Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin,East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation |
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| Institution: | 1. Geocoastal Group, School of Geosciences, The University of Sydney, NSW 2006, Australia;2. College of Science, Technology and Engineering, James Cook University, Townsville, QLD 4811, Australia;3. College of Science, Technology and Engineering, James Cook University, PO Box 6811, Cairns, QLD 4870, Australia;4. Environment Department, University of York, York YO10 5DD, UK;5. Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18002 Granada, Spain;1. Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Ministry of Education, Wuhan 430074, China;2. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China;3. College of Marine Science and Technology, China University of Geosciences (CUG), Wuhan, Hubei 430074, PR China;4. Natural Resources Canada, Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia, B2Y 4A2, Canada;5. Shenzhen Branch of CNOOC Ltd., Guangzhou 510240, China;6. Sanya Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China;1. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;2. 3D Seismic Lab. School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, United Kingdom;3. MARUM, Center for Marine Environmental Sciences, University of Bremen, Bremen 28359, Germany;4. Sanya Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China;5. University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures. |
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| Keywords: | Submarine slope failure Mass transport deposit (MTD) Gas hydrate Bottom-simulating reflector (BSR) Gas/fluid flow Gas hydrate stability zone (GHSZ) Seismic data interpretation Ulleung basin East (Japan) sea |
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