Abstract
We report the occurrence of Early Paleocene to Early Oligocene radiolarian fossils from mudstones of the Sagamiko Group in the Kanto Mountains, central Japan. The Sagamiko Group is dominantly a monotonous succession of sandstone and mudstone layers, and is split into two stratigraphic units: (1) coarse and fine sandstones with intercalated phyllitic black mudstones of the Gongenyama Formation, and (2) phyllitic black mudstones with intercalated medium to fine sandstones of the Seto Formation. Well-preserved radiolarian fauna were found in mudstones from four localities in these formations: samples G-015, S-001, S-002, and S-021. The G-015 fauna from the Gongenyama Formation contains Amphisphaera aotea, A. coronata, Lithelius minor, and Lithomespilus coronatus, suggesting an approximate Early Paleocene age. The fauna from the Seto Formation consists of Lophocyrtis aspera, Podocyrtis mirabilis, P. mitra, and Theocyrtis perpumila in samples S-001 and S-002, and Lophocyrtis (?) cavifundus, Theocorys bianulus, and Theocyrtis careotuberosa in sample S-021, corresponding to Middle Eocene and latest Late Eocene to Early Oligocene ages respectively. This is the first recorded evidence for Lower Paleocene units in the southernmost Kanto Mountains. In general, the distribution of Paleocene units in the Shimanto Belt is restricted, and the origin of the Lower Paleocene rocks, whether as part of an accretionary complex or as a forearc basin deposit, is still controversial. Detailed geologic mapping and stratigraphy of the Gongenyama Formation are needed to understand its depositional setting.
