9:45 AM - 10:00 AM
[MIS13-04] Paleoenvironmental reconstruction and analysis of historical tsunami deposits in the coastal lowlands of the Tsukahara area in Minami-Soma City, Japan
Keywords:tsunami deposits, Minamisoma City, paleoenvironment, grain size distribution, total sulfer, radiocarbon age
Many investigations related to tsunami deposits conducted on the Pacific coast of the Tohoku area, Japan, have identified several tsunami events. Reports from the Odaka Ward, in Minamisoma City, have gradually clarified tsunami recurrence in this area. The paleoenvironmental analysis performed in this study identified historical tsunami deposits in the coastal lowland of the Tsukahara area at the northern Odaka River.
A total of seven cores from six drilling sites were recovered from the field survey and transported to the laboratory. The cores were described concerning grain size of the sandy deposits and total sulfur content of muddy samples, and X-ray fluorescence core scanning, radiocarbon dating, and X-ray computed tomography analyses were performed.
A total of eight depositional facies were identified in the cores, which were found to be resultant of deposits from the 2011 tsunami, paddy soil, flood plains, upper/lower salt marshes, barrier sand body, lagoons, and events intercalated in other depositional facies according to the total sulfur values and core descriptions. Historical reconstruction based on sedimentary analysis revealed that from BC13 to BC16 the area was a calm lagoon environment in which muddy sediments formed, after which a coastal barrier sand body of approximately 1 km in length developed northward from coarser materials derived from the Odaka River. Lagoon filling and drying changed the lagoon environment into a saltmarsh environment during approximately the 6th century AD. After the area changed completely into non-marsh land and became dry, the flood plain environment expanded. Finally, artificial changes were introduced via rice paddies construction and the area was widely covered by artificial paddy soil. On 11 March 2011, the region was targeted by a large tsunami, which deposited new material. Since then, most of these deposits have been removed during public reconstruction efforts.
Event deposits intercalated with upper saltmarsh deposits were correlated with landward and seaward sites. The observed distribution limits of the event deposits were of approximately 1.5 km from the present coastline. X-ray fluorescence scanning analysis of the event deposits revealed high peaks of calcium, sulfur, manganese, and strontium, which are a proxy of marine clastics, and a high silicon content compared with the over- and underlying layers. These results along with radiocarbon dating data indicated that the event layer intercalated with upper saltmarsh deposits was formed by the tsunami during the Kofun Period (6th to 8th centuries AD).Therefore, the event can be considered a historical tsunami.
A total of seven cores from six drilling sites were recovered from the field survey and transported to the laboratory. The cores were described concerning grain size of the sandy deposits and total sulfur content of muddy samples, and X-ray fluorescence core scanning, radiocarbon dating, and X-ray computed tomography analyses were performed.
A total of eight depositional facies were identified in the cores, which were found to be resultant of deposits from the 2011 tsunami, paddy soil, flood plains, upper/lower salt marshes, barrier sand body, lagoons, and events intercalated in other depositional facies according to the total sulfur values and core descriptions. Historical reconstruction based on sedimentary analysis revealed that from BC13 to BC16 the area was a calm lagoon environment in which muddy sediments formed, after which a coastal barrier sand body of approximately 1 km in length developed northward from coarser materials derived from the Odaka River. Lagoon filling and drying changed the lagoon environment into a saltmarsh environment during approximately the 6th century AD. After the area changed completely into non-marsh land and became dry, the flood plain environment expanded. Finally, artificial changes were introduced via rice paddies construction and the area was widely covered by artificial paddy soil. On 11 March 2011, the region was targeted by a large tsunami, which deposited new material. Since then, most of these deposits have been removed during public reconstruction efforts.
Event deposits intercalated with upper saltmarsh deposits were correlated with landward and seaward sites. The observed distribution limits of the event deposits were of approximately 1.5 km from the present coastline. X-ray fluorescence scanning analysis of the event deposits revealed high peaks of calcium, sulfur, manganese, and strontium, which are a proxy of marine clastics, and a high silicon content compared with the over- and underlying layers. These results along with radiocarbon dating data indicated that the event layer intercalated with upper saltmarsh deposits was formed by the tsunami during the Kofun Period (6th to 8th centuries AD).Therefore, the event can be considered a historical tsunami.