Title Santonian Fossils from the Upper Cretaceous Atokura Formation In
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Santonian fossils from the Upper Cretaceous 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 late Cretaceous 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: Ammonoidea 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 - 144 - Bulletin of Tokyo Gakugei University, Division of Natural Sciences, Vol. 64 (2012) MATSUKAWA, OBATA: Santonian fossils from the Upper Cretaceous Atokura Formation, Japan 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. - 145 - Bulletin of Tokyo Gakugei University, Division of Natural Sciences, Vol. 64 (2012) MATSUKAWA, OBATA: Santonian fossils from the Upper Cretaceous Atokura Formation, Japan 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. - 146 - Bulletin of Tokyo Gakugei University, Division of Natural Sciences, Vol. 64 (2012) MATSUKAWA, OBATA: Santonian fossils from the Upper Cretaceous Atokura Formation, Japan