Abstract
In the Daisetsuzan Mountains which are mainly composed of Quaternary andesitic lavas a number of block streams and block slopes are recognized. The block stream on the Chubetsudake NW slope is one of them. This study aims to elucidate the origin and the process of movement of this block stream on the basis of the data on the morphological and sedimentological characteristics of the block stream and the size and orientation measurements of boulders.
The block stream shows following characteristics.
1) Most of deposits have an openwork texture.
2) Ridges and furrows are formed on the surface of the block stream.
3) Transverse ridges and furrows are found on the upper part of the block stream and longitudinal ones are found on the lower part. But at the central winding part of the block stream ridges and furrows are transvers outside and longitudial inside.
4) The block stream is rarely covered with vegetation except lichen.
5) Brocken trees are found at the lower part of the block stream.
6) Two tephra layers are recognized on the boulder surface. The age of the upper layer is ca. 200 y. B. P. and that of the lower one is thought to be several hundreds years ago at most because the two tephra layers are only interbedded with a thin humus layer.
The results of the size and orientation measurements of boulders can be summarized as follows.
1) Boulders are better sorted on the several convex part in the longitudinal profile than remaining parts.
2) At the several localities in the longitudinal profile the orientation of boulders tends to parallel to the slope direction.
From the above, the origin of the block stream is thought to be a rockslide-avalanche. The directions of the ridges and furrows at the central winding part of the block stream suggest that the velocity of the movement was lower outside and higher inside.
The ages of the two tephra layers indicate that the block stream was probably formed around several hundreds years ago.
In the Daisetsuzan Mountains which contains young volcanoes there is a high possibility that a volcanic earthquake might trigger a rock avalanche. The last eruption of Asahidake is inferred to take place about 500 years ago. As this age approximates to the tephrochronologically inferred age of the block stream, the volcanic activity of Asahidake possibly caused the rockslide-avalanche on the Chubetsudake NW slope.
