The use of tephras for stratigraphic correlation in the eastern margin of Japan Sea

Abstract

This study is based on the analysis of tephra samples collected by piston-coring along the eastern margin of Japan Sea. The characterization of marker- tephras constitutes the basis of tephra studies and it is carried out using a combination of parameters, such as chemical composition, petrography, and stratigraphic and geographic distribution. Twelve piston-cores distributed from the north of central Honshu, offshore Kanazawa City, to the west of Hokkaido, more than 500km away, were used in this study. Samples were analyzed by SEM-EDS and their chemical composition and glass shard shapes were compared with those of cataloged marker-tephras in and around Japan. Main marker-tephras such as Aso-1, K-Tz, Aso-4, AT, and U-Oki were recognized and provided a good tephra correlation along the eastern margin of Japan Sea. Age control using C14 of foraminiferal tests and the identification of diatom assemblage supported the correlation between tephras and were used together to construct sedimentation rate curves. Seafloor topography strongly controls the accumulation rate, which are different between troughs and ridges in the latter condensed sections are present. Four unknown tephras were identified in this study and were called Joetsu-1 (Jo-1), Joetsu-2 (Jo-2), Okushiri-1 (Ok-1) and Okushiri-2 (Ok-2). Their petrographical and chemical characteristics are now available.