Preliminary Report on the Formative Process of the Alluvial Lowland along the Upper Muko River, Eastern Part of Hyogo Prefecture, West Japan

Abstract

　The alluvia llowland along the Muko River in the southwestern part of the Sasayama basin is composed of clay and silt beds named the Benten Kokudo possibly of a lacustrine origin. We determined the subsurface structure of this lowland using the electric prospecting method, and the depositional ages of the alluvia ldeposit by volcanic glass analysis and 14C dating. The deposit is 2 to 7 meters thick and immediately above the gravel beds more than 20 meters thick. Along the section where the river cuts the Furuichi lineament striking northeast to southwest, the base and surface of the gravel beds have probably been displaced by about 20 meters and 1 to 2 meters, respectively. This suggests the possibility of late Quaternary active faulting along the lineament. The deposit is divided into two lithostratigraphic units : the upper fluvial unit and lower finer lacustrine unit deposited at approximately 25000 to 6300 yBP. The upper unit was deposited after 2930 yBP and is restricted to the lowland area close to the present river course. The subsurface structure and depositional ages show the complicated historical changes of this lowland somewhat different from the previous interpretation that it is simply of a lacustrine origin.