9:30 AM - 9:45 AM
[HGM02-03] Landform classification in the Kinoe River Basin at the northern part of the Daisen volcano, Tottori, south-western Japan.
Keywords: landform classification, the Daisen volcano, Kinoe river basin, Hohkoku pyroclastic flow , Nawa pyroclastic flow , river terraces
Introduction
The Kinoe River, which flows at the northern foot of Mt. Daisen, flows north from Mt. Kabutogasen. Arakawa (1984) created a geomorphological map of the alluvial fans at the northwestern foot of the Daisen volcano, and Yamamoto (2017) summarized a geological map of the Daisen volcano. Although flat surfaces are clearly observed in the Kinoe River basin, their landform classification has not been fully clarified. The purpose of this study is to clarify the history of landform formation by classifying the landforms of the Kinoe River Basin and its surrounding flat surfaces.
Method
For topographic classification, we observed the landforms and outcrops on site in conjunction with aerial photograph interpretation and contour map reading of 1:25,000 and 1:10,000 topographic maps, and mineral analysis of tephra samples for identification. In addition, using the map of the Geospatial Information Authority of Japan, we calculated the elevation at every 5m horizontal distance from the mouth of the Kinoe River, projected it onto the line along the Kinoe River to create the riverbed longitudinal profile, and also projected the topographical surfaces. In addition, boring logs were examined for each topographic surface.
Results
The pyroclastic flow surface has an undulating topographical form in the longitudinal and cross sections, while the fluvial terrace surface has a flat tilting topography. Pyroclastic flow deposits are sparsely packed with sub-angular pebbles and boulders in a hard muddy matrix, and fluvial terrace deposits are densely packed with round gravel in a sandy matrix. The Kinoe River Basin is divided into two pyroclastic flow surfaces and a group of fluvial terrace surfaces dissecting them (Fig. 1). The upper pyroclastic flow surface on both sides of the Kinoe River has been dissected, while the lower pyroclastic flow surface on the left bank side of the Kinoe River is relatively well preserved. The two pyroclastic flow surfaces intersect at about 2.9 km, and the borehole log at about 2.4 km shows that the lower pyroclastic flow deposit cover the upper pyroclastic flow one.
The fluvial terrace surfaces formed by eroding the upper pyroclastic flow surface were divided into 2 surfaces on the upstream side and 3 surfaces on the downstream side, most of which showed gradients of 2.6 to 3.2 degrees(Fig. 1). On the other hand, the fluvial terrace surfaces that eroded the lower pyroclastic flow surface were named Daisen-cho Tanaka surface and Iwaigaki surface. The Daisen-cho Tanaka surface showed a gradient of 1.4 degrees, which is the same as the present riverbed.
At the outcrop at the entrance of Bush Clover's Ranch(Fig. 1), the Sanbe Kisuki pumice layer was confirmed directly above the fluvial terrace deposits that eroded the lower pyroclastic flow surface. The Sanbe-Kisuki pumice layer unconformably overlies the Nawa pyroclastic flow deposits (Arakawa, 1984) , which has a fall age of 110,000-115,000 years ago (Machida and Arai, 2003). Therefore, the lower pyroclastic flow was identified as the Nawa pyroclastic flow. The Upper Pyroclastic Flow was newly named Hohkoku Pyroclastic Flow of unknown age. In addition, the drilling log of Daisen-cho Tanaka surface shows that black humus soil with a thickness of more than 1 m was deposited directly above the alluvial fan gravel layer, suggesting that the Daisen-cho Tanaka surface emerged about 10,000 years ago. Carbon isotope dating of the black humus soil is required.
Conclusion
In the Kinoe River basin, the Hohkoku pyroclastic flow surface (new name), the Nawa pyroclastic flow surface, and the fluvial terrace surface group dissecting them were classified for the first time. The Hohkoku pyroclastic flow was deposited, and three fluvial terraces were left in the process of subsequent valley formation. Afterwards the Nawa pyroclastic flow buried this valley about 110,000 years ago, river incision progressed and an alluvial fan was formed in the lower reaches called Daisen-cho Tanaka surface (new name), followed by the Iwaigaki surface (new name).
References
Arakawa, H. (1984) Geographic Review Series A, 57, 831-855. ; Machida, H. and Arai, F. (2003) University of Tokyo Press. 336pp. ; Yamamoto, T. (2017) Geological Survey Research Report, 68, 1-16.
The Kinoe River, which flows at the northern foot of Mt. Daisen, flows north from Mt. Kabutogasen. Arakawa (1984) created a geomorphological map of the alluvial fans at the northwestern foot of the Daisen volcano, and Yamamoto (2017) summarized a geological map of the Daisen volcano. Although flat surfaces are clearly observed in the Kinoe River basin, their landform classification has not been fully clarified. The purpose of this study is to clarify the history of landform formation by classifying the landforms of the Kinoe River Basin and its surrounding flat surfaces.
Method
For topographic classification, we observed the landforms and outcrops on site in conjunction with aerial photograph interpretation and contour map reading of 1:25,000 and 1:10,000 topographic maps, and mineral analysis of tephra samples for identification. In addition, using the map of the Geospatial Information Authority of Japan, we calculated the elevation at every 5m horizontal distance from the mouth of the Kinoe River, projected it onto the line along the Kinoe River to create the riverbed longitudinal profile, and also projected the topographical surfaces. In addition, boring logs were examined for each topographic surface.
Results
The pyroclastic flow surface has an undulating topographical form in the longitudinal and cross sections, while the fluvial terrace surface has a flat tilting topography. Pyroclastic flow deposits are sparsely packed with sub-angular pebbles and boulders in a hard muddy matrix, and fluvial terrace deposits are densely packed with round gravel in a sandy matrix. The Kinoe River Basin is divided into two pyroclastic flow surfaces and a group of fluvial terrace surfaces dissecting them (Fig. 1). The upper pyroclastic flow surface on both sides of the Kinoe River has been dissected, while the lower pyroclastic flow surface on the left bank side of the Kinoe River is relatively well preserved. The two pyroclastic flow surfaces intersect at about 2.9 km, and the borehole log at about 2.4 km shows that the lower pyroclastic flow deposit cover the upper pyroclastic flow one.
The fluvial terrace surfaces formed by eroding the upper pyroclastic flow surface were divided into 2 surfaces on the upstream side and 3 surfaces on the downstream side, most of which showed gradients of 2.6 to 3.2 degrees(Fig. 1). On the other hand, the fluvial terrace surfaces that eroded the lower pyroclastic flow surface were named Daisen-cho Tanaka surface and Iwaigaki surface. The Daisen-cho Tanaka surface showed a gradient of 1.4 degrees, which is the same as the present riverbed.
At the outcrop at the entrance of Bush Clover's Ranch(Fig. 1), the Sanbe Kisuki pumice layer was confirmed directly above the fluvial terrace deposits that eroded the lower pyroclastic flow surface. The Sanbe-Kisuki pumice layer unconformably overlies the Nawa pyroclastic flow deposits (Arakawa, 1984) , which has a fall age of 110,000-115,000 years ago (Machida and Arai, 2003). Therefore, the lower pyroclastic flow was identified as the Nawa pyroclastic flow. The Upper Pyroclastic Flow was newly named Hohkoku Pyroclastic Flow of unknown age. In addition, the drilling log of Daisen-cho Tanaka surface shows that black humus soil with a thickness of more than 1 m was deposited directly above the alluvial fan gravel layer, suggesting that the Daisen-cho Tanaka surface emerged about 10,000 years ago. Carbon isotope dating of the black humus soil is required.
Conclusion
In the Kinoe River basin, the Hohkoku pyroclastic flow surface (new name), the Nawa pyroclastic flow surface, and the fluvial terrace surface group dissecting them were classified for the first time. The Hohkoku pyroclastic flow was deposited, and three fluvial terraces were left in the process of subsequent valley formation. Afterwards the Nawa pyroclastic flow buried this valley about 110,000 years ago, river incision progressed and an alluvial fan was formed in the lower reaches called Daisen-cho Tanaka surface (new name), followed by the Iwaigaki surface (new name).
References
Arakawa, H. (1984) Geographic Review Series A, 57, 831-855. ; Machida, H. and Arai, F. (2003) University of Tokyo Press. 336pp. ; Yamamoto, T. (2017) Geological Survey Research Report, 68, 1-16.