Geostatistical analysis of seafloor depressions on the southeast margin of New Zealand's South Island — Investigating the impact of dynamic near seafloor processes on geomorphology

• Geostatistical analysis methods applied to multibeam bathymetry and seismic data
• Geomorphology of seafloor depressions has been quantitatively characterised.
• No direct correlation between gas venting and formation of seafloor depressions
• Likely mechanism of depression formation: groundwater flux linked to current flow

Seafloor depressions are widespread on the present-day continental slope along the southeast coast of New Zealand's South Island. The depressions appear to be bathymetrically constrained to depths below 500 m, correlating to the top of the gas hydrate stability zone, and above 1100 m. Similar depressions observed on the Chatham Rise are interpreted to have formed as a result of gas hydrate dissociation, leading to the hypothesis that a similar origin can be applied for the depressions investigated in this study. Our investigation, however, has found limited geophysical or geochemical evidence to support this hypothesis.The objective of this paper is to examine whether a causal relationship can be established between potential mechanisms of depression formation and the present-day seafloor geomorphology. Geostatistical analysis methods applied to multibeam bathymetry and interpretation of 3D seismic data have been used to empirically describe the geomorphology of the seafloor depressions and investigate potential correlations between geomorphology and other processes such as current flow along the shelf and slope in this region and underlying polygonal fault systems.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 an alternative mechanism that may have been responsible for the formation of these structures. Based on the evidence presented in this study, the most likely mechanism responsible for the formation of these seafloor depressions is groundwater flux related to the interaction of current systems and the complex geomorphology of submarine canyons on the southeast coast of the South Island.