The Fukuchiyama Basin is one of the small intermontane basins distributed in the Meso-Paleozoic terrain of the northwestern part of the Kinki District. The basin is filled with the Quaternary deposits having wide sedimentary surfaces which are regarded as the extensions of the“Higher Terrace”widely spreading in the central part of the Kinki. The followings have been clarified by the present studies. 1) The Quaternary deposits in this basin reaches more than 50m in thickness consisting of five sedimentary units succeeded conformably, which are collectively called the Fukuchiyama Formation. Each unit was formed as a small fan developed along the marginal part of the basin or as fluvial deposit along the main river course flowing into the basin. 2) Fig. 13 is the summary of the results of the studies of the Fukuchiyama Formation from viewpoints of paint fossil and paleomagnetism. No evidence of the Metasequoia flora was found, which shows that this formation is correlative to the Upper Part of the Osaka Group. The Sabia bed, intercalated in the upper horizon of the formation, contains the plant remains of warmer or subtropical types and is thought to correspond to the Ma 8 horizon of the Osaka Group. 3) The paleomagnetic polarity of the Fukuchiyama Formation is normal except one narrow horizon showing reverse event. This is the character of the Brunhes Normal Epoch. 4) The Fukuchiyama Formation closely resembles to the Meimi Formation in the Harima Basin in various points, which has a FT age of 0.49±0.09 Ma in the lowest horizon. 5) The age of the Fukuchiyama Formation is inferred to be the Middle Pleistocene, about 0.6 or 0.7 Ma to 0.2 or 0.15 Ma, considering that the Brunhes Normal is generally dated 0.7 Ma to present with brief revesals at 0.11 and 0.02 Ma and Ma 8 horizon of the Osaka Group is intercalated with a volcanic ash layer having a FT ale of 0.38±0.04Ma.
The Asono Formation is a lacustrine deposit consisting of diatomaceous siltstone, tuffaceous sandstone, tuff and conglomerate. The formation abuts unconformably against volcanic basement rocks, including the Kumamureyama rhyolite that has a FT age of 0.73±0.14Ma. The upper part of the Asono Formation contains a lava flow which has a pseudo-pillow structure. This flow is called the Uchiyama hornblende andesite and its FT age is 0.34±0.17Ma. Thus the Asono Formation is inferred to deposit during the Middle Pleistocene. The fossil diatoms from the Asono Formation are all freshwater species. Six zones have been identified, based on the predominant species. Zone A: Cylotella zone Zone B: Stephanodiscus zone Zone C: Cyclotella zone Zone D: Opephora zone Zone E: Melosira zone Zone F: Stephanodiscus zone The fossil diatom assemblages show the water in Lake Asono was alkaline throughout deposition of the formation. The alkalinity was higher and the eutrophy more advanced during depostion of the Zone E and F sediments than the other sediments. Pollen analysis indicates probably cooler water temperature at the time of Zones B and F than the other zones.