Radiolarians as tracers for provenance of gravels in Lower Cretaceous molasse (Outer Zone of SW Japan)

This paper discusses the results and the recent status of the study by radiolarians as tracers for erosional events, especially for those of Jurassic and pre-Jurassic accretionary complexes in the Outer Zone of SW Japan. As a test case to clarify the provenance of radiolarian-bearing gravels in conglomerates, the author reviewed the work previously done to trace the source rocks within oceanic-plate stratigraphy successions from monomictic chert-pebble conglomerates of the Lower Cretaceous Ryoseki-Monobegawa group. The group is a molasse typical of paralic sedimentary facies in the Outer Zone of SW Japan.De l'utilisation des radiolaires comme indicateurs de provenance des graviers dans les molasses crétacées inférieures (zones externes du SW Japon). Le présent article discute des implications qu'a l'utilisation récente des radiolaires comme indicateurs d'événements érosifs, en particulier de ceux de complexes d'accrétion jurassiques et anté-jurassiques au SW Japon. Comme étude de cas pour éclairer la provenance de graviers à radiolaires des conglomérats, l'auteur effectue une revue des séries stratigraphiques synthétiques à partir des conglomérats monogéniques à galets de cherts du groupe Ryoseki-Monobegawa du Crétacé inférieur. Ce groupe molassique est de faciès sédimentaire paralique typique des zones externes du SW Japon

TAXONOMY AND FAUNAL AFFINITY OF LATE CARNIAN ­ RHAETIAN CONODONTS IN THE SOUTHERN CHICHIBU BELT, SHIKOKU, SW JAPAN

Conodonts from the latest Carnian - Middle Norian pelagic limestone and Late Norian - Rhaetian bedded chert-section at Hisaidani, southern Chichibu Belt, Shikoku, SW Japan, are described. Taxonomy of conodont-genera Metapolygnathus Hayashi, Ancyrogondolella Budurov, MockinaKozur and Misikella Kozur & Mock is discussed. Middle Norian Mockina sakurae n. sp., M. shamiseni n. sp. and Late Norian M. hisaidaniensisn. sp. are described.The affinities of the Hisaidani - fauna are rather Tethyan. The Early - Middle Norian conodont-association at Hisaidani is dominated by Ancyrogondolella spatulata (Hayashi). Towards the Middle and Late Norian, affinities with Pacific taxa are present. The three new species provide also a provincial character that typifies the pre-accretionary Izanami plateau from which the rocks of Hisaidani were derived.&nbsp

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

Carnian-Rhaetian Conodonts from SW Japan

Vengono descritti in questo articolo conodonti provenienti dai calcari pelagici del Carnico terminale - Norico medio e dalle selci stratificate del Norico superiore - Retico della sezione di Hisaidani, Chichibu Belt meridionale, Shikoku, Giappone sudoccidentale. Viene discussa la tassonomia dei generi Metapolygnatbus Hayashi, Ancyrogondolella Budurov, Mockina Kozur e Misikella Kozur & Mock. Sono proposte tre nuove specie: Mockina sakurae n. sp. e M. shamiseni n. sp. del Norico medio e M. hisaidaniensis n. sp. del Norico superiore. L'affinità della fauna di Hisaidani è abbastanza orientata in senso tetidiano. L'associazione a conodonti di età Norica inferiore e media è dominata da Ancyrogondolella spatulata (Hayashi), mentre verso I'alto, nel Norico medio e superiore sono presenti taxa della provincia pacifica. La presenza di tre nuove specie suggerisce anche un provincialismo che caratterizza la situazione pre-accrezione del plateau Izanami da cui derivano le successioni rocciose di Hisaidani.Conodonts from the latest Carnian - Middle Norian pelagic limestone and Late Norian - Rhaetian bedded chert-section at Hisaidani, southern Chichibu Belt, Shikoku, SW Japan, are described. Taxonomy of conodont-genera Metapolygnathus Hayashi, Ancyrogondolella Budurov, Mockina Kozur and Misikella Kozur & Mock is discussed. Middle Norian Mockina sakurae n. sp., M. shamiseni n. sp. and Late Norian M. hisaidaniensis n. sp. are described. The affinities of the Hisaidani - fauna are rather Tethyan. The Early - Middle Norian conodont-association at Hisaidani is dominated by Ancyrogondolella spatulata (Hayashi). Towards the Middle and Late Norian, affinities with Pacific taxa are present. The three new species provide also a provincial character that typifies the pre-accretionary Izanami plateau from which the rocks of Hisaidani were derived

Izanami Plateau : Japan's Triassic low latitude carbonates

The Izanami plateau is the oceanic origin of Triassic pelagic limestones in Jurassic and Early Cretaceous accretionary complexes (AC) of Japan. Well dated by conodonts and radiolarians, the Triassic ocean type carbonate plateau, from where blocks, in the Mino-Tamba (Ashio), Mikabu and Chichibu belts, are derived, has a characteristic history of deposition related to seamount, hot-spot and plate-motion. The Izanami plateau is characterized by the changing nature of its substrate, from Paleozoic platform carbonates in more internal areas, to pillow basalts in more external zones, and by the younging of the base of the Triassic carbonates covering that substrate. In the more internal zones, depositional hiatuses relate to post - Aegean and Early Carnian emersions. Lower - Middle Norian condensation preceded the drowning of the platform in the more external zones, where Late Norian and Rhaetian strata consist of bedded chert only. Faunistically, the realm of the Izanami plateau has more Tethyan than Pacific conodont affinities with some specific endemism. The low latitude Izanami plateau extended over a large area, off the Yangtse block, prior to its inclusion in Jurassic to Early Cretaceous AC

Analysis of mesoscopic deformation structures in mélange with special respect to the stages of syn- and post- sedimentation : A case study in the Southern Chichibu Terrane in eastern Shikoku, Southwest Japan

In this study, the term "mélange" is used as describing several muddy matrical rocks that are characterized by a chaotic, relatively disturved,"block in matrix" fabric. To find a clue to clarify the origin and the mechanism of the mélanges in the Southern Chichibu Terrane in Southwest Japan, the author examined the analyses of the mesoscopic deformation structures of mélanges at Warabiishi coast in eastern end of Shikoku, where widely crops out the Early Cretaceous succession including muddy chaotic rocks with many blocks of limestone. chert and greenstone. The results can be summarized as follows. 1. The deformation stages of the Warabiishi mélange is subdivided. into three stages. 1) The first stage characterized by rupturing of limestone blocks to form jigsaw-puzzle structure under the condition of high confining- and high pore water- pressures possibly caused by mud-diapirism before the succession of the Warabiishi Unit formed. 2) The second stage is the syn-sedimentary deformations which occur mixing by grabity flows and slidings of non- and semi- consolidated deposits along the lower continental slope to form olistostromes and slump deposits with turbidites. 3) The third stage is the formation of shear structus such as right-stepping en échelon shear fractures (Riedel shear fractures with compressive bridges) and clockwise-back-tilted sigmoidal blocks with ductile shear tails by E-W trending left-lateral simple shearing on condition that the sediments are relatively dewatered and consolidated after the sedimentation of the Warabiishi unit has finished. 2. Based on the sedimentary structures and facies analyses, Warabiishi area of the Southern Chichibu Terrane in Early Cretaceous age is situated in southward inclined lower continental slope with submarine fans where the sediments are supplied by sediment gravity flows as such turbidites and olistostromes partly originated in mud-diapears. 3. The faults controling the formation of southward facing imbricate structure of the Southern Chichibu Terrane moved not only as thrusts but also as E-W trending left-lateral simple shearig faults according to the presence of the above Riedel shear fractures and sigmoidal blocks of the third stage in the Warabiishi mélange along the Butsuzo Tectonic Line. 4. The above-mentioned tectonic features in addition to the southward younging of the mélanges age in the Southern Chichibu Terrane are concordant phenomena with the accretional zone settigs of active convergent margin of the plate tectonics model