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.

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The Ena basin is located in the easternmost part of the intra-arc basin of South-west Japan, and accumulated the Late Cenozoic Second Setouchi Supergroup. This paper reveals the occurence and evolution of the Ena basin on the basis of the analysis of distribution and lithofacies of the basin fills, the Upper Cenozoic Seto Group and the Quaternary. The Ena basin is a composite fault basin of several smaller-scale basement blocks with peripheries 5 to 10 km long. The basin has been formed through a series of faulting, collapsing and tilting. Such block movements had occured intermittently in the Pliocene, and accelerated in the Quaternary. The evolutional feature of the Ena basin controlled by the block movements may be common to the whole intra-arc basin of Southwest Japan.

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This paper discusses the Quaternary stratigraphy and tectonic movements around the northeastern marginal area of the Niigata Plain. Representative Quaternary in this area is divided into the Shimokonakayama, Nashinoki, Kushigata, Yokooka, Oosawa, and Harukiyama Formations in ascending order. They are overlain by the river terrace and alluvium deposits. The Shimokonakayama Formation comprises gravels and sands, and is about 10m in thickness. This formation unconformably overlies the Pliocene. The Nashinoki Formation is composed mainly of silt and sands, with channel fill gravels and colluvial gravels. This formation with thickness of 30m unconformably overlies the Shimokonakayama Formation. The Kushigata, Oosawa and Harukiyama Formations are debris flow deposits which are composed of boulder gravels. The Yokooka Formation comprises well sorted fine sands. The stratigraphic correlations between the study area and adjacent areas are clarified. Representative correlations are follows. The Shimokonakayama and Nashinoki Formation are correlated with lower Pleistocene Yamadera and middle Pleistocene Sasagami Formations, respectively. The Kushigta and Yokooka Formations are correlated with middle Pleistocene Gozu gravel bed and "Ancient Sand Dunes on higher terraces" respectively. In the southern part of the study area, upper part of the Quaternary deposit includes a large amount of grvels derived from the Kushigata Range, and each Quaternary formation overlaps the older strata accompanying the uplifting of the Kushigata Range and subsidence of the basin. These geologic features are very similar to one of the Sasagami area, where situated on the Shibata-Koide Tectonic Line. In the northern part of the study area, however, the newer layers hardly overlap the older strata in the hilly land, and the Quaternary deposits have a small amount of gravels derived from the hills back of the basin. The northern part of study area is situated in the north end of the Shibata-Koide Tectonic Line. According to these Quaternary features, block uplifting and subsiding is generally obvious on the Shibata-Koide Tectonic Line, but the north end of the line is characterized by gentle folding instead of block uplifting and subsiding.

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