札幌南西山地における3つの地すべりに関する14C年代
1986 年 59 巻 2 号 p. 98-107
詳細
1986 年 59 巻 2 号 p. 98-107
It is essential to obtain radiocarbon ages for the investigation of the period when landslide frequently occurred. Landsliding processes must be considered on the basis of microlandforms within a landslide area.
Many landslides are distributed in the mountains southwest of Sapporo, Hokkaido. In this paper, topographical features of three landslides along the Izariirisawa River, west of Nakayama Pass and on the NE slope of Muineyama are described, and the radiocarbon ages are presented in relation to the occurrence of these landslides.
The small-scale landslide along the Izariirisawa River occupies an area of 8.2 ha and is composed of such simple landform elements as a pair of scarp and sliding block. The radiocarbon ages of wood from the landslide deposits suggests that this landslide occurred at about 11, 000 y. B. P.. The landslide west of Nakayama Pass has an area of 29.4 ha and comprises four sliding blocks. As these blocks are dissected to show the almost same topographic features, it seems possible that these blocks were formed within a same period. The radiocarbon age of wood in a block is older than 31, 250 y. B. P. The landslide occupying an area of 379.4 ha on the NE slope of Muineyama is topographically composed of main scarp, main sliding block and depositional surface of debris flow. Such micro-landforms as mounds, ridges etc. can be recognized both on the main sliding block and on the depositional surface of debris flow. This suggests that the sliding process responsible for the formation of this landslide occurred more complicatedly than that of the other landslides. The samples for radiocarbon dating were found as follows : peat in the bog formed on the sliding block, a piece of wood in the landslide deposits and a piece of charcoal in the debris flow deposits. The radiocarbon ages of these samples show that debris flow occurred at around 8, 400 y. B. P. prior to the occurrence of landslide on the upper slope at around 7, 100_??_6, 200 y. B. P. This suggests that landslide did not always occurr together with debris flow.
