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[HQR05-P07] Sedimentary characteristics of the Fuji-Sagamigawa Lahar deposit estimated from lithological composition of matrix
Keywords:lahar deposit, Fuji-Sagamigawa lahar, Sagami River, Katsura River, aggregate
The Fuji-Sagamigawa Mud Flow (Lahar) originated from the Fuji Volcano (Older Fuji) around 22 ka (Machida, 2009) flowed along the Sagami River (Katsura River) valley. We attempted to recognize sedimentary characteristics of the Fuji-Sagamigawa Lahar deposit (FSLD) based on lithological composition of matrix samples of FSLD.
FSLD is almost covered by younger lava flows of the Fuji Volcano and exposes on valley cliff of the upper Katsura River and the Shakunagare River, a branch of the Katsura River (e.g., Takada et al., 2016). Our recent survey revealed that the FSLD has the lower boulder layer below the well-known pebble layer at least upstream side of the Otsuki (e.g., Shirai et al., 2022).
Most of the outcrops of FSLD introduced by the Research Group for Geology and Geomorphology of the Sagamihara City (1990) have been hidden by vegetation and retaining wall, recently. Instead of the hidden outcrops, we discovered several new outcrops and took matrix samples from site-1 to site-3 (Figure) for measurement of lithological composition of matrices in the upper pebble layer (site-1) and the lower boulder layer (sites-2 and -3). Additionally, matrix samples were obtained from site-A to site-C to examine that partially exposed boulder layers belong the lower FSLD or not.
After washing with ultrasonic cleaner (4 to 12 hours), grains with 1 to 2 mm diameter were selected from matrix samples by sieving. Measured grains were composed by basalt, scoria, basement rocks (tuff and others), and aggregate of medium-sized grains. Aggregate implies deposition of mixture of sandy-muddy grains and rework (retransport-redeposition) of lahar deposit. Variation in aggregate ratio of FSLD matrix samples may be useful to estimate characteristics of lahar units composing FSLD.
Acknowledgment: A part of this study is performed as an investigations of the Research Center for Volcanic Hazards and Their Mitigation (Research Center for Hazard Mitigation in Volcanic Islands and Urban Areas), Tokyo Metropolitan University.
References
Machida (2009): Nature in History of Sagamihara City. p.159-165. Sagamihara City.
Research Group for Geology and Geomorphology of the Sagamihara City (1990): Report on the geology and geomorphology of the Sagamihara (4). Sagamihara City, 63p.
Shirai et al. (2022): 21st International Sedimentological Congress Abstract Book, T3-40859.
Takada et al. (2016): Geological Map of Fuji Volcano (2nd edition), Miscellaneous Map Series 12. Geological Survey of Japan, AIST.
FSLD is almost covered by younger lava flows of the Fuji Volcano and exposes on valley cliff of the upper Katsura River and the Shakunagare River, a branch of the Katsura River (e.g., Takada et al., 2016). Our recent survey revealed that the FSLD has the lower boulder layer below the well-known pebble layer at least upstream side of the Otsuki (e.g., Shirai et al., 2022).
Most of the outcrops of FSLD introduced by the Research Group for Geology and Geomorphology of the Sagamihara City (1990) have been hidden by vegetation and retaining wall, recently. Instead of the hidden outcrops, we discovered several new outcrops and took matrix samples from site-1 to site-3 (Figure) for measurement of lithological composition of matrices in the upper pebble layer (site-1) and the lower boulder layer (sites-2 and -3). Additionally, matrix samples were obtained from site-A to site-C to examine that partially exposed boulder layers belong the lower FSLD or not.
After washing with ultrasonic cleaner (4 to 12 hours), grains with 1 to 2 mm diameter were selected from matrix samples by sieving. Measured grains were composed by basalt, scoria, basement rocks (tuff and others), and aggregate of medium-sized grains. Aggregate implies deposition of mixture of sandy-muddy grains and rework (retransport-redeposition) of lahar deposit. Variation in aggregate ratio of FSLD matrix samples may be useful to estimate characteristics of lahar units composing FSLD.
Acknowledgment: A part of this study is performed as an investigations of the Research Center for Volcanic Hazards and Their Mitigation (Research Center for Hazard Mitigation in Volcanic Islands and Urban Areas), Tokyo Metropolitan University.
References
Machida (2009): Nature in History of Sagamihara City. p.159-165. Sagamihara City.
Research Group for Geology and Geomorphology of the Sagamihara City (1990): Report on the geology and geomorphology of the Sagamihara (4). Sagamihara City, 63p.
Shirai et al. (2022): 21st International Sedimentological Congress Abstract Book, T3-40859.
Takada et al. (2016): Geological Map of Fuji Volcano (2nd edition), Miscellaneous Map Series 12. Geological Survey of Japan, AIST.