Geomorphic Development of the Yukuhashi Alluvial Plain, Northeastern Kyushu

Abstract

The Yukuhashi Plain, about 13km and 10km across from east to west and north to south, respectively, is located at the northeastern part of Kyushu and faces western Seto Inland Sea. The plain is surrounded by the mountains composed of Paleozoic limestone, metamorphics, Mesozoic granites and Quaternary volcanics except in its eastern margin. The plain is divided into two parts in consideration of its geomorphic features. The northern part of the plain consists of alluvial lowlands along the Obase and the Nagao Rivers flowing from west to east. The southern part of the plain is characterized by alluvial fans formed by the Harai and the Ima Rivers flowing south to north.The sequence of Recent deposits along the shoreline is shown in Fig. 4. The thickness of the deposits exceeds 16.5m at the hurried valley. From their textural characteristics, the deposits are divided into five layers as follows:1) Lowermost Sand and Gravel layer mainly consists of rounded gravels which are 0.5-4cm in diameter. Along the Harai rever, the layer is distributed to near surface.2) Lower Sand layer containing marine shells is considered as foreset bed of the deltaic sediment. At the lower part of the layer is dated 7, 710±210 14C years B. P. Marine diatom species are prominent at the upper part of the layer.3) Middle Mud layer containing marine shells (e.g. Tegillarca granosa) and humic materials is made up of silt or sandy silt. The layer also contains a lot of marine and brackish diatom species (e. g. Cyclotella striata, Melosira sulcata etc.), therefore the layer is considered to have been formed as bottomset bed sediments. At the period of the Middle Mud deposition, Postglacial transgression was reached at its maximum stage, and the inner bay widely spread in the plain. The period is estimated around 4, 800 years B. P.4) Upper Sand layer holds shell-fragments. This layer is considered to indicate the sedimentary environment of delta front. After the deposition of Middle Mud layer, deltaic sedimentation has been continuing.5) Uppermost Mud layer consists of organic silt deposited at backmarsh or lagoonal environment.In the plain, marine and brackish diatom species are appeared up to 1.8m above sea level. It denotes the highest sea level of the Postglacial transgression in the Yukuhashi Plain, but its age is unknown. The present high tide which exceeds 2m above mean sea level in this area may suggest that the present sea level is the highest during the Postglacial transgression.