<国立科博専報>志摩半島白亜系の時代論

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The Izumigawa Formation and the Lower Member of the Gokashoura Formation of Nansei Group in the western Shim Peninsula (Fig. 2) both contain brackish molluscs of the Shiroi type (MATSUKAWA, 1979) and neritic trigonians. Thus, they seem to represent sediments of Neocomian age. The two formations resemble each other in petrological characters of sandstone (SAKA et al., 1979). Judging from their lithofacies and index fossils, both formations are nearly at the same stratigraphic level. Both are nearly contemporaneous or, at least, may be regarded as successive sediments in the same basin during the Neocomian. The Lower Mudstone Member of the Gokashoura Formation can be assigned to the Barremian, because it contains two echinoid species, Aphelaster serotinus TANAKA et SHIBATA and Heteraster yuasensis (TANAKA et 0KUBO) (Plate 9). The Lower Cretaceous strata distributed in the western Kii Peninsula (OBATA and OGAWA, 1976) and the Nansei Group in the western Shima show similar bio- and lithofacies (Figs. 3,4). As a whole, Barremian to lower Aptian (?) sediments are approximately 700m in thickness in both areas. The quartzose sandstone of the Nishihiro Formation in western Kii and that of the Gokashoura Formation closely resemble each other petrologically, and they are regarded as deltaic sediments near a river mouth (SAKA et al., 1979). Three possible interpretations are presented to explain the occurrences of marine invertebrates and brackish molluscs in the Arida, western Kii, the Shima, and the Sanchu areas (Fig. 5). They will be useful for future studies of these areas until a definite solution is found. It is necessary to obtain enough fossil evidence in the upper half of the Gokashoura Formation in the Shima Peninsula as well as in the upper half of the Nishihiro Formation in the Arida area, in order safely to define the upper limit of the Shiroi type molluscan occurrence.
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<国立科博専報>志摩半島白亜系の時代論

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