Keywords:Jikumaru terraced paddy field, Top 100 Terraced Paddy Fields in Japan, Onogawa Group, Aso pyroclastic flow, Oita Bungoono Geopark
The Jikumaru terraced paddy field in the Oita Bungoono Geopark is selected as one of the Top 100 Terraced Paddy Fields in Japan. This area is placed between the Ono and Ogata Rivers and the terraced paddy fields are developed in the small and shallow valleys. The bedrock of this area consists of the sandstone and conglomerate of the Late Cretaceous Onogawa Group Ryozen Formation. The non- to weakly welded Aso-3 and Aso-4 pyroclastic flow deposits covered over the Onogawa Group. The Aso-4 pyroclastic flow deposit along the Ono and Ogata Rivers is densely welded. It indicates the sedimentary rocks of the Onogawa Group formed small hills before the Aso-3 and Aso-4 pyroclastic flows. The pyroclastic flow deposits are thin and easy to erode. Accordingly, the small and shallow valleys where terraced paddy fields are developed are formed in this area. The formation of the Jikumaru terraced paddy field is related to the geology and geomorphology of this area and it may be one of the important elements of the Oita Bungoono Geopark.