Abstract
In Lake Okotanpe, Hokkaido, sediment cores were obtained at 15 points from the lake bottom. A fresh volcanic ash layer was seen at the bottom part of twelve cores among them. Its thickness of about 1 mm suggests that the volcanic ash layer originates in the ash-fall from the 1977 eruption of Usu Volcano, Hokkaido. Hence, in order to clarify quantitatively the origins of core sediments, the Usu volcanic ash (1977-Us-II), and a surf icial soil and a riverbed material in the lake drainage area were mineralogically compared with the core sediments according to the following items.
(i) mineral composition of sand (250>d>37μ) and clay (d<2μ)
(ii) geometry of each sand mineral
(iii) chemical composition of fresh volcanic glass
The clay mineral composition and the chemical composition of volcanic glass were obtained with X-ray diffractometer and electron probe microanalyser, respectively. From the results involving vertical distributions of grain size and ignition loss of sediment core, the following conclusions can be drawn:
(1) The volcanic ash layer of sediment core was formed from the 1977 volcanic ash-fall of Usu Volcano.
(2) A “volcanic ash-mixed” layer on the pure ash one consists of the Usu volcanic ash and original lake sediments. The sediments of the two origins were both supplied from the surrounding drainage area through torrential rains.
(3) The lake-infilling is in considerable progress because of relatively heavy rains after the 1977 ash fall. The annual sediment accumulation of 1981 and 1982 is 15.2 and 10.1 cm yr-1, respectively, averaged for three representative cores.
(4) A spatial distribution of average sedimentation rate (cm yr-1 ) for 1977-1983 was obtained from the twelve cores. This distribution pattern in the lake can be consistently explained by sedimentary processes of river-induced turbidity currents and sediment dispersion during the river sediment discharge, as presented by CHIKITA (1986).
