Boundary between the Kazusa and Shimosa groups beneath the central Kanto Plain: Recognition of MIS 12 in the GS-KS-1 core (Koshigaya, Saitama Prefecture, central Japan)

Abstract

We examined a sediment core recovered from Koshigaya, central Kanto Plain, central Japan, with the aim of understanding the stratigraphic location of MIS 12, which corresponds to the boundary between the Kazusa and Shimosa groups of the Boso Peninsula. Detailed analyses of sedimentary facies, pollen and diatom assemblages, and tephrochronology indicate that MIS 12 could be assigned to a bay ravinement surface at which stratigraphic level coarse sediments (e.g., river-channel deposits) are absent. Because the equivalent of the upper part of the Kazusa Group immediately below the bay ravinement surface consists of alternating beds of marine and non-marine sediments similar to those of the Shimosa Group, MIS 12 in the central region of the Kanto Plain is less significant in terms of lithostratigraphic subdivisions than that in the stratotype of the Boso Peninsula.