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[MIS15-P04] Stratigraphic sequence and ages of dammed-lake and debris flow deposits caused by Oya-Kuzure rockslide: new data from tributary Tachisawa River in the uppermost Abe River System
Keywords:Oya-Kuzure, Slope failure, 14C age, Dammed-lake deposits
1. Introduction
The Oya-Kuzure rockslide (OYA) is a huge landslide located at the headwaters of the Abe River (AR). The OYA is believed to be triggered by the CE1707 Hoei earthquake (M8.4). However, there are other opinions that the OYA had preceded before CE1707. In a few tributaries of the headwater of AR, dammed-lake deposits by rockslide-debris flow deposits supplied from the OYA are exposed. Here, we present new evidences regarding stratigraphic sequence and ages of debris flow and dammed-lake deposits found in Tachisawa River (TR). Then we will discuss the possibility that the OYA had occurred before CE1707.
2. Outline of local geology
In an area of the confluence of the AR and TR, a series of terrace features consisting of thick debris are developed. Whereas sandstone and shale are widely distributed around the OYA and the upper AR basin, basalt and limestone are limitedly exposed in the TR basin. This geological setting is useful for determining the source of debris .
3. Method
We conducted geological survey and 14C dating.
4. Stratigraphic sequence of debris flow deposits and dammed-lake deposits
An outcrop locality consisting of debris flow deposits and dammed-lake deposits is on the right bank of Tachisawa River, at which 0.5 km upstream from the confluence of the AR and the TR (Figure). A sandy silt layer (SL) is exposed on the present riverbed just below the outcrop. A hand drilling revealed that the SL continues to a depth of 3.90 m from the riverbed or more (lower limit unknown). An alternation of silt and gravel (SG) 4 m thick is placed on the SL with conformity. The SG contains lithic fragments of basalt and limestone, thus the SG was supplied from the TR basin. A poorly sorted coarse-grained debris (DF) 3 m thick consisting only sandstone and shale is placed on the top of SG with conformity. The top surface of DF can be regarded as the eroded terrace surface at this outcrop. There are several other terraces on the higher level (north side) of this outcrop , and the highest terrace surface among them coincides with the higher surface developing along the AR. Since the lithofacies of the DF and the debris flow deposits forming the highest terrace along the AR is quite similar, as well as the lithic types of rock clast of them are same. The DF was a debris flow origin by the OYA.
5. 14C ages
Two wood samples taken from 2.82 m deep (Sample A) and 0.80 m deep (B) below the present riverbed of SL show CE1646 or later, and CE1444-1620 (IntCal20; 2σ), respectively. Other two wood samples (Samples C & D) close to the present riverbed of SL is CE1479 to 1635 and CE1644 or later, respectively. A further wood sample (E) at the top of SG is CE1660 or later. The cause of age reversal is under consideration, but if younger values of A, D, and E will be adopted, the SL and SG are thought to be after the mid-1600s CE. No sample was found from the DF.
6. Age of dammed-lake deposits and onset of the Oya-Kuzure rockslide
From the viewpoint of geographical position, altitude, lithofacies , and lithic types of gravels, the SL and SG are judged to be dammed-lake deposits that occurred in the TR basin owing to the outlet from the TR to the AR was dammed (local landslides that can cause damming do not exist between or around the outcrop and confluence). Even if the DF was debris flow in CE1707 from the OYA, the fact that dammed-lake deposits 7.9 m thick is present below the DF means that debris accumulation of the AR valley had begun before CE1707. The onset of the OYA, which had a significant effect on the river conditions of the AR and its tributaries, was before CE1707.
The Oya-Kuzure rockslide (OYA) is a huge landslide located at the headwaters of the Abe River (AR). The OYA is believed to be triggered by the CE1707 Hoei earthquake (M8.4). However, there are other opinions that the OYA had preceded before CE1707. In a few tributaries of the headwater of AR, dammed-lake deposits by rockslide-debris flow deposits supplied from the OYA are exposed. Here, we present new evidences regarding stratigraphic sequence and ages of debris flow and dammed-lake deposits found in Tachisawa River (TR). Then we will discuss the possibility that the OYA had occurred before CE1707.
2. Outline of local geology
In an area of the confluence of the AR and TR, a series of terrace features consisting of thick debris are developed. Whereas sandstone and shale are widely distributed around the OYA and the upper AR basin, basalt and limestone are limitedly exposed in the TR basin. This geological setting is useful for determining the source of debris .
3. Method
We conducted geological survey and 14C dating.
4. Stratigraphic sequence of debris flow deposits and dammed-lake deposits
An outcrop locality consisting of debris flow deposits and dammed-lake deposits is on the right bank of Tachisawa River, at which 0.5 km upstream from the confluence of the AR and the TR (Figure). A sandy silt layer (SL) is exposed on the present riverbed just below the outcrop. A hand drilling revealed that the SL continues to a depth of 3.90 m from the riverbed or more (lower limit unknown). An alternation of silt and gravel (SG) 4 m thick is placed on the SL with conformity. The SG contains lithic fragments of basalt and limestone, thus the SG was supplied from the TR basin. A poorly sorted coarse-grained debris (DF) 3 m thick consisting only sandstone and shale is placed on the top of SG with conformity. The top surface of DF can be regarded as the eroded terrace surface at this outcrop. There are several other terraces on the higher level (north side) of this outcrop , and the highest terrace surface among them coincides with the higher surface developing along the AR. Since the lithofacies of the DF and the debris flow deposits forming the highest terrace along the AR is quite similar, as well as the lithic types of rock clast of them are same. The DF was a debris flow origin by the OYA.
5. 14C ages
Two wood samples taken from 2.82 m deep (Sample A) and 0.80 m deep (B) below the present riverbed of SL show CE1646 or later, and CE1444-1620 (IntCal20; 2σ), respectively. Other two wood samples (Samples C & D) close to the present riverbed of SL is CE1479 to 1635 and CE1644 or later, respectively. A further wood sample (E) at the top of SG is CE1660 or later. The cause of age reversal is under consideration, but if younger values of A, D, and E will be adopted, the SL and SG are thought to be after the mid-1600s CE. No sample was found from the DF.
6. Age of dammed-lake deposits and onset of the Oya-Kuzure rockslide
From the viewpoint of geographical position, altitude, lithofacies , and lithic types of gravels, the SL and SG are judged to be dammed-lake deposits that occurred in the TR basin owing to the outlet from the TR to the AR was dammed (local landslides that can cause damming do not exist between or around the outcrop and confluence). Even if the DF was debris flow in CE1707 from the OYA, the fact that dammed-lake deposits 7.9 m thick is present below the DF means that debris accumulation of the AR valley had begun before CE1707. The onset of the OYA, which had a significant effect on the river conditions of the AR and its tributaries, was before CE1707.