大分平野の地形発達と地殻変動

Central Kyushu, which is located in the westernmost part of the Median Tectonic Line, has been investigated because of its geological and geomorphological importance in regional structures. Oita coastal plain is situated in the eastern part of Central Kyushu and terrace morphology is well developed in the area.<br> The present writer tried to clarify the mode of recent crustal movement in relation to the geological structure, after careful consideration of geomorphic history of the area.<br> Terrace surfaces distributing in Oita coastal plain are either marine or fluvial in origin. They are classified into seven levels, namely I, II, Ill, IV, V, VI and VI terrace surfaces in descending order. These surfaces were formed after Ozai sub-stage which is contemporaneous with Pyobugaura stage in southern Kanto.<br> Among these terrace surfaces, the V surface is most widely developed and its conti-nuity is very well. This surface consists of marine silt, and sand and pebble overlying the Oita Group with unconformity. The silt member filling the preceding dissected valley is called oka Formation and a bay environment is indicated from the molluscan fossils included in the member. Therefore, the V surface is considered to have been formed during the last major interglacial stage of Pleistocene age.<br> The I and II surfaces are distributed along the rivers, and these surfaces are of fluvial origin. The III surface was constructed during the ecstatic rise of sea-level in Pleistocene, preceding the V surface, judging from its distribution, altitude and deposit facies. Hereafter, the slow lowering and staying of sea-level followed and the IV surface was formed. The VI surface is a erosional river terrace surface with narrow area. The VII surface was formed during the last glacial stage, since the inclination of its longitudinal profile is the steepest among all terraces in this area, and the surface is buried by alluvium.<br> In Oita coastal plain, fault systems in two directions are observed. They are Median Tectonic Line direction (NE-SW) and N-S direction diagonal to the former. The former system is mainly recognized at the northern part of Saganoseki peninsula, but its recent activity is not conspicuous.<br> On the other hand, the latter system is presumed to be active in recent years from the displacements of terrace surfaces. In the area to the east of the Ono river, the most continuous V surface (17 m-41 m) increases its altitude westward, but the surface continuity is interrupted at the Ono River by a fault (Ono-gawa Fault) belonging to the latter system. The surface also increases again its altitude westward in the area to the west of the Ono River. There are double tiltings in both sides of the fault. The Ill, IV and VI surfaces are deformed in the same mode as the V surface. This west-ward tilting is clearly recognized on the surfaces below the II, and it is considered to be resulted from the recent activity of the Ono-gawa Fault.<br> The IV surface in Tsurusaki-daichi has been displaced by the Yokoo Fault, named by the present writer, being nearly parallel to the Ono-gawa Fault, with the same mode of deformation as by the Ono-gawa Fault. It seems that the activity of the Yokoo Fault associates with the recent activity of the Ono-gawa Fault.<br> Fault system of the Median Tectonic Line direction is recognized in the southern part of the study area where the Mesozoic Ono-gawa Group is distributed, and dextral displacements along some faults of the system can be deduced from the stream anomaly.