岡山市北部吉備高原地域に分布する古第三系・新第三系の分布・堆積相と古地理変遷

The deposition of the Paleogene to Neogene sediments in the Kibi Plateau region played an important role to form the Kibi Plateau Surface. Study area is situated in the southern part of the Kibi Plateau. Field work has been continued more than 30 years and traced distribution of the deposits which make clear the paleo-topography. Sedimentary facies of deposits were observed to understand depositional environments. Without Fission-Track zircon dating from tuff samples, we could not divide and define the formations. The Paleogene Kibi Group and the latest Paleogene to Neogene Bihoku Group are distributed in the study area. The Kibi Group is unusual deposits which filled steep valley in mountain area and comprise the Tomiyoshi Formation (36 to 34 Ma) and the Tsudaka Formation (29 to 27 Ma). The formations are fluvial deposits and have similar litho-facies. The distribution of the formations indicate drainage system when deposited. Conglomerates with imbricate structure are the major component of the formations and sandstones and mudstones (sometimes with plant fragments and rootlets) are associated. The Bihoku Group is composed of the fluvial Yagane Formation (24.5 to 25.5 Ma) and the shallow marine Nichiouji Formation (15 to 16 Ma). The formations have the same depositional basin of low relief valley. The Yagane Formation is composed of conglomerate, sandstone, mudstone and coaly mudstone. The Nichiouji Formation is composed of well sorted sandstone with marine fossils.