Abstract
The Lower Cretaceous Choshi Group, Upper Cretaceous Nakaminato Group and Paleogene Oarai Formation show isolated distribution in the eastern end of the Kanto Plain along the Pacific coast, central Japan. The current status of their geological studies and significance are reviewed, because they are important for reconstructing the tectonic relation between Northeast Japan and Southwest Japan. On the basis of age, stratigraphy and depositional facies, there are no correlative strata with the above in the Cretaceous-Paleocene Yezo forearc basin sediments of Northeast Japan. Despite the difference in facies, the Barremian to Upper Aptian Choshi Group seems to be an eastern extension correlative with the Sanchu Group of the Chichibu Belt in the northern Kanto Mountains, Southwest Japan. The Upper Campanian to Lower Maastrichtian Nakaminato Group has some characteristics common to the Izumi Group along the southern part of the Inner Zone of Southwest Japan. It forms a fault-bounded basement-uplifting block associated with the Oarai Formation and Miocene sediments within the southern extension of the Tanakura Tectonic Zone. The Oarai Formation is thought to be Paleogene in age based on isotopic age of gravel, plant fossils and geologic setting. It may be correlative with the Yorii and Kanohara Formations in the northern Kanto Mountains, judging from lithofacies and tectonic setting.
