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[HDS10-P07] Relationship between a natural dam at the Yomogisawa River in the headwater area of the Abe River and the Oya-Kuzure rockslide with reference to new chronological information
Keywords:Oya-Kuzure, lacustrine sediment, 14C age
1. Introduction
The Oya-Kuzure is a huge landslide site located at the headwaters of the Abe River in Shizuoka Prefecture. This landslide is believed to be triggered by the CE1707 Hoei earthquake (est. M 8.6) (Shizuoka River Office, Ministry of Construction, 1988). There are, however, other arguments about the age of Oya-Kuzure, and all of them are based solely on historical documents without geological and chronological data. Machida (1959; Geogr. Rev. Jpn.) pointed out that lacustrine sediments are buried under the present riverbed near the confluence of the Oya and Mikochi Rivers upstream of the Abe River. He also showed that other lacustrine sediments are distributed along a few tributaries, whereas details were unknown.
In this presentation, we report landslide sediments originating from Oya-Kuzure and a resultant, natural dams, as well as the dammed lacustrine sediments found in Yomogisawa River, one of small tributaries of the Oya River, and show radiocarbon ages of those sediments. Then we will discuss the age of the Oya-Kuzure.
2. Landforms and geology
Oya-Kuzure has a following an area of 1.8 km2, a headscarp width of 1.8 km, and a vertical difference of 800 m. The Oya River originates from the Oya-Kuzure, and the Mikochi River is combined to form the Abe River. As Oya-Kuzure has supplied a large number of debris, fill terrace is developed in this area.
The geology is mainly composed of alternating beds of sandstone and shale, but igneous rocks and serpentinite are limitedly distributed in the upper reaches of Yomogisawa River (Sugiyama and Matsuda, 2014).
3. Description of landslide and lacustrine sediments
An outcrop locality consisting of gravel layers and sandy silt layers is present on the left bank of Yomogisawa River, at which 1.1 km upstream from the confluence of Yomogisawa and Oya Rivers. The top surface (ground surface) of this outcrop corresponds to the terrace surface about 4 m high from the present riverbed of Yomogisawa River.
A paleosol layer S with a large amount of deformed wood fragments is confirmed at the bottom of outcrop. The paleosol S continues subsurface of outcrop and its thickness is estimated to 0.3 m or more. Above the paleosol S, an angular gravel layer A (max. thickness ca. 3 m) consisting of granule to boulder clast is observed. The gravel layer A becomes thinner in the upstream side. A sandy silt layer X (max. th. 0.6 m) and a sand layer Y (max. th. 0.5 m) are laid on the gravel layer A in this order, and both layers X and Y gradually become thinner in the downstream side contrary to the layer A. Layers X and Y abut on layer A and are finally disappear. Layers X and Y are covered by a gravel layer B (max. th. 1.5 m) with granule to boulder of debris flow origin.
Layers A and B can be distinguished by roundness and lithology of clastic materials. Layer A shows a lower degree of roundness than that of layer B, suggesting that layer A was caused by and moved by rock avalanche behavior. Layer B contains serpentinite and igneous rocks derived from the upper part of Yomogisawa River, suggesting its source is restricted to the Yomogisawa River watershed.
Layer A is thought to be rock avalanche sediments supplied from the Oya-Kuzure and formed a natural dam obstructing the stream of Yomogisawa River. Layers X and Y are considered to be dammed lacustrine sediments by this natural dam. In addition, it is considered that the paleosol S is derived from disturbed soil at which layer A moved down and flowed into Yomogisawa River, or from a small-scale landslide occurred nearby the outcrop before formation of the natural dam.
Paleosol S, and layers A and X contain fossil wood fragments. We consequently measured 14C age of those samples.
4. 14C age
The wood samples showed the following ages. The calendar year was calculated by OxCal and INTCAL20 and shows a 2σ error interval (1σ for 14C year). Paleosol layer S= cal CE1396-1441 (521±23 14C) BP, gravel layer A = cal CE1506-1596/1616-1655 (297±23 14C) BP, sandy silt layer X = cal CE1644-1680/1740-1753/1762-1800 (220±20 14C) BP.
5. Discussion
No stratigraphic reversal of 14C ages are confirmed. Our age data can be considered as appropriate. Layer A may be up to 200 years older from CE1707, and it is probable that a landslide activity of Oya-Kuzure had preceded several hundred years before CE1707. On the other hand, the age of layer X includes CE1707, but it has a wide range and may have been deposited from the middle of CE1600’s to around CE1800. Therefore, in addition to 1707 Hoei earthquake, CE1498 Meio Tokai (est. M8.2-8.4), CE1605 Keicho Tokai (est. M7.9), and CE1703 Genroku (M7.9-8.2) earthquakes are candidate triggers of rock avalanche from the Oya-Kuzure site.
The Oya-Kuzure is a huge landslide site located at the headwaters of the Abe River in Shizuoka Prefecture. This landslide is believed to be triggered by the CE1707 Hoei earthquake (est. M 8.6) (Shizuoka River Office, Ministry of Construction, 1988). There are, however, other arguments about the age of Oya-Kuzure, and all of them are based solely on historical documents without geological and chronological data. Machida (1959; Geogr. Rev. Jpn.) pointed out that lacustrine sediments are buried under the present riverbed near the confluence of the Oya and Mikochi Rivers upstream of the Abe River. He also showed that other lacustrine sediments are distributed along a few tributaries, whereas details were unknown.
In this presentation, we report landslide sediments originating from Oya-Kuzure and a resultant, natural dams, as well as the dammed lacustrine sediments found in Yomogisawa River, one of small tributaries of the Oya River, and show radiocarbon ages of those sediments. Then we will discuss the age of the Oya-Kuzure.
2. Landforms and geology
Oya-Kuzure has a following an area of 1.8 km2, a headscarp width of 1.8 km, and a vertical difference of 800 m. The Oya River originates from the Oya-Kuzure, and the Mikochi River is combined to form the Abe River. As Oya-Kuzure has supplied a large number of debris, fill terrace is developed in this area.
The geology is mainly composed of alternating beds of sandstone and shale, but igneous rocks and serpentinite are limitedly distributed in the upper reaches of Yomogisawa River (Sugiyama and Matsuda, 2014).
3. Description of landslide and lacustrine sediments
An outcrop locality consisting of gravel layers and sandy silt layers is present on the left bank of Yomogisawa River, at which 1.1 km upstream from the confluence of Yomogisawa and Oya Rivers. The top surface (ground surface) of this outcrop corresponds to the terrace surface about 4 m high from the present riverbed of Yomogisawa River.
A paleosol layer S with a large amount of deformed wood fragments is confirmed at the bottom of outcrop. The paleosol S continues subsurface of outcrop and its thickness is estimated to 0.3 m or more. Above the paleosol S, an angular gravel layer A (max. thickness ca. 3 m) consisting of granule to boulder clast is observed. The gravel layer A becomes thinner in the upstream side. A sandy silt layer X (max. th. 0.6 m) and a sand layer Y (max. th. 0.5 m) are laid on the gravel layer A in this order, and both layers X and Y gradually become thinner in the downstream side contrary to the layer A. Layers X and Y abut on layer A and are finally disappear. Layers X and Y are covered by a gravel layer B (max. th. 1.5 m) with granule to boulder of debris flow origin.
Layers A and B can be distinguished by roundness and lithology of clastic materials. Layer A shows a lower degree of roundness than that of layer B, suggesting that layer A was caused by and moved by rock avalanche behavior. Layer B contains serpentinite and igneous rocks derived from the upper part of Yomogisawa River, suggesting its source is restricted to the Yomogisawa River watershed.
Layer A is thought to be rock avalanche sediments supplied from the Oya-Kuzure and formed a natural dam obstructing the stream of Yomogisawa River. Layers X and Y are considered to be dammed lacustrine sediments by this natural dam. In addition, it is considered that the paleosol S is derived from disturbed soil at which layer A moved down and flowed into Yomogisawa River, or from a small-scale landslide occurred nearby the outcrop before formation of the natural dam.
Paleosol S, and layers A and X contain fossil wood fragments. We consequently measured 14C age of those samples.
4. 14C age
The wood samples showed the following ages. The calendar year was calculated by OxCal and INTCAL20 and shows a 2σ error interval (1σ for 14C year). Paleosol layer S= cal CE1396-1441 (521±23 14C) BP, gravel layer A = cal CE1506-1596/1616-1655 (297±23 14C) BP, sandy silt layer X = cal CE1644-1680/1740-1753/1762-1800 (220±20 14C) BP.
5. Discussion
No stratigraphic reversal of 14C ages are confirmed. Our age data can be considered as appropriate. Layer A may be up to 200 years older from CE1707, and it is probable that a landslide activity of Oya-Kuzure had preceded several hundred years before CE1707. On the other hand, the age of layer X includes CE1707, but it has a wide range and may have been deposited from the middle of CE1600’s to around CE1800. Therefore, in addition to 1707 Hoei earthquake, CE1498 Meio Tokai (est. M8.2-8.4), CE1605 Keicho Tokai (est. M7.9), and CE1703 Genroku (M7.9-8.2) earthquakes are candidate triggers of rock avalanche from the Oya-Kuzure site.