A study on sea-level changes and tectonic movements during the late Quaternary deduced from the uranium-series method of dating for emerged coral reefs

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This study was performed on coral reef terraces to compare regionally the relative sea level changes and tectonic movements from the Ryukyu through the Philippine Islands. The results revealed that the "Western Pacific Region" has experienced significantly different tectonic movement during ca. 210 ky, as follows :1.Based on the α-spectrometric ^<230>Th/^<234>U method of dating, the lowest Pleistocene reef terrace in Kikai Island was proved to have been formed in different depositional environment during Marine Isotope Stage 5b (MIS-5b) through MIS-3, with lowering the relative sea level, namely from the insular shelf where fore-reef sediments were deposited to shallow water environment where reefs were formed.Holocene reef terrace along Araki Coast of Kikai overlies thin coral limestone and can be sub-divided into six steps, have been emerged at 〜7.4, 6.6, 4.0, 3.0 ky or later, respectively.2.^<230>Th/^<234>U method of dating was successively applied to reef-building corals collected from the lowest reef terrace of Aguni, Tarama and Ishigaki Islands, which were assigned to MIS-7, MIS-5c and MIS-5e, respectively.3.U-series dates from 0.93±0.04 to 8.57±0.13 ky were obtained from hermatypic corals collected from several marine terraces (at a maximum altitude of 45m) along the west coast of Taiwan. This fact indicates that the upheaval rate of Taiwan during Holocene is highest in the world.4.The reef terrace assigned to MIS-5e is distributed at the altitude of 13-14 m in the Panglao Island adjacent to the southwestern end of Bohol Island, whereas the reef terrace of the same age is attained to 29 m at the Pamilacan Island. It means that the tectonic movement may occur among small-scaled blocks. Such a tectonic movement can be observed in Holocene reef terraces and the other paleo-sea level indicators such as tidal notches and a cluster of micro-atoll along northwest coasts of the Luzon Island.