Preliminary report on the age and depositional process of the Fukuchi Peaty Beds in the upper reach of the Fukuchi River in the eastern part of the Chugoku Mountains, Western Japan

Abstract

The Fukuchi Peaty Beds have exposed along the upper reach of the Fukuchi River in the Chugoku Mountains at Shirakuchi, Shiso City, Hyogo Prefecture. They are composed of organic sandy clay with minor amounts of sand, and include a lot of plant fossils. Morphology and refractive index of glass shards contained in the beds have revealed they had deposited between 20 and 7.3 ka. Three AMS-14C dates from the plant fossils also indicate the deposition had begun at 9,140 to 9,435 cal BP in the early Holocene and continued at least until 8,457 to 8,638 cal BP with a very rapid sedimentation rate of 7 to 16 mm/yr. The landslide occurred at Shirakuchi between 9,140 and 9,435 cal BP is thought to be due to either a large earthquake by the Yamasaki fault zone or more probable torrential rainfall. The debris flow deposits dammed up the Fukuchi River, resulting in an elongate lake about 200-m wide and 500-m long at its maximum. This ancient lake had been buried rapidly with debris supplied from surrounding mountain slopes and plant remains. The Fukuchi Peaty Beds are thought to be parts of such filling deposits of the lake.