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.
Tephra units since 0.3 Ma in the Tochigi-Ibaraki region have been re-described with regard to their stratigraphy, compositions, ages and volumes, adding unpublished data. They were erupted from Takahara, Nantai, Akagi, Iiji and Haruna volcanoes. This study has listed up all tephra units, more than 0.1 km3 DRE in volume, from these volcanoes. The largest unit is the ca. 44-ka Akagi-Kanuma tephra (Ag-KP), whose minimum magma volume is 2 km3 DRE. Furthermore, this study has revised the tephrostratigraphy of the KR1 core in the central Tochigi by Kuwahara (2009), because his study included miscorrelations of some tephra units. New tephrostratigraphy is going to be a base of correlation for Quaternary strata and long-term evaluation for this region.
Towada volcano is an active volcano located in the northern part of the Northeast Japan arc. We carried out fission-track (FT) and AMS radiocarbon (14C) dating of tephras from Towada and Hakkoda volcanoes to establish a detailed eruptive history during the Pre-caldera and Caldera-forming stages of Towada volcano. The obtained FT ages were 0.23 ± 0.05 Ma for Shirobeta tephra (WP), 0.19 ± 0.05 Ma for T-6 tephra, 0.08 ± 0.03 Ma for Zarame 1 tephra (ZP1), and 0.09 ± 0.03 Ma for Okuse Pyroclastic Flow Deposits. Although these FT ages are consistent with stratigraphy, they have large error ranges of several tens of thousands of years. Therefore, further examinations including application of other chronological dating methods are necessary to obtain more precise estimates of the eruptive age of these tephras. The 14 obtained 14C age was 17,730 ± 70 BP for Biscuit 2 tephra (BP2). Based on stratigraphical consistency, this C age is appropriate for the eruptive age of BP2.