Gas-related sedimentary features from the eastern Canadian continental shelf

Abstract

The first discovery of pockmarks (gas-escape craters) was in muds of the basins of the central Scotian Shelf off eastern Canada. Since then, pockmarks have been found in many continental shelf environments of the world. They may be used as a hydrocarbon exploration tool and recently their role as foci of intense biological chemosynthetic activity has attracted considerable attention. Pockmarks occur in the Gulf of Maine, Passamaquoddy Bay, eastern Scotian Shelf, Scotian Slope, Laurentian Channel, Gulf of St Lawrence, Halibut Channel and on Labrador Shelf. Their formation in Passamaquoddy Bay may be enhanced through earthquake activity as their greatest density occurs in association with a large fault zone. Pockmarks on Labrador Shelf and in Laurentian Channel may have been formed preferentially as a consequence of disturbance of the seabed by grounded moving icebergs. Pockmarks in Placentia Bay, Newfoundland occur in an area of large megaflutes at the seabed and the pockmarks may have provided an initial seabed roughness for subsequent formation of the megaflutes by a tsunami-generated turbidity current. Subsurface zones of gas-charged Quaternary sediments are often associated with areas of pockmarks. Gas-charged sediments in the nearshore frequently appear as thick Holocene deposits occurring in areas overlying buried channel systems. In the nearshore, biogenic methane is the probable source. In other areas, such as Downing Basin on the Grand Banks of Newfoundland, the gas-charged sediments contain ethane, butane and methane, suggesting a deeper hydrocarbon source. Active gas venting has been observed at the seabed in Downing Basin without the formation of pockmarks. Seabed observations from research submersibles and bottom photography have identified areas of white filamentous bacteria on sandy sediments sometimes associated wtih circular patches of dense benthic communities. Seismic reflection profiles from these areas also indicate the presence of shallow gas (bright spots) within Tertiary age sediments. Taken in their entirely, the broad distribution of gas-related features suggests that the major basins of the eastern Canadian continental shelf vent gas, but pockmarks or other morphologic venting features are only produced where soft cohesive sediments are present at the seabed. The chronic release of gas from the seabed may contribute to the biological productivity of the offshore areas as well as provide a significant, but as yet unquantified, volume of methane and other gases to the ocean and atmosphere.