Three nannofossil datum planes of the Early Pleistocen e are identified in the cores from subsurface at Irabu Island, South Ryukyus. The result allows for the first time to date the initial sedimentation of the reef-related Riukiu Limestone there. It represents the age (1.2M a) turning from siliciclastics to shallow-water carbonate regime. Implication of this transition time is discussed briefly in relation with both local neotectonis m resulting in the birth of a backarc basin (Okinawa Trough) and regional to global reor ganization of paleoceanographic environments.
