新・古第三系の分布に着目した岡山県中西部における 吉備高原地域の地形特性

The Kibi Plateau Surface is associated with the Paleogene gravel river bed deposits of the Kibi Group and the Miocene marine deposits of the Katsuta Group and the Bihoku Group. These Paleogene and Miocene units are valley fill deposits. The Kibi Group is composed of formations which is deposited by more than 7 different depositional events during earliest to latest Paleogene Period. Repeated cycles of “erosion - formation of new valley system - deposition and fill up the valley” created the low relief topographic surface. The deposition of the Miocene deposits might complete the Kibi Plateau Surface. Detailed field works have done in middle western part of Okayama Prefecture. The Kibi Plateau Surface in study area is gently dipping from NW (about 600m high) to SE (about 100m high). The distributions of the Paleogene and Miocene deposits fit the surface. At the southern margin of the Kibi Plateau, the surface is a little inclined (from 400m high to 100m high) where the bottom of paleo-valleys of the Paleogene deposits are subparallel to the surface. It suggests that the gentle tilting of the Kibi Plateau is caused by an up-warping.