Sedimentary basins of the Seto Group(Morphology and formative mechanism of late Cenozoic sedimentary basins, especially of tilting basin)

Abstract

The Seto Group, late Miocene to early Pleistocene in age, is widely exposed in the Tono district, Gifu Prefecture, and the Owari district, Aichi Prefecture. Our studies have revealed structures of sedimentary basin of the Seto Group in the Tono district. The sedimentary basin is an assemblage of small collapse basins which are enclosed with high angle unconformities. These small collapse basins are quadrangular in shape, and their sides range from 500 m to 2 km in length. The first-stage collapse occurred in the late Miocene to generate the small sedimentary basins sporadically. Fine-grained sediment of the lower part of the Seto Group was supplied from the surrounding hinter land into the basins. Subsequently, the second-stage collapse took place in the early Pliocene to form a chain of small basins. These small sedimentary basins were linked up and produced a big river valley. The big river transported coarse sediment of the upper part of the Seto Group from its upper stream into the fluvial linked-basins. Further, we clarify the geohistorical relationship among the Seto Group, the Tokai Group, and Tokai-ko (Lake Tokai). The paleogeographic change is also discussed.