9:45 AM - 10:00 AM
[BPT04-04] Lithostratigraphy and ostracod fossils from the Permian-Triassic boundary section of pelagic paleo-atoll carbonates
Keywords:end-Permian mass extinction, Kamura Section, hot acetolysis, microbialite, palaeobiogeography, "shallow-water refuge" hypothesis
The end-Permian mass extinction (EPME; c.a. 252 Ma ago) is the largest mass extinction event in the Earth’s history. Large numbers of marine species went extinct because of drastic environmental changes possibly caused by Siberian Traps volcanism. This study focuses on the Kamura Limestone in Takachiho Town, Miyazaki Prefecture, Japan to evaluate environmental changes in shallow pelagic settings across the EPME. The Kamura Limestone was originally deposited at the top of an atoll located in pelagic Panthalassa. Previous studies investigated the sedimentology (Sano and Nakashima, 1997) as well as bio- and chemo-stratigraphy (conodont fossil and carbonate carbon isotope; e.g., Zhang, et al., 2019). In addition, various benthic taxa including ostracods and gastropods have been reported based on observation of thin sections. These fossils would be potentially informative on the oceanic paleoenvironment and biotic recovery processes across the EPME.
Ostracoda, a group of small aquatic crustacean, is known as a good bioindicator of paleoenvironment and paleogeography because of their low mobility throughout their life history and high degree of habitat specialization depending on factors such as depth and temperature. To explain ostracod survival after the EPME, Forel et al. (2013) proposed the “microbialite refuge” hypothesis arguing that microbial reefs provided condition sufficient for survival of ostracods. Also, Qiu et al. (2019) proposed the “shallow-water refuge” hypothesis suggesting that shallow oceans acted as refuge for ostracods. This study aims to recover ostracod fossil species and show their assemblages in the Kamura Limestone.
The studied section, the Kamura Section (Kamura B section of Sano and Nakashima, 1997) spans the uppermost part of Mitai Formation to the Basal Member of the Kamura Formation ranging in age from the Changhsingian (the uppermost Permian) to the Griesbachian (the lowermost Triassic). Careful investigation of the outcrop especially around the EPME horizon was conducted, and detailed lithostratigraphy was reconstructed. The studied section consists of dolostone, dolomitic thrombolite, thrombolite and bioclastic wacke-packstone in ascending order. Lens-shaped shelly packstone and alternations of thrombolite and bioclastic wacke-packstone were present in the upper part of the thrombolite. Dolomitic parts were observed in the Mitai Formation, and the basal Kamura Formation. In these dolomitic parts, lime-mudstone and thrombolite which preserve lithofacies before dolomitization were partly recognized.
Ostracod fossils were extracted from collected rocks by hot acetolysis method (Crasquin-Soleau et al., 2005). From 11 of 15 samples, 53 species belonging to 23 genera were recognized. Some ostracods such as Silenites? zhejiangensis have been reported from the Permian–Triassic boundary sections in South China. It implies the Kamura Section may be geographically or oceanographically close to the South China Block which was located in the eastern Paleo-Tethys. On the other hand, many of the species are endemic taxa, implying that the Kamura Section was located in a distinct bioprovince, while it had interactions with the South China bioprovince by ocean currents. In general, ostracod species composition in the Kamura Section varies depending on sedimentary facies. We divided ostracod species compositions into the following four parts in ascending order: (1) from the lime-mudstone, Bairdiacypris spp. were relatively rich, but no dominant species could be recognized; (2) from the dolomitic thrombolite, Paracypris cf. gaetanii was dominant; (3) from the lower part of thrombolite, P. cf. gaetanii and Bairdia spp. were dominant; (4) from the upper part of thrombolite, Bairdia spp. were dominant. Also, Palaeocopida such as Geffenina sp.1 were dominant in the lens-shaped shelly packstone in the thrombolite. According to Crasquin-Soleau et al. (2006), the Kamura Section was deposited in the external infralittoral zone. However, the fossils in the lens-shaped shelly packstone of this study probably came from the internal infralittoral zone. Abundant ostracods from both thrombolite and shelly packstone indicate that the “shallow-water refuge” hypothesis is applicable to explain ostracod survival in the top of paleo-atoll, which is consistent with South China. In contrast, ostracod genera composition changes in thrombolite from the Kamura Section differs from those of South China (Ji et al., 2023), possibly indicating local environmental differences.
Ostracoda, a group of small aquatic crustacean, is known as a good bioindicator of paleoenvironment and paleogeography because of their low mobility throughout their life history and high degree of habitat specialization depending on factors such as depth and temperature. To explain ostracod survival after the EPME, Forel et al. (2013) proposed the “microbialite refuge” hypothesis arguing that microbial reefs provided condition sufficient for survival of ostracods. Also, Qiu et al. (2019) proposed the “shallow-water refuge” hypothesis suggesting that shallow oceans acted as refuge for ostracods. This study aims to recover ostracod fossil species and show their assemblages in the Kamura Limestone.
The studied section, the Kamura Section (Kamura B section of Sano and Nakashima, 1997) spans the uppermost part of Mitai Formation to the Basal Member of the Kamura Formation ranging in age from the Changhsingian (the uppermost Permian) to the Griesbachian (the lowermost Triassic). Careful investigation of the outcrop especially around the EPME horizon was conducted, and detailed lithostratigraphy was reconstructed. The studied section consists of dolostone, dolomitic thrombolite, thrombolite and bioclastic wacke-packstone in ascending order. Lens-shaped shelly packstone and alternations of thrombolite and bioclastic wacke-packstone were present in the upper part of the thrombolite. Dolomitic parts were observed in the Mitai Formation, and the basal Kamura Formation. In these dolomitic parts, lime-mudstone and thrombolite which preserve lithofacies before dolomitization were partly recognized.
Ostracod fossils were extracted from collected rocks by hot acetolysis method (Crasquin-Soleau et al., 2005). From 11 of 15 samples, 53 species belonging to 23 genera were recognized. Some ostracods such as Silenites? zhejiangensis have been reported from the Permian–Triassic boundary sections in South China. It implies the Kamura Section may be geographically or oceanographically close to the South China Block which was located in the eastern Paleo-Tethys. On the other hand, many of the species are endemic taxa, implying that the Kamura Section was located in a distinct bioprovince, while it had interactions with the South China bioprovince by ocean currents. In general, ostracod species composition in the Kamura Section varies depending on sedimentary facies. We divided ostracod species compositions into the following four parts in ascending order: (1) from the lime-mudstone, Bairdiacypris spp. were relatively rich, but no dominant species could be recognized; (2) from the dolomitic thrombolite, Paracypris cf. gaetanii was dominant; (3) from the lower part of thrombolite, P. cf. gaetanii and Bairdia spp. were dominant; (4) from the upper part of thrombolite, Bairdia spp. were dominant. Also, Palaeocopida such as Geffenina sp.1 were dominant in the lens-shaped shelly packstone in the thrombolite. According to Crasquin-Soleau et al. (2006), the Kamura Section was deposited in the external infralittoral zone. However, the fossils in the lens-shaped shelly packstone of this study probably came from the internal infralittoral zone. Abundant ostracods from both thrombolite and shelly packstone indicate that the “shallow-water refuge” hypothesis is applicable to explain ostracod survival in the top of paleo-atoll, which is consistent with South China. In contrast, ostracod genera composition changes in thrombolite from the Kamura Section differs from those of South China (Ji et al., 2023), possibly indicating local environmental differences.