5:15 PM - 6:45 PM
[SVC30-P12] Revisiting the tephra characteristics and correlation of young Ontake lower tephra group
Keywords:tephrochronology, Ontake volcano, Major element composition
Ontake volcano is an active volcano located in central Japan and has been active since the Middle Pleistocene (Kioka et al., 1998). The activity of Ontake volcano is divided into old and young stages. It experienced repeated explosive felsic magma eruptions for approximately 100–80 ka in young stage. The stratigraphy of this stage is summarized by Kimura (1987) and Takemoto et al. (1987). However, the fundamental petrographic characteristics contributing to correlation and identification are not sufficient. Recently, Matsu'ura and Komatsubara (2024) pointed out that the possibility of past miscorrelation of the On-Pm1 tephra. In tephrochronology, characterization of tephra beds and accurate identification is fundamental to constructing tephrochronological frameworks contributing to the reconstruction of eruptive history and Quaternary geology/geomorphology. In this study, we revisit the description and petrological characteristics of the young Ontake lower tephra group and then reexamine the identification of tephra beds in the distal area.
We conducted field surveys and tephra samplings in 17 outcrops and one borehole core in the 40-620 km range from Ontake volcano. All samples were washed, clashed, and sieved, and mineral assemblage was described using 62-120 µm particles. Refractive indexes of volcanic glass shards were measured using RIMS2000 (Tokyo Metropolitan University), and the major element composition of glass shards was analyzed using WDS-EPMA (JXA-8200, Marine Core Research Institute, Kochi University).
We described nine plinian pumice fall deposits in the proximal area (On-Km, On-Pm1, On-So, On-Yb, On-Kt, On-In, On-Ot, On-Ng, On-Tt, from lower to upper). The lower tephras (On-Km to On-Yb) contain biotite (bi), but the upper tephras (On-Kt to On-Tt) don't contain it. Hornblende (ho) and orthopyroxene (opx) were found in almost tephras. Some tephras contain clinopyroxene (cpx). The refractive index tends to be low around 1.500 for the lower tephras which contains bi, and high for the upper tephras. Based on the major element composition of glass shards, it is possible to clearly distinguish each tephra by using elements such as Al2O3, FeO, and CaO. In particular, in the SiO2-Al2O3 diagrams, we can divide into two trends: a High-Al trend (On-Km to On-Yb) and a Low-Al trend (On-Kt to On-Tt).
We reexamined the tephra identification using this database. As a result, Tz-3 tephra (Minagawa, 1969) recognized at Tozurahara, Sagamihara City, Kanagawa Prefecture was found to be correlated to the On-Ot tephra, and the newly discovered tephra at Taneichi, Hirono-cho, Iwate Prefecture was correlated to the On-Kt tephra. We also confirmed that the tephras in Onikobe, Osaki City, Miyagi Prefecture, and the Isawa Wakago Atago, Oshu City, Iwate Prefecture are also correlated to the On-Kt tephra, that consistent with Matsu'ura and Komatsubara (2024). It is expected that tephras derived from Ontake volcano will be correlated and identified in more distal and new outcrops in the future using this database.
We conducted field surveys and tephra samplings in 17 outcrops and one borehole core in the 40-620 km range from Ontake volcano. All samples were washed, clashed, and sieved, and mineral assemblage was described using 62-120 µm particles. Refractive indexes of volcanic glass shards were measured using RIMS2000 (Tokyo Metropolitan University), and the major element composition of glass shards was analyzed using WDS-EPMA (JXA-8200, Marine Core Research Institute, Kochi University).
We described nine plinian pumice fall deposits in the proximal area (On-Km, On-Pm1, On-So, On-Yb, On-Kt, On-In, On-Ot, On-Ng, On-Tt, from lower to upper). The lower tephras (On-Km to On-Yb) contain biotite (bi), but the upper tephras (On-Kt to On-Tt) don't contain it. Hornblende (ho) and orthopyroxene (opx) were found in almost tephras. Some tephras contain clinopyroxene (cpx). The refractive index tends to be low around 1.500 for the lower tephras which contains bi, and high for the upper tephras. Based on the major element composition of glass shards, it is possible to clearly distinguish each tephra by using elements such as Al2O3, FeO, and CaO. In particular, in the SiO2-Al2O3 diagrams, we can divide into two trends: a High-Al trend (On-Km to On-Yb) and a Low-Al trend (On-Kt to On-Tt).
We reexamined the tephra identification using this database. As a result, Tz-3 tephra (Minagawa, 1969) recognized at Tozurahara, Sagamihara City, Kanagawa Prefecture was found to be correlated to the On-Ot tephra, and the newly discovered tephra at Taneichi, Hirono-cho, Iwate Prefecture was correlated to the On-Kt tephra. We also confirmed that the tephras in Onikobe, Osaki City, Miyagi Prefecture, and the Isawa Wakago Atago, Oshu City, Iwate Prefecture are also correlated to the On-Kt tephra, that consistent with Matsu'ura and Komatsubara (2024). It is expected that tephras derived from Ontake volcano will be correlated and identified in more distal and new outcrops in the future using this database.