四国中・東部の前期白亜紀非海生二枚貝動物群

Cretaceous non-marine faunas in Central and Eastern Shikoku can be classified into the Masaki and Sakashu faunal groups on the basis of their taxonomic components and a new stratigraphy and tectonic subdivision of the Chichibu Superbelt. The non-marine bivalves from the Monobegawa Group in the Masaki Belt belong to the Masaki Faunal Group which are identified the Hauterivian Tatsukawa fauna, Late Barremian Yunoki fauna and Early Aptian Hibihara fauna. The non-marine bivalves from the Nankai and Takegatani groups of the Sakashu Belt belong to the Sakashu Faunal Group which are identified the Hauterivian Shobu fauna. The three faunas of the Masaki Faunal Group in Central and East Shikoku are found in stratigraphic order. The Masaki Faunal Group is characterized by the brackish- and freshwater mollusks. On the other hand, the Sakashu Faunal Group is characterized by the brackish-water mollusks. It suggests that salinity is the main differential factor between the Hauterivian faunas in the Masaki and Sakashu faunal groups. A Hauterivian mixed assemblage of the Tatsukawa and Shobu faunas was found in Central Shikoku. The finding suggests that the two faunas were formed in different salinity environment and adjoining areas.四国中・東部秩父累帯下部白亜系の非海生二枚貝類について種構成や群集の生息環境を考察し，再編された地帯区分との対応を検討した．これらの白亜紀非海生動物群は新たな秩父累帯の地帯区分及び種構成に墓づいて，互いに共通種を持たない正木動物群(Masaki Faunal Group)と坂州動物群(Sakashu Faunal Group)に区分される．正木帯に属する物部川層群の非海生二枚貝類は正木動物群に帰属し，立川フォーナ(Hauterivian)，柚ノ木フォーナ(Late Barremian)，日比原フォーナ(Early Aptian)で構成される．坂州帯に属する南海層群，竹ヶ谷層群の非海生二枚貝類は坂州動物群に帰属し，菖蒲フォーナ(Hauterivian)で特徴づけられる．四国中・東部における正木動物群は，汽水生一淡水生軟体動物で特徴づけられ，坂州動物群は汽水生軟体動物で特徴づけられる．このことは，坂州動物群は正木動物群より塩分濃度の高い環境で生息したことを示す．また，四国中央部では両動物群の混在が確認され，正木動物群と坂州動物群は，隣接して存在し，塩分濃度の異なる環境に生息したことが推測される

Stratigraphy of the Nankai Group in the Tosayamada area, Central Kochi, and correlation with the Monobegawa Group

The purpose of our study is to clarify stratigrapical correlation of the Nankai and Monobegawa Groups. This paper presents the results of srratigraphy, sedimentary facies and geologic time. The Cretaceous System of the Chichibu Terrain in the Tosayamada area is divided into the Monobegawa and Nankai Groups. The Monobegawa Group conformably overlies the Yasuba Formation(Permian) can be subdivided into three formations which are the Ryoseki, Monobe and Hibihara Formation. The Nankai Group can be subdivided into two formation which are the Funadani and lgenoki Formations. These are bounded to the North by Suita Tectonic line with serpentinite and to the South by Permian accretionary complex. The lower part of the Funadani Formation consists of 150m of massive conglomerate and sandstone. The upper part of this formation consists of black mudstone which bear ammonite and some bivalve fossils. The lgenoki Formation conformably overlies the Funadani Formation. The lower part of the Igenoki Formation consists of alternate layers sandstone and mudstone in which occur abundant bivalves and ammonites. They indicate the lower part to be Aptian. Some bivalves and ammonites are obtained from black mudstone which is the upper part of this formation. lgenoki Formation lies in fault contact with Permian accretionary complex which is named Yukigamine Formation. Stratigrafical sequences of both groups are very similar, therefore, the Nankai group is member of Kurosegawa Tarrain

Lower Cretaceous litho-and bivalve-stratigraphy of the Sakawa-Ochi area, Kochi Prefecture, SW Japan

The Ochi-Sakawa area is one of the classic fields of Paleozoic and Mesozoic stratigraphy in the tectonic Kurosegawa Belt in of Eastern Shikoku, Japan. The Mesozoic is characterized by shallow marine and brackish water deposits, in which Triassic and Jurassic shallow marine deposits are famous for their abundant megafossils, the Cretaceous deposits remaining until now largely unknown. Located within the Chichibu Superbelt* the Cretaceous formations of the Ochi-Sakawa area consists of two tectonic blocks, being separated by a north-south trending fault, within the Yoshigahira and Sakashu belts. In both blocks, the Lower Cretaceous formations consist in ascending order of the Kaisekiyama Formation (Hauterivian), the Tosakamo Formation (Barremian-Aptian), the Kurohara Formation (Aptian), and the Yotsushiro Formation (Albian). Brackish water bivalves occur in the Kaisekiyama Formation and lower part of the Tosakamo Formation. Higher up, in the in the lower and middle parts of the Tosakamo Formation, Barremian shallow marine bivalves and in the upper part of the formation, Aptian marine bivalves occur. The Kurohara Formation yields Aptian shallow marine bivalves and the Yotsushiro Formation yields Albian marine bivalves and ammonites. The Early Cretaceous faunas of the Sakawa block is characterized by the so-called Sakashu and Masaki faunal groups, while that of the Ochi block contains elements of the so-called Masaki faunal group. The differentiation of these faunas suggests a faunal transition between different environments within the Yoshigahira and Sakashu belts. *Kumosoyama, Masaki, Yoshigahira, Sakashu and Nakagawa belt

四国中央部黒瀬川帯混在型群集

The Early Cretaceous bivalve fauna from the Kurosegawa Belt in Central Shikoku was studied with respect to the faunal affinity. In the Kurosegawa Belt, two faunal-types of Hauterivian bivalves have been recognized (Kozai et al., 2002). The one, Tatsukawa-type from the Monobegawa Group is distributed in the Northern Kurosegawa Belt. Another one, Shobu-type from the Nankai and Takegatani groups is distributed in the southern Kurosegawa Belt. Based on the specific compositon, the former is regarded as "Tethyan fauna", and the later is regarded as "northern Tethyan fauna" of Tashiro (1994, 2000). The specific compositions of these two faunas are quite different except for the cosmopolitan species. A transitional fauna between the Tatsukawa-type and Shobu-type was newly discovered in Hauterivian sandstone bed of brackish facies in the north-south intermediate part of the Kurosegawa Belt. Among the six local species, Protocardia ibukii, Costocyrena otsukai, Pulsidis antiqua and Tetoria Sanchuensis are the Tatsukawa-type elements. Isodomella matsumotoi and Eomiodon matsumotoi are the Shobu-type elements. In addition to the finding of the marine Barremian "Tethyan"-"Northern Tethyan" transitional bivalve-fauna, new discovery of the Hauterivian brackish trasitional-fauna is significant for the analysis of paleobiogeography and tectonics during and after the Early Cretaceous. Especially, the existence of the transitional fauna indicates that the "Tethyan" and "Northern Tethyan" faunas are paleobiogeographicaly closer

Mollusks from the Barremian Lower Hanoura Formation, Tokushima Prefecture

Early Cretaceous Bivalves from the Lower Hanoura Formation of the Monobegawa Group in Tokushima, East Shikoku, are described. Forty eight species of thirty seven genera, including four new species, are dealt with. The species are as follows: - Nucula sp., Nuculopsis shidoensis (Yabe and Nagao), Portlandia sanchuensis (Yabe and Nagao), Mesosaccella choshiensis Hayami, Cosmetodon nipponicus (Nagao), Cucullaea obliquata (Amano), Nanonavis yokoyamai (Yabe and Nagao), Barbatia hiroyasensis sp. nov., Glycymeris matsumotoi Tashiro, Modiolus obatus sp. nov., Amygdalum ishidoense (Yabe and Nagao), Pinna sp. cf. P. robinaldia d’Orbigny, Pterinella shinoharai Hayami, Gervillia forbesiana d’Orbigny, Isognomon ichikawai Hayami, Entolium sanchuense Hayami, Entolium ikedai Tashiro, Entolium nishikawai, sp. nov., Neithea hanourensis Tashiro and Kozai, Neithea atava (Römer), Parvamussium kimurai (Hayami), Plicatula monobensis Tashiro and Kozai, Plicatula kiiensis Hayami, Limatula Nagaoi Hayami, Amphydonte sp. cf. A. subhaliotoidea (Nagao), Lopha nagaoi Hayami, Rastellium carinatum (Lamarck), Rutitrigonia yeharai Kobayashi, Pterotrigonia pocilliformis (Yokoyama), Astarte subsenecta Yabe and Nagao, Yabea shinanoensis (Yabe and Nagao), Yabea sp. aff. Y. shinanoensis (Yabe and Nagao), Eriphyla sp. aff. E. oblonga Freneix, Anthonyaelongata sp. nov., Globocardium sphaeroideum (Forbes), Granocardiumbrevis Tashiro and Kozai, Granocardium ishidoense (Yabe and Nagao), Leptosolen amabilis Tashiro and Kozai, Ptychomya densicostata Nagao, Scittila japonica Hayami, Scitilla dericatostriata Tashiro and Kozai, Caestocorbla shikamai Hayami, Panopea plicata (Sowerby), Pholadomya miyamotoi Nagao, Gonyomya subarchiaci Nagao, Goniomya hayamii Tashiro and Kozai, Plectomya concentrica Tashiro and Kozai, Plectomya aritagawana Hayam

Discovery of Triassic microfaunas at Mae-Sot, NW Thailand (Preliminary report)

The Middle and Late Triassic radiolarian faunas were extracted from a vitric tuff rich laminated bedded-chert in the Mae-Sot and Umphang areas of NW Thailand. The radiolarian faunas from the bedded-chert succession are characterized by Triassocampe postdeweveri (Early Ladinian), Pachus multinodosus (Early Norian), Sarla natividadensis (Middle Norian) and Canoptum rhaeticum (Norian-Rhaetian), individually. The Triassic chert-sequence is overlain by the ‘Jurassic base-conglomerate’, an ill-sorted breccia in a reddish-silt matrix. The limestone and chert clasts in the conglomerate yield Early - Late Triassic conodonts and Middle - Late Triassic radiolarians, respectively. Chert clasts in the conglomerate yield among others Norian - Rhaetian radiolarians like Pachus multinodosus and Canoptum rhaeticum, while Early - Late Triassic conodonts as Platyvillosus costatus, Cratognathodus cf. cuspidatus, Metapolygnathus polygnathiformis, M. carpathicus, Ancyrogondolella quadrata and A. spatulata are found in limestone clasts. The silici-pelagic origin of the clasts suggests the presence of an ocean before the end Triassic orogeny along the Mae Sariang Zone that amalgamated the parts of the Shan-Thai block. This first finding of Late Triassic (Norian - Rhaetian) radiolarians from bedded-cherts, next to the Middle Triassic and older radiolarian faunas, adds another element to the reconstruction of the sequence now comprised in the Mae Sariang Zone, W of the Nan-Uttaradit Suture. The occurrence of Triassic limestone, as that of the Chaiburi Formation in the Phatthalung area of the Mae Sariang Zone or the Kodiang Limestone in the ‘Western zones’, may elucidate the question about the provenance of the Triassic conodont-bearing limestone clasts in the Jurassic base-conglomerate that seals the Mae-Sariang Zone. The newly dated Triassic sequence is further sealed by the continental-shelf deposits of the Toarcian - early Bajocian Hua Fai Group