【10/4更新】2022年4月1日からのCiNii ArticlesのCiNii Researchへの統合について

Paleogene Foraminifera from Hokkaido, Japan Part 1. Lithostratigraphy and Biostratigraphy including Description of New Species

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Foraminiferal assemblages from Paleogene-outcropping areas in Hokkaido, Japan are described, giving a full account of their systematics and stratigraphic distribution. Employing five benthic foraminiferal events which are useful for intra-regional stratigraphic correlation and some other important foraminiferal changes, a sequence of Eocene to lower Oligocene strata is biostratigraphically divided into eight benthic foraminiferal zones. These zones are named, in upward sequence, as follows : The Elphidium asanoi-Reophax tappuensis Assemblage-zone, Elphidium ishikariense-Bulimina yabei Assemblage-zone, Haplophragmoides tanaii-Haplophragmoides subevolutus Assemblage-zone, Haplophragmoides umbilicatus-Haplophragmoides subevolutus Assemblage-zone, Bulimina schwageri-Haplophragmoides umbilicatus Assemblage-zone, Haplophragmoides subevolutus-Cyclammina pacifica Assemblage-zone, Bulimina schwageri-Gyroidina yokoyamai Assemblage-zone, and Nonion ezoensis-Cyclammina pacifica Assemblage-zone. The discovery of nine planktonic foraminiferal species in these Paleogene strata provides a better age determination than that given by previous workers. The major part of the Poronai and Momijiyama Formations in the Yubari area and the Utsunai Formation in the Nakatombetsu area are considered to lie within an Upper Eocene to Lower Oligocene interval. By means of the distribution of the above-mentioned zones and ages, correlation of Paleogene strata in Hokkaido has been carried out and it is concluded that the "Paleo-Poronai Sea (Kaiho, 1983)" transgressed northward and reached the Tempoku region (northern most region of Japan) during a late Eocene to early Oligocene period. Additionally, radiolarian abundance, lithofacies and benthic foraminifera of the Poronai and Momijiyama Formations in the Yubari area clearly exhibit a southward deepening. Most of the Poronai and Momijiyama Formations are considered to represent outer neritic to bathyal depths on the western continental slope of central Hokkaido. A rapid flourishing of calcareous foraminifera recognized in the vicinity of Eocene/Oligocene boundary in the studied area may have a relation to the rapid deepening of CCD in that time proposed by van Andel and Moore (1974). In the section of Systematic Paleontology, a total of 142 species and 3 subspecies are described. Of these, 39 species and 1 subspecies are proposed as new to science.

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