Revised stratigraphy for the Pleistocene system beneath the Higashi-Ibaraki plateau, NE Japan, and its relationship to the sea-level change in MIS 5-7: redefinition of the Ibaraki, Miwa, Natsumi and Kasagami Formations based on subsurface geology and correlation of tephra layers.

Abstract

The Pleistocene strata beneath the Higashi-Ibaraki plateau were formed under the influence of glacioeustasy. They consist mainly of muddy estuarine deposits filling an incised valley and overlying shoreface sandy deposits, and were called as the “Miwa Formation” by previous studies. However, the “Miwa Formation” includes several glacioeustasy cycles and should be divided into new stratigraphic divisions. In this paper, I have examined lithofacies data of 46 boreholes along the KasumigauraDosui tunnel and 19 ones along the Kita-Kanto and Higashi-Mito Highways, and recognize 6 incisedvalley systems within the Pleistocene strata. Furthermore, I have measured the major element compositions of 9 tephra samples from outcrops of the Pleistocene strata and detected the 95-ka KikaiTozuhara, 0.13-Ma Akagi-Mizunuma 8, 0.22-Ma Iiji-Moka and 0.24-Ma Ata-Toihama tephras. As the result, I have redefined the Ibaraki, Miwa, Natsumi and Kasagami Formations, in descending order. These formations were deposited in Marine Isotope Stage 5c, 5e, 7a-c, and 7e, respectively.