Deep sea environment of Kuroko formation as indicated by the benthic foraminifera from the Hokuroku district, Japan.
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A preliminary interpretation on the depth of seawater during the Kuroko mineralization was made by examining the foraminiferal assemblages in the Middle Miocene sedimentary rocks in the Hokuroku district. In drill core samples of the M3 mudstone, M2 mudstone, and T2 tuff units from the Matsuki mine and from several other areas in the district, two fairly distinct benthic assemblages are recognized: a dominantly calcareous Melonis-Cibicidoides-Pullenia assemblage and a dominantly arenaceous Martinottiella-Spirosigmoilinella-Sigmoilopsis assemblage. The available evidence, such as the data from the Deep Sea Drilling Project, suggests that the species of the Melonis assemblage lived at lower bathyal depths (2, 500-4, 000 m) and above the carbonate com-pesation depth, while the species of the Martinottiella assemblage lived at abyssal depths near to or below the carbonate compensation depth (about 4, 000 m).<BR>At least two cycles of alternation of the Melonis and Martinottiella assemblages were observed in the Matsuki cores. Such alternations may have been caused by fluctuations in the depth of seafloor, in the carbonate compensation depth, or in combination of both. The working hypothesis is advanced that the alternations of the foraminiferal assemblages were due to fluctuations in the depth of seafloor. The inferred paleobathymetric curve for the Hokuroku "basin" shows that the depth of seawater ranged from 3, 500 m to in excess of 4, 000 m during the m<SUB>3</SUB>, M<SUB> 2</SUB>, and T<SUB> 2</SUB> phases of sedimentation and during the Kuroko mineralization.
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