168 / 2025-04-13 09:45:42

Lithostratigraphy and ostracod fossil occurrences of the Permian–Triassic boundary carbonates deposited in pelagic paleo-atoll

Kamura Limestone,Hot acetolysis,refuge,end-Permian mass extinction,Microbialites

Theme 3: Earth-Life Co-evolution Through Time > Session 3f: Rise of Modern-type Ecosystems in Triassic

The end-Permian mass extinction (EPME) is the largest mass extinction event in the Earth’s history. Previous studies about EPME have focused on mainly Tethys Ocean regions. Meanwhile, research on other oceanic region would also be worth to provide deeper insights into the state of the Earth at the EPME. This study aims to reveal paleoenvironment and biotic recovery process across the EPME at the pelagic Panthalassa using the paleo-atoll carbonate rocks and recovered ostracod fossils.

The Kamura Limestone in Takachiho Town, Miyazaki Prefecture, Japan, which was originally deposited at the top of an atoll located in pelagic Panthalassa across the EPME was chosen for study area. 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). The studied section consists of dolostone, dolomitic thrombolite, thrombolite and bioclastic packstone in ascending order. Thrombolite forms alterations with lens-shaped shelly packstone and bioclastic wacke–packstone in the upper part, possibly implying gradual demise of microbial reef. In dolomite and dolomitic thrombolite, lime-mudstone and thrombolite which preserve original lithofacies were partly recognized.

53 species belonging to 23 genera ostracod fossils were recognized from the Kamura Section by hot acetolysis. Some recovered ostracod species have been reported from the Permian–Triassic boundary sections in South China, but many of them are endemic taxa. This implies that the Kamura Section was located in a distinct bioprovince, while it had interactions with the South China bioprovince. Ostracods compositions can be divided into the following four parts in ascending order: (1) from the lime-mudstone, Bairdiacypris spp. were relatively rich; (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. From the Triassic lens-shaped shelly packstone, Geffenina sp. 1 was dominant, but this genus was considered to have disappeared before the latest Permian in Tethys. This means that the pelagic ocean acted as ‘refuge’ for ostracods in the EPME.

Sano, H. and Nakashima, K., 1997. Lowermost Triassic (Griesbachian) microbial bindstone-cementstone facies, southwest Japan. Facies, 36, 1, 1-24.