Abstract
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.
