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[MIS11-P07] Deposits formed by Holocene seawater incursion events on the Wakasa Bay coast
Keywords:Tsunami deposit, ITRAX, Wakasa Bay, Fukui Prefecture, Niigata–Kobe Tectonic Zone (NKTZ), Sea of Japan
The Wakasa Bay, which faces the Sea of Japan and is the study area of this research, has a higher strain rate than the surrounding areas. Therefore, there are many active submarine faults that can cause tsunamis. To understand the tsunami inundation history in the Wakasa Bay area, we conducted coring survey and obtained four core samples in a coastal wetland of Tsuruga Bay, a bay in the eastern part of Wakasa Bay. Two event layers (layer A and B) and one plant debris layer were found in the sediment cores. To understand the genesis of the layers, we described the sedimentary facies by naked eye observation and X-ray CT imaging of the sediment cores, and analyzed chemical composition with XRF core scanner (ITRAX). The ages of the event layers were obtained by radiocarbon dating.
The upper event layer A is 5–10 cm thick and mainly consists of fine sand. It has a sharp lower boundary and parallel lamination. The results of chemical analysis showed that Si, indicating the presence of quartz sand, was detected in the layer A. In addition, S, S/Rb, Sr/Rb, Ca, Ca/Rb, K, and (Ca-Fe)/Ca, which are proxies of seawater, were detected more at the layer A than the upper and lower mud layers. The value of Ti/Ca, a proxy of terrestrial clastic debris, at the layer A is lower than that of the upper and lower mud layers. Therefore, the layer A is interpreted to have been deposited by a seawater incursion event such as a tsunami or storm surge. The depositional age of layer A is 5500–6150 cal yr BP.
The lower event layer B is 5–30 cm thick and mainly consists of coarse sand and granule. It shows no clear lamination. Although Sr was detected slightly more at the layer B than the upper organic-rich mud layer, other proxies that are considered to represent seawater incursion were less detected at the layer B. Therefore, no clear relationship was revealed between the deposition of the layer B and seawater incursion. The age of layer B is 5750–5900 cal yr BP.
The plant debris layer is 10–20 cm thick. Although no sand and gravel were found by the naked eye observation and CT imaging, S/Rb, Sr/Rb, Ca, and Ca/Rb, which are proxies of seawater, were detected relatively more at the plant debris layer than the upper and lower layers. On the other hand, Ti/Ca, a proxy of a terrestrial source, was detected relatively lower than the upper and lower sediments. Therefore, this plant debris layer may be formed by a seawater incursion event such as a tsunami or storm surge.
At present, it is unclear whether the seawater incursion events were caused by a tsunami or other events such as a storm surge. The age of the layer A found by this study is partially overlaps with an event deposit reported by KEPCO et al. (2012) at Inogaike, Tsuruga City, Fukui Prefecture (5320–5600 cal yr BP), which is also located on the coast of Wakasa Bay. Therefore, it is possible that the layer A and the event deposit at Inogaike were formed by the same event.
The upper event layer A is 5–10 cm thick and mainly consists of fine sand. It has a sharp lower boundary and parallel lamination. The results of chemical analysis showed that Si, indicating the presence of quartz sand, was detected in the layer A. In addition, S, S/Rb, Sr/Rb, Ca, Ca/Rb, K, and (Ca-Fe)/Ca, which are proxies of seawater, were detected more at the layer A than the upper and lower mud layers. The value of Ti/Ca, a proxy of terrestrial clastic debris, at the layer A is lower than that of the upper and lower mud layers. Therefore, the layer A is interpreted to have been deposited by a seawater incursion event such as a tsunami or storm surge. The depositional age of layer A is 5500–6150 cal yr BP.
The lower event layer B is 5–30 cm thick and mainly consists of coarse sand and granule. It shows no clear lamination. Although Sr was detected slightly more at the layer B than the upper organic-rich mud layer, other proxies that are considered to represent seawater incursion were less detected at the layer B. Therefore, no clear relationship was revealed between the deposition of the layer B and seawater incursion. The age of layer B is 5750–5900 cal yr BP.
The plant debris layer is 10–20 cm thick. Although no sand and gravel were found by the naked eye observation and CT imaging, S/Rb, Sr/Rb, Ca, and Ca/Rb, which are proxies of seawater, were detected relatively more at the plant debris layer than the upper and lower layers. On the other hand, Ti/Ca, a proxy of a terrestrial source, was detected relatively lower than the upper and lower sediments. Therefore, this plant debris layer may be formed by a seawater incursion event such as a tsunami or storm surge.
At present, it is unclear whether the seawater incursion events were caused by a tsunami or other events such as a storm surge. The age of the layer A found by this study is partially overlaps with an event deposit reported by KEPCO et al. (2012) at Inogaike, Tsuruga City, Fukui Prefecture (5320–5600 cal yr BP), which is also located on the coast of Wakasa Bay. Therefore, it is possible that the layer A and the event deposit at Inogaike were formed by the same event.