Distribution Change of Sand and Gravel during the Holocene, in the Abe River Plain, Shizuoka Prefectere, Central Japan

Abstract

In order to estimate sediment yield change during the Holocene, the author investigated distribution change of sand and gravel on the basis of analysis of a large number of bore-hole logs in an area supplied with a great deal of sand and gravel. In such areas the change of Holocene deposits distribution is expected to be influenced by environmental change in catchment areas.The Abe River Plain, extending about 12km from north to south and 7km from east to west, is located in the central part of the Honshu Island and is known as a typical torrential river. The upper parts of this river dissect rugged mountains, the Akaishi Mountains, forming steep mountain slopes where landslides have frequently occurred. And the accumulation terraces, which were attributed to the rapid erotion, are distributed along the river (Machida, 1959).Most of the plain is occupied with alluvial fans formed by the Abe River and other small rivers, and the alluvial formations of the plain are dominated by sand and gravel. But the seaward front of sand and gravel dominated area varies in several levels in the alluvial formation. Figs. 11-a, b, c, d, e, f represent the distribution of sand and gravel at intervals of 5m from the level of -20m (which means 20m below the present sea-level) to 0m, and +3m (which means 3m above the present sea-level), which provides the lowest surface on the plain.The results obtained from these 6 figures are as follows;(1) From the level of -20m to that of -10m and from that of -5m to that of +3m, the sand and gravel deposition area is reduced. On the other hand it is expanded from the level of -10m to that of -5m.(2) The area of sand and gravel is the largest on the level of -5m and on that of +3m the smallest.(3) The above trend of progression and retrogression of seaward front of sand and gravel dominated area does not correspond to the general trend of sea-level change during the Holocene.These results suggest the increase of sand and gravel supply of the Abe River around the age represented by the level of -5m. It calls for the reexamination of the assumption that the rate of deposition has kept constant rate during the Holocene in the Honshu Island.