[2poster26-52] T6［Topic Session］Latest Studies in Sedimentary Geology

[T6-P-15] Documentation of channel morphology and backscatter intensity facies along the Shionomisaki Canyon, Nankai Trough, using newly acquired multi-beam bathymetry data and surface sediment cores

*Mari Hamahashi¹, Hiroaki Koge², Konosuke Terui³, Rina Fukuchi⁴, Kiichiro Kawamura¹, Hanaya Okuda⁵, Asuka Yamaguchi³, Yohei Hamada⁵, Akira Ijiri⁶, Kanji Tatsumi⁶, Takahiro Hosokawa⁷ (1. Yamaguchi University, 2. National Institute of Advanced Industrial Science and Technology, 3. University of Tokyo, 4. Naruto University of Education, 5. Japan Agency for Marine-Earth Science and Technology, 6. Kobe University, 7. Kochi University)

Keywords:Shionomisaki Canyon, submarine canyon, multi-beam bathymetry, seafloor sediment, fluvial geomorphology

The geomorphology of submarine channels along the Nankai subduction zone exhibits complex nature that are likely the result of dynamic climatic, tectonic, and oceanic forcings. Submarine canyons are considered to be important passageways for sediment transport from the hinterland and continental shelf, feeding into the trench floor and ocean basin. The Shionomisaki Canyon (SC), with walls of up to ~600 m in height, is a major submarine canyon that has incised into the forearc basin and accretionary prism along the Nankai Trough. The evolution of submarine canyons in active margins and their depositional/incisional processes, however, are yet poorly constrained (Puig et al. 2014). Along the SC, complex forms of sediment waves are inferred, and instability of canyon walls is suggested by multiple scars, gullies, steep flanks and debris flows. During YK23-10S Cruise, high-resolution multi-beam bathymetry data covering ~4600 km² around the full length (upstream to downstream) of the SC was acquired onboard the R/V Yokosuka, using a Kongsberg EM 122 Multi-narrow Beam Echo Sounder, operating at sonar frequencies of 12 kHz, with 432 beams by dual swath, and swath width of 120°. Together with bathymetry survey, sub-bottom profiler data was acquired using the 3300-HM (EdgeTech) Subbottom Profile Subsystem. The motivation of our research is to characterize the seafloor of the contemporary SC using the newly acquired MBES dataset, focusing on the channel morphology and backscatter seismic facies of seafloor sediments. Generally, the intensities of acoustic signals backscattered from the seafloor indicate seafloor characteristics such as interface roughness, acoustic impedance, and surficial heterogeneity, related to seafloor composition, grain size, and small-scale topographies (Lamarche et al. 2011). In this study, we investigate the seafloor sediment distribution from the derived relationship between sediment mean grain size and backscatter angular response. For this purpose, we conduct analyses of facies and grain size using the push core sediments acquired at the channel floor and adjacent terrace surface during the dives of the R/V Shinkai 6500, which provided a unique opportunity to sample seafloor sediments along the SC.
Reference: Puig, P. et al (2014), Ann Rev Mar Sci; Lamarche, G. et al (2011), Cont Shelf Res