Late Quaternary Sediments around Lake Aoki, Nagano, Central Japan, and the Origin of the Lake

摘要

The late Pleistocene and Holocene sediments around Lake Aoki are divided into the Ya- busawa Formation, Kuzuresawa Formation, Kamishiro Gravel-sand Formation, Sanosaka Debris Deposits, Aokiko Terrace Deposit, and Aokiko Deposits, on the basis of lithology and tephro-stratigraphic correlation. The Yabu- sawa Formation and its correlatives are mainly composed of silt with abundant plant remains, deposited in a cold period more than 50,000 years ago. They were formed by a meandering river which flowed from south to north as a relatively wide valley. The gravel sediments of the Kuzuresawa Formation and the middle part of the Kamishiro Gravel-sand Formation were formed by a braided river. A giant land- slide took place at the eastern margin of the Nishina Mountains and dammed up the river about 30,000 years ago. This landslide deposits made the Sanosaka Hill. The deep depression in the south is Lake Aoki, which has maintained a deep water con- dition until the present. The furrow in the north of the hill was filled gradually by a large amount of clastic sediment through river streams, forming the Kamishiro Basin.