Large pockmarks on the Angola passive continental margin (Sahling, 2008)

Pockmarks are morphological features on the seafloor that have a bowl-like

shape, i.e. a shallow depression in the seafloor. They can be circular to oval and

span a wide range of sizes, from only several meters in diameter to over one

kilometer and from only few centimeters in depth to over 100 m. Having first been

discovered in the 1970s on the Scotian Shelf they have since been found on large

parts of the continental margins worldwide. This is due to improvements in

hydroacoustic methods such as the swath bathymetry echosouder allowing for

efficient high resolution mapping of the seafloor along with ever increasing

research and exploration activity in the world's oceans.

Pockmarks have been linked to gas escape from the seafloor since their

discovery. Theories about their formation include the sudden escape of a large

volume of gas (methane formed in organic-rich sediments), gas hydrate

dissociation leading to collapse structures as well as a constant bubble stream

escaping from the seafloor and preventing local sedimentation.

My PhD project deals with seepage features and their tectonic as well as

sedimentological setting on the West African passive continental margin. Gas

seepage there is linked to salt tectonic activity providing pathways like faults for

gas from depth to the seafloor, supplying seepage features including large scale

pockmarks. I apply a variety of geophysical methods in my study including

multichannel seismic data, high-frequency echosounder and swath bathymetry

echosounder systems.

Stefan Wenau, GLOMAR PhD-student

Pockmarks - gas emission structuresat the seafloor