日本の白亜系の大型化石による分帯と西欧模式地との対比

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  • Mega-fossil zonation of the Cretaceous System in Japan and correlation with the standards in Western Europe

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Ammonites and certain other mollusks are useful for the zonation and inter-regional correlation of the Lower Cretaceous in Japan. The faunas are mostly of Tethyan or Sub-mediterranean affinity but Boreal elements are occasionally intermingled. In addition to them North Pacific element scome into beds of certain ages. There are ammonite bearing continuous sequences from the Tithonian to Berriasian (or even to Lower Valanginian?) in southern Kitakami and Abukuma areas on the Pacific side of Northeast Japan and also in certain places of Southwest Japan. For the precise correlation, the faunules (Protacanthoceras-Parakilianella-Spiticeras-Thurmanniceras) should be restudied. The main part of the Valanginian and Lower Hauterivian are free from ammonite and represented generally by the non-marine beds (Ryoseki Formation etc.) or by an unconformity. Upper Hauterivian is indicated by the bed with Crioceratites ishiwarai in Kitakami and then that with Pseudothurmannia hanouraensis in Shikoku. The marine Barremian are well distributed from Hokkaido to Kyushu, of which the Kimigahama (Choshi), Ishido, Arida, and Monobe Formations are better studied. The bed with Pulchellia ishidoensis-Barremites difficile, that with Shasticrioceras nipponicum-Paracrioceras aff. P. elegans and then that with Heteroceras aff. H. astieri-Barremites strettostoma in ascending order are approximately correlated with the related zones in the strato types of Europe, although palaeontologic descriptions are utterly needed. The uppermost Barremian and the lowest Aptian seem to be lacking or represented by non-marine beds and there are unconformities of various grade in many places. The main part of the Aptian is represented by the Choshi and the Miyako Groups which exhibit successively the Zones of Australiceras aff. A. gigas-Dufrenoyia aff. D. dufrenoyi-Tropaeum bowerbanki, Epicheloniceras martinioides, Parahoplites aff. P. vectense, Hypacanthoplites subcornuerianus and Diadochoceras nodosocostatiforme-Eodouvilleiceras matsumotoi. They are correlated with the standard zones in southern England and France. Some of the faunas have been already monographed and others await descriptions. The Albian faunas have been rather poorly reported and they do not contain the hoplitid ammonites which are important zonal indices in the Boreal province of Europe, Douvilleiceras mammillatum from the upper part of the Miyako Group indicates the upper Lower Albian and another faunule in the Yatsushiro Formation of Kyushu contains Epileymeriella, Platyknemiceras and Brewericeras, suggesting upper Lower Albian. No good indicator of the Middle Albian has been described, although there should be some. The Upper Albian is represented better by marine beds in various areas from Hokkaido to Kyushu. Dipoloceras, Diplasioceras and Adkinsites are found in its lower part and Mortoniceias (Deiradoceras), M. (Mortoniceras) and certain species of Hamites, Mariella, Pseudohelicoceras etc. in its upper part, but a further zonal subdivision is difficult. The Upper Cretaceous is afforded with fairly rich faunas in Hokkaido and other areas in Japan, of which ammonites of various habitats, inoceramids and certain others are useful for the zonation andinterregional correlation. They include world-wide species, Indo-Pacific, North Pacific elements and at limited levels several elements of Tethyan affinity. The scheme of zonation has been presented (e.g., MATSUMOTO, 1977; TAKAYANAGI & MATSUMOT0, 1981), but it is tentative and should be revised as our work proceeds. The zones defined by ammonites of near shore shallow sea facies are useful for the correlation with the standard zones in western Europe and else where, those defined by ammonites of off-shore facies and certain inoceramid groups are so for the correlation with the zones in other areas around the North Pacific or with in the Indo-Pacific realm. In the Cenomanian the ascending Zones of Graysonites wooldridgei, Mantelliceras japonicum, Acanth

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