COASTAL TERRACES OF THE SADO ISLAND, JAPAN

Abstract

The Sado Island with 857 km2 in area is situated in the Japan Sea, about 35 km off Honshu. This islandd consists of two regions, Osado and Kosado. The former region which lies in the northern half is larger than the latter and also higher in altitude, and considered to be a tilted block with steeper foreslopes on the east side. This area is composed mainly of volcanic rocks such as andesite, liperite, basalt, and Miocen green tuff. Surrounding this island are coastal terraces developed widely. The author surveyed these coastal terraces and their deposits in the Osado region, in order to clarify their distribution, altitude, and nature and to consider the geomorphic history. The main results are summarized as follows: 1) The coastal terraces are divided into six plains owing to their altitude and continuity. They are as follows: the first terrace (160-220 m in altitude), second (80-140 m), third (60-120 m), fourth (35-70 m), fifth (25-40 m), and the sixth (5-8 m). 2) These terraces are well developed in the western coastal region in general. On the east side, however, they are no more than a fragmentary distribution. Among the terraces of the Osado region, the third and fourth terraces show a broad and cotinuous distribution. The sixth terrace is also developed all around the island in spite of the limited width. 3) Considering from the thickness and facies of the terrace deposits, the nature of the terrace plains can be classified into four groups. A a In the western coastal region, every terrace plain is of a marine origing formed by striking abrasion of the open sea. B: On the coast along the Mano Bay, the terraces are thought to be of a marine origin formed by the cut and built action of waves in the inland bay. C: In the Kuninaka Plain, the second and third terraces are elevated fans built by the rivers from the northern mountains. On the contrary, the fourth terrace is a dissected coastal plain which consists of thicker neritic sediments deposited in the inland bay. D: In the eastern coastal region, marine terraces are hardly recognized, and most of the flat plains are very small elevated fans consisting of poorly sorted boulders transported by the torrential streams down from the steep slopes. 4) The sixth terrace continues to the alluvial plain of the Kuninaka area, composed of marine alluvial deposits containing the fauna which indicates warmer water temperautre than the present sea in the vicinity and having about 70 m in thickness. Therefore, the formation of the sixth terrace plain is to be correlated with the transgression of early alluvial age which is recognized at many places in Japan. As for the fourth terrace which has a wide distribution and thicker neritic sediments, it may be considered that it was built by the transgression preceeding the regression of the last glacial age just before the alluvial trans-gression. 5) The differences in the character of terrace plains which were mentioned in 3) seem to haue been influenced chiefly by the asymmetry of topography and the condition whether open-sea-facing or not. Rock control is recognized where stacks are found on the terrace plains in the volcanic rock areas of Osado. Concerning the formation of the terrace as a whole, however, it is of the secondary importance, since topographic differences such as those of slopes seem to be the primary differentiating causes. 6) On every marine terrace plain, the elevations of the ancient strand lines increase in an accelerated manner towards the middle part of both sides of the Osado coasts, and this tendency is more evident on the older terraces. This means the upwarping movement with the axis at the middle part of the island has continued since the formation of the first terrace plain.