Uranium-series Age of the Highest Marine Terrace of the Upper Pleistocene on Kikai Island, Central Ryukyus, Japan

Abstract

The uplifted Pleistocene marine terraces in Kikai Island have been investigated using radiometric dating since the 1960s. The highest terrace was called the Hyakunodai Terrace, which is divided into three blocks by faults, assigned in this study as Block C, Block B and Block A in ascending order of height. Previous studies indicate that Block A was formed during Marine Isotope Stage (MIS) 5e and that MIS 5c corals were distributed on the cliff between Blocks A and B based on Uranium-series dating. We carried out careful fieldwork and report more precise α-spectrometric U-series ages of Blocks A and B. Fossil corals showed eight Mid- to Late Pleistocene dates ranging from 154.8±6.4 to 97.7±3.0 ky (2σ) and three Middle Pleistocene ages (>450 ky). Mid- to Late Pleistocene dates are divided into three age groups, 154.8±6.4-142.7±5.8 ky, 122.1±3.8 ky and 108.2±3.2-97.7±3.0 ky, corresponding to MIS 6, 5e and 5c, respectively. While all age groups were obtained from Block A, Block B only contains MIS 5c and Middle Pleistocene corals. The MIS 5c outcrop at Block A contains clear reef structure (bindstone) and the coral assemblages indicate depositional depth ranging from 5 to 15 m. These facts suggest that the highest terrace was formed during MIS 5c. Exposure of MIS 5e or 6 corals is possibly due to erosional effect during MIS 5c transgression.