From Middle Triassic Bedded Chert in Shikoku, Japan
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Jour. Geol. Soc. Japan, Vol. 110, No. 12, p. 758－764, December 2004 Lingularia sp.（Brachiopoda）from Middle Triassic bedded chert in Shikoku, Japan Abstract Rie S. Hori ＊ and A lingulid brachiopod Lingularia sp. is described from Middle Tri- Hamish J. Campbell ＊＊ assic（Anisian）bedded chert in the Northern Chichibu Terrane, Shikoku, Japan. The fossil occurrence is unusual because of its asso- Received January 14, 2004 ciation within deep-sea sediments. Lingulids are well known from Early Accepted October 25, 2004 and Middle Triassic successions and their apparent abundance may ＊ Department of Earth Sciences, Faculty of Sci- relate to the recovery following the Permian/Triassic（P/T）extinction ence, Ehime University, Matsuyama 790-8577, event. This find in Japan tends to support the view that lingulids Japan favoured dysoxic conditions and became widespread in vacant niches ＊＊ Institute of Geological and Nuclear Sciences, of the ocean after the P/T boundary anoxic event as“opportunistic PO Box 30-368, Lower Hutt, New Zealand taxa”. Key words: bedded chert, dysoxic condition, Lingularia, Permian/Triassic boundary, Middle Triassic stone of Imoto 1984 Fig.1 . The Toishi-type siliceous Introduction （ ）（ ） ‘ ’ shale is observed between sequences of the Upper Permi- Bedded chert sequences are distributed widely in Meso- an and Lower Triassic bedded chert, and is occasionally zoic accretionary complexes in Japan. Cherts are general- associated with carbonaceous black shale, pyrite nodules, ly considered to be ancient deep-sea sediments with and dolostones. This remarkable lithology is interpreted extremely low sedimentation rates and long accumulation to indicate development of extreme oxygen depleted times（e.g. Matsuda and Isozaki, 1991）. They commonly （dysoxic to anoxic）oceanic environmental conditions on preserve rich assemblages of microfossils, such as radio- the deep ocean floor resulting from the end-Permian larians, sponge spicules, conodonts and fish teeth. These extinction event（e.g. Isozaki, 1996, 1997; Kakuwa, 1996）. microfossils permit determination of depositional age of the original sediment and constrain tectonic histories of chert-bearing Mesozoic complexes（e.g. Matsuda et al., 1980; Matsuoka, 1984; Otsuka, 1985; Wakita, 1985）. How- ever, macrofossils from these bedded cherts are extreme- ly rare with only one previous record by Iijima et al. （1988）. These workers described Brachyoxylon（fossil wood）within the Ashio Terrane, exposed in the upper- most part of the Adoyama Chert, Kuzu area of central Japan. The specimen was collected from a horizon that is 20 cm below the base of the Middle Jurassic Maki Sand- stone. Iijima et al.（1988）speculated that the fragment of wood was derived from an adjacent landmass associated with submarine slumping. Bedded chert sequences from Mesozoic accretionary complexes in the Chichibu and Tamba－Mino－Ashio Ter- ranes range in age from Late Carboniferous to Jurassic. Within them, the Permian/Triassic（P/T）boundary is rec- Fig.1. Lithostratigraphic column of the Triassic bedded chert ognized by the presence of a distinctive clay-rich litholo- sequence indicating the stratigraphic level of the bed with Lingu- gy referred to as the‘Toishi-type’ siliceous shale/clay- laria sp. Time scale after Kozur（2003）. CThe Geological Society of Japan 2004 758 Jour. Geol. Soc. Japan, 110（ 12 ） Lingularia from Triassic chert in Shikoku 759 Fig.2. Geological map of Shikoku Island and an index map of the Kumosoyama area. The Lingularia sp. bearing bedded chert locality is indicated. The geological map is modified from Yamakita（1986）. Isozaki（1997）has argued that the facies change from the Kumosoyama, near Kamiyama Town, Tokushima Prefec- ‘Toishi-type’siliceous shale（claystone）to Middle Trias- ture, Shikoku, Japan（Fig.2）. The specimen was collected sic bedded chert deposition indicates the slow environ- from a chert horizon within a Middle Triassic bedded mental recovery from anoxic to normal oxic conditions, chert sequence in the Tenjinmaru Formation. The fossil following the P/T boundary event. During the Middle Tri- locality is within a roadside outcrop along Route 193, assic（Anisian）recovery period, a rhythmic bedded Kumoso Road, on the northwest flank of Mt. Kumosoya- sequence of alternating chert and relatively thick shale ma, Shikoku（Fig.2）. characteristically occur just above the‘Toishi-type’ In the Kumosoyama area, the Jurassic accretionary siliceous shale. This sequence differs from normal or com- complex of the Northern Chichibu Terrane is widely dis- mon regularly-bedded chert by having centimetre-thick tributed and has been mapped in terms of two formations: shale beds. the Tenjinmaru and Takashiroyama Formations（Yamaki- We report here on the discovery of a single brachiopod ta, 1986）. The Tenjinmaru Formation consists of bedded specimen, Lingularia sp., from an Anisian bedded chert chert-siliceous rocks and sandstone-mudstone units. It in Shikoku（Fig.1）. This is a most unusual find and is the forms relatively coherent sequences. The Takashiroyama first record of a brachiopod fossil from Triassic bedded Formation is comprised of basic volcanic rocks with asso- chert in Japan. However, occurrences of Lingulacea, gen- ciated lenses or blocks of chert and limestone. Geological erally referred to as“Lingula”, are not unusual among sketch maps and microfossils localities from this area are Lower to Middle Triassic sequences globally（e.g. Rodland also available in Isozaki（1981, 1987）and Suyari et al. and Bottjer, 2001）. Lingula has been interpreted as an （1982）. opportunistic‘disaster’taxon that flourished in marine At the fossil locality（Fig.2）, typical‘Toishi-type’ environments following the P/T extinction event. The find siliceous shale is observed, consisting of a sequence of recorded herein is relevant to these phenomena in terms black and grey siliceous shale beds that overlies a Permi- of the deep-sea sedimentary record. an bedded chert sequence. Up sequence, these shale beds are overlain by a Middle Triassic regular-bedded chert Sample Material facies. The‘Toishi-type’siliceous shale contains Early The brachiopod was discovered in an exposure on Mt. Triassic conodonts such as Neospathodus cf. dieneri and 760 Hori, R. S. and Campbell, H. J. 2004―12 slightly more pale in colour with an interior-internal aspect, and the other is darker coloured with a more exte- rior-external aspect. The fossils represents a single spec- imen of one valve of an inarticulate brachiopod identified here as Lingularia sp.（Fig.4）. The shell is not entire: the mid to upper left hand portion（5×1 mm）of the commis- sure is missing, as if it were torn off（possibly predated）, and the edge of the shell is irregular in shape.（See details in the systematic description below）. Discussion Several possible scenarios explaining the presence of Lingularia within the lower Middle Triassic bedded chert sequence of the Northern Chichibu Terrane are consid- ered below: 1）Lingularia may have been relatively abundant in Fig.3. Photomicrograph of a thin section of the Lingularia-bearing oceanic environments during early Middle Triassic shale-chert couplet. time, when deep-sea floors were dysoxic for at least T ＝ Triassocampe sp.（Radiolaria）; upper dark area ＝ shale; ten million years following the P/T boundary anoxic lower pale area ＝ chert. Scale bar ＝ 200 μm. event. 2）The Lingularia specimen may be regarded as an iso- Neohindeodella cf. nevadaensis（Yamakita, 1986）. Mid- lated one-off redeposited‘float’fossil derived from a dle Triassic radiolarian fossils such as Triassocampe spp. source area up-slope within the chert/shale deposi- occurr from associated bedded cherts with the siliceous tional environment. mudstone（Suyari et al, 1982）. In the present study, we 3）The great regression associated with the P/T boundary have also recognized primitive types of Triassocampe may have provided a habitat（e.g. marginal sea） and fragments of conodonts（? juvenile forms of Neogon- favourable to Lingularia in the area proximal to the dollella）from a chert bed closely associated with the sedimentary basin where the bedded cherts accumu- shale bed from which the brachiopod fossil was collected. lated. According to Kozur and Mostler（1994）, primitive Tri- 4）Lingularia may have been a natural component of the assocampe first appear at the boundary between the Pel- deep ocean fauna in the Triassic time. sonian（middle Anisian）and Illyrian（late Anisian）. The last scenario is the most unlikely because Lingu- Therefore, the bedded chert horizon from which the bra- laria are uncommon in these rocks; they have not been chiopod fossil was collected is considered to be of early reported previously from Triassic chert sequences, or Middle Triassic age. This age determination is consistent indeed chert sequences in general. At the end of Permian with previous reports based on radiolarian studies（e.g. time, sea level dropped ca. 200 m. It also dropped during Suyari et al., 1982）. middle Anisian time, but much less so and certainly not The brachiopod-bearing rock is a regularly-bedded enough to dramatically alter Lingularia habitat area chert facies consisting of alternating grey chert beds（2-3 （Erwin, 1993.）. Therefore, the third scenario is not con- cm thick）and yellowish grey shale beds（1-2 cm thick）. sidered plausible. Petrographically, the shale comprises mainly spherical The occurrence of Lingularia sp. from bedded chert is radiolarian tests, sponge spicules, a small number of con- unusual because it is preserved within a deep-sea facies odonts, and clay minerals, such as illite and white mica. that almost certainly accumulated below CCD. However, The chert comprises rich radiolarian tests, sponge occurrences of lingulids traditionally assigned to the spicules, rare conodonts, clay minerals and microcrys- genus“Lingula”, are not rare from Early to Middle Tri- talline quartz（Fig.3）. Stylolites are occasionally devel- assic marine sedimentary sequences（e.g. Rodland and oped within the chert Bottjer, 2001）. Accordingly, the first scenario is the most The fossil was discovered within a fragmental piece of reasonable explanation. chert measuring ca. 45×20×15 mm.