Abstract
Pliocene paleogeography of the western Shinjo Basin, Yamagata Prefecture was reconstructed by analyzing sedimentary facies, paleocurrent directions, and thicknesses of the depositional sequences. It revealed the regression history of the Shinjo Basin, which was originally formed as a part of a NW-deepening rift basin crossing the area now raised as the Dewa Hills. A rapid regression was recognized in the southwest of the basin in the Early Pliocene, ca. 5 Ma, then in the north, and finally in the middle part of the basin. The paleogeographic change is attributed to the inception of uplift of the Dewa Hills at 5 Ma. The uplift started in the south, then in the north prior to the central part of the Dewa Hills. This differential uplifting history has determined the location of the Mogami River which incises the central Dewa Hills.
