琉球列島,宝島・小宝島の第四紀後期離水サンゴ礁と完新世後期の海水準

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Marine terraces with coral reef limestone are common in the tectonically active Ryukyu island arc and provide an important opportunity for reconstruction of late Quaternary eustasy, crustal movement and fossil coral reef formation. Takara and Kodakara islands are the northernmost coral islands with both Holocene and Pleistocene reef caps in the Pacific ocean. They are located at a decisive site to restore the paleoenvironment of Kuroshio warm current oceanic region. Both islands are situated on a tilted island shelf and seem to have been rising up periodically since the last glacial age (Eemian) at the nearly constant long-term tilting rate of 3.8×10^<-8>/year, due to accumulation of sudden uplift associated with major earthquakes. This tilting rate (R) is estimated from the following equation, R={(A_k-S)-(A_t-S)}÷15,000÷T=(A_k-A_t)÷15,000÷T, where A is the elevation of correlated terrace for Takara (t) and Kodakara (k) islands, T is the age of the terrace, S is the level of the paleo sea level relative to present sea level in meters, 15,000 is the distance in meters between two islands. The tilting rate should be calculated in ascending order geomorphologically to obtain the long-term rate. Pleistocene reef terraces are developed typically in Takara island, that is Nagabatake at 53 m above sea level (Barbados III), Nakabaru at 35 m a.s.l. (Barbados II), and Ohara surface at 21 in a.s.l. (Barbados I). Nagabatake formation includes subordinate deltafan gravel member (Kannon gravel, 30 m thick) accompanying eustatic sea-level rise. Nagabatake and Nakabaru reef tracts seem to have been formed as fringing reefs without moat, but Ohara surface is the erosional one cut on the former formations. Holocene terraces are distinctly developed in Kodakara island, that is Haebaru at 9.6 m a.s.l. (3,425-2,378 radiocarbon years B.P.), Tsukuridomari surface at 2.4 m a.s.l. (2,378-1,330 Y.B.P.). The separation of these two terraces took place around 2,378 Y.B.P. by the enormous seismic uplift of which interval is culculated about 10,000 years. Holocene formations in Kodakara island consist of Fuchinotomari, Haebaru (5,235-2,378 Y.B.P.) and Tsukuridomari formation (2,378-1,330 Y.B.P.) in ascending order, and Fuchinotomari formation is buried by Haebaru formation. Kodakara data showed a gentle submergence rate or an almost stillstand at present sea level in the last 3,500 years, but suggested a more rapid submergence between 3,500 and 5,500 years B.P.. Although submergence rates are different, this abrupt decrease in the rise of sea level at 3,500 radiocarbon years B.P. is recognized also in the adjacent coast of Florida. While the Holocene highest terrace of Takara and Kodakara islands was formed around 3,500 Y.B.P., the one of the southern part of Boso peninsula near Tokyo, the type locality of "Holocene submergence (Jomonian transgression)", shows the age of around 6,500 Y.B.P. The age, height and number of terrace in the last 7,000 years presumably depend on the amount of seismic uplift and the recurrence interval of earthquake rather than sea-level fluctuations in the tectonically active regions such as Japanese islands.
地学団体研究会の論文
1979-07-25

琉球列島,宝島・小宝島の第四紀後期離水サンゴ礁と完新世後期の海水準

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