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[HQR03-P07] Fuji-Sagamigawa Lahar deposit distributed from Otsuki to Uenohara cities, Yamanashi Prefecture
Keywords:lahar deposit, Fuji–Sagamigawa mud flow, Sagami River, Katsura River
The Fuji-Sagamigawa Mud Flow (Lahar) originated from the Fuji Volcano (Older Fuji) around 22 ka (Machida, 2009) distributes along the Sagami river valley. Detailed field survey and description on the Fuji-Sagamigawa Lahar deposit (FSLD) at mainly downstream side of the Otsuki City were carried out by Research Group for Geology and Geomorphology of the Sagamihara City (1990). Our recent survey revealed that the FSLD has the lower boulder (and cobble) part below the well-known pebbly part at least upstream side of the Otsuki (e.g., Shirai et al., 2022). It makes necessary to reexamine lithofacies and distribution of the FSLD at downstream side of the Otsuki.
Most of the outcrops of FSLD introduced by the research group on 1990 have been hidden by vegetation and retaining wall, recently. Instead of the hidden outcrops, we discovered two broad outcrops (sites-1 and -2 in Figure) newly.
Site-1 (ca. 5km downstream from the Otsuki): FSLD consists of upper pebbly layer (>14 m thick) composed by multiple units and lower boulder layer (>2 m thick). Boulders in the lower layer are ca. 1 m in diameter. Pebbly layer shows fining upward succession from ca. 3 cm to ca. 1 cm in diameter.
Site-2 (ca. 15 km downstream from the Otsuki): FSLD (>6 m thick) overlies fluvial terrace deposit and consists mainly of pebble. The pebble layer looks like a continuum (a single unit) with fining upward succession from ca. 3 cm to ca. 1 cm in diameter. Boulders with ca. 40 cm diameter are contained in the basal part.
It is estimated that the lower boulder layer at the site-1 and the whole pebble layer at the site-2 should have been the main body of the FSLD. It means that the lahar decreased its transport capacity from the site-1 to the site-2 with ca. 10 km distance. However, the FSLD in the Sagamihara ca. 30 km downstream side from the site-2 are overlain by boulders in ca. 70 cm diameter (e.g., Takehara et al., 2017), which were transported by the lahar, and it suggests increase in transport capacity of the lahar. It is necessary to investigate successively for the understanding flow process of the Fuji-Sagamigawa Lahar.
References
Research Group for Geology and Geomorphology of the Sagamihara Cit (1990) Report on the geology and geomorphology of the Sagamihara (4). Board of Education, Sagamihara City, 63p.
Machida (2009) Nature in History of Sagamihara City. Sagamihara City, 159-165.
Shirai et al. (2022) 21st International Sedimentological Congress Abstract Book, T3-40859.
Takehara et al. (2017) Bulletin of the Sagamihara City Museum, 25, 60-73.
Most of the outcrops of FSLD introduced by the research group on 1990 have been hidden by vegetation and retaining wall, recently. Instead of the hidden outcrops, we discovered two broad outcrops (sites-1 and -2 in Figure) newly.
Site-1 (ca. 5km downstream from the Otsuki): FSLD consists of upper pebbly layer (>14 m thick) composed by multiple units and lower boulder layer (>2 m thick). Boulders in the lower layer are ca. 1 m in diameter. Pebbly layer shows fining upward succession from ca. 3 cm to ca. 1 cm in diameter.
Site-2 (ca. 15 km downstream from the Otsuki): FSLD (>6 m thick) overlies fluvial terrace deposit and consists mainly of pebble. The pebble layer looks like a continuum (a single unit) with fining upward succession from ca. 3 cm to ca. 1 cm in diameter. Boulders with ca. 40 cm diameter are contained in the basal part.
It is estimated that the lower boulder layer at the site-1 and the whole pebble layer at the site-2 should have been the main body of the FSLD. It means that the lahar decreased its transport capacity from the site-1 to the site-2 with ca. 10 km distance. However, the FSLD in the Sagamihara ca. 30 km downstream side from the site-2 are overlain by boulders in ca. 70 cm diameter (e.g., Takehara et al., 2017), which were transported by the lahar, and it suggests increase in transport capacity of the lahar. It is necessary to investigate successively for the understanding flow process of the Fuji-Sagamigawa Lahar.
References
Research Group for Geology and Geomorphology of the Sagamihara Cit (1990) Report on the geology and geomorphology of the Sagamihara (4). Board of Education, Sagamihara City, 63p.
Machida (2009) Nature in History of Sagamihara City. Sagamihara City, 159-165.
Shirai et al. (2022) 21st International Sedimentological Congress Abstract Book, T3-40859.
Takehara et al. (2017) Bulletin of the Sagamihara City Museum, 25, 60-73.