63. Record O F a Cretaceous Ammonite from the Akaishi Mountains, Central Japan

No. 7] Proc. Japan Acad., 54, Ser. B (1978) 321

63. Record o f a Cretaceous Ammonite from the Akaishi Mountains, Central Japan

By Tatsuro MATSUMOTO, Hakuyu OKADA, and Sadahiko SAKURAI

(Communicated by Teiichi KoBAYASHI, M. J. A., Sept. 12, 1978)

Introduction. The Mesozoic part of the Shimanto Group is ex- tensively exposed on the Pacific side of Southwest Japan but is generally poor in mega-fossils, except for a special case of the Uwaj ima Group in western Shikoku. Enthusiastic hunting by country people, however, has occasionally met with lucky finds of ammonites and other mega-fossils. They have tied up with specialists in palaeontology to give rise to scientifically competent results. In 1977 one of us (the last author, S.S.) was afforded by such a lucky find of an ammonite, while he was studying a local field geology for an educational purpose. He took contact with the first junior author (H.O.) of Shizuoka University, who in turn sent the obtained ammonite specimen to the senior author (T.M.) for a palaeontologic study. We revisited twice the fossil locality and neighbourhoods to look at the geology, without, however, getting more specimens. Before going further, we thank Professor Emeritus Teiichi Kobayashi, M. J. A., for his encouragement to report this record and to Dr. Akira Tokuyama for his kind suggestion about the geology of the Akaishi Mountains. This study was in part supported by a Grant- in-Aid for Scientific Researches from the Ministry of Education (Grant No. 246037) to H. Okada. Geological setting. The fossil locality is in a small gully run- ning northeastward to a branch at Tadaki of the River Futamata, which is a tributary of the River Tenryu. It is at about 5750 m northeast from the Kashima Bridge of Highway 152 in the city of Tenryu (Shizuoka Prefecture) (Fig. 1). This locality is situated in the Akaishi Mountains or the Akaishi Massif. As is clearly shown in the recently published geological map of Shizuoka Prefecture (Tsuchi, ed., 1974), the Akaishi Massif is made up of (A) Sambagawa and Chichibu belts of mainly Palaeozoic rocks in the northwestern part, (B) Shimanto belt of Mesozoic rocks in the central main part, and (C) the Setogawa belt of Lower Tertiary rocks in the southeastern part. The structural trends of these rocks are predominantly NE-SW, but the general distribution is laterally displaced by two faults of N-S trend in the western part (Kimura, 1961). The River Tenryu runs approximately in parallel with these 322 T. MATSUMOTO, H. OKADA, and S. SAKURAI [Vol. 54(B),

Fig. 1. Map showing the location of ammonite (small solid circle). Fig. 2. Geological framework of the Akaishi Massif (adapted from Tsu- ch i (ed.), 1974). A-C : See text. D : Neogene and Quaternary. Location of ammonite indicated by a small solid circle.

N-S faults and the newly found fossil locality is in this displaced part of the Shimanto belt (Fig. 2). The rocks in this displaced part of the Shimanto belt were indicated as the Komyo Formation (or "Group") in the Geological Map of Toyohashi, scale 1: 200,000 (compiled by the Geological Survey of Japan, 1955). This formation was described as consisting of sand- stone, clayslate, chert, schalstein and limestone and regarded at least partly as Jurassic in age. Tokuyama (Tsuchi, ed., 1974) has revised the stratigraphy of the Shimanto Group in the Akaishi Mountains in ascending order as follows : (1) Pelitic crystalline schist (2) Phyllitic black shale (3) Lower turbidite (with metabasalt in lower part) (4) Tuffaceous shale (with andesitic tuff) (5) Upper turbidite (6) Black shale What was indicated as Komyo Formation in the previous map in the displaced part along the wrench fault has been revised to the third formation by Tokuyama, who regarded the age of formations (2) to (6) as Cretaceous and that of formation (1) as possibly Jurassic from No. 7] Cretaceous Ammonite from the Akaishi Mountains 323

Fig. 3. Hauericeras sp. cf. H (Gardeniceras) rembda (Forbes). Septal sutures and a constriction (dotted). Bar indicates 1 mm. (T.M. delin.)

his comprehensive viewpoints. The evidence of fossils was, , however, very poor. The strata exposed at and around the fossil locality of Tadaki are highly contorted and cleaved black shale with detached beds of sand- stone. Some limestone, chert and basaltic green rocks are met with, although their exact stratigraphic position has not yet been deter- mined. The black shale contains calcareous nodules of various sizes and the ammonite was found in one of these nodules. That nodule is rounded, fairly large, about 150 cm x 200 cm in diameters, and contains some other indeterminable molluscan shells. It is sitting on a slope of the gully, but the gully is so narrow that its derivation from the black shale exposed in the gully is doubtless. In fact there are more calcareous nodules enclosed in situ in the shale, although mega- fossils were not found in them. Whether the formation exposed at and around the fossil locality of Tadaki really belongs to the third formation ("Lower turbidite") of Tokuyama or otherwise may be questionable, because the exposed rocks are not of typical turbidite f acies. Palaeontologic note. The ammonite specimen obtained from the dark grey calcareous nodule at Tadaki is fragmentary. It is a part (about a quarter) of a whorl of about 30 mm in diameter. It is higher than broad, having parallel, almost flat to slightly convex flanks and a sagittate and then keeled venter. Surface is smoothish, but for a well marked constriction, which is gently prorsiradiate and sig- moidal on the flank and considerably projected on the venter. Suture is well exposed on the main part of the flank, except for the destroyed umbilical part, showing such a pattern as illustrated in Fig. 3. Despite the incomplete preservation, the observed characters show that this ammonite belongs to the genus Hauericeras of the 324 T. MATSUMOTO, H. OKADA, and S. SAKURAI [Vol. 54(B),

Desmoceratidae and that it resembles closely H. (Gardeniceras) rembda (Forties). For the time being it can be called Hauericeras (Gardeniceras) cf. rembda (Forties). It is now preserved at Shizuoka University (No. S G 78001). Hauericeras (Gardeniceras) rembda (Forties) originally occur- red in the Valudayur Bed (Ariyalur Group) of southern India (Forties, 1846; Kossmat, 1898). Sastry et al. (1968) established the Zone of Hauericeras rembda in the Ariyalur Group and correlated it with the Lower Maastrichtian. The same species occurs also in the Lower Maastrichtian of Madagascar (Collignon, 1971). Hauericeras (Gardeniceras) cf. rembda has already been recorded in Japan from the Lower Sandy Siltstone of the Hakobuchi Group in Hokkaido and the Shimonada Sandy Siltstone of the Izumi Group in Awaj i Island of Southwest Japan (Matsumoto and Obata,1955) . These formations are correlated with the Maastrichtian. The genus Hauericeras, including the subgenera Hauericeras (s.s.) and Gardeniceras (Matsumoto and Obata, 1955), ranges from Coniacian to Maastrichtian. Accordingly the present find of H, sp. cf. H. (G.) rembda from Tadaki supports the Tokuyama's assignment of the main part of the Shimanto Group in the Akaishi Mountains to the Cretaceous and furthermore suggests the presence of the Maastrichtian in a certain part of the Shimanto Group.

References

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