Abstract
The stratigraphic divisions of the Upper Pleistocene Shimosa Group forming the Hitachi Terraces in Ibaraki Prefecture are still poorly constrained, as their correlation with the type area of the group on Boso Peninsula has not been established. The Kioroshi Formation is the upper unit of the Shimosa Group. We have conducted a tephrochronological investigation of the Kioroshi Formation, including a stratigraphic and petrological study, and determination of volcanic glass major element chemistry and the refractive indices of glass, hornblende, and orthopyroxene. We have attempted to correlate tephra layers with those that can be expected to exist in a westerly source area around the north Kanto region and in marine cores off Kashima in the Pacific Ocean. Tephras ArP, OiP, and Ob that are intercalated with the Kenjaku Member in the lower unit of the Kioroshi Formation can be correlated with tephras Nk-Yt and Nk-Nm distributed in the westerly source area and TAu-9 in the southwestern Kanto region, respectively. Tephra KtP that is intercalated with the Namegata Member in the upper Kioroshi Formation can possibly be correlated with tephra 20 (1) in a marine core off Kashima. On the basis of these correlations, tephras ArP, OiP, and KtP can be interpreted to have been deposited during early MIS 5e, MIS 5e, and early MIS 5d, respectively. The Kenjaku and Namegata members in the Kioroshi Formation are estimated to have been deposited during MIS 6–5e and early MIS 5d, respectively.
