THE GEOMORPHOLOGICAL DEVELOPMENT OF THE NORTH-WESTERN OSAKA PLAIN

Abstract

The Osaka Basin which is located in the east end of the Inland Sea of Japan, is filled with the Plio-Pleistocene deposits, so called the Osaka Group, of clay, sands and gravels. It graddually becomes of marine origin with alternation of marine and nonmarine stratum. The Plio-Pleistocene is exposed in the platforms and the hills in and around the Osaka Plain of which the height ranges from several tens meters to about 200 meters. On the sides of the surrounding mountains, we observe the disturbed Osaka Group, influenced by the uplifting movement of the mounatins. Generally speaking, it is complicatedly disturbed by f lexture, fold and overthrust, but it is safely concluded that the Plio-Pleistocene is dislocated to rise generally from the deltaic plain to the margins of the basin, and, so to say, that the Plio-Pleistocene deposits slightly decline to disappear into the bottom of the bay, or below the deltaic deposits. In another view, the the distribution of the deltaic deposits can be said to be restricted to the depressed area, provided by the basin building movement. The hills in the uplifted area present tectonic morphology. The late Pleistocene platforms of which the inclination be comes steeper with their being older (higher), cut the Osaka qroup which dips in the same direction. Both the surfaces or the prolonged surfaces of the platforms cross together and sink into the deltaic deposits in the depressed area. On the other hand, the Pleistocene below the deltaic deposits, it is suggested by boring, have sedimentary cycle of clay and gravels or coarse materials. It can be said hypothetically that the sedimentary cycle is possibly of eustatic origin. To say conclusively, the geomorphology of the hills is under the control of tectonic movements, but eustatic control is over the sedimentation below the delta. The several hundreds of boing data in which the locations of the main ones are shown in Fig. 1, show us that each layer is slightly declined toward the central part of the bay. And as are shown in Fig. 2, the deeper the layers are, the steeper the inclination. The clayey deposits decrease in the thickness toward the uplifted area of the hills of the Plio-Pleistocene. Such inclination is significanty accordant to that of gravels on the platforms. It is due to basin building movements (or tilting movement). The correlation of the data of boreholes makes us conceive that two epoch-making ground surfaces still remain to be buried below the deltaic deposits. The Fig. 3 shows the younger buried surface which is formed during the lowering of sea-level of the last glacial age in the consideration of the radiocarbon chronology of a piece of wood on the gravels. The surface is terraced, as is shown in Fig. 3. The terrace emerges from the deltaic deposits, forming the platform, with steep cliffs, which is dotted surface of Fig. 1. The older surface below the younger is shown in Fig. 5. It cuts flat the underlying layers as shown in Fig. 2, c. The older surface is considered to be correlated with the upper platform in the plain. The clayey or silty deposits containing shells between the two gravels (Fig. 2, o) is accumulated during the transgressive period. It is the basin-building movement of the Osaka plain that preserved the old landforms in the depressed place., byburying them in the materials from the uplifting area. The late-pleistocene geomorphological process of the north-western Osaka Plain can be schematically represented as shown in Fig. 6, (I). The surface of gravels is formed in the order of I, II, III. In case the quantity of the deposits in the depressed area, is negligibly scanty, or the relative rise of sea-level is so rapid that sedimentation can not keep up with it, the scheme of it will be (II) of Fig. 6. In rapanese Islands, such tilting movement in depositional plain seems to be commen process.