Abstract
In order to clarify the geomorphic development of the Koriyama Basin and northwest part of Abukuma Mountains, Northeast Japan, geomorphological and geological survey were carried out. Low-relief surfaces named Funahiki, Kumagami, Miharu, Upper Moki, and Lower Moki low-relief surfaces (Koike, 1969) were recognized in this study. Kumagami and Miharu surfaces were covered by the Miharu pyroclastic flow deposits (Kubo et al., 2003) dated at 4.82+-0.12 Ma and 4.86+-0.12 Ma by K-Ar dating method. Upper Moki and Lower Moki low-relief surfaces were not covered only by the Miharu pyroclastic flow deposits but also by Ashino pyroclastic flow deposits (1.5-2.0 Ma), one of the Shirakawa pyroclastic flow deposits group. Koriyama Basin located west side of Abukuma Mountains was filled with sands and silts. It was only clarified that deposition of these sediments forming Koriyama Surface terminated at 120-60 ka by previous study. In this study, it is found that these sediments contain Nishigo pyroclastic flow deposits, one of the Shirakawa pyroclastic flow deposits group. The geomorphic development established from formation of low-relief surfaces and deposition of basin filling sediments by tephrochronology can provide data for the reconstruction of the history of Abukuma Mountains.
