Late Cenozoic Calcareous Nannoplankton Biostratigraphy in the Southern Kwanto Region, Japan

The Pliocene-Pleistocene sediments are widely developed in the central part of the Boso Peninsula, especially well exposed along the Obitsu and Yoro Rivers, and their thickness attain to more than 4,000 meters in the maximum part. In the central part of the peninsula the early Pliocene Toyooka Group is separated from the early Pleistocene Kazusa Group with the Kurotaki Unconformity. At the northern end of the peninsula the Pleistocene Inubo Group is overlain with the middle to late Pleistocene Katori Formation. The Anno Formation of the Toyooka Group is characterized with the joint occurrence of Reticulofenestra pseudoumbilica and discoasters. In the Kazusa Group Gephyrocapsa oceanica occurs through the sequences. The upper part of the Inubo Group yields stably Gephyrocapsa protohuxleyi. Based on the calcareous nannoplankton fossil evidences, the Anno Formation is assigned to the Lower Pliocene, the Kazusa Group to the Lower Pleistocene and the Inubo Group to the Lower-Middle Pleistocene. The hyatus of the Kurotaki Unconformity is estimated at a maximum of 1 million years. Sedimentation ratio of the Kazusa Group is also estimated at 6-7 meters/1,000 years