Reexamination on stratigraphy of the Middle to Upper Pleistocene Shimosa Group beneath the Tsukuba Upland, Ibaraki Prefecture, central Japan

The stratigraphy of the Middle to Upper Pleistocene Shimosa Group beneath the Tsukuba Upland area of Ibaraki Prefecture, Kanto Plain, central Japan has been reexamined by performing detailed facies observations and analyzing intercalated tephra layers and fossil pollen assemblages in sediment cores. The Pleistocene deposits in the sediment cores are divided into five depositional cycles, each composed of fluvial and coastal sediments that formed under the influence of glacioeustacy. These cycles are correlated with the Yabu, Kamiizumi, Kiyokawa (Kamiiwahashi), Kioroshi, and Joso formations in ascending order based on the tephroand palyno-stratigraphy. Of note, the incised valley fill beneath the Tsukuba Upland area, which was previously correlated with the Kamiiwahashi Formation (MIS 7), corresponds to the Kioroshi Formation (MIS 5e). The valley was incised during MIS 6, which was one of the most prominent sea level lowstands of the last 400 ky. The new correlation is consistent with the distribution of equivalent incised valley fills in the Kioroshi Formation beneath the Kanto Plain in Chiba and Saitama prefectures.