POCKMARKS AND NATURAL GAS IN COASTAL MAINE – THE SEARCH FOR SOURCES

Coastal Maine contains estuarine and nearshore deposits of gas-charged Holocene sediments covering ca. 311 km². Well-developed pockmark fields occur in Penobscot Bay, Passamaquoddy Bay, Blue Hill Bay, and in many smaller sites. In 2001-2002 we conducted sidescan sonar surveys, seismic reflection profiling, and vibracoring designed to elucidate origins and evolution of pockmark fields. We targeted well-developed pockmark fields in Penobscot Bay, a large estuary, and Muscongus Bay, a neutral embayment. Muscongus Bay contains large areas of abundantly gas-charged sediments, but with only one or two small pockmarks known. Our initial hypothesis for the Muscongus Bay target area, an area informally named Cranberry Basin, was that bedrock ridges may have contained an early Holocene lake and/or restricted estuary at times of sea level lower than ca. 30 m. We have recovered 46 vibracores in Penobscot Bay and 10 in Muscongus Bay, in addition to 5 and 9 vibracores, respectively, from earlier projects. The upper 3-4 m of the Holocene section in Penobscot Bay are typically olive-brown, relatively well oxidized mud with water content of 40-60%, and with abundant organic content (5-7% LOI) in the form of fine detrital organic material. Near-surface shear strengths are generally 1.5-2 kg/cm². The massive mud contains few intact mollusk fragments or foraminifera. Common zones with up to 4 cm diameter may be burrow traces, or possibly gas vent structures. Slightly indurated “pipes” in cores and dredged from the seafloor resemble published examples from known gas venting regions. Cranberry Basin cores are distinctively darker grey, reduced muds with very high organic contents (10-12%) and water contents (60-70%). Near-surface shear strength is <1 kg/cm². They contain abundant bivalves and gastropods, benthic foraminifera, with scattered sand lenses, shell hash lenses, and siliceous sponge spicules. They also contain abundant natural gas. At least one core contains detrital organic matter and abundant salt-marsh foraminifera. There is no evidence for lake sediments in the cores opened to date. The distinctly different sediments and degree of pockmark formation in the two embayments may be related to riverine inputs, paleogeography, or physical circulation. We rule out seismicity or groundwater as driving mechanisms.