MIS 5c Maker Tephra-bearing Beds in the Upper Pleistocene Joso Formation, Shimosa Group in the Sashima and Tsukuba Uplands, Kanto Plain, Japan

Abstract

Marine and fluvial terraces in the Kanto Plain mostly formed in marine oxygen isotope stage (MIS) 5 under the concurrent influence of eustatic sea-level changes and Kanto basin-forming movements. In the central part of the plain, however, it remains unknown when and how terraces composed of the upper Pleistocene Joso and Kioroshi Formations, Shimosa Group developed. Terrace surfaces are classified by integrating geomorphic, sedimentary facies, and tephra analyses focusing on the southern Sashima and Tsukuba Uplands, including the northwestern Shimosa Upland. Terrace surfaces are classified into levels I to III. Terrace sediments distributed over levels I to II from the top in the Sashima, Tsukuba, and Shimosa Uplands are divided into two formations: Kioroshi Formation composed of beach facies and Joso Formation composed of flood plain and fluvial channel facies.Found in the Joso Formation were tuffaceous silt layer and pumiceous sand layer, including tephra grain correlated with On-Pm1 (c. 96 ka: Aoki et al., 2008) and Nk-Ma (c. 100 ka: Yamamoto, 2012) in the southern Sashima and Tsukuba Uplands, which allow fluvial terraces formed in the Central Kanto Plain after MIS 5c to be identified.