A REVIEW OF SHALLOW-WATER POCKMARK DISTRIBUTION AND ORIGINS IN THE NORTHWESTERN GULF OF MAINE

Seafloor depressions, called pockmarks, appear in late19th century bathymetric surveys, and were observed as "channels" in seismic profiles in Penobscot Bay in 1959. Widespread geophysical mapping changed early beliefs that pockmarks were unusual to an appreciation that they are common. Along the Maine coast, 79 large fields of natural gas are mapped, totaling more than 300 km2. Fewer than 10 fields have abundant pockmarks, but most fields are not explored. Fluid escape is the identified mechanism of pockmark formation; groundwater and methane were each postulated as driving forces. We favor methane because we have chemically measured and acoustically observed gas, and know no mechanism to deliver groundwater to existing fields. Disagreement exists over the current activity of the fields. Most gas fields lack pockmarks, and scarce observations of gas release suggest that fluid venting may be rare. Side scan images and anecdotes of gas venting in Belfast Bay, time-series maps showing the filling and creation of pockmarks, and 27 degree mean pockmark wall slopes suggest at least annual activity. Some gas fields lacking pockmarks possess breached seals; breaching of overlying seals may ultimately end pockmark activity. Pockmarks are not known from Coastal Plain estuaries, so if gas escape forms pockmarks, methane in formerly glaciated terrane requires a special source. Glacial-marine sediment and drowned wetlands are possible candidates. The upper, ice-distal "glacial"-marine sediment is often fossiliferous and organic-rich. Lakes and bogs commonly occur on this substrate on land, and presumably did so in present-day estuarine settings when sea level was lower. Lake and wetland environments commonly exist at the upper end of modern estuaries, and bays with pockmarks, like Somes Sound, were once demonstrably lakes. Thus, we believe gas and pockmark fields mark a significant transformation of terrestrial environments to marine as a result of sea level rise.