100 , 2016 , Department of Geography, Tokyo Metropolitan University
Reconstruction of evolution of the Middle Pleistocene fluvial terraces in the drainage of the Natori River in Northeast Japan is important to understand how climate, sea-level changes, and crustal movements have controlled geomorphic development, particularly tectonic separation between the Tohoku Backbone Range and the basins located at the foot of the range. The purpose of this study is to clarify the chronological framework of the geomorphic development of these terraces. To reconstruct geomorphic evolution within a chronological framework, marker tephras; Adachi-Medeshima (Ac-Md), Tsubonuma Air-fall Pumice 4-2 (TbP4-2: MIS 7.4–5.2), Tsubonuma Air-fall Lithic Fragment (Tblf:MIS 7.4–5.2), Magarizaka Ash (MgA: MIS 7.4 or MIS 7.2) and the Tsubonuma Air-fall Pumice1 (TbP1: MIS 9), in descending order, were used. This study classified and newly defined the terrace surfaces in the study area into Imo-toge Surface, Moto-isago 1 and 2 surfaces, Aobayama 1 to 4 surfaces, Dainohara Surface and lower surfaces from older to younger. Imo-toge and Moto-isago 1 surfaces would have been formed as fluvial fans before MIS 9 based on stratigraphic relationship with the TbP1. Based on the deformation of the Moto-isago 1 Surface, tectonic separation of the Kawasaki Basin from the Tohoku Backbone Range finished afterMIS 10. Aobayama 1 to 4 and Dainohara surfaces would have evolved in the period from MIS 7.5 to MIS 5.2 reflecting climate or eustatic sea-level changes.