1995 Volume 47 Issue 4 Pages 285-301
The ages and formation processes of volcanic-fans in the foot of Mt. fwaki in the Aomori prefecture, Northeast Japan, were discussed.
The author recognized seven periods of volcanic-fans formation based on the stratigraphy of fan deposits and the six key tephra layers.
The key tephras are the Hachinohe Pumice (ca. 13ka.), the Ofudo Pumice (ca. 26ka.), the Upper brown loam, the Toya Ash (ca. 100ka.), the Lower brown loam and the Shinboshi Pumice formation (probably during the Riss Glacial Age).
From the examination of facies of the fan deposit and the comparison between the volume of debris and the volume of dissecting valleys in the side slope of the volcano, the most of the fan deposits may not be the normal sediments derived from the dissecting valleys but the volcanic debris flow, lahar or other kinds of debris flow caused by volcanic activities.
The history of the volcanic-fans formation in the northern foot of Mt. Iwaki are supposed as follows;
Period I (before the fall of Shinboshi Pumice formation): The deposit during the Period I might be provided as debris avalanche. The deposits of terrace I were composed of angular gravels, fractured huge blocks and many flow mounds.
Period III (after the formation of the Lower brown loam and before the fall of Toya Ash): The deposit during the Period III might be provided as debris flow. The sand and gravel deposits of terrace III show the facies of poor sorting, sub-round to round gravel shape. Partly the terrace III deposits are composed of coarse sand layers.
Period V (after the formation of the Upper loam and before the flow of Ofudo Pumice): The deposit during Period V might be provided as lahar. The deposits of terrace V were composed of tuffacious sandy gravel beds, including burned woods. Its facies is similar to the facies of deposits of period III.
Period VI (after the flow of Ofudo Pumice and before the flow of Hachinohe Pumice): The deposit during Period VI might be provided as pyroclastic flow. The deposits were composed of poorly sorted sub-angular gravels and the alternation of volcanic sand and fine sand layers. Terrace VI develops in the dissecting valleys.
Period VII (after the flow of Hachinohe Pumice): The most of the deposits were the normal fluvial deposits provided by stream flow through the dissecting valleys.
It is newly revealed that the most of deposits of volcanic-fans in the northern foot of Mt. Iwaki are the sediments occured by the volcanic activities.