琉球列島への黒潮流入と第四紀サンゴ礁形成

要旨

The oldest Quaternary coral-reef limestone of the Ryukyu Islands have been dated 700, 000 to 600, 000yrs BP by the electron spin resonance (ESR) method. Prior to this date, the last time fossil coral reef was built in the Eocene time. The Shimajiri Group, mudstone and sandstone of the Late Miocene to 1.6 million years ago, was deposited in bathyal ironment. A 1, 000km long land bridge, the ‘China-Okinawa Land Bridge’, was formed in the Early Pleistocene Time. Coral-reef building since then is discussed here. Coral reef could not have been built in the marginal area of coral-reef building like the Ryukyu Islands, before the Kuroshio or Japan Current entered into the Okinawa Trough situated behind the land bridge. This reason is explained using the three-dimensional flow characteristics of the present Kuroshio Current. The collision of the Luzon Volcanic Arc with Taiwan made the boundary area between the Ryukyu Island Arc and Taiwan deeply bend and pulled apart the Yonaguni Depression on it. The depression introduced the current to the back-arc basin as at the present day. As a result tropical waters swept the Ryukyu Islands, and Quaternary coral-reef building began 700, 000 to 600, 000yrs BP.

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