An Early Maastrichtian (Latest Cretaceous) Ammonoid Fauna from the Soya Hill Area, Hokkaido, Northern Japan
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むかわ町穂別博物館研究報告第 32 号（2017 年 3 月）7–41 頁 The Bulletin of the Hobetsu Museum, no.32 (Mar., 2017), p.7–41. An early Maastrichtian (latest Cretaceous) ammonoid fauna from the Soya Hill area, Hokkaido, northern Japan YASUNARI SHIGETA1, MASATAKA IZUKURA2 and YUKIYASU TSUTSUMI1 1Department of Geology and Paleontology, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki, 305- 0005, Japan (e-mail: [email protected]) (e-mail: [email protected]) 2N28 E21-1-3 Higashi-ku, Sapporo, Hokkaido 065-0028, Japan (e-mail: [email protected]) Abstract An early Maastrichtian (latest Cretaceous) ammonoid fauna is reported from the middle part of the Etanpakku Formation of the Yezo Group exposed along the Onishibetsu and Sarukotsu rivers in the Soya Hill area, northernmost Hokkaido, northern Japan. The fauna comprises nine ammonoid species belonging to eight genera: Neophylloceras hetonaiense Matsumoto, Tetragonites popetensis Yabe, T. terminus Shigeta, Gaudryceras izumiense Matsumoto and Morozumi, Anagaudryceras matsumotoi Morozumi, Pachydiscus sp., Diplomoceras sp., Nostoceras sp. and Baculites regina Obata and Matsumoto. The fauna correlates with the lower part of the Gaudryceras izumiense Zone of the upper lower Maastrichtian in the Izumi Group of Southwest Japan. Zircon geochronology also reveals that the ages of the tuffs from the lowest and uppermost parts of the Etanpakku Formation in the Soya Hill area are 72.6±1.6 Ma and 70.6±1.2 Ma, respectively, which infer a late Campanian to earliest middle Maastrichtian age. Key wards: ammonoid, Cretaceous, Hokkaido, Maastrichtian, Soya Hill (Recieved 1 October 2016) Introduction an early Maastrichtian ammonoid fauna including Gaudryceras izumiense Matsumoto and Morozumi, Strata of the Campanian–Maastrichtian Yezo 1980 and Baculites regina Obata and Matsumoto, Group are widely distributed in northernmost Hokkaido 1963, which are common to the Izumi Group in (Osanai et al., 1959; Matsushita et al., 1964; Takahashi Southwest Japan. In this paper, we document the fauna and Ishiyama, 1968; Matsumoto and Ohara, 1971). Cape and discuss its biostratigraphic implications. We also Soya at the northernmost point of Hokkaido has been provide a zircon-based geochronology of the Etanpakku a classic locality for Campanian ammonoid research Formation based on its intercalated tuffs. since the early work of Jimbo (1894). Matsumoto and Miyauchi (1984) described 30 ammonoid species from Outline of stratigraphy the western coastal area of the cape, and Matsumoto (1984b) established a biostratigraphic scheme for the The Yezo Group along the upper course of the Campanian of Hokkaido. Although Ando and Ando Onishibetsu and Sarukotsu rivers can be divided into (2002) reported the occurrence of several ammonoid two units, the Karibetsu and Etanpakku formations specimens of probable Maastrichtian age from the Soya in ascending order, as defined by Matsushita et al. Hill area, ammonoids representative of this particular (1964). These strata are cut and displaced by two stage have not well been studied in northernmost major faults striking NNW–SSE (Figures 2, 3). The Hokkaido. Karibetsu Formation, consisting of mudstone and To ensure precise stratigraphic attribution of sandy mudstone, is mainly exposed in the western part Maastrichtian ammonoids in northernmost Hokkaido, of the area, whereas the Etanpakku Formation, which field expeditions were carried out by the authors (Y. S. is composed of various types of sandstone as well as and M. I.) in 2015 along the Onishibetsu and Sarukotsu sandy mudstone and conglomerate, is distributed in the rivers in the Soya Hill area (Figure 1). We discovered middle part of the area between the two major faults. ©2016 By the Hobetsu Museum 7 Yasunari Shigeta et al. B 142°0′ E 140 E B 144 E A Cape Soya 45 N 45 N HOKKAIDO 43 N HB Ichannai R. 100 km 45 ° 20′ N Masuhoro R. Onishibetsu River Ekope R. Sarukotsu R. 5 km Karibetsu R. Figure 1. Index maps showing distribution of Yezo Group (black areas) in Hokkaido (A) and the studied areas in the Soya Hill area (B). HB, Hobetsu. The Etanpakku Formation is in fault contact with the northward. Karibetsu Formation in the western part and with the Miocene Soya Formation in the eastern part of the Karibetsu Formation area. The Karibetsu Formation is equivalent to the The structure in the middle part of the area in the Orannai Formation in the Cape Soya area (Osanai upper course of the Onishibetsu River between the two et al., 1959) and the Kamikoma Formation in the major faults consists of an anticline with strata on the Nakatonbetsu area (Osanai et al., 1963), which is western limb striking N5–10° westward and dipping located approximately 40 km southeast of the Soya Hill 10–30° westward, while beds of the eastern limb area. strike N20–30° westward and dip 10–30° eastward. Exposure.—Uppermost course of the Onishibetsu In contrast, the structure in the upper course of the and Sarukotsu rivers, and middle course of the Sarukotsu River area consists of a syncline with strata Rokugousen River, a tributary of the Onishibetsu River on the northern limb striking N50–60° westward and (Figures 2, 3). dipping 10–20° southward, while those of the southern Thickness.—Greater than 25 m in the Rokugousen limb strike N30–40° eastward and dip 10–20° River section. 8 Maastrichtian ammonoids from Soya Hill A 142°0′ E 142°10′ E 12 28 28 25 30 5 Onishibetsu River 45 ° 18′N 10 25 Rokugousen River 500 m 20 B 142°0′ E 142°10′ E 3 I 2 1 Onishibetsu River 6 4 5 II 8 45 ° 18′N III V 7 9 IV 10 Rokugousen River 500 m 20 Figure 2. Geological map (A) and locality map (B) showing study sections (I through V) and fossil localities along the Onishibetsu River and its tributaries in the Soya Hill area, Hokkaido. See Figure 3 for legend. Stratigraphic relationship.—The Karibetsu Etanpakku Formation Formation is conformably overlain by the Etanpakku The Etanpakku Formation is equivalent to the Formation. The uppermost part is exposed along the Heitarouzawa Formation in the Nakatonbetsu area axis of an anticline in the Rokugousen River section, (Osanai et al., 1963). but the stratigraphic position of the formation’s Exposure.—Upper course of the Onishibetsu and outcrops in the uppermost course of the Onishibetsu Sarukotsu rivers and their tributaries (Figures 2, 3). and Sarukotsu rivers is uncertain. Thickness.—Greater than 600 m. Lithology.—The formation consists of dark Stratigraphic relationship.—The Etanpakku grey, massive mudstone (Figure 4), but detailed Formation conformably overlies the Karibetsu sedimentological features are uncertain because of poor Formation, but its top portion is in fault contact with exposures. the Karibetsu Formation in the uppermost course of the Fossils.—Although a few calcareous concretions Onishibetsu River. were observed in the mudstone, the formation has not Lithology.— The lower part of the formation (200- yet yielded megafossils. m thick) consists of gray to greenish gray, fine-grained, bedded or massive sandstone intercalated with a 3-m 9 Yasunari Shigeta et al. A 142°10′ E Sarukotsu River 12 12 Miocene Soya Formation 18 15 Cretaceous Etanpakku Fm. 18 Karibetsu Fm. 45 ° 13′N fault anticline 500 m Etanpakku Mt. syncline B 142°10′ E Sarukotsu River VI 11 12 15 13 14 VII 45 ° 13′N VI study section fossil locality 500 m Etanpakku Mt. tuff sampling locality Figure 3. Geological map (A) and locality map (B) showing study sections (VI, VII) and fossil localities along the Sarukotsu River and its tributary in the Soya Hill area, Hokkaido. thick white, vitric tuff bed and alternating beds of grained, bedded or massive sandstone (Figure 4), but coarse-grained sandstone and conglomerate composed some of the sandstone beds are characterized by low- mainly of rounded to subrounded pebbles and cobbles angle, hummocky cross-stratification. A 2–3-m thick in greenish gray, medium- to course-grained sandstone white, vitric tuff bed is intercalated in the uppermost matrix (Figure 4). part. The middle part of the formation (250-m thick) Fossils.—From the lower part of the formation, consists of gray to greenish gray, fine- to course- inoceramid bivalves were collected from the alternating grained, bedded or massive sandstone in association beds of coarse-grained sandstone and conglomerate and with dark gray, intensely bioturbated muddy sandstone fine-grained, massive sandstone (Figures 5, 7A–C). and sandy mudstone beds (Figure 4). Spherical The muddy sandstone in the middle part of the calcareous concretions containing ammonoids, bivalves formation is fossiliferous and inoceramid bivalves and gastropods are common in the muddy sandstone. (Figures 5, 6, 7D–S, 8) as well as the following The upper part of the formation (150-m thick) ammonoids were collected from calcareous concretions also consists of gray to greenish gray, fine- to course- (Figures 5, 6, 12–24): Neophylloceras hetonaiense 10 Maastrichtian ammonoids from Soya Hill I v v v v v v v v 3 II III 2 VII 15 1 V VI 6 8 9 12 14 13 Etanpakku Formation YEZO GROUP 11 7 5 4 IV 50 m v v v v v v v v 10 K. Fm. Legend conglomerate muddy sandstone v v v v tuff coarse to medium- sandy mudstone outcrop fossil grained sandstone fossil from float concretion fine-grained sandstone mudstone tuff sample Figure 4. Columnar sections showing localities from which fossils and tuff samples were collected in the Soya Hill area, Hokkaido. Matsumoto, 1942a, Tetragonites popetensis Yabe, 1903, Baculites regina Obata and Matsumoto, 1963, T. terminus Shigeta, 1989, Gaudryceras izumiense Diplomoceras sp. and Nostoceras sp. Matsumoto and Morozumi, 1980, Anagaudryceras No fossils were found in the upper part of the matsumotoi Morozumi, 1985, Pachydiscus sp., formation. 11 Yasunari Shigeta et al. sp. inoceramid bivalves Neophylloceras hetonaiense Gaudryceras izumiense Anagaudryceras matsumotoi Diplomoceras Baculites regina Tetragonites popetensis Tetragonites I-V terminus Tetragonites v v v v v v v v 3 70.5±1.1 Ma MID.