10:45 〜 11:00
[MIS23-25] Stratigraphy and paleo-environmental study of Paleogene sequences in Hokkaido, Japan
キーワード：古第三紀、層序、渦鞭毛藻
Paleogene period is characterized by the transitional interval from warm greenhouse period to cool icehouse period in the climate history of earth. Although detailed stratigraphic and paleo-environmental studies of Paleogene sequences have been carried out in the Atlantic, Indian oceans and Tethys Sea, while little is known about the paleo-environmental change in the northwest Pacific Ocean.
Thick Paleogene sequences are exposed in the central and eastern Hokkaido, Japan. These sequences were accumulated in the fore-arc and/or intra-arc basins in the northwestern Pacific Ocean. In this study, we analyzed U-Pb ages of tuff beds and carbon isotope, benthic foraminifera and dinoflagellate cyst fossils of the Paleogene sequences of the Nemuro, Poronai, Urahoro and Onbetsu groups. Based on our U-Pb ages and carbon isotope stratigraphy and previous work of calcareous nannofossil biostratigraphy, the middle-upper parts of the Nemuro, Poronai, Urahoro and Onbetsu groups are correlated with Danian to Ypresian, Lutetian to Rupelian, Bartonian to Rupelian and lower to middle part of Rupelian, respectively. Four major environmental events are identified in the studied sequences as below.
(1) Danian-Selandian: This period is characterized by eutrophication of surface ocean, represented by prominent increase in P/G ratio of dinoflagellate in the Nemuro Group. This interval corresponds to the warm-climate period of early Paleocene.
(2) PETM: No significant faunal change is identified around this period in the Nemuro Group. Since exposure around this horizon is quite poor, there is a possibility that exact PETM horizon is not exposed in this studied section.
(3) Middle Eocene Climatic Optimum (MECO): This interval is correlated with the basal part of the Poronai Formaiton. On set of deposition of the Poronai Formation might reflect the sea level rise in this period although tectonic subsidence might have been the major contribution of the Poronai basin formation.
(4) Bartonian cooling: In the middle part of Poronai Formation is marked by occurrences of glendnite. This interval yields abundant benthic foraminifera of Bullimina. These evidences suggest that cooling and increased primary productivity occurred in this region.
(5) Eocene/Oligocene boundary: Prominent sedimentary facies change from hemipelagic mudstone to shallow-marine sandstone occurred across the boundary between the Poronai and Momijiyama formations. These facies change may reflect sea-level fall around the Oi-1 glaciation. The same horizon is identified in the upper part of the Urahoro Group.
Thick Paleogene sequences are exposed in the central and eastern Hokkaido, Japan. These sequences were accumulated in the fore-arc and/or intra-arc basins in the northwestern Pacific Ocean. In this study, we analyzed U-Pb ages of tuff beds and carbon isotope, benthic foraminifera and dinoflagellate cyst fossils of the Paleogene sequences of the Nemuro, Poronai, Urahoro and Onbetsu groups. Based on our U-Pb ages and carbon isotope stratigraphy and previous work of calcareous nannofossil biostratigraphy, the middle-upper parts of the Nemuro, Poronai, Urahoro and Onbetsu groups are correlated with Danian to Ypresian, Lutetian to Rupelian, Bartonian to Rupelian and lower to middle part of Rupelian, respectively. Four major environmental events are identified in the studied sequences as below.
(1) Danian-Selandian: This period is characterized by eutrophication of surface ocean, represented by prominent increase in P/G ratio of dinoflagellate in the Nemuro Group. This interval corresponds to the warm-climate period of early Paleocene.
(2) PETM: No significant faunal change is identified around this period in the Nemuro Group. Since exposure around this horizon is quite poor, there is a possibility that exact PETM horizon is not exposed in this studied section.
(3) Middle Eocene Climatic Optimum (MECO): This interval is correlated with the basal part of the Poronai Formaiton. On set of deposition of the Poronai Formation might reflect the sea level rise in this period although tectonic subsidence might have been the major contribution of the Poronai basin formation.
(4) Bartonian cooling: In the middle part of Poronai Formation is marked by occurrences of glendnite. This interval yields abundant benthic foraminifera of Bullimina. These evidences suggest that cooling and increased primary productivity occurred in this region.
(5) Eocene/Oligocene boundary: Prominent sedimentary facies change from hemipelagic mudstone to shallow-marine sandstone occurred across the boundary between the Poronai and Momijiyama formations. These facies change may reflect sea-level fall around the Oi-1 glaciation. The same horizon is identified in the upper part of the Urahoro Group.