Abstract
2015 Kanto-Tohoku heavy rain gave damage of levee breaking of Kinu River at Misaka, Joso City in Ibaraki Prefecture, Japan. Around the levee-broken site, natural levee, flood plain is developed and related fluvial landform such as back marsh and abandoned old channel are observed. To know the history of landform devolopment, it is important to date the fluvial landform deposits. The flood that broke the levee also formed clevasse channel and such the deposits under the surface was appeared; therefore, the deposit (fine sand) at the elevation 14.2 m, above the average Tokyo Bay sea level, were sampled at site A and dated using Carbon 14 method. As a result, the age 420-537 cal AD (2 sigma, 95.4%) was obtained. At the site B, where is ca.60m away from the site A, the dating showed 360-273 cal BC (2 sigma, 58.9%) and 262-199 cal BC (2 sigma, 36.5%). Site B is dark gray clay layer 30cm in thick, and sand layer was above the dark blue clay layer. Therefore, the sand layer is thoght to be deposited before 420 AD but after 199 BC; however, unconformity was difficult to be found between the sand layer and the dark blue clay layer.
