1989 Volume 43 Issue 6 Pages 402-416
The Kobiwako Group, Pliocene to Pleistocene in age, is distributed in the Iga and the Omi basins. The sedimentary basin of this group originated around the Ueno basin in early Pliocene times, about four millon years ago, and subsequently migrated to presentday Lake Biwa. Four stages are distinguished with respect to the migration of the sedimentary basin. The first to forth stages are represented by the sedimentary basins of the Ueno, the Iga and the Ayama Formations, of the Koga Formation, of the Gamo and the Kusatu Formations and of the Katata and the Takashima Formations, respectively. The basin in each stage is an association of the depressed fault blocks which are divided by the faults and flexures trending in north-northwest, east-southeast and north-northeast directions. Along the margin of the basin of each stage, steep angle uncomformities are common. Their traces are rather stright and trend in the same directions as the faults and flexures in the sedimentary basin. Debris flow and/or talus deposits consisting of poorly sorted angular to subangular gravels is common along the steep unconformities. Faults with same trends as those of the unconformity are usually found in the basement rocks covered by the steep unconformity. Thus it can be inferred that the sedimentary basin of the Kobiwako Group arose through fault movemens and rapid subsidence of the fault blocks, and that the northward migration of the sedimentary basin was controlled by sifting of the area of fracturing and subsidence of basements. Each block in the sedimentary basin appears to have been tilting during the deposition of the group, because the vertical profile of the sediments on each block shows a wedge-like shape. Not only lacustrine clay but also deltatic sand buried thickly up the subsiding zones on the tilting blocks. The sedimentary basin of the Osaka Group also shows fault block structure. Lateralvariation in thickness of the group indicates that subsidence of the basin progressed through tilting of the fault block since its embryonic stage. From above mentioned facts, it can be stated that the Pliocene to Pleistocene sedimentary basins in the Kinki district arose through block movements and tilting subsidence of faultblocks. Fault blocks composing the Pliocene to Pleistocene sedimentary basins in the Kinki dis-trict are sized about 30 km in the northern parts of the basins, and more than 10 km in the southern parts. But they are only a few kilometers in the middle part of the basins. Because the belt of small scale block coincides approximately with the boundary of basement geology between the Ryoke Complex and the Mino-Tamba Belt, such a decrease in dimension of fault block appears to have been affected by pre-Neogene geologic structures.
