Maokouan-Wuchiapingian) Boundary in Mid-Oceanic Paleo-Atoll Limestone of Kamura and Akasaka, Japan

104 Proc. Japan Acad., 77, Ser. B (2001) [Vol. 77(B),

Middle-Upper Permian (Maokouan-Wuchiapingian) boundary in mid-oceanic paleo-atoll limestone of Kamura and Akasaka, Japan

By Yukio ISOZAKIt) and Ayano OTA

Department of Earth Science & Astronomy, The University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo 153-8902 (Communicated by Tatsuro MATSUMOTO,M.J.A., June 12, 2001)

Abstract: Nearly 10 million years before the Permo-Triassic boundary (PTB; ca. 251 Ma) characterized by the greatest mass extinction in the Phanerozoic, the Middle-Upper Permian boundary marked another big biotic decline almost comparable in magnitude to the PTB event. Two stratigraphic sections spanning across the Maokouan (Middle Permian)-Wuchiapingian (Upper Permian) boundary (MWB) were newly found in paleo-atoll limestone within the Jurassic accretionary complex in Kamura and Akasaka, Japan. These two sections share almost identical litho- and biostratigraphy that records a remarkable biotic extinction of large-shelled fusultnids and a sharp lithologic change exactly across the MWB. These new data, as the first evidence from the shallow-water mid-oceanic realm, suggest that a quick environmental change occurred in a global scale across the MWB. A thin, acidic tuff recognized at the MWB horizon in the paleo- atoll limestone has a potential utility as a key bed for global correlation and suggests a possible link between the end-Permian biosphere crisis and the explosive acidic volcanism.

Key words: Mass extinction; Permo-Triassic boundary; limestone; superocean; seamount; paleoenvi- ronment.

Introduction. The Permian period (Paleozoic) paleoenvironmental information on the supercontinent was punctuated by the greatest mass extinction in the Pangea including its big embayment named Tethys but Phanerozoic by which numerous marine and non- with almost nothing on the superocean Panthalassa (or marine biota were terminated abruptly.' Previous bio- paleo-Pacific) that occupied nearly 70% of the Earth's stratigraphical researches focused mainly on the surface then. Changhsingian (Permian)-Griesbachian (Triassic) bound- The Paleozoic-Mesozoic accretionary complexes in ary (PTB; ca. 251 Ma) event per se. Nonetheless, one of Japan contain numerous fragments of ancient oceanic the biggest biotic declines in the Phanerozoic occurred crust, such as deep-sea chert, basaltic greenstones of also in the later half of the Permian before the PTB, i.e., ancient hotspot seamount and paleo-atoll limestone at the Maokouan (Middle Permian)-Wuchiapingian (Fig. l).7~ These represent sole source of information on (Upper Permian) boundary (MWB) that predates the the lost superocean because ancient oceanic plates PTB by nearly 10 million years.2) 3) The greatest magni- loaded these rocks have already subducted beneath tude of the end-Permian mass extinction may have Asia and completely disappeared from the Earth's sur- resulted from the two distinct extinction events face. The finding of deep-sea chert spanning across the occurred within a short time interval less than 10 million PTB from Japan provided a new insight into the sudden years.3~ extinction of Paleozoic radiolarians (zooplankton) in The PTB crisis has been analyzed in several well- the mid-oceanic realm and extraordinarily long-term preserved and fossiliferous shelf carbonate sections (ca. 20 million years) oxygen depletion (anoxia) in mostly in South China and other Paleo-Tethyan mar- deep superocean.8 As the onset timing of the deep-sea gins.4~° These rocks provide us with valuable pieces of anoxia was dated to be around the MWB, these two distinct geologic phenomena both in a global scale may have t) Correspondence to: Y. Isozaki. been related each other, probably in a cause-effect rela- No. 6] Middle-Upper Permian boundary in paleo-atoll limestone 105

Fig. 1. Simplified plate tectonic framework of a ridge-trench system (modified from references7~'9~). Mid-oceanic shallow-water limestone commonly develops on top of seamount of hotspot origin, forming atoll-like framework. By successive subduction of oceanic plate, fragments of mid-oceanic atoll limestone are secondarily incorporated into an accretionary complex along an active continental margin. The Jurassic accretionary complex in Japan contains numerous blocks of Permian limestone derived from mid-oceanic paleo-atoll and some preserve important stratigraphic boundaries, such as the Permo-Triassic boundary (PTB) and Middle-Upper Permian boundary (MWB).

tionship. Thus detailed analysis is needed also on the the same accretionary complex. Their current occur- MWB event in order to clarify the ultimate cause of the rence in Japan is a consequence of long-lasting westward end-Permian crisis. movement of the seamount-loading oceanic plate fol- The present authors lately found two new strati- lowed by oceanic subduction-accretion along the Asian graphic sections of mid-oceanic paleo-atoll limestone that continental margin in the Jurassic (Fig. 1) .7) span across the MWB in Southwest Japan. These strati- Among many accreted Permian limestone graphic records are critical in deciphering the initial blocks/lenses, some preserve geologically important stage of the end-Permian paleoenvironmental change, in horizons, such as the PTB and MWB. The Permian particular as the first evidence on mid-oceanic surface limestone in Kamura in Miyazaki prefecture and that in environment of the superocean at that timing. In this Akasaka in Gifu prefecture both have a well-preserved article, we describe the characteristics of the MWB section covering the MWB transition interval for a transition beds in these sections and briefly discuss detailed stratigraphical research (Fig. 2). The lime- their geologic implications. As this short report focuses stone in Kamura consists of the Middle to Upper solely on the MWB transition beds, detailed description Permian and Early to Late Triassic," 13) retaining the of the two study sections will be published elsewhere. clearly exposed PTB horizon between the Changh- Paleo-atoll limestone in Kamura and singian and Griesbachian.14) The MWB was previously Akasaka. Surrounded by the strongly deformed unknown in this limestone but we lately found a new sec- Jurassic mudstone and sandstone, the Permian limestone tion that exposes continuous sequence spanning from occurs as kilometric-sized large allochthonous the Maokouan to Changhsingian via Wuchiapingian.l5) block/lens in the Jurassic accretionary complex in This section at Shioinouso, in the north of Kamura, is Southwest Japan. The origin of such limestone is called the Kamura section in this article. presumed in an atoll-like environment on Permian The Permian limestone in Akasaka is well known for seamounts primarily located somewhere in a mid- the abundant occurrence of various Maokouan fossils oceanic realm on the basis of the following geological such as fusulinid, rugose coral, bivalve, and gastro- constraints.9)'10) They are 1) shallow-water bioclastic pod.16) The limestone consists mainly of Maokouan, and nature of carbonates, 2) complete absence of coarse- the overlying Wuchiapingian occurs sporadically in grained terrigenous clastics, and 3) intimate association small exposures.l7)'18) Previously reported MWB sec- with oceanic island (seamount) type basaltic (OIB) tions in Akasaka were already destroyed by quarry greenstones in many other Permian limestone bodies in development, however, we newly found another section 106 Y. ISOZAKI and A. OTA [Vol. 77(B),

Fig. 2. Stratigraphic columns of the Middle-Upper Permian boundary (or Maokouan- Wuchiapingian boundary; MWB) sections in the mid-oceanic paleo-atoll limestone in Kamura and Akasaka, Japan. Letters L, Y, C and R indicate occurrence of fususlinids, stand- ing for Lepidolina. Yabeina, Codonofusiella, and Reichelina, respectively. Note the remarkable change in lithology across the MWB and intercalation of a thin acidic tuff bed at the boundary horizon in both sections. No. 6] Middle-Upper Permian boundary in paleo-atoll limestone 107

preserving the Maokouan to Wuchiapingian transition across the MWB. In addition, the intimate association of interval. This section newly found at Ichihashi in the volcanic tuff with the MWB beds indicates a possible link north of Akasaka is called the Akasaka section in this between volcanism and global environmental change. article. Correlation of the MWB tuff. The newly found Maokouan-Wuchiapingian boundary (MWB) MWB tuff should be noted as a useful key bed for corre- beds. The Kamura and Akasaka sections share an lating the MWB horizon because it occurs just below the almost identical litho- and biostratigraphy across the FAD (first appearance datum) horizon of the MWB (Fig. 2). The Kamura section consists of the Wuchiapingian fusulinids. The MWB tuff in shallow- Maokouan dark gray limestone (the Iwato Formation) water paleo-atoll limestone is correlated to an acidic tuff and the overlying Wuchiapingian light gray limestone horizon in accreted Permian deep-sea chert/mudstone. (the Mitai Formation).15) Likewise, the Akasaka section The Permian deep-sea chert-mudstone sequence in the comprises the Maokouan dark gray limestone (the Akiyoshi belt and its outlier (Kurosegawa belt) in Japan Upper Formation) and the overlying Wuchiapingian has also an acidic tuff horizon around the Middle-Late light gray limestone (the Uppermost Formation). 17)'18) Permian transition level dated by radiolarians There is a sharp lithologic contrast between the (Follicucullus monacanthus Zone) 20)22) These lines of Maokouan and Wuchiapingian rocks in both sections; i.e., evidence support that the fall of the MWB air-borne tuff the former is characterized by the dark gray to black have occurred extensively in mid-oceanic realm of the limestone enriched in organic carbon (the Kinshozan superocean Panthalassa. facies19~),while the latter by the light gray dolomitic lime- Moreover, this MWB tuff bed has a potential utility stone. in global correlation. In many Middle-Late Permian sec- The upper part of the Maokouan comprises the tions in South China, a distinct "shale" bed is identified Yabeina-Lepidolina Zone that is characterized by abun- just at the MWB horizon. This 2 meter thick "shale" bed dant large-shelled (up to 10 mm in diameter) fusulinids called the Wampo Shale was first described as the basal including Lepidolina, Yabeina, and Colania, together unit of the Wuchiaping limestone (or Wuchiaping with large bivalves (probably Shikamaia). The topmost Formation) at its stratotype in the Liangshan mountain part of the dark gray limestone (approx. 10 meter of Shaanxi proiince, China.28~ This bed occurs exactly thick) is nearly identical in lithology with the underlying between the uppermost Maokou limestone and the Yabeina-Lepidolina Zone but this interval yields no basal Wuchiaping limestone (Fig. 3). Our preliminary index fusulinid. The Wuchiapingian is represented by the analysis recently identified abundant euhedral vol- Codonofusiella-Reichelina Zone in which only small- canogenic minerals such as quartz, zircon, and apatite in shelled (less than 2 mm in diameter) fusulinids, such as the Wampo Shale in several sections in Sichuan Codonofusiella and Reichelina, occur. province.24~ Abundant clay minerals (mostly illite) A thin bed of acidic tuff is newly recognized probably originated from volcanic glasses. These indicate between the Maokouan limestone (the barren interval that the Wampo Shale represents an ancient volcanic above the Yabeina-Lepidolina Zone) and the overlying tuff, in particular that of rhyolitic to dacitic composition. Wuchiapingian limestone (Codonofusiella-Reichelina The Wampo bed is the thickest among all acidic tuffs in Zone) in both sections. The tuff bed is 5 mm thick in the the Upper Permian in South China, as others are no more Akasaka section, and less than 2 mm thick in the than 15 cm thick. Thus the MWB tuff bed in the Kamura. It is strongly weathered into light greenish gray Kamura and Akasaka sections is correlated to the clayey bed. Despite its thinness, this tuff is quite distinct Wampo (tuff) bed in South China. because there is none such tuff bed throughout the MWB volcanism. The occurrence of MWB tuff in entire Permian paleo-atoll limestone in Kamura and the Kamura and Akasaka sections indicates a regional Akasaka. delivery of air-borne volcanic ash in the mid-oceanic It is noteworthy that a remarkable change exists realm of Panthalassa. The average convergence rate of across the MWB in lithology and in faunal content of the plates along the East Asian subduction zone is regarded paleo-atoll limestone, and that two study sections share to be around 3 cm/year. The time interval between the the same stratigraphic record disregarding their current primary deposition of the MWB beds (ca. 260 Ma) and isolation for more than 500 km. This coeval change in the secondary accretion to Asia (ca. 160 Ma) is estimat- two separated sections suggests that the surface envi- ed to be ca. 100 million years. Thus the primary position ronment changed drastically in the mid-superocean of the Middle-Upper Permian limestone in Kamura and 108 Y. ISOZAKI and A. OTA [Vol. 77(B),

Fig. 3. Correlation chart of the Kamura and Akasaka sections with the stratotype section of the Wuchiaping (limestone) Formation and Wampo bed in the Liangshan mountain in Shaanxi province, China. Note the occurrence of the MWB acidic tuff both in the mid-oceanic paleo-atoll limestone in Japan and the continental shelf limestone in South China. These 3 sections are biostratigraphically correlated in high precision.

Akasaka was somewhere in the middle of the super- paleo-atoll limestone and in continental shelf ocean, nearly 3000 km off Jurassic East Asia. This sug- sequences. This implies that the MWB global environ- gests the volcanism across MWB may have been excep- mental change including the mass extinction may have tionally violent, in particular, explosive enough to dis- been led by a large scale acidic thus explosive volcanism. tribute volcanic ash to such a remote mid-oceanic Acknowledgements. The present authors would realm. like to thank Dr. Tatsuro Matsumoto, M.J. A., (Prof. Judging from the thickness of the MWB tuff in Emeritus of Kyushu University) for critical reading of our South China, the source volcanism may have occurred in manuscript. Dr. Kametoshi Kanmera (Prof. Emeritus of western South China or more westerly region. The sub- Kyushu University), Prof. Toshio Koike (Yokohama duction-related arc volcanism along the eastern margin National University) and Dr. Tetsuo Matsuda (Kyoei of Pangea appears unlikely to be responsible for laying Consultant Co.) gave us valuable suggestions on the thick ash bed throughout South China. Concerning the Upper Permian stratigraphy in Japan. This article is ded- large igneous province (LIP) in Asia around the MWB icated to late Dr. Kozo Watanabe for his remarkable con- timing, possible candidates include the Emeishan tribution to the biostratigraphical researches on the basalt in western South China and Panjal Traps in Kamura limestone. northern India.25~The geochemical nature of such flood basalt derived from mantle plume, however, is quite distinct from that of the MWB tuff. As none of flood basalt References can be the direct source of the MWB acidic tuff, further 1) Erwin, D. H. (1994) Nature 367, 231-236. research is needed to locate a possible site of the MWB 2) An, Y .G., Zhang, J., and Shang, Q. H. (1994) Mem. Can. Assoc. acidic volcanism. Petr. Geol. 17, 813-822. As the timing of this volcanism coincides not only 3) Stanley, S., and Yang, X. (1994) Science 266,1340-1344. with the mass extinction in shallow-water continental 4) Iranian-Japanese Research Group (1981) Mem. Fac. Sci., margin2~'3~but also with the onset of deep-sea anoxia,8~ Kyoto Univ., Ser. Geol. Miner. 47, 61-133. the MWB environmental change may have occurred in a 5) Sheng, J. Z., Chen, C. Z., Wang, Y. G., Rui, L., Liao, Z. T., global scale. In particular, the MWB acidic volcanism may Bando, Y., Ishii, K., Nakazawa, K., and Nakamura, K. have left a profound impact on the biosphere then. A (1984) Jour. Fac. Sci., Hokkaido Univ., Ser. 4 21,133-181. remarkable lithologic and faunal change occurred just 6) Sweet, W. C., Yang, Z. Y., Dickins, J. M., and Yin, H. F. (eds.) across the MWB tuff horizon, both in mid-oceanic (1992) Permo-Triassic Events in the Eastern Tethys. Cambridge Univ. Press, Cambridge, pp. 1-181. No. 6] Middle-Upper Permian boundary in paleo-atoll limestone 109

7) Isozaki, Y., Maruyama, S., and Furuoka, F. (1990) Tectono- 17) Sakagami, S. (1981) Proc. Japan Acad. 56B, 25-29. physics 181,179-205. 18) Ozawa, T., and Nishiwaki, N. (1992) 31st IGC Field Trip 8) Isozaki, Y. (1997) Science 276, 235-238. Guidebook, vol. 1, pp. 189-195. 9) Isozaki, Y. (1997) Res. Rept. Kanagawa Pref. Mus. Nat. Hist. 8, 19) Toriyama, R. (1967) Geohistory I, Asakura Publ., Tokyo, pp. 117-130 (in Japanese with English abstract). 254-305 (in Japanese). 10) Sano, H., and Nakashima, K. (1997) Facies 36,1-24. 20) Isozaki, Y. (1987) Jour. Geosci. Osaka City Univ. 30, 51-131. 11) Kambe, N. (1963) Rept. Geol. Surv. Japan, no. 198, pp. 1-66. 21) Tokuoka, T., Naka, K., Nishimura, K., Pillai, D. D. L., and 12) Kanmera, K., and Nakazawa, K. (1973) Mem. Can. Assoc. Petr. Ishiga, H. (1988) Geol. Rep. Shimane Univ. 7, 49-68 (in Geol. 2,100-119. Japanese with English abstract). 13) Watanabe, K., Kanmera, K., and Nakashima, K. (1979) In 22) Nishimura, Y., Itaya, T., Isozaki, Y., and Kameya, A. (1989) Biostratigraphy of Permian and Triassic Conodonts and Mem. Geol. Soc. Japan, no. 33, pp. 143-166 (in Japanese Holothurian Sclerites in Japan (eds. Koike, T., and Igo, H.). with English abstract). Commit. Prof. M. Kanuma's Retirement Ceremony, Tokyo, 23) Lu, Y. H. (1956) Scientia Sinica 5, 733-761. pp. 127-137 (in Japanese). 24) Isozaki, Y., Kubo, T., and Yao, J. X. (unpublished data). 14) Koike, T. (1996) Trans. Proc. Palaeont. Soc. Japan, NS, no. 25) Chung, S. L., Jahn, B. M., Wu, G. Y., Lo, C. H., and Bolin, C. 181, 337-346. (1998) Amer. Geophys. Union, Geodynamic Ser., no. 27, 15) Ota, A., Kanmera, K., and Isozaki, Y. (2000) Jour. Geol. Soc. pp. 47-58. Japan 107, 853-864 (in Japanese with English abstract). 16) Akasaka Research Group (1956) Chikyu-Kagaku, no. 26127, 10-18 (in Japanese with English abstract).