Abstract
The Shigaraki-Yamato Plateau is situated in the central part of the Kinki Triangle (HUZITA, 1962) characterized by many active faults and alternate alignment of small-scale mountains and basins. This plateau is one of such mountain blocks uplifted by faulting. The basement rocks of this area is mainly composed of the granitic rocks of the Ryoke Complex. The thin sedimentary blankets distributed in this area can be divided into the following four groups from the geohistorical view-points : the First Setouchi Supergroup (early to middle Miocene), the Muro Group (middle to late Miocene), the Second Setouchi Supergroup (Plio-Pleistocene) and terrace deposits (late Pleistocene). The chronological variety of these sedimentary blankets advantages us to consider the change of the "movement rate " of the basement rock bodies since the early Miocene. In this paper, some problems concerning such sedimentary blankets are described based on the regional geological map (1:100,000) of this area prepared for the study of neotectonics mentioned above. They are summarized as follows: The Jigokudani, Tsugeno and Soni Groups, all of which belong to the Setouchi Volcanic Rocks, were clarified to be correlated one another with the detailed stratigraphic relationships among them. Consequently, the Muro Group is newly presented as a pyroclastic rock group including the previous three groups. The Ofuku Gravel Bed, which shows the typical mountain gravels facies and is distributed in the western part of the Shigaraki Plateau, were ascertained to be correlated to the Shigaraki and Kitamata Gravel Beds, both of which are called "the high level gravel bed" by KONDO (1968). Therefore, the stratigraphical situation of them is very likely considered to be the lower part of the Iga Formation of the Kobiwako Group. The low-relief eroded surface, which is one of the topographical features of "plateau", is considered to have been formed during some cycles of transgression and regression since the Miocene. The original form of the surface, however, might existed as a primary peneplain before the Miocene. The present low-relief eroded surface can be therefore regarded as a sort of stripped peneplain. The faults, most of which are reverse faults in outcrops, can be divided into four groups in their trends, namely E-W, NE-SW, N-S and NWW-SEE trends. The maximum vertical displacement after the sedimentation of the Second Setouchi Supergroup attains to 300-350 meters. Inferred from the fact that the remarkable structural difference between the First and Second Setouchi Supergroups cannot be recognized, the major stage of faulting is considered to be at least after the late Pliocene. This stage coincides almost with the general tectonic phase in the Kinki Triangle (HUZITA et al., 1973)
