Recent Progress on Coral Reef Terrace Researches at Kikai Island, Southwestern Japan. A Chronological and Sedimentary Study on the Pleistocene Series in Kikai Island, Central Ryukyus, Southwestern Japan.

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  • Omura Akio
    Department of Earth Sciences, Faculty of Science, Kanazawa University
  • Sasaki Keiichi
    Department of Earth Sciences, Faculty of Science, Kanazawa University
  • Terao Daisuke
    Department of Earth Sciences, Faculty of Science, Kanazawa University
  • Murakami Kazuo
    Department of Earth Sciences, Faculty of Science, Kanazawa University

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  • 喜界島のサンゴ礁段丘に関する最近の成果　　喜界島の更新世堆積物とそのウラン系列年代について
  • キカイジマ ノ コウシンセイ タイセキブツ ト ソノ ウラン ケイレツ ネンダイ ニ ツイテ

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Abstract

The Pleistocene series in Kikai Island, given the name Riukiu Limestone by Hanzawa (1935) and afterward redefined as the Ryukyu Group by Nakagawa (1969), is chronologically divided into two members, which are middle and late Pleistocene in age, respectively. The late Pleistocene reefal limestone has been subdivided into five time-stratigraphic units as a result of the U-series dating by Omura (1988).<br>This study summarizes a total of 35α spectrometric 230Th/234U dates ranging from ca. 40 to ca. 70ky for corals collected from the coral limestone corresponding to the framestone/bindstone of Embry and Klovan (1971). The dating results imply that coral limestones younger than the oxygen isotope stage 5a (Shackleton and Opdyke, 1973) can be correlated to the bufflestone/bindstone/framestone/rudstone (Nakamori et al., 1995) underlying Terrace IV through Terrace IIIb (or IIIC<sub>lower</sub>: Omura et al., 1995; Chappell et al., 1996) of the Huon Peninsula, Papua New Guinea. Such coral limestones are sporadically distributed at an altitude of ca. 10 to ca. 65m in the island, and overlie the fore-reef sediments which are assigned to stage 3 to 5e by 230Th/234U dating on some gravels of hermatypic corals or ahermatypic solitary corals (Omura, 1983).<br>The fore-reef sediments underlying the coral limestones correlative to oxygen isotope stage 3 through 4 were composed of coarse-grained detrital limestone (rudstone/floatstone), fine-grained detrital limestone (grainstone/floatstone), rhodolith limestone (floatstone/rudstone), Operculina limestone (floatstone/rudstone/grainstone/packstone), and bryozoan limestone (grainstone/packstone/wackestone), in descending order. They are coincident with near reef sand (occurring at depths shallower than about 90m), rhodolith and large foraminifera gravelly sand (on the open sea shelf in the depth range from ca. 60 to ca. 150m), and bryozoan sand (on the open sea terrace and upper shelf slope in water depth of ca. 60 to ca. 200m), respectively, among the present-day carbonate sediments in the western offshore of Miyako Islands (Tsuji, 1993; Tsuji et al., 1993). For the reason, a series of the above-mentioned fore-reef sediments is thought to be an upwardshallowing sequence which was deposited successively at the upper shelf slope to the outer edge of the reef via the outer and inner shelves; in other words, the sea-floor becomes gradually shallower as a result of the regression from isotope stages 5e through 3.

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