[H-GM02] Geomorphology

[HGM02-02] Huge block deposits at river mouths and on marine terraces in the south coast of Yaku Island, southern Japan

*Hiroshi Shimazu¹ (1.Department of Geography, Faculty of Geo-Environmental Science, Rissho University)

Keywords:huge block, granite, deposits, river mouth, marine terrace, Yaku Island

There are many huge rounded blocks larger than 3m, which are the Yakushima granitic rocks, on the valley floors of the rivers in Yaku Island. These blocks were produced on the valley side slopes in granitic area and transported to the valley floors. Downstream change of diameter of the blocks along the rivers and blocks on the riverbed in the sedimentary rock area show that these blocks were transported by fluvial process. Rounded huge blocks, same as blocks on the valley floor, are found in the south coast of Yaku Island, especially at the river mouths and on the beach near the river mouths. Several levels of marine terraces surround Yaku Island. There are rounded huge blocks also on the marine terraces and in the deposits of the terraces in the south coast. The sites of these deposits are located in the sedimentary rock area and are 1 to 1.5 km away from the geological boundary. Thus landslides on the slopes in the granitic area are not the cause of the blocks. The deposits including the huge blocks on the marine terraces, which are seems to develop during MIS5e, indicates that the huge blocks were transported by fluvial processes to the river mouths and shallow sea floor.