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.

Breaking and abrasion mechanisms act on the sands and gravels transported in riverine environments as pristine finer grains are produced. To elucidate the characteristics of these grain-producing mechanisms, the relationships between rock type, grain size, and roundness of detritus were examined. The downstream changes in roundness of both sand and gravel fractions (0.5-128 mm in diameter) were investigated at two tributaries of the Watarase River, central Japan, under similar fluvial and geological conditions. The tendency that grain roundness and ratio of chert and shale (ch/sh) became lower as the grain size became finer indicates that pristine angular grains produced from broken and/or abraded coarser grains increased in the finer grain size fraction. At downstream sites, where more transported grains were deposited, the authors identified the phenomenon that the roundness values were highly similar within certain consecutive grain size fractions; i.e., “roundness in saturation.” This indicates an increase in angular grains produced from transported coarser grains and an increase in abraded and rounded grains become in attainment of equilibrium during transport. The onset of roundness value saturation, i.e., no additional increase of grain roundness during the transport process, would differ according to rock type. In this study, the roundness value of chert and shale was 0.4 (sub-angular) and 0.6 (sub-rounded), respectively. These values would be unique to rivers in the Japanese islands characterized by short river lengths and steep gradients.