和泉山脈の形成と大阪層群

Abstract

The author intends in this study to clarify the formation of the Inland Sea in Plio-Pleistocene time in relation to the activity of the Median Tectonic Line (M. T. L.). The M. T. L. is believed to have been active through several stages, but details of its activity during Plio-Pleistocene time have not been known well. He investigated the Plio-Pleistocene sediments, the Osaka Group on the northern area of the Izumi Range which is bordered, on the south, by fault scarps of the M. T. L. formed in Plio-Pleistocene stages. This study was made by relating the change of facies of the Osaka Group with the uplift of the Izumi Range.As the result, the followings could be concluded:1. The Osaka Group in this area is divided into Uchihata members (the Pliocene), Amano members (the Plio-Pleistocene) and Semboku members (the late Pleistocene) in ascending order.2. The Uchihata members contain the gravels derived from the area to the south of the M. T. L. In this stage, the Izumi Range had not uplifted yet and the rivers from the southern area crossed the M. T. L.3. The M. T. L. became active after the sedimentation of Uchihata members. Along the M. T. L. was formed the structural valley of the Kino river on the south and the Izumi Range on the north. The latter supplied the surrounding area with coarse gravels. The Amano members were formed in this stage.4. In late Pleistocene time has developed the structure of the Osaka Group accompanied with faults, flexures and anticlines of N-S or NE-SW trend by E-W compression. The Semboku members deposited in this stage.