Oblique intersection of lithostratigraphic units and a tephra layer in the Pleistocene Shimosa Group beneath central Kanto Plain, central Japan.

Abstract

The borehole core (GS-KM-1) was collected at Kitamoto City, Saitama Prefecture, central Kanto Plain to investigate stratigraphy and facies distribution of the Pleistocene Shimosa Group in the shallow subsurface of the Tokyo metropolitan area. A cummingtonite-rich tephra layer was found at the top of Yabu Formation marine facies. This tephra layer is intercalated in the middle part of the Yabu Formation, corresponding to the sea-level highstand of MIS 9, at Urawa to the south of Kitamoto and in the terrestrial muddy facies of the Kamiizumi Formation, which conformably overlies the Yabu Formation, at Gyoda to the north of Kitamoto. This suggests that the terrestrial muddy facies of the Kamiizumi Formation were deposited during the late-MIS 9 marine regression. It should be noted that lithostratigraphic units and the time plains obliquely intersect when correlating each formation of the Shimosa Group over a wide area.