Seabed pockmarks and subsurface fluid migration at multiple scales: Investigations using hydroacoustic and seismic data

Fluid migration from deep sedimentary basins towards Earth’s surface has various implications for hydrocarbon accumulations and influences slope stability, climate and ecological systems. Fluids seeping from the seafloor can give insight into deep crustal and tectonic processes and can significantly change the seafloor morphology as well as the chemical composition of the overlying ocean. The timing of fluid migration in many sedimentary basins is often poorly constrained and the composition of fluids involved in the formation of migration pathways and seafloor venting structures is difficult to determine. Likewise, the effect of upward migrating fluids on the surrounding host sediments and their diagenetic processes is an under-investigated field. Although gas bubbles venting from the seafloor are well constrained and easily identified in hydroacoustic data, the detection of submarine groundwater discharge sites often relies on oral traditions and visual reports from fishermen who recognize anomalies (schlieren/streaks) on the sea-surface. In many regions (e.g. organic muds, hydrothermal fields), submarine groundwater discharge is accompanied by gas venting. However, the processes involved in simultaneous gas and water discharge, as well as their relative contributions to geomorphological structures, are generally not well understood. Decreasing acoustic resolution with depth requires a multi-scale approach to gain a better understanding of the various fluids and migration processes involved at different depths. Using different hydroacoustic systems and frequencies, I examine fluid migration pathways in the subsurface and various morphological expressions that seeping fluids create on the surface during discharge. I integrate well logs, surface sediment grab samples, as well as sediment cores and geochemical porewater analysis to validate and ground truth the hydroacoustic and seismic observations. I apply these methods to datasets from two study areas: the Canterbury Basin, east of New Zealand’s South Island, and Eckernförde Bay in the Baltic Sea of Northern Germany. In the Canterbury Basin, my analyses reveal a wide variety of subsurface migration pathways as well as surface structures related to fluid migration. I show how diagenetic processes of fine-grained sediments are dramatically changed in a 2 km radial distance around a conduit feeding a sediment volcano. This change manifests itself in the suppression of polygonal faulting, and is a result of either 1) a significant change in differential stress induced by buoyant upward migrating fluids that accumulate at depth, or 2) permeable stringers intruding into the surroundings of the feeding pipe and therefore facilitating the dewatering of the enclosing sediments. On the surface of the Oligocene Marshall Paraconformity, I find pockmarks as well as discharged sediments emplaced by sediment volcanism. While the pockmarks appear to be related to dewatering mechanisms of the underlying strata, the sediments emplaced on the same surface seem to be sourced from Cretaceous strata. Several sediment intrusions into Paleocene sediments are similarly sourced from Cretaceous lithologies and affect the overlying fault orientation. Also, I find that recent fluid migration pathways are likely to be responsible for shallow gas accumulations on the continental slope of the Canterbury Basin. On the present-day seafloor, there are numerous pockmarks on the shelf and slope that have been modified by currents. The pockmarks form as a result of gas and/or groundwater seepage, but the contribution of gas versus offshore groundwater could not be unequivocally determined for the Canterbury Basin. In Eckernförde Bay, in contrast to the Canterbury Basin, I was able to hydroacoustically distinguish areas of submarine groundwater discharge and areas additionally affected by gas seepage. Using very high-resolution multibeam data and sub-bottom profiling, I observed and characterised a new type of pockmark that is associated with submarine groundwater and gas discharge. I determined that, in gaseous muddy sediments, submarine groundwater discharge results in unusually consistent and exceptionally shallow free gas that can even be detected with high-frequency 400 kHz multibeam systems