[S-CG55] Ocean Floor Geoscience

[SCG55-P04] Topographic and geological features of the seafloor from offshore of the mouth of the Fuji River to offshore of Tagonoura Port

*Nina Arai¹, Yusuke Sato², Yukihiko Nakano², Reiko Nagata², Soshi Shibao¹, Genju Yamamoto³, Yuka Yokoyama³, Asahiko Taira³, Izumi Sakamoto³ (1.Graduate School of Oceanography,Tokai University, 2.Marine Works Japan Ltd., 3.School of Marine Science and Technology,Tokai University)

Keywords:Suruga Bay, Fuji River, Spur, Lava

Suruga Bay is the deepest bay in Japan, located in Shizuoka Prefecture. The bay mouth from Irozaki to Omaezaki is approximately 56 km long and 60 km deep, with a surface area of 2,300 km², the deepest point being approximately 2,500 m. Many rivers flow into Suruga Bay from the west, including first-class rivers such as the Oi River, Abe River, Fuji River, and Kano River. Among them, the Fuji River, one of the three most rapid rivers in Japan, supplies a large amount of sediment and forms a vast submarine fan offshore. In this study, seafloor topography (Multi Beam Echo Sounder) and geological survey (seafloor observation by underwater camera) were conducted to clarify the mechanism of sediment supply from the mouth of the Fuji River. The results revealed the existence of 16 ridge-like spur with a north-south axis from shallow to deep water areas on the seafloor from offshore Fuji River to offshore Tagonoura Port. The characteristics of the shape of the sea legs are: 1) The distribution density of sea legs is greater in the eastern part than in the western part. 2) The distance from the coast to the starting point of the sea legs becomes shorter toward the east. 3) Dendritic microstructure development can be seen on the surface of the western part of the seafloor. 4) It was found that terraces with cliffs of 10 m to 50 m in specific height developed in the eastern part of the study area, starting at a depth of 110 m to 150 m. In addition, 5) the seafloor video taken by an underwater camera showed black bedrock exposed in the cliff topography. Reflection sonic survey results from previous studies indicate that the spur in the study area are based on the Fujikawa-oki Formation (Middle to Upper Pleistocene), overlain by the Holocene (Sato and Arai, 2016). Based on the bathymetry data and seafloor video obtained in this survey, bathymetry data from previous studies, and the results of the reflection sonic survey, the formation factor of the seafloor base in the western part of the study area is unknown, but the shape of the upper part of the seafloor is thought to be related to repeated flow path changes of the Fuji River in the fan-shaped area of the Fuji River. There are several possible reasons for the formation of the cliff topography that exists to the east of the study area. Among them, the horseshoe shape of this cliff suggests that it may be a slide cliff caused by a landslide topography or by different geological conditions in the lower part of the spur. According to the results of onshore borehole investigations obtained in previous studies, 1) on the east side of the mouth of the Fuji River, andesite to basalt lava from the Fuji volcanic eruption is intercalated at depths of 145 m to 150.7 m, which is known to be more than 12,000 years old from radiocarbon dating results, while the lava However, such lava is not seen on the west side of the mouth of the Fuji River (Ozaki et al., 2016). Therefore, it is considered that the lava did not reach the west side of the mouth of the Fuji River. 2) The black color of the cliffs in the seafloor images taken at depths of approximately 130 m to 165 m suggests that they are basalts from the volcanic eruptions of Mt.fuji. Furthermore, 3) Based on the cross section of the cliff topography created and the results of the onshore borehole survey in the previous study, the water depth at which the cliff topography is formed includes the depth at which lava is distributed. The above three reasons suggest that the lava may be distributed along the cliff topography in the eastern part of the study area. References: Sato and Arai (2016) S-5. Seamless Geoinformation of Coastal Zone "Northern Coastal Zone of Suruga Bay" , Ozaki et al. (2016) S-5. Seamless Geoinformation of Coastal Zone "Northern Coastal Zone of Suruga Bay".