鮮新世における日本海への暖流の流入 : 富山市八尾町の三田層産軟体動物群の検討を通じて  [in Japanese] Influx of warm-water current to Japan Sea during the Pliocene : Based on analyses of molluscan fauna from the Mita Formation in Yatsuo-machi of Toyama City  [in Japanese]

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Abstract

鮮新世の中期における日本海への暖流の流入の時期や場所を明らかにすべく,富山県三田層産軟体動物群を検討した.三田層は岩相的に細粒~中粒砂岩よりなり,3枚の凝灰岩層(OT3, MT1, MT2)を挟む.軟体動物化石を多産するOT3凝灰岩層とMT2凝灰岩層の間の層準は石灰質ナンノ化石のNN16帯の下部~中部に対比される.三田層から産出した軟体動物群中には51種の寒流系種に加え,OT3凝灰岩層の上位層準からMT2凝灰岩層の直上の層準にかけて24種の暖流系種が認められた.こうした種構成は現在より高温で薄い暖流が,寒流に起因する冷水塊上を流れていたためと思われる.すべての層準から暖流系種を産出することから,4~2.2Maの間には暖流が恒常的に流入していたと考えられる.さらに,三田層と鍬江層の暖流系共通種が秋田県天徳寺層よりも多いことや古生物地理学的設定から,暖流は津軽海峡ではなく,対馬海峡から流入したと推定される.

Molluscan fauna from the Mita Formation in Toyama Prefecture was examined for elucidating the timing and pass ways of the warm-current influx to the Japan Sea in the middle Pliocene. The Mita Formation consists of fine- to medium-grained sandstone with three tuff beds (OT3, MT1 and MT2). The horizons between OT3 (about 4 Ma) and MT2 (2.2-2.3 Ma) yielding many molluscan fossils can be assigned to the lower to middle part of NN16 nannofossil zone. The fauna includes many characteristic species of the Plio-Pleistocene Omma-Manganji fauna as well as some Miocene relict species. Besides 51 cold-water species, 24 warm-water species were found from the every stratigraphic horizon above OT3 and to that just above MT2. The Mita fauna also includes some common species with the Plio-Pleistocene warm-water Kakegawa fauna in the southwestern Pacific side of Japan. Such species composition might be affected by the shallow warmer current than today flowing over the cold-water mass. Judging from every horizons yielding the warm-current species, the warm current always flowed since about 4 Ma to 2.2Ma. Moreover, because of more numerous common warm-water species between the Mita and Kuwae faunas than the Kuwae and the northern Tentokuji ones, and of their biogeographical setting, it can be inferred that the warm current entered through around the Tsushima Strait, not through the Tsugaru Strait.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 114(9), 516-531, 2008-09-15

    The Geological Society of Japan

References:  58

Codes

  • NII Article ID (NAID)
    110006990197
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • Article Type
    ART
  • ISSN
    00167630
  • NDL Article ID
    9710825
  • NDL Source Classification
    ZM49(科学技術--地球科学--地質)
  • NDL Call No.
    Z15-174
  • Data Source
    CJP  NDL  NII-ELS  J-STAGE 
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