Detrital zircon U-Pb ages were measured for nine non-fossiliferous sandstones from the uppermost Izumi Group, a “Late Cretaceous” forearc clastic sequence in the eastern Izumi Mountains of western Kii Peninsula, SW Japan. Seven out of nine sandstones yielded Paleocene grains. These results confirm that the uppermost Izumi Group was deposited in the Paleogene (Selandian-Thanetian or younger), extending the total depositional duration of the group to ca. 27 myr, almost double the previous estimate. The new age data raise the possibility that a stratigraphic interval across the Cretaceous-Paleogene extinction boundary may be preserved within the group. The occurrence of Paleogene strata further constrains the onset of the low-angle Median Tectonic Line to post-Paleocene.