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3.8 “常総粘土”（Jc）

命名・定義

本報告では徳橋・遠藤（1984）にならい，木下層及び姉崎層と新期関東ローム層の間に挟在する凝灰質粘土層を“常総粘土”と呼ぶ．なお，「姉崎」図幅地域などでは，“常総粘土”は木下層を覆い姉崎層と一部同時異相関係にあるが，本地域を含む下総台地南縁部では本層は姉崎層の上位に認められる．

層相・分布

下総下位面分布域に分布する．“常総粘土”は主に灰白色―茶灰色を呈する厚さ数10cmの凝灰質粘土からなる（第3.33図）．所によって原地性植物根化石を含むほか，褐鉄鉱の薄い層を挟む．本地域では層厚が薄く，下位の姉崎層最上部の泥層と識別しがたい．木更津市三光路（Loc. 50）では本層中に後述の御岳第一テフラ（On-Pm1）が確認された．

テフラ

御岳第一テフラ（On-Pm1 : Kobayashi, 1960）

木更津市三光路（Loc. 50）において厚さ数cmの黄白色―黄色の風化軽石層として認められた．重鉱物は角閃石が多く，黒雲母を含む．角閃石の屈折率（n2）は1.680-1.688（1.683-1.684）である（第3.7表）．このテフラの降下年代は，町田・新井（2003）によると約100kaとされ，海洋酸素同位体ステージ5cに相当する．

","text":"3.8 “常総粘土”（Jc）\n命名・定義\n本報告では徳橋・遠藤（1984）にならい，木下層及び姉崎層と新期関東ローム層の間に挟在する凝灰質粘土層を“常総粘土”と呼ぶ．なお，「姉崎」図幅地域などでは，“常総粘土”は木下層を覆い姉崎層と一部同時異相関係にあるが，本地域を含む下総台地南縁部では本層は姉崎層の上位に認められる．\n層相・分布\n下総下位面分布域に分布する．“常総粘土”は主に灰白色―茶灰色を呈する厚さ数10cmの凝灰質粘土からなる（第3.33図）．所によって原地性植物根化石を含むほか，褐鉄鉱の薄い層を挟む．本地域では層厚が薄く，下位の姉崎層最上部の泥層と識別しがたい．木更津市三光路（Loc. 50）では本層中に後述の御岳第一テフラ（On-Pm1）が確認された．\nテフラ\n御岳第一テフラ（On-Pm1 : Kobayashi, 1960）\n木更津市三光路（Loc. 50）において厚さ数cmの黄白色―黄色の風化軽石層として認められた．重鉱物は角閃石が多く，黒雲母を含む．角閃石の屈折率（n2）は1.680-1.688（1.683-1.684）である（第3.7表）．このテフラの降下年代は，町田・新井（2003）によると約100kaとされ，海洋酸素同位体ステージ5cに相当する．","blocks":[{"type":"heading","depth":1,"text":"3.8 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