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.

Quaternary geohistory inferred by seismic stratigraphy of a carbonate province in an active margin, off Miyako Island, south Ryukyus, Japan

“…Calcareous nannoplankton data from borehole cores suggest that the Riukiu Limestone in the survey area was deposited between ca. 1.2 Ma and 0.39 Ma (Sado et al, 1992).…”

“…The Shimajiri Crustal Movement occurred between approximately 1.7 Ma and 1.2 Ma (the early Pleistocene). These ages represent the uppermost part of the Shimajiri Group (Ujiie and Oki, 1974) and the lowest part of the Riukiu Limestone (Sado et al, 1992) of the survey area, respectively. This event resulted in anticlinal uplift of the Ryukyu Arc and associated with NW -SE trending faulting (Furukawa, 1983).…”

Influx of the Kuroshio Current into the Okinawa Trough and Inauguration of Quaternary Coral-Reef Building in the Ryukyu Island Arc, Japan.

“…These ages are identical with one deduced from a compilation of nannofossil stratigraphy (Koba, 1980;Koba and Kaigara, 1989). Sado et al (1992) stated using nannodata developed by Takayama and Sato (1987) that the reef limestone in Irabu-jima Island dated back to 1.2 million years ago. They deduced the date from nannodata of three first appearances, three extinctions, and one rich occurrence.…”