Coastal Terraces and Rias along the Kesennuma Bay, Miyagi Prefecture

Abstract

The Kesennuma Bay, which has two branch embayments To-wan (eastern bay) and Sei-wan (western bay), is in the southern part of the Sanriku Rias coast, which is considered to be a submerged marginal area of Kitakami Mountains. It is generally said that terraces develop poorly along the Rias coasts. Along the coast near the Kesennuma Bay, however, terraces and constituting deposits are distributed rather well. The results of the author's study on terraces and relating matters are as follows.1) Coastal terraces are classified into Mitsumine Terrace, Shinjo Terrace, Matsuiwa Terace, Iwatsuki Terrace and Katahama Terrace from the upper to the lower.2) The terrace deposits excepting that of Katahama Terrace consist of gravels which are maturely weathered into Kusari (decayed) Gravel, and silt and sand lenses. Matsuiwa Terrace and Iwatsuki Terrace are composed of fluvial deposits iii the lower part and marine in the upper beds. Uncomformity between the bedrock surfaces and terrace deposits indicates valley forms like a buried valley beneath the alluvial plains.Red weathered crusts and soils are observed on the terraces higher than Katahama Terrace. From the above mentioned facts the author estimated that Matsuiwa Terrace and Iwatsuki Terrace were formed at least during the stages of high sea levels which followed the low sea level stages.3) Katahama Terrace surface is about 6m high above sea level, and a few meters higher than coastal lowland surface.4) Recent drowned valley is shown along the To-wan which is embranced by the Karakuwa Peninsular and the Kesennuma Oshima island (Fig. 6). The embayment of Ria shoreline at the Kesennuma Bay had been formed before these terraces were built. From the distribution of Senganda Formation it is considered that the formation of Ria shoreline at this area is dated back to Plio-Pleistocene.