Benthonic Foraminiferal Biostratigraphy of the Standard Areas of Middle Miocene in the Pacific Side Province, Central Japan

Abstract

The benthonic foraminiferal assemblages from the Tomioka and Nishiyatsushiro groups distributed in the Pacific side province, Central Japan, selected by IKEBE et al. (1977) as the composite stratotype of the Kaburan Stage (Middle Miocene) and subdivided by CHIJI and KONDA (1978) into two substages, the lower Kaburan and upper Fujian substages, were studied. The following nine and four assemblage zones were recognized in ascending order, respectively. Tomioka Group Melonis-Stilostomella Zone Cyclammina-Discammina compressa Zone Cibicidoides pseudoungerianus-Stilostomella lepidula Zone Martinottiella communis-Spirosigmoilinella compressa Zone Stilostomella-Trifarina kokozuraensis Zone Cassidulina norcrossi-Trifarina kokozuraensis Zone Uvigerina proboscidea-Lenticulina lucida Zone Uvigerina proboscidea-Praeglobobulimina pupoides Zone Ammonia tochigiensis-Amphycoryna tubulata japonica Zone Nishiyatsushiro Group Melonis pompilioides-Ammodiscus incertus Zone Globobulimina auriculata-Melonis nicobarensis Zone Globobulimina auriculata-Nodosaria longiscata Zone Melonis pompilioides-Stilostomella ketienziensis Zone The relation between these assemblage zones of the first group and those of the latter is shown in Fig. 9, together with the horizons of the first (or last) occurrences of important planktonic foraminifera recognized in these sequences. Judging from the stratigraphic distribution of planktonic and benthonic foraminifera, it may be concluded; (1) the sedimentary environment of the Tomioka Group was the upper bathyal zone under the influence of the warm water current from the late Early Miocene to the early Middle Miocene, but afterwards, it became shallower and changed to the outer or middle neritic zone, and a shifting of the current so as to allow the appearance of cold water species of planktonic foraminifera took place during the middle--late Middle Miocene in the area; (2) on the other hand, the sedimentary environment of the Nishiyatsushiro Group has continued to be the upper bathyal zone throughout that time under the warm water condition. The fuanal changes of benthonic foraminifera in the Globorotalia peripheroacuta/Globorotalia miozea (s.s.) and Globorotalia bykovae/Globorotalia menardii zones reported by Maiya and Murata (1977) from the Hiki Hill and the Daigo-Omiya area of the Kanto district seem to resemble those recognized in the Globorotalia peripheroacuta/Globigerina nepenthes Interval-Zone of the Tomioka Group. It may be said, therefore, that the marine regression from the north and west of the Kanto district began in the middle stage of the Middle Miocene. Besides, referring to the faunal character of benthonic foraminifera from the Middle Miocene sequences distributed not only in the Kanto district but in the adjacent areas of the Pacific side province, Central Japan, some problems which should be reinvetigated in future were discussed.