西太平洋地域の大型有孔虫動物群集の変遷 : 特に第三紀を中心として

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The Larger Foraminifera found from the Tertiary sediments in the Tethyan, America and Indo-Pacific provinces have been studied intensively during the past fifty years. That some elements of the Larger Foraminifera are common as similar faunas in each province indicates that their mutual occurrence is probably due to their migration and transportation during the younger stages by warm ocean currents. For regional correlation based upon the Larger Foraminifera, the chronostratigraphy of them is by the Letter Stage. The Letter Stage has been a means for dividing the Tertiary of the East Indies and now is extended to cover the entire Indo-Pacific region. The purpose of this paper is to consider the Letter Stage based upon an assemblage of Larger Foraminifera from the known stratigraphic distribution in the western Pacific Ocean and to clarify previous confusion. Here, discussions are based upon the Oppel-zone composed of an assemblage of selected genera and/or species of the Larger Foraminifera, the Phylo-zone of the genus Nephrolepidina, and the common association with the Larger and Planktonic Foraminifera. The Letter Stage established in the East Indian region of lower latitudes can be applied satisfactorily to the vicinity of 42° north latitude in the western Pacific Ocean (see Fig. 1). Accordingly, Letter Stage Tb is determined by the first appearance of Spiroclypeus vermicularis, Biplanispira absurda, B. mirabilis, B. fulgeria, B. inflata, Pellatispira orbitoides and P. provaleae, and corresponds to P14 of the planktonic foraminiferal zonation of Blow as judged from the consideration of IDA (1974). Based upon the first appearance of Nummulites fichteli and Gypsina globulus, and the disappearance of Pellatispira and Discocyclina, this stratigraphic positiond (=Tc) corresponds to P17 of the planktonic formainiferal zonation of Blow. Td is determined by the first appearance of Eulepidina papuaensis, Lepidocyclina isolepidinoides and Cycloclypeus oppenoorth. It probably corresponds to P19 of the planktonic foraminiferal zonation. Tel-4 is determined by the disappearance of Nummulites fichteli, and corresponds to N1 of the planktonic foraminiferal zonation. Te5 can be determined by the horizon nearer to the disappearance of Heterostegina borneensis andSpiroclypeus margaritatus, and it probably corresponds to N 4 of the planktonic foraminiferal zonation. Tf1 is determined by the appearance of Nephrolepidina rutteni, and Tf2 by the disappearance of N. verbeeki, N. japonica, N. douville, Miogypsina globulina, M. polymorpha, M. borneensis, Flosculinella bontangensis and Austrotrillina howchini. They correspond to N6 and N9 of the planktonic foraminiferal zonation, respectively. Tf3 is determined by the disappearance of Nephrolepidina rutteni in the lower latitude areas, but in the middle latitudes, Tf3 cannot be determined by the Larger Foraminifera. The former may be pointed to N13 to N14 of the planktonic foram iniferal zonation. Tg is determined by the disappearance of Nephrolepidina rutteni in the lower latitude areas, but by the appearance of Alveolinella quoii and Borelis schlumbergeri in the middle latitudes. The former may be pointed to the horizon nearer to the middle part of N15 of the planktonic foraminiferal zonation. In Tokunoshima island, Ryukyu-retto, Japan, in the vicinity of 27°30 north latitude, the Itokina Formation at the basal part of the Ryukyu Group is represented by the larger foraminiferal fauna such as Borelis pulchrus, Archaias sp., Peneroplis pertusus and Cycloclypeus carpenteri. Therefore, Th should be positioned before the appearance of these assemblages. Also, according to IKEBE et al. (1972), the base of the Ryukyu Group corresponds to the basal part of N22 of the planktonic foraminiferal zonation.
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西太平洋地域の大型有孔虫動物群集の変遷 : 特に第三紀を中心として

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