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311.
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 相似文献
312.
313.
Vertical profile of polycyclic aromatic hydrocarbons in a sediment core from a reservoir in Osaka City 总被引:1,自引:0,他引:1
Miho Ishitake Hiroshi Moriwaki Kenshi Katahira Osamu Yamamoto Kenshiro Tsuruho Hideo Yamazaki Shusaku Yoshikawa 《Environmental Geology》2007,52(1):123-129
Concentrations of polycyclic aromatic hydrocarbons (PAHs) were measured in a dated sediment core from a reservoir at Osaka
City, Southwest Japan. The sediment core consisted of deposits collected over a period of almost 70 years whose PAH content
would serve as a historical record of atmospheric environment at Osaka City. Total PAH concentrations varied from 4.2 to 26 mg kg−1 dry wt, and peaked in the 1940s, reflecting the occurrence of a large fire due to air attacks during World War II. The results
indicated that warfare had the largest impact on atmospheric environment in Osaka City. Total PAH concentrations decreased
in the post-war period except for a small peak. In the 1950s, there was a downward trend from the 1970s to the present. These
trends can be ascribed to the growth of industrial activities and the regulation of atmospheric pollutant emissions, respectively. 相似文献