Abstract
Seafloor depressions are widespread on the present-day continental slope along the south-east coast of New Zealand's South Island. The depressions appear to be bathymetrically constrained to depths below 500 m and above 1100 m. Similar depressions observed on the Chatham Rise are interpreted to have formed as a result of gas hydrate dissociation, primarily due to the correlation of the depth range to the predicted gas hydrate stability zone in the region. This lead to the hypothesis that a similar origin can be applied for the depressions investigated in this study. However, this investigation has found limited geophysical or geochemical evidence to support this hypothesis.
The objective of this study is to examine whether a causal relationship can be established between potential mechanisms of depression formation and observations based on existing and newly acquired data. This has been done using newly acquired data from the R/V Sonne, R/V Polaris II and R/V Tangaroa, in combination with existing data sets from previous surveys in the region. A combination of multiple geophysical survey techniques have been utilised in this study to conduct the first detailed investigation of these structures. Multibeam bathymetry and backscatter have been used to produce high resolution maps of the seafloor geomorphology and to carry out automated supervised segmentation of substrate classes. Sediment samples and underwater images have been used to ground truth substrate classifications. Sediment samples have also been used to conduct geochemical analysis to assess whether evidence of paleogeochemical methane is present. Subsurface profiles in the form of multichannel boomer seismic, parasound seismic, 2D and 3D seismic lines have been used to investigate underlying structural controls such as polygonal fault systems and understand the stratigraphic framework of the seafloor depressions.
Although the results of our analysis do not preclude that the seafloor depressions formed as a result of gas hydrate dissociation, neither does our geophysical or geochemical evidence support the theory. Therefore, we propose that an alternative mechanism may have been responsible for the formation of these structures. The morphometric variability of the seafloor depressions on the Otago Shelf and Chatham Rise indicates that either these structures were formed as the result of the influence of multiple mechanisms, or that they formed at different times and are at different stages in formation. Based on the evidence presented in this study, we propose that the interaction of multiple mechanisms is the most likely explanation for the formation of these seafloor depressions. Multiple mechanisms have played a role in the formation of these structures, including fluid and / or gas venting, groundwater flux and subsurface structural controls. We propose that the primary mechanism of formation for the smaller seafloor depressions is groundwater flux related to artesian seepage of meteoric groundwater, however this mechanism cannot fully explain the formation of the giant structures observed on the Central Chatham Rise. We therefore propose that in this area gas venting from hydrates or other sources may have been a factor. Subsequent to their initial formation the seafloor depressions have been modified and maintained by the action of regional oceanic currents.