中央構造線に沿う第二瀬戸内期の堆積場 : その時代と変遷  [in Japanese] Age and tectonic development of the Plio-Pleistocene sedimentary basins along the Median Tectonic Line, Southwest Japan  [in Japanese]

紀伊半島から四国にかけての中央構造線活断層系に沿った地域には, 河川ないし小規模な湖沼成の鮮新・更新統が細長く分布している。これらの地層の年代は, 植物化石やテフラの広域対比などから, 3 Ma頃から前・中期更新世境界前後までに及ぶと推定される。3 Ma頃から中央構造線に沿って, 小規模な堆積場が形成され始めたと考えられる。また2 Ma頃からは広く堆積盆地が形成され, この頃から断層活動も明瞭になったと推定される。これらの堆積盆地は前期更新世末期あたりまで存続し, その後少なくとも一部の地域では隆起してしまう。これにあわせて古い堆積場の形成に関与していた断層も不活発になったとみられる。

The Plio-Pleistocene sediments are distributed along active fault system of the Median Tectonic Line from central Kii Peninsula to Shikoku, Southwest Japan. Those are named the Shobudani Formation, Yudani Formation of the Osaka Group, Moriyama and Dochu Formations, Okamura Formation and Gunchu Formation from east to west. Each formation consists of a sequence of fluvial or lacustrine deposits. The age of the stratais estimated to range from the Middle Pliocene (ca, 3 Ma) to around the boundary of the Early-Middle Pleistocene on the basis of magnetostratigraphy, fission track dating, plant macrofossil assemblages and the correlation of widespread tephras. But 3 Ma as the lower limit age is not exact because it is based on plant fossils data only. It is estimated that small sedimentary basins were formed from about 3 Ma along the Median Tectonic Line, being separeted from the basins around the Osaka Bay on the northeast. The new sedimentary basins widely developed from about 2 Ma and continued till about the end of the Early Pleistocene, but after then some parts uplifted. It is not clear whether the present depositional areas were defined at that time or have developed later. The northern or southern margin of the basins formed during 2-1 Ma are bounded by the faults whose topographic features are not sharp. The time of the faulting was examined in the Kinokawa Valley area of Kii Peninsula as an example. Along the faults angular gravels belonging to the Shobudani Formation have deposited. The upper part of the formation has covered the fault cutting the lower part in the east area, while the upper part conformably overlies the lower part and is cut by the fault in the west area. Therefore it is estimated that those faults were active at nearly the same time when the sedimentary basin developed.