A Find O F Triassic Molluscs from the Buko Limestone Formation, Chichibu, Saitama Prefecture
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No. 2] Proc. Japan Acad., 54, Ser. B (1978) 41 9. A Find o f Triassic Molluscs from the Buko Limestone Formation, Chichibu, Saitama Prefecture By Minoru TAMURA,Shigema KAWADA,Atsushi TAKEDA, Hisakazu OGURI,Tel SAITO,Kozo TSUCHIDA, and Makoto INOUE (Communicatedby Teiichi KOBAYASx1,M.J. A., Feb. 13, 1978) The limestone forming the northern slope of Mt. Buko has long been quarried by Chichibu Cement Co., Ltd., and other companies for supplying cement producers and other limestone users. The limestone there has long been thought to be a member of the Chichibu Palaeozoic System from the lithostratigraphic point of view. However, the result from a very recent joint geological survey carried out by Chichibu Cement Co., Ltd., Nihon Cement Co., Ltd. and Mitsubishi Mining and Cement Co., Ltd. should have undermined this generally accepted assumption on the geologic age of the Buko Limestone Formation. That is to say, a fossil-rich horizon was happened to be found at the basal part of the Buko Limestone Formation by the joint geological survey party and the fossils obtained were determined by Tamura and turned out to be the Tethys Upper Ladinic-Carnic affinis. As it is a very significant contribution to the study of the geological evolution of Japan, the authors herewith decided to disclose the results from the survey. Mt. Buko, 1336 m high is an independent mountain in the Kwanto Massif and situated on the southeastern rim of the Chichibu Basin, western part of Saitama Prefecture. The northern slope of the east- westerly running main ridge of Mt. Buko is nearly entirely formed by the Buko Limestone Formation striking east-westerly and dipping steeply toward the north. The Buko Limestone Formation conform- ably overlies the Utozawa Schalstein Formation with the boundary line running parallel to the main ridge of Mt. Buko on the southern shoulder of the southern slope of the ridge, and contacts the supposedly overlying Kamikagemori Formation composed of siliceous limestone, cherts, slates and sandstones, with a fault. The alternating transition between the Buko Limestone and the Utozawa Schalstein Formation was observed not only at the surface but also in the tunnels of the Une Mine of Ryoko Lime Industry Co., Ltd, and the Buko Mine of the Buko Mining Co., Ltd. Fossils were found both in the limestone lens of 1 to 2 m thick and in the subjacent tuffaceous slate at the very base of the Buko Lime- 42 M. TAMURA et al. [Vol. 54(B), stone Formation. The first discovery place of the fossil horizon is Station 1 at about 680 m high place above the present quarry of the Utoo Mine. The place is shown in the geologic map (Fig. 2). Fossils from the tuffaceous slate of Station 1 are better in the state of preser- vation than those from the superjacent limestone. Due to the well development of joints oblique to the bedding, fine specimens were hardly obtained from this station. Station 2 at about 1,285 m high, along the trail to the Mitake Shrine on the summit of Mt. Buko, is better than Station 1 in yielding of well preserved fossil. Numerous fossils were chiefly obtained from the tuffaceous slate. Fossils found in marl concretions intercalated in this horizon are still possessing shell part. The following species were determined : Bakevillia sp. B. cf. b ouei (Hauer ) Gervillia (Cultriopsis) angulata Munster Entolium subdemissum Munster Plicatula ( Pseudo p lacuno ~ sis ) s.p P. (P.) fssistriata (Winkler) Unionites (?) sp. cf. Unionites m~nsteri Wissmann Costatoria sp. Gruenewaldia wohrmanni (Bittner) Myophoricardium sp. M. aff. lineatum (Wohrmann ) Pleuromya sp. P. cf. elon g ata Schlotheim Gen, and sp. inlet. A Gen, and sp, inlet. B Crinoid stem (Pentacrinus) Species determined were almost comm on to the both stations. Gervillia (Cultriopsis) angulata, Unionites (?) sp, are predominant in Station 1 but aff. Myophoricardium linen tum in Station 2. Gruene- wardia wohrmanni is a predominant species in both stations. Gervillia (Cultriopsis) angulata, Gruenewaldia wohrmanni and Myophoricardium lineatum were all known from Carnian, and Ento- lium subdemissum was recorded from Ladinian to Upper Triassic. The Buko fauna listed above has many common species to the faunas from St. Cassian Formation of Alps (A. Bittner, 1895) and other formations of the same age and further to the fauna reported by A. Lerman (1960) from Ramon, South Israel. The geologic age of St. Cassian Formation was revised by M. Urlichs (1977) as Upper- most Ladinian. Judging from the above, the age of the Buko fauna is thought to be comparable to the Ladino-Carnic fauna reported by M. Tamura (1972) from the Konose Group in the drainage area of the Kuma River, Kyushu. The fauna is poor in preservation. The speci- No. 2] Triassic M olluscs from Buko Limestone, Saitama 43 Fig. 1. Geologic map of the Mt. Buko area. 1, Mt. Buko; 2, Mitake Shrine; 3, Utoo Mine; 4, Ubukawa Mine; 5, Buko Mine; 6, Une Mine. I, Station 1; II, Station 2. A, Kamigagemori Form.; A-1, Chert; A-2, Slate; A-3, Siliceous slate; B, Buko Limestone Form.; C, Utozawa Schalstein Form.; D, Fossil Locality; E, Fault. Fig. 2. Index map of the surveyed area. 1, Tokyo; 2, Mt. Buko; 3, Chichibu; 4, Kumagaya; 5, Hanno; 6, Omiya. Figs. 3-5. Gruenewardia wohrmanni (Bittner). Fig. 3. Rubber external cast of 1. valve (lower) and r. valve (upper). Fig. 4a, b. Rubber external cast of r, valve and its internal mold. Fig. 5. Rubber external cast of 1. valve. Figured samples are all from St. 2. Figs. 6-8. Entorium subdemissum Munster. Fig. 6. Internal mold of 1. valve. Fig. 7. Internal mold of 1, valve. Fig. 8. Rubber external cast of 1. valve. Samples of Figs. 6 and 7 are from St. 2 and sample of Fig. 8 from St. 1. Figs. 9-11. Gervillia (Cultriopsis) angulata Wohrmann. Fig. 9a, b. External mold of 1. valve and its internal mold. Fig. 10. R. valve. Fig. 11. R. valve. Figured samples are all from St. 1. 44 M. TAMURA et al. [Vol. 54(B), mens determined as Gruenewaldia decussate seem to include the form standing nearer to G. wohrmanni in the Buko fauna. Costatoria sp. of the Bukozan fauna seems to perhaps be identical with Costatoria gold f ussi reported by M. Hirata and K. Ichikawa (1966) from Yoshitsugu, Kagami Machi, Kami Gun, Kochi Prefecture. The host rock of this fossil was thought to be derived from the argillaceous limestone of the Sambosan Group. More precise correlation of the Buko fauna will be able to finish by further study now commenced by Tamura. It should be added that there was a Carno-Early Noric fauna reported by K. Ichikawa (1954) from the Arai Formation, Itsukaichi, Tokyo, about 25 km southeast of Mt. Buko. No common species were known between the Arai and the Buko faunas. References Bittner, A. (1895) : Lamellibranchiaten der Alpinen Trias. Abh. geol. Reich- sanst., Bd. 18, Heft 1, Wien. Hirata, M., and Ichikawa, K. (1966) : Triassic fossil found from the northern rim of the Butsuzo tectonic line at Yoshitsugu, Kagami Machi, Kami Gun, Kochi Prefecture (in Japanese). Combined vol. of Rept. No. 62, Kansai Branch and Rept. No. 45, Nishi-Nihon Branch, Geol. Soc. Japan. Ichikawa, K. (1954) : Triassic mollusca from the Arai Formation at Iwai near Itsukaichi, Tokyo Prefecture, Japan. Japan. Jour. Geol. Geogr., 24. Lerman, A. (1960) : Triassic Pelecypods from southern Israel and Sinai. Bull. Res. Counc. Israel, G. (Geo-Sci.), 9 G. Tamura, M. (1972) : Myophorian fossils discovered from the Konose Group, Kumamoto Prefecture, Japan, with a note on Japanese Myophoriids. Mem. Fac. Educ. Kumamoto Univ., No. 21, Sec. 1. Urlichs, M. (1977) : Zur Altersstellung der Pachycardientuffe and der Unteren Cassianer Schichten in den Dolomites (Italien). Mitt. 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