An Estuarine Pockmark Field in Previously Glaciated Belfast Bay, Maine: Vibracoring and Seismic Reflection Results

Pockmarks are crater-like seafloor features that occur globally in a variety of geological settings. One setting in which these features are found is estuaries on previously glaciated coasts. These seabed features are associated with fluid escape, in this case specifically the release of methane. Many questions persist regarding pockmark field origin, and pockmark activity and evolution. Using 210 km of CHIRP seismic reflection profiles and 125 km² of high-resolution swath bathymetric data we collected 27 1.5-4.5 m-long vibracores in the Belfast Bay, Maine pockmark field. Because of the underlying geology, natural gas here must be of a biogenic origin. Within the center of the pockmark field a distinct, previously unidentified acoustic layer was imaged by seismic data and which we hypothesize is the source bed for the biogenic methane. Vibracores collected from the periphery of the field are characterized by relatively low concentrations of organic material throughout the following four stratigraphic units: 1) 0.3 - 2 m of watery mud; 2) 0.3 - 0.5 m of hard pipe-like clay nodules, 2-10 cm in diameter, within a watery mud matrix; 3) 0.1-0.3 m of low water content, low (<0.5%) organic content, oxidized mud with clasts, fissures and cracks; 4) a basal sequence of reduced mud with low water content and low concentration of organic matter (<0.5%). Initial interpretations of this peripheral pockmark stratigraphy involve the deposition of glaciomarine mud (reduced mud), formation and preservation of a degassed paleosol (oxidized mud), the disruption/entrainment of desiccated mud nodules and subsequent burial by estuarine mud. This stratigraphy differs from that exhibited by cores collected in the center of the pockmark field. This implies that distinct mechanisms for pockmark formation may be active within one pockmark field and surficial expression of the features may depend upon modern sedimentary processes and not upon specific initial origins.