5:15 PM - 7:15 PM
[BPT04-P02] Stratigraphic reconstruction of Lower Triassic deep-sea sedimentary rock formations in the Mino-Tanba-Ashio Belt
1. Introduction
The Permian-Triassic mass extinction (ca. 252 Ma) is the largest extinction event in the Phanerozoic [1]. After this extinction, the recovery of marine ecosystem took 5 million years of the Early Triassic [1]. The recovery of benthos in equatorial pelagic deep Panthalassa was reported based on an ichnofabrics in a few sections of Spathian age (late Olenekian of the Early Triassic) [2][3]. However, this recovery has been only confirmed in a limited area. It is uncertain if the recovery is a widespread phenomenon in equatorial pelagic deep sea.
To gain more information of the deep-sea section and reveal the environmental records of the Early Triassic, we have conducted the geological survey of the Hoshakuji section in Gifu Prefecture, Japan.
2. Geological Setting and Methods
The Hoshakuji section [4] (hereafter HS section) is located in the Mino-Tanba-Ashio Belt of the Jurassic accretionary complex in the central Japan. We determined the age of the HS section by using the newly collected conodont fossils. To extract fossils, we used “chip method” [5][6] and the NaOH treatment method [7].
3. Results and Discussion
We reconstructed a stratigraphic section from the two outcrops, which were divided into three subsections HS-1, HS-2 and HS-3 from south to north. Subsections HS-1, HS-2 and HS-3 consist mostly of siliceous claystone layers and thin black claystone layers. HS-1 includes a dolostone layer in the lower part. HS-2 and HS-3 have thin intercalations of chert layers. Furthermore, alternations of siliceous claystone and black claystone layers were seen in the lower part of HS-3. This layer shows weak bioturbation corresponding to ichnofabric index 2 of Droser and Bottjer (1986) [8]. Conodont fossils occurred from throughout these subsections. HS-1 yielded Smithian (early Olenekian of the Early Triassic) conodonts including Conservatella conservativa. On the other hand, HS-3 yielded Spathian (upper Olenekian of the Early Triassic) conodonts including form species "Triassospathodus" symmetricus.
Observation of thin section revealed the presence of pyrite in the black claystone layers and siliceous claystone layers of all subsections. Because sulphate reduction forming pyrite requires anoxia and sufficient organic matter in seawater, the presence of pyrite suggest that the depositional environment of the HS section was oxygen-poor at least dysoxic in the sediment interface and organic matter sinking flux was high. Further, the appearance of bioturbation in HS-3 suggests benthic organism activities at the seafloor. It suggests that recovery of benthos activity had started at least by the Spathian. Similar trend of Spathian bioturbation recovery has been reported from other sections of the pelagic deep-sea Panthalassa and other oceanic regions such as the Paleo-Tethys [9][10]. It implies the recovery of benthic organisms was a widespread and common phenomenon of Spathian after the mass extinction event.
4. References
[1] Chen and Benton, 2012. Nature Geoscience, 5, 375-383.
[2] Kakuwa, Y., 2008. Global Planeta. Change, 63, 40–56.
[3] Muto, S., 2021. Global Planeta. Change, 197, 103402.
[4] Yamakita et al., 2010. Abstract, The 2010 Annual Meeting of the Palaeontological Society of Japan, C23, p.47 (in Japanese)
[5] Takemura et al., 2001. Topics on Palaeontology, 2, 17–24. (in Japanese)
[6] Muto et al., 2018. Palaeogeogr., Palaeoclimatol., Palaeoecol., 490, 687–707.
[7] Rigo et al., 2023. Paleontol., 66, e12672.
[8] Droser and Bottjer, 1986. Journal of Sedimentary Research, 56, 558-559.
[9] Foster et al., 2018. Palaios, 33, 266-281.
[10] Sun et al., 2015. Palaeogeogr., Palaeoclimatol., Palaeoecol., 427, 62-78.
The Permian-Triassic mass extinction (ca. 252 Ma) is the largest extinction event in the Phanerozoic [1]. After this extinction, the recovery of marine ecosystem took 5 million years of the Early Triassic [1]. The recovery of benthos in equatorial pelagic deep Panthalassa was reported based on an ichnofabrics in a few sections of Spathian age (late Olenekian of the Early Triassic) [2][3]. However, this recovery has been only confirmed in a limited area. It is uncertain if the recovery is a widespread phenomenon in equatorial pelagic deep sea.
To gain more information of the deep-sea section and reveal the environmental records of the Early Triassic, we have conducted the geological survey of the Hoshakuji section in Gifu Prefecture, Japan.
2. Geological Setting and Methods
The Hoshakuji section [4] (hereafter HS section) is located in the Mino-Tanba-Ashio Belt of the Jurassic accretionary complex in the central Japan. We determined the age of the HS section by using the newly collected conodont fossils. To extract fossils, we used “chip method” [5][6] and the NaOH treatment method [7].
3. Results and Discussion
We reconstructed a stratigraphic section from the two outcrops, which were divided into three subsections HS-1, HS-2 and HS-3 from south to north. Subsections HS-1, HS-2 and HS-3 consist mostly of siliceous claystone layers and thin black claystone layers. HS-1 includes a dolostone layer in the lower part. HS-2 and HS-3 have thin intercalations of chert layers. Furthermore, alternations of siliceous claystone and black claystone layers were seen in the lower part of HS-3. This layer shows weak bioturbation corresponding to ichnofabric index 2 of Droser and Bottjer (1986) [8]. Conodont fossils occurred from throughout these subsections. HS-1 yielded Smithian (early Olenekian of the Early Triassic) conodonts including Conservatella conservativa. On the other hand, HS-3 yielded Spathian (upper Olenekian of the Early Triassic) conodonts including form species "Triassospathodus" symmetricus.
Observation of thin section revealed the presence of pyrite in the black claystone layers and siliceous claystone layers of all subsections. Because sulphate reduction forming pyrite requires anoxia and sufficient organic matter in seawater, the presence of pyrite suggest that the depositional environment of the HS section was oxygen-poor at least dysoxic in the sediment interface and organic matter sinking flux was high. Further, the appearance of bioturbation in HS-3 suggests benthic organism activities at the seafloor. It suggests that recovery of benthos activity had started at least by the Spathian. Similar trend of Spathian bioturbation recovery has been reported from other sections of the pelagic deep-sea Panthalassa and other oceanic regions such as the Paleo-Tethys [9][10]. It implies the recovery of benthic organisms was a widespread and common phenomenon of Spathian after the mass extinction event.
4. References
[1] Chen and Benton, 2012. Nature Geoscience, 5, 375-383.
[2] Kakuwa, Y., 2008. Global Planeta. Change, 63, 40–56.
[3] Muto, S., 2021. Global Planeta. Change, 197, 103402.
[4] Yamakita et al., 2010. Abstract, The 2010 Annual Meeting of the Palaeontological Society of Japan, C23, p.47 (in Japanese)
[5] Takemura et al., 2001. Topics on Palaeontology, 2, 17–24. (in Japanese)
[6] Muto et al., 2018. Palaeogeogr., Palaeoclimatol., Palaeoecol., 490, 687–707.
[7] Rigo et al., 2023. Paleontol., 66, e12672.
[8] Droser and Bottjer, 1986. Journal of Sedimentary Research, 56, 558-559.
[9] Foster et al., 2018. Palaios, 33, 266-281.
[10] Sun et al., 2015. Palaeogeogr., Palaeoclimatol., Palaeoecol., 427, 62-78.