集号)

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Pleistocene coral reef terraces were divided into four to seven time units on Kikai, Hateruma and Yonaguni Islands in the Ryukyu Arc, by the α-spectrometric 230Th/234U dating of hermatypic corals. Such a unit can be correlated with its counterpart in the Huon Peninsula, New Guinea, and Barbados, West Indies, which was formed during the high sea stand corresponding to isotope stages 3, 5 or 7 (Shackleton and Opdyke, 1973). Units exposed on an island are controlled in number by the amount of uplift. For instance, seven terraces are identified on Kikai which is characterized by the most vigorous upheaval during the last 130, 000 years, whereas only four Pleistocene units are occurred on Hateruma Island. The former shorelines on three islands were determined in addition to those on Kita- and Minami-Daito Islands resting on the Philippine Sea Plate, by using topographic and geologic features, such as inner margin of coral reef terrace, raised surf bench, and litho- and bio-facies of a limestone unit. Their present altitudes are most useful for determining the emergent timing and estimating quantitatively the amount or the rate of uplift of an island.Among three islands in the Ryukyus, Kikai emerged most recenthy after the last interglacial maximum, and Hateruma appeared as an island above sea level after the penultimate interglacial. The oldest coral reef on both islands is considered to have been formed as a table reef, and afterwards fringing reefs delveloped, during high sea stand of isotope stages 5 c to 3. Both islands have increased in area with the apparent regression of the sea, caused by the uplift of the islands themselves. All coral reefs on Yonaguni were formed as fringing reefs surrounding hilly land underlain by the Yaeyama Group (Middle Miocene). In other words, Yonaguni had already existed before the time when the oldest reef grew; hence it has a much longer geologic history as an island than Kikai and Hateruma. Kita- and Minami-Daito Islands are also considered to have a long history, judging from the difference in elevation between the highest point underlain by the limestone formed as a part of the pre-existent atoll (older than 4, 600ka: Koba et al., 1991) and the present elevation (12m) of the shoreline of the last interglacial maximum.The present maximum altitude of the last interglacial shoreline was determined to be >224m, 41m, 30m, and 10-12m on Kikai, Hateruma, Yonaguni and Daitos, respectively. Such divergence is due to the difference in the amount of uplift during the last 130, 000 years. The rate of their uplift can be concluded not to be constant, at least during the same time span. The direction and the degree of tilting also differ among five islands, because of the difference in tectonic setting. In other word, each island has a very forceful individuality in terms of geologic and tectonic history.
日本第四紀学会の論文

サンゴ礁段丘の地形層序と構成層の230Th/234U年代測定からみた過去30万年間の古海面変化 (黒潮圏の第4紀古環境特集号)

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