Benthonic Smaller Foraminifera from the Oil Fields of Northern Japan

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Surface and subsurface geology undertaken parallel with micropaleontological studies of the Foraminifera has resulted in the discrimination of foraminiferal zonules. These zonules extend over wide areas and show remarkable uniformity in their specific compositions and stratigraphic levels within the respective geological columns of the different areas distributed from Akita Prefecture in the north, to Niigata Prefecture in the south via Yamagata Prefecture. Zoning by Foraminifera during Tsugawa to Nishiyama time in the Niigata oil-fields and from Nishikurosawa to Tentokuji time in the Akita and Yamagata oil-fields was rendered possible by the guide species. Among the deposits younger than the Tentokuji-Nishiyama time, zoning was based upon differences in the fauna and changes in their assemblages and frequency. Faunal changes in both time and space have proved to be suitable for analyses of the conditions under which the Foraminifera once lived. The Foraminifera are good indicators of change in environmental conditions and for sectioning of the different stratigraphic units along the coast of the Japan Sea. The Foraminifera studied in this article amounts to 114 genera and 360 species, among which one genus and 35 species or subspecies are described as new to science. Details are given as to their precise levels in the Neogene Tertiary formations dealt with in this work. Indications are given as to rock facies of the formations, place of depository of the specimens and localities of their occurrences.

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