Stratigraphic positions of tephra layers from the Shiobara Caldera in the Middle Pleistocene Yabu Formation, Shimosa Group, beneath central Kanto Plain, central Japan

Abstract

A tephra correlated with the So-OT tephra from Shiobara caldera in northern Tochigi Prefecture was identified in the Yabu Formation of the Middle Pleistocene Shimosa Group in the GS2012-OMY core, drilled at Fukasaku, Saitama City, Saitama Prefecture, on the central Kanto Plain, Japan. This tephra and the So-TN tephra immediately below it have been recognized at several locations on the Kanto Plain, indicating they occur between the Yb1 and Yb5 marker tephras in the Yabu Formation. Furthermore, the So-OT tephra is stratigraphically located just above the sea-level highstand of Marine Isotope Stage 9.3. Recently, the importance of the Yabu Formation on the Kanto Plain has been increasing from both applied geological and hydrogeological viewpoints as a foundation for structures and as an aquifer. The identification of additional marker tephras in the Yabu Formation is expected to improve our understanding of its distribution.