Abstract
Quaternary laminated and stratified mud formations distributed along the Anegawa River running on the western foot of Mt. Ibukiyama, eastern Shiga Prefecture, are interpreted as dammed-lake deposits on the basis of their lithology, distribution, and the presence of landslide debris accumulated below the western and southern slopes of Mt. Ibukiyama.
The mud formations are subdivided into the younger and older lake deposits. The former, in part, consists of thinly laminated sediments including clastic fragments, diatom frustules, siderite framboids, and organic materials. The laminaewere formed annually by the cyclic repetition of blooming of diatom in early spring, siderite formation in summer as anoxic lake condition, and later clay mineral accumulation, plus periodical inputs of coarse clastic materials and organic matters from the catchment area. The 14C ages of plant remains embedded in the younger and older lake sediments are scattered around 5, 000 cal BP and about 30, 000-40, 000BP, respectively. The age data together with the duration of the varved sediments accumulation, fall age of tephra, and descriptions in ancient documents indicate that the large-scale landslides on the western slope of Mt. Ibukiyama forming a dammed lake along the Anegawa River and accumulating thick sediments in the lake have occurred at least two times.
