17:15 〜 19:15
[BCG07-P02] 北部北上帯門馬ユニットのジュラ紀付加体における海洋プレート層序基底部の年代決定
キーワード：石炭紀、コノドント、玄武岩、チャート、水酸化ナトリウム法
The Jurassic accretionary complex of Japan contains oceanic rocks derived from the pelagic realm of Panthalassa. These rocks provide palaeoenvironmental records of a vast oceanic realm as well as constraints on plate motion and palaeogeography. Reconstruction of the lower part of the oceanic plate stratigraphy in Jurassic accretionary complexes of Japan is based on microfossil occurrences from deep-sea chert and palaeo-atoll limestone and other associated sedimentary rocks. The oldest microfossils from chert are Pennsylvanian (late Carboniferous) conodonts and radiolarians1,2,3. These upper Carboniferous cherts are sometimes associated with basaltic rocks, based on which the basal part of the oceanic plate stratigraphy was speculated to comprise basaltic rocks and overlying bedded chert, both of Pennsylvanian age. Although this reconstruction is widely accepted4, the lithostratigraphic relationship between basaltic rocks and Carboniferous cherts had not been detailed. Recently, Muto et al. (2024) described a basalt–chert sequence including a transitional lithofacies such as tuffaceous chert in the Kadoma Unit of the North Kitakami Belt. The age of the basalt was determined as Bashkirian or early Moscovian, while that of bedded chert was shown to encompass the Sakmarian to late Kungurian or early Roadian. Thus, the basal part of the oceanic plate stratigraphy in Jurassic accretionary complexes of Japan was detailed for the first time. The present study provides additional biostratigraphic data to further enhance the age assignment of the basalt–chert sequence in the Hayasaka Highlands.
The studied section is exposed along the Nekosokomatasawa River in the eastern part of Morioka City, Iwate Prefecture, Tohoku Region, Japan. The lithostratigraphy of the section comprises four units: the basalt, lower red bedded chert, upper red bedded chert and grey bedded chert units, in ascending order 5. We investigated the lower red bedded chert unit, which is ~15 m thick and is in contact with the basalt unit with a minor fault. Muto et al.5 found no age diagnostic fossils from this interval. In the present study, we utilized a new method of microfossil extraction using heated NaOH solutions6. We recovered conodont fossils from all the treated samples. On the other hand, while the NaOH solution method is known to be effective in extracting radiolarians from chert, none of our samples yielded radiolarians. The conodont fossils obtained in the present study include Neognathodus symmetricus (Lane), Neognathodus sp. cf. N. symmetricus (Lane), Neognathodus sp., Gondolella sp. and idiognathodontidae with a median groove and accessory nodose lobe. These conodonts indicate the late Bashkirian (early Pennsylvanian), according to biostratigraphic studies in Ukraine7, USA8 and South China9. Based on new and previous data, the basalt unit of the studied section is the Bashkirian and the lower red bedded chert unit is mostly upper Bashkirian. The studied basalt-chert sequence indicates for the first time that the Jurassic accretionary complex of Japan includes a portion of the oceanic plate that was formed in the Bashkirian.
1. Ishiga, H. Earth Sci. J. Assoc. Geol. Collab. Japan 36, 333–339 (1982).
2. Igo, H. & Kobayashi, F. Trans. Proc. Palaeontol. Soc. Japan. New Ser. 96, 411–426 (1974).
3. Ehiro, M., Yamakita, S., Takahashi, S. & Suzuki, N. J. Geol. Soc. Japan 114, 121–139 (2008). (in Japanese)
4. Kojima, S. et al. in The Geology of Japan (eds. Moreno, T., Wallis, S., Kojima, T. & Gibbons, W.) 61–100 (Geological Society of London, 2016).
5. Muto, S., Ito, T. & Ozeki, M. 49, 107–123 (2024).
6. Rigo, M. et al. Palaeontology 66, e12672 (2023).
7. Nemyrovska, T. I. Scr. Geol. 25, 1–115 (1999).
8. Barrick, J. E., Lambert, L. L., Heckel, P. H., Rosscoe, S. J. & Boardman, D. R. Stratigraphy 10, 55–72 (2013).
9. Wang, Z. & Qi, Y. Riv. Ital. di Paleontol. e Stratigr. 109, 379–397 (2003).
The studied section is exposed along the Nekosokomatasawa River in the eastern part of Morioka City, Iwate Prefecture, Tohoku Region, Japan. The lithostratigraphy of the section comprises four units: the basalt, lower red bedded chert, upper red bedded chert and grey bedded chert units, in ascending order 5. We investigated the lower red bedded chert unit, which is ~15 m thick and is in contact with the basalt unit with a minor fault. Muto et al.5 found no age diagnostic fossils from this interval. In the present study, we utilized a new method of microfossil extraction using heated NaOH solutions6. We recovered conodont fossils from all the treated samples. On the other hand, while the NaOH solution method is known to be effective in extracting radiolarians from chert, none of our samples yielded radiolarians. The conodont fossils obtained in the present study include Neognathodus symmetricus (Lane), Neognathodus sp. cf. N. symmetricus (Lane), Neognathodus sp., Gondolella sp. and idiognathodontidae with a median groove and accessory nodose lobe. These conodonts indicate the late Bashkirian (early Pennsylvanian), according to biostratigraphic studies in Ukraine7, USA8 and South China9. Based on new and previous data, the basalt unit of the studied section is the Bashkirian and the lower red bedded chert unit is mostly upper Bashkirian. The studied basalt-chert sequence indicates for the first time that the Jurassic accretionary complex of Japan includes a portion of the oceanic plate that was formed in the Bashkirian.
1. Ishiga, H. Earth Sci. J. Assoc. Geol. Collab. Japan 36, 333–339 (1982).
2. Igo, H. & Kobayashi, F. Trans. Proc. Palaeontol. Soc. Japan. New Ser. 96, 411–426 (1974).
3. Ehiro, M., Yamakita, S., Takahashi, S. & Suzuki, N. J. Geol. Soc. Japan 114, 121–139 (2008). (in Japanese)
4. Kojima, S. et al. in The Geology of Japan (eds. Moreno, T., Wallis, S., Kojima, T. & Gibbons, W.) 61–100 (Geological Society of London, 2016).
5. Muto, S., Ito, T. & Ozeki, M. 49, 107–123 (2024).
6. Rigo, M. et al. Palaeontology 66, e12672 (2023).
7. Nemyrovska, T. I. Scr. Geol. 25, 1–115 (1999).
8. Barrick, J. E., Lambert, L. L., Heckel, P. H., Rosscoe, S. J. & Boardman, D. R. Stratigraphy 10, 55–72 (2013).
9. Wang, Z. & Qi, Y. Riv. Ital. di Paleontol. e Stratigr. 109, 379–397 (2003).