Depositional features and transportation mechanism of valley-filling Iwasegawa and Kaida debris avalanches, Japan |
| |
| Authors: | Shinji Takarada Tadahide Ui Yuko Yamamoto |
| |
| Institution: | (1) Hokkaido Branch, Geological Survey of Japan, Kita-8, Nishi-2, Kita-ku, Sapporo 060-0808, Japan Fax: +81-11-709-1817, e-mail: takarada@gsj.go.jp, JP;(2) Department of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Kita-10, Nishi-8, Kita-ku, Sapporo 060-0810, Japan, JP;(3) Nippon Denso Co., Ltd., 1-1 Showa-cho, Kariya, Aichi 448-0029, Japan, JP |
| |
| Abstract: | The depositional features of two valley-filling debris avalanche deposits were studied to reveal their transportation and
depositional mechanisms. The valley-filling Iwasegawa debris avalanche deposit (ca. 0.1 km3) is distributed along the valleys at the southeastern foot of Tashirodake Volcano, northern Honshu, Japan. Debris-avalanche
blocks range in size from <35 m proximally to <10 m in the distal zone and consist dominantly of fragile materials. Debris-avalanche
matrix percentages increase from 35–60% in the proximal zone to 95% in the distal zone. The debris-avalanche matrix is greater
in volume (80–90%) at the bottom and margins of the deposit. Normal grading of large clasts and reverse grading of wood logs
and branches occur within the debris-avalanche matrix. Preferred orientation of 311 wood logs and branches within the deposit
coincide with the interpreted local flow direction. The basal part of the deposit is characterized by (1) erosional features
and incorporated clasts of underlying material; (2) a higher proportion (30–50%) of incorporated clasts than the upper part;
and (3) reverse grading of clasts.
The valley-filling Kaida debris avalanche deposit (50 000 y B.P., >0.3 km3) is distributed along the valleys at the eastern-southeastern foot of Ontake Volcano, central Japan. Debris-avalanche blocks
range in size from <25 m proximally to <7 m in the medial zone. Debris-avalanche matrix percentages increase from 50–70% in
the proximal zone to 80% in the distal zone. The debris-avalanche matrix is more abundant (80–90%) at the bottom part of the
deposit. Deformation structures observed in the debris-avalanche blocks include elongation, folding, conjugate reverse faults,
and numerous minor faults in unconsolidated materials. Lithic components within the debris-avalanche matrix tend to have a
higher percentage of plucked clasts from the adjacent underlying formations.
A Bingham "plug flow" model is consistent with the transportation and depositional mechanisms of the valley-filling debris
avalanches. In the plug of the debris avalanche, fragile blocks were transported without major rupturing due to relatively
small shear stresses in regions of small strain rate. The debris-avalanche matrix was mainly produced by shearing at the bottom
and margins of the avalanche. Valley-filling debris avalanches tend to have smaller debris-avalanche blocks and larger amounts
of debris-avalanche matrix than do unconfined debris avalanches. These differences may be due to disaggregation of debris-avalanche
blocks by shearing against valley walls and interaction between debris-avalanche blocks and valley walls. Oriented wood logs
and branches, reverse grading of clasts at the base, and a higher proportion of incorporated clasts at the base are interpreted
to result from shearing along the bottom and valley walls.
Received: 25 March 1998 / Accepted: 10 October 1998 |
| |
| Keywords: | Debris avalanche Valley filling Debris-avalanche block Debris-avalanche matrix Wood orientation Basal structure Plug flow |
| 本文献已被 SpringerLink 等数据库收录！ |
|
