5:15 PM - 6:45 PM
[MIS20-P04] Depositional environment and paleotsunami history of Ukedo at Namie Town, Fukushima Prefecture
Keywords:tsunami deposit
This presentation discusses the evolution of event deposits and depositional environments based on the results of excavations carried out in 2021 and 2023 in the Ukedo area, Namie Town, Fukushima Prefecture. Paleotsunami deposits in this area has been examined preliminary by Imaizumi et al. (2008). The Nm-NK tephra (Yamamoto, 2003) from 3700 BC to 3000 BC has been detected, which indicate preservation of a continuous geological record during the last 6500 years. Findings of paleotsunami deposits in this area can be correlated with that in the Sendai Plain, which will contribute to better understanding of paleotsunami history of the central part of the Japan Trench.
In 2021, six cores (UD1 – UD6) were taken along a shore-normal transect, and in 2023 seven cores (UK1-6) were retrieved along another transect 300 m north of the 2011 transect. The site UK4, which contained largest number of event sand layers, composed of a soil layer (0.5 m in thickness), an organic silt (1.5 m) and dune or beach ridge deposits consisting of fine- to medium-grained sands. At UK4, Eight sand layers and three gray silt layers were identified as event deposits within the organic silt. Radiocarbon dating was carried out on these sandy event deposits from UK4 and other 2023 cores, confirming that they can be correlated with the UD cores. The gray silt layers exhibited differences in their depositional facies between 2021 and 2023 cores; thickness of the gray silt at site UD2 was about 1 cm, meanwhile that at site UK4 was 3 cm. The number of the silt layers was larger at UK4. This may be explained by the lower elevation of the site UK4 comparing with the site UD2; UK4 can be associated with local topographic depression, which tend to accommodate more sediments. Differences in mineralogy and sedimentary structure were observed between these silt layers. In this presentation, diatom and CNS data from the cores will be analyzed to examine sources of the gray silt , depositional environments and palaeo-tsunami history of Ukedo area.
In 2021, six cores (UD1 – UD6) were taken along a shore-normal transect, and in 2023 seven cores (UK1-6) were retrieved along another transect 300 m north of the 2011 transect. The site UK4, which contained largest number of event sand layers, composed of a soil layer (0.5 m in thickness), an organic silt (1.5 m) and dune or beach ridge deposits consisting of fine- to medium-grained sands. At UK4, Eight sand layers and three gray silt layers were identified as event deposits within the organic silt. Radiocarbon dating was carried out on these sandy event deposits from UK4 and other 2023 cores, confirming that they can be correlated with the UD cores. The gray silt layers exhibited differences in their depositional facies between 2021 and 2023 cores; thickness of the gray silt at site UD2 was about 1 cm, meanwhile that at site UK4 was 3 cm. The number of the silt layers was larger at UK4. This may be explained by the lower elevation of the site UK4 comparing with the site UD2; UK4 can be associated with local topographic depression, which tend to accommodate more sediments. Differences in mineralogy and sedimentary structure were observed between these silt layers. In this presentation, diatom and CNS data from the cores will be analyzed to examine sources of the gray silt , depositional environments and palaeo-tsunami history of Ukedo area.