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[HQR04-P08] Landform evolution since the Middle Pleistocene estimated at the lower reaches of the Himekawa River, central Japan
Keywords:Fluvial terrace, Himekawa River
Studies on fluvial terrace have been conducted in many areas of the Japanese islands, and their relationship with climate changes and sea-level changes has been discussed. However, studies focusing on gravel shape (form and roundness) of fluvial terrace deposithave been seldom carried out. In addition to topographic classification and construction of chronology, the authors compared gravel shape of fluvial terraces in the lower reaches of the Himekawa River in the western Niigata Prefecture with gravel shape of the present Himekawa river bed.
Throughout the study, some results were obtained;
(1) The form of the pebbles on the present Himekawa River show that ratio of spherical (cubic) and disc shapes decrease and rod shape increases downstreamward. Local increase in ratio of spherical (cubic) and disc shapes in the middle reaches near by the confluence with the Urakawa River should be influenced by a giant slope-failure “Hieda-Yama-Kuzure” where lots of breccia and boulders were supplied AD1911. It suggests that the effects of giant slope-failure and debris flow may be detected from shape of fluvial gravels.
(2) Fluvial terrace in the studied area are divided as H1~3, M1~3, and L surface in descending order. Volcanic glass shards and minerals originated from AT, DKP, Aso-4, K-Tz, and Tt-D were detected from the tephric loess (so-called loam) in this area. Detection of K-Tz and Tt-D from the basal part of the loam layer on the M2 terrace means that the ages of the M1 and M2 terraces become older than the previous estimation.
(3) A buried valley was discovered on the eastern side of the present Himekawa River as a result of the field survey. The buried valley is estimated as the paleo-Himekawa River channel during the Middle Pleistocene.
(4) The H1~H3 terraces contain sub-angular cubic andesite pebbles just like as the middle reaches of the present Himekawa River, so it is considered that sub-angular cubic andesite pebbles were transported by debris flow from the Umikawa volcanic rocks area near the study area.
Consequently, landform evolution in the lower reaches of the Himekawa River is estimated as follow;
Himekawa River flows eastern side of the present Himekawa River valley separated by the ridge of the basement rock during the Middle Pleistocene. The interglacial sea-level rise buries the basement ridge below the alluvial plain deposits. Then, downward valley erosion during the subsequent sea-level fall made the present Himekawa River valley due to tilting to north–northwest direction. During the middle Pleistocene, a large amount of andesite gravels derived from slope failures in the Umikawa volcanic rocks area.
Throughout the study, some results were obtained;
(1) The form of the pebbles on the present Himekawa River show that ratio of spherical (cubic) and disc shapes decrease and rod shape increases downstreamward. Local increase in ratio of spherical (cubic) and disc shapes in the middle reaches near by the confluence with the Urakawa River should be influenced by a giant slope-failure “Hieda-Yama-Kuzure” where lots of breccia and boulders were supplied AD1911. It suggests that the effects of giant slope-failure and debris flow may be detected from shape of fluvial gravels.
(2) Fluvial terrace in the studied area are divided as H1~3, M1~3, and L surface in descending order. Volcanic glass shards and minerals originated from AT, DKP, Aso-4, K-Tz, and Tt-D were detected from the tephric loess (so-called loam) in this area. Detection of K-Tz and Tt-D from the basal part of the loam layer on the M2 terrace means that the ages of the M1 and M2 terraces become older than the previous estimation.
(3) A buried valley was discovered on the eastern side of the present Himekawa River as a result of the field survey. The buried valley is estimated as the paleo-Himekawa River channel during the Middle Pleistocene.
(4) The H1~H3 terraces contain sub-angular cubic andesite pebbles just like as the middle reaches of the present Himekawa River, so it is considered that sub-angular cubic andesite pebbles were transported by debris flow from the Umikawa volcanic rocks area near the study area.
Consequently, landform evolution in the lower reaches of the Himekawa River is estimated as follow;
Himekawa River flows eastern side of the present Himekawa River valley separated by the ridge of the basement rock during the Middle Pleistocene. The interglacial sea-level rise buries the basement ridge below the alluvial plain deposits. Then, downward valley erosion during the subsequent sea-level fall made the present Himekawa River valley due to tilting to north–northwest direction. During the middle Pleistocene, a large amount of andesite gravels derived from slope failures in the Umikawa volcanic rocks area.