The geomorphic history of the ainoura plain, kyushu, japan, based on excavation of the monzen ruins

Abstract

The complex coastline of north-western Kyushu, Japan, consisting of cliffs, steep-sided inlets and archipelagos, has resulted from subsidence in mountainous terrain; the region's populated river valleys,having narrow alluvial plains, often suffer rock avalanches. The Ainoura plain is one such alluvial plain. Excavation of the Monzen ruins on the Ainoura Plain by the Nagasaki Prefectural Board of Educationhas revealed the plain's geomorphic history regarding how the gravels and associated muds and sands were deposited during the last several thousand years. Regarding alluvial succession, mud and sand firstfilled in the trough, possibly as part of the transgression caused by the Holocene climatic optimum, and gravel later began to replace parts of the sand and mud layers by successive erosion and sedimentation,thereby raising riverbed level. It was confirmed that such conglomerate from the Ainoura plain was derived from the Hokusho landslide area in the hinterland because there is no conglomerate layer inthe Hino plain (a small plain southeast of the Ainoura plain). The Ainoura River's thalweg shows that the Monzen ruins are located at a sudden reduction in its gradient, thereby accounting for the thicknessof coarse clastic material in that area. The reconstructed Ainoura plain's paleoshoreline during the Holocene climatic optimum is also reported. The paleoshoreline did not move much until people beganto reclaim land; it would thus be expected that the similar deposition would happened on the Ainoura plain inside this paleoshoreline.