The Middle Pleistocene Basin-fills and Higher Terraces in the Takanosu Basin, Northeastern Japan

Abstract

The Takanosu Basin is located in the northern part of the Dewa Hills, the inner zone of the Northeastern Japan Arc, and filled with a series of deposits in which the Yuguruma Formation takes the uppermost part.
The Yuguruma Formation is composed mainly of lacustrine blueish gray tuffaceous silt and contains intercalated gravel, sand, acid tuff, and lignite layers. Its total thickness is estimated to beabout 200 meters or more. Significant tectonic deformation of the formation is not recognizable. Paleomagnetic measurements indicate that the Yuguruma Formation is Middle Pleistocene in age, and its deposition finished a little later than 0.35 Ma.
There are three higher terrace surfaces in the Takanosu Basin, i.e., the T-H1, T-H2, and T-H3 surfaces. The T-H1 surface provides the depositional surface of the Yuguruma Formation. The T-H1, T-H2, and T-H3 are indirectly dated as ca. 0.35-0.44 Ma, ca. 0.20 Ma, and ca. 0.16-0.19 Ma respectively, on the basis of accumulation rate of weathered tephra which covers the surfaces and intercalates dated Late Pleistocene marker-tephra layers.
The presence of the higher terraces underlain by the thick sediments indicates a change in tectonism from subsidence since early Pliocene time (the end of Funakawa age) to uplift all over the basin in Middle Pleistocene time, because the Yuguruma Formation emerged and the T-Hl surface was formed around 0.35 Ma. The tectonic development of the besin has still continued to deformed the T-H3 surface and Late Pleistocene T-Lla surface.