Seismic studies of paleo-pockmarks on the Chatham Rise, New Zealand

Abstract

This study investigates over 2400 buried pockmark features on the Chatham Rise through the analysis and interpretation of 2D seismic and Parasound data. The main objectives of this research are to establish what caused the formation of buried pockmarks on the Chatham Rise and to determine when the pockmarks were formed. The study area is located on the Western Chatham Rise, near the Canterbury Shelf off the East Coast of New Zealand. The pockmark fields were revealed through multibeam bathymetry data collected from surveys during the past 20 years. Previously, the pockmarks on the Chatham Rise were thought to have been produced by the release of methane through gas hydrate dissociation. However, recent geochemical investigations showed no indication of methane in the sediment cores. Current hypotheses for the formation of the pockmarks include groundwater fluid flow and the release of carbon dioxide (CO2) modulated by CO2 hydrates. This thesis presents the results of the spatial analysis of the pockmarks on the Western Chatham Rise and whether there are any links between the location of pockmark formation and regional geology. The interpretation of 2D seismic and Parasound data improves the current understanding of the stratigraphy along the Chatham Rise and Canterbury Shelf areas and places the pockmark field in a regional geologic context. The structures beneath the pockmarks were investigated to determine how the features were formed. The spatial configuration of the pockmarks was analysed vertically for stacking and laterally for potential ties to specific horizons, particularly horizons associated with Milankovitch order climate cycles. Results show a preference of pockmarks around specific horizons and clustering of pockmarks above extensively faulted geologic units. New ideas behind the cause of the pockmarks in this field are discussed. Results suggest a fluid migration system could be responsible for transporting a fluid or gas from depth to the seafloor, with the fluid escape at the seafloor causing the pockmark. Possible fluids or gases responsible for pockmark formation the Chatham Rise include methane or other hydrocarbon gases.