Abstract
The Seto Group, one of the representative Upper Cenozoic fluvial sediments in Japan, is distributed in the area from the eastern Nagoya to the Tono district. The group is important to the discusssion of the Late Cenozoic tectonics of the Southwest Japan arc, because it records the formational process of the intra-arc basin of the arc. The stratigraphy of the group, however, have been confused around Ena City in the Tono district. Through a detailed field survey, this paper reveals the lithostratigraphy of the group around Ena City. To improve the confused stratigraphy, the principal reference section of the Toki Sand and Gravel Formation in the Seto Group is designated at Ohi, Ena City. The formation attains to 92m thick there, and generally to more than 130m. It is divided into the Lowermost Gravel, Lower Gravel, Middle Sand and Mud, and Upper Gravel Members in ascending order. Stratigraphical, paleontological and radioisotopic data show that the formation deposited mainly during the Pliocene. The formative process of the Ena basin is to be reconstructed through the systhesis of the revealed stratigraphy and structural analysis of the basin, in the coming paper.
