fossils from the Upper Atokura Title Formation in Kwanto Mountains, Japan, and their significances( fulltext )

Author(s) MATSUKAWA,Masaki; OBATA,Ikuwo

Citation 東京学芸大学紀要. 自然科学系, 64: 143-152

Issue Date 2012-09-28

URL http://hdl.handle.net/2309/131823

Publisher 東京学芸大学学術情報委員会

Rights Bulletin of Tokyo Gakugei University, Division of Natural Sciences, 64: 143 - 152，2012

Santonian fossils from the Upper Cretaceous Atokura Formation in Kwanto Mountains, Japan, and their significances

Masaki MATSUKAWA* and Ikuwo OBATA**

Department of Environmental Sciences

(Received for Publication; May 25, 2012)

MATSUKAWA, M. and OBATA, I.: Santonian fossils from the Upper Cretaceous Atokura Formation in Kwanto Mountains, Japan, and their significances. Bull. Tokyo Gakugei Univ. Div. Nat. Sci., 64: 143-152 (2012) ISSN 1880-4330

Abstract

Santonian ammonoid and inoceramid, from the Nakanokaya Formation and/or Atokura Formation in northern Kwanto Mountains, Japan, are described herein: Polyptychoceras (Polyptychoceras) cf. obstrictum and Inoceramus (Inoceramus) amakusensis. These fossils indicate that the age interpretation of the Nakanokaya Formation and/or Atokura Formation previously should be changed to Santonian of the international standard from Gyliakian of the Japanese standard (= Cenomanian to Turonian). This suggests that both the sedimentary basins of the Sanchu Cretaceous and the Atokura and its equivalent formations were provided deposits from nearly the same rocks in the provenance not in the same age, but in the different age.

Key words: Santonian, Atokura Formation, Kwanto, Polyptychoceras (Polyptychoceras) cf. obstrictum, Inoceramus (Inoceramus) amakusensis

Department of Environmental Sciences, Tokyo Gakugei University, 4-1-1 Nukuikita-machi, Koganei-shi, Tokyo 184-8501, Japan

Introduction origin of the Atokura Formation has been accepted, because some Cretaceous molluscan fossils were discovered (Arai The Cretaceous strata are separately distributed in the et al., 1963). Afterward, the Cretaceous strata including the northern part of the Kwanto Mountains, central Japan. Atokura Formation were interpreted to be moved as nappe The strata are locally subdivided into some formations. from the original depositional basin after In the Shimonita Area, the Cretaceous strata including the (Arai and Takagi, 1998). This supported the original idea Atokura Formation were disputed about its origin, whether of allochthonous sedimentary body moved as nappe by allochthonous sedimentary body moved as nappe or Fujimoto et al. (1953). igneous, in its early study era (Sugiyama, 1943, Fujimoto As to the discussion of location of original sedimentary et al., 1953). Then, the view of allochthonus sedimentary basin of the Atokura Formation and the comparison with

* Department of Environmental Sciences, Tokyo Gakugei University (4-1-1 Nukuikita-machi, Koganei-shi, Tokyo, 184-8501, Japan) ** Fukada Geological Institute, 2-13-12 Hon-Komagome, Bunkyo-Ku, Tokyo 113-0021, Japan

－ 143 － Bulletin of Tokyo Gakugei University, Division of Natural Sciences, Vol. 64 (2012) MATSUKAWA, OBATA: Santonian fossils from the Upper Cretaceous Atokura Formation, Japan other Cretaceous sedimentary basin, certain geological Alternation Member, Nakanokaya Conglomerate and age of the formation has been required. The geological the Upper Mudstone Member. They included both the age was, however, used only the list of fossils without Kanoya and Nakanokaya formations into the following paleontological systematics and pictures as its evidence. three members, the Lower Sandstone Member, Middle On the present paper, we show the geological age of the Alternation Member and the Upper Mudstone Member of Cretaceous strata including the Atokura Formation with the the Atokura Formation. paleontological systematics of ammonite and inoceramid So, we try to discuss the biostratigraphic positions of and pictures. both fossil localities with biostratigraphic resolving power of ammonites and inoceramids. This shows the geological Geological setting age of the Atokura Formation also.

Ammonite specimens were collected from mudstone beds Systematic paleontology and sandy part in mudstone beds along middle stream of the Kanazawa Valley in Shimonita Town, Gunma Abbreviations. H, whorl height; B, whorl breadth; B/ Prefecture (Figs. 1, 2). Those beds were subdivided into H, breadth/height ratio. All specimens are housed in the the upper member of the Atokura Formation (Arai et Department of Environmental Sciences, Tokyo Gakugei al., 1963), and into the Middle Alternation Member of University, Koganei, Tokyo (TGUSE). the Atokura Formation (Arai and Takagi, 1998). Then, inoceramids were collected from mudstone beds at Class: Cephalopoda Zittel, 1884 southeastern ridge of the Mt. Yotsumata-yama (Figs. 1, Order: Zittel, 1884 2). The fossil locality corresponds to the area mapped as Suborder: Ammonitina Hyatt, 1889 the Nakanokaya Formation (Arai et al., 1963), and Upper Superfamily: Turrilitaceae Gill, 1871 Mudstone Member of the Atokura Formation (Arai and Family: Diplomoceratidae Spath, 1926 Takagi, 1998). Subfamily: Polyptychoceratinae Matsumoto, 1938 Opinions are diverse concerning stratigraphic levels of As Wright and Matsumoto (1954) remarked, Scalarites two fossil localities: (1) The level of ammonite locality is my be derived from Hyphantoceras, or from some common higher than the level of inoceramid locality if we follow ancestor of Hyphantoceras and Bostrychoceras, and in Arai et al. (1963), (2) Vice versa if we follow Arai and turns leads to the group of the Upper Cretaceous form, Takagi (1998), this is caused by different stratigraphic Polyptychoceras Yabe, 1927 and Subptychoceras Shimizu, interpretation of the Atokura Formation. Namely, Arai 1935, for which, and also for Scalarites itself, the family et al. (1963) mentioned that the Cretaceous strata in the Polyptychoceratidae Matsumoto (1938, p. 193) was northern part of the Kwanto Mountains are divided into established. The North American group of Solenoceras the Kanoya, Nakanokaya and Atokura formations. They may be bituberculate derivatives of Polyptychoceras or confirmed that the Kanoya Formation is conformably possibly independent offshoots of some later nostoceratid covered by the Nakanokaya Formation. Then, they of Emperoceras type. They started that the family includes interpreted that the Atokura Formation is higher than the also a few forms whose names require to be stabilized. Nakanokaya Formation on the basis of suggesting ages by Fourty two years afterward, Wright et al. (1996, p. ammonites and inoceramids, because they did not confirm 250, 253) adopted the subfamily Polyptychoceratinae stratigraphic relationship between the succession from with simple definition for eight genera. We follow the the Kanoya to Nakanokaya formations and the Atokura classification by Wright et al. (1996). Formation. On the other hand, Arai and Takagi (1998) showed the succession of the Atokura Formation, and Genus: Polyptychoceras Yabe, 1927 revised definition of the formation and its distribution Subgenus: Polyptychoceras Yabe, 1927 exhibiting as the geological map. They defined the Atokura Polyptychoceras (Polyptychoceras) cf. obstrictum (Jimbo, Formation consists of five units in ascending order, the 1894) Atokura Conglomerate, Lower Sandstone Memebr, Middle

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Fig. 1. Ammonite (1) and inoceramid (2) fossil localities are shown in two geological maps drawn by Arai et al. (1963) (above) and by Arai and Takagi (1998)(below). A-P of Arai et al. (1963) (above) are as follows; A, Quaternary (mainly Terrace deposits), B, Tertiary (Tomioka Group), C-E, Upper Cretaceous Urakawan Series (Atokura Formation), C, upper member of the Atokura Formation, D, lower member of the Atokura Formation, E, basal conglomerate of the Atokura Formation (Atokura conglomerate), F, Kawaiyama Quartz Diorite, G, Hornfers, H, Upper Cretaceous Gyliakian Series (Nakanokaya formation and Nakanokaya conglomerate), I, Lower Cretaceous? (Kanoya formation), J, Chichibu Paleozoic strata, K, Limestone in the Chichibu Paleozoic strata, L. Mikabu type volcanic rocks, M, Sambagawa Schist, N, Serpentinite, O, zoo-fossil locality, P, phyto- fossil locality.

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Fig. 2. Map showing fossil localities. Geological map in box at right below shows two Cretaceous strata of Sanchu Cretaceous and Atokura and/or Nakanokaya formations in Shimonita area.

Fig. 3A-H Material. Six specimens, TGUSE-MM 6005, 6006, 6007, 6008, 6009, 6010 from loc. l, Kanazawa Valley, Compare. Shimonita Town (M. Matsukawa Coll.), the upper member 1894. Hamites obstrictus Jimbo, pl. 7, figs. 2, 2b. of the Atokura Formation (Arai et al., 1963) or the Middle 1954. Polyptychoceras obstrictum (Jimbo), Wright and Alternation Member of the Atokura Formation (Arai and Matsumoto, fig. 3. Takagi, 1998). They are imperfect. TGUSE-MM 6008 is 1963. Polyptychoceras obstrictum (Jimbo), Matsumoto, pl. observable whorl breadth, but whorls of other specimens 66, fig. 2. are depressed.

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Fig. 3. A-H. Polyptychoceras (Polyptychoceras) cf. obstrictum, A, TGUSE-MM 6005, B, TGUSE-MM 6006, C, TGUSE-MM 6007, D, TGUSE-MM 6008, E and F, TGUSE-MM 6009, E is a cross section view, G and H TGUSE-MM 6010, H is a latex peel from G, from locality 1. I, Fairly smooth ammonite gen. et sp. indet., from locality 1. J, Bivalvia gen. et sp. indet., from locality 1. K, L, TGUSE-MM5228 and M, TGUSE-MM5229, Inoceramus ( I .) amakusensis, K is a latex peel from L, from locality 2. Scale bars show 1 cm.

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Measurements (in mm). of whorl section, but the lectotype of Polyptychoceras

nd rd Specimen 2 shaft 3 shaft obstrictum has elliptical whorl sections. The present Length Min. Height Breadth B/H Length Max. Height specimens are different from the lectotype (Matsumoto, TGUSE-MM 6005 69.1 7.3 -- 63.9 12.6 1963, p. 30, pl. 46, fig. 5) of Polyptychoceras haradanum TGUSE-MM 6006 37.1 8.2 -- 17.6 10.0 (Yokoyama, 1890, p. 182, pl. 20, fig. 5) from the Upper TGUSE-MM 6007 ------49.9 13.6 Cretaceous of Hokkaido, Japan, because the lectotype has TGUSE-MM 6008 25.2 19.6 7.7 0.39 -- --

TGUSE-MM 6009 ------30.4 12.8 broad and coarser ribbing and elliptical whorl sections.

TGUSE-MM 6010 ------15.6 8.4 Based on the ribbing, the present is allied to Polyptychoceras obstrictum rather than P. haradanum. Description. Specimens are small size, less than 70 mm Occurrence. Santonian (Atokura Formation) of Gunma in length, and are preserved second and third shafts. Shafts Prefecture. are nearly straight, parallel, not touch, and follow to more or less open hook. Shaft gap is very narrow. Whorl section Class: Bivalvia Linné, 1758 is roughly oblong. Sides of shaft are ornamented with ribs Subclass: Pteriomorphia Beurlen, 1944 and constrictions. Ribs are rectiradiate, fine and dense in the Order: Pteroida Newell, 1965 second shaft, but rursiradiate, thick and coarse in the third shaft. Suborder: Pteriina Newell, 1965 They become gradually coarser near hook. Constrictions are Superfamily: Pteriacea Gray, 1847 observable as ridge on second shaft, and as faint hollow on the Family: Inoceramidae Giebel, 1852 third shaft. Constrictions irregularly present seven to ten ribs on Genus: Inoceramus J. Sowerby, 1814 the second shaft, and three to five ribs on the third shaft. Subgenus: Inoceramus J. Sowerby, 1814 Remarks. The present specimens are similar to the illustrated specimen, lectotype (Matsumoto, 1963, p. 45, Inoceramus (Inoceramus) amakusensis Nagao and pl. 66, fig. 2) of Polyptychoceras obstrictum (Jimbo, 1894, Matsumoto, 1940 p. 184, pl. 7, figs. 2, 2b) from the Upper Cretaceous in Hokkaido in having fine ribs, and rectiradiate constrictions 1940. Inoceramus amakusensis, Nagao and Matsumoto, p. on the second shaft and rursiradiate constrictions on the 13-16, pl. 3, fig. 6; pl. 4, figs. 1, 3, 4; pl. 5, fig. 1. third shaft. But the present specimens have coarser ribs 1962. Inoceramus amakusensis, Nagao and Matsumoto, on the third shaft than the lectotype of Polyptychoceras Matsumoto and Ueda, p. 161-164, fig. 12, pl. 22, obstrictum. The present specimens have roughly oblong figs. 1-3.

Fig. 4. Comparison of stratigraphic schemes of Arai et al. (1963) and Arai and Takagi (1998). Solid circles 1 and 2 showing ammonite and inoceramid localities. Arrow reveals geological age.

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Material. Two external moulds of right valve, TGUSE- suggested that the locality 1 is higher than the locality 2 on MM 5228, 5229 (I. Obata and M. Matsukawa Coll.), from the basis of Arai et al. (1963), but vice versa on the basis locality 2, mudstone beds of the Nakanokaya Formation of of Arai and Takagi (1998) (Figs. 1 and 4). The specimens Arai et al. (1963) and the Upper Mudstone Member of the of Inoceramus (I.) amakusensis occur in the Nakanokaya Atokura Formation of Arai and Takagi (1998). Specimens Formation (Arai et al., 1963) or the Upper Mudstone are imperfect and small size, less than 26 mm in length. Member of the Atokura Formation (Arai and Takagi, 1998). Based on Toshimitsu et al. (1995), Inoceramus amakus̱ ensis Measurements. (in mm) is zonal species of the Inoceramus amakusensis Zone Specimen Length Height H/L assigned to the Santonian. So, they should be assigned to TGUSE-MM 5228 25.1+ 31.9+ 1.3+ the Santonian. TGUSE-MM 5229 17.4+ 17.6 + 1.0+ Based on Hayakawa and Tashiro (1994), some species Descriptive remarks. Among the syntypes of Nagao of Polyptychoceras occur in four ino̱ ceramid zones in and Matsumoto, 1940, (UMUT. MM 6514 is designated ascending order, Inoceramus amakusensis Zone, I. as the lectotype by Matsumoto and Ueda [fig. 12]). The japonicus Zone, I. orientalis nagaoi Zone and I. schmidti specimens are characterized by an oblong outline, long Zone of the Yezo Group, Hokkaido, and three inoceramid hinge line, almost rectangle hinge angle, and strong zones in ascending order, I. amakusensis Zone, I. japonicus concentric undulatory growth rings, with minor rings. Zone and I. balticus cf. toyajoanus Zone of the Himenoura Undulations are low, broad, irregular in size and distance. Group, Kyushu. They are assigned to the Santonian and Outer undulations are rounded-top. Regarding oblong Campanian in age. outline and the presence of minor rings in middle growth So, the succession from the Nakanokaya Formation to stage, the characters suggest that the specimen can be the upper member of the Atokura Formation classified by classified as I. amakusensis described by Nagao and Arai et al. (1963), and from the Middle Alternation Member Matsumoto (1940). The present specimens are similar to to the Upper Mudstone Member of the Atokura Formation younger shell of the lectotype of Inoceramus amakusensis classified by Arai and Takagi (1998) is safely correlated by in having oblong outline and strong concentric undulatory the fossil indices to the Santonian in age. growth rings. But juvenile shell of the lectotype is more Hitherto, Arai et al. (1963) concluded that ages of swollen than the present specimen. the Nakanokaya Formation and Atokura Formation are The specimen differ from the illustrated specimen of probable Lower Cretaceous and Senonian (Coniancian to Inoceramus teshioensis (Nagao and Matsumoto, 1940, p. Maastrichitian), respectively, based on fossil evidence. 274-277, pl. 24, figs. 6, 7, 9; pl. 26, figs. 5, 6, 7) from the The following species, Inoceramus cfr. teshioensis, I. cfr. Upper Cretaceous Yezo Group in Hokkaido, because the uwajimensis, Bivalvia gen. et sp. indet., Acanthotrigonia former is characterized by oblong outline and irregular cfr. dilapsa, ammonoidea gen. et sp. indet., Echinoidea distance of undulations, but the latter is characterized by gen. et sp. indet., were listed from the Nakanokaya obtuse hinge angle and regular distance of undulations. Formation, and ammonites similar to the genus Scaphites Occurrence. Santonian (Nakanokaya Formation or and Polyptychoceras from the upper member of the Atokura Formation) of Gunma Prefecture. Atokura Formation. Then, they probably correlated the Nakanokaya Formation to the Gyliakian of the Japanese Discussion standard corresponding to Cenomanian to Turonian of the international standard, and the Atokura Formation to (1) Geological age of the Atokura Formation the Senonian. Unfortunately, these specimens have not Stratigraphic levels of fossil localities of ammonites and been exhibited as pictures on the paper, and have not been inoceramids correspond to the Nakanokaya Formation and described with their repository. So, we can not reconfirm the upper member of the Atokura Formation were classified these specimens at the present stand. by Arai et al. (1963), and the Middle Alternation Member and Upper Mudstone Member of the Atokura Formation (2) Location of the original sedimentary basin of the were classified by Arai and Takagi (1998). Their levels are Atokura Formation

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Arai et al. (2000) inferred that the sedimentary basin Chikyukagaku (Earth Science) 64, 18-31 (in Japanese with of the Atokura Formation was nearly located to the English abstract). sedimentary basin of the Sanchu Cretaceous in the Arai, H., Murakami, S., Takagi, H., 2000. Provenance of the Atokura Kwanto Mountains based on age constraint and mineral Formation, Kanto Mountains, central Japan. Memoires of compositions of sandstone. They employed the age Geological Society of Japan 56, 123-136 (in Japanese with constraints that the Sanchu Cretaceous is Hauterivian to English abstract). Turonian and the Atokura Formation and its equivalent Arai, H., Takagi, H., 1998. Geotectonic development of the Atokura formations distributed in the northern part of the Kwanto Nappe: restoration of the overfold in the Shimonita area, Kanto Mountains is Turonian to Campanian. Although Arai et al. Mountains. Journal of the Geological Society of Japan 104, (1998) referred Takei (1963) and Koizumi (1991) for age 861-876 (in Japanese with English abstract). of the Sanchu Cretaceous, and The Conveners of the Paleo- Beurlem, K., 1944. Beiträg zur Stommesgeschichte der Muscheln. Mesozoic of the Annual Meeting at Saitama (1995) for the Bayerische Akademie der Wissenschaften, 1-2, 133-145. Atokura Formatoin, the ages on those references cited from Fujimoto, H., Watanabe, K., Sawa, H., 1953. The nappe theory with Obata et al. (1976) and Takei et al. (1977) for the Sanchu reference to the northern part of the Kwanto Mountainland. Cretaceous, and from Arai et al. (1963) and Ishii et al. Bulletin of Chichibu Museum of natural History, no. 3, 1-41, (1991). However, the Sanchu Cretaceous is correlative with pls. 1-3 (in Japanese with English abstract). the Hauterivian to Turonian (Takei et al., 1977; Matsukawa, Gray, J.E., 1847. A list of the genera of Recent , their 1983; Matsukawa and Tomishima, 2009), and the Atokura synonyms and types. Zoological Society of London, Formation is correlative with the Santonian discussed Proceedings. 15, 129-219. above. This suggests that both the sedimentary basins Gill, T., 1871. Arrangement of the families of mollusks. Smithonian were provided deposits from nearly the same rocks in the Miscellaneous Collections 227, xi + 1-49. provenance not in same age, but in different age. Giebel, C.G., 1852. Allgemeine Paläontologie: Entwurf einer Systematischen Darstellung der Fauna und Flora der Vorwelt. Conclusions Ambrosius Abel, Leipzig, 413 p. Hayakawa, H., Tashiro, M., 1994. Taxonomy, mode of occurrence 1. Ammonite and inoceramid specimens were collected and stratigraphic position of Polyptychoceras from the from the Cretaceous strata in northern Kwanto Himenoura Group. Research Report of Kochi University 43, Mountains. They are identified as Polyptychoceras 167-176 (in Japanese with English abstract). (Polyptychoceras) cf. obstrictum and Inoceramus Hyatt, A., 1889. Genesis of the Arietitidae. Smithsonian Contributions (Inoceramus) amakusensis, and are described with to Knowledge 673, xi + 238 pp. systematics. Ishii, A., Takahashi, O., Kaneko, Y., Matsukawa, M., 1991. Review 2. The Cretaceous strata including the Atokura, Kanoya of the Shoya Formation. Abstract of the 98th meeting of the and Nakanokaya formations are correlative with the Geological Society of Japan, Matsuyama, 147 (in Japanese) Santonian of the Upper Cretaceous based on these Jimbo, K., 1894. Beitränge zur Kenntniss der fauna der species. Kreiderformation von Hokkaido. Palaeontologische 3. Cretaceous sedimentary basins of the Atokura Formation, Abhandlungen, Jena, 6, 149-194, pls. 2-9. and the Sanchu Cretaceous were provided deposits from Koizumi, K., 1991. Stratigraphy and geologic structure of the nearly the same rocks in the provenance not the same Cretaceous System in the eastern half of the Sanchu Graben, age, but in the different age. Kanto Mountains, central Japan. Journal of Geological Society of Japan 97, 799-815 (in Japanese with English abstract). References Matsukawa, M., 1983. Stratigraphy and sedimentary environments of the Sanchu Cretaceous, Japan. Memoirs of the Ehime Arai, F., Hayama, Y., Hayashi, S., Hosoya, T., Ibe, H., Kanazawa, University, Natural Science, series D, 9, 1-50. K., Kizaki, Y., Kin, G., J., Takahashi, K., Takahashi, T., Takei, Matsukawa, T., Tomishima, K., 2009. Evaluation of studies of K., Toya, K., Yamashita, N., Yoshiba, K., 1963. On the Atokura stratigraphy and sedimentary environments of the Sanchu Conglomerate on the Shimonita District, Gunma Prefecture. Cretaceous since Matsukawa (1983). Bulletin of the Tokyo

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Gakugei University, Division of Natural Science, 61, 119-144 Takei, K., Takizawa, F., Takeuchi, M., Fujiwara, H., 1977. (in Japanese with English abstract). Cretaceous System in the western part of the Sanchu graben, Matsumoto, T., 1938. A biostratigraphic study on the Cretaceous Kanto mountains. Journal of the Geological Society of Japan deposits of the Naibuti Vally, South Karahuto. Proceeding of the 83, 95-113 (in Japanese with English abstract). Imperial Academy of Japan, Tokyo 14, 190-194. The Conveners of the Paleo-Mesozoic of the Annual Meeting at Matsumoto, T. (ed.), 1963. A survey of the fossils from Japan Saitama, 1995. The results of recent researches on the Paleo- illustrated in classical monograph (Primary a nomenclatorial Mesozoic of the Kanto Mountains, central Japan. Earth Science revision). Palaeontological Society of Japan 24 Anniversary (Chikyu Kagaku) 49, 271-291 (in Japanese with English volume. 29-32, pls. 44-51; 41-48, pl. 60-68. abstract). Matsumoto, T., Ueda, Y., 1962. Appendices palaeontological notes. Toshimitsu, S., Matsumoto, T., Noda, M., Nishida, T., Maiya, S., In: Ueda, Y., The type Himenoura Group. Memoires of the 1995. Toward an integrated mega-, micro- and magneto- Faculty of Science, Kyushu University, series D, Geology, 7, stratigraphy of the Upper Cretaceous in Japan. Journal of the 129-178, pls. 22-27. Geological Society of Japan 101, 19-20 (in Japanese with Nagao, T., and Matsumoto, T., 1940. A monograph of the Cretaceous English abstract). Inoceramus of Japan parts 1 and 2. Journal of Faculty of Wright, C. W., Matsumoto, T., 1954. Some doubtful Cretaceous Science, Hokkaido Imperial University, series 4, 4, 241-299, ammonite genera from Japan and Saghalien. Memoires of the pls. 23-34. 1-64, pls. 1-21. Faculty of Science, Kyushu Univesity (series D, Geology) 4, Newell, N.D., 1965. Classification of the Bivalvia. American 107-134, figs. 1-22, pls. 7, 8. Museum Novitates 2206, 1-25. Wright, C.W., Calloman, J.H., Howarth, M.K., 1996. Cretaceous Obata, I., Matsukawa, M., Tsuda, H., Futakami, M., Ogawa, Y., Ammonoidea. in: Brousius, E., Hardesty, J., Keim, J., Kerns, J. 1976. Geological age of the Cretaceous Ishido Formation, and Renteria, K. (Eds.), Treatise on Invertebrate Paleontology, Japan. Bulletin of the National Science Museum, Tokyo, series Part L. Mollusca 4 (revised). The Geological Society of C, 2, 121-140. America, Inc. and The University of Kansas Press, Lawrence, Shimizu, S., 1935. The Upper Cretaceous ammonites socalled xx + 362 pp. Hamites in Japan. Proceedings of the Imperial Academy, Tokyo Yabe, H., 1927. Cretaceous stratigraphy of the Japanese Islands. 11, 271-273, 12 figs. Science Report of the Tohoku Imperial University, series 2, Sowerby, J. de C., 1814. Article XI. Proceedings of Philosophical Geology, 11, 27-100, figs. 1-5, pls. 3-9. Societies, Linnean Society, Annals of Philosophy, 4, 6, 448. Yokoyama, M., 1890. Versteinerungen aus der japanischen Kreide, Spath, L. F., 1926. On new ammonites from the English Chalk. Palaeontographica Band B, 36, 159-202, pls. 18-25. Geological magazine 63, 77-83. Zittel, K.A. von, 1884. Cephalopoda. in: Zittel, K.A. von (Ed.), Sugiyama, R., 1943. So-called Atokura conglomerate in Shimonita Handbuch der Palaeontologie, 1(2). Oldenbourg, Munich/ Town, Gunma Prefecture. Bulletin of Tokyo Science Museum, Leipzig, pp. 329-522. no. 7, 30 p. (in Japanese without English title). Takei, K., 1963. Stratigraphy and geological structure of the eastern part of the Sanchu Graben, Kwanto Mountains. Journal of Geological Society of Japan 69, 130-146 (in Japanese with English abstract).

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関東山地北部の跡倉層から産出したサントニアン階の化石とその意義

松 川 正 樹・小 畠 郁 生

環境科学分野

要 旨

関東山地北部の群馬県下仁田地域に分布する白亜系からアンモナイトとイノセラムが産出し， Polyptychoceras (Polyptychoceras) cf. obstrictum と Inoceramus (Inoceramus) amakusensisとして記載された。これは，後期白亜紀の Santonian期を示す。これにより，関東山地北部に分布する白亜系と山中白亜系（Hauterivian 期～Turonian期）が異な る時代に同様な後背地から供給された堆積物により形成されたと解釈される。

キーワード: サントニアン階，跡倉層，関東山地，Polyptychoceras (Polyptychoceras) cf. obstrictum，Inoceramus (Inoceramus) amakusensis

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