<国立科博専報>上部白亜系宇和島層群の層序

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In the Shimanto terrane, the Outer zone of Southwest Japan, are developed thick and monotonous geosynclinal sediments, generally showing the zonal arrangement with a trend of NE-SW or so, that were accumulated during a period of time from late Mesozoic to mid-Tertiary. These sediments have been collectively called the Shimanto Supergroup. The Shimanto Supergroup of the Uwajima district, western part of Shikoku, comprises the Lower Cretaceous Kitanada Group and Chikanaga Formation, and the Upper Cretaceous Shitaba, Okuura and Hokezu Fromations and Uwajima Group (Fig.1.). Generally speaking, pelitic rocks are predominant in the Lower Cretaceous whereas sandstones with frequent intercalations of conglomerates are developed as well as pelitic rocks in the Upper Cretaceous. The Uwajima Group has been noted for its abundant occurrence of fossils and remarkable change in lithofacies. It is composed of thick neritic sediments and structurally characterized by block faulting, showing occasional brachy folds. Based on the cycles of sedimentation, the group is subdivided into six formations, i.e., Chiyoura, Tenjinzaka, Narukawa, Furushiroyama, Ishibiki and Yorimatsu, in ascending order, although they locally vary in lithofacies, thickness and occurrence of fossils (Figs. 4,6,8). From the evidence of inocerami and ammonite fossils contained, the Chiyoura, Tenjinzaka, Narukawa, Furushiroyama and Ishibiki Formations are assigned to the Inoceramus uwajimensis Zone, and the Yorimatsu Formation, to the Inoceramus amakusensis Zone. In short, the geological age of the Uwajima Group may range from the Coniacian to the Santonian. Another interesting fact is that the ornate ammonites, e.g. Forresteria, Protexanites and Subprionotropis, which occur commonly in neritic environments are found in the Uwajima Group. Smooth and thin-shelled ammonites, i.e., phylloceratids, desmoceratids and gaudryceratids, and heteromorph ones, e.g. Baculites, Scaphites and Polyptychoceras, are fairly common in the Group (Fig.8). In view of the sedimentary environment, the occurrence of ornate ammonites in the Group should be noted in connection with the Lower Cretaceous ornate species hitherto reported from the Shimanto terrane. The present paper also discusses the comparison of the Cretaceous strata in the Shimanto terrane between Shikoku and Kyushu.
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<国立科博専報>上部白亜系宇和島層群の層序

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