A review of radiolarian biostratigraphy for the oil-bearing Neogene of Japan. Advances in biostratigraphic resolution.

During the last decade, the Neogene radiolarian zonation of Japan has been greatly improved by combination of biostratigraphic studies from both onshore sections and offshore deep-sea cores. This improvement is achieved by advancement of knowledge on taxonomy, the nature of evolutionary lineages, and the paleomagnetic dating of biostratigraphic events. The biostratigraphic framework established here consists of three zones of the tropical area for Lower Miocene and 14 zones and subzones of the subarctic area for Middle Miocene to Pleistocene. These zones provide an average age resolution of about 1.6 m.y. between 16 and 7Ma and about 0.8 m.y. between 7 and 2Ma. The Lower Miocene radiolarian biostratigraphy of Japan is poorly known due to a lack of good reference sections.These biostratigraphic results revealed that Neogene radiolarian assemblages of Japanese oil-field belong to the subarctic biogeographic province of the North Pacific rather than temperate one. However, due to the presence of many undescribed species and few quantitative assemblage data, details of radiolarian ecological and paleoenvironmental aspects of the Neogene of Japan are not known. Characterization of radiolarian assemblages and paleoenvironmental interpretation will be a considerable subject on the Japanese Neogene in future.The lower limit of the Eucyrtidium inflatum Zone is modified herein.