5:15 PM - 7:15 PM
[SVC34-P07] Sources and emplacement mechanisms of debris avalanche deposits at the foot of Bandai and Nekoma volcanoes, Central Fukushima Prefecture, Japan
Keywords:Bandai volcano, Nekoma volcano, debris avalanche
Debris avalanche is a phenomenon in which sediment generated by the collapse of a volcano moves at high velocity over long distances in a water-unsaturated state. It is important to clarify the history of debris avalanches associated with the collapse of volcanic bodies to discuss the formation history of volcanic edifice and long-term geomorphic development at the foot of volcano, as well as for disaster prevention measures. In this study, topographic and geological surveys were carried out at the foot of Bandai and Nekoma volcanoes in central Fukushima Prefecture to clarify the distribution of debris avalanche deposits. Petrographic characteristics of lava fragments in debris avalanche deposits were determined and compared with the chemical composition of volcanic edifice constituent rocks reported by Kimura et al. (2002) and Yamamoto (2011) to estimate the source. Based on these data and previous studies, the average denudation rate that the volcanic edifice suffers from collapse was calculated and the effect of the collapse on the denudation of volcanic edifice that have different degrees of activity and cleavage was discussed.
At the northern foot of volcanoes, two debris avalanche deposits, called the Oshizawa debris avalanche deposit, are distributed. The ages of the debris avalanche deposits are reported to be 0.7-0.5 Ma (Mimura, 2002) and 0.11-0.089 Ma (Koarai et al., 1994), respectively. In this study, these strata are referred to as the Oshizawa debris avalanche deposit-1 (OsDAD-1) and the Oshizawa debris avalanche deposit-2 (OsDAD-2) from the lower levels.
The chemical composition of lava fragments collected from the OsDAD-1 are only consistent with the rocks constituting the old edifice of Nekoma volcano on the SiO2 vs. K2O diagram, suggesting that the source of the Oshizawa debris avalanche-1 is the old edifice of Nekoma volcano.
The chemical compositions of lava fragments collected from the OsDAD-2 are consistent with both the rocks of the new edifice of Nekoma volcano and old edifice of Bandai volcano, suggesting that either volcanic edifice (or the contact area of both volcanic edifice) is the source, but the details are unknown.
At the southern foot of volcanoes, the debris avalanche deposits distributed are, in decreasing order, the Okinashima debris avalanche deposit (OkDAD, 46 ka; Yamamoto and Sakaguchi, 2023), the Iwane debris avalanche deposit (IwDAD), and the Furukan-non debris avalanche deposit (FkDAD, 18~17 ka; Yamamoto and Sakaguchi, 2023), which are all considered to be derived from the Bandai volcano (Chiba and Kimura 2001).
The chemical compositions of the lava fragments collected from the FkDAD are consistent with those of the new edifice of Nekoma volcano, not Bandai volcano, suggesting that the source of the Furukan-non debris avalanche is the new edifice of Nekoma volcano.
Although the Okinashima debris avalanche and the Furukan-non debris avalanche are different in size, they have similar H/L values, suggesting that they had similar fluidity. However, both debris avalanche deposits have different topographic and geological features, suggesting that they have different flow mechanisms.
OkDAD is characterized by hummocks throughout the depositional area and the presence of block facies in the sediments. The proportion of block facies in the total sediments is about 90% of the outcrop in the center of the depositional area (8.5 km from the source) and about 20% of the outcrop at the margins of the depositional area (9.3 km from the source).
FkDAD does not show clear hummocks in the depositional area. The sediments have no block facies at many locations and the facies are massive and non-selective. Although no hummocks were observed at the sites where block facies were observed, the thickness of the beds locally increased, suggesting that the sediments were deposited in a valley filling manner.
At the northern foot of volcanoes, two debris avalanche deposits, called the Oshizawa debris avalanche deposit, are distributed. The ages of the debris avalanche deposits are reported to be 0.7-0.5 Ma (Mimura, 2002) and 0.11-0.089 Ma (Koarai et al., 1994), respectively. In this study, these strata are referred to as the Oshizawa debris avalanche deposit-1 (OsDAD-1) and the Oshizawa debris avalanche deposit-2 (OsDAD-2) from the lower levels.
The chemical composition of lava fragments collected from the OsDAD-1 are only consistent with the rocks constituting the old edifice of Nekoma volcano on the SiO2 vs. K2O diagram, suggesting that the source of the Oshizawa debris avalanche-1 is the old edifice of Nekoma volcano.
The chemical compositions of lava fragments collected from the OsDAD-2 are consistent with both the rocks of the new edifice of Nekoma volcano and old edifice of Bandai volcano, suggesting that either volcanic edifice (or the contact area of both volcanic edifice) is the source, but the details are unknown.
At the southern foot of volcanoes, the debris avalanche deposits distributed are, in decreasing order, the Okinashima debris avalanche deposit (OkDAD, 46 ka; Yamamoto and Sakaguchi, 2023), the Iwane debris avalanche deposit (IwDAD), and the Furukan-non debris avalanche deposit (FkDAD, 18~17 ka; Yamamoto and Sakaguchi, 2023), which are all considered to be derived from the Bandai volcano (Chiba and Kimura 2001).
The chemical compositions of the lava fragments collected from the FkDAD are consistent with those of the new edifice of Nekoma volcano, not Bandai volcano, suggesting that the source of the Furukan-non debris avalanche is the new edifice of Nekoma volcano.
Although the Okinashima debris avalanche and the Furukan-non debris avalanche are different in size, they have similar H/L values, suggesting that they had similar fluidity. However, both debris avalanche deposits have different topographic and geological features, suggesting that they have different flow mechanisms.
OkDAD is characterized by hummocks throughout the depositional area and the presence of block facies in the sediments. The proportion of block facies in the total sediments is about 90% of the outcrop in the center of the depositional area (8.5 km from the source) and about 20% of the outcrop at the margins of the depositional area (9.3 km from the source).
FkDAD does not show clear hummocks in the depositional area. The sediments have no block facies at many locations and the facies are massive and non-selective. Although no hummocks were observed at the sites where block facies were observed, the thickness of the beds locally increased, suggesting that the sediments were deposited in a valley filling manner.