SEA-LEVEL CHANGES DURING THE HOLOCENE AND GEOMORPHIC DEVELOPMENTS OF THE SENDAI COASTAL PLAIN, NORTHEAST JAPAN

Abstract

The Sendai coastal Plain is located on the Pacific side of northeastern part of Honshu Island, extending about 50 km long from north to south and 10 km wide from east to west. In this paper, the author studied the structure of alluvial formations and depositional processes of the Sendai coastal plain based on the analysis of boringlogs, radiometric datings and field survey. A curve of sea-level changes during the Holocene was restored and the geographical changes of shorelines were considered for the last 10, 000 years. The Holocene sea-level change in the Sendai region is summarized as follows. The rapid rise of sea-levels continued until 8, 000 yr BP and then rate of rising became slower. The sea-level reached nearly the present level at 4, 500 yr BP. Since then, it has been rather stable with slight fluctuations (Fig. 6). The Sendai coastal plain is consisted of alluvial formations ca. 60 m in thickness which are clearly divided into two kind of sediments ; marine or terrestrial in origin (Fig. 5). Based on the boundary surface between these two sediments, the geographical changes of shorelines during the Holocene is restored as follows (Fig. 7). (1) By the period when the sea-level reached about 30 m below the present one (ca. 9, 000 yr BP), the sea invaded landward and it passed the location just below the present shoreline in some places. This transgression continued until about 8, 000 yr BP. (2) After the period when the sea-level reached about 10 m below the present one (ca. 8, 000 yr BP), the Sendai coastal plain stopped decreasing in extent and began to expand seaward by filling up the shallow sea with terrestrial sediments, although the sea-level was still rising (ca. 8, 000 yr BP-4, 500 yr BP). (3) Following the former period, the shoreline continued to progress seaward intermittently and reached the present position for the period from 4, 500 yr BP to the present. The author emphasized that the period of the highest sea-level in the Holocene and that of the maximum areal extent of the sea did not coincide with each other in time. This time-gap is considered to depend on the conditions of the amount of loads supplied from nearby rivers. The beginning time of a seaward expansion of the coastal plain was earlier in the area supplied with larger amount of loads than in the area with smaller amount.