Elemental Concentrations and <sup>14</sup>C Ages of Cemented Materials in Beachrocks from Zamami, Aka, and Tokashiki Islands, West of Okinawa

The formation ages of beachrocks have been determined from the radiocarbon ages of fossil coral and shell samples, however, each age measured only indicates an extinction age. Therefore, the ages of fossil coral and shell samples show apparently different ages from the true formation ages of each beachrock. Based on the analyses of nine cemented materials in total collected from Zamami, Aka, and Tokashiki Islands, west of Okinawa Island, it is recommented to determine the formation age of conglomeratic beachrock based on cemented materials. Elemental concentrations are measured with ICP-MS and show high Ca concentrations of between 25.2 and 55.1 wt%, suggesting high CaCO3 concentrations. These high CaCO3 concentrations made it possible to form conglomeratic beachrocks. The radiocarbon ages of cemented materials yield an age of between 3764 ± 62 yr BP and 871 ± 60 yr BP, indicating the oldest and the youngest formation ages of beachrocks.