Upper Jurassic to Lower Cretaceous Ikenoue Formation around the Kurosegawa Terrane in the western Kii Peninsula, Southwest Japan

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The Ikenoue Formation in the western Kii Peninsula, Southwest Japan, is formally defined here. From viewpoint of the Kurosegawa Terrane, as the tectonic nappe above the Jurassic sedimentary complex of the Chichibu Terranes, significance of the Ikenoue Formation is discussed.The Ikenoue Formation is a geological unit characterized by well-bedded clastic sequences of early Late Jurassic to middle Early Cretaceous time. The Ikenoue Formation can be divided into three members based on lithology. The lower, middle and upper members of the formation are respectively correlated with the Tatego Member of the Obiki Formation, the Yura Formation and the kamiya Formation in the Chuki Group of the Southern Chichibu Terrane by the radiolarian fossil age. The lower and middle members of the Ikenoue Formation are correlated with the Late Jurassic well-bedded clastic sequences, which overlie the constituents of the Kurosegawa Terrane. Thus, the Ikenoue Formation was formed as cover sediments on the Kurosegawa and pre-Late Jurassic Southern Chichibu Terranes.The Kurosegawa and pre-Late Jurassic Southern Chichibu Terranes must have already been conjugated when the Ikenoue Formation was deposited. It is, therefore, concluded that the Kurosegawa Terrane was formed as a tectonic nappe above the Jurassic sedimentary complex of the pre-Late Jurassic Southern Chichibu Terrane during early Late Jurassic time.
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