Here we report foraminifera and calcareous algae discovered within calcareous clastic gravels of the Sekinoiriya Formation in the Yokozemachi Group of the Neogene Chichibu Basin. The fossil assemblages are comparable to those of the Takaoka Formation in the eastern margin of the Kanto Mountains and the Cretaceous deposit of the Sanchu Graben. The fossils indicate that the Cretaceous system existed to the east of the Chichibu Basin as a source of clastics during the deposition of this formation.
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