[S-GL30] 地球年代学・同位体地球科学

[SGL30-10] 浮遊性有孔虫層序N7帯/N8帯境界の年代：古地磁気層序とU-Pb年代測定による検討

*星 博幸¹、大信田 彦磨²、林 広樹³、岩野 英樹⁴、檀原 徹⁴、栗原 行人⁵、柳沢 幸夫⁶ (1.愛知教育大学自然科学系、2.愛知県立海翔高等学校、3.島根大学、4.京都フィッション・トラック、5.三重大学、6.産総研)

キーワード：地質年代尺度、一志層群、磁気層序、中新世、浮遊性有孔虫化石層序、U-Pb年代

One of major differences between Miocene portions of GTS2004 and GTS2012 (GTS = Geologic Time Scale) is the age assigned for the N7/N8 planktonic foraminiferal zonal boundary; ages of 16.97 Ma and 16.38 Ma were assigned, respectively, by GTS2004 and GTS2012. The difference is mainly due to lack of reliable assignment of the N7/N8 boundary to the magnetostratigraphy, and this issue needs to be resolved for better refining the geologic time scale. Here we present results of an integrated study that includes new planktonic foraminiferal, magnetostratigraphic, and U-Pb isotopic data from an onland sedimentary sequence of the Ichishi Group in central Japan. Our stratigraphically dense sampling allowed us to locate the stratigraphic position of the FAD of Praeorbulina sicana that defines the N7/N8 boundary. We conducted zircon U-Pb dating on a thin ash bed located at 1.8 m above the zonal boundary and obtained a precise date of about 17.0 Ma. Further, our magnetostratigraphic investigation revealed a reverse-normal-reverse (R-N-R) polarity sequence for the studied section and demonstrated that both the N7/N8 boundary and the nearby dated ash bed are located within the upper reverse polarity zone. On the basis of these data, it is safe to conclude that: (1) the R-N-R polarity sequence of the studied section is correlated to a portion of Chronozones C5Dr-C5Dn-C5Cr, (2) the N7/N8 boundary is within Chronozone C5Cr, and (3) the age of the N7/N8 boundary is about 17.0 Ma. We suggest this should be incorporated into a forthcoming revision of the geologic time scale.