関東平野中央部の地下および房総半島で新たに発見された前期更新世の含菫青石テフラ

抄録

The research result by the Geological Research Group of Central Kanto Plain is already reported about the core samples collected from boreholes at Kasukabe, Itakura, Ora, and Yoshimi. The cordierite bering tephras were newly found from those core samples. Moreover, the same tephra was found out also from outcrop of Isumi-cho, Chiba Prefecture. As for cordierite bering tephras, the Joetsu Ash etc. are known until now. And the Joetsu Ash is 1.3Ma or 1.1Ma in age. In this study, newly found codierite bering tephras are described with grain-size composition, mineral assemblage, refractive index of volcanic glass and chemical composition of cordierites. These features show that newly found codierite bering tephras probably can be identified with the Joetsu Ash. This fact agrees with the consideration of the Geological Research Group of Central Kanto Plain (1994) that there is the underground structure of the form of the stairs which has the deepest portion in the Kasukabe area. Moreover, since codierite bering tephra was found directly under O-18 volcanic ashes layer in Boso Peninsula, the upper part of D layer of drilling cores is identified with the part of the Otadai Formation.