[M-IS13] New frontiers in geology

[MIS13-P05] Geochemistry of clastic sediment in orogenic belt: A case study of Otsuchi area, Northeast Japan

*Kei Sugiura¹, Hikaru Sawada², Mutsumi Iizuka³, Mana Yasui⁴, Takafumi Kasaya² (1.Tohoku University, 2.Japan Agency for Marine-Earth Science and Technology, 3.Hokkaido University, 4.Waseda University)

Keywords:Clastic sediment, High field strength elements, Otsuchi Bay, North Kitakami Belt

The chemical composition of coastal and river sediments is influenced by the geology of their origin and the fractionation of clastic material during sedimentary processes. This chemical composition and particle size distribution are critical to understanding the various sedimentary processes. In the case of large-scale rivers flowing over continents, the chemical composition and particle size distribution of coastal sediments generally progresses toward fractionation and converges to a fine-grained, mudstone composition. In contrast, in the case of rivers in island arcs along active subduction zones, such as in Japan, the river flow paths are short and the geology of provenances is variable. In order to understand the clastic sediments of coastal areas of island arcs, it is necessary to increase geochemical studies in regions with complex geology. Therefore, we report the results of bulk geochemical analyses and particle size distributions of the sediment samples collected from inside and outside of Otsuchi Bay and the three rivers flowing into the bay in the Sanriku coastal area located on the fore-arc side of Northeast Japan. As it is known that Otsuchi Bay and its vicinity were seriously damaged by 11 March 2011 tsunami, tsunami deposits have been reported in the bay sediments. It is clear that the geochemical analysis of these three riverine sediments is well explained by the mixing of rocks of their respective provenances. The chemical composition and particle size distribution of the sediments within Otsuchi Bay indicates that coarse-grained quartz has been removed from the sediments. It is suggested that sediments from Unosumai River, which have the largest flow area, increase the abundance of fine-grained mafic minerals such as chlorite, saponite, and smectite. Seabed sediments from outside of Otsuchi Bay contain many shells, and interpretation of the present results was not easy. However, it was found that seabed sediments collected from sites closer to the mouth of the bay were coarse-grained and their chemical compositions are similar to granite and sandstone from the provenance area. On the other hand, seabed sediments from offshore sites are fine-grained and more closely related to mudstone in chemical composition. Our study did not detect any fractionation of chemical composition or particle size distribution characteristic of tsunami deposits in the bay sediments. These results suggest that differences of geology in the source areas of the sediments and the particle size distribution of each mineral that composes them should be considered together to understand coastal sediments that form along subduction zones, including the tsunami deposit.