Stratigraphy of Late Quaternary Sediments and the Depositional Environment in the Sakaide Plain, Kagawa Prefecture, Japan

The Sakaide Plain, located in the northeastern part of Shikoku Island, is one of the largest plains around the Seto Inland Sea. The late Quaternary sediments in the Sakaide Plain are divided into the Honmachi Formation, the Sakaide Formation, and the Ayagawa Formation. The Sakaide Formation is subdivided into three members: lower, middle, and upper members of the Pleistocene. The upper member consists mainly of muds deposited in marshes around 25, 000yrs BP. The overlying Ayagawa Formation is also subdivided into three members: the Takaya-cho muds member deposited as lagoonal sediments, the Takaya-cho sands member as marine sediments, and the Hayashida-cho sands member as marginal marine sediments in tidal flats. The volcanic ash named “Sakaide 5”, which can be correlated to Aira-Tn volcanic ash (AT), was found in the Sakaide Formation upper member. Another volcanic ash named “Ayagawa”, which can be correlated to Kikai-Akahoya volcanic ash (K-Ah), was found in the Ayagawa Formation. The delta of the Ayagawa river, a major part of the Sakaide Plain, was built by covering the Takaya-cho muds member before 6, 300yrs BP. The bottom set beds of the delta were formed around 6, 300yrs BP. The foreset beds were deposited until 5, 500yrs BP, and then the top-set beds covered these beds after 5, 500yrs BP.