A New Age Assignment of Riukiu Limestone (Pleistocene) by Calcareous Nannofossils from the Borehole Cores at Irabu Island, South Ryukyus, Japan.

Abstract

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. I.