Terraces and Volcanic Ashes along the Middle and Lower Courses of the Mabechi River

Abstract

The Mabechi River drains the northwestern part of Iwate Prefecture and the southeastern part of Aomori Prefecture, Northeast Honshu, Japan. This paper describes the terraces and volcanic ash layers distributed along the middle and lower courses of the trunk stream (Fig. 1), and their chronological interrelation (Figs. 2 and 3).
The Hachinohe Volcanic Ash covers the surface of the Tamonoki and the older terraces and comprises ash-flow tuff which builds a flat-top topography at the foot of the valley walls. Along the middle stream, the main part of the Fukuoka Terrace consists of this ash-flow tuff. Some features of the ash-flow tuff of the Hachinohe Ash are reported.
Decision of the ages of the terraces is based on the radio-carbon date of the natural charcoal from the Hachinohe Ash, 12, 700±270yr. B. P.(GaK-205), and the standard sequence of development of the terraces along the Pacific coast of Japan by Nakagawa (1961b). The Takadate Terrace is dated to the last high sea-level stage of the late Pleistocene and the Nejo and Tamonoki Terraces are dated to have developed during the oscillations of the low sea-level stage of the latest Pleistocene. This latter date corresponds to the Last Glacial stage.