Pleistocene Tephrostratigraphy and Underground Sediments under the Koriyama Basin at the Southern Part of Northeast Japan

Abstract

 Recent advance of Quaternary tephra studies in Northeast Japan enables us to reconstruct depositional histories in inland basins back to Middle to Early Pleistocene. The Koriyama Basin located in the southern part of the Northeast Japan arc is filled with the Koriyama Formation (KF) forming dissected Koriyama Upland. Previous studies have reported that KF is Pleistocene sediments composed of gravel, sand, and silt. However, comparing to numerous chronological studies in the coastal areas of Northeast Japan, chronological data for this basin-fill sediment are not enough. This causes in difficulties for the reconstruction of landform development in this area. Here, we present Early to Middle Pleistocene tephrostratigraphy beneath the Koriyama Basin. An all-core boring (KR-11-1) was carried out at the Fukushima Prefectural Koriyama-kita Technical High-school (FKTH) on the Koriyama Upland, where the Upper Part of KF (UKF: 0-46.31 m), the Lower Part of KF (LKF: 46.31-69.60 m) and the Shirakawa Formation (SF: 69.60-100.33 m) were detected. In UKF, we recognized two tephras, that is, Sn-SK (37.63-37.67 m; 0.17-0.27 Ma) and So-OT (38.24-38.40 m; 0.31-0.33 Ma). A pyroclastic flow deposit (KR8038) was found in 69.60-80.38 m in depth. It was correlated to U8 tephra (0.910-0.922 Ma) in the Umegase Formation of the Kazusa Group, marine sediments in the Boso Peninsula, south Kanto. We also recognized other two tephras, Hu-TK (0.15-0.20 Ma) and Sn-MT (0.18-0.26 Ma) from stored core (FKTH-A and FKTH-B cores) samples drilled at the FKTH. Tephrostratigraphy in KF and SF beneath the Koriyama Basin indicates that LKF deposited during 0.922-0.31 Ma, and UKF started to deposit before 0.33-0.31 Ma.