2022 年 128 巻 1 号 p. 215-220
The upper Miocene to Pleistocene Shimajiri Group, composed mainly of siltstone and sandstone, occurs on some of the Ryukyu Islands in southwestern Japan. This group is thought to have been deposited in a shelf-slope to fore-arc basin setting before the accumulation of Pleistocene coral reef deposits (Ryukyu Group). We investigated the calcareous nannofossil biostratigraphy of the Shimajiri Group in the Offshore Okinawa 1-x well, drilled near the northeastern margin of the Okinawa-Miyako Submarine Plateau, Ryukyu Islands. A sample from 280 m, at the base of Ryukyu Group, contains Gephyrocapsa parralella, which first occurred at 0.987 Ma. Discoaster quinqueramus and D. berggrenii, which define both the top and bottom of the calcareous nannofossil zone NN11, are found throughout the Shimajiri Group, consistent with a late Miocene age between 8.10 Ma and 5.53 Ma. A large sedimentary gap between the Shimajiri and Ryukyu groups suggests that part of the Okinawa-Miyako Submarine Plateau may have been above sea level during the early Pliocene, implying deposition of the Shimajiri Group here was completed earlier than in other regions. This study provides key constraints on the Cenozoic geological history and phylogeography of the Ryukyu Islands.
