1 0 0 0 OA 大阪平野の発達史 : ^<14>C年代データからみた
Having carried out the study of the latest alluvial deposits (the Nanba Formation) in Osaka since the 1950s, the writers revealed that the developmental history of the Osaka Plain can be divided into nine stages. In this paper, numerous data on fossils, sedimentary facies, topography, prehistorical remains and historical facts are synthesized under the stratigraphical successions and the radio-carbon dates. Nine successive stages in the development of the Osaka Plain, with approximate radio-carbon dates, are as follows. 1. The stage of the paleo-Osaka Plain (ca. 20000 y.B.P., Wurm maximum, Pre-Jomon age): The geomorphological features of the Paleo-Osaka Plain are guessed by the relief map of the base of the latest alluvial deposits (Fig. 4). The river system of the Paleo-Osaka River was widely developed. 2. The stage of the Paleo-Kawachi Plain (ca. 9000 y.B.P., earliest Jomon age): The sea-level was over 20m lower than the present sea-level. Osaka Bay already existed, but the Kawachi area was not covered by sea-water as yet. 3. The stage of Kawachi Bay I (ca. 7000-6000 y.B.P., the first half of early Jomon age): The sea-level reached the same level as the present one. The Paleo-Kawachi Plain was widely covered by sea-water. A sand-bar was developed on the north of the Uemachi Upland (Fig. 6). 4. The stage of Kawachi Bay I (ca. 5000-4000 y.B.P., latest early Jomon-middle Jomon age): The sea-level was in the same level as one of Kawachi Bay I or in a slightly higher level than that. The delta of the Yodo River was well developed. The sand-bar was further developed to the north of the Uemachi Upland (Fig. 7). 5. The stage of Kawachi Lagoon (ca. 3000-2000 y.B.P., latest Jomon〜the first half of Yayoi age): Owing to the development of the sand-bar north of the Uemachi Upland, Kawachi Lagoon was formed. Standella capillacea (tropical to subtropical, brackish water species) at the inlet and Corbicula sandai (Lake Biwa, freshwater species) in the inner part are characteristic (Fig. 8). 6. The stage of Lake Kawachi I (ca. 1800-1600 y.B.P., late Yayoi〜early Kofun age): Through the subsequent development of the sand-bar on the north of the Uemachi Upland. Kawachi Lagoon changed into Lake Kawachi. The delta of the Yodo River finally reached the sand-bar (Fig. 9). 7. The stage of Kawachi Lake II (Nintoku〜Nara age): In the time of the Emperor Nintoku (ca. A.D. 400), Ookawa (the Oo River) was, excavated at the base of the aforesaid sand-bar. A part of Kawachi Lake near the excavated outlet was an important port called "Nagarafunase" till the later Nara age. 8. The stage of the Osaka Plain I (After Nara age): Each delta of rivers filled the sea-area behind the line of formation of the offshore-bar (Fig.9). The Plain began to be developed in west Osaka. 9. The stage of the Osaka Plain I (After Muromachi〜early Edo age): Each delta was rapidly developed beyond the aforesaid line. This stage is the present stage of the Osaka Plain.