Stratigraphic correlation between the Plio-Pleistocene Yabuta and Junicho Formations using volcanic ash beds, and diatom and calcareous nannofossil biostratigrahy of lower part of the Junicho Formation in northwestern Toyama Prefecture, Central Japan

摘要

The Yabuta Formation consists of the early to late Pliocene marine sediments distributed along the Nadaura coast, and the Junicho Formation includes late Pliocene to early Pleistocene marine sediments near Asahiyama hill, in northwestern part of Toyama Prefecture.We examined volcanic ash beds in the Yabuta and Junicho Formations by petrographical method and measuring chemical component of volcanic glass. Two pairs of the volcanic ash beds (MT2-T1, UN-T3) can be correlated between the Yabuta and Junicho Formations, and the middle-upper part of the Yabuta Formation can be correlated to the lower part of the Junicho Formation. Fossil diatoms from the lower part of the Junicho Formation showed that the boundary between the Neodenticula koizumii-Neodenticula kamtschatica Zone and the N. koizumii Zone is around the T3 volcanic ash bed that is widely traced in the Hokuriku Region. Calcareous nannofossil biostratigraphy of the lower part of the Junicho Formation indicates that Datum A (2.75 Ma) is demarcated about 4 meter below the T3 volcanic ash bed. A widespread tephra of the Habutaki I-MT2 ash layer is situated about 14 m below the Datum A in the Junicho Formation. 藪田層は富山県灘浦海岸沿いに分布する鮮新世前期～後期の地層で, 十二町層は同県氷見市朝日山周辺に分布する鮮新世後期～更新世前期の地層である.筆者らは両層に挟在する火山灰層の記載岩石学的検討と化学分析, および十二町層下部の珪藻化石・石灰質ナンノ化石の検討し, 次の結果を得た.(1)藪田層と十二町層間でMT2-T1,UN-T3の2組の火山灰層が対比されることから, 藪田層の中部・上部が十二町層の下部に対比される.(2)珪藻化石層序の結果から北陸地域に広く追跡されているT3火山灰層は珪藻化石帯N. koizumii-N. kamtschatica帯とN. koizumii帯の境界付近に挟在する.またT3の4 m下位に石灰質ナンノ化石層序のDatum A(2.75 Ma)が位置する.(3)十二町層下部で認めたT1火山灰層は広域火山灰層, 土生滝I-MT2火山灰層と対比され, 石灰質ナンノ化石層序のDatum Aより14 m下位に位置する.