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[SVC34-P04] Differences in sedimentological characteristics of pyroclastic flow deposits and lahar deposits in the downstream of Yukawa River, Asama Volcano, Central Japan
Keywords:Asama Volcano, Pyroclastic flow, Lahar, Facies, Granulometry, Sediment composition
The Komoro 1st pyroclastic flow deposit (K01-Pfl: ca.17 ka) is distributed at the southern foot of Asama volcano. In the downstream of the Yukawa River in the Saku Basin, K01-Pfl is 10–30 m thick, and is overlined by its secondary deposit (SDPF). Kawauchi and Aramaki (1979) reported that the SDPF is a hydrologic deposit composed of pumice flow materials. In other words, the SDPF is classified as a lahar deposit. Lahar refers to the movement of volcaniclastic materials mediated by water, including debris flows (DF), hyperconcentrated flows (HCF), and stream flows (SF). There have been no detailed studies on the SDPF, and the distinction between the SDPF and K01-Pfl remains unclear. In this study, we focused on K01-Pfl and SDPF to clarify the differences in facies, granulometry, and componentry.
Outcrops were observed at 15 locations on the terraces downstream of the Yukawa River. The deposits were described and classified into units based on the facies. The matrix of each unit (excluding gravels larger than 64 mm) was sampled for granulometric and componentry analysis. For granulometric analysis, the samples were sieved using a sieve shaker, from −4 to 4φat 1φ intervals. In the componentry analysis, particles ranging from 0 to 1φ were examined under a binocular microscope. The particles were classified into pumice, scoria, volcanic glass, colorless minerals, colored minerals, and lithic fragments.
The pyroclastic flow deposits and the overlying lahar deposits were observed along the riverbanks. Deposits that were massive, poorly sorted, developed degassed pipes, and rich in pumice, were determined as pyroclastic flow deposits. According to Kawauchi and Aramaki (1979), the study area is not within the range of the Komoro 2nd pyroclastic flow deposit. Therefore, this pyroclastic flow deposits were identified as K01-Pfl. The lahar deposits were characterized by a rich in lithic fragments and mainly consisted of DF, HCF, and SF facies. The facies exhibiting matrix supported, poorly sorted, and massive or coarse-tail normal grading were considered as DF facies. The facies displaying a lateral succession of gravels in a sandy matrix with unclear parallel bedding was considered as HCF facies, while the facies with well sorted sandy matrix, massive or clearly parallel bedding were considered as SF facies. The lahar deposits were predominantly observed in outcrops on the topographic surface about 5 m below the depositional surface of K01-Pfl. Since they were hydrologic deposits overlying K01-Pfl, the lahar deposits were identified as the SDPF reported by Kawauchi and Aramaki (1979).
The following differences were observed in the granulometry and componentry of K01-Pfl and SDPF. The K01-Pfl was characterized by a high proportion of fine grains (>4φ), while SDPF was poor in fine grains (>4φ), in common with the DF, HCF, and SF facies. Compared to SDPF, K01-Pfl contained fewer lithic fragments and more pumice, scoria, and volcanic glass. K01-Pfl, with mean diameter ranging from 1.0 to 2.4φ with a deviation from 2.0 to 2.4φ, while SDPF, with mean diameter from −2.0 to 1.5φ with a and deviation from 1.3 to 2.6φ, both plotted in distinctly different areas on the mean diameter-deviation diagram. The observed differences in granulometry can be explained by the process that the particles with smaller hydraulic settling sizes of SDPF were transported further downstream by water current, as noted by Nagahashi and Kataoka (2014).
Therefore, the identification of pyroclastic flow or lahar deposits can be determined through granulometric and componentry analyses although these deposits are difficult to distinguish based on only facies analysis.
References
Kawauchi, S., and Aramaki, S. (1979) 1:50,000 scale map Komoro and its explanatory text. Geological Survey of Japan.
Nagahashi, Y., and Kataoka, K.S. (2014) Tephrology (part 4) : Grain morphology and component of tephra and volcaniclastic particles. The Quaternary Research, 53, 229-234.
Outcrops were observed at 15 locations on the terraces downstream of the Yukawa River. The deposits were described and classified into units based on the facies. The matrix of each unit (excluding gravels larger than 64 mm) was sampled for granulometric and componentry analysis. For granulometric analysis, the samples were sieved using a sieve shaker, from −4 to 4φat 1φ intervals. In the componentry analysis, particles ranging from 0 to 1φ were examined under a binocular microscope. The particles were classified into pumice, scoria, volcanic glass, colorless minerals, colored minerals, and lithic fragments.
The pyroclastic flow deposits and the overlying lahar deposits were observed along the riverbanks. Deposits that were massive, poorly sorted, developed degassed pipes, and rich in pumice, were determined as pyroclastic flow deposits. According to Kawauchi and Aramaki (1979), the study area is not within the range of the Komoro 2nd pyroclastic flow deposit. Therefore, this pyroclastic flow deposits were identified as K01-Pfl. The lahar deposits were characterized by a rich in lithic fragments and mainly consisted of DF, HCF, and SF facies. The facies exhibiting matrix supported, poorly sorted, and massive or coarse-tail normal grading were considered as DF facies. The facies displaying a lateral succession of gravels in a sandy matrix with unclear parallel bedding was considered as HCF facies, while the facies with well sorted sandy matrix, massive or clearly parallel bedding were considered as SF facies. The lahar deposits were predominantly observed in outcrops on the topographic surface about 5 m below the depositional surface of K01-Pfl. Since they were hydrologic deposits overlying K01-Pfl, the lahar deposits were identified as the SDPF reported by Kawauchi and Aramaki (1979).
The following differences were observed in the granulometry and componentry of K01-Pfl and SDPF. The K01-Pfl was characterized by a high proportion of fine grains (>4φ), while SDPF was poor in fine grains (>4φ), in common with the DF, HCF, and SF facies. Compared to SDPF, K01-Pfl contained fewer lithic fragments and more pumice, scoria, and volcanic glass. K01-Pfl, with mean diameter ranging from 1.0 to 2.4φ with a deviation from 2.0 to 2.4φ, while SDPF, with mean diameter from −2.0 to 1.5φ with a and deviation from 1.3 to 2.6φ, both plotted in distinctly different areas on the mean diameter-deviation diagram. The observed differences in granulometry can be explained by the process that the particles with smaller hydraulic settling sizes of SDPF were transported further downstream by water current, as noted by Nagahashi and Kataoka (2014).
Therefore, the identification of pyroclastic flow or lahar deposits can be determined through granulometric and componentry analyses although these deposits are difficult to distinguish based on only facies analysis.
References
Kawauchi, S., and Aramaki, S. (1979) 1:50,000 scale map Komoro and its explanatory text. Geological Survey of Japan.
Nagahashi, Y., and Kataoka, K.S. (2014) Tephrology (part 4) : Grain morphology and component of tephra and volcaniclastic particles. The Quaternary Research, 53, 229-234.