Japan Geoscience Union Meeting 2019

Presentation information

[J] Poster

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS20] Mountain Science

Mon. May 27, 2019 3:30 PM - 5:00 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Keisuke Suzuki(Department of Environmental Sciences, Faculty of Science, Shinshu University), Yoshihiko Kariya(Department of Environmental Geography, Senshu University), Akihiko SASAKI(Department of Geography and Environmental Studies, Kokushikan University), Chiyuki Narama(Niigata University, Department of Science)

[MIS20-P01] Development of fluvial terraces and landslides since the late Pleistocene in the Shirata River basin, southeastern side of Amagi Volcano, Izu Peninsula, Japan

*Nao Tokumoto1, Yoshihiko Kariya2, Kuniyasu Mokudai3 (1.Undegraduate, Senshu Univ., 2.Senshu Univ., 3.Japan Geoservice Inc.)

Keywords:the last glacial period, fill terrace, erosional terrace, landslide, tephrochronology

This study clarified geomorphic features of debris flow-fluvial terraces and landslide complexes on the southeastern side of Amagi Volcano in Izu Peninsula, central Japan. Terrace forms can be classified into three: H, M, and L surfaces from descending altitude order. Among them, the M surface is common and its geology is unique. The M surface is composed of thick and poorly sorted angular gravel beds associated with debris flow processes. The L surface is an erosion surface of higher terraces. A longitudinal profile of the M surface seems to connect with large landslide bodies in the upper reach of stream on the volcano and to continue below the present seafloor off the mouth of such a stream. Some of large landslides show secondary movement of bodies and dissected headscarps. Furthermore, the M surface is covered by a tephra layer supplied from a nearby monogenetic volcano in 19 ka. These facts show that the M surface had been formed by debris flow events filling the stream in the late Pleistocene when the sea-level was lowered. Then an increase in precipitation in the area and a decrease in bedload supply into the stream, as well as sea level rise, would have caused down cutting of the floor of stream and created a terrace form of the M surface during the period from the latest Pleistocene to the early Holocene. Because of further down cutting, the L surface had been formed. Detailed descrption of the H surface is required in future.