Maokouan-Wuchiapingian) Boundary in Mid-Oceanic Paleo-Atoll Limestone of Kamura and Akasaka, Japan
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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) character- ized by the greatest mass extinction in the Phanerozoic, the Middle-Upper Permian boundary marked anoth- er 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 extinc- tion 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 dis- tinct 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