Abstract
At the northwestern foot of the Daisen volcano, which has a relative height of about 1, 700m above the foot, many alluvial fans different in age widely develop. The surfaces are divided into six groups according to their ages; Older fan I (OF I), Olderr fan II (OF II), Middle fan (MF), Younger fan I (YF I), Younger fan II (YF II) and Youngest fan (YTF). The ages were estimated from the stratigraphic relations of the fan deposits to marker tephras.
The MF, YF I and YF II fans are developed downstream of the valleys, in which the Nawa pyroclastic flow deposits (Nw), the Makibara pyroclastic flow deposits (Mk) and the Odori pyroclastic flow deposits (Od) are distributed, respectively. The fan deposits are composed of the materials supplied from the related pyroclastic flow deposits.
The process of MF, YF I and YF II fan formation is summarized as follows:
1. A pyroclastic flow containing many rock fragments of cobble to boulder size was deposited on the bottom of a radial valley dissecting the body of the volcano.
2. When the pyroclastic flow deposit was loose, or not welded, the river immediately started to erode it.
3. The gravel and sand were transported from the downstream tip of the pyroclastic flow deposits mainly in the form of debris flow.
4. The debris flows formed alluvial fans at the foot of the volcano, where the slope becomes gentle and wide plains develop.
The ages of the fan surfaces of MF, YF I and YF II are 90, 000_??_100, 000 y. B. P., ca. 20, 000 y. B. P. and 7, 000_??_10, 000 y. B. P., respectively. The ages of the pyroclastic flow deposits of Nw, Mk and Od are ca. 100, 000 y. B. P., 25, 000_??_30, 000 y. B. P. and ca. 17, 000 y. B. P., respectively. Therefore, the time needed for the fan formation is estimated as less than 5±103 or 1±104 years. This indicates that the period of fan formation at the foot of volcano is very short as compared with the life of a large volcano, which is in the order of 105 years.
