2019 Volume 78 Issue 1 Pages 3-14
The purpose of this study is to reconstruct the paleo-environmental changes in the last 10,000 years and to identify the event deposits in sediments of an estuary in the southern part of Minamisoma City, Fukushima Prefecture, northeast Japan. The area was used for growing rice until it was inundated by the tsunami of March 11, 2011. A 15-m-long core of the Holocene sediments and underlying Pleistocene sandy siltstone was taken and sedimentary facies were identified based on the core description. Soft X-ray photograph, grain size analysis, and radiometric 14C dating were conducted to assess the paleo environment and sources of each event deposit. The environment changed as follows: (1) 10,000-8,500 cal. yr BP: the area changed from backshore to sandy and muddy tidal flat following the Last Glacial Maximum, (2) 8,500-6,000 cal. yr BP: tidal flat environment changed to central basin of the estuary during the Holocene maximum transgression, and (3) 6,000 cal. yr BP-present: salt marsh environment formed after the sea level rise. A few event sand beds intercalated in the deposits. The event sand beds contain muddy rip-up clasts and marine shell fragments, and represent lamination, grading structures, and periodic sedimentation. These characteristics suggest that they are possible tsunami deposits.
