HR: 15:25h
AN: B53C-08    [Abstracts]
TI: Modern Gas Vents in the St. Lawrence Estuary (Eastern Canada); Linking Palaeozoic Rocks, Quaternary Sediments and the Marine Environment.
AU: * Duchesne, M J
EM: mduchesn@nrcan.gc.ca
AF: Geological Survey of Canada-Québec, 490, rue de la Couronne, Québec City, Qc G1k 9A9, Canada
AU: pinet, N
EM: npinet@nrcan.gc.ca
AF: Geological Survey of Canada-Québec, 490, rue de la Couronne, Québec City, Qc G1k 9A9, Canada
AU: Bolduc, A
EM: abolduc@nrcan.gc.ca
AF: Geological Survey of Canada-Québec, 490, rue de la Couronne, Québec City, Qc G1k 9A9, Canada
AU: Lavoie, D
EM: delavoie@nrcan.gc.ca
AF: Geological Survey of Canada-Québec, 490, rue de la Couronne, Québec City, Qc G1k 9A9, Canada
AU: Campbell, D C
EM: cacampbe@nrcan.gc.ca
AF: Geological Survey of Canada-Atlantic, 1, Challenger Drive (P.O. Box 1006), Dartmouth, NS B2Y 4A2, Canada
AB: Since 2003, the seafloor and the subsurface of the St. Lawrence Estuary (eastern Canada) were surveyed with various geophysical tools such as high and very-high resolution seismic reflection systems, sidescan sonar and multibeam echo sounder; moreover several cores were collected. The most striking features on the multibeam bathymetric imagery are numerous pockmarks (n>750) ranging in diameter from tens to hundreds of meters. The use of multi-resolution geophysical systems allowed the documenting of their fractal nature as micro and macro seeps are observed. Pockmarks are isolated, associated with submarine landslides, linearly distributed or included in cluster features. Sidescan sonar coverage confirms that some vents are active whereas backscatter images show several highly- reflective pockmarks. Carbonate crusts were sampled in some of these highly-reflective features whereas in other pockmarks similar crusts, located at various depths in the sedimentary column, indicate possible intermittent chemosynthetic microbial activity through recent times. Seismic sections permitted to image gas chimneys beneath the pockmarks that are rooted into the bedrock and gas-related amplitude anomalies within the Quaternary succession. In addition several seismic anomalies are found over either bedrock highs or slightly- dipping features imaged near the top of the bedrock. The pockmarks distribution correlates with both the bedrock geology and the thickness of the Quaternary deposits. Pockmarks are preferentially found above the Paleozoic autochtonous domain (St. Lawrence platform) and located where the Quaternary sediment cover is the thinnest (<200 m) although seismic anomalies attributed to the presence of gas, are observed over the entire basin including where the Quaternary succession is the thickest (>420 m). The results of the study suggest that gas release in the marine environment is likely of thermogenic origin and controlled by one or several of the following parameters: 1) the occurrence of a leaking seal layer in some parts of the basin, 2) the lithostatic pressure induced by the Quaternary succession or 3) the physical properties of the underlying breached Paleozoic reservoirs.
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL (0460)
DE: 4811 Chemosynthesis
DE: 4840 Microbiology and microbial ecology (0465)
DE: 8169 Sedimentary basin processes
SC: Biogeosciences [B]
MN: 2007 Fall Meeting