中部琉球喜界島の地史 : 琉球石灰岩産サンゴ化石のウラン系列年代測定のまとめとして  [in Japanese] Geologic history of the Kikai Island, Central Ryukyus, Japan : Summary of uranium-series dating of fossil corals from the Riukiu Limestone  [in Japanese]

Uranium-series dating of fifty-three coral samples imply that the Pleistocene Riukiu Limestone (HANZAWA, 1935) on Kikai Island, Central Ryukyus, can be generally divided into two members. The one is Middle Pleistocene in age, assigned to older than 250 ka B.P. by the ^<230>Th/^<234>U method, although very limited part of this member seems to be formed during the penultimate interglacial, about 200 ka B.P. The other, formed after the last interglacial, is composed of five reef complexes, each of which was dated to be ca. 40, 50 to 65, ca. 83, ca. 101, and 120 to 130 ka B.P., respectively. Overlaying directly the basement, Upper Pliocene Somachi Formation (NAKAGAWA, 1969), the Middle Pleistocene Member composes the major part of the Riukiu Limestone in the extensive area on the island. On the contrary, everyone of the Upper Pleistocene reef complexes is likely to be preserved as a small-scaled limestone unit and to have the relation of paraconformity to the Middle Pleistocene Member. The Riukiu Limestone on Kikai is thus considered to have been formed during seven stages of high sea stand, three interglacials correlative to the oxygen isotopic stages 5, 7, and probably 9, and four interstadials after the last interglacial. Because three coral samples dated to be 121±5, 126±6, and 138±5 ka, occurred on the highest terrace (Hyakunodai) including the highest point (224 m in altitude) of the island, there is very little doubt that Kikai had been entirely submerged during the last interglacial. After that, the island has gradually increased in size with the apparent lowering of sea level. The present elevation of uplifted coral reefs dated suggests that Kikai has been highly upheaved during the last 120〜130 ka in comparison with upheaval or Hateruma. For instance, the difference of more than 170 m can be seen between Kikai and Hateruma Islands on height of the reef formed during the last interglacial. The principal cause of such a great discrepancy is to be sought of the difference in vertical displacement between two islands. The difference of elevation speed estimated for several stages suggests that the rate of vertical displacement has not necessarily been uniform during the last 130 ka and has gradually increased up to the present. The upheaval of Kikai is thought to have started after formation of the Middle Pleistocene Member dated as more than 250 ka B.P. by the ^<230>Th/^<234>U method and 400〜550 ka B.P. by the ESR method.