5:15 PM - 6:45 PM
[HQR05-P12] River Piracies between the Abu River and the Saba River,
Landform Evolurion in the upper region of the Saba River
Keywords:river piracies, landform evolution, river terrace, Aso-4 pyroclastic flow deposits
The river capture between the Abu River and the Saba River in the western Chugoku Mountains is pointed out based on the trace of the middle terrace (M terrace) at the altitude of 330 m. The age of the river capture is estimated to be between 130 and 85 ka, based on Aso-4 (86 ka) and Aso-3 (133 ka), which are distributed along the Abu River (Yamauchi and Shiraishi 2014). However, Aso-4 pyroclastic flow deposits (pfd) has been found at the lower terrace (Ls1 terrace) at 274 m (Tsuji et al. 2022) close to the elbow of the capture. Then the age of the river capture should be constrained by the Aso-4. However, the history of landform evolution of the Saba River and the Honoki Rivers (the branch river of the Abu River) has not been discussed. This study discusses the history of landform evolution based on geographical and geological survey.
The terraces in the upper region of the Saba River are classified M (Mα, Mβ), Ls1, Ls2, Ls3 and Lc. The Mβ terrace has a relative height of about 40 m from the recent Saba River bed.
The terraces along the Honoki River are classified M (Mα), La1, and La2. The Mα terrace is distributed at altitude of 320 to 330 m near the elevation of the divide in valley (alt. 328 m).
The Mα terrace has thicker sediments than the Mβ terrace and is mainly composed of matrix-supported gravel layers.
At the Ls1 terrace outcrop (alt. 275 m), a conglomerate layer is found below Aso-4 pfd, and the sub-rounded gravel to breccia layer containing breccia originating from the branch river is recognized above Aso-4 pfd. The soil layer overlying the breccia layer contain thin Aira-Tn tephra (AT, 30 ka). The Aso-4 pfd and AT are distributed at almost the same altitude.
The Lc terrace is distributed downstream of Chojagahara lava dome (165 ka, bottom at alt. 170 m) where 7 km downstream from the elbow, and the collapse deposits include the andesite originated from Chojagahara lava dome.
From the above, the following findings were obtained.
・The age of the river capture is before the deposition of the Aso-4 pfd.
・The riverbed height had been stagnant at alt. 275 m at the outcrop at the Ls1 during the period before Aso-4 and the AT. The reasons for this stagnant is thought to be the dam-up caused by the formation of Chojagahara lava dome. Also, it is possible that the lava dome collapsed before AT deposition and was scraped down to the current level of the Saba River bed.
・Before the river capture, it is necessary to consider the steep topographic barrier at the downstream of the elbow along the Paleo-Saba River based on the bottom altitude of Chojagahara lava dome.
The terraces in the upper region of the Saba River are classified M (Mα, Mβ), Ls1, Ls2, Ls3 and Lc. The Mβ terrace has a relative height of about 40 m from the recent Saba River bed.
The terraces along the Honoki River are classified M (Mα), La1, and La2. The Mα terrace is distributed at altitude of 320 to 330 m near the elevation of the divide in valley (alt. 328 m).
The Mα terrace has thicker sediments than the Mβ terrace and is mainly composed of matrix-supported gravel layers.
At the Ls1 terrace outcrop (alt. 275 m), a conglomerate layer is found below Aso-4 pfd, and the sub-rounded gravel to breccia layer containing breccia originating from the branch river is recognized above Aso-4 pfd. The soil layer overlying the breccia layer contain thin Aira-Tn tephra (AT, 30 ka). The Aso-4 pfd and AT are distributed at almost the same altitude.
The Lc terrace is distributed downstream of Chojagahara lava dome (165 ka, bottom at alt. 170 m) where 7 km downstream from the elbow, and the collapse deposits include the andesite originated from Chojagahara lava dome.
From the above, the following findings were obtained.
・The age of the river capture is before the deposition of the Aso-4 pfd.
・The riverbed height had been stagnant at alt. 275 m at the outcrop at the Ls1 during the period before Aso-4 and the AT. The reasons for this stagnant is thought to be the dam-up caused by the formation of Chojagahara lava dome. Also, it is possible that the lava dome collapsed before AT deposition and was scraped down to the current level of the Saba River bed.
・Before the river capture, it is necessary to consider the steep topographic barrier at the downstream of the elbow along the Paleo-Saba River based on the bottom altitude of Chojagahara lava dome.