9:35 AM - 9:50 AM
[MIS18-03] Pockmark formation in shallow methane hydrate-bearing area off the Tango Peninsula, Japan
Keywords:shallow methane hydrate, Oki Trough, off Tango Peninsula, seismic reflection survey, pockmark
The term pockmark refers to a geographical hollow shape, regardless of its cause of formation. In a field of marine geology, previous studies on the distribution of pockmarks have often reported that fluid carries away part of sediment along its flow path resulting in the creation of pockmarks and have discussed the fluid’s origins and paths. Matsumoto (2009) reported that the accumulation of shallow methane hydrate (MH) in the off-Joetsu area disturbed the strata and was released due to its buoyancy, it resulted in the formation of pockmarks.
The shallow MH extends from the seafloor to a depth of ~100 m and is known to be intermittently distributed along the coast of the Sea of Japan and the Hokkaido region in Japan. We, AIST, has previously conducted investigations on the presence of MH in Japan and has now begun examining the technology and an assessment the environmental impact of development of MH. Within this flamework, we consider the cause of formation of pockmarks in the off-Tango Peninsula area, with the aim of understanding behavior of fluids and sediments involved in the occurrences of the shallow MH.
The research area, located ~100 km offshore to the north of the Wakasa Bay and the Tango Peninsula, forms the northern edge of a topographic high that extends northward. Steep landslides have developed on the western slope, which forms a boundary with the Oki Trough. The gentle eastern slope forms a boundary with the Wakasa Basin, where broad and horseshoe-shaped gentle landslides have been observed. Acoustic investigations, using research vessels and Autonomous Underwater Vehicle, and a high-resolution three-dimensional seismic reflection survey (HR3D) have revealed densely distributed pockmarks and highlighted their subseafloor characteristics in the research area. The circular pockmarks range in diameter from several tens of meters to 500 m and appear to be distributed in a linear array above the bathymetric high and irregularly on the gentle eastern slope.
Although pockmarks were clearly recognizable in the topography, they could not be detected on the seafloor using backscatter intensity, thereby suggesting that they were not accompanied by hard materials produced by the methane/seawater reaction. An unconformity was recognized in the HR3D profile, which was interpreted as the boundary between the Neogene and Quaternary (Yamamoto 1993). The sub-seafloor structures just below the pockmarks usually exhibit downward bends of continuous strata, with no recognizable collapses. The bending structures were traceable to deeper depth. The bottoms of the bends lay mostly above the unconformity. The pockmarks on the western slope appeared to be associated with high-angle faults that connected the unconformity to layers near the seafloor. These were sometimes accompanied by high-amplitude reflectors. Some pockmarks on the gentle eastern slope were also associated with high-angle faults, whereas others were larger in diameter and were distributed over a wider area without high-angle faults. The centers of the bends tend to shift their location to the northeastward slightly toward a shallower depth.
As landslides were recognized on the gentle eastern slope, it is possible that some of the sediments in the area moved out of the bathymetric high. At the same time, a tensile stress in north-east direction may have occurred in this area. As there were laterally continuous strata below the pockmarks, the bending structures may not be the result of vertical removal of sediment along fluid paths or accumulation and loss of sufficient amounts of MH, as reported previously. The pockmarks in this area may have been caused either by a gentle and continuous fluid discharge that prevented the growth of the sediment layer locally or by the lateral migration of sediments affected by landslides. This study was conducted as part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry).
The shallow MH extends from the seafloor to a depth of ~100 m and is known to be intermittently distributed along the coast of the Sea of Japan and the Hokkaido region in Japan. We, AIST, has previously conducted investigations on the presence of MH in Japan and has now begun examining the technology and an assessment the environmental impact of development of MH. Within this flamework, we consider the cause of formation of pockmarks in the off-Tango Peninsula area, with the aim of understanding behavior of fluids and sediments involved in the occurrences of the shallow MH.
The research area, located ~100 km offshore to the north of the Wakasa Bay and the Tango Peninsula, forms the northern edge of a topographic high that extends northward. Steep landslides have developed on the western slope, which forms a boundary with the Oki Trough. The gentle eastern slope forms a boundary with the Wakasa Basin, where broad and horseshoe-shaped gentle landslides have been observed. Acoustic investigations, using research vessels and Autonomous Underwater Vehicle, and a high-resolution three-dimensional seismic reflection survey (HR3D) have revealed densely distributed pockmarks and highlighted their subseafloor characteristics in the research area. The circular pockmarks range in diameter from several tens of meters to 500 m and appear to be distributed in a linear array above the bathymetric high and irregularly on the gentle eastern slope.
Although pockmarks were clearly recognizable in the topography, they could not be detected on the seafloor using backscatter intensity, thereby suggesting that they were not accompanied by hard materials produced by the methane/seawater reaction. An unconformity was recognized in the HR3D profile, which was interpreted as the boundary between the Neogene and Quaternary (Yamamoto 1993). The sub-seafloor structures just below the pockmarks usually exhibit downward bends of continuous strata, with no recognizable collapses. The bending structures were traceable to deeper depth. The bottoms of the bends lay mostly above the unconformity. The pockmarks on the western slope appeared to be associated with high-angle faults that connected the unconformity to layers near the seafloor. These were sometimes accompanied by high-amplitude reflectors. Some pockmarks on the gentle eastern slope were also associated with high-angle faults, whereas others were larger in diameter and were distributed over a wider area without high-angle faults. The centers of the bends tend to shift their location to the northeastward slightly toward a shallower depth.
As landslides were recognized on the gentle eastern slope, it is possible that some of the sediments in the area moved out of the bathymetric high. At the same time, a tensile stress in north-east direction may have occurred in this area. As there were laterally continuous strata below the pockmarks, the bending structures may not be the result of vertical removal of sediment along fluid paths or accumulation and loss of sufficient amounts of MH, as reported previously. The pockmarks in this area may have been caused either by a gentle and continuous fluid discharge that prevented the growth of the sediment layer locally or by the lateral migration of sediments affected by landslides. This study was conducted as part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry).