Coastal Terraces along the Noheji Bay, Northeasten Japan

Abstract

Along the western coast of the Noheji Bay, several terraces are observed. One of them was named Noheji Terrace (Nakagawa 1963), whose deposit is called Tanabu Formation. It was assumed that the terrace had been formed during the time of the high sea level within Riss-Wiirm interglacial stage.
According to the investigation by the present author, however, the terrace can be classified into two levels ; the upper is Tateno Terrace (fill-top terrace of Tateno Gravel bed which is of fluvial origin) and the lower Noheji Terrace (Noheji Formation which is marine). A higher terrace is named Kuchihirotai Terrace and a lower one is designated as Kuchihiro Terrace. The lower terrace is a few meters higher than coastal lowland, and is covered with two layers of volcanic ash. The lower layer is reddish brown clayey ash, and the upper is chiefly composed of yellowish brown pumice grains.
The layer is correlated with Hachinohe Volcanic Ash (Oike 1954), and the upper is Chibiki Pumice (Nakagawa 1963).
In the eastern coast of the Noheji Bay and Oshimashiruchi coastal area, terraces which correspond to Kuchihiro Terrace are found, and are called Yokohama Terrace and Shiruchi lower Terrace. Judged from volcanic ash underlying the terraces, these terraces are formed in Late Pleistocene.
Excepting Kuchihiro Terrace, the terraces are dissected by numerous shallow valleys, and transformed into a hilly land. Gentle slopes of shallow valley are covered with reddish brown clayey ash mentioned above. It is consequently concluded that these valleys had already been cut before the fall of Hachinohe Volcanic Ash.