Passamaquoddy Bay is a large (575 km2) embayment located in southwestern New Brunswick, Canada.
One of its more remarkable features is the multitude of depressions or “pockmarks’ on its floor (Pecore and Fader, 1990). Formative mechanisms may include:
- Methane ebullition entraining sediments which are dispersed by local currents;
- Seismic activity which resuspends sediment following the discharge of subterranean liquids (including methane);
- Meteorite impact;
- Glacial erosion and tectonic activity leading to the formation of hill – hole pairs;
- Thermogenic gas that accumulates behind a gas hydrate seal and is explosively released as a glacial period ends.
According to Fader (1990) “…pockmarks have been found in many continental shelf environments of the world and their role as foci of intense chemosynthetic activity has attracted considerable attention”. There are estimated to be approximately 11,000 pockmarks in Passamaquoddy Bay covering an area of approximately 87 km2. They occur on Holocene clay, the uppermost unit of the seafloor in two patches separated by a ridge of till-covered bedrock, (Pecore and Fader, 1990). Some are aligned in parallel rows while others occur in clusters and, in some cases, smaller pockmarks occur inside larger ones. Their diameter ranges from 1 to 300 m with an average diameter of 29m and they occur in water depths of 20-40m. While their average depth is 3.5m, the range is substantial, the deepest being at least 50m.
Fig 3. Grey scale multibeam image of Passamaquoddy Bay pockmark areas.