Kd 38 volcanic ash bed intercalated in the lower part of the Kazusa Group in the Boso Peninsula, central Japan : Reexamination for the correlated volcanic ash bed

Kd 38 pyroclastic key bed which is intercalated near the boundary of Pliocene-Pleistocene series is a widespread volcanic ash bed and important key bed for the stratigraphy of the Kazusa Group. Redefined Kd38 pyroclastic key bed as th eKd38 volcanic ash bed were reexamined for correlation in thirteen localitied in the Boso Peninsula based on their lithofacies and petrographic properties. Thirteen localities are divided three Orikizawa-Koshikoma areas based on lithofacies of Kd38 volcanic ash. Lithofacies and petrographic properties of Kd38 volcanic ash are different at each area. However, Kd38 volcanlc ash bed in different localities can be correlated by correlation of the Kd38.2 volcanic ash bed below the Kd38 volcanic ash bed and relationship of lithofacies and petrographic properties. Their differences of lithofacies and petrographic properties of the Kd 38 volcanic ash bed at each localities reflects the difference in depositional processes of this volcanic ash after fallout.鮮新-更新統境界付近に挟在する広域火山灰層として重視されている，上総層群下部の火砕鍵層Kd38をKd38火山灰層として再定義し，房総半島東部から西部に至る13地点において対比の再検討を行った．Kd38火山灰層は調査を行った各地点において，岩相および記載岩石学的性質が異なるが，佐野～会所地域，横瀬～追原地域，折木沢～小志駒地域の各地域内での岩相および記載岩石学的性質はほぼ類似する，さらにそれぞれの地域間においては，下位の特徴的な性質を持っKd38．2火山灰層の対比とKd38火山灰層のユニット区分による岩相と記載岩石学的性質の関係から対比される．このような各地点におけるKd38火山灰層の岩相や記載岩石学的性質の違いは，降灰後の堆積過程の違いを反映している