11:00 AM - 1:00 PM
[HCG25-P01] Characteristics of deposition by dredged soil dumping in deepsea: an example from Sagami Bay off Misaki
Keywords:dredged soil dumping, side-scan sonar, Sagami Bay
A large number of spot-like structures with a diameter of about 150 m are discovered over several square kilometer regions on the side-scan sonar images obtained from the Sagami Bey off Misaki at a water depth of 700 m, and interpreted to be caused by expulsion from subseafloor or landsliding. However, additional survey and interview reveal that these structures were artificial due to dredged soil dumping.
More than one hundred fifty spot-like structures showing strong backscattering with a diameter of 100-300 m were recognized on the small sedimentary basin off Misaki by side-scan sonar IZANAGI (11, 12 kHz). Because the eastern part of the Sagami Bay is located in the hanging wall of the subduction zone and these spot-like structures were similar to gas-escape pockmark structures, they were interpreted to be caused by gas or sediment expulsion (Tokuyama, 2000, Aquabiology). Seafloor survey using ROV NSS during KY04-11 cruise revealed that strong backscattering was attributed to aggregation of gravels on flat basin floor. Sediment core samples collected from strong backscattering regions are mainly composed of mud without any stratigraphic discontinuity and did not lead to elucidation of gravel aggregation process (Ashi, 2005, Chikyu Monthly). ROV Hyper-Dolphin survey during NT05-19 cruise conducted seafloor observation and rock sampling. The gravel samples are late Miocene dark gray siltstones and conglomerates composed of sand and mud matrix and volcanic granules, and shows low roundness surrounded by flat surfaces. The strong backscattering regions show normal seafloor without any deformation such as steps and fissures. Thus, it is unlikely that aggregation of blocks was caused by sediment eruptions. Although gravels restricted in the seafloor suggest recent landsliding, lack of current mark leads to possibility of artificial construction. Therefore, hearing investigation of Yokosuka Civil engineering office was conducted and proved that localities of dredged soil dumping from the Miura fishery harbor almost coincides with blocky seafloor distribution. Although artificial construction was not expected at first because of far from the coast more than 10 km and wide distribution about several square kilometers, we also should pay attention to artificial construction even in deepsea for interpretation of seafloor acoustic imagery.
More than one hundred fifty spot-like structures showing strong backscattering with a diameter of 100-300 m were recognized on the small sedimentary basin off Misaki by side-scan sonar IZANAGI (11, 12 kHz). Because the eastern part of the Sagami Bay is located in the hanging wall of the subduction zone and these spot-like structures were similar to gas-escape pockmark structures, they were interpreted to be caused by gas or sediment expulsion (Tokuyama, 2000, Aquabiology). Seafloor survey using ROV NSS during KY04-11 cruise revealed that strong backscattering was attributed to aggregation of gravels on flat basin floor. Sediment core samples collected from strong backscattering regions are mainly composed of mud without any stratigraphic discontinuity and did not lead to elucidation of gravel aggregation process (Ashi, 2005, Chikyu Monthly). ROV Hyper-Dolphin survey during NT05-19 cruise conducted seafloor observation and rock sampling. The gravel samples are late Miocene dark gray siltstones and conglomerates composed of sand and mud matrix and volcanic granules, and shows low roundness surrounded by flat surfaces. The strong backscattering regions show normal seafloor without any deformation such as steps and fissures. Thus, it is unlikely that aggregation of blocks was caused by sediment eruptions. Although gravels restricted in the seafloor suggest recent landsliding, lack of current mark leads to possibility of artificial construction. Therefore, hearing investigation of Yokosuka Civil engineering office was conducted and proved that localities of dredged soil dumping from the Miura fishery harbor almost coincides with blocky seafloor distribution. Although artificial construction was not expected at first because of far from the coast more than 10 km and wide distribution about several square kilometers, we also should pay attention to artificial construction even in deepsea for interpretation of seafloor acoustic imagery.