RIAS COAST AND COASTAL TERRACES OF SANRIKU, NORTHEASTERN JAPAN

Abstract

The southern Sanriku coast is a typical rias coast in Japan with serrate shoreline ranging from Miyako Bay to Ojika Peninsular and poorly developed coastal terraces, is in striking contrast to the northern Sanriku coast with broad raised coastal terraces and nearly straight shoreline. The rias coast is considered to be the submerged maturely dissected margins of Kitakami mountains mainly consisting of pre-Tertiary rocks, and the coastal terraces along the embayments of the rias coast prove the uplifts of the area. The geomorphological history of the area seems to be a more complex one. The development of coastal landform of the area are divided into two stages; the stage of drowned embayments and the stage of coastal terraces. Judging from the Senganda. Formation (uppermost Pliocene) in the western coast of Kesennuma Bay and the piedmont steps or lower peneplain (pre-Pleistocene) in the northern Sanriku coast, the forming of rias coast outline as submergence coast is dated back to Plio-Pleistocene. It could be assumed that the submergence had been brought by a tectonic movement. At the following stage the coastal terraces have been formed inside the main embayments, and the stage is regarded as the stage of modification of rias coast, when the rias coast was modified by a series of submergence and emergence occurred alternatively, which is proved by the terraces surfaces and terrace deposits. Glacio-eustatic change of sea level seem to have been the principal factor in the geomorphic evolution during the stage, especially during the latter half of the stage. The coastal terraces can be classified into five levels; from the upper to the lower, Takada, Marumori, Sakari, Ofunato and Kadonohama Terraces at Ofunato region, and Toyomane, Omoe, Namiita and Lower Terraces at Miyako region. The levels of coastal terraces at both regions may be correlated respectively. Broad raised terraces developing along the northern Sanriku coast from Miyako to Omoto are designated as Settai, Taro and Furusato Terraces from the upper to the lower. These terraces are characterized by the higher elevations and the southward inclinations of their surfaces. This region appears to have uplifted by crustal movement more than the southern rias coast region where has been less influenced during the terrace forming stage. At Miyako region, higher terraces are well developed, but lower ones poorly, and the deposits can be observed at a few locations. At Ofunato region, on the contrary, the terrace surfaces are narrow in general, but the deposits of the lower terraces are observable at many locations. Hence, the process of terrace forming is well recognized at this region. Especially the deposits of Sakari and Ofunato Terraces are composed of fluvial beds in lower part and marine beds atop. Dissecting valleys, which had been cut during the regression of sea, were buried beneath these deposits. There are no critical evidence on the chronology of these ter-races, but according to the height of Ofunato Terrace surface and the red weathered crusts and soils covering Ofunato Terrace which is the lowest terrace among the terraces covered by the red weathered crusts and soils, it may be assumed that Ofunato Terrace had been built during the time of Riss-Wurm interglacial. Recent drowned valleys, corresponding to final stage of the history of Sanriku rias coast, are found inside the main bays. Some of them are obscured by subsequent deposition or remain as gorges without deposition. These valleys which were cut under the condition of low sea level during the last glacial, have been drowned by the post-glacial transgression.