Japan Geoscience Union Meeting 2015

Presentation information

Oral

Symbol M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS30] Evolution of the Pelagic Realm

Tue. May 26, 2015 9:00 AM - 10:45 AM 304 (3F)

Convener:*Atsushi Matsuoka(Department of Geology, Faculty of Science, Niigata University), Toshiyuki Kurihara(Graduate School of Science and Technology, Niigata University), Yasuhiro Kato(Department of Systems Innovation, Graduate School of Engineering, University of Tokyo), Tetsuji Onoue(Earth and Environmental Sciences, Graduate School of Science and Technology, Kumamoto University), Katsunori Kimoto(Japan Agency for Marine-Earth Science and Technology), Tatsuo Nozaki(Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology), Hayato Ueda(Department of Geology, Niigata University), Kenta Kobayashi(Department of Geology, Faculty of Science, Niigata University), Takashi Hasegawa(Division of Global Environmental Science and Engineering, Graduate School of Natural Science and Technology, Kanazawa University), Chair:Hayato Ueda(Department of Geology, Niigata University), Satoshi Takahashi(Department of Earth and Planetary Science, University of Tokyo)

9:00 AM - 9:15 AM

[MIS30-01] Biostratigraphy and carbon isotope stratigraphy of the Olenekian-Anisian pelagic deep-sea section from Ogama, Ashio Belt

*Shun MUTO1, Satoshi TAKAHASHI1, Satoshi YAMAKITA2, Noritoshi SUZUKI3, Yoshiaki AITA4, Megumi SAITO5, Nozomi SUZUKI5, Yoshimi KUBOTA5 (1.Department of Earth and Planetary Science, The University of Tokyo, 2.Department of Earth Sciences, University of Miyazaki, 3.Institute of Geology and Paleontology, Tohoku University, 4.Geology Lab, Faculty of Agriculture, Utsunomiya University, 5.Department of Geology and Paleontology, National Museum of Nature and Science)

The recovery of life after the end-Permian mass extinction occurred around Anisian (early Middle Triassic). At the Olenekian-Anisian (Lower-Middle Triassic) boundary (O-AB), the lithologic change from claystone dominant facies to radiolarian-rich bedded chert facies has been reported from the Panthalassic pelagic deep-sea sediments in Japan. This trend has been interpreted to reflect a biotic recovery of the pelagic Panthalassa (Isozaki, 1997, Takahashi et al., 2009). However, spatial variations in the lithofacies of these pelagic deep-sea sediments have seldom been investigated in detail, due to the scarcity of well-preserved sections. This study reports a high-resolution microfossil- and carbon isotope-based stratigraphy around the O-AB from a new pelagic deep-sea section (Ogama section) situated in the Tochigi Prefecture, eastern Japan. The lower 8.5 m of the Ogama section is composed of alternating claystone and chert, while the upper 9 m is composed of bedded chert. Age diagnostic fossils show that the alternating claystone and chert interval can be correlated to the Spathian (upper Olenekian: uppermost Lower Triassic) to middle Anisian (lower Middle Triassic), and the bedded chert interval to the middle (to upper) Anisian. Organic carbon isotope values show a positive peak near the lowest occurrence of the conodont Ch. timorensis. This fact accords with the carbonate carbon isotope record reported from shallow-water marine carbonate sections.
The lithological transition from alternating claystone and chert to bedded chert facies near the OA-B has also been reported from the well-studied sections in the Inuyama area, Central Japan (compiled in Ikeda et al., 2010). Although the timing of this lithologic transition is roughly coincident for the two study areas, a conspicuous difference in lithology is observed around the O-AB. In the Ogama section, the O-AB falls within a 4 m thick, organic-rich black claystone dominant interval. In contrast, the O-AB interval in the Inuyama area is composed mainly of grey siliceous claystone. This suggests that production and/or preservation of organic matter was greater at the depositional grounds of the Ogama section. Further comparison of lithologic characters of the two study areas could promote the understanding of the pelagic oceanic environments around the O-AB.