東北本州日本海地域の後期新生代底生有孔虫群集の変遷  [in Japanese] Late Cenozoic benthic foraminiferal assemblages in the Japan Sea coastal region of northeast Honshu, Japan  [in Japanese]

秋田地域を中心とする東北本州日本海域の上部新生界の化石資料から, 日本海形成以降の底生有孔虫群集の変遷を述べた。日本海は台島期に始まり, 西黒沢期に大きく海域が広がった。西黒沢期には浅海域に熱帯-亜熱帯性浅海群集が, 深海部には南方系深層水群集が分布した。女川期に入り浮遊性有孔虫群集は寒流系のものに変わり, 底生群集も一般に深海性砂質有孔虫群集に変わったが, 南方系深層水群集は混合して存続し船川期の始めになって消滅して, 鮮新世型深層氷群集に変わった。女川期後期以降東北本州は徐々に隆起して鮮新世末には陸化し, 西の日本海側に位置する男鹿半島では更新世後期まで海成堆積が行われた。

Late Cenozoic faunal changes of benthic foraminifera in the Japan Sea coastal region of northeast Honshu, Japan, are descrided on the basis of land sections in relation to the paleoenvironmental history of the Japan Sea since the birth of the sea. According to recent paleomagnetic studies, the Japan Sea opened up during Early to Middle Miocene time, where by drifting the Japanese Islands away from the Asian Continent. The earliest evidence of marine incursion in the Japan Sea region comes from fossil diatoms of the Daijima age occurring in late Early Miocene sediments (Koizumi, 1988). The diatom flora was collected from localities that yielded of leaves of land plants named the Daijima-type flora, and it was composed of a mixture of marine, brackish and fresh water species suggesting an estuarine condition. Distinct marine faunas appeared at the beginning of the Middle Miocene as shown by the occurrence of planktonic foraminifers of Zone N. 8 age (Maiya, 1978). The benthic foraminiferal assemblages in the earliest Middle Miocene (N. 8 to N. 10) strata, that are an early to middle Nishikurosawa age, consist of 1) sub-tropical shallow-water assemblage that includes many Middle Miocene species as well as such larger foraminifera as Miogypsina and Operculina, 2) deep water assemblage of southern origin characterized by several deep water species living in the present-day Pacific off southwestern Japan, and 3) transitional fauna of the two. This occurrence of deep water (middle bathyal) fauna during the earliest developmental stage of the Japan Sea is quite remarkable. In the late Nishikurosawa age (N. 11), all the shallow-sea areas submerged deeper, and this subsidence culminated in the deep sea basin of the Onnagawa age during late Middle to Late Miocene time. At the middle period of the Middle Miocene, a transition period from the Nishikurosawa to Onnagawa age, sea conditions changed greately from warm to cold water sas shown by a change in the planktonic foraminiferal faunas, suggesting the curtailment of inflow of warm current into the sea owing to the closing seaways located in the southern margin. Sediments of the Onnagawa age are characterized by siliceous shale and diatomite in the northern Japan. The benthic foraminiferal fauna also changed its character, containing largely agglutinated species inhabiting lower middle bathyal to lower bathyal depths. Recently, however, a unique fauna was discovered in the Tsugaru Peninsula from the Genpachimori Formation, which is correlatable with the upper part of the Onnagawa Formation. The fauna showsa character transitional from the lower Middle Miocene Nishikurosawa age faunas to those of the Pliocene-Pleistocene Funakawa, Kitaura and Wakimoto ages in terms of inclusion of characteristic species of both faunas. Spriosigmoilinella compressa, an important species among agglutinated foraminifera of the lower Middle to Upper Miocene of the Japan Sea region, is considered also a member of southern deep water. These facts suggest that the deep-water Nishikurosawa fauna did not become extinctal together, but that it was able to survive in some deep water areas of the sea and changed gradually its character to give rise to the Pliocene-Pleistocene fauna. However, the warm water planktonic fauna disappeared in the middle of Middle Miocene. In the Pliocene Funakawa age, the benthic foraminiferal fauna consists of a mixture of upper middle to upper bathyal calcareous species and agglutinated species. Although this fauna indicates a marine condition shallower than that of the preceding Onnagawa age, this apparent shallowing is considered not to reflect shallowing of the Japan Sea basin but to have resulted from the uplift of the eastern margin of the sea. During the Upper Miocene to Pliocene interval, the northeast Honshu gradually uplifted earlier than western, Japan Sea side, such as the area of the Oga Peninsula. Thus, by the beginning of Pleistocene, northeast Honshu became emerged, accompa