NEW INSIGHTS INTO THE STRATIGRAPHY OF A NEARSHORE POCKMARK FIELD, BELFAST BAY, ME

Biogenic natural gas deposits and circular seafloor depressions, or pockmarks, are globally occurring phenomena recognized in a variety of continental margin settings including deltas, shelf basins, and areas of petroleum production. Pockmarks are also widespread in mid-latitude estuaries, especially in formerly glaciated regions, such as the Gulf of Maine. Despite their ubiquity, pockmarks are one of the least understood underwater landforms on the coast of northeastern North America. Pockmarks are sourced by methane escaping from organic-rich sediments in the deep Holocene sections of Belfast Bay, Maine and similar settings in the region. The source beds are early Holocene terrestrial (bog or lake) or estuarine (salt marsh, tidal flat, or bay) sediments. In seismic profiles of the northwestern Gulf of Maine, acoustic wipeout zones, which indicate the presence of gas in Holocene sediments, are also common. However, in coastal Maine, biogenic gas fields often lack pockmarks, and pockmarks without associated gas are also observed. This disassociation is attributed to gas migration along stratigraphically controlled permeable beds and subsequent gas escape through “pipes” in Holocene mud. Recently gathered high resolution Chirp seismic profile data, in conjunction with an inferometric side scan sonar remapping of the bay's bathymetry, shows a distinct seismic facies above the Pleistocene/Holocene unconformity. This unit ranges in thickness from 0 m to 3 m, and has layered bedding. We speculate that this reflector represents the Holocene source bed and/or the pathway for gas escape pockmark formation. With seismic profile data and future collection of vibracores in Belfast Bay, Maine, we hope to better stratigraphically constrain the evolution of these innershelf geomorphic features.