著者

田辺団体研究グループ

出版者

地学団体研究会

雑誌

地球科學 (ISSN:03666611)

巻号頁・発行日

vol.47, no.1, pp.1-16, 1993-01-25

被引用文献数

2

---

The Miocene Tanabe Group, which consists of shallow-marine siliciclastic sediments, is distributed in the southwestern part of the Kii Peninsula, Japan. The group is about 1,500m thick, and is divided into the Asso and the Shirahama Formations in ascending order. The Asso Formation is mainly composed of mudstone and conglomerate, and the Shirahama Formation of sandstone, mudstone and alternating beds of them. A thick conglomeratic sedimentary wedge, which was deposited in a fan-delta system (the Shimomisu Fan-delta), is developed in the northern area of the distribution of the group. To clarify the sedimentary history of the fan-delta, at first, sedimentary facies were recognized and classified based mainly on grain-size and sedimentary structures. Consequently, the facies associations and the sedimentary bodies were recognized and defined based on the occurrence and combinations of sedimentary facies. The spatial and stratigraphic interrelationship among sedimentary facies, facies associations and sedimentary bodies were analyzed in detail. The following results are obtained. The sediments of the Shimomisu Fan-delta consist of 3 sedimentary bodies, which are in turn consist of 6 facies associations. The sedimentary history of the Shimomisu Fan-delta is summarized as follows: STAGE OF GENESIS: The drowned valleys facing to the ocean were formed as the sea-level rise began. The debris flow deposits and marsh muds, and then sands and rounded gravels in the neritic environments were deposited in the valley. STAGE OF PROGRADATION: After the burial of valleys, the rate of the sea-level rise decreased. Gravely deposits supplied from the north formed a fan-delta system which prograded toward the south. Gravely deposits were deposited as foreset beds, resting on a downlap surface, which were underlain by the bottomset muds. The fan-delta divided the sedimentary basin into embayed sea in the northern margin and open shelf in the resting area. STAGE OF AGGRADATION: The rate of the sea-level rise again increased, and the supply of sediments were continued. Therefore, the rate of the progradation decreased, and sediments were accreted vertically in the neritic area, forming the topset beds of the fan-delta. STAGE OF ABANDONMENT: The rapid increase in the rate of the sea-level rise took place in the later half of Blow's N 8 stage, and the fan-delta system was abandoned and the shelf mud facies covered throughout the basin.