Abstract
Correlation of the Nanatani to Uonuma Formations of 13Ma-1Ma in the Niigata region on the Sea of Japan side was comprehensively investigated using subaqueous tephra marker layers. The examined areas are the Sado Island, Nishiyama-Chuo Oil Field, Niitsu-Shitada Hills, Higashiyama-Uonuma Hills, Hachikoku Oil Field, Yoneyama region, Higashikubuki Hill, Arai region and Nishikubiki region. The tephrostratigraphical investigation was proved to be effective especially in the sedimentary basin prevailing marked contemporaneous heterotopic facies relations from fluvio-lacustrine to bathyal environments as in the Niigata basin. The age estimation of the predominant ash layers was also attempted based on the correlation of widespread ash layers to those in the Himi, Tokai and Kazusa Groups, and on the data of the magnetostratigraphy and diatom and nannofossil zones. The shallow marine to fluvio-deltaic environments (the Uonuma Group) were appeared initially in the southern Higashikubiki and the Uonuma Hills in 2.4-2.2Ma, and the shallow facies progressively extended toward the Nishiyama-Chuo Oil Field in 2.2-1.0Ma.
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