Anthracoporella-Palaeoaplysina mound in Upper Carboniferous mid-Panthalassan atoll-type carbonates in a Jurassic accretionary complex, central Japan

摘要

The Carboniferous-Permian Kano-yama limestone in the Jurassic Chichibu Accretionary Complex, central Japan, contains an algal-microbial mound composed of Anthracoporella, Palaeoaplysina, Tubiphytes, Archaeolithoporella, and bryozoans. The limestone is characterized by abundant micrite, implying a low-energy back-reef depositional environment. The fusuline fauna includes Daixina sokensis, Carbonoschwagerina minatoi and C. morikawai, which are representative of the Gzhelian Stage of the Carboniferous. Anthracoporella and Palaeoaplysina are interpreted to be the main bafflers in the mound. The lower horizon of the mound is characterized by a binding structure of Tubiphytes, Archaeolithoporella and bryozoans, which enabled the formation of this Anthracoporella and Palaeoaplysina mound. Compared with the Akiyoshi-Taishaku limestone, a well-documented Panthalassan atoll-type carbonate, the occurrence of Palaeoaplysina is a common feature of these limestones. Based on their accretion ages, the Kano-yama limestone is estimated to have been located several thousand kilometers west of the Akiyoshi-Taishaku limestone in the Panthalassa Ocean during the latest Carboniferous. The occurrence of Palaeoaplysina in both the Akiyoshi-Taishaku and Kano-yama limestones indicates that the distribution of Palaeoaplysina, which is known to be a cool-water genus, extended over thousands of kilometers in the Panthalassa during the period of the Gondwana glaciation.