The northern coast of Ariake Bay, which is part of the Tsukushi Plain in Kyushu, Japan, is underlain by thick Quaternary deposits. However, the Late Middle Pleistocene succession had not been found. Recently, the Middle Pleistocene formation (Shagarami Formation) including a tephra bed (Shagarami tephra) was discovered under Aso-3 tephra in a drill core from a site near the mouth of Rokkaku River. This study revealed sedimentary facies of the Shagarami Formation, petrographic characteristics and LA-ICPMS-FT age of the Shagarami tephra. The Shagarami Formation can be divided into two sequence units on the basis of sedimentary facies, and it contains oyster fossils and mud-drape indicative of deposition in marine environments. The Shagarami tephra found in the top of the lower part of the Shagarami Formation mainly comprises pumice, and a zircon LA-ICPMS-FT age of 0.33 ± 0.06 Ma (1σ) was obtained. Therefore, lower and upper parts of the Shagarami Formation were deposited in different marine transgression and regression sequences in the Late Middle Pleistocene, respectively.
