東部瀬戸内堆積区の形成と淡路島の隆起

抄録

Geographic Outline AWaji Island lies from NE to SW, dividing the east Inland Sea. The central range running in same direction continues to the Rokko mountains which are thought to have been elevated by crustal movement in Pleistocene (Rokko Movement). In the southern part of this island is the Yuzuruha mountains of Cretaceus sedimentary rocks which are a part of the series of mountain ranges on the north of Median Tectonic Line (MTL). In the plains of this island are seen hills of Pliocene deposits (the Awaji group) elevated from below the bottom of the Inland Sea.
Purpose of Study The authors intended in this study to make clear the process of the formation of the east Inland Sea and Awaji Island. In order, they carried out the investigation on following points :
1. Relation between the Miocene deposits and the age of the formation of the erosion surface on the mountains in the north.
2. Sedimentary process of the Awaji group.
3. Faults and deformation of the Awaji group.
Result of study The results obtained are as follows :
1. The morphology of the basement rocks (granitic rocks) in the north had been already gentle and undulating before the Kobe group deposited, caused by the Miocene transgression. The thickness of it reached a few hundreds meter in the north, but after regression the Kobe group was erosed down to basement. The mountains in the north were flattened again in this age.
2. In Pliocene time, the area of the east Inland Sea depressed and deposited the Awaji group of gravel, sand, lacustrine silt and clay. In early stage, the lake began to be supplied with sand, and gravel from the area to the north of the island. In last stage, rivers originated to the south of MTL carried gravel (Goshiki-hama gravel) over MTL. to the lake.
3. Goshikihama gravel is supposed to be contemporaneous with the Uchihata gravel, which is the lowest of the Osaka group (Yoshiki OKA 1978). The lower part of the Awaji group is suggested to be older than the Osaka group.
4. The thickness of the Awaji group reaches 800 meter in the north and 300 meter or more in the middle part of the island. It indicates the depression of the Inland sea in Pliocene time.
5. The fault movement in this island took place after the sedimentation of the Awaji group. Reverse and transcurrent faults by E-W compression have been discriminative since the middle of Pleistocene age.
6. The faults can be divided, by strike, into three types, as follows :
Type I (N60°E) The faults of this type caused the uplift of the Yuzuruha mountains in the south.
Type II (N-SNE-SW) This type of faults caused the uplift of the central range of the island.
Type III (N20°W) This type of faults are active reverse faults cut through the main range of the island.