METHANE SOURCE BED CHARACTERIZATION IN 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, estuaries, shelf basins, and areas of petroleum production. Associated with methane escape, 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. In Belfast Bay, Maine and similar settings in the northern Gulf of Maine, pockmarks are sourced by methane escaping from organic-rich sediments deep within the Holocene. We hypothesize that the source beds are early Holocene terrestrial (bog or lake) or estuarine (salt marsh, tidal flat, or bay) sediments that accumulated at a time of lower-than-present sea level. Results from a high-resolution Chirp seismic survey that gathered over 210 km of shore-parallel tracklines show a distinct seismic facies above the Pleistocene/Holocene unconformity. This unit ranges in thickness from 0 m to 3 m, and exhibits layered bedding. We hypothesize that this reflector represents the Holocene source bed and/or the pathway for gas-escape pockmark formation. Recently a vibracore survey collected 27 cores in precisely targeted locations identified by the Chirp profiles. Currently undergoing analysis, initial vibracore investigations indicate the presence of gas and identify a permeable bed that may act as a conduit. The permeable layer is 20-40 cm thick and is characterized by a fining-upward sequence of coarse material ranging from cobbles to coarse sand. Articulated Mya arenaria buried in life position rest atop the permeable layer and will be radiocarbon dated. With the continuation of vibracore analysis we anticipate the identification of a distinct organic layer as a source for the methane.