10:45 AM - 12:15 PM
[SGL23-P07] Distribution and refractive index of the Aso-4 pyroclastic flow deposits around Chikuzen-Maebaru region, North Kyushu Island
Keywords:Aso-4 pyroclastic flow, Itoshima Peninsula, Aso Volcano, Quaternary, Widespread tephra
The Aso-4, ca. 85~90 ka, is the largest eruption occurred at the Aso volcano. The Aso-4 pyroclastic flow deposit (Aso-4 pfl) widely covers central to northern Kyushu Island. The Aso-4 pfl is divided into several subunits. Among them. the Yame pumice flow and the Tosu orange pumice flow are widely distributed (Watanabe 1978; Matsuo & Horikawa 2009).
There small Pleistocene mounds in the alluvial lowland, Itoshima Lowland of the Chikuzen Maebaru district. Because the mounds have only less than 1m above the alluvial lowland, it is difficult to distinguish the mounds from Holocene natural levees inferred from only their surface morphology. Although Shimoyama et al. (1984) suggested that these mounds are composed by the Aso-4 pfl based on only mineral compositions, they had not analyzed refractive indices of volcanic glass and heavy minerals. Refractive indices are important for not only confirming the Aso-4 pfl but also identifying the Aso-4 pfl subunits (Matsuo & Horikawa 2009). If the subunits could be identified, it may lead to understanding of the flow and depositional processes of individual Aso-4 pyroclastic flow. We investigated on distribution of Aso-4 pfl based on refractive indices in the Chikuzen Maebaru district.
We found the Aso-4 pfl at six sites in the study area. These sites are distributed at the small mounds or the lower middle Pleistocene terraces. On the upper middle and high Pleistocene terraces, the Aso-4 pfl was not found. The small mounds are distributed in the Itoshima Lowland and small valley of Nagano River, western part of the study area. Although the thickness of the Aso-4 pfl is not confirmed because the drilled core has not reached the bottom, but the maximum thickness reaches more than 1.5 m. The Aso-4 pfl is commonly involve orthopyroxene [Opx] and hornblende [Ho]. Refractive indices of Opx and Ho are measured to be n2=1.685-1.690 and γ=1.699-1.701, respectively. In addition, volcanic glass is bubble-wall or parallel-type (Kishi & Miyawaki 1996) are found, showing refractive indices of n=1.505 to 1.510. These mineral compositions and refractive indices suggest that they are most likely to be the Aso-4 pfl.
The Aso-4 pfl identified in Shito of Itoshima City in the western part of the Itoshima Lowland consists of white to white very fine sand. This mostly composed of volcanic glass with a small amount of Opx and Ho. The refractive indices of volcanic glass are characterized by two peaks around 1.506-1.507 and 1.509-1.511 respectively. Besides, brownish clay with very fine sand from Taku, Itoshima City, shows similar refractive indices. These features suggests that the Aso-4 pfl found in the study area are corresponding to the Yame pumice deposits (Matsuo & Horikawa 2009).
Between the Aso volcano and the study area, the Sefuri Mountains with 800 to 1000 m height are distributed. Because the Aso-4 pfl is not distributed on the upper middle and high Pleistocene terrace, it is unlikely that the Aso-4 pyroclastic flow had cross the Mountains entirely. Distribution of the Aso-4 pfl in the study area indicates two possible of their flowing routes: 1) from the east side of the Itoshima Lowland, and 2) over a part of the mountain pass in the Mountains. In the Fukuoka Plain, the Aso-4 pfl are distributed and indicates flowed in the west-northwest direction inferred from buried fallen woods in the Aso-4 pfl (Shimoyama 1994). The study area is located on the extension of pyroclastic flow from the Fukuoka Plain. On the other hand, pyroclastic flows flowed westward in the Saga Plain, southern side of the Sefuri Mountains (Shimoyama 1994). In addition, the Aso-4 pfl was reported at around 600 m at Fuji Town, westward of the Saga Plain (Nagaoka 2001). The valley of the Nagano River faces the Nagano Pass of the Sefuri Mountains, which has ~600 m in elevation and located at the 9 km north from Fuji Town. This indicates possibility that the Aso-4 pyroclastic flow had flowed thorugh the pass.
There small Pleistocene mounds in the alluvial lowland, Itoshima Lowland of the Chikuzen Maebaru district. Because the mounds have only less than 1m above the alluvial lowland, it is difficult to distinguish the mounds from Holocene natural levees inferred from only their surface morphology. Although Shimoyama et al. (1984) suggested that these mounds are composed by the Aso-4 pfl based on only mineral compositions, they had not analyzed refractive indices of volcanic glass and heavy minerals. Refractive indices are important for not only confirming the Aso-4 pfl but also identifying the Aso-4 pfl subunits (Matsuo & Horikawa 2009). If the subunits could be identified, it may lead to understanding of the flow and depositional processes of individual Aso-4 pyroclastic flow. We investigated on distribution of Aso-4 pfl based on refractive indices in the Chikuzen Maebaru district.
We found the Aso-4 pfl at six sites in the study area. These sites are distributed at the small mounds or the lower middle Pleistocene terraces. On the upper middle and high Pleistocene terraces, the Aso-4 pfl was not found. The small mounds are distributed in the Itoshima Lowland and small valley of Nagano River, western part of the study area. Although the thickness of the Aso-4 pfl is not confirmed because the drilled core has not reached the bottom, but the maximum thickness reaches more than 1.5 m. The Aso-4 pfl is commonly involve orthopyroxene [Opx] and hornblende [Ho]. Refractive indices of Opx and Ho are measured to be n2=1.685-1.690 and γ=1.699-1.701, respectively. In addition, volcanic glass is bubble-wall or parallel-type (Kishi & Miyawaki 1996) are found, showing refractive indices of n=1.505 to 1.510. These mineral compositions and refractive indices suggest that they are most likely to be the Aso-4 pfl.
The Aso-4 pfl identified in Shito of Itoshima City in the western part of the Itoshima Lowland consists of white to white very fine sand. This mostly composed of volcanic glass with a small amount of Opx and Ho. The refractive indices of volcanic glass are characterized by two peaks around 1.506-1.507 and 1.509-1.511 respectively. Besides, brownish clay with very fine sand from Taku, Itoshima City, shows similar refractive indices. These features suggests that the Aso-4 pfl found in the study area are corresponding to the Yame pumice deposits (Matsuo & Horikawa 2009).
Between the Aso volcano and the study area, the Sefuri Mountains with 800 to 1000 m height are distributed. Because the Aso-4 pfl is not distributed on the upper middle and high Pleistocene terrace, it is unlikely that the Aso-4 pyroclastic flow had cross the Mountains entirely. Distribution of the Aso-4 pfl in the study area indicates two possible of their flowing routes: 1) from the east side of the Itoshima Lowland, and 2) over a part of the mountain pass in the Mountains. In the Fukuoka Plain, the Aso-4 pfl are distributed and indicates flowed in the west-northwest direction inferred from buried fallen woods in the Aso-4 pfl (Shimoyama 1994). The study area is located on the extension of pyroclastic flow from the Fukuoka Plain. On the other hand, pyroclastic flows flowed westward in the Saga Plain, southern side of the Sefuri Mountains (Shimoyama 1994). In addition, the Aso-4 pfl was reported at around 600 m at Fuji Town, westward of the Saga Plain (Nagaoka 2001). The valley of the Nagano River faces the Nagano Pass of the Sefuri Mountains, which has ~600 m in elevation and located at the 9 km north from Fuji Town. This indicates possibility that the Aso-4 pyroclastic flow had flowed thorugh the pass.