High resolution analysis of cooling process in northern Japan during the Middle Miocene period based on microfossils.

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Based on the lithostratigraphy and foraminiferal biostratigraphy on the Middle Miocene shallow-marine deposits in northern Japan, from Hokkaido to Kanto and Hokuriku provinces, the following results were obtained.1) Several Miocene shallow-marine deposits in Hokkaido are discriminated into three categories of their ages deposited, before 15Ma, 15-12Ma, and after 12Ma. 2) The Togeshita Formation was deposited at sublittoral environment in its earlier (only in the central part of Rumoi) and middle stages (whorl part), but at bathyal depth in the others. 3) A subtropical larger foraminifer, Miogypsina, is found in the Ooanzaigawa Formation of Kaminokuni region, southeast Hokkaido. 4) The Ginzan Formation in Obanazawa, Northeast Province, is a shallowmarine deposits in 11.5〜10.5 Ma. Its assemblage indicates mild-temperate climate. 5) The assemblages in the Kobana Formation in Karasuyama, Kanto Province, indicate transgressive succession in warm-temperate shallow marine. 6) Sampling, sedimentary facies analysis and foraminiferal analysis are performed for the Kurosedani, Higashibessho, Tenguyama formations in Tonami region, the Nanao, Sekinohana, Suso, Higashi-innai, and Hojuji formations in Noto Peninsula. 7) A foraminiferal database system is constructed and tentatively began to use it. 8) The cooling event after the Mid-Neogene climatic optimum was diachronous. It began in ca. 12Ma at northern and eastern Hokkaido, but after 10Ma at middle part of Tohoku Province of northern Japan.