Abstract
Inawashiro-ko Formation is defined by a sediment core, INW2012 (drilling depth : 37.13m below lake floor (blf)) taken from the central part of Lake Inawashiro-ko, Fukushima Prefecture, Japan. The present study reports litho- and tephro-stratigraphy and results of AMS radio-carbon dating of the INW2012 sediment core. The Inawashiro-ko Formation is divided into three stratigraphic units : the Lower part (37.13- 26.60m blf) consisting of medium sand-sandy silt (vertically varied in grain size) with granule and wood fragments ; the Middle part (26.60-24.89 m blf) consisted of very fine sand-silt with a fining upward trend ; and the Upper part (24.89- 0.00m blf) composed of dense alternations of brighter and darker clay layers including clay-sand layers and tephra beds which are clearly distinguished from clay-dominant layers. The Lower part is of fluvial origin before formation of the lake. The Middle part was deposited during the initial stage of the lake formation, whereas the Upper part formed under deep-water condi-tions similar to the present lake. Six tephra beds can be correlated with previously reported tephras such as Aira-Tn (AT), Asama-Kusatsu (As-K), Towada-Chuseri (To-Cu), Numazawa-Numazawako (Nm-NK), Haruna-Futatsudake-Shibukawa (Hr-FA), and Haruna-Futatsudake-Ikaho (Hr-FP) tephras in chronological order. On the basis of 14C dating, Lake Inawashiro-ko formed at 42calkBP, and the sedimentation rate in the Upper part is estimated to be 0.3-1.0mm/yr.
